4 ms·

TIL: Phone networks already carried digital signals in the 60s. I absolutely did not expect that.

p3n1s 3y ago
"Fast-forward to 1962. Ma Bell begins using T-1s to connect its switching centers. T-1s are a digital communications link, not analog. The entire phone network goes digital. In a modern phone network, the only analog communication is between an individual house's phone line and the phone company's box." This is misleading. The phone network used analog links (typically microwave relay) for long distance almost exclusively for many years (into the 80s) after the introduction of T1. T1 itself is not terribly well suited to very long distance links though and can only be pressed into it with repeaters every mile or so.
- throwaway167 3y ago
  Hence towers with dishes on the outside. Long perplexed why they had canvas stretched across, and didn't point up: https://en.m.wikipedia.org/wiki/Microwave#/media/File%3AFrazier_Peak%2C_tower_and_Honda_Element.jpg https://en.m.wikipedia.org/wiki/Microwave#/media/File%3AFraz...
  - jjjjmoney 3y ago
    Our Charter Spectrum “cable” connection goes over a microwave link like this. How do I know? Well, we had a pretty big wildfire a couple years ago that burned a microwave tower a couple hours away. The official response was that they couldn’t get internet back in town until they rebuilt the tower. Our internet was down for a couple weeks until it was safe enough for them to get back to the tower site.
  - Sharlin 3y ago
    Microwave relay links predate spaceflight and even when comms satellites became a thing, it made (and still makes) little sense to beam regional TV/radio/phone up to geosynchronous and back, due to cost and latency. There was no good replacement to terrestrial microwave until fiber optic became a thing. In a very real sense microwave relays are the direct descendant of the optical telegraph (aka "clacks" in Discworld lingo).
  - ac29 3y ago
    > Long perplexed why they had canvas stretched across Its a radome, used to protect the antenna from weather.
- h2odragon 3y ago
  130VDC on the copper, apparently. Had "the second longest T1 in the service area" for a while there. BellSouth declined to renew the contract in 2008, the business folded, etc. In 2013 I was digging through the old building, and the T1 line card in the wiring closet was still powered. Nowadays the relay boxes all the way into town are mostly hanging open or have been repeatedly run over. I wonder how much of the rest of that infrastructure they just abandoned in place.
  - Aloha 3y ago
    the 130V was to power either repeaters or the smart jack.
- dylan604 3y ago
  >The phone network used analog links (typically microwave relay) for long distance almost exclusively for many years (into the 80s) after the introduction of T1 The reason should be obvious. Digging long distance trenches and all of the property buying/leasing and right of ways/easements and all of the other fun stuff makes it a total nightmare. Radio broadcast just makes much more financial sense as well as logistically.
- brookst 3y ago
  Were the microwave links really analog signaling?
  - NovemberWhiskey 3y ago
    The Long Lines network used frequency modulation and frequency division multiplexing and was completely analog. It was really only advancements in semiconductor lasers and optical fiber manufacturing that drove its abandonment.
- the_third_wave 3y ago
  > The phone network used analog links (typically microwave relay) for long distance Those microwave links represent layer 0 in the connection which is by definition always analogue, the same is/was true for T-1 which used alternate mark inversion [1] signalling over a 4-wire circuit. It is the type of signal carried over the link which decides whether it is considered "digital" or "analogue". T-1 carried 24 time-divided PCM channels ("digital"), microwave links could and can carry both frequency-divided multiplexed analogue channels ("analogue") as well as time-divided PCM or packet-switched channels ("digital"). [1] https://en.wikipedia.org/wiki/Bipolar_encoding#Alternate_mark_inversion https://en.wikipedia.org/wiki/Bipolar_encoding#Alternate_mar...
fragmede 3y ago
Computers are old. The future is here, it's just unevenly distributed. I take a self-driving car multiple times a week yet anyone outside my bubble hasn't ever ridden one and thinks that technology doesn't work yet. I have conversations with an LLM, but people on the other side of the world don't even have laptops or reliable Internet access. Military people have jetpacks to move around on. Inequality is as old as dirt, unfortunately.
- flyinghamster 3y ago
  It also strikes me how much the old phone network, and its cultural aspects, have faded into the past - particularly when I drive past some derelict bit of telecom infrastructure. Even 30 years ago, it was amusing to watch kids try to deal with a rotary-dial phone. Now even landlines are an endangered species, and area codes are barely relevant. Finding a working telco pay phone these days would be like finding a unicorn. Meanwhile, on a random roadtrip through rural Illinois yesterday, I was seeing "5G UC" on my cell phone, out in the middle of nowhere. Unevenly distributed, indeed!
  - dylan604 3y ago
    >Finding a working telco pay phone these days would be like finding a unicorn. Even if I found one, I would only be able to make a collect call. I never carry loose change on me, and I haven't had a calling card in decades.
    - actionfromafar 3y ago
      There used to be a way to pay with credit card IIRC?
      - dylan604 3y ago
        Not that I was ever exposed to. Calling cards were just account numbers with a monthly statement. If you used a credit card for that, then maybe, but to me paying with a credit card suggests a per call transaction. That I have not experienced. But the world is a bigger place than just me, so maybe it was available in other areas?
retrac 3y ago
Telecommunications were the first application for nearly every principle and technology now used in computers. In the 1930s, Claude Shannon was trying to optimize the phone network's automatic switching fabric built out of millions and millions of relays when he, more or less by accident, proved that switching circuits are equivalent to Boolean algebra, in a deep sense, and so the appropriate arrangement of switches, can compute anything that can be computed. The first major commercial application for the vacuum tube was not in radio, but in amplifying long-distance telephone links. They would quickly find use in frequency multiplexing telephone lines, too. [1] Similarly, one of the first applications of vacuum tubes as a high-speed electronic switch was for multiplexing telegraph lines, and much of the initial research on high-reliability low-power tubes able to switched fully on without damage began, with the telephone and telegraph companies. The first computers were built out of relays and vacuum tubes specially designed for telecommunications. The transistor was invented at Bell Labs. The planned application was the telephone network. The very same year, the theory behind modern error correcting codes was developed at Bell Labs too. These find extensive use today in data storage in computers, but the original goal was arbitrarily accurate transfer of telegraph data over noisy links. Digital transmission of photographs dates to the late 1920s too; they wanted to send images over extremely noisy long-distance telegraph links. So record the intensity of light over each spot of the image on paper tape. Then send the tape over the wire. And reverse on the other end. [2] I've come to understand the computer revolution as really the story of the telecommunication revolution. Computers serendipitously came about with advances in telecommunications, enabled by the same technologies, and indeed, often driven by the needs of telecommunications. (One of the very first applications for a computer in an embedded context - dating to the late 1950s in experiment - the telephone exchange, of course.) [1] https://long-lines.net/tech-equip/misc/Graybar103-0693.jpg https://long-lines.net/tech-equip/misc/Graybar103-0693.jpg [2] http://www.hffax.de/history/html/bartlane.html http://www.hffax.de/history/html/bartlane.html
- Merrill 3y ago
  At one time teletype switching was done in centers where the incoming line was connected to a tape punch. The operator tore the tape off the punch, read the destination address punched at the beginning of the tape, and put the tape on a tape reader connected to an outgoing line to another center closer to the destination. https://en.wikipedia.org/wiki/Tape_relay https://en.wikipedia.org/wiki/Tape_relay IP routers now do this faster.
queuebert 3y ago
Isn't Morse code technically a digital signal, albeit with an extremely low bit rate?
- dreamcompiler 3y ago
  Yes. It's sloppy PWM with a non-ASCII character code. It's optimized for human encoding/decoding rather than machine; the pulse widths are not globally uniform like they would be in a machine-oriented code, which is what I mean by "sloppy."
deleted 3y ago
[deleted]

"Fast-forward to 1962. Ma Bell begins using T-1s to connect its switching centers. T-1s are a digital communications link, not analog. The entire phone network goes digital. In a modern phone network, the only analog communication is between an individual house's phone line and the phone company's box." This is misleading. The phone network used analog links (typically microwave relay) for long distance almost exclusively for many years (into the 80s) after the introduction of T1. T1 itself is not terribly well suited to very long distance links though and can only be pressed into it with repeaters every mile or so.

throwaway167 3y ago
Hence towers with dishes on the outside. Long perplexed why they had canvas stretched across, and didn't point up: https://en.m.wikipedia.org/wiki/Microwave#/media/File%3AFrazier_Peak%2C_tower_and_Honda_Element.jpg https://en.m.wikipedia.org/wiki/Microwave#/media/File%3AFraz...
- jjjjmoney 3y ago
  Our Charter Spectrum “cable” connection goes over a microwave link like this. How do I know? Well, we had a pretty big wildfire a couple years ago that burned a microwave tower a couple hours away. The official response was that they couldn’t get internet back in town until they rebuilt the tower. Our internet was down for a couple weeks until it was safe enough for them to get back to the tower site.
- Sharlin 3y ago
  Microwave relay links predate spaceflight and even when comms satellites became a thing, it made (and still makes) little sense to beam regional TV/radio/phone up to geosynchronous and back, due to cost and latency. There was no good replacement to terrestrial microwave until fiber optic became a thing. In a very real sense microwave relays are the direct descendant of the optical telegraph (aka "clacks" in Discworld lingo).
- ac29 3y ago
  > Long perplexed why they had canvas stretched across Its a radome, used to protect the antenna from weather.
h2odragon 3y ago
130VDC on the copper, apparently. Had "the second longest T1 in the service area" for a while there. BellSouth declined to renew the contract in 2008, the business folded, etc. In 2013 I was digging through the old building, and the T1 line card in the wiring closet was still powered. Nowadays the relay boxes all the way into town are mostly hanging open or have been repeatedly run over. I wonder how much of the rest of that infrastructure they just abandoned in place.
- Aloha 3y ago
  the 130V was to power either repeaters or the smart jack.
dylan604 3y ago
>The phone network used analog links (typically microwave relay) for long distance almost exclusively for many years (into the 80s) after the introduction of T1 The reason should be obvious. Digging long distance trenches and all of the property buying/leasing and right of ways/easements and all of the other fun stuff makes it a total nightmare. Radio broadcast just makes much more financial sense as well as logistically.
brookst 3y ago
Were the microwave links really analog signaling?
- NovemberWhiskey 3y ago
  The Long Lines network used frequency modulation and frequency division multiplexing and was completely analog. It was really only advancements in semiconductor lasers and optical fiber manufacturing that drove its abandonment.
the_third_wave 3y ago
> The phone network used analog links (typically microwave relay) for long distance Those microwave links represent layer 0 in the connection which is by definition always analogue, the same is/was true for T-1 which used alternate mark inversion [1] signalling over a 4-wire circuit. It is the type of signal carried over the link which decides whether it is considered "digital" or "analogue". T-1 carried 24 time-divided PCM channels ("digital"), microwave links could and can carry both frequency-divided multiplexed analogue channels ("analogue") as well as time-divided PCM or packet-switched channels ("digital"). [1] https://en.wikipedia.org/wiki/Bipolar_encoding#Alternate_mark_inversion https://en.wikipedia.org/wiki/Bipolar_encoding#Alternate_mar...

fragmede 3y ago

Computers are old. The future is here, it's just unevenly distributed. I take a self-driving car multiple times a week yet anyone outside my bubble hasn't ever ridden one and thinks that technology doesn't work yet. I have conversations with an LLM, but people on the other side of the world don't even have laptops or reliable Internet access. Military people have jetpacks to move around on. Inequality is as old as dirt, unfortunately.

flyinghamster 3y ago
It also strikes me how much the old phone network, and its cultural aspects, have faded into the past - particularly when I drive past some derelict bit of telecom infrastructure. Even 30 years ago, it was amusing to watch kids try to deal with a rotary-dial phone. Now even landlines are an endangered species, and area codes are barely relevant. Finding a working telco pay phone these days would be like finding a unicorn. Meanwhile, on a random roadtrip through rural Illinois yesterday, I was seeing "5G UC" on my cell phone, out in the middle of nowhere. Unevenly distributed, indeed!
- dylan604 3y ago
  >Finding a working telco pay phone these days would be like finding a unicorn. Even if I found one, I would only be able to make a collect call. I never carry loose change on me, and I haven't had a calling card in decades.
  - actionfromafar 3y ago
    There used to be a way to pay with credit card IIRC?
    - dylan604 3y ago
      Not that I was ever exposed to. Calling cards were just account numbers with a monthly statement. If you used a credit card for that, then maybe, but to me paying with a credit card suggests a per call transaction. That I have not experienced. But the world is a bigger place than just me, so maybe it was available in other areas?

retrac 3y ago

Telecommunications were the first application for nearly every principle and technology now used in computers. In the 1930s, Claude Shannon was trying to optimize the phone network's automatic switching fabric built out of millions and millions of relays when he, more or less by accident, proved that switching circuits are equivalent to Boolean algebra, in a deep sense, and so the appropriate arrangement of switches, can compute anything that can be computed. The first major commercial application for the vacuum tube was not in radio, but in amplifying long-distance telephone links. They would quickly find use in frequency multiplexing telephone lines, too. [1] Similarly, one of the first applications of vacuum tubes as a high-speed electronic switch was for multiplexing telegraph lines, and much of the initial research on high-reliability low-power tubes able to switched fully on without damage began, with the telephone and telegraph companies. The first computers were built out of relays and vacuum tubes specially designed for telecommunications. The transistor was invented at Bell Labs. The planned application was the telephone network. The very same year, the theory behind modern error correcting codes was developed at Bell Labs too. These find extensive use today in data storage in computers, but the original goal was arbitrarily accurate transfer of telegraph data over noisy links. Digital transmission of photographs dates to the late 1920s too; they wanted to send images over extremely noisy long-distance telegraph links. So record the intensity of light over each spot of the image on paper tape. Then send the tape over the wire. And reverse on the other end. [2] I've come to understand the computer revolution as really the story of the telecommunication revolution. Computers serendipitously came about with advances in telecommunications, enabled by the same technologies, and indeed, often driven by the needs of telecommunications. (One of the very first applications for a computer in an embedded context - dating to the late 1950s in experiment - the telephone exchange, of course.) [1] https://long-lines.net/tech-equip/misc/Graybar103-0693.jpg https://long-lines.net/tech-equip/misc/Graybar103-0693.jpg [2] http://www.hffax.de/history/html/bartlane.html http://www.hffax.de/history/html/bartlane.html

Merrill 3y ago
At one time teletype switching was done in centers where the incoming line was connected to a tape punch. The operator tore the tape off the punch, read the destination address punched at the beginning of the tape, and put the tape on a tape reader connected to an outgoing line to another center closer to the destination. https://en.wikipedia.org/wiki/Tape_relay https://en.wikipedia.org/wiki/Tape_relay IP routers now do this faster.

queuebert 3y ago

Isn't Morse code technically a digital signal, albeit with an extremely low bit rate?

dreamcompiler 3y ago
Yes. It's sloppy PWM with a non-ASCII character code. It's optimized for human encoding/decoding rather than machine; the pulse widths are not globally uniform like they would be in a machine-oriented code, which is what I mean by "sloppy."

deleted 3y ago

[deleted]