When the tower was constructed in 1887, multiplexing technology was probably not available (I'm not so sure of the timeline in Europe). By 1913 it likely would have come into use. However, multiplexing really isn't a factor here, as the tower seems to have been built to serve local loops. Since these loops go to subscriber telephone sets, there's no option for multiplexing without expensive and maintenance-intensive equipment at customer premises. Multiplexing of local loops is called "pair gain" and wouldn't be developed until later, and it was never really that popular in most phone systems. Outside of suburban areas, it's typical that each copper pair runs directly to the exchange. Historically, and today, there is rarely any active equipment (or since the 1950s or so even passive conditioning) on local loops, they're just wires from the exchange to the phone.
As for why you didn't see similar constructions in other cities, this was definitely an unusually large telephone office for the time. In the US, a city exchange of the late 20th century would usually have just hundreds of lines, many of them multi-party. Telephone companies scaled up by building more exchanges, rather than a single very large one. When they got into these kinds of subscriber numbers at an exchange, the F1/F2 cable scheme was in use to avoid this kind of wiring. It does seem to be the case that telephone adoption was unusually rapid in Sweden, I find one (poorly sourced) claim that there were some 4,800 telephone subscribers in Stockholm in 1886 which would very likely make it the most telephone-rich city in the world. The situation of the tower seems to have developed in part because its builder, Allmänna, was consolidating the Stockholm telephone market through acquisitions and made a decision to centralize the many acquired customers onto on exchange.
What I'm a little confused about here is the lack of cables. The other big reason you didn't see constructions like this in the US, even in places like New York City, is because subscriber loops were quickly moved into lead-sheathed, paper-insulated multi-pair cables. These could contain hundreds of pairs. Cables were pretty much reaching maturity when this tower was built. I am curious as to the reason that multi-pair cables were not adopted more quickly in Stockholm, but it may be as simple as the considerable investment in this tower making open wire the preferred option for its short lifespan. In any case, the common claim that underground cables obsoleted the tower rings hollow to me, or at least misses an important detail, as aboveground cables were already in use in the 1880s. I suspect that modernization to cables was just deferred in Stockholm until it happened to also make sense to move to duct or pipe systems. In the US, it was more common that telephone exchanges switched to overhead (aerial) cable to manage exactly the wire sprawl issue that this tower exemplifies, and then only later (if ever) started to bury cables.
> As for why you didn't see similar constructions in other cities, this was definitely an unusually large telephone office for the time
For some perspective here - it took until the mid-80s for most of Germany to be connected to a phone line. That is, the 1980s.
I recently talked about that with my father after I found a postcard from one of my uncles from the early 80s confirming meeting and dinner plans. While I remember them always having a phone they were one of the households only connected in the mid 80s - which in retrospect explains some of the things I've found odd about them when talking to them by phone. It was a new thing for them.
(My parents got connected early on - my mother used to work for the post office in the phone exchange, and one of the perks of the job was priority for getting a phone line. Which also explained why we had an old grey phone, while pretty much all my friends had a relatively modern - for the time - one: they all only somewhat recently got phones)
I just wanted to say that after the first paragraph, I wondered who this comment was written by, and then I realised I knew the answer already. There was no need for me to even check.
Comments mention multiplexing and that’s not wrong but the real reason for the vast number of wires is amplifiers, or rather the lack of practical ones at the time. You had to transmit at high enough power to overcome losses and still be able to hear at the destination.
Each wire carries just one signal at a power that would easily interfere with others, they needed relatively thick wires separated from each other. You see pictures of poles with lots of cross bars carrying lots of wires in this period.
Once amplification was practical they could use the thin telephone wires bundled together in a cable, each wire carrying a much fainter signal that can be easily amplified as needed.
Amplification existed but it took the vacuum tube to get it affordable and reliable for each circuit to have its own amplification.
Does that mean the quality of the voice calls in that era was better than later systems? Since it's logical to have loss of quality when a weak signal is amplified.
Right, quality was poor and the transmitter and receiver were part of the problem. They experimented with different methods, but part of the problem was that without amplification they needed a transmitter that worked at the full voltage necessary to travel the full distance.
As someone obsessed with Networking and Networking Topology I am so so fascinated by analog telephone networks. Just the idea of long-distance connections being made via physical wires connecting at switches by human operators feels so raw. I can't even imagine such a thing. When I was younger analog telephone was being phased out. I still remember my dad having to go buy calling cards at convenience stores so he could make long distance calls back home. And then one day there were no more calling cards.
I can imagine lots of lessons learned from telephone networking helped shape ideas around computer network design.
More specifically, packet-based rather than switched telecoms.
Using a switch, each connection is a literal physical circuit. Multiplexing allows multiple circuits per phone line (through frequency separation at the carrier-frequency level), and much early telephony research involved increasing the multiplexing capacity and consequent issues.
With packet-switched networks, the only circuits are the interconnects between routers, and each individual data packet can take a different route. Subject to quality of service / service level agreements (QoS / SLA), it's possible to support far more distinct individual connections over packet-switched networks, though there still remains a maximum total bandwidth. For time-sensitive modalities (e.g., realtime voice or video), excess traffic leads to congestion and buffering, distortion, or interference, so limits remain. But they're far more generous than with circuit-based networks.
Put another way, packets give a greater assurance of establishing a link between any two nodes, whilst circuits give a greater assurance of a minimum bandwidth floor between those nodes. If you can get a connection, circuit-based line quality is often (though not always) superior, in terms of consistency, clarity, and low latency.
That doesn't necessarily line up with my understanding of reality.
In the US, a PRI can handle up to exactly 23 concurrent, native g.711u phone calls. That's it's capacity: No more, and no less. It's always 23, with each concurrent call using exactly 64kbps of symmetric bandwidth....just because that's the number of B channels provided.
But if we take that same PRI and make it do IP packets instead using MLPPP, then our capacity is actually reduced. By adding the magic of packet switching, we also add overhead. And with that added overhead, we can only get only get ~19 g.711u calls through that same circuit.
(Now, sure: In a bigger picture, that PRI may be better utilized as an IP pipeline than as a dedicated telephony circuit. It's certainly more flexible that way.
But packetization is not something that automatically improves capacity. It often does the opposite.)
This is well outside my area of expertise, assess accordingly.
From some quick DDGing, PRI (primary rate interface) is one of several possible options for a packet-switched telephony system, and is specifically contrasted with SIP VOIP connections in this article:
PRI is TDM. It isn't packet-switched; there are no packets on the voice-carrying B channels. It uses timeslots, instead.
> NB, "always 23" ... unless you're in Europe, in which case 23 === 30.
I believe I was very specific about the locality I was referring to, but I am appreciative of the seemingly-disingenuous pedantry that is apparently unfettered by such arcane concepts as "context".
Where TDM == "time-division multiplexing". Expanding acronyms, particularly those from less-mainstream concepts and fields, is appreciated.
It wasn't clear from your comment (or the source I'd read) that PRI is circuit based.
The fact that circuits can be multiplexed (whether through frequency-separation, timeslicing, or other mechanisms, see <https://en.wikipedia.org/wiki/Multiplexing> for general MUXology) is irrelevant. They are still circuits. And there are limits to how many circuits can be maintained at a single time, regardless of the apparent endpoint combinatorial possibilities.
Packet-based switching has far greater capacity and flexibility, as I, my sources, and several other comments to this thread, have noted.
Pedantry aside, you seem to be confirming my initial point.
Theoretically, yes. However, in practice, no. One of the great wins of packetization is that your infrastructure becomes generic in the sense that it is application-independent, allowing all subsequent improvements in eg. compression protocols to be applied retroactively. Therefore, if you would apply for example a variable bit rate, modern CODEC to the voice streams it's likely you would fit substantially more than a naïve set of voice channels on the raw PRI. The main costs are not in overhead, but enhanced session setup and round-trip latency and increased processing overheads at the edges. However, modern electronics offer embedded solutions making this ~free and a few ms are tolerable. IIRC voice is fine at <200ms latency, more or less, though less is obviously better.
The philosophical battle was centralist vs decentralised, for-purpose vs generic infrastructure, single-mode vs. multi-mode network applications, circuit-switched guaranteed QoS vs. best-effort packets, waterfall vs. iterative development and investment. Packets won for general purpose networking, because the nature of physics and bureaucracy meant the cost and time savings for operators and users were substantial.
Using a different codec can improve call-carrying capacity of any compatible link, for sure. But that's wiggling a different variable: Packetization-vs-not is a whole different game than g.711u vs g.723 is.
And I agree, completely: There's very good reasons for packet-switched networks having won over TDM technologies like PRI, with IP to tie it altogether. What we've arrived at is beautifully generic in that the packets themselves don't care at all about what combination of lower layers were used to get them from A to B. However it gets there, it's just IP. That's really neat.
Latency-wise, I'm a little torn: With the latency involved in a normal phone call on my normal cell phone today, I find that I "talk over" people on phone calls more than I did decades ago in the TDM days. The pacing is very different than it was.
Sometimes, when the echo cancellation fails at some level, I can hear myself echo from the far end of a call.
That echo is annoying, but I mention it because it is something that lets me hear the latency of the call. It's often on the order of 500ms RTT...which is quite a lot. We never had latency like that in the TDM days for domestic calls.
(Historically, in TDM world: The voice data always arrived at the right time and there was also always a place to put it at the right time. It was real-time instead of best-effort, so there just wasn't any utility to having any large buffers along the way: The timing was either resolutely correct or it didn't work at all. Things are a lot mushier today.)
At this early time (this is not long after the invention of the phone itself) - none, really. Stockholm had a much higher number of telephones per home, but not very long after this operators put the cables underground and that was that.
Does this mean that probably somewhere below that tower, there were operator stations that would have allowed any of the 5500 lines to connect to any of the other lines? How many simultaneous calls would that even be? Correct me if I borked this, but 5500!/(2^(5500/2)), perhaps?
It seems plausible that if the phone had only just been invented, you'd initially set up small systems that would in fact allow any line to connect to any line. That'd be fine for maybe even a few dozen lines. But as the image shows, that doesn't scale too well.
The total number of connections would all but certainly be far greater than the maximum number of simultaneous calls which could be supported.
Under PSTN, public-switched telephone networks, a not-infrequent occurrence, especially when calling long-distance, was to get a message "all circuits are busy". When each call was literally a circuit, and the "switch" (the central telephone exchange) made and broke those circuits as calls began and ended, my understanding (not my area of expertise, but one of some interest) is that this meant that all available interchange connections were occupied. For long-distance, this was typically far lower than for local calls (most phone traffic is local), and for international calls, lower still. The first transatlantic telephone cable could support only 36 simultaneous calls, in 1956. Calls were short, expensive, and all but exclusively for business and government subscribers.
I'd expect that the Stockholm exchange probably supported a few hundred simultaneous calls, probably a few (a dozen or so perhaps) per operator, who had to physically connect each call.
A lot of phone systems saturated themselves when things first shut down. Probably not physical copper constraints but the virtual interconnects between providers:
Through the 1990s (and prior) this was not uncommon, particularly on high-traffic days, often holidays (in the US: Mother's Day, Thanksgiving, and Christmas), when calling long-distance. Also during natural disasters, when local calls might also be affected.
There were also systems outages, such as the 1988 Hinsdale switching station which took out phone service to most of the Chicago area (both local and long-distance):
"1988 PHONE CRISIS TIED TO 1 BROKEN POWER LINE" (1988)
Turns out that that switch was a SPOF:
To make sure such a crisis doesn`t happen again, Illinois Bell Telephone Co. announced Friday it is embarking on a $80-million, five-year program to construct a complete duplicate telephone network system throughout its Chicago suburban operation and to redesign its fire protection systems.
If you have n nodes, each one needs to connect to n-1 other nodes. But the connection from A to B is the same as the one from B to A. Therefore, there are n(n-1)/2 total connections.
Even a large confluence of connections like this would likely still have had local switchboards.
When you made a call, your local operator would have connected you either to a local number on their own board, or to another local board as needed. That second operator would have then connected you to the desired number.
Each board would only have limited connection lines to each other board (or to a branch exchange). So if all the connections from board A to board B (or to the branch exchange) were in use, the caller would have to try again later.
It did to an extent, they built the old copper network in tiers. I don't know the exact numbers and I'm sure they varied by area, but the general idea was - your home phone would connect to a local exchange, which served just dozens of local homes, and that exchange would connect to a bigger exchange somewhere higher up the network over a bundle of circuits. And that architecture repeated for a few layers.
But it wasn't 1:1, so you would have lets say 100 homes connected to a local exchange, and that local exchange would have say 20 lines to the next exchange in the network. That placed limits on the amount of concurrent connections you could have from one area - if 21 homes all tried to call people in the next city over, at least one of them would get a signal that all circuits are full and they would have to try again later. It drastically reduced the amount of lines you need between local exchanges though.
I guess it helped that phone calls were quite expensive, so people generally made very short calls. I haven’t really thought about this before but one of the main reasons for the pricing system could have been the facts that you mentioned.
In Sweden, the pricing system was tiered. Same area code (roughly: same municipality) = lowest rate. Neighbouring area codes = higher rate. Outside of that = highest rate. The rate was halved after 6pm. A reason for lowering the rates in the evening might have been that there were far less business users calling after 6pm.
One of the reasons I remember the pricing system is that my parents would not be happy if I dialed in to a modem pool before 6pm :)
Before I was born, the telephone company in Sweden (Televerket, later Telia) started to upgrade their system to use digital telephone exchanges (AXE). But there were of course still some kind of hard limit for how many concurrent calls they could handle, so I guess that’s why they kept the pricing system for a while.
This is partly speculation on my part, so feel free to correct me if I’m wrong.
Yep, that's right. The long distance trunks were a more limited resource so the telcos charged more per minute to use them. After digital exchanges came around it was less of a factor, but I think the pricing structure stuck around for a while.
You'd think that at least initially, individual towns would stand up fully connected (albeit small) but isolated networks. That before very long, the idea of connecting one town to the next would occur, and it would be realized that you only need a relatively small number of "long distance" lines, connected between the existing switchboards. At which point, if you were wiring up a city, you'd follow that pattern; tiered layers, as you say. It stands to reason then, that Stockholm's system must have started very early, and had absolutely explosive growth, to get to a situation like that tower.
They mostly did, but the limit on distance is pretty tight - according to Wikipedia [0] local loops were limited to 5 km in length (without extra equipment). I imagine that Stockholm's system here both started early and was in a very dense neighborhood of Stockholm, where direct wiring like this was still a tenable solution.
A core design principle of the Internet was the ability to automatically route around damage. The requirement came from a desire to be robust against nuclear attack. Very few 20th century networking systems could do this. Star topology or ring topology all had intrinsic SPoF.
When the tower was constructed in 1887, multiplexing technology was probably not available (I'm not so sure of the timeline in Europe). By 1913 it likely would have come into use. However, multiplexing really isn't a factor here, as the tower seems to have been built to serve local loops. Since these loops go to subscriber telephone sets, there's no option for multiplexing without expensive and maintenance-intensive equipment at customer premises. Multiplexing of local loops is called "pair gain" and wouldn't be developed until later, and it was never really that popular in most phone systems. Outside of suburban areas, it's typical that each copper pair runs directly to the exchange. Historically, and today, there is rarely any active equipment (or since the 1950s or so even passive conditioning) on local loops, they're just wires from the exchange to the phone.
As for why you didn't see similar constructions in other cities, this was definitely an unusually large telephone office for the time. In the US, a city exchange of the late 20th century would usually have just hundreds of lines, many of them multi-party. Telephone companies scaled up by building more exchanges, rather than a single very large one. When they got into these kinds of subscriber numbers at an exchange, the F1/F2 cable scheme was in use to avoid this kind of wiring. It does seem to be the case that telephone adoption was unusually rapid in Sweden, I find one (poorly sourced) claim that there were some 4,800 telephone subscribers in Stockholm in 1886 which would very likely make it the most telephone-rich city in the world. The situation of the tower seems to have developed in part because its builder, Allmänna, was consolidating the Stockholm telephone market through acquisitions and made a decision to centralize the many acquired customers onto on exchange.
What I'm a little confused about here is the lack of cables. The other big reason you didn't see constructions like this in the US, even in places like New York City, is because subscriber loops were quickly moved into lead-sheathed, paper-insulated multi-pair cables. These could contain hundreds of pairs. Cables were pretty much reaching maturity when this tower was built. I am curious as to the reason that multi-pair cables were not adopted more quickly in Stockholm, but it may be as simple as the considerable investment in this tower making open wire the preferred option for its short lifespan. In any case, the common claim that underground cables obsoleted the tower rings hollow to me, or at least misses an important detail, as aboveground cables were already in use in the 1880s. I suspect that modernization to cables was just deferred in Stockholm until it happened to also make sense to move to duct or pipe systems. In the US, it was more common that telephone exchanges switched to overhead (aerial) cable to manage exactly the wire sprawl issue that this tower exemplifies, and then only later (if ever) started to bury cables.
This article has more photos of the tower, but unfortunately not much more technical history: https://rarehistoricalphotos.com/the-stockholm-telephone-tow...
And this includes some photos of other parts of the Stockholm telephone network. The tower was not the only impressive construction required to manage this many open-wire pairs: https://thehistoryinsider.com/when-the-sky-over-stockholm-wa...
> As for why you didn't see similar constructions in other cities, this was definitely an unusually large telephone office for the time
For some perspective here - it took until the mid-80s for most of Germany to be connected to a phone line. That is, the 1980s.
I recently talked about that with my father after I found a postcard from one of my uncles from the early 80s confirming meeting and dinner plans. While I remember them always having a phone they were one of the households only connected in the mid 80s - which in retrospect explains some of the things I've found odd about them when talking to them by phone. It was a new thing for them.
(My parents got connected early on - my mother used to work for the post office in the phone exchange, and one of the perks of the job was priority for getting a phone line. Which also explained why we had an old grey phone, while pretty much all my friends had a relatively modern - for the time - one: they all only somewhat recently got phones)
Is that East Germany or West Germany?
West Germany
I just wanted to say that after the first paragraph, I wondered who this comment was written by, and then I realised I knew the answer already. There was no need for me to even check.
Not only was the tower demolished, some 700 buildings in that central area were in efforts to modernise the city.
https://en.wikipedia.org/wiki/Redevelopment_of_Norrmalm?wpro...
Comments mention multiplexing and that’s not wrong but the real reason for the vast number of wires is amplifiers, or rather the lack of practical ones at the time. You had to transmit at high enough power to overcome losses and still be able to hear at the destination.
Each wire carries just one signal at a power that would easily interfere with others, they needed relatively thick wires separated from each other. You see pictures of poles with lots of cross bars carrying lots of wires in this period.
Once amplification was practical they could use the thin telephone wires bundled together in a cable, each wire carrying a much fainter signal that can be easily amplified as needed.
Amplification existed but it took the vacuum tube to get it affordable and reliable for each circuit to have its own amplification.
Does that mean the quality of the voice calls in that era was better than later systems? Since it's logical to have loss of quality when a weak signal is amplified.
I suspect the quality was poor in the 19th century due to not yet knowing how to make good receivers and mouthpieces.
Right, quality was poor and the transmitter and receiver were part of the problem. They experimented with different methods, but part of the problem was that without amplification they needed a transmitter that worked at the full voltage necessary to travel the full distance.
That is exactly how I envision the clacks based on Pratchett's descriptions. Maybe without exactly that many wires ...
For me, this picture is more of China Miéville than Pratchett.
But clacks are optical telegraphs [1]; they communicate by semaphores, not wires.
[1] https://en.wikipedia.org/wiki/Optical_telegraph
Wouldn’t the number of wires be…. zero? The clacks are optical. They’re semaphore.
It is speculated that this was an inspiration for the Citadel in Half-Life: Alyx VR game:
https://www.reddit.com/r/HalfLife/comments/e809fn/cant_help_...
As someone obsessed with Networking and Networking Topology I am so so fascinated by analog telephone networks. Just the idea of long-distance connections being made via physical wires connecting at switches by human operators feels so raw. I can't even imagine such a thing. When I was younger analog telephone was being phased out. I still remember my dad having to go buy calling cards at convenience stores so he could make long distance calls back home. And then one day there were no more calling cards.
I can imagine lots of lessons learned from telephone networking helped shape ideas around computer network design.
And that's why we invented multiplexing and even better: store and forward packets.
More specifically, packet-based rather than switched telecoms.
Using a switch, each connection is a literal physical circuit. Multiplexing allows multiple circuits per phone line (through frequency separation at the carrier-frequency level), and much early telephony research involved increasing the multiplexing capacity and consequent issues.
With packet-switched networks, the only circuits are the interconnects between routers, and each individual data packet can take a different route. Subject to quality of service / service level agreements (QoS / SLA), it's possible to support far more distinct individual connections over packet-switched networks, though there still remains a maximum total bandwidth. For time-sensitive modalities (e.g., realtime voice or video), excess traffic leads to congestion and buffering, distortion, or interference, so limits remain. But they're far more generous than with circuit-based networks.
Put another way, packets give a greater assurance of establishing a link between any two nodes, whilst circuits give a greater assurance of a minimum bandwidth floor between those nodes. If you can get a connection, circuit-based line quality is often (though not always) superior, in terms of consistency, clarity, and low latency.
That doesn't necessarily line up with my understanding of reality.
In the US, a PRI can handle up to exactly 23 concurrent, native g.711u phone calls. That's it's capacity: No more, and no less. It's always 23, with each concurrent call using exactly 64kbps of symmetric bandwidth....just because that's the number of B channels provided.
But if we take that same PRI and make it do IP packets instead using MLPPP, then our capacity is actually reduced. By adding the magic of packet switching, we also add overhead. And with that added overhead, we can only get only get ~19 g.711u calls through that same circuit.
(Now, sure: In a bigger picture, that PRI may be better utilized as an IP pipeline than as a dedicated telephony circuit. It's certainly more flexible that way.
But packetization is not something that automatically improves capacity. It often does the opposite.)
This is well outside my area of expertise, assess accordingly.
From some quick DDGing, PRI (primary rate interface) is one of several possible options for a packet-switched telephony system, and is specifically contrasted with SIP VOIP connections in this article:
<https://enterprise.spectrum.com/support/faq/voice-and-collab...>
NB, "always 23" ... unless you're in Europe, in which case 23 === 30.
More on PRI vs. SIP: <https://enterprise.spectrum.com/support/faq/voice-and-collab...>.
(Other than being able to spell PRI and the above articles, I've no knowledge on the topic.)
PRI is TDM. It isn't packet-switched; there are no packets on the voice-carrying B channels. It uses timeslots, instead.
> NB, "always 23" ... unless you're in Europe, in which case 23 === 30.
I believe I was very specific about the locality I was referring to, but I am appreciative of the seemingly-disingenuous pedantry that is apparently unfettered by such arcane concepts as "context".
Where TDM == "time-division multiplexing". Expanding acronyms, particularly those from less-mainstream concepts and fields, is appreciated.
It wasn't clear from your comment (or the source I'd read) that PRI is circuit based.
The fact that circuits can be multiplexed (whether through frequency-separation, timeslicing, or other mechanisms, see <https://en.wikipedia.org/wiki/Multiplexing> for general MUXology) is irrelevant. They are still circuits. And there are limits to how many circuits can be maintained at a single time, regardless of the apparent endpoint combinatorial possibilities.
Packet-based switching has far greater capacity and flexibility, as I, my sources, and several other comments to this thread, have noted.
Pedantry aside, you seem to be confirming my initial point.
I firmly disagree with your initial point.
We have data flowing at rate N. It fits perfectly and precisely through a pipe of N size, with no room to spare (as is the way of a PRI).
We add to this data additional packetization overhead that was never present before.
So where we once had N, we now have N + >0.
That's more than N. The pipe is still only N, though -- it hasn't changed at all. The data no longer fits; it cannot fit.
Capacity is thus reduced by the addition of packetization.
Theoretically, yes. However, in practice, no. One of the great wins of packetization is that your infrastructure becomes generic in the sense that it is application-independent, allowing all subsequent improvements in eg. compression protocols to be applied retroactively. Therefore, if you would apply for example a variable bit rate, modern CODEC to the voice streams it's likely you would fit substantially more than a naïve set of voice channels on the raw PRI. The main costs are not in overhead, but enhanced session setup and round-trip latency and increased processing overheads at the edges. However, modern electronics offer embedded solutions making this ~free and a few ms are tolerable. IIRC voice is fine at <200ms latency, more or less, though less is obviously better.
The philosophical battle was centralist vs decentralised, for-purpose vs generic infrastructure, single-mode vs. multi-mode network applications, circuit-switched guaranteed QoS vs. best-effort packets, waterfall vs. iterative development and investment. Packets won for general purpose networking, because the nature of physics and bureaucracy meant the cost and time savings for operators and users were substantial.
Using a different codec can improve call-carrying capacity of any compatible link, for sure. But that's wiggling a different variable: Packetization-vs-not is a whole different game than g.711u vs g.723 is.
And I agree, completely: There's very good reasons for packet-switched networks having won over TDM technologies like PRI, with IP to tie it altogether. What we've arrived at is beautifully generic in that the packets themselves don't care at all about what combination of lower layers were used to get them from A to B. However it gets there, it's just IP. That's really neat.
Latency-wise, I'm a little torn: With the latency involved in a normal phone call on my normal cell phone today, I find that I "talk over" people on phone calls more than I did decades ago in the TDM days. The pacing is very different than it was.
Sometimes, when the echo cancellation fails at some level, I can hear myself echo from the far end of a call.
That echo is annoying, but I mention it because it is something that lets me hear the latency of the call. It's often on the order of 500ms RTT...which is quite a lot. We never had latency like that in the TDM days for domestic calls.
(Historically, in TDM world: The voice data always arrived at the right time and there was also always a place to put it at the right time. It was real-time instead of best-effort, so there just wasn't any utility to having any large buffers along the way: The timing was either resolutely correct or it didn't work at all. Things are a lot mushier today.)
What kind of solutions were employed in other cities? Does anyone know? Cities like London and New York etc must have had much more telephone lines.
Madrid (Spain) had an equivalent structure https://historiatelefonia.com/2014/11/21/templetes-telefonic...
At this early time (this is not long after the invention of the phone itself) - none, really. Stockholm had a much higher number of telephones per home, but not very long after this operators put the cables underground and that was that.
Does this mean that probably somewhere below that tower, there were operator stations that would have allowed any of the 5500 lines to connect to any of the other lines? How many simultaneous calls would that even be? Correct me if I borked this, but 5500!/(2^(5500/2)), perhaps?
It seems plausible that if the phone had only just been invented, you'd initially set up small systems that would in fact allow any line to connect to any line. That'd be fine for maybe even a few dozen lines. But as the image shows, that doesn't scale too well.
Well, on further consideration, I arrived at N!/((2^(N/2)*(N/2)!)
Still, pretty astronomical.
The total number of connections would all but certainly be far greater than the maximum number of simultaneous calls which could be supported.
Under PSTN, public-switched telephone networks, a not-infrequent occurrence, especially when calling long-distance, was to get a message "all circuits are busy". When each call was literally a circuit, and the "switch" (the central telephone exchange) made and broke those circuits as calls began and ended, my understanding (not my area of expertise, but one of some interest) is that this meant that all available interchange connections were occupied. For long-distance, this was typically far lower than for local calls (most phone traffic is local), and for international calls, lower still. The first transatlantic telephone cable could support only 36 simultaneous calls, in 1956. Calls were short, expensive, and all but exclusively for business and government subscribers.
<https://hamhistory.org/first-transatlantic-telephone-cable/>
I'd expect that the Stockholm exchange probably supported a few hundred simultaneous calls, probably a few (a dozen or so perhaps) per operator, who had to physically connect each call.
Indeed, I'm old enough to remember "all circuits are busy."
You lived through covid too?
A lot of phone systems saturated themselves when things first shut down. Probably not physical copper constraints but the virtual interconnects between providers:
https://productioncommunity.publicmobile.ca/t5/Get-Support/A...
https://archive.ph/D9qQ4
Through the 1990s (and prior) this was not uncommon, particularly on high-traffic days, often holidays (in the US: Mother's Day, Thanksgiving, and Christmas), when calling long-distance. Also during natural disasters, when local calls might also be affected.
There were also systems outages, such as the 1988 Hinsdale switching station which took out phone service to most of the Chicago area (both local and long-distance):
"1988 PHONE CRISIS TIED TO 1 BROKEN POWER LINE" (1988)
Turns out that that switch was a SPOF:
To make sure such a crisis doesn`t happen again, Illinois Bell Telephone Co. announced Friday it is embarking on a $80-million, five-year program to construct a complete duplicate telephone network system throughout its Chicago suburban operation and to redesign its fire protection systems.
<https://www.chicagotribune.com/1989/03/11/1988-phone-crisis-...>
If you have n nodes, each one needs to connect to n-1 other nodes. But the connection from A to B is the same as the one from B to A. Therefore, there are n(n-1)/2 total connections.
Even a large confluence of connections like this would likely still have had local switchboards.
When you made a call, your local operator would have connected you either to a local number on their own board, or to another local board as needed. That second operator would have then connected you to the desired number.
Each board would only have limited connection lines to each other board (or to a branch exchange). So if all the connections from board A to board B (or to the branch exchange) were in use, the caller would have to try again later.
There are more telecommunication lines now than ever. We’ve just gotten really good at organizing them?
We've gotten really good at multiplexing lots of connections over a single line.
I believe the concept of multiplexing made the tower obsolete, orher than the subterranean cables of course.
I’m not sure that plain old telephone service allowed multiplexing, so it was probably just the latter
It did to an extent, they built the old copper network in tiers. I don't know the exact numbers and I'm sure they varied by area, but the general idea was - your home phone would connect to a local exchange, which served just dozens of local homes, and that exchange would connect to a bigger exchange somewhere higher up the network over a bundle of circuits. And that architecture repeated for a few layers.
But it wasn't 1:1, so you would have lets say 100 homes connected to a local exchange, and that local exchange would have say 20 lines to the next exchange in the network. That placed limits on the amount of concurrent connections you could have from one area - if 21 homes all tried to call people in the next city over, at least one of them would get a signal that all circuits are full and they would have to try again later. It drastically reduced the amount of lines you need between local exchanges though.
Interesting!
I guess it helped that phone calls were quite expensive, so people generally made very short calls. I haven’t really thought about this before but one of the main reasons for the pricing system could have been the facts that you mentioned.
In Sweden, the pricing system was tiered. Same area code (roughly: same municipality) = lowest rate. Neighbouring area codes = higher rate. Outside of that = highest rate. The rate was halved after 6pm. A reason for lowering the rates in the evening might have been that there were far less business users calling after 6pm.
One of the reasons I remember the pricing system is that my parents would not be happy if I dialed in to a modem pool before 6pm :)
Before I was born, the telephone company in Sweden (Televerket, later Telia) started to upgrade their system to use digital telephone exchanges (AXE). But there were of course still some kind of hard limit for how many concurrent calls they could handle, so I guess that’s why they kept the pricing system for a while.
This is partly speculation on my part, so feel free to correct me if I’m wrong.
Yep, that's right. The long distance trunks were a more limited resource so the telcos charged more per minute to use them. After digital exchanges came around it was less of a factor, but I think the pricing structure stuck around for a while.
You'd think that at least initially, individual towns would stand up fully connected (albeit small) but isolated networks. That before very long, the idea of connecting one town to the next would occur, and it would be realized that you only need a relatively small number of "long distance" lines, connected between the existing switchboards. At which point, if you were wiring up a city, you'd follow that pattern; tiered layers, as you say. It stands to reason then, that Stockholm's system must have started very early, and had absolutely explosive growth, to get to a situation like that tower.
They mostly did, but the limit on distance is pretty tight - according to Wikipedia [0] local loops were limited to 5 km in length (without extra equipment). I imagine that Stockholm's system here both started early and was in a very dense neighborhood of Stockholm, where direct wiring like this was still a tenable solution.
[0] https://en.wikipedia.org/wiki/Plain_old_telephone_service
It absolutely did, in 3kHz bands. That's how you could also sometimes hear someone else talking.
Single point of failure?
A lot of old telephony systems were full of SPoFs.
A core design principle of the Internet was the ability to automatically route around damage. The requirement came from a desire to be robust against nuclear attack. Very few 20th century networking systems could do this. Star topology or ring topology all had intrinsic SPoF.
This is actually amazing
NK logo and all