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Coaxial Goes to War

112 points by Sniffnoy 5y ago

mjh2539 5y ago
What a great read. Who would have thunk the history of coaxial cables could be so interesting?
RF_Savage 5y ago
That was a very interesting read on AT&T's coaxial long distance network and what it was used for.
ggm 5y ago
Minimum radial bend is a big deal in wave guides. If you look at TV transmitter masts you may well see pretty fat cables and ducting. The ducting can be a wave guide or just weather cover for cables. Thick Ethernet was coax, fat coax, marked where to put the taps for maximum signal strength.
- jonsen 5y ago
  ... where to put the taps for maximum signal strength It's not that signal strength varies along the cable. It's about minimizing the effect of reflections. Some reflection is unavoidable when you tap into the cable. "Transceivers should be installed only at precise 2.5-meter intervals. This distance was chosen to not correspond to the signal's wavelength; this ensures that the reflections from multiple taps are not in phase.": https://en.m.wikipedia.org/wiki/10BASE5 https://en.m.wikipedia.org/wiki/10BASE5
- teh_klev 5y ago
  Once upon a time in a former life I had an industrial customer that used 10BASE5 and it ran all around their factory. One of the many jobs I did for these folks was installing vampire taps and AUI drop cables. As you mention the optimal placing of a tap should be on these marks, or as close to as possible. Unfortunately the reality was that where these cables ran and where the taps needed to be fitted and where the dots actually existed never really aligned well, especially in high up confined crawl spaces. We soon discovered that 10BASE5 is pretty forgiving even if you install a tap in the least optimal positions, e.g. dead centre between the dots. Previous installers even managed to install two to three taps more or less next to each other and there were no complaints about performance degradation.
  - jonsen 5y ago
    But, one day one new tap takes the whole network down.
    - ggm 5y ago
      We used to joke that fixing campus CATV cable meant removing the end-tap over night to let the surplus electricity drip out. The thing is, during this time 2mbit telco cabling sometimes was a gas filled conduit, and your comms data frame included pipes with brass taps, and small buckets: they got leaks, and you had to drain water sometimes.
jimmySixDOF 5y ago
There a lot of DCO Exchanges that still refer to facilities for all major fiber routes as part of the "Long Lines" infrastructure out of respect for these original coax runs.
ryanmercer 5y ago
That page reminds me so much of TOTSE (oh how I miss you) with a dash of Sick Sad World thrown in. Ahhh the nostalgic feels.
- jcrawfordor 5y ago
  TOTSE was definitely very influential on my interest in the telecom system back in the early '00s. To this day, researching things like phone switch replacement projects I'll often end up finding text files that I remember being on TOTSE back when.
varjag 5y ago
Because phone conversations required around 15kHz of bandwidth (assuming no companding, which was not yet done at the time) this imposed a big limit... Is that really true? In my home country the speech band was passed at 300-3000Hz, meaning 2x2.7KHz full duplex OTW. 15KHz sounds unnecessarily luxurious for speech.
- ajb 5y ago
  Maybe this is counting both directions separately, and doubling again because of sampling vs nyquist? (Not sure the latter actually applies if you're doing everything in analogue, which they would have been at the start). They probably also didn't have very sharp filters at the time, so couldn't optimally pack the channels.
  - varjag 5y ago
    There is no sampling/discretization on analog phone line. And since they mention crosstalk issues this has to be analog.
    - jcrawfordor 5y ago
      Yes, at the time the only real theoretical bandwidth requirement was the highest audio frequency they wanted to be able to pass, since this was all a trivial analogue setup. In practice there were a surprising number of extra constraints, some things you'd expect like the rolloff of the filters and amplifiers, but others are the kind of thing that only come up when you have very long cable runs with hundreds of amplifiers... they had to think very carefully about how specific harmonics would accumulate through all the amplifiers and place carriers accordingly, so that the harmonics that accumulated to a significant degree would fall in unused frequency ranges. All of this tended to increase the practical bandwidth requirements, or in other words get you fewer channels than you would expect on a given cable. A sort of interesting issue is that the worst crosstalk problems were always at low frequency, so most of these carriers operated entirely above audio frequency. The audio frequency was thus an available circuit, but the quality was poor, so it was usually used for order wire or other internal purposes. This had the very convenient property on many open wire and cable systems that a lineman could connect their butt set directly to the pair and use it to call an exchange troubleshooting desk, the butt set didn't need to have any awareness or support for the carrier technology since the AF band was otherwise unused. This is very similar to how DSL works today.
jakedata 5y ago
There seems to be some homage being paid to the old L-3 system by the company that has been named variously L-3 communications, L3 Technologies and L3Harris "for the last initials of its founders". Probably an inside joke for those in the know. https://en.wikipedia.org/wiki/L3_Technologies https://en.wikipedia.org/wiki/L3_Technologies

Coaxial Goes to War

112 points by Sniffnoy 5y ago

mjh2539 5y ago

What a great read. Who would have thunk the history of coaxial cables could be so interesting?

RF_Savage 5y ago

That was a very interesting read on AT&T's coaxial long distance network and what it was used for.

ggm 5y ago

Minimum radial bend is a big deal in wave guides. If you look at TV transmitter masts you may well see pretty fat cables and ducting. The ducting can be a wave guide or just weather cover for cables. Thick Ethernet was coax, fat coax, marked where to put the taps for maximum signal strength.

jonsen 5y ago
... where to put the taps for maximum signal strength It's not that signal strength varies along the cable. It's about minimizing the effect of reflections. Some reflection is unavoidable when you tap into the cable. "Transceivers should be installed only at precise 2.5-meter intervals. This distance was chosen to not correspond to the signal's wavelength; this ensures that the reflections from multiple taps are not in phase.": https://en.m.wikipedia.org/wiki/10BASE5 https://en.m.wikipedia.org/wiki/10BASE5
teh_klev 5y ago
Once upon a time in a former life I had an industrial customer that used 10BASE5 and it ran all around their factory. One of the many jobs I did for these folks was installing vampire taps and AUI drop cables. As you mention the optimal placing of a tap should be on these marks, or as close to as possible. Unfortunately the reality was that where these cables ran and where the taps needed to be fitted and where the dots actually existed never really aligned well, especially in high up confined crawl spaces. We soon discovered that 10BASE5 is pretty forgiving even if you install a tap in the least optimal positions, e.g. dead centre between the dots. Previous installers even managed to install two to three taps more or less next to each other and there were no complaints about performance degradation.
- jonsen 5y ago
  But, one day one new tap takes the whole network down.
  - ggm 5y ago
    We used to joke that fixing campus CATV cable meant removing the end-tap over night to let the surplus electricity drip out. The thing is, during this time 2mbit telco cabling sometimes was a gas filled conduit, and your comms data frame included pipes with brass taps, and small buckets: they got leaks, and you had to drain water sometimes.

jimmySixDOF 5y ago

There a lot of DCO Exchanges that still refer to facilities for all major fiber routes as part of the "Long Lines" infrastructure out of respect for these original coax runs.

ryanmercer 5y ago

That page reminds me so much of TOTSE (oh how I miss you) with a dash of Sick Sad World thrown in. Ahhh the nostalgic feels.

jcrawfordor 5y ago
TOTSE was definitely very influential on my interest in the telecom system back in the early '00s. To this day, researching things like phone switch replacement projects I'll often end up finding text files that I remember being on TOTSE back when.

varjag 5y ago

Because phone conversations required around 15kHz of bandwidth (assuming no companding, which was not yet done at the time) this imposed a big limit... Is that really true? In my home country the speech band was passed at 300-3000Hz, meaning 2x2.7KHz full duplex OTW. 15KHz sounds unnecessarily luxurious for speech.

ajb 5y ago
Maybe this is counting both directions separately, and doubling again because of sampling vs nyquist? (Not sure the latter actually applies if you're doing everything in analogue, which they would have been at the start). They probably also didn't have very sharp filters at the time, so couldn't optimally pack the channels.
- varjag 5y ago
  There is no sampling/discretization on analog phone line. And since they mention crosstalk issues this has to be analog.
  - jcrawfordor 5y ago
    Yes, at the time the only real theoretical bandwidth requirement was the highest audio frequency they wanted to be able to pass, since this was all a trivial analogue setup. In practice there were a surprising number of extra constraints, some things you'd expect like the rolloff of the filters and amplifiers, but others are the kind of thing that only come up when you have very long cable runs with hundreds of amplifiers... they had to think very carefully about how specific harmonics would accumulate through all the amplifiers and place carriers accordingly, so that the harmonics that accumulated to a significant degree would fall in unused frequency ranges. All of this tended to increase the practical bandwidth requirements, or in other words get you fewer channels than you would expect on a given cable. A sort of interesting issue is that the worst crosstalk problems were always at low frequency, so most of these carriers operated entirely above audio frequency. The audio frequency was thus an available circuit, but the quality was poor, so it was usually used for order wire or other internal purposes. This had the very convenient property on many open wire and cable systems that a lineman could connect their butt set directly to the pair and use it to call an exchange troubleshooting desk, the butt set didn't need to have any awareness or support for the carrier technology since the AF band was otherwise unused. This is very similar to how DSL works today.

jakedata 5y ago

There seems to be some homage being paid to the old L-3 system by the company that has been named variously L-3 communications, L3 Technologies and L3Harris "for the last initials of its founders". Probably an inside joke for those in the know. https://en.wikipedia.org/wiki/L3_Technologies https://en.wikipedia.org/wiki/L3_Technologies