The other day, purely by happenstance, I came across a video made in 1969, published by the AT&T Archives, called Life of the Telephone Operator. There is a fifteen to twenty segment portion of that video which shows all these enormous banks of this beautiful analog electrical equipment just clicking and whirring away. I was immediately transfixed. I was born in 83. By the time I even cared about phones, it was already 93, and by the time I began to really use them, it was 99. So, all of this stuff phased out just as I was coming into this world. So I never knew about it, and never thought about it. But when I saw that analog equipment just clicking away, and learned that these central office/ switching buildings were everywhere, I was just blown away. I had to know more. I had to know how it all worked. I want you to know that I voraciously devoured every second of this hour and 40 minute video. You did such a beautiful job demonstrating the line finder. This all just takes my breath away! Why does it amaze me so much that we had the capability of undertaking the monumental task of meeting the country's telephony needs by building these Rube Goldberg-esque analog machines? Thank you so much for making this video. I am already aware that you have others. The second I finish posting this comment, I'm moving on to the video about the step by step dialing to Pennsylvania 6500. Thank you so, so, so much for publishing these videos for weirdos like me who just have to learn about obsolete technology simply because it is so completely amazing and fascinating. Thank you!!!
You’re welcome! And thank you very much for your generous words of appreciation. I daresay very few have watched this video all the way through. Those that do must have a very keen interest in the subject and a real thirst for details. I am particularly delighted when a young person such as yourself has such passion for our electromechanical telephone heritage. I didn’t see any other videos that attempted to explain this equipment in depth, so I thought since I have the equipment and the time (long since retired), I would make an attempt. When I receive a comment like yours, it makes all the effort worthwhile. I hope you enjoy the other videos. Thanks for watching.
@@shaggydogg3786 The Bell System was heavily involved in the development of relay based computers in the 1930's and 1940's. Such computers formed the backbone of their common control crossbar telephone switching systems, which began production in 1938. I briefly describe this in my video "The Crossbar Switch", but I lack the equipment to do a thorough video on this subject.
Well, we had traffic engineers, and equipment engineers, who monitored the network, and planned additions to keep up with demand. When I was a boy, not everyone had a phone. There were party lines, and sometimes in an emergency, you called someone close, and they ran and got your party. These old switches didn't last that long. In my short career, we upgraded from ringdown magneto sets, to operator handled, to automatic dial (this switch), to common control, to direct long distance dialing, to electronic, to digital. All that in just over thirty years. My career was with the Bell System. I was fortunate enough to work most of my time in switching. I worked many central offices of various types. I worked SXS, 5XB, 1ESS, and 1AESS. Later, I did translations for all of them, plus digital switches. It was great fun. With your curiosity and enthusiasm, you'd have made a great switchman and trouble shooter. SXS, what is shown here, means Step by Step, because your can actually follow a call visually from the line finder, to each selector, to the connector, and watch them operate. If you were there long enough, you could identify a broken rotary pawl from anywhere in the office by ear. My most fun times were in SXS and 5XB as a switchman. After that, it got to be more theory and trouble numbers. Sorry you missed a great party.
I always imagined simple stepper relays wired one into the other, Wow, its Way more than that. The fact it worked as dependably as it did is simply mind blowing!!! It is the highest testament to the mechanical and electrical engineers who designed it, in 1920 no less. It should humble modern engineers. Thank you, you put a whole lot of work into something that the few rare people who dont look at cats all day long will watch.
The development of the SXS switching system was started in 1889 by Almon Strowger. The workings of a two motion stepping device was so intricate that Almon Strowger was unable to get a reliable working version built. It was the brilliant people that he hired, Alexander Keith and brothers John and Charles Erickson, that actually built and patented the first recognizable version of the SXS switch along with the rotary dial, both in 1896. Strowger’s company evolved into Automatic Electric (AE). AE continually made massive design improvements, such that by 1920 it had evolved into a reliable, efficient and cost effective system. It was at this point (1920) that the Bell System caved in and began purchasing AE SXS equipment. The Bell System’s manufacturing arm, Western Electric, began manufacturing its own version of SXS in 1926 and introduced the SXS line finder in 1928.
This technology was used in many big cities (Cleveland) was one until cutover to #1 ESS in the early 70s. At the Clevoh45 this required 45 techs, after cutover to modern digital electronic technology that was 3 people on the day shift.
I learned step by step in the Air Force back in 1973. I remember taking most of a day in class tracing schematics just to get from off hook to receiving dial tone. Thanks for the excellent explanation and thanks for taking me back in time.
I worked for decades in the telephone industry. I am extremely familiar with the GTE Leich All Relay TPS (terminal per station) switching platform; still have a few relay bars from it. Nice seeing the relay switching equipment in operation.
Electromechanical switching equipment served the public extremely well for over 6 decades. Now it's intriguing to see and listen to all the moving parts that have long gone from our lives. Thanks for watching.
This one is the most fascinating to me. I think I've watched it more than a dozen times. It annoys me no end that I worked around these central offices so many years and never once wondered how they worked. But I was an outside plant kind of guy and the CO was somebody else's job. How I wish I could go back and spend some time in there knowing what I now know about them. Thank you so much for creating these videos. I know these are difficult to produce and take a lot of time but I hope you have more information that you can share though I can' t imagine what it might be.
Many thanks (well at least 12) for watching.You're comments make it all worth while for me. My videos have now covered all of the switching equipment I have. However, I may do a video over the winter on the detailed operation of a rotary dial 3-slot payphone connected to my demo unit. I haven't seen anything on you tube that provides the level of detail I would like to see.
That would be great. I worked on that type only a couple of times. Single slot was everywhere when I was hired in 1975. But it's the same, I knew how my end of it worked but not the CO end.
I watched this all the way through - twice! And can hardly wait to move on to the selectors and connectors!! For years I have always wondered how this equipment works since peering through the windows of a central office when I was growing up in Detroit, back in the days of step-by-step and crossbar switches. Thanks so much for sharing your knowledge and equipment with us... This is so fascinating and quite a pleasant experience to understand and admire the ingenuity that went into these devices at this level of detail.
It is very satisfying to me when I hear from someone who has watched this video all the way through (and in your case, twice) and found it fascinating. It makes all the effort I put into it worthwhile. I hope you enjoy the other videos. Thanks for watching.
It’s fairly simple. The first selector is controlled by the dial pulses for its vertical steps then it automatically cuts in to complete a circuit to the second selector. The second selector is also controlled by dial pulses for its vertical steps then cuts in for a connection to the connector. The connector is completely controlled by dial pulses for vertical steps then for the cut in steps. I could tell what bay I was in by listening to the switches. The line finders were fast. The selector was slow vertically following the dial pulses then fast on the cut in. The connector was slow vertically and on horizontal cut in following the rotary dial pulses. It’s fun to remember this but as useless as tits on a boar hog.
Yes, I have a very keen interest. Thank you for such a detailed video explaining it, I always wondered how a handset going off-hook invoked the line finder process and this has been of great help. I will say, there must have been a TON of effort to film and edit this. Thanks again.
Thank you for taking the time to make this set of videos. I have always wondered about the differences between the Western Electric and Automatic Electric step-by-step system. I have 200 lines worth of AE step and have done videos of Audichron time machines.
ABSOLUTELY LOVE THIS VIDEO! It could have saved my parents dial telephone 50 years ago if I had seen it then. I wanted to know how telephones worked so I took it apart and broke the plastic plunger in the hookswitch. Thanks!
I was home alone l disassembling my working mom's phone when I was 10 years old because, of course, I too had to know how it worked. The whole time I was disassembling it I was holding the switchhook so that the dial tone wouldn't play and the scary off hook alarm wouldn't happen. I was so scared of the off hook alarm as I was convinced that it would tip somebody off that I was breaking rules and taking the phone apart. I was sitting on her bed and had the whole cover off, holding the switch hook down and was staring at it the innards... when the phone rang in my face and so startled me I remember the whole thing 50 years later! I had that whole thing reassembled in 30 seconds I bet! I always wondered if some psychic with a great sense of humor just couldn't help herself! Nobody ever called during the day and I was home from school pretending to be sick that day... I decided someone must always be watching! Only three years later I was gleefully hacking in any old phone in our house and adding extensions in those hardwired don't-touch days. I later became an electrical engineer and my first job was to design D4 channel Bank cards and learned all about telephony and designing products that survive lightning strikes and AC crosses. I once got to tour a still working step-by-step Newport RI central office as they were preparing to migrate to a digital switch in 1985. The sea of chattering relays and continuous 20hz ringing monitor bell that confirmed the alternator was spinning was memorable. our tour guide also told us that the technicians we saw wouldn't be working there in another couple of months and that the people managing the new switch would be in some office somewhere else.It made me feel a little sad and made me wonder if the channel bank cards I was designing we're headed for obsolescence... Still reading? I have more stories my wife isn't interested in!
It really says something about the reliability of electro-mechanical relays, cams, levers and ratchets. At least there were no software glitches! There's a simplified version of this used in vintage jukeboxes. They were called stepper units. And they were used in conjunction with the wall boxes because the wall boxes only had four wires going to the Jukebox. A series of pulses from the pulse generator which is very similar to this where you have a wiper running over a series of contacts. The Jukebox would interpret those pulses through a device similar to this video and it would make a selection. It was a real pain in the ass to work on so I can have some real appreciation for what's showing in your video here.
Just a comment about electromechanical reliability. I worked at a CO that had 45 technicians working around the clock to keep a 40,000 line Office working. After final cutover to digital, electronic technology. There were 3 techs on day shift.
@@calbob750 Electro-mechanical devices do have their issues. Mainly physically wearing out, contamination and oxidation on contacts, etc. My point is they're surprisingly reliable considering how primitive they are. The number of these units the phone company had, probably tens of thousands, I'm positive when they went to solid state, maintenance dropped by 90%. I currently have a Bluetooth issue with my phone in my car. The BT can't "locate" the car sound system half the time without restarting the phone. I found the solution in that little remnant of 70 year old analog technology: the headphone jack direct to the aux input. Works every time. 😄
Rotary phone system went offline about 20 yrs ago in my reich , but in 2022 i will be building my household rotary based telephone system has an intercom phone system. im going to injoy watching this 2 hour long film just to learn the history behind the idea of rotary phones :D
The mechanical switching technology is always fascinating, I'm intrigued, how many lines do you have how big is your ranch? a mechanical switchboard is robust and reliable, but requires the use of disk telephones, the external line can also be VoIP with an ATA adapter, cmq it is not difficult to replace it with a modern PBX switchboard and with twisted-pair telephones, reusing the existing network , cmq I don't know if I'm wrong, but a wireless system or with GMRS radio with license or extended wi-fi network and smatphone with VoIP app I see it more modern and practical, it does not require you to go to the phone in the place where it is installed
Excellent explanation what will happen in a second of dialling a phonenumber. Thanks very much for sharing. Amazing how ingenuity has once come to life. The more I learn by these great videos, the more I'm I'm impressed by the craftsmanship and knowledge that founded our era.
Not to say, the seekers before those pioneers. Also in our time, we have zillions of little inventions that form a great leaps of evolution. Almost, sure that preceding the result, of each improvement, thousands of failures likely existed. From your video, I learned and realise that not only the sequence but also the little smart (necessary electro/mechanical) interacting tricks that made it possible to make a connection and detect unavailability. Thanks for explaining all of that which will require substantial more time to comprehend it as a whole. It looks so simple but is mind-blowing looking at the details.
Everyone stands on the shoulders of those that went before them. You might be interested to know we were doing digital multiplexing over 50 years ago. Some people look at these switches and call them Rube Goldberg, but the planning and engineering that went into them were far from it. You have to crawl before you can run. We were conceiving and planning digital switches while we were cleaning switch banks, changing wipers and cords, and welding pitted relay contacts.
You can see from this why the rotary dial phone took so long to record the numbers. The lag was from the electromechanical technology delay making the connection.
I had a friend up in Alaska that worked in a CO with about 5000 subscribers and used these Strowger Steppers. He explained it to me and I almost understood it. I loved the sound of the racks of relays stepping away, knowing that the 10 clicks per second of a line finder meant someone was dialing a number.
Actually, its the selectors and connectors that respond to the rotary dial pulses, not the line finder. But I understand what you mean about loving the sound of the switches in motion.
Thank you for this amazing and in-depth explanation! :)) Electromechanical devices like this are fascinating. I'm sure a lot more mental and physical work went into designing and developing these devices than designing anything "digital" today.
You wouldn't think, but development is a prossess for some sharp people & this is the epitaph of the type of switchgear my uncle continuously maintained to develop further for compulsory needs-crazy cool stuff, The Transistor still had to materialz with really sharp people.
Well, the SxS epitaph was a long time coming. It lived on in the North American PSTN until 2002, when the last one was decommissioned. Thanks for watching.
Thanks for this detailed explanation of a complex mechanism. I built a small exchange myself using a number of uniselectors as first selectors. That configuration is a lot easier to understand than the linefinder alternative.
I'm glad you liked the level of detail. But it's understandable since you are a switch enthusiast. It's always nice to hear from a fellow switcher. Thanks for watching.
The mechanical switching technology is always fascinating, reliable and robust and immune to any electromagnetic impulse, both natural and artificial, certainly a modern switchboard even with continuous maintenance after 100 years like this one no longer works and is also irreparable, just think of the electrilitic capacitors on the cards, of course it is more practical, it has a thousand functions and is DTMF but if it is broken it is useless, nice that some switchboard has remained operational as a museum, in 100 years there will be no electronic switching systems working in museums.
Well, there is no joy in staring at a box with no moving parts and no sounds (except perhaps a continuous hum). The new equipment is, of course absolutely amazing in what it can do. But at a show, my demonstration unit attracts plenty of attention simply because it has many moving parts that can be seen and a lot of noises that can be heard, all of which is under control of the interaction by the users operating the rotary dial telephone sets. Thanks for watching.
This has got to be the BEST video I have ever watched on this topic. Thank you so much for taking the time to put this together. It is clear, concise and informative. Cheers!
I can't wait to find some time to watch this! I never really thought about how phone lines worked pre-digit phones, as in how they worked at the call station/center.
To get the full picture, you'll also need to watch my other two long videos, one on the selector and then the one on the connector. Just search my channel name, Hicken65, on You Tube.
One of my old friends became an engineer with the local Bell operating company. Years ago I asked him about the reliability of the #1 ESS with all of those discrete semiconductors, resistors, and capacitors on circuit cards. Do you have to replace them often? "No, almost never. The least reliable part is the reed switch arrays."
Well done sir.. I did watch the whole vid (but did fast forward at certain parts). As a die designer, I’ve seen many dies which made similar stamped parts, beryllium copper flat springs. Also, my father was an electronics / electro mechanics teacher. Was taught by him in my early days.. always liked seeing devices like these. All those people involved in its development and manufacturing, etc. Thanks for your expertise.
I love this video! Very informative. I am working on building a very scaled down working electromechanical telephone exchange. When it's done, all of its parts will all be polished up and mounted to a custom made brass and wood "stand". All the wires will be properly "stitched" and of course there will be multiple different phones all connected and working. It's a labor of love, so to speak, but these devices have fascinated me for years and years. The one thing I haven't been able to find as of yet are schematics and drawings of individual stepping switches, line finders, etc. Any info or websites you may know of would be greatly appreciated.
Go to the "Telephone Collectors International" website. Click on "TCI Library". Click on "BC Switching Library" and search from there for Schematic Drawings (SD's) and Circuit Descriptions (CD's) for various manufacturers. Good luck with your project and thanks for watching.
Takes me back to my apprenticeship and City & Guilds telephony exams. "Draw and describe the operation of the rotary hunting and testing mechanism of a two motion final selector"
I worked at the Clevoh45 in the 70s. It was a Panel office. Ohio Bell kept this 1920s technology going until the early 70s when it cutover to #1 ESS. The manning went from 45 technicians to 3 after the final cutover to other technologies. I’m impressed with the knowledge shown in this video. I doubt if the average CO Apparatusman or Central Office Technician has as comprehensive knowledge of the technology that they worked on.
You're probably right. I felt I knew more after researching and producing the video than I did when I worked on SXS. Mind you, I never took any formal SXS training and just learned from the "old guys" as I went along. Also, I didn't spend a lot of time in the SXS. Our CO had both a large SXS and a large #5 Xbar. I spent most of my time in the Xbar. Thanks for your feedback. It's always nice to hear from a fellow switchman.
The other amazing thing is that you’d need to have so many selectors for the prefix of the phone number. In the city I grew up in we had 42x-xxxx, 45x-xxxx, 47x-xxxx. There were about 25,000 people. Now there are 500,000 people and many more exchanges. However if you divide 25,000 into 3 it means each prefix had to support 8,333 people. That’s a lot! The digit 4 was ignored but the 2 would be significant. You’d need many selectors if many callers dialled 42x-xxxx. It’s mind boggling.
Very comprehensive video, thank you! I wonder what kind of make- and break-contact symbols these are and how common they were used in what area. Do you have any info on that? Were these used only in early American SXS-Systems, or also in later Panel and Crossbar switch circuits? Was there a standard for notating these circuits? And what symbols were used outside of the US? I know several others, some of them might be much more common today, at least in Europe.
These symbols are known as detached contact symbols. On schematic diagrams, the relay contacts were moved away from the relay coils and placed where they were functionally utilized in the circuit, which greatly simplified the analysis of the circuit. This approach was developed by the Bell System Labs in the early 1950's. You can read all about it in an article in the Bell Labs Record (BLR) of July 1954, titled "Detached Contact Schematics". You can search online for the Bell Labs Record. Thanks for watching.
Do you have a silimlar video for panel and crossbar switches? This is very helpful in understanding the actual electrical path of the call. All other videos I see bypass the detail and just say "here's the sender and the marker" but don't go into detail HOW they determine a free line finder or circuit.
No, I do not have any of the panel or crossbar equipment necessary to make a video. There is quite a bit of literature online if you search hard enough.
The development of the SXS switching system was started in 1889 by Almon Strowger. It took about 30 years for it to mature into a reliable, efficient and cost effective system, with most of the design improvements made by Automatic Electric. By 1920, it was accepted by the Bell System, who introduced the SXS line finder switch around 1928. Improvements continued through to the 1970's, just as large scale removals of this equipment began. At their peak, the number of SXS lines in the Bell System reached about 25 million.
@@Hicken65 Until the advent of cheap digital electronic devices, the SxS system was probably the most economic switching system for most situations. The British Post Office had a choice when converting London exchanges (offices) to automatic - use Bell's Panel equipment or use SxS with 'directors' (a bunch of 2 motion selectors, uniselectors and relays which converted the three initial digits to a 1 to 6 digit routing code). BPO chose SxS because there was already British manufacturing facilities for the equipment and employees were familiar with it. Not sure what the raw economics would have been, but the cost effectiveness of Panel over SxS could not have have been too significant to cause BPO to choose Panel. Even Bell replaced a Panel office with SxS.
@@netking66 I don't think Panel was ever used outside of the Bell System. In the early 1920's, The bell Telephone Company of Canada (which was closely affiliated with, but not a true member of the Bell System) was making plans for their gradual conversion to dial, beginning with Toronto. The company was heavily pressured by the Bell System to install Panel. However, the Chief Engineer was having none of it. He had recently joined BTCO from the telco in Nova Scotia, where SXS had already been successfully installed and was working well .Also, BTCO's manufacturing arm, Northern Electric was already licensed to manufacture Automatic Electric SXS. Toronto's dial conversion continued with SXS for the next 25 years.
@@Hicken65 Before Panel, Bell System developed a system called 'Rotary' (not to be confused with rotary dials). Like Panel it was power driven via shafts and clutches and used 10x30 outlet selectors (or 10x20 for final selectors). Also like Panel it used revertive impulse control where the selector sent impulses back to a register which signalled the selector to stop when in the right position. Presumably Panel's 25x20 outlet selectors was seen as an advantage since a 10000 line office just needed two stages instead of the usual three although an initial stage would be needed in most cases for trunks to other offices. Despite this Rotary went into production with the first switch being installed in New Zealand (and that switch may have remained in service until replaced by a digital switch) and Rotary was used in Paris and other European cities. Possibly Bell preferred to offer Rotary to overseas markets meaning that Panel was never used in these markets. Also Ericsson had its own 25x20 outlet power driven system which was conceptually rather similar to Panel.
I don't have any videos on the relay replacement procedures. However, a changeout wasn't particularly difficult. The relay assemblies can be removed by disconnecting the wires and undoing two screws that hold the relay to the switch base. Similarly, the contact spring assemblies can be removed from the relay heel plate by undoing two screws. Everything is easy to reach with the switch dismounted.
Would it be accurate to saw that the "relays" are solenoids? aka: transform electric current into magnetic force that cause movement? (normally, a relay is considered to be an electrically activated switch to let current flow or not).
The relay in the SXS switching system is a device that contains a solenoid as one of its components. The solenoid is an electromagnet that consists of a copper wire wound onto an iron core. The relay also has an armature, a moveable magnetic metal arm that is attracted to the solenoid when the solenoid is energized by an electric current, and the relay also has a set of electrical contacts that can be closed or opened by the movement of the armature.
@@Hicken65 Thanks. If there is both mechanical action and electrical contact, I guess the term "relay" works,. he old AT&T videos also use the term "relay".
Well, it certainly was a tremendous amount of time and effort to produce. However, to have someone watch it all the way through makes it all worthwhile. The average watch time is about 12 minutes. Thanks for watching.
@@Hicken65 don't put too much stock into the TH-cam analytics. I can watch the video in its entirety and you might get a view count but the watch time would report zero, bringing your average way down. In other words, don't feel bad that people aren't watching your video when sometimes they really are. Also, I've seen view counts and likes get lowered and even erased!
@@KC9UDX Yes, I've noticed some strange things happening with the stats as well. It doesn't bother me that many people don't watch my long videos all the way through. I realize my target audience is very small.
My dad spent many years during the 60's with these switches. Said the scariest noise he ever heard was when they had a cascade type event that locked up the switches in the whole office. The place went dead silent, at that moment they knew the SHTF...
These systems were typically engineered to allow about 10% of the telephone lines to be in use simultaneously. Unusual events that caused this limit to be exceeded by a large amount would cause a rapid deterioration of service.
@Hicken65 , wow, grew up with step by step service in Baden, PA but never remember the phone service going down because too many people were using it! Why did they continue to manufacture step by step switches in the 1960's when crossbar #5 was the newest & best switching?
@@jgrysiak6566 SXS switches were still needed to handle the growth in the towns and cities where these systems were in service. Generally, #5 XB was not being used to replace SXS because it was not cost effective to do so, especially in one exchange towns. In cities, XB was often added to provide for growth with the existing SXS remaining in place. In these locations SXS equipment was still added for trunk growth. Large scale SXS replacement began in the 1980's with the introduction of digital switching equipment and their associated lucrative feature rich capabilities.
@Hicken65 , Really wanted a pushbutton phone & was fascinated with them in the 1970's but sadly was not available until Oct 1980, when Baden, PA cut over to #2BESS from SxS. Finally got my dream of getting a pushbutton phone in the fall 1982, my parents wanted me to take it back but then eventually they liked it & kept it. Lol
@Hicken65 , there was a ton of growth in the Baden, PA SXS office in the 1970s. If they would have built a #5 crossbar switch, would they have had to assign another nxx to the town?
So this sort of is making sense. So one line finder is required per call and active the whole time the call is in progress. So whole bank of finders can handle 20 calls. Do I have that right? Only 20 calls per 200 subscribers seems a bit low. So is expected usage of the system only 10% of the amount of subscribers? Or would there be more banks of finders for 200 subscribers? Well 198?
One linefinder per call tied up for the entire duration of the call is correct. So one fully equipped line finder group could handle 20 simultaneous calls. Typical busy hour usage was 10%, sometimes less. Many offices did not need to be fully equipped with 20 line finders.There was also a 30 capacity linefinder group available for heavily business oriented offices. Telcos also had the option of loading a line finder group with less than the maximum of 198 working lines, where 20 line finders proved to be insufficient.
Those numbers were not lightly arrived at. 20% was the average they forecasted as a design element starting point. But, remember not all terminals were necessarily assigned. That was carefully watched. We had service observing and traffic engineering, who would constantly monitor usage and overflow within all LF groups. They did line load control, and network balancing frequently, to insure the subscribers would not overload their particular group. High usage customers, like some businesses, might be in a less densely assigned Line Finder Group.
Just a comment from a former COT. This limitation on the number of calls that could be placed at one time usually wasn’t a problem. In the Cleveland area the one exception was when Santa Claus was on the local TV station. At 3 o’clock he would ask kids to call him! The CO would tie up because of hundreds of kids all trying to call at the same time. There was an emergency option called line load control. Activating this only allowed essential subscribers like Fire, Police and Medical to get dial tone and make calls. This was on Panel Switching.
Hahahaha..... And you guys thought the Space Shuttle was complicated ! What I find amazing is that a single Microprocessor can do all the hardware logic through software. But this hardware of the past is absolutely brilliantly designed. Wow 😯
Modern digital switching systems are absolutely amazing, but not interesting to look at in action, unless you enjoy looking at motionless boxes with a hum. Many people find the old electromechanical systems enjoyable to watch and listen to and to even understand how they worked. There is a great deal of reading material describing the SXS technology. My videos are an attempt to bring this literature to life. Thanks for watching.
Ah yes...I cut my telephony teeth on SXS in the 1970's. I loved the clatter of the exchange and could often pinpoint troubled switches just by the change in the rhythmic flow of the normal office sounds. Later in the #1 and #5 ESS offices, I was never as happy as those earlier noisy days. Perhaps you could include discussion of class of service or other control conditions thorough use of normal post cams and contacts at some time.?
Always good to hear from a fellow switchman. It was great to detect an irregularity and get it fixed before a trouble ticket was issued. Unfortunately, my line finder is not equipped with the optional class of service screening hardware. I do demonstrate the the use of the normal post springs and cams in their digit absorbing application in the selector. This is in my video "The Selector In The Step-By-Step Telephone Switching System Part 2". You can find it by searching my channel name Hicken65. Thanks for watching.
Ah well. If complicated technical subjects could be learned by watching a video this would be a vastly different world. How long was the Bell System training school to teach competence on these systems?
I never had any formal training on SXS, I just learned from the old guys in the central office. So I don't know how long the training course was. I did take the training course for No. 5 Crossbar, which was 11 weeks. Thanks for watching.
@@Hicken65 to actually comprehend a schematic I always had to make a series of 11x17 copies and mark them up with different color highlighters to highlight the circuit path as well as the contacts of a particular relay. I had never seen the use of / and X for NO and NC contacts before. (Or is it the other way around?) But it is easy understand that back in the day of hand drawn schematics / and X were far easier to draw than two small circles with a bar connecting them for NC and with the bar hovering over the circles for NO. Where'd the tip, ring and sleeve convention originate?
The original schematics for SXS equipment were conventional "attached contact" drawings in which each relay's coils and all of its contacts were shown in one location on the drawing.That is, the relay's contacts and the wires that connected to them were depicted directly above or below the relay coil. This was difficult to read and trace. A different method was employed for crossbar systems called "detached contact" schematics. The relay contacts were moved away from their coils and depicted in locations that pertained to their functionality in a circuit. This allowed complex circuits to be broken out into many Functional Schematics (FS). So for example, a completing marker in a No. 5 Crossbar office, which has over 1500 relays and many thousands of contacts, was broken down into hundreds of pages of FS's to facilitate trouble analysis. In each FS, the detached contacts were shown as a short vertical line for a break contact (which has a through path for electricity when the relay is released and a broken path when the relay is operated) and an "X" for a make contact (which has a broken path when the relay is released and a through path when the relay is operated). Some of the SXS schematics were converted to detached contact type, which made reading them much easier. It is the detached contact methodology that I am using in my video. In the days of early telephony, when manual cord switchboards were dominant, the male plug on the end of each cord was a 3-wire plug. So the metallic portion of the plug that fitted into a jack had 3 insulated sections. One wire was connected to the section at the very end or "tip"of the plug. Another wire was connected to a narrow circular "ring" section behind the tip. The last wire was connected to a long cylindrical section that enclosed all three wires like a "sleeve".
Lenny... awesome video. Back in the day I made what I call the switch bobbin (brass grooved part) Have one on my desk as a reminder of my screw machine day. Would you call this an analog to digital switch system?
I'm not quite sure what you're asking but I'll answer what I think you're after. The SXS system was replaced by another analog system called crossbar. Crossbar was replaced by a hybrid analog/digital system that had a digital CPU but an analog switching network (#1 ESS and SP-1).These were followed by all digital switches such as DMS 100 and #5 ESS.
@@greenlawnfarm5827 Possibly, in the tiny relay contact points. These relay contacts were precious metal alloys. There were two major types called No. 1 Contact Metal (platinum, gold and silver) and No. 2 Contact Metal (palladium, platinum, tungsten). One or both types may be present in these step-by-step relays.
What isn't much covered in the video, to add to the complexity, are the blocking events or the ratio of line finders to subscribers which in the explanation seems to be about 10 or 20 line finders for 100 subscribers. In other words, the entire rest of the system can be idle but in that group of wires, 10 or 20 subscribers can block the other 90 or 80. That is a bit unlikely to happen but in a local emergency, such as a house on fire, it is likely that all of the neighbors are going to be vying for the same line finders.
The number of subscriber lines sharing up to 20 line finders is 200, not 100. So 20 lines could block the other 180, and often did in the busy hours, but for very short intervals of time, generally less than three seconds.The system was intentionally designed to have 98.5% of the calls get dial tone within three seconds in the busy hour, which meant that 1.5 % of the calls would be delayed greater than three seconds.This was deemed to be a reasonable cost/service relationship that was mandated and monitored by the government. This service criterion only applied to "normal" traffic. Any abnormal traffic due to external events, such as severe weather, emergency situations, serious breaking news, etc. were excluded from the service statistics, and thus not included in the provisioning process. Your remark about the neighbors vying for the same line finders would only occur in a small office with one or two line finder groups. Subscribers sharing the same feeder cable into the central office were distributed over as many line finder groups as possible, so that in the event of a cable cut, in which there would be many short circuits, the effect on any line finder group would be minimalized.
Well, when you've mastered the line finder, be sure to take a look at my detailed videos of the selector and the connector. Just search my channel name, Hicken65, on TH-cam. Thanks for watching.
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This was really fascinating. The complexity and expense of Central Offices helps explain why pre-1990 socialist India with govt. telco could never keep up with demand for new subscriber connection. Waiting period for new phone service in 1970's and 80's was still 2 to 6 years. Then along came affordable cell phones and everyone got one. And imagine how the paltry connectivity in the pre-cell phone era of India negatively impacted the speed of business. Well, supposedly one could pay a bribe and get a telco hookup faster. This vid serves as a great hyper focused example of how socialism sucks and capitalism rules, specifically it sucks in ways people might not know since they are not industry insiders, yet all are affected negatively.
Well certainly the cost to equip landline telephone service for such a large population would have been enormous. In addition to the actual switching equipment there are the the costs of the power supplies, buildings, transmission equipment and all the outside plant cables. Thanks for an interesting perspective and thanks for watching.
I was a technician for Western Union at a centralized telephone bureau and we had a switch similar to this one. That was a long time ago. I miss working on mechanical things and tube equipment. Now just replace the circuit board and throw the bad one away.
Hi. I must have watched about 20 various videos on how telephones work etc., and even I can't believe that I watched ALL of this one as interesting as it is. 1hr 40mins is a long time, but I stuck with it 'cos I wanted to know. I've got most of it, and of course I now realise that the relays are THE crucial parts; what with introducing deactivation delays, and multiple contacting. Much of this makes more sense of what I used to know as a child from the late 50's and early 60's dials clicking on their return, sparks on 2 wires. Oh what fun. The one main question I have now is this. How on earth did an undertaker conceive of such a switch, indeed an entirely automated telephone exchange when the 'birth' of electricity was barely only 50yrs earlier? Very few people were THAT familiar with electricity back then. So, a bit like Filo T Farnsworth, a 17yr old farm boy conceiving the electronic television principle, how did Strowger do this too? Thanks for sharing this video. Regards Mark in the UK
Strowger certainly had a good understanding of electromagnets, pawls and ratchets and his concept of how to apply these components in an automatic telephone selecting device was quite an achievement. However, his device was a very rudimentary one in which each electromagnet was directly operated by the calling subscriber, necessitating 5 wires between each telephone and the central office. Telegraph type keys on the telephone set had to be pushed once for each vertical and rotary step taken by the selecting device. His device did not contain any of the intricacies of the switches in my videos. As simple as his two-motion selecting switch was, Strowger was unable to get a reliable working model of it built, and had to design and file another patent for a single motion (rotary only) switch, which was the one installed in the world’s first automatic exchange at LaPorte, Indiana in 1892. Strowger left the telephony business in 1896, due to failing health just as the brilliant people he had hired actually designed and produced a workable two-motion stepping switch and the first rotary dial. It took the next 30 years to fully develop and refine the step-by-step switching system to the level depicted in my videos. To fully understand the system, you need to see my other long videos on the selector and the connector. Just search on my channel name, Hicken65, in You Tube. Thanks for watching.
@@Hicken65 Strowger's company became the Automatic Electric Co, which furnished and further developed the step-by-step equipment. Bell System at one time was buying AE up to the mid-1930s.These bore a WE nameplate, but was really identical to AE,
@@rayfridley6649 Undoubtedly, AE gets all the credit for SXS design up to the mid 1920's. However, Western Electric began manufacturing SXS equipment in 1926. From that time on, the Bell System made many significant improvements to the SXS system, some of which were adopted by AE. The design of the two motion SXS line finder belongs to the Bell System. AE only had the Keith Line Switch and the rotary line switch until Bell came out with the SXS switch line finder.
Don’t forget Western Electric and Bell Labs developed most of the technology in a Central Office. There is a switching system that was used in big cities called Panel. It was developed and installed in the 1920’s. Many COs used Panel until cutover to ESS or Crossbar in the 1960s or 70s. The knowledge shown in this video exceeds anything I experienced in the CO or in training at Bell System training centers.
I work in a switch in the UK and the noise isn't from switches and relays but rather from the air con units for the PDH and SDH networks. As time moves on, SDH and PDH is meeting the same fate as the switches and relays as their twilight days draw in with telco providers switching their users to IP based solutions
Very intensive and great video. The complexity of this mechanical exchange is mind boggling, yet it was very reliable. We had a step by step exchange until the late 1980s when it was replaced with a digital system. I used to pay attention to the sounds when dialling a number. Now I understand why the last 2 digits dialled did not produce any clicks. All of the previous digits before the last two digits would have a thump sound, and that would be the wiper spinning over to an available path. I’m still puzzled by some aspects of this. If the wiper scans over the circuits testing them for ground until it finds one that’s free/available, how come it would not make a click in the ear of the people already using those circuits on other calls as it swipes past them? It would seem as if it’s physically cutting into the line and then moving on to the next one. Very strange. In my city all phone numbers started with the digit 4. On the step system the 4 did not need to be dialled. You could just dial the last 6 digits. I noticed when I picked up the phone and dialled the 4, it would remove the dial tone but there would be no thump. This means there was no horizontal sweep happening. When dialling the second digit you would hear the horizontal sweep. Or of you left the 4 off and dialled 6 you would hear the thump of the vertical wiper action. I’m not sure how they did this, but they seemed to deactivate the first dialled digit but only if it was the digit 4. If it was any other digit dialled it would activate the horizontal wiper movements. After watching this video, that part puzzles me.
When a selector is rotary hunting for an idle trunk, its tip and ring wipers are open (electrically dead). Only the sleeve lead is alive. For your questions on short dialing, you need to look at my video "The Selector In The Step-By-Step Telephone Switching System Part 2" which explains the concept of digit absorption. You'll find it by searching on my channel name Hicken65.
Hicken65 Thank you that makes sense that the tip and ring would be dead or yes it would interfere with calls as it swipes across the terminals. I’m assuming one of the magnetic relays removes the power from the tip and ring as it would need to be connected once it found a vacant line. I’m so interested in this but there’s so much going on that it’s hard to retain everything. The switch is doing a ton of things. It’s hard to believe this was designed so long ago. Not much longer after electricity was discovered. Not only would this take a very analytical mind, but the mechanical design - the dogs - the spring - the failsafe grounded circuit so you can’t dial more than 10 pulses. It would require enormous skills to mechanically put together something like that. There’s no CPU or computer behind all of this. One magnet is activated by the result of another.
The workings of a two motion stepping device was so intricate that Almon Strowger was unable to get a reliable working version built. It was the brilliant people that he hired, Alexander Keith and brothers John and Charles Erickson, that actually built and patented (U.S. patent 638249) the first recognizable version of the SXS switch. I never cease to be intrigued by watching and listening to one in action.
@@Hicken65 In looking over the patent drawings, the mechanism invented in 1895 is essentially the same as later makes. It's a bit of a wonder to me that WE did not re-design the relays, etc. (The panel system had a relay design without bearings that was later deployed in crossbar.) Presumably the patents had expired by 1921 when the Bell System started deploying SXS offices. They held out for a long time with manual operators. I think this was considered a service feature. The Keith/Erickson design is clearly the work of experienced mechanical designers. Strowger had a funeral parlor to run. Curiously, the panel system was deployed in 1923. COs were deployed with both systems, but it is not clear to me what advantages panel had in large metro COs. My Midwestern city of a million only had SXS offices until crossbar came in (about 1961.) When it first arrived, the local techs did not understand it well. My grandmother's phone in a growing part of the area went from SXS to crossbar. It did strange things and didn't always work predictably. One day she called a Bell supervisor and complained. He gave her a spiel on how wonderful this technology was. Being a non-technical person (but saintly) she listened and then replied, "and wouldn't it be wonderful if it worked!" The crossbar exchange was not replacing SXS but supplementing it in new areas. Only later, of course, it was all replaced by ESS. One city exchange was, after 55 years in service, replaced by a #5 ESS. It was not worn out, but for lower maintenance and especially for new features that could not be implemented easily in SXS, the ESS was purchased. I was a MTS at Bell Labs and one of my contributions helped lead to the invention of Ethernet. Some years ago the CTO of Cisco said, "Eventually everything becomes Ethernet." You could easily interface a computer to a crossbar switch, and it would not be an unreasonable design. But today it is hard to compete with Internet Protocol, with the latest switch chip able to run 64 channels each at 400 billion bits per second.
@@AriBenDavid The Bell System used the Automatic Electric SXS systems for the first few years. WE began making its own version around 1926. Strowger left the industry around 1897 due to ill health. He died in 1902. I've never been involved with the panel switch and don't know much about it. I believe it had the ability to store the dialed digits, perform translation tasks and route the calls using dial senders. This gave it much more flexibilty than the direct control SXS. NYC had about 200 exchange names in the 1920's, too difficult to be handled by SXS. The crossbar problem you mentioned must have been a local issue. I worked with crossbar for many years and found it to be extremely reliable. As for interfacing a computer with a crossbar switch, this was very successfully accomplished by Northern Electric at the beginning of the 1970's. The system was called SP-1 (Stored Program - 1). It was sold all across Canada and to many independent telco's in the U.S.
Plessey was a big player in telephone switching, mostly with crossbar, analogue electronic and digital.The company acquired an interest in SXS technology when they merged with Automatic Telephone & Electric Company Ltd. and Ericsson Telephones Ltd. in 1961. Thanks for watching.
I used to test these Strowger switches at a company called Whiteley Electrical Radio Company located in Mansfield, before they were sent to Plessey in Beeston. I believe Plessey took over a company known as Ericsson’s. We only performed point to point continuity tests. The relays were later set by highly paid experts. Happy days, about 1968.
The UK also used two motion (Strowger) type linefinders, likely in the larger exchanges. In the Bell System, the first SXS systems to be used (circa 1920) were the Automatic Electric designed systems that used either the Keith Plunger line switch or the Uniselector, which was called a Rotary Line Switch in North America.The Bell System developed the two-motion line finder around 1926.
@@Thereishope664 @Hicken65 I started work with Post Office Telephones in 1973 as an apprentice. It is true that, certainly with the later 2000-type 2-motion selectors (which had square fronted rather than rounded relay covers), the line finders were huge uniselectors, but since a large number of the older exchanges that hadn't yet been updated had the 1000-type 2-motion selectors which were identical to those shown in this video, it is quite likely that we also used the same type for line finders. I vividly remember servicing both types of 2-motion strowger type selectors, but these were all 1st, 2nd, 3rd, 4th or Final Selectors. The 1000-type selectors had a bit of a habit of chewing their wipers, especially if set up in a hurry, but the 2000-type selectors rectified this by only wiping in one direction. When the call dropped, the selector would drive the rotary motion to the end of the bank, where it would drop and return to normal beneath the lowest bank of contacts. It meant the wipers were much more reliable. I have fond memories of working on the selectors. The 1000-type required you to unbolt them from the bank by removing those two nuts, but the 2000-type were a much simpler exercise to 'unjack' them from the bank. The latter also had a larger number of jack connection points to allow a variety of tests while the selector was in place. But the best thing was that, back then, we had a fully stocked stores. We always had fully calibrated tools, and there were hundreds of them, and a full set of spare parts for every conceivable selector and relay set in the exchange. You removed the selector from the bank and placed it in its test cradle at the work bench, then took a set of measurements. If you were not able to 'adjust' the device back to 'service' tolerance limits, then it had to be stripped down and the worn parts replaced. I noticed the the shaft of the selector you were demonstrating was worn. That would have affected timings, so in our day that shaft, including all the brass ratchet attachments would have been replaced. Every relay and armature had a specific set of metrics to be measured. How far the armature moved when it operated, the gap when fully operated between the armature and the coil face, and it was all adjustable. Each set of contacts had a specific spring force measured in grammes, which were all part of ensuring the timings of the relays were correct. All those ratchets and pawls had specific clearances but, when they'd all been set and you ran the tests, the machine would perform to factory specification. There was so much to learn, but I was there right at the end of Strowger. Most new build was Crossbar and then System X. Only 10-year plan expansions were still being installed with new Strowger equipment. I saw the writing on the wall and moved out into more generalised electromagnetic systems work, and eventually electronics.
I think it would have been very interesting to see how this equipment was manufactured. Great precision with terminating the color-coded wires was required.Thanks for watching.
@Marcel H One of my jobs was to check the wiring connections made by Northern Electric installers on switch extensions. They were generally pretty good, but I used to make money on bets with them that they had made an error, particularly on a day after one of their notorious parties.
I used to get a kick out of doing a linefinder chain test. The fun part of this step switch is that it was invented by a paranoid undertaker by using a collar box. He thought the local operator was directing calls to his competitor.
I wouldn't say paranoid, it was well known operators in small single both offices in small towns got up to a lot of nonsense. It was harder in a larger office with coworkers and actual supervisors, though I'm sure they found a way. And given he had people close to him tricked by this, using the other funeral home by accident, after asking to speak to his..
Well, I know that North Electric manufactured All- Relay and Crossbar systems and have seen ample documentation to support this. I have no reason to justify stating your claim that North Electric manufactured Step-By-Step equipment is invalid, I just haven't seen any documentation to support it. However, Step-By-Step equipment was licensed to many manufacturers around the world, so perhaps North Electric was one of them.
@@ryanbelknap814 No. Rotary dials generate dial pulses, which simply open and close the loop to the central office. Touch-Tone telephone sets have push buttons which generate the DTMF tones.
At 1:05, discussing the D relay staying closed if there is another dial tone request using the path, it seems to me that these double-winding relays function as OR gates, since either or both of the windings can keep the relay energized. Is this correct?
Yes this is correct.You can call the primary winding A and give it a value of 1 when it is energized and 0 when it is not energized. You can call the secondary winding B and give it a value of 1 when it is energized and 0 when it is not energized. You can call the set of D relay contacts Q and give them a value of 1 when they are operated and a value of 0 when they are released. If A is 0 and B is 0, Q is 0; if A is 1 and B is 0, Q is 1; if A is 0 and B is 1, Q is 1; if A is 1 and B is 1, Q is 1.
watched every second. circuits with an X are normally/fail open (power doesn't go through), and those without are normally/fail closed (power goes through), right?
Yes. The X is a make contact set, which is normally open and closed when the relay is operated. The short line is a break contact set, which is normally closed and open when the relay is operated. Thanks for watching ( especially the whole video).
You can join the "Telephone Collectors International " email group and post a message detailing what parts you are making available. There are quite a few switch collectors in that group.
So, since the G-relay isn't incorporated into the Linefinder switch, is it an essential part (or the only method) to determine the location of the calling line? I have recently aquired my first linefinder switch and would like to see it hunt and locate a "calling line". I understand that this process works with the commutator but I don't understand how these two parts work together to find the line. Could you explain?
Each G relay is associated with 20 lines assigned to the same bank level on a Line Finder (LF). When a G relay operates, it identifies the level that those lines are assigned to by grounding the appropriate commutator segment. It also grounds a “start” lead at a particular entry point in the LF preference sequence chain so that selection of a LF starts at the LF in which the lines requesting service are at the lowest level on the bank in order to minimize hunt time and switch wear. In a demonstration unit like mine with only one LF and two lines, the G relays are not required. Normally, a pair of make contacts on the line relay provide a ground path to operate the G relay. In turn, a pair of make contacts on the G relay provide a ground path to the LF start lead, which causes the A relay in the first idle LF in the preference chain to operate. I simply use these same line relay make contacts to ground the LF start lead directly. Both of my telephone lines are assigned to the same level on the LF bank. So instead of having a G relay provide ground to the appropriate commutator segment, I wired a permanent ground to that commutator segment. Note, however, for your LF to function properly, that is, to hunt for and connect to your calling telephone line and remain connected until you hang up, you need a line circuit for each line (which contain a line relay and a cutoff relay) and a first selector attached the output tip, ring and sleeve leads of the LF.
I am asking this for my dad: We are having some difficulties with the 355A Step by Step switching system. Looking for 224BN F-Relays, also for Line Finders ED 32000, marking on the frames in place show ED 32000-30-G3 985D which are part of the 355A switching system. My question now is when you acquired your first line finder switch, what are the markings on the frame and what are company’s or manufacturer location? Information would be greatly appreciated. Future plans we would probably need 2 line finders.
Sorry for the delay in replying. I didn't get an email notification for this comment and saw it by chance today. My line finder is a different type than yours. Mine is an ED-33013-30--G11 985D, which would work in a No. 1 SXS or a 355A. The markings on the frame are shown at the 13:27 point in the video. I don't see any manufacturers name of my line finder. However, the ED number identifies it as a Northern Electric or Western Electric product. NE was licensed to manufacture WE products in Canada. My line finder is made by NE since it came out of a Bell Canada central office. Not sure what you are looking for, but if it is to purchase a new line finder, you are out of luck. Neither NE nor WE exist any more and no company is manufacturing SXS switches. Tahnks for watching.
Ugh, at the beginning of this video, the constant switching and repeating of the same clips for minutes at a time is dizzying as hell and extremely annoying.
Almon Strowger had the concept for a step-by-step switching device and obtained a patent for it. So most of the world's two motion (vertical/rotary) electromechanical telephone switches are referred to as "Strowger" switches, including the one in my video. The No. 5 Crossbar system was introduced in 1948 and became the major growth vehicle in the Bell System for 30 years. By the late 1970's it was serving more lines in the Bell System than step-by-step. Crossbar growth was slowed down in the 1970's by hybrid analog/digital systems in the 1970's and by all-digital systems in the 1980's. Crossbar was completely (or nearly so) phased out in the Bell System by the end of the 1990's.
In the big offices, there were three or four CO techs in the office during the day. They spent most of their time performing preventative maintenance routines on the switches, and the rest of the time handling trouble reports. Small offices were unmanned but a route man would visit daily to handle troubles and wire in new subscribers.
Do the 200 lines represent subscriber lines or outbound lines? If it's outbound lines, how are the subscriber lines connected to the line finders? I assume each subscriber can connect to n-number (e.g. 10) line finders per each 50 to 100 groups of subscriber lines to reduce 1-to-1 scaling complexity?
The 200 lines are subscriber lines.They are wired to the bank terminals of a line finder group which contains 20 banks and up to 20 line finders. The 200 lines are multipled across all 20 banks so that they can be "found" and connected to any one of the line finders in the group. So 200 lines have shared access to up to 20 Line finders.
@@Hicken65 Ahhhh....got it. So the sleeve connections help in the determination if a line finder is already engaged or not. So unlike a crossbar switch, the line finder stays engaged for the duration of the call then releases upon on-hook.
@@lohphat The sleeve lead provides four main functions in the SXS system. On the line finder bank, it identifies the location of the line requesting service; on the connector bank, it identifies the busy/idle status of a line being called; it blocks a dial tone request from being initiated by a line when an incoming call is being answered by that line; and it acts as the control wire that keeps a switch train operated for the duration of a call. So the line finder, selectors and connector involved on a call are all held operated for the duration of the call by the sleeve wire.
@@Hicken65 So the sleeve is energized when active so that when the wiper scans across it's looking for an inactive sleeve lead? Then as it progresses through the connectors, idle trunks and connectors are detected with an inactive sleeve....what prevents two connectors finding an idle trunk at the same time and both grabbing it thinking they're the first? Are two calls ever linked by mistake?
@@lohphat I suppose double connections may have occurred from time to time, but I'm not aware that it was ever an issue to be concerned about. The change in the electrical condition on the sleeve lead occurs within about 50 ms of seizure of a circuit, so it would have to be a very close race to create a double connection.
I watched this and for the life of me can't figure out where the subscriber's line is wired in. When the received is taken off hook, where is that line first connected to?
The diagram at the 40:00 minute mark shows a telephone set wired to 3 different places. First, it is wired to a line relay winding in a line circuit; second, it is wired to the bank terminals of a line finder group; and third, it is wired to the bank terminals of a connector group. When you go off-hook, you bridge the tip and ring leads together inside the telephone set, which completes the circuit path to operate the line relay in the line circuit.
@@tgcoder I'm not sure what you're asking. All subscriber lines come into the office in exactly the same way as the one in the diagram. If there were 1000 lines in the office, there would be 1000 telephones wired to 1000 different cable pairs, 1000 different line circuits, 1000 different line finder group bank terminals and 1000 different connector group bank terminals.
Wow, that's expensive provisioning. Typical Bell System city offices would have 16 to 20 per 200 lines. Phone contests would tend to overload connectors, not so much the linefinders. Neither linefinders nor connectors were provisioned to handle special events. For most of their history they were provisioned based on the busy hour load average of three months.
Hicken65 we had a radio station in town that would have elaborate contests. When the announcement happened so many tried to get in every line finder. of in the entire town went up. Finally company sued to make them at least to take 10th call. Not the 300 or so they normally did. I had cleared a trouble and went to sub to test to house. Not knowing the long awaited contest had kicked. All I heard was tweedling for 10 minutes. I started backtracking to see what I had screwed up.
The company I worked for always provisioned 18 line finders for 100 lines. GoS was 1/100. Dial tone returned to the caller in 3 seconds. A choke network was employed in later years to restrict calls able to complete to contest dial ins.
@@bboogaar In the Bell System, the engineering criterion was that 1.5% of the calls would get dial tone delay greater than 3 seconds in the average busy season busy hour(ABSBH). This wasn't an upper limit, it was the actual objective. In many towns, the ABSBH originating load per main station would only be 1.1 to 1.5 ccs (hundred call seconds). Applying these types of loads to the traffic capacity tables would produce a requirement for 12 to 15 line finders in each 200-line size group. So there was quite a difference in provisioning methodology between the Bell and non-Bell companies.
As an X inside plant an outside plant cabling person how in the hell did they get an analog switch dialing rotary to do predictive programming????? somebody please answer that
In the 1970's, telephone companies began installing electronic devices in between the linefinders and the first selectors. These devices could receive and store dial pulses, perform translations, and add or delete digits, then outpulse the changes to the first selector or other switch as needed.
SXS and Xbar were completely separate and different switching systems. However they both formed part of the Public Switched Telephone Network (PSTN). They were able to communicate with each other via trunks, and there was no problem placing calls from one type to the other. It was common for a large central office to have both SXS and Xbar in the same building.The SXS was an earlier design than the Xbar. In the Bell System, SXS dominated the manual to dial conversions during the 1920's through the 1940's. In the 1950's and 1960's, new switching systems required for dial conversion and growth were dominated by Xbar. It wasn't until the 1970's that Xbar overtook SXS in the number of lines in service.
@@Hicken65 So, only a few metro areas? What were its advantages in NYC, for instance? Quite a strange contraption ! Crossbar seems more like an ESS with its complex shared markers.
@@AriBenDavid The markers were remarkable devices. They were special purpose analog computers composed of around 1000 wire spring relays. They could process a call setup in about 1/3 to 1/2 seconds.
Well, the 7 minute Dial Tone video doesn’t show or explain the operation of the relays, magnets, ratchets, pawls, interrupters, VON springs, 11th rotary step, bank multiple slip, subgrouping, transfer chain, open chain multiple, functions of the Group & Alarm Relay circuit or selection sequence process.
Haha, no no, sorry, I know the dial tone one doesn't. But I guess I thought my memory recalled a video from you that was specifically about the relays in the line finder. Anyway, I guess I'll have to go back and have a new look at the list of your videos too get reminded. Anyway, thanks, because you definitely put a lot of good work into these videos!
You're welcome. I have now made 3 very detailed videos of the 3 major types of switches, which are the selector, the connector and the line finder. They each perform completely different functions. However, the electromechanical operations involving the vertical and rotary stepping actions are very similar and use basically the same apparatus. I felt it best to keep each video independent and show the stepping details in each.
Thank you I have been searching for this info and could not find it. By the way cleaning banks was my worst routine to do. Now all I have to do is find the different chemicals we used in the flushing unit . @@Hicken65
I'm assuming you're talking about long distance billing. Up through the 1940's the billing was accomplished by the long distance switchboard operators who made up call detail tickets which were then sent to the accounting department. With the advent of Direct Distance Dialing (DDD), adjunct equipment called Automatic Message Accounting (AMA) was installed at the toll centres that produced call details on reels of punched paper tape that were then shipped off to the accounting department. SXS end offices were equipped with adjunct equipnment called Automatic Number Identification (ANI) that forwarded the calling telephone number to the AMA equipment at the toll centre.
@@user2C47 I am not aware of any ANI videos. There are probably quite a few text descriptions online. You could begin with the "Bell Laboratories Record" which is easily found on a Google search. The May 1958 issue has an ANI description.
@@user2C47 Automatic Number Identification was a signaling protocol for billing. Most local calling, before Automatic Message Accounting Full Detailed Billing, was flat rate. Local switches did not have the capability of billing. If you had a toll switch, like a Crossbar Tandem, that switch may have had that billing function. In that case, the local switch and the tandem switch would have interoffice trunking, especially for billing, called ANI Trunks. The signaling protocol was set to first send the calling party number, and then the called party number. The tandem would take that info and cause an AMA record to be taken, in the form of punches on a continuous paper tape, later upgraded to magnetic tape, and much later to a data link to the billing department. Once we went to Stored Program Control electronic (computer) switches, they all did their own billing, and toll switches didn't need to any more.
Well, you're the first one to identify that Robert Redford still that I pulled from the movie. He's actually in a crossbar switching system central office, not a step-by-step. He uses the 1013A test set that he stole in two scenes, first in a Holiday Inn PBX room and secondly, in the NYC Telephone Company CO. There are other old movies that do have scenes inside step-by-step CO's.
I love your reference to phone numbers at 1:40:09. PE6-5000 the Pennsylvania Hotel song by Glen Miller: th-cam.com/video/0OUkNOwpXtc/w-d-xo.html. and BW4-5780 a song by the Marvelettes in 1962: th-cam.com/video/Us18AUBM2RI/w-d-xo.html
PE6-5000 and BE4-5789 are the two numbers that can be dialed on my SXS demo unit. I have two short videos that demonstrate calls being dialed through the SXS equipment to these numbers. At the end , a 30 second excerpt of these songs is provided upon answering the call, although I used The Andrews Sisters for the PE6-5000 call because I wanted vocal lyrics to be heard.
I would have done a 6 screen mosaic shot so you can follow the sequence without switching full screen. Showing each step full screen is making me queasy.
@@Hicken65 I think it was meant as constructive criticism. I don't know if you have the capability to do what he's asking; it would be extremely time consuming for me to do something like that. And it might not be the only solution. But the method used doesn't seem to be very helpful, and I found myself getting queasy as well. I'm not epileptic and frankly this is the first time in my life I've experienced something like this from watching a video. Again, I'm not complaining, just trying to say that this might not be the best way to do this in the future.
Well, it certainly wasn't my intent to make viewers queasy. It never occurred to me that it would, having watched it many times on my 27" IMAC. I don't know if I could have done the mosaic approach. I don't have expensive software, just the Apple imovie 11 that came with the computer.
@@Hicken65 I never would have expected it to happen. I was very surprised that it did. I wonder if increasing the time on each frame would help. I sort of guess that what causes it is that my brain can't figure out what I'm even looking at in such a short time.
The other day, purely by happenstance, I came across a video made in 1969, published by the AT&T Archives, called Life of the Telephone Operator. There is a fifteen to twenty segment portion of that video which shows all these enormous banks of this beautiful analog electrical equipment just clicking and whirring away.
I was immediately transfixed.
I was born in 83. By the time I even cared about phones, it was already 93, and by the time I began to really use them, it was 99. So, all of this stuff phased out just as I was coming into this world. So I never knew about it, and never thought about it.
But when I saw that analog equipment just clicking away, and learned that these central office/ switching buildings were everywhere, I was just blown away.
I had to know more. I had to know how it all worked.
I want you to know that I voraciously devoured every second of this hour and 40 minute video. You did such a beautiful job demonstrating the line finder.
This all just takes my breath away! Why does it amaze me so much that we had the capability of undertaking the monumental task of meeting the country's telephony needs by building these Rube Goldberg-esque analog machines?
Thank you so much for making this video. I am already aware that you have others. The second I finish posting this comment, I'm moving on to the video about the step by step dialing to Pennsylvania 6500. Thank you so, so, so much for publishing these videos for weirdos like me who just have to learn about obsolete technology simply because it is so completely amazing and fascinating. Thank you!!!
You’re welcome! And thank you very much for your generous words of appreciation. I daresay very few have watched this video all the way through. Those that do must have a very keen interest in the subject and a real thirst for details. I am particularly delighted when a young person such as yourself has such passion for our electromechanical telephone heritage. I didn’t see any other videos that attempted to explain this equipment in depth, so I thought since I have the equipment and the time (long since retired), I would make an attempt. When I receive a comment like yours, it makes all the effort worthwhile. I hope you enjoy the other videos. Thanks for watching.
You also might be interested in the early analog computers developed in the 1930’s and 40’s. I agree this obsolete equipment is fantastic.
@@shaggydogg3786 The Bell System was heavily involved in the development of relay based computers in the 1930's and 1940's. Such computers formed the backbone of their common control crossbar telephone switching systems, which began production in 1938. I briefly describe this in my video "The Crossbar Switch", but I lack the equipment to do a thorough video on this subject.
Well, we had traffic engineers, and equipment engineers, who monitored the network, and planned additions to keep up with demand. When I was a boy, not everyone had a phone. There were party lines, and sometimes in an emergency, you called someone close, and they ran and got your party. These old switches didn't last that long. In my short career, we upgraded from ringdown magneto sets, to operator handled, to automatic dial (this switch), to common control, to direct long distance dialing, to electronic, to digital. All that in just over thirty years. My career was with the Bell System. I was fortunate enough to work most of my time in switching. I worked many central offices of various types. I worked SXS, 5XB, 1ESS, and 1AESS. Later, I did translations for all of them, plus digital switches. It was great fun. With your curiosity and enthusiasm, you'd have made a great switchman and trouble shooter. SXS, what is shown here, means Step by Step, because your can actually follow a call visually from the line finder, to each selector, to the connector, and watch them operate. If you were there long enough, you could identify a broken rotary pawl from anywhere in the office by ear. My most fun times were in SXS and 5XB as a switchman. After that, it got to be more theory and trouble numbers. Sorry you missed a great party.
I always imagined simple stepper relays wired one into the other, Wow, its Way more than that. The fact it worked as dependably as it did is simply mind blowing!!! It is the highest testament to the mechanical and electrical engineers who designed it, in 1920 no less. It should humble modern engineers. Thank you, you put a whole lot of work into something that the few rare people who dont look at cats all day long will watch.
The development of the SXS switching system was started in 1889 by Almon Strowger. The workings of a two motion stepping device was so intricate that Almon Strowger was unable to get a reliable working version built. It was the brilliant people that he hired, Alexander Keith and brothers John and Charles Erickson, that actually built and patented the first recognizable version of the SXS switch along with the rotary dial, both in 1896. Strowger’s company evolved into Automatic Electric (AE). AE continually made massive design improvements, such that by 1920 it had evolved into a reliable, efficient and cost effective system. It was at this point (1920) that the Bell System caved in and began purchasing AE SXS equipment. The Bell System’s manufacturing arm, Western Electric, began manufacturing its own version of SXS in 1926 and introduced the SXS line finder in 1928.
NO s
SAME ENGINEERS CREATING THINGS. Dont confuse with lame engineers who got their degree because of equal opportunity bull...
This technology was used in many big cities (Cleveland) was one until cutover to #1 ESS in the early 70s. At the Clevoh45 this required 45 techs, after cutover to modern digital electronic technology that was 3 people on the day shift.
My job was to install, maintain and repair step by step telephone equipment in the sixties. Now I am 76. Cheers from New Zealand
It's always great to hear from a fellow switch tech. Thanks for watching.
I learned step by step in the Air Force back in 1973. I remember taking most of a day in class tracing schematics just to get from off hook to receiving dial tone. Thanks for the excellent explanation and thanks for taking me back in time.
Well, the SXS switching system is indeed a time machine. Thanks for watching.
Forty-two steps from off-hook to dial tone, as I remember. Army Signal Corps 66 to 69.
My grandfather worked for and retired from the Bell System as a central office technician. He spent his working life looking after these switches.
A very worthy occupation in its day. I hope he found it satisfying. Thanks for watching.
Electromechanical amazingness on such incredible scale may never happen again in human history.
Well, SXS was certainly a great achievement for the timeframe in which most of its major development occurred (i.e.1896-1920).
That is an amazingly complicated piece of electro-mechanical switching gear. Thank you for taking the time to explain it all.
Well, thank you for taking the time to watch it.
I worked for decades in the telephone industry. I am extremely familiar with the GTE Leich All Relay TPS (terminal per station) switching platform; still have a few relay bars from it. Nice seeing the relay switching equipment in operation.
Electromechanical switching equipment served the public extremely well for over 6 decades. Now it's intriguing to see and listen to all the moving parts that have long gone from our lives. Thanks for watching.
I'm amazed that anyone ever figured out how to make this work-much less make it work reliably. Truly amazing. Thanks for sharing!
And thanks for watching.
Add to that making it mass producable for the many banks of the many central offices
Wow, there's a lot of work involved.
Easy to understand, illustrated in detail, very good, I'm thrilled! Thank you for this video.👍
Thank you for your kind words and thanks for watching.
This one is the most fascinating to me. I think I've watched it more than a dozen times. It annoys me no end that I worked around these central offices so many years and never once wondered how they worked. But I was an outside plant kind of guy and the CO was somebody else's job. How I wish I could go back and spend some time in there knowing what I now know about them. Thank you so much for creating these videos. I know these are difficult to produce and take a lot of time but I hope you have more information that you can share though I can' t imagine what it might be.
Many thanks (well at least 12) for watching.You're comments make it all worth while for me. My videos have now covered all of the switching equipment I have. However, I may do a video over the winter on the detailed operation of a rotary dial 3-slot payphone connected to my demo unit. I haven't seen anything on you tube that provides the level of detail I would like to see.
That would be great. I worked on that type only a couple of times. Single slot was everywhere when I was hired in 1975. But it's the same, I knew how my end of it worked but not the CO end.
I watched this all the way through - twice! And can hardly wait to move on to the selectors and connectors!! For years I have always wondered how this equipment works since peering through the windows of a central office when I was growing up in Detroit, back in the days of step-by-step and crossbar switches. Thanks so much for sharing your knowledge and equipment with us... This is so fascinating and quite a pleasant experience to understand and admire the ingenuity that went into these devices at this level of detail.
It is very satisfying to me when I hear from someone who has watched this video all the way through (and in your case, twice) and found it fascinating. It makes all the effort I put into it worthwhile. I hope you enjoy the other videos. Thanks for watching.
It’s fairly simple. The first selector is controlled by the dial pulses for its vertical steps then it automatically cuts in to complete a circuit to the second selector. The second selector is also controlled by dial pulses for its vertical steps then cuts in for a connection to the connector. The connector is completely controlled by dial pulses for vertical steps then for the cut in steps. I could tell what bay I was in by listening to the switches. The line finders were fast. The selector was slow vertically following the dial pulses then fast on the cut in. The connector was slow vertically and on horizontal cut in following the rotary dial pulses. It’s fun to remember this but as useless as tits on a boar hog.
Yes, I have a very keen interest.
Thank you for such a detailed video explaining it, I always wondered how a handset going off-hook invoked the line finder process and this has been of great help.
I will say, there must have been a TON of effort to film and edit this. Thanks again.
Yes, it was a lot of work, but all of it was a labour of love. Thanks for watching.
Thank you for taking the time to make this set of videos. I have always wondered about the differences between the Western Electric and Automatic Electric step-by-step system. I have 200 lines worth of AE step and have done videos of Audichron time machines.
It's always nice to hear from a fellow switcher hobbyist. Thanks for watching.
ABSOLUTELY LOVE THIS VIDEO! It could have saved my parents dial telephone 50 years ago if I had seen it then. I wanted to know how telephones worked so I took it apart and broke the plastic plunger in the hookswitch. Thanks!
You're welcome, and thanks for watching.
I was home alone l disassembling my working mom's phone when I was 10 years old because, of course, I too had to know how it worked. The whole time I was disassembling it I was holding the switchhook so that the dial tone wouldn't play and the scary off hook alarm wouldn't happen. I was so scared of the off hook alarm as I was convinced that it would tip somebody off that I was breaking rules and taking the phone apart. I was sitting on her bed and had the whole cover off, holding the switch hook down and was staring at it the innards... when the phone rang in my face and so startled me I remember the whole thing 50 years later! I had that whole thing reassembled in 30 seconds I bet!
I always wondered if some psychic with a great sense of humor just couldn't help herself! Nobody ever called during the day and I was home from school pretending to be sick that day... I decided someone must always be watching!
Only three years later I was gleefully hacking in any old phone in our house and adding extensions in those hardwired don't-touch days.
I later became an electrical engineer and my first job was to design D4 channel Bank cards and learned all about telephony and designing products that survive lightning strikes and AC crosses.
I once got to tour a still working step-by-step Newport RI central office as they were preparing to migrate to a digital switch in 1985. The sea of chattering relays and continuous 20hz ringing monitor bell that confirmed the alternator was spinning was memorable. our tour guide also told us that the technicians we saw wouldn't be working there in another couple of months and that the people managing the new switch would be in some office somewhere else.It made me feel a little sad and made me wonder if the channel bank cards I was designing we're headed for obsolescence...
Still reading? I have more stories my wife isn't interested in!
@@JamieMurdock90 Aah, the good old days.
It really says something about the reliability of electro-mechanical relays, cams, levers and ratchets. At least there were no software glitches!
There's a simplified version of this used in vintage jukeboxes. They were called stepper units. And they were used in conjunction with the wall boxes because the wall boxes only had four wires going to the Jukebox. A series of pulses from the pulse generator which is very similar to this where you have a wiper running over a series of contacts. The Jukebox would interpret those pulses through a device similar to this video and it would make a selection. It was a real pain in the ass to work on so I can have some real appreciation for what's showing in your video here.
That's really interesting. If you have one of these jukeboxes, you should video this action and put it on You Tube.
Just a comment about electromechanical reliability. I worked at a CO that had 45 technicians working around the clock to keep a 40,000 line Office working. After final cutover to digital, electronic technology. There were 3 techs on day shift.
@@calbob750
👍
@@calbob750
Electro-mechanical devices do have their issues. Mainly physically wearing out, contamination and oxidation on contacts, etc. My point is they're surprisingly reliable considering how primitive they are. The number of these units the phone company had, probably tens of thousands, I'm positive when they went to solid state, maintenance dropped by 90%.
I currently have a Bluetooth issue with my phone in my car. The BT can't "locate" the car sound system half the time without restarting the phone. I found the solution in that little remnant of 70 year old analog technology: the headphone jack direct to the aux input. Works every time. 😄
Rotary phone system went offline about 20 yrs ago in my reich , but in 2022 i will be building my household rotary based telephone system has an intercom phone system. im going to injoy watching this 2 hour long film just to learn the history behind the idea of rotary phones :D
Well, I hope you enjoy it. Thanks for watching and good luck with your project.
The mechanical switching technology is always fascinating, I'm intrigued, how many lines do you have how big is your ranch? a mechanical switchboard is robust and reliable, but requires the use of disk telephones, the external line can also be VoIP with an ATA adapter, cmq it is not difficult to replace it with a modern PBX switchboard and with twisted-pair telephones, reusing the existing network , cmq I don't know if I'm wrong, but a wireless system or with GMRS radio with license or extended wi-fi network and smatphone with VoIP app I see it more modern and practical, it does not require you to go to the phone in the place where it is installed
Excellent explanation what will happen in a second of dialling a phonenumber. Thanks very much for sharing.
Amazing how ingenuity has once come to life.
The more I learn by these great videos, the more I'm I'm impressed by the craftsmanship and knowledge that founded our era.
There are many unsung heroes that pioneered the first 100 years of the Industrial Revolution. Thanks for watching.
Not to say, the seekers before those pioneers. Also in our time, we have zillions of little inventions that form a great leaps of evolution. Almost, sure that preceding the result, of each improvement, thousands of failures likely existed.
From your video, I learned and realise that not only the sequence but also the little smart (necessary electro/mechanical) interacting tricks that made it possible to make a connection and detect unavailability.
Thanks for explaining all of that which will require substantial more time to comprehend it as a whole. It looks so simple but is mind-blowing looking at the details.
@@ptrooms People like yourself that take the time to understand some of the details make all my efforts worthwhile. Thanks for watching.
Everyone stands on the shoulders of those that went before them. You might be interested to know we were doing digital multiplexing over 50 years ago. Some people look at these switches and call them Rube Goldberg, but the planning and engineering that went into them were far from it. You have to crawl before you can run. We were conceiving and planning digital switches while we were cleaning switch banks, changing wipers and cords, and welding pitted relay contacts.
You can see from this why the rotary dial phone took so long to record the numbers. The lag was from the electromechanical technology delay making the connection.
Very interesting. It brings back memories because I worked on these machines for AT&T in Dayton Ohio. I retired from AT&T after 35 years.
Well, you can visit this video anytime you feel like a trip down memory lane. Thanks for watching.
Thank you for this video. This is the first time I'm getting to understand the actual details of how this works.
Thanks. It makes the effort all worthwhile.
I had a friend up in Alaska that worked in a CO with about 5000 subscribers and used these Strowger Steppers. He explained it to me and I almost understood it. I loved the sound of the racks of relays stepping away, knowing that the 10 clicks per second of a line finder meant someone was dialing a number.
Actually, its the selectors and connectors that respond to the rotary dial pulses, not the line finder. But I understand what you mean about loving the sound of the switches in motion.
@@Hicken65 Yes, I realized that when I wrote it; the line finder operates quite a bit faster than the selector producing a buzz followed by clicking.
Thank you for this amazing and in-depth explanation! :)) Electromechanical devices like this are fascinating. I'm sure a lot more mental and physical work went into designing and developing these devices than designing anything "digital" today.
I have a 1964 jukebox. It's all electromechanical !!!!
Could have used this 40 years ago when i was actively fixing these electro-mechanical wonders !!
Me too! Thanks for watching.
You wouldn't think, but development is a prossess for some sharp people
& this is the epitaph of the type of switchgear my uncle continuously maintained to develop further for compulsory needs-crazy cool stuff,
The Transistor still had to materialz
with really sharp people.
Well, the SxS epitaph was a long time coming. It lived on in the North American PSTN until 2002, when the last one was decommissioned. Thanks for watching.
Thanks for this detailed explanation of a complex mechanism. I built a small exchange myself using a number of uniselectors as first selectors. That configuration is a lot easier to understand than the linefinder alternative.
I'm glad you liked the level of detail. But it's understandable since you are a switch enthusiast. It's always nice to hear from a fellow switcher. Thanks for watching.
The mechanical switching technology is always fascinating, reliable and robust and immune to any electromagnetic impulse, both natural and artificial, certainly a modern switchboard even with continuous maintenance after 100 years like this one no longer works and is also irreparable, just think of the electrilitic capacitors on the cards, of course it is more practical, it has a thousand functions and is DTMF but if it is broken it is useless, nice that some switchboard has remained operational as a museum, in 100 years there will be no electronic switching systems working in museums.
Well, there is no joy in staring at a box with no moving parts and no sounds (except perhaps a continuous hum). The new equipment is, of course absolutely amazing in what it can do. But at a show, my demonstration unit attracts plenty of attention simply because it has many moving parts that can be seen and a lot of noises that can be heard, all of which is under control of the interaction by the users operating the rotary dial telephone sets. Thanks for watching.
This has got to be the BEST video I have ever watched on this topic. Thank you so much for taking the time to put this together. It is clear, concise and informative. Cheers!
Thank you. Producing this video took a long time, but it was a labour of love. Thanks for watching.
I can't wait to find some time to watch this!
I never really thought about how phone lines worked pre-digit phones, as in how they worked at the call station/center.
To get the full picture, you'll also need to watch my other two long videos, one on the selector and then the one on the connector. Just search my channel name, Hicken65, on You Tube.
One of my old friends became an engineer with the local Bell operating company. Years ago I asked him about the reliability of the #1 ESS with all of those discrete semiconductors, resistors, and capacitors on circuit cards. Do you have to replace them often? "No, almost never. The least reliable part is the reed switch arrays."
Well done sir.. I did watch the whole vid (but did fast forward at certain parts).
As a die designer, I’ve seen many dies which made similar stamped parts, beryllium copper flat springs. Also, my father was an electronics / electro mechanics teacher. Was taught by him in my early days.. always liked seeing devices like these. All those people involved in its development and manufacturing, etc. Thanks for your expertise.
You're welcome, and thanks for watching.
I love this video! Very informative. I am working on building a very scaled down working electromechanical telephone exchange. When it's done, all of its parts will all be polished up and mounted to a custom made brass and wood "stand". All the wires will be properly "stitched" and of course there will be multiple different phones all connected and working. It's a labor of love, so to speak, but these devices have fascinated me for years and years. The one thing I haven't been able to find as of yet are schematics and drawings of individual stepping switches, line finders, etc. Any info or websites you may know of would be greatly appreciated.
Go to the "Telephone Collectors International" website. Click on "TCI Library". Click on "BC Switching Library" and search from there for Schematic Drawings (SD's) and Circuit Descriptions (CD's) for various manufacturers. Good luck with your project and thanks for watching.
@@Hicken65 WOW! What a treasure trove of information!!! Thank you so much. I have also joined as a new member.
I worked on this type of switching in the 60's, I thought it was a beautiful combination of mechanics and electricity.
Yes. It's a delight to watch and listen to.
Takes me back to my apprenticeship and City & Guilds telephony exams.
"Draw and describe the operation of the rotary hunting and testing mechanism of a two motion final selector"
Well, I hope you passed. Thanks for watching
I worked at the Clevoh45 in the 70s. It was a Panel office. Ohio Bell kept this 1920s technology going until the early 70s when it cutover to #1 ESS. The manning went from 45 technicians to 3 after the final cutover to other technologies. I’m impressed with the knowledge shown in this video. I doubt if the average CO Apparatusman or Central Office Technician has as comprehensive knowledge of the technology that they worked on.
You're probably right. I felt I knew more after researching and producing the video than I did when I worked on SXS. Mind you, I never took any formal SXS training and just learned from the "old guys" as I went along. Also, I didn't spend a lot of time in the SXS. Our CO had both a large SXS and a large #5 Xbar. I spent most of my time in the Xbar. Thanks for your feedback. It's always nice to hear from a fellow switchman.
@@Hicken65 Are those "Old Guys" the same as the Big Old Guys [who] Bowl Saturdays?
@@danielsandoval2846 No, my old guys have long since shuffled off their mortal coils.
Bell of Pennsylvania in Pittsburgh started cutting over panel offices to #1ESS in Dec. 1975. A great Xmas present
The other amazing thing is that you’d need to have so many selectors for the prefix of the phone number. In the city I grew up in we had 42x-xxxx, 45x-xxxx, 47x-xxxx. There were about 25,000 people. Now there are 500,000 people and many more exchanges. However if you divide 25,000 into 3 it means each prefix had to support 8,333 people. That’s a lot! The digit 4 was ignored but the 2 would be significant. You’d need many selectors if many callers dialled 42x-xxxx. It’s mind boggling.
In a large exchange, it worked out to about a half a switch (of all types, line finders, selectors and connectors) per telephone line.
montage too fast, got seizures
Cool
Very comprehensive video, thank you!
I wonder what kind of make- and break-contact symbols these are and how common they were used in what area. Do you have any info on that? Were these used only in early American SXS-Systems, or also in later Panel and Crossbar switch circuits? Was there a standard for notating these circuits? And what symbols were used outside of the US? I know several others, some of them might be much more common today, at least in Europe.
These symbols are known as detached contact symbols. On schematic diagrams, the relay contacts were moved away from the relay coils and placed where they were functionally utilized in the circuit, which greatly simplified the analysis of the circuit. This approach was developed by the Bell System Labs in the early 1950's. You can read all about it in an article in the Bell Labs Record (BLR) of July 1954, titled "Detached Contact Schematics". You can search online for the Bell Labs Record. Thanks for watching.
Excellent article. Thank you for this reference! 😊
Do you have a silimlar video for panel and crossbar switches? This is very helpful in understanding the actual electrical path of the call. All other videos I see bypass the detail and just say "here's the sender and the marker" but don't go into detail HOW they determine a free line finder or circuit.
No, I do not have any of the panel or crossbar equipment necessary to make a video. There is quite a bit of literature online if you search hard enough.
Thank you for the video. I've been looking for this level of explanation for years, and you did it very well!
I'm glad it met your expectations. Thanks for watching.
I love videos like this one. Thank you for posting.
Well, thank you for watching.
I'm thinking of the effort to design the entire system (without computers). And to build these highly reliable components.
The development of the SXS switching system was started in 1889 by Almon Strowger. It took about 30 years for it to mature into a reliable, efficient and cost effective system, with most of the design improvements made by Automatic Electric. By 1920, it was accepted by the Bell System, who introduced the SXS line finder switch around 1928. Improvements continued through to the 1970's, just as large scale removals of this equipment began. At their peak, the number of SXS lines in the Bell System reached about 25 million.
@@Hicken65 Until the advent of cheap digital electronic devices, the SxS system was probably the most economic switching system for most situations. The British Post Office had a choice when converting London exchanges (offices) to automatic - use Bell's Panel equipment or use SxS with 'directors' (a bunch of 2 motion selectors, uniselectors and relays which converted the three initial digits to a 1 to 6 digit routing code). BPO chose SxS because there was already British manufacturing facilities for the equipment and employees were familiar with it. Not sure what the raw economics would have been, but the cost effectiveness of Panel over SxS could not have have been too significant to cause BPO to choose Panel. Even Bell replaced a Panel office with SxS.
@@netking66 I don't think Panel was ever used outside of the Bell System. In the early 1920's, The bell Telephone Company of Canada (which was closely affiliated with, but not a true member of the Bell System) was making plans for their gradual conversion to dial, beginning with Toronto. The company was heavily pressured by the Bell System to install Panel. However, the Chief Engineer was having none of it. He had recently joined BTCO from the telco in Nova Scotia, where SXS had already been successfully installed and was working well .Also, BTCO's manufacturing arm, Northern Electric was already licensed to manufacture Automatic Electric SXS. Toronto's dial conversion continued with SXS for the next 25 years.
@@Hicken65 Before Panel, Bell System developed a system called 'Rotary' (not to be confused with rotary dials). Like Panel it was power driven via shafts and clutches and used 10x30 outlet selectors (or 10x20 for final selectors). Also like Panel it used revertive impulse control where the selector sent impulses back to a register which signalled the selector to stop when in the right position. Presumably Panel's 25x20 outlet selectors was seen as an advantage since a 10000 line office just needed two stages instead of the usual three although an initial stage would be needed in most cases for trunks to other offices. Despite this Rotary went into production with the first switch being installed in New Zealand (and that switch may have remained in service until replaced by a digital switch) and Rotary was used in Paris and other European cities. Possibly Bell preferred to offer Rotary to overseas markets meaning that Panel was never used in these markets. Also Ericsson had its own 25x20 outlet power driven system which was conceptually rather similar to Panel.
@@netking66 I was aware of the Rotary System, but never saw one or studied its operation. Thanks for the additional information.
Have you done a vid on how they cleaned, and or replacing a dead coil, or blade on these switches? Must have been some task...
I don't have any videos on the relay replacement procedures. However, a changeout wasn't particularly difficult. The relay assemblies can be removed by disconnecting the wires and undoing two screws that hold the relay to the switch base. Similarly, the contact spring assemblies can be removed from the relay heel plate by undoing two screws. Everything is easy to reach with the switch dismounted.
Would it be accurate to saw that the "relays" are solenoids? aka: transform electric current into magnetic force that cause movement?
(normally, a relay is considered to be an electrically activated switch to let current flow or not).
The relay in the SXS switching system is a device that contains a solenoid as one of its components. The solenoid is an electromagnet that consists of a copper wire wound onto an iron core. The relay also has an armature, a moveable magnetic metal arm that is attracted to the solenoid when the solenoid is energized by an electric current, and the relay also has a set of electrical contacts that can be closed or opened by the movement of the armature.
@@Hicken65 Thanks. If there is both mechanical action and electrical contact, I guess the term "relay" works,. he old AT&T videos also use the term "relay".
I wouldn’t warn my audience that I’m going to be talking for an hour and 40 minutes. Having said that, I appreciated the warning.
Beautiful description of a beautiful system. Thank you :3
Well, thank you for your comment and for watching.
Made it through! My head is mush, but I gathered much. Man, that must've been quite a project to edit that video together.
Well, it certainly was a tremendous amount of time and effort to produce. However, to have someone watch it all the way through makes it all worthwhile. The average watch time is about 12 minutes. Thanks for watching.
@@Hicken65 don't put too much stock into the TH-cam analytics. I can watch the video in its entirety and you might get a view count but the watch time would report zero, bringing your average way down. In other words, don't feel bad that people aren't watching your video when sometimes they really are. Also, I've seen view counts and likes get lowered and even erased!
@@KC9UDX Yes, I've noticed some strange things happening with the stats as well. It doesn't bother me that many people don't watch my long videos all the way through. I realize my target audience is very small.
thanks for taking the time to make this amazing video! It has a lot of details that are really useful! I really truly appreciate it! :)
You're welcome. Glad you enjoyed it.
My dad spent many years during the 60's with these switches. Said the scariest noise he ever heard was when they had a cascade type event that locked up the switches in the whole office. The place went dead silent, at that moment they knew the SHTF...
These systems were typically engineered to allow about 10% of the telephone lines to be in use simultaneously. Unusual events that caused this limit to be exceeded by a large amount would cause a rapid deterioration of service.
@Hicken65 , wow, grew up with step by step service in Baden, PA but never remember the phone service going down because too many people were using it!
Why did they continue to manufacture step by step switches in the 1960's when crossbar #5 was the newest & best switching?
@@jgrysiak6566 SXS switches were still needed to handle the growth in the towns and cities where these systems were in service. Generally, #5 XB was not being used to replace SXS because it was not cost effective to do so, especially in one exchange towns. In cities, XB was often added to provide for growth with the existing SXS remaining in place. In these locations SXS equipment was still added for trunk growth. Large scale SXS replacement began in the 1980's with the introduction of digital switching equipment and their associated lucrative feature rich capabilities.
@Hicken65 , Really wanted a pushbutton phone & was fascinated with them in the 1970's but sadly was not available until Oct 1980, when Baden, PA cut over to #2BESS from SxS. Finally got my dream of getting a pushbutton phone in the fall 1982, my parents wanted me to take it back but then eventually they liked it & kept it. Lol
@Hicken65 , there was a ton of growth in the Baden, PA SXS office in the 1970s. If they would have built a #5 crossbar switch, would they have had to assign another nxx to the town?
So this sort of is making sense. So one line finder is required per call and active the whole time the call is in progress. So whole bank of finders can handle 20 calls. Do I have that right? Only 20 calls per 200 subscribers seems a bit low. So is expected usage of the system only 10% of the amount of subscribers? Or would there be more banks of finders for 200 subscribers? Well 198?
One linefinder per call tied up for the entire duration of the call is correct. So one fully equipped line finder group could handle 20 simultaneous calls. Typical busy hour usage was 10%, sometimes less. Many offices did not need to be fully equipped with 20 line finders.There was also a 30 capacity linefinder group available for heavily business oriented offices. Telcos also had the option of loading a line finder group with less than the maximum of 198 working lines, where 20 line finders proved to be insufficient.
Those numbers were not lightly arrived at. 20% was the average they forecasted as a design element starting point. But, remember not all terminals were necessarily assigned. That was carefully watched. We had service observing and traffic engineering, who would constantly monitor usage and overflow within all LF groups. They did line load control, and network balancing frequently, to insure the subscribers would not overload their particular group. High usage customers, like some businesses, might be in a less densely assigned Line Finder Group.
Just a comment from a former COT. This limitation on the number of calls that could be placed at one time usually wasn’t a problem. In the Cleveland area the one exception was when Santa Claus was on the local TV station. At 3 o’clock he would ask kids to call him! The CO would tie up because of hundreds of kids all trying to call at the same time. There was an emergency option called line load control. Activating this only allowed essential subscribers like Fire, Police and Medical to get dial tone and make calls. This was on Panel Switching.
Hahahaha..... And you guys thought the Space Shuttle was complicated ! What I find amazing is that a single Microprocessor can do all the hardware logic through software. But this hardware of the past is absolutely brilliantly designed. Wow 😯
Modern digital switching systems are absolutely amazing, but not interesting to look at in action, unless you enjoy looking at motionless boxes with a hum. Many people find the old electromechanical systems enjoyable to watch and listen to and to even understand how they worked. There is a great deal of reading material describing the SXS technology. My videos are an attempt to bring this literature to life. Thanks for watching.
Ah yes...I cut my telephony teeth on SXS in the 1970's. I loved the clatter of the exchange and could often pinpoint troubled switches just by the change in the rhythmic flow of the normal office sounds. Later in the #1 and #5 ESS offices, I was never as happy as those earlier noisy days.
Perhaps you could include discussion of class of service or other control conditions thorough use of normal post cams and contacts at some time.?
Always good to hear from a fellow switchman. It was great to detect an irregularity and get it fixed before a trouble ticket was issued. Unfortunately, my line finder is not equipped with the optional class of service screening hardware. I do demonstrate the the use of the normal post springs and cams in their digit absorbing application in the selector. This is in my video "The Selector In The Step-By-Step Telephone Switching System Part 2". You can find it by searching my channel name Hicken65. Thanks for watching.
Ah well. If complicated technical subjects could be learned by watching a video this would be a vastly different world.
How long was the Bell System training school to teach competence on these systems?
I never had any formal training on SXS, I just learned from the old guys in the central office. So I don't know how long the training course was. I did take the training course for No. 5 Crossbar, which was 11 weeks. Thanks for watching.
@@Hicken65 to actually comprehend a schematic I always had to make a series of 11x17 copies and mark them up with different color highlighters to highlight the circuit path as well as the contacts of a particular relay. I had never seen the use of / and X for NO and NC contacts before. (Or is it the other way around?) But it is easy understand that back in the day of hand drawn schematics / and X were far easier to draw than two small circles with a bar connecting them for NC and with the bar hovering over the circles for NO.
Where'd the tip, ring and sleeve convention originate?
The original schematics for SXS equipment were conventional "attached contact" drawings in which each relay's coils and all of its contacts were shown in one location on the drawing.That is, the relay's contacts and the wires that connected to them were depicted directly above or below the relay coil. This was difficult to read and trace. A different method was employed for crossbar systems called "detached contact" schematics. The relay contacts were moved away from their coils and depicted in locations that pertained to their functionality in a circuit. This allowed complex circuits to be broken out into many Functional Schematics (FS). So for example, a completing marker in a No. 5 Crossbar office, which has over 1500 relays and many thousands of contacts, was broken down into hundreds of pages of FS's to facilitate trouble analysis. In each FS, the detached contacts were shown as a short vertical line for a break contact (which has a through path for electricity when the relay is released and a broken path when the relay is operated) and an "X" for a make contact (which has a broken path when the relay is released and a through path when the relay is operated). Some of the SXS schematics were converted to detached contact type, which made reading them much easier. It is the detached contact methodology that I am using in my video.
In the days of early telephony, when manual cord switchboards were dominant, the male plug on the end of each cord was a 3-wire plug. So the metallic portion of the plug that fitted into a jack had 3 insulated sections. One wire was connected to the section at the very end or "tip"of the plug. Another wire was connected to a narrow circular "ring" section behind the tip. The last wire was connected to a long cylindrical section that enclosed all three wires like a "sleeve".
Lenny... awesome video. Back in the day I made what I call the switch bobbin (brass grooved part) Have one on my desk as a reminder of my screw machine day. Would you call this an analog to digital switch system?
I'm not quite sure what you're asking but I'll answer what I think you're after. The SXS system was replaced by another analog system called crossbar. Crossbar was replaced by a hybrid analog/digital system that had a digital CPU but an analog switching network (#1 ESS and SP-1).These were followed by all digital switches such as DMS 100 and #5 ESS.
Did they put any gold in it?
@@greenlawnfarm5827 Possibly, in the tiny relay contact points. These relay contacts were precious metal alloys. There were two major types called No. 1 Contact Metal (platinum, gold and silver) and No. 2 Contact Metal (palladium, platinum, tungsten). One or both types may be present in these step-by-step relays.
@@Hicken65 ok.
amazing video, thank you for taking the time to make this
Glad you enjoyed it. Thanks for watching.
What isn't much covered in the video, to add to the complexity, are the blocking events or the ratio of line finders to subscribers which in the explanation seems to be about 10 or 20 line finders for 100 subscribers. In other words, the entire rest of the system can be idle but in that group of wires, 10 or 20 subscribers can block the other 90 or 80. That is a bit unlikely to happen but in a local emergency, such as a house on fire, it is likely that all of the neighbors are going to be vying for the same line finders.
The number of subscriber lines sharing up to 20 line finders is 200, not 100. So 20 lines could block the other 180, and often did in the busy hours, but for very short intervals of time, generally less than three seconds.The system was intentionally designed to have 98.5% of the calls get dial tone within three seconds in the busy hour, which meant that 1.5 % of the calls would be delayed greater than three seconds.This was deemed to be a reasonable cost/service relationship that was mandated and monitored by the government. This service criterion only applied to "normal" traffic. Any abnormal traffic due to external events, such as severe weather, emergency situations, serious breaking news, etc. were excluded from the service statistics, and thus not included in the provisioning process. Your remark about the neighbors vying for the same line finders would only occur in a small office with one or two line finder groups. Subscribers sharing the same feeder cable into the central office were distributed over as many line finder groups as possible, so that in the event of a cable cut, in which there would be many short circuits, the effect on any line finder group would be minimalized.
@@Hicken65 If the 20 slots were busy did the relay system wait for a free LF or did the caller get fast-busy immediately?
The system waited for a free line finder.
Impressive machine. I wonder how long and how many prototypes it took to engineer something like this.
See my reply to Steve Koehn below.
Electromechanical marvel! I almost understood, I think I'll just need to watch it a second time to get it all
Well, when you've mastered the line finder, be sure to take a look at my detailed videos of the selector and the connector. Just search my channel name, Hicken65, on TH-cam. Thanks for watching.
You don’t need to get it all. This video defines the context. That’s worth understanding!
Bardzo bardzo bardzo bardzo dużo różnych informacji Powiedziałbym nawet że za wiele dla normalnego laika ostatnio widziałem film o tym samym urządzeniu ale zawierał się on w około 10 minutach i bez problemu zmieścił tam główne założenia zasady działania całego mechanizmu myślę że to normalnie wystarczy A co do tego filmu to wygląda on bardziej jak opis książkowy to jeżeli tak to najchętniej oglądało by się go jeżeli by został podzielony na jakieś sekcje aczkolwiek jeżeli bym oglądał ten film dosłownie cały to na pewno jest wartościowy
Zrobiłem krótkie filmy dla osób, które chcą tylko przegląd. Zrobiłem długie filmy dla ludzi, którzy chcą każdy szczegół. Dziękujemy za oglądanie.
This was really fascinating. The complexity and expense of Central Offices helps explain why pre-1990 socialist India with govt. telco could never keep up with demand for new subscriber connection. Waiting period for new phone service in 1970's and 80's was still 2 to 6 years. Then along came affordable cell phones and everyone got one. And imagine how the paltry connectivity in the pre-cell phone era of India negatively impacted the speed of business. Well, supposedly one could pay a bribe and get a telco hookup faster. This vid serves as a great hyper focused example of how socialism sucks and capitalism rules, specifically it sucks in ways people might not know since they are not industry insiders, yet all are affected negatively.
Well certainly the cost to equip landline telephone service for such a large population would have been enormous. In addition to the actual switching equipment there are the the costs of the power supplies, buildings, transmission equipment and all the outside plant cables. Thanks for an interesting perspective and thanks for watching.
I was a technician for Western Union at a centralized telephone bureau and we had a switch similar to this one. That was a long time ago. I miss working on mechanical things and tube equipment. Now just replace the circuit board and throw the bad one away.
Unfortunately, in the end, nearly all of these SXS switches were also thrown away.
@@Hicken65 It's like seeing old Linotype machines being scrapped. They are pieces of art.
Hi. I must have watched about 20 various videos on how telephones work etc., and even I can't believe that I watched ALL of this one as interesting as it is. 1hr 40mins is a long time, but I stuck with it 'cos I wanted to know. I've got most of it, and of course I now realise that the relays are THE crucial parts; what with introducing deactivation delays, and multiple contacting. Much of this makes more sense of what I used to know as a child from the late 50's and early 60's dials clicking on their return, sparks on 2 wires. Oh what fun.
The one main question I have now is this. How on earth did an undertaker conceive of such a switch, indeed an entirely automated telephone exchange when the 'birth' of electricity was barely only 50yrs earlier?
Very few people were THAT familiar with electricity back then.
So, a bit like Filo T Farnsworth, a 17yr old farm boy conceiving the electronic television principle, how did Strowger do this too?
Thanks for sharing this video.
Regards Mark in the UK
Strowger certainly had a good understanding of electromagnets, pawls and ratchets and his concept of how to apply these components in an automatic telephone selecting device was quite an achievement. However, his device was a very rudimentary one in which each electromagnet was directly operated by the calling subscriber, necessitating 5 wires between each telephone and the central office. Telegraph type keys on the telephone set had to be pushed once for each vertical and rotary step taken by the selecting device. His device did not contain any of the intricacies of the switches in my videos. As simple as his two-motion selecting switch was, Strowger was unable to get a reliable working model of it built, and had to design and file another patent for a single motion (rotary only) switch, which was the one installed in the world’s first automatic exchange at LaPorte, Indiana in 1892. Strowger left the telephony business in 1896, due to failing health just as the brilliant people he had hired actually designed and produced a workable two-motion stepping switch and the first rotary dial. It took the next 30 years to fully develop and refine the step-by-step switching system to the level depicted in my videos. To fully understand the system, you need to see my other long videos on the selector and the connector. Just search on my channel name, Hicken65, in You Tube. Thanks for watching.
@@Hicken65 Strowger's company became the Automatic Electric Co, which furnished and further developed the step-by-step equipment. Bell System at one time was buying AE up to the mid-1930s.These bore a WE nameplate, but was really identical to AE,
@@rayfridley6649 Undoubtedly, AE gets all the credit for SXS design up to the mid 1920's. However, Western Electric began manufacturing SXS equipment in 1926. From that time on, the Bell System made many significant improvements to the SXS system, some of which were adopted by AE. The design of the two motion SXS line finder belongs to the Bell System. AE only had the Keith Line Switch and the rotary line switch until Bell came out with the SXS switch line finder.
Don’t forget Western Electric and Bell Labs developed most of the technology in a Central Office. There is a switching system that was used in big cities called Panel. It was developed and installed in the 1920’s. Many COs used Panel until cutover to ESS or Crossbar in the 1960s or 70s. The knowledge shown in this video exceeds anything I experienced in the CO or in training at Bell System training centers.
I work in a switch in the UK and the noise isn't from switches and relays but rather from the air con units for the PDH and SDH networks. As time moves on, SDH and PDH is meeting the same fate as the switches and relays as their twilight days draw in with telco providers switching their users to IP based solutions
Yeah, its deja vu all over again.LOL
Very intensive and great video. The complexity of this mechanical exchange is mind boggling, yet it was very reliable. We had a step by step exchange until the late 1980s when it was replaced with a digital system. I used to pay attention to the sounds when dialling a number. Now I understand why the last 2 digits dialled did not produce any clicks. All of the previous digits before the last two digits would have a thump sound, and that would be the wiper spinning over to an available path. I’m still puzzled by some aspects of this. If the wiper scans over the circuits testing them for ground until it finds one that’s free/available, how come it would not make a click in the ear of the people already using those circuits on other calls as it swipes past them? It would seem as if it’s physically cutting into the line and then moving on to the next one. Very strange. In my city all phone numbers started with the digit 4. On the step system the 4 did not need to be dialled. You could just dial the last 6 digits. I noticed when I picked up the phone and dialled the 4, it would remove the dial tone but there would be no thump. This means there was no horizontal sweep happening. When dialling the second digit you would hear the horizontal sweep. Or of you left the 4 off and dialled 6 you would hear the thump of the vertical wiper action. I’m not sure how they did this, but they seemed to deactivate the first dialled digit but only if it was the digit 4. If it was any other digit dialled it would activate the horizontal wiper movements. After watching this video, that part puzzles me.
When a selector is rotary hunting for an idle trunk, its tip and ring wipers are open (electrically dead). Only the sleeve lead is alive. For your questions on short dialing, you need to look at my video "The Selector In The Step-By-Step Telephone Switching System Part 2" which explains the concept of digit absorption. You'll find it by searching on my channel name Hicken65.
Hicken65 Thank you that makes sense that the tip and ring would be dead or yes it would interfere with calls as it swipes across the terminals. I’m assuming one of the magnetic relays removes the power from the tip and ring as it would need to be connected once it found a vacant line. I’m so interested in this but there’s so much going on that it’s hard to retain everything. The switch is doing a ton of things. It’s hard to believe this was designed so long ago. Not much longer after electricity was discovered. Not only would this take a very analytical mind, but the mechanical design - the dogs - the spring - the failsafe grounded circuit so you can’t dial more than 10 pulses. It would require enormous skills to mechanically put together something like that. There’s no CPU or computer behind all of this. One magnet is activated by the result of another.
The workings of a two motion stepping device was so intricate that Almon Strowger was unable to get a reliable working version built. It was the brilliant people that he hired, Alexander Keith and brothers John and Charles Erickson, that actually built and patented (U.S. patent 638249) the first recognizable version of the SXS switch. I never cease to be intrigued by watching and listening to one in action.
@@Hicken65 In looking over the patent drawings, the mechanism invented in 1895 is essentially the same as later makes. It's a bit of a wonder to me that WE did not re-design the relays, etc. (The panel system had a relay design without bearings that was later deployed in crossbar.) Presumably the patents had expired by 1921 when the Bell System started deploying SXS offices. They held out for a long time with manual operators. I think this was considered a service feature. The Keith/Erickson design is clearly the work of experienced mechanical designers. Strowger had a funeral parlor to run. Curiously, the panel system was deployed in 1923. COs were deployed with both systems, but it is not clear to me what advantages panel had in large metro COs. My Midwestern city of a million only had SXS offices until crossbar came in (about 1961.) When it first arrived, the local techs did not understand it well. My grandmother's phone in a growing part of the area went from SXS to crossbar. It did strange things and didn't always work predictably. One day she called a Bell supervisor and complained. He gave her a spiel on how wonderful this technology was. Being a non-technical person (but saintly) she listened and then replied, "and wouldn't it be wonderful if it worked!" The crossbar exchange was not replacing SXS but supplementing it in new areas. Only later, of course, it was all replaced by ESS. One city exchange was, after 55 years in service, replaced by a #5 ESS. It was not worn out, but for lower maintenance and especially for new features that could not be implemented easily in SXS, the ESS was purchased. I was a MTS at Bell Labs and one of my contributions helped lead to the invention of Ethernet. Some years ago the CTO of Cisco said, "Eventually everything becomes Ethernet." You could easily interface a computer to a crossbar switch, and it would not be an unreasonable design. But today it is hard to compete with Internet Protocol, with the latest switch chip able to run 64 channels each at 400 billion bits per second.
@@AriBenDavid The Bell System used the Automatic Electric SXS systems for the first few years. WE began making its own version around 1926. Strowger left the industry around 1897 due to ill health. He died in 1902. I've never been involved with the panel switch and don't know much about it. I believe it had the ability to store the dialed digits, perform translation tasks and route the calls using dial senders. This gave it much more flexibilty than the direct control SXS. NYC had about 200 exchange names in the 1920's, too difficult to be handled by SXS. The crossbar problem you mentioned must have been a local issue. I worked with crossbar for many years and found it to be extremely reliable. As for interfacing a computer with a crossbar switch, this was very successfully accomplished by Northern Electric at the beginning of the 1970's. The system was called SP-1 (Stored Program - 1). It was sold all across Canada and to many independent telco's in the U.S.
My first job was with plessey telecoms travelling around the country helping to construct this type of equipment
Plessey was a big player in telephone switching, mostly with crossbar, analogue electronic and digital.The company acquired an interest in SXS technology when they merged with Automatic Telephone & Electric Company Ltd. and Ericsson Telephones Ltd. in 1961. Thanks for watching.
I used to test these Strowger switches at a company called Whiteley Electrical Radio Company located in Mansfield, before they were sent to Plessey in Beeston. I believe Plessey took over a company known as Ericsson’s. We only performed point to point continuity tests. The relays were later set by highly paid experts. Happy days, about 1968.
Here in the UK we used uni-selectors to enable telephones to seize a line.
The UK also used two motion (Strowger) type linefinders, likely in the larger exchanges. In the Bell System, the first SXS systems to be used (circa 1920) were the Automatic Electric designed systems that used either the Keith Plunger line switch or the Uniselector, which was called a Rotary Line Switch in North America.The Bell System developed the two-motion line finder around 1926.
@@Hicken65 Thank's for the info.
@@Thereishope664 @Hicken65 I started work with Post Office Telephones in 1973 as an apprentice. It is true that, certainly with the later 2000-type 2-motion selectors (which had square fronted rather than rounded relay covers), the line finders were huge uniselectors, but since a large number of the older exchanges that hadn't yet been updated had the 1000-type 2-motion selectors which were identical to those shown in this video, it is quite likely that we also used the same type for line finders. I vividly remember servicing both types of 2-motion strowger type selectors, but these were all 1st, 2nd, 3rd, 4th or Final Selectors.
The 1000-type selectors had a bit of a habit of chewing their wipers, especially if set up in a hurry, but the 2000-type selectors rectified this by only wiping in one direction. When the call dropped, the selector would drive the rotary motion to the end of the bank, where it would drop and return to normal beneath the lowest bank of contacts. It meant the wipers were much more reliable. I have fond memories of working on the selectors. The 1000-type required you to unbolt them from the bank by removing those two nuts, but the 2000-type were a much simpler exercise to 'unjack' them from the bank. The latter also had a larger number of jack connection points to allow a variety of tests while the selector was in place.
But the best thing was that, back then, we had a fully stocked stores. We always had fully calibrated tools, and there were hundreds of them, and a full set of spare parts for every conceivable selector and relay set in the exchange. You removed the selector from the bank and placed it in its test cradle at the work bench, then took a set of measurements. If you were not able to 'adjust' the device back to 'service' tolerance limits, then it had to be stripped down and the worn parts replaced. I noticed the the shaft of the selector you were demonstrating was worn. That would have affected timings, so in our day that shaft, including all the brass ratchet attachments would have been replaced. Every relay and armature had a specific set of metrics to be measured. How far the armature moved when it operated, the gap when fully operated between the armature and the coil face, and it was all adjustable. Each set of contacts had a specific spring force measured in grammes, which were all part of ensuring the timings of the relays were correct. All those ratchets and pawls had specific clearances but, when they'd all been set and you ran the tests, the machine would perform to factory specification. There was so much to learn, but I was there right at the end of Strowger. Most new build was Crossbar and then System X. Only 10-year plan expansions were still being installed with new Strowger equipment. I saw the writing on the wall and moved out into more generalised electromagnetic systems work, and eventually electronics.
Amazing
Thanks, and thanks for watching.
Love the look of the wire used in this, is that wierd? Haha, Great video, thanks for sharing.
I think it would have been very interesting to see how this equipment was manufactured. Great precision with terminating the color-coded wires was required.Thanks for watching.
@Marcel H One of my jobs was to check the wiring connections made by Northern Electric installers on switch extensions. They were generally pretty good, but I used to make money on bets with them that they had made an error, particularly on a day after one of their notorious parties.
I used to get a kick out of doing a linefinder chain test. The fun part of this step switch is that it was invented by a paranoid undertaker by using a collar box. He thought the local operator was directing calls to his competitor.
The chain test was a great demonstration device when people were given tours through the CO.
I wouldn't say paranoid, it was well known operators in small single both offices in small towns got up to a lot of nonsense. It was harder in a larger office with coworkers and actual supervisors, though I'm sure they found a way.
And given he had people close to him tricked by this, using the other funeral home by accident, after asking to speak to his..
There was a North Electric plant in Johnson City TN that manufactured these switches.
Well, I know that North Electric manufactured All- Relay and Crossbar systems and have seen ample documentation to support this. I have no reason to justify stating your claim that North Electric manufactured Step-By-Step equipment is invalid, I just haven't seen any documentation to support it. However, Step-By-Step equipment was licensed to many manufacturers around the world, so perhaps North Electric was one of them.
@@Hicken65 yes they sold out to Alcatel in the mid 80's and moved operations to North Carolina.
Are the step by step switching system required for a phone system
I really don't understand what you're asking here. Perhaps you could expand your question.
@@Hicken65 I did think of another question related to phones is rotary dial dtmf dual tone multie frequency
@@ryanbelknap814 No. Rotary dials generate dial pulses, which simply open and close the loop to the central office. Touch-Tone telephone sets have push buttons which generate the DTMF tones.
@@Hicken65 could u make the step by step switching system the things that u said u connected tip ring to the phone
@@ryanbelknap814 I'm sorry, but I don't understand your question.
At 1:05, discussing the D relay staying closed if there is another dial tone request using the path, it seems to me that these double-winding relays function as OR gates, since either or both of the windings can keep the relay energized. Is this correct?
Yes this is correct.You can call the primary winding A and give it a value of 1 when it is energized and 0 when it is not energized. You can call the secondary winding B and give it a value of 1 when it is energized and 0 when it is not energized. You can call the set of D relay contacts Q and give them a value of 1 when they are operated and a value of 0 when they are released. If A is 0 and B is 0, Q is 0; if A is 1 and B is 0, Q is 1; if A is 0 and B is 1, Q is 1; if A is 1 and B is 1, Q is 1.
watched every second. circuits with an X are normally/fail open (power doesn't go through), and those without are normally/fail closed (power goes through), right?
Yes. The X is a make contact set, which is normally open and closed when the relay is operated. The short line is a break contact set, which is normally closed and open when the relay is operated. Thanks for watching ( especially the whole video).
I have some extra step by step parts where could I send them. I don't want to throw them out. Thanks
You can join the "Telephone Collectors International " email group and post a message detailing what parts you are making available. There are quite a few switch collectors in that group.
I'm quite interested in your spare parts. Please send me a message.
@@peterspotts4055 Sorry alot of people jumped on my offer already and they are gone
So, since the G-relay isn't incorporated into the Linefinder switch, is it an essential part (or the only method) to determine the location of the calling line? I have recently aquired my first linefinder switch and would like to see it hunt and locate a "calling line". I understand that this process works with the commutator but I don't understand how these two parts work together to find the line. Could you explain?
Each G relay is associated with 20 lines assigned to the same bank level on a Line Finder (LF). When a G relay operates, it identifies the level that those lines are assigned to by grounding the appropriate commutator segment. It also grounds a “start” lead at a particular entry point in the LF preference sequence chain so that selection of a LF starts at the LF in which the lines requesting service are at the lowest level on the bank in order to minimize hunt time and switch wear. In a demonstration unit like mine with only one LF and two lines, the G relays are not required. Normally, a pair of make contacts on the line relay provide a ground path to operate the G relay. In turn, a pair of make contacts on the G relay provide a ground path to the LF start lead, which causes the A relay in the first idle LF in the preference chain to operate. I simply use these same line relay make contacts to ground the LF start lead directly. Both of my telephone lines are assigned to the same level on the LF bank. So instead of having a G relay provide ground to the appropriate commutator segment, I wired a permanent ground to that commutator segment. Note, however, for your LF to function properly, that is, to hunt for and connect to your calling telephone line and remain connected until you hang up, you need a line circuit for each line (which contain a line relay and a cutoff relay) and a first selector attached the output tip, ring and sleeve leads of the LF.
I am asking this for my dad:
We are having some difficulties with the 355A Step by Step switching system. Looking for 224BN F-Relays, also for Line Finders ED 32000, marking on the frames in place show ED 32000-30-G3 985D which are part of the 355A switching system.
My question now is when you acquired your first line finder switch, what are the markings on the frame and what are company’s or manufacturer location? Information would be greatly appreciated. Future plans we would probably need 2 line finders.
Sorry for the delay in replying. I didn't get an email notification for this comment and saw it by chance today. My line finder is a different type than yours. Mine is an ED-33013-30--G11 985D, which would work in a No. 1 SXS or a 355A. The markings on the frame are shown at the 13:27 point in the video. I don't see any manufacturers name of my line finder. However, the ED number identifies it as a Northern Electric or Western Electric product. NE was licensed to manufacture WE products in Canada. My line finder is made by NE since it came out of a Bell Canada central office. Not sure what you are looking for, but if it is to purchase a new line finder, you are out of luck. Neither NE nor WE exist any more and no company is manufacturing SXS switches. Tahnks for watching.
Ugh, at the beginning of this video, the constant switching and repeating of the same clips for minutes at a time is dizzying as hell and extremely annoying.
The information given here is what's important. Just saying. It's really well explained.
Is a step-by-step switch the same as a Strowger switch? Also, wasn't step-by-step switching replaced by No.5 Crossbar? I'm pretty sure it was.
Almon Strowger had the concept for a step-by-step switching device and obtained a patent for it. So most of the world's two motion (vertical/rotary) electromechanical telephone switches are referred to as "Strowger" switches, including the one in my video. The No. 5 Crossbar system was introduced in 1948 and became the major growth vehicle in the Bell System for 30 years. By the late 1970's it was serving more lines in the Bell System than step-by-step. Crossbar growth was slowed down in the 1970's by hybrid analog/digital systems in the 1970's and by all-digital systems in the 1980's. Crossbar was completely (or nearly so) phased out in the Bell System by the end of the 1990's.
@@Hicken65 Thanks so much.
In the Cleveland area Step by Step was replaced by #1 Crossbar, #5 Crossbar and later #1 ESS. All Western Electric and Bell Labs technologies.
How often did these units require maintenance?
In the big offices, there were three or four CO techs in the office during the day. They spent most of their time performing preventative maintenance routines on the switches, and the rest of the time handling trouble reports. Small offices were unmanned but a route man would visit daily to handle troubles and wire in new subscribers.
Do the 200 lines represent subscriber lines or outbound lines? If it's outbound lines, how are the subscriber lines connected to the line finders? I assume each subscriber can connect to n-number (e.g. 10) line finders per each 50 to 100 groups of subscriber lines to reduce 1-to-1 scaling complexity?
The 200 lines are subscriber lines.They are wired to the bank terminals of a line finder group which contains 20 banks and up to 20 line finders. The 200 lines are multipled across all 20 banks so that they can be "found" and connected to any one of the line finders in the group. So 200 lines have shared access to up to 20 Line finders.
@@Hicken65 Ahhhh....got it. So the sleeve connections help in the determination if a line finder is already engaged or not.
So unlike a crossbar switch, the line finder stays engaged for the duration of the call then releases upon on-hook.
@@lohphat The sleeve lead provides four main functions in the SXS system. On the line finder bank, it identifies the location of the line requesting service; on the connector bank, it identifies the busy/idle status of a line being called; it blocks a dial tone request from being initiated by a line when an incoming call is being answered by that line; and it acts as the control wire that keeps a switch train operated for the duration of a call. So the line finder, selectors and connector involved on a call are all held operated for the duration of the call by the sleeve wire.
@@Hicken65 So the sleeve is energized when active so that when the wiper scans across it's looking for an inactive sleeve lead? Then as it progresses through the connectors, idle trunks and connectors are detected with an inactive sleeve....what prevents two connectors finding an idle trunk at the same time and both grabbing it thinking they're the first? Are two calls ever linked by mistake?
@@lohphat I suppose double connections may have occurred from time to time, but I'm not aware that it was ever an issue to be concerned about. The change in the electrical condition on the sleeve lead occurs within about 50 ms of seizure of a circuit, so it would have to be a very close race to create a double connection.
Good video, but the opening monatge sequence honestly nearly started off a migraine. Its really jarring..
I had to close my eyes in order to focus on what was being said.
I watched this and for the life of me can't figure out where the subscriber's line is wired in. When the received is taken off hook, where is that line first connected to?
The diagram at the 40:00 minute mark shows a telephone set wired to 3 different places. First, it is wired to a line relay winding in a line circuit; second, it is wired to the bank terminals of a line finder group; and third, it is wired to the bank terminals of a connector group. When you go off-hook, you bridge the tip and ring leads together inside the telephone set, which completes the circuit path to operate the line relay in the line circuit.
The diagram only shows one subscriber. At what point in the diagram, do all of the subscriber lines come in?
@@tgcoder I'm not sure what you're asking. All subscriber lines come into the office in exactly the same way as the one in the diagram. If there were 1000 lines in the office, there would be 1000 telephones wired to 1000 different cable pairs, 1000 different line circuits, 1000 different line finder group bank terminals and 1000 different connector group bank terminals.
GTE usually allocated 20 line finders per 100 dns. Phone contests quickly overloaded this.
Wow, that's expensive provisioning. Typical Bell System city offices would have 16 to 20 per 200 lines. Phone contests would tend to overload connectors, not so much the linefinders. Neither linefinders nor connectors were provisioned to handle special events. For most of their history they were provisioned based on the busy hour load average of three months.
Hicken65 we had a radio station in town that would have elaborate contests. When the announcement happened so many tried to get in every line finder. of in the entire town went up. Finally company sued to make them at least to take 10th call. Not the 300 or so they normally did. I had cleared a trouble and went to sub to test to house. Not knowing the long awaited contest had kicked. All I heard was tweedling for 10 minutes. I started backtracking to see what I had screwed up.
@@w.p8960 Indeed, a very unusual situation.
The company I worked for always provisioned 18 line finders for 100 lines. GoS was 1/100. Dial tone returned to the caller in 3 seconds. A choke network was employed in later years to restrict calls able to complete to contest dial ins.
@@bboogaar In the Bell System, the engineering criterion was that 1.5% of the calls would get dial tone delay greater than 3 seconds in the average busy season busy hour(ABSBH). This wasn't an upper limit, it was the actual objective. In many towns, the ABSBH originating load per main station would only be 1.1 to 1.5 ccs (hundred call seconds). Applying these types of loads to the traffic capacity tables would produce a requirement for 12 to 15 line finders in each 200-line size group. So there was quite a difference in provisioning methodology between the Bell and non-Bell companies.
Thank You!
You're welcome and thanks for watching.
As an X inside plant an outside plant cabling person how in the hell did they get an analog switch dialing rotary to do predictive programming????? somebody please answer that
In the 1970's, telephone companies began installing electronic devices in between the linefinders and the first selectors. These devices could receive and store dial pulses, perform translations, and add or delete digits, then outpulse the changes to the first selector or other switch as needed.
In Pittsburgh Elliot was called dogtown - things are funnier than you think🎵💥!, (not first time on it)
Not many next gens. left - STILL- NS.
Was this system used in the crossbar networks?
SXS and Xbar were completely separate and different switching systems. However they both formed part of the Public Switched Telephone Network (PSTN). They were able to communicate with each other via trunks, and there was no problem placing calls from one type to the other. It was common for a large central office to have both SXS and Xbar in the same building.The SXS was an earlier design than the Xbar. In the Bell System, SXS dominated the manual to dial conversions during the 1920's through the 1940's. In the 1950's and 1960's, new switching systems required for dial conversion and growth were dominated by Xbar. It wasn't until the 1970's that Xbar overtook SXS in the number of lines in service.
@@Hicken65 What was the market penetration of panel?
@@AriBenDavid It maxed out at 6 million lines. To my knowledge it was never utilized outside of the Bell System.
@@Hicken65 So, only a few metro areas? What were its advantages in NYC, for instance? Quite a strange contraption ! Crossbar seems more like an ESS with its complex shared markers.
@@AriBenDavid The markers were remarkable devices. They were special purpose analog computers composed of around 1000 wire spring relays. They could process a call setup in about 1/3 to 1/2 seconds.
Subbed, you are amazing.
Well, thank you, and thanks for watching.
What's the difference between this video and your old video besides the new click-click-click through your whole 2nd paragraph?
Well, the 7 minute Dial Tone video doesn’t show or explain the operation of the relays, magnets, ratchets, pawls, interrupters, VON springs, 11th rotary step, bank multiple slip, subgrouping, transfer chain, open chain multiple, functions of the Group & Alarm Relay circuit or selection sequence process.
Haha, no no, sorry, I know the dial tone one doesn't. But I guess I thought my memory recalled a video from you that was specifically about the relays in the line finder. Anyway, I guess I'll have to go back and have a new look at the list of your videos too get reminded. Anyway, thanks, because you definitely put a lot of good work into these videos!
You're welcome. I have now made 3 very detailed videos of the 3 major types of switches, which are the selector, the connector and the line finder. They each perform completely different functions. However, the electromechanical operations involving the vertical and rotary stepping actions are very similar and use basically the same apparatus. I felt it best to keep each video independent and show the stepping details in each.
Does anybody remember the name of the solvent holder tool that hooked up to the steps of the ladder to clean the banks of the switches?
It was the KS16297 Flushing Unit.
Thank you I have been searching for this info and could not find it. By the way cleaning banks was my worst routine to do. Now all I have to do is find the different chemicals we used in the flushing unit .
@@Hicken65
I couldn't find any documentation that identified the makeup of the fluid, other than it contained 3% oil. The part number is KS16436 L1.
Thanks!@@Hicken65
Little oil and steam separate if course
Been in room full of them working back in day.. Sure are load..
It was a wonderful rhythmic clatter that let you know all was well.
ohhhh now I understand
In Pittsburgh Elliot was called dogtown - things are funnier than you think🎵💥!
With the complexity of this switch alone, i wonder how billing for each number was accomplished
I'm assuming you're talking about long distance billing. Up through the 1940's the billing was accomplished by the long distance switchboard operators who made up call detail tickets which were then sent to the accounting department. With the advent of Direct Distance Dialing (DDD), adjunct equipment called Automatic Message Accounting (AMA) was installed at the toll centres that produced call details on reels of punched paper tape that were then shipped off to the accounting department. SXS end offices were equipped with adjunct equipnment called Automatic Number Identification (ANI) that forwarded the calling telephone number to the AMA equipment at the toll centre.
@@Hicken65 Is there a video describing how ANI worked?
@@user2C47 I am not aware of any ANI videos. There are probably quite a few text descriptions online. You could begin with the "Bell Laboratories Record" which is easily found on a Google search. The May 1958 issue has an ANI description.
@@user2C47 Automatic Number Identification was a signaling protocol for billing. Most local calling, before Automatic Message Accounting Full Detailed Billing, was flat rate. Local switches did not have the capability of billing. If you had a toll switch, like a Crossbar Tandem, that switch may have had that billing function. In that case, the local switch and the tandem switch would have interoffice trunking, especially for billing, called ANI Trunks. The signaling protocol was set to first send the calling party number, and then the called party number. The tandem would take that info and cause an AMA record to be taken, in the form of punches on a continuous paper tape, later upgraded to magnetic tape, and much later to a data link to the billing department. Once we went to Stored Program Control electronic (computer) switches, they all did their own billing, and toll switches didn't need to any more.
Found two of the lines that need repar
Blinklys rele?
Tingeklokke?
Tattoo marskin?
Norwegian for turn signal relay? I really don't understand the point of your comment.
@35:44
Three Days of the Condor.
(That's how I got here!)
Well, you're the first one to identify that Robert Redford still that I pulled from the movie. He's actually in a crossbar switching system central office, not a step-by-step. He uses the 1013A test set that he stole in two scenes, first in a Holiday Inn PBX room and secondly, in the NYC Telephone Company CO. There are other old movies that do have scenes inside step-by-step CO's.
I love your reference to phone numbers at 1:40:09. PE6-5000 the Pennsylvania Hotel song by Glen Miller: th-cam.com/video/0OUkNOwpXtc/w-d-xo.html. and BW4-5780 a song by the Marvelettes in 1962: th-cam.com/video/Us18AUBM2RI/w-d-xo.html
PE6-5000 and BE4-5789 are the two numbers that can be dialed on my SXS demo unit. I have two short videos that demonstrate calls being dialed through the SXS equipment to these numbers. At the end , a 30 second excerpt of these songs is provided upon answering the call, although I used The Andrews Sisters for the PE6-5000 call because I wanted vocal lyrics to be heard.
Pics are to fast and pointless. Show the line being made not this flashing screen stuff
I would have done a 6 screen mosaic shot so you can follow the sequence without switching full screen. Showing each step full screen is making me queasy.
Well, viewer discretion is advised.
@@Hicken65 I think it was meant as constructive criticism. I don't know if you have the capability to do what he's asking; it would be extremely time consuming for me to do something like that. And it might not be the only solution.
But the method used doesn't seem to be very helpful, and I found myself getting queasy as well. I'm not epileptic and frankly this is the first time in my life I've experienced something like this from watching a video. Again, I'm not complaining, just trying to say that this might not be the best way to do this in the future.
Well, it certainly wasn't my intent to make viewers queasy. It never occurred to me that it would, having watched it many times on my 27" IMAC. I don't know if I could have done the mosaic approach. I don't have expensive software, just the Apple imovie 11 that came with the computer.
@@Hicken65 I never would have expected it to happen. I was very surprised that it did. I wonder if increasing the time on each frame would help. I sort of guess that what causes it is that my brain can't figure out what I'm even looking at in such a short time.
You can change your You Tube playback setting to .5 of .25 to see if that helps.
@0:40 wow...that's a LOT of editing!!
Indeed it was.
Just edited once then multiplied by the number. Like six of one single edit, for the number 6. 👍