Its incredible that you got an old timer that knows this tech. to actually set it up and literally pass along his knowledge to you. All you have to do is figure out how to make proper percussion with it! 😁 and sample the hell out of it!
back in the day I used to install Northern Telecom DMS Switches to replace entire central offices full of those old step switches... rows & rows of them, 20ft tall, clicking away.
Digital Multiplex System...the world's first all-digital exchange. Still in use in some places. There is a guy in the US who has a complete working one he takes to shows.
@@LOOKMUMNOCOMPUTER You are doing a great job Sam, your museum is really choc full of interesting stuff, and it's been awesome to see it being built from the ground up, all thru the pandemic too. Props to ya man.
What an excellent video. To listen to someone like Richard explain the system is extremely enjoyable, please thank him on my behalf. I had a brief interaction with this type of equipment in the mid-70s, working for a company that built similar racks for British Railways, who had their own phone system, as well as other customers who used this type of equipment.
This is one of the most enjoyable things I've watched recently, brilliant engineering and demonstrated so, so well. I hope to see more telecoms stuff, a definite subscribe from me!
still don't understand how only a few wires can make so many phones work.. but i've seen something i never would have seen once again by following your channel. awesome!
See, it's things like this that make me so happy to be a Patreon supporter 😁 I've met a couple of retired phone engineers who've loved the job so much that they've continued hacking the tech in their own time, I can't think of many other professions that would garner that sort of commitment!
It's surprising what "level" explanation has been defined as.. for example, the USA has 7 digits for a local telephone number after the area code [many locations have (many) overlapping area codes].. the first three digits are the exchange, the thousands digit "D" digit, is the level here, if a locality has a need for fewer than 1000 telephone numbers all will usually be assigned in the same level unless a specific reason exists to do otherwise, once that level becomes too full, another level might be added. In the old days terminals ending in 9 mostly 99 were held for telephone company purposes and not assigned to customers.. we had XX98 and the school district had the same XX but XX97, my mother said when she was home during the school day, we would get lots of wrong numbers for the school, back in the day instead of something like a final selector, a hunting selector was installed and all numbers were sequential, eventually this was overcome with "terminals" all assigned to one number on the additional selector.. eliminating the need for number waste and placing all numbers sequential (look at the mess when a company had 5 lines with hunting, number 6 was a different customer, and the one who had 5 lines needed more. Many areas (I think the UK was one) had phone numbers sized to fit the community needs, like 487 could be your number, but a bigger town your family was from could have 5 number phone numbers.. eventually with advanced dialing features and standardization this was changed. I don't know if all "levels" in the Strowger system went to unobtainable signal right away when it was so, some companies built out the system so you didn't know till a full number was dialed.. What puzzles me is tandems, we had two exchange codes, in the same building, in the step days.. one was 796, a different rate zone, the other was 257, more commercial customers.. both calls were local to each other, plus a lot of 25X numbers further into the same city in a different wire center and a different rate center, when 796 called 257 did it take a trunk to the tandem for 25X and another back, to a different part of the same originating building, or was it trapped with a double connection and dropped when a 7 was or wasn't dialed next. As for 796, a oddity occurred, we has 79X in many different wire centers, one was with 373, but it served the mall customers as 373 would fill out with too many customers, and at that time direct dialing to each department was becoming widely used.. to think, how did the network handle itself with codes so scattered. It used to be you would dial PLaza 3-XXXX for example, or PLaza5-XXXX and the same building handled your call.
For an in depth look at Strowger switching, you may want to check out my 3 very long videos of the line finder, selector and connector. These are the Bell System version of these devices. Just search my channel name, Hicken65 on RUclips.
yeah very helpful thanks a lot as i got hold of another rackl that is not all together for a bit of a challenge. so i will be watching these thanks a lot!
You are so great to see things like preserving these thing. The person donating the rack needs a medal as see so much skipped and my other half would have kicked me out if I came home with a rack like this. Please feed back to him what a great person. So was he a training coach?
I always used to think that all these switches worked with number pairs. That is, if you were dialling something like 62, the arm would go up to the 6th level then move across 2 spaces. But it seems a lot more complicated than that - it's going up and trying to find an available link to the next selector!
I was all my life amazed by this telephone rotary system : the rotary mechanism on the telephone receiver/speaker itself and the - in some way or another - "relais"-elevator-system that finally needs to make a passing through all right numbers connection... You can make a safe-open-you system with this once you get it knowable alright about the real operational working of it... It's as connection-making by pushing all the right dots of a QR-code at the same time and than, getting that 'Sesam, open you"-awe moment. Just a thought.
A ops B ops C,then the switch goes vertically...however a 3 year apprenticeship and further 2 years specialisation with the GPO in 1973 to be able to strip down and rebuild these switches,and fault diagnose.These strowger switches were very labour intensive.Then almost overnight they were obsolete,and another learning curve as the first electronic PABX switches were introduced,the first one being the Monarch switch,that the GPO had on their portfolio.
Fascinating stuff! I was always interested in telephony and phreaking when I was a kid, but unfortunately most of that tech was pretty much obsolete by the time I was old enough to try to put it into practice, though I did get to do some of it. 😜 Anyways, great to see you have an interest in old phone tech as well. Cheers
Oildag is graphite in an oil based solution, just as Aquadag is graphite in a water based solution, used for the graphite coating of CRT's, and called the same.
its true what they say ask a man a question about his expertise and he will gladly explain everything I work with a guy like him and also was a true gent you can't beat years on the job
What happens when it gets to the final selector, I missed it and can’t seem to make sense when I replay. where does it go? I’m the same as what you said at the start, I’m just listening in ore to the retired engineer 😂
The final selector routed the final two digits of the number you were calling so if for example the last two digits were 6 and 4,, when you dialled the 6, the selector would step up six times and stop without turning into the contacts like the previous selectors. This then meant it was ready for you to tell it the last digit. When you dialled the 4, it would then step into the contacts four times and so then apply ringing to the number ending 64 and wait for the called number to answer. When the called number picked up the phone, it cut off the ringing and connected you through. The final selector gave access to the 100 telephone numbers ending from 11 (up 1 and across 1) up to 00 (up 10 and across 10) in the group starting with the hundreds digit you dialled on the previous selector. In those days, if you were calling someone in the same town, and that had town had 6 digit telephone numbers, you only had to dial the six digits, not the whole national number as we always use now. When you picked up your phone to maybe call someone with the number 547364, several parts happened to get you a free "first selector" (there wasn't one per phone line) and once you got to that point, you would hear dial tone as the exchange was ready for you to dial your number selection. When you dialled the 5, that selector would step up 5 times then automatically around the 10 steps on tat row looking for a free "second selector" and would hold that for you to dial your second digit and so it went on through to the final selector as described which too the final two digits. The point being, when you initially picked up the phone to make a call, the exchange had no idea where you were going to call as it could have been anywhere in the World. Once you made your choice on the "first selector", you gave it a clue that you were calling one of the customers served off this exchange (or a nearby exchange in the same Dialling code area as was often the case) and had narrowed it down from anyone in the World to one of up to possibly about 700000 (in theory) customers off this telephone exchange or a nearby one as mentioned. That is because if the first number you dialled were 1, 9 or 0, it would take you elsewhere. (In the UK, 1 being services like the telephone operator or directory inquiries etc; 9 could take you to the emergency number but also could route your call to nearby telephone exchanges without going through the trunk network; 0 would take you to the trunk network and possibly on to International) I'm sure no one will read this far as I've gone on a bit here but if you're still with me, there were, in the UK, variations to this which are important. There was an option for a customer to have multiple lines working on the same number. They could for example have the published number 547361 but have 10 lines so that customers could still get through to them even if the already had a few calls active. When they took this service up, someone in the exchange would get a request to "enable" this by making some connections on the back of the wiring on that group of final selectors so that when you dialled the 6 as before, it would still step up six times and wait for the last digit. However, when you then dialled the 1, it would act more like the previous selectors in the chain, it would step to the first position and test to see if it were busy as before but instead of giving you the "engaged tone" at that point and waiting for you to clear down, it would step on to what in theory was 547362 and check that one, if that were busy it would step on to the next and so on until it had checked the whole "enabled" group of lines that customer were paying for. There was also the facility to have more than 10 lines but a different type of final selector was needed for those customers called an "eleven and over" final selector. The same principle applied but the switch was able to test 10 lines to see if they were busy and if they were, it would automatically drop out of the end of the contact bank and step up to the next level above and check those andso on depending on how many lines that customer had until it had checked them all. The other big variation on all this was called "Director" which routed calls differently in large urban areas, London, Manchester, Glasgow etc where a high proportion of calls made were in the urban area but I.ve gone on with my man-splaining quite enough and even though we had to learn about Director in college, I never worked on it.
@@peterhanahoe4913 thank you so much for you detailed reply, it had cleared up a few thing even if I did need to read it twice to digest it all. My question is (sorry) wouldn’t all the selectors get confused if 20 calls came in at the same time for example or where there many banks of selectors they could come through, how does the call from one home telephone not bump into many other calls being put through? As technically the phone line might not be engaged?
@@BTTT-c9q Thanks for at least reading my epic. I hope I can clear this one up. The first thing to remember is that not everyone with a phone line would want to make a call at the same time and so there didn't need to be enough selectors in the exchange to cope with that level of traffic. I think I said earlier that there were several things that happened before the caller got dial tone from the "first selector". People who payed a business tariff and so had busier lines, mainly in the daytime, were allocated a "uni-selector" which in the scheme of things at least, was between the line relays which detected the customer picking up the handset, and the "first selector". In a similar manner to the way the other selectors worked, but not looking like them, this uni-selector, which had a wheel-like appearance, had wiper arms that would spin around a semi-circular bank of contacts which had the "first selectors" connected to them. Once activated, it would spin along the contacts until it detected a free "first selector" to connect to which would then give the customer dial tone as described previously. As I said, business lines would have their own dedicated uni-selector in most cases but there were other priority lines that would be allocated one but there wasn't one for every line. Quieter, lines with less usage, would have yet another piece of equipment in the mix. In groups of 50, they would be connected through a thing called a "line-finder" and each "line-finder" would have five uni-selectors. In this way, only five people out of the 50 could make an outgoing call at any one time but this was almost always found to be enough. If at any stage in this process, a "line-finder" couldn't find a free uni-selector. A uni-selector couldn't find a free "first selector" etc and got to the last step in the contact bank, the caller would then hear "equipment engaged tone". Most customers didn't really recognise this from normal bust tone when a line was engaged but instead of even bleeps, it was long and short ones alternating. Also, when this last step was hit by any stage in the whole process, a pulse would operate an "overflow meter". A small counter that would step on one position. There were many overflow meters in a big exchange, readings had to be taken weekly and passed on up far action to be taken by the traffic department. You would see in this video that once a call is established, those selectors remain in position. In that position, they are not accessible to anyone else making a call and test "busy" to any other selectors going through their banks of contacts trying to set up a call, When the call is ended, the selectors on that one call all drop back to normal at once as seen in the video and become "Free" again. The point here being that once a call was set up, all the equipment involved in routing it stayed exclusive to that call until it ended (except under fault conditions) The traffic department had the job of calculating what was the minimum number of selectors in each section of the switching that could be available to not cause traffic overflows . Going back to the uni-selector, I think ( I may have forgotten the number here, d'oh) they spun through a bank of 12 contacts, each of which had "first selectors" connected to them. Those 12 sets of contacts would be "commoned together" on the back of the shelf so you might have 20 uni-selectors all choosing from the same 12 "first selectors" when trying to set up a call, the whole thing relying on not all the customers wanting to make a call at once and the traffic department calculating the correct number that were likely to want to make a call at the same time and o providing the correct amount of equipment to accommodate this. I Getting back to your question about 20 or so calls coming it at once, well the likelihood of them coming in in the same fraction of a second is very low. If two people tried to make a call at the same time, from the same group of uni-selectors as described, one would almost certainly be slightly ahead of the other even if only by milliseconds and so would seize the "first selector" before the other and so the other call would step over that one and grab the next free one. The same really goes for calling a number just as they try to make a call. The incoming call would either get through or it wouldn't. If the exchange hadn't detected the person being called lifting the handset as the final selector stepped onto their line, the call would be connected through. If it had detected them lifting the handset, the caller would get a "busy tone". It wasn't that uncommon for people to lift their phone to make a call, not listen for dial tone and so not realise a call had just been connected to them, and keep dialling (the pulses going nowhere) , to find someone tring to talk to them when the dialing had finished. Lots of thing that shouldn't happen in theory still happened under fault conditions. Also worth remembering, what I am talking about here was called Strowger, non-director. I did mention in my previous reply that the big cities used director which routed calls differently and also there were small exchanges, usually in villages or country areas, which were Strowger but again routed calls in a different way. these would be places with only a few hundred lines and were called UAX (Unmanned Automatic Exchange" but there were variations on that too. The whole system was constantly evolving and so there were several types of exchanges all working at the same time and inter-connecting with each other obviously. I hope this helps..
Before they added the extra numbers of course. We used to stand outside the exchange listening to the noises going on inside - 43 years later I find out what’s going on.
Super glad people like Richard are preserving tech like this. I feel like without his explanation, this would have just been alien tech to me.
i am a former post office engineer, and very jealous of you having a small demonstation exchange, i hope you and others enjoy it
Time to plan a visit to the museum!
I could listen to retired engineers for hours, man.
Its incredible that you got an old timer that knows this tech. to actually set it up and literally pass along his knowledge to you. All you have to do is figure out how to make proper percussion with it! 😁 and sample the hell out of it!
back in the day I used to install Northern Telecom DMS Switches to replace entire central offices full of those old step switches... rows & rows of them, 20ft tall, clicking away.
RIP Nortel
Digital Multiplex System...the world's first all-digital exchange. Still in use in some places. There is a guy in the US who has a complete working one he takes to shows.
@@douro20 System X is hard to beat.
@@KarlHamilton Karl the Meridian system😀I did some courses at Nortel in Maidenhead UK.
Thank you Richard for describing everything as you assembled it!
The "Yeah" at the end, lol, I felt that :D
Hahaha yep ! It was a lot to take in haha
@@LOOKMUMNOCOMPUTER You are doing a great job Sam, your museum is really choc full of interesting stuff, and it's been awesome to see it being built from the ground up, all thru the pandemic too. Props to ya man.
What an excellent video. To listen to someone like Richard explain the system is extremely enjoyable, please thank him on my behalf. I had a brief interaction with this type of equipment in the mid-70s, working for a company that built similar racks for British Railways, who had their own phone system, as well as other customers who used this type of equipment.
There are some amazing people out there, glad you found Richard! Lots to learn here!
This is one of the most enjoyable things I've watched recently, brilliant engineering and demonstrated so, so well. I hope to see more telecoms stuff, a definite subscribe from me!
Fascinating stuff. Telephone exchanges have always intrigued me!
still don't understand how only a few wires can make so many phones work.. but i've seen something i never would have seen once again by following your channel. awesome!
Awesome! cant wait to come and visit! all of the things i love under one roof....
See, it's things like this that make me so happy to be a Patreon supporter 😁 I've met a couple of retired phone engineers who've loved the job so much that they've continued hacking the tech in their own time, I can't think of many other professions that would garner that sort of commitment!
What a splendid chap Richard was. So good to hear someone explain this old tech with such affection
It's surprising what "level" explanation has been defined as.. for example, the USA has 7 digits for a local telephone number after the area code [many locations have (many) overlapping area codes].. the first three digits are the exchange, the thousands digit "D" digit, is the level here, if a locality has a need for fewer than 1000 telephone numbers all will usually be assigned in the same level unless a specific reason exists to do otherwise, once that level becomes too full, another level might be added. In the old days terminals ending in 9 mostly 99 were held for telephone company purposes and not assigned to customers.. we had XX98 and the school district had the same XX but XX97, my mother said when she was home during the school day, we would get lots of wrong numbers for the school, back in the day instead of something like a final selector, a hunting selector was installed and all numbers were sequential, eventually this was overcome with "terminals" all assigned to one number on the additional selector.. eliminating the need for number waste and placing all numbers sequential (look at the mess when a company had 5 lines with hunting, number 6 was a different customer, and the one who had 5 lines needed more.
Many areas (I think the UK was one) had phone numbers sized to fit the community needs, like 487 could be your number, but a bigger town your family was from could have 5 number phone numbers.. eventually with advanced dialing features and standardization this was changed.
I don't know if all "levels" in the Strowger system went to unobtainable signal right away when it was so, some companies built out the system so you didn't know till a full number was dialed..
What puzzles me is tandems, we had two exchange codes, in the same building, in the step days.. one was 796, a different rate zone, the other was 257, more commercial customers.. both calls were local to each other, plus a lot of 25X numbers further into the same city in a different wire center and a different rate center, when 796 called 257 did it take a trunk to the tandem for 25X and another back, to a different part of the same originating building, or was it trapped with a double connection and dropped when a 7 was or wasn't dialed next. As for 796, a oddity occurred, we has 79X in many different wire centers, one was with 373, but it served the mall customers as 373 would fill out with too many customers, and at that time direct dialing to each department was becoming widely used.. to think, how did the network handle itself with codes so scattered. It used to be you would dial PLaza 3-XXXX for example, or PLaza5-XXXX and the same building handled your call.
THIS IS SICKKKK!!!! I love learning about stuff like this, so beautifully analogue
Tim Hunkin talked about a step-by-step exchange which was still going in the early '90s in one part of the UK...I forget where.
For an in depth look at Strowger switching, you may want to check out my 3 very long videos of the line finder, selector and connector. These are the Bell System version of these devices. Just search my channel name, Hicken65 on RUclips.
yeah very helpful thanks a lot as i got hold of another rackl that is not all together for a bit of a challenge. so i will be watching these thanks a lot!
You are so great to see things like preserving these thing. The person donating the rack needs a medal as see so much skipped and my other half would have kicked me out if I came home with a rack like this.
Please feed back to him what a great person. So was he a training coach?
This is amazing. Thank you for this video. Extremely cool. And thank you to Richard for sharing his knowledge with us.
I always used to think that all these switches worked with number pairs. That is, if you were dialling something like 62, the arm would go up to the 6th level then move across 2 spaces. But it seems a lot more complicated than that - it's going up and trying to find an available link to the next selector!
I was all my life amazed by this telephone rotary system : the rotary mechanism on the telephone receiver/speaker itself and the - in some way or another - "relais"-elevator-system that finally needs to make a passing through all right numbers connection... You can make a safe-open-you system with this once you get it knowable alright about the real operational working of it... It's as connection-making by pushing all the right dots of a QR-code at the same time and than, getting that 'Sesam, open you"-awe moment. Just a thought.
This was (excuse the pun) phreaking awesome. Thanks Sam, and thank you Richard for sharing your knowledge, brilliant viewing :D
Magnificent, the entire vide. God bless you Richard for passing on the knowledge and God bless the author for sharing it.
A ops B ops C,then the switch goes vertically...however a 3 year apprenticeship and further 2 years specialisation with the GPO in 1973 to be able to strip down and rebuild these switches,and fault diagnose.These strowger switches were very labour intensive.Then almost overnight they were obsolete,and another learning curve as the first electronic PABX switches were introduced,the first one being the Monarch switch,that the GPO had on their portfolio.
I'd love to see a video taking a closer look on that amazing sounding alarm buzzer!
Fascinating stuff! I was always interested in telephony and phreaking when I was a kid, but unfortunately most of that tech was pretty much obsolete by the time I was old enough to try to put it into practice, though I did get to do some of it. 😜 Anyways, great to see you have an interest in old phone tech as well. Cheers
Fantastic! Well done Richard.
Ps The days of feeler gauges and spring tensioners,a 4 week course on faultfinding and strip down and rebuild.A tough course.
Fascinating!
Oildag is graphite in an oil based solution, just as Aquadag is graphite in a water based solution, used for the graphite coating of CRT's, and called the same.
9:25 the Teletubbies look absolutely fascinated, 😄. Needless to say, I am too.
its true what they say ask a man a question about his expertise and he will gladly explain everything
I work with a guy like him and also was a true gent
you can't beat years on the job
How did they do register for billing purposes?
Any schematics that came with this you can share? Very cool!
I need to find one of those modules...
Magical. Memories of working on AXEs
16:24 Weird laugh that sounds like those telephone switches
This is awesome.
What part is the crossbar switches?
Love this .
Its crazy just how mechanically complicated telephone relays are. Its incredible to think that a small computer could do the job of this huge machine.
What happens when it gets to the final selector, I missed it and can’t seem to make sense when I replay. where does it go? I’m the same as what you said at the start, I’m just listening in ore to the retired engineer 😂
The final selector routed the final two digits of the number you were calling so if for example the last two digits were 6 and 4,, when you dialled the 6, the selector would step up six times and stop without turning into the contacts like the previous selectors. This then meant it was ready for you to tell it the last digit. When you dialled the 4, it would then step into the contacts four times and so then apply ringing to the number ending 64 and wait for the called number to answer. When the called number picked up the phone, it cut off the ringing and connected you through. The final selector gave access to the 100 telephone numbers ending from 11 (up 1 and across 1) up to 00 (up 10 and across 10) in the group starting with the hundreds digit you dialled on the previous selector.
In those days, if you were calling someone in the same town, and that had town had 6 digit telephone numbers, you only had to dial the six digits, not the whole national number as we always use now. When you picked up your phone to maybe call someone with the number 547364, several parts happened to get you a free "first selector" (there wasn't one per phone line) and once you got to that point, you would hear dial tone as the exchange was ready for you to dial your number selection. When you dialled the 5, that selector would step up 5 times then automatically around the 10 steps on tat row looking for a free "second selector" and would hold that for you to dial your second digit and so it went on through to the final selector as described which too the final two digits. The point being, when you initially picked up the phone to make a call, the exchange had no idea where you were going to call as it could have been anywhere in the World. Once you made your choice on the "first selector", you gave it a clue that you were calling one of the customers served off this exchange (or a nearby exchange in the same Dialling code area as was often the case) and had narrowed it down from anyone in the World to one of up to possibly about 700000 (in theory) customers off this telephone exchange or a nearby one as mentioned. That is because if the first number you dialled were 1, 9 or 0, it would take you elsewhere. (In the UK, 1 being services like the telephone operator or directory inquiries etc; 9 could take you to the emergency number but also could route your call to nearby telephone exchanges without going through the trunk network; 0 would take you to the trunk network and possibly on to International)
I'm sure no one will read this far as I've gone on a bit here but if you're still with me, there were, in the UK, variations to this which are important. There was an option for a customer to have multiple lines working on the same number. They could for example have the published number 547361 but have 10 lines so that customers could still get through to them even if the already had a few calls active. When they took this service up, someone in the exchange would get a request to "enable" this by making some connections on the back of the wiring on that group of final selectors so that when you dialled the 6 as before, it would still step up six times and wait for the last digit. However, when you then dialled the 1, it would act more like the previous selectors in the chain, it would step to the first position and test to see if it were busy as before but instead of giving you the "engaged tone" at that point and waiting for you to clear down, it would step on to what in theory was 547362 and check that one, if that were busy it would step on to the next and so on until it had checked the whole "enabled" group of lines that customer were paying for. There was also the facility to have more than 10 lines but a different type of final selector was needed for those customers called an "eleven and over" final selector. The same principle applied but the switch was able to test 10 lines to see if they were busy and if they were, it would automatically drop out of the end of the contact bank and step up to the next level above and check those andso on depending on how many lines that customer had until it had checked them all.
The other big variation on all this was called "Director" which routed calls differently in large urban areas, London, Manchester, Glasgow etc where a high proportion of calls made were in the urban area but I.ve gone on with my man-splaining quite enough and even though we had to learn about Director in college, I never worked on it.
@@peterhanahoe4913 thank you so much for you detailed reply, it had cleared up a few thing even if I did need to read it twice to digest it all. My question is (sorry) wouldn’t all the selectors get confused if 20 calls came in at the same time for example or where there many banks of selectors they could come through, how does the call from one home telephone not bump into many other calls being put through? As technically the phone line might not be engaged?
@@BTTT-c9q Thanks for at least reading my epic. I hope I can clear this one up. The first thing to remember is that not everyone with a phone line would want to make a call at the same time and so there didn't need to be enough selectors in the exchange to cope with that level of traffic. I think I said earlier that there were several things that happened before the caller got dial tone from the "first selector". People who payed a business tariff and so had busier lines, mainly in the daytime, were allocated a "uni-selector" which in the scheme of things at least, was between the line relays which detected the customer picking up the handset, and the "first selector". In a similar manner to the way the other selectors worked, but not looking like them, this uni-selector, which had a wheel-like appearance, had wiper arms that would spin around a semi-circular bank of contacts which had the "first selectors" connected to them. Once activated, it would spin along the contacts until it detected a free "first selector" to connect to which would then give the customer dial tone as described previously. As I said, business lines would have their own dedicated uni-selector in most cases but there were other priority lines that would be allocated one but there wasn't one for every line. Quieter, lines with less usage, would have yet another piece of equipment in the mix. In groups of 50, they would be connected through a thing called a "line-finder" and each "line-finder" would have five uni-selectors. In this way, only five people out of the 50 could make an outgoing call at any one time but this was almost always found to be enough.
If at any stage in this process, a "line-finder" couldn't find a free uni-selector. A uni-selector couldn't find a free "first selector" etc and got to the last step in the contact bank, the caller would then hear "equipment engaged tone". Most customers didn't really recognise this from normal bust tone when a line was engaged but instead of even bleeps, it was long and short ones alternating. Also, when this last step was hit by any stage in the whole process, a pulse would operate an "overflow meter". A small counter that would step on one position. There were many overflow meters in a big exchange, readings had to be taken weekly and passed on up far action to be taken by the traffic department. You would see in this video that once a call is established, those selectors remain in position. In that position, they are not accessible to anyone else making a call and test "busy" to any other selectors going through their banks of contacts trying to set up a call, When the call is ended, the selectors on that one call all drop back to normal at once as seen in the video and become "Free" again. The point here being that once a call was set up, all the equipment involved in routing it stayed exclusive to that call until it ended (except under fault conditions)
The traffic department had the job of calculating what was the minimum number of selectors in each section of the switching that could be available to not cause traffic overflows . Going back to the uni-selector, I think ( I may have forgotten the number here, d'oh) they spun through a bank of 12 contacts, each of which had "first selectors" connected to them. Those 12 sets of contacts would be "commoned together" on the back of the shelf so you might have 20 uni-selectors all choosing from the same 12 "first selectors" when trying to set up a call, the whole thing relying on not all the customers wanting to make a call at once and the traffic department calculating the correct number that were likely to want to make a call at the same time and o providing the correct amount of equipment to accommodate this. I
Getting back to your question about 20 or so calls coming it at once, well the likelihood of them coming in in the same fraction of a second is very low. If two people tried to make a call at the same time, from the same group of uni-selectors as described, one would almost certainly be slightly ahead of the other even if only by milliseconds and so would seize the "first selector" before the other and so the other call would step over that one and grab the next free one. The same really goes for calling a number just as they try to make a call. The incoming call would either get through or it wouldn't. If the exchange hadn't detected the person being called lifting the handset as the final selector stepped onto their line, the call would be connected through. If it had detected them lifting the handset, the caller would get a "busy tone". It wasn't that uncommon for people to lift their phone to make a call, not listen for dial tone and so not realise a call had just been connected to them, and keep dialling (the pulses going nowhere) , to find someone tring to talk to them when the dialing had finished. Lots of thing that shouldn't happen in theory still happened under fault conditions.
Also worth remembering, what I am talking about here was called Strowger, non-director. I did mention in my previous reply that the big cities used director which routed calls differently and also there were small exchanges, usually in villages or country areas, which were Strowger but again routed calls in a different way. these would be places with only a few hundred lines and were called UAX (Unmanned Automatic Exchange" but there were variations on that too. The whole system was constantly evolving and so there were several types of exchanges all working at the same time and inter-connecting with each other obviously.
I hope this helps..
Absolutely fascinating.... Try making mobile calls on this..
So much more fun than computers.
Excellent
I'm curious to know where the actual audio ringing/busy tone heard in the earpiece comes from
Hi, it’s generated by the white motor in the bottom of the shelf …
classic maglite
did you get any schematics?
I bet you've wrote his number down EVERYWHERE!
Find memories of maintaining Strowger exchanges
woohoo
377 numbers away from my phone number in s Benfleet
Before they added the extra numbers of course. We used to stand outside the exchange listening to the noises going on inside - 43 years later I find out what’s going on.