[seagreen]

Thank you for the information!

> Circuit switched analog POTS has been dead for a long time, but delays were limited to amplifier delays (almost nothing) and speed of light in wires or radio.

Two questions about this: it sounds like there are still a fair amount of landlines around if this article is accurate: https://www.cnn.com/2024/02/05/tech/landline-phone-service-p... Do you mean that people still have landlines, but they're no longer circuit switched analog? And would that only be for long distance, with local landline calls still using circuit switched analog, or would it be for both?

Also, as to your latter point about old school POTS having almost no delays other than speed of light: does that hold for long distance calls as well? I get the impression that there could be added latency for long distance POTS calls (but have had trouble finding sourced numbers on anything yet).

[toast0]

The central office switch that a landline is connected to is almost certainly doing analog/digital conversion. Digital switches were introduced in the 1970s and Analog exchanges were all but extinct by 2000 (according to [4], the last step by step exchange in Canada was replaced in 2002). If the call remains old school POTS, it will be essentially a T1/PRI call from that point on. Circuit switched digital calling was phased in over time, as CO switches were replaced. Chances are, a landline is actually handled by a remote terminal out in the field, and a/d converted there.

Long distance (to most destinations) calling probably switched to digital trunks faster than local calling, because long distance calling was already separate equipment from local calling, and replacing central office switches is a big process (including a pretty intense cutover [1]).

Long distance calling would have more equipment in the path than a local call, of course, and may not have had very optimal routing paths compared to today. I found a route map from the 1960 AT&T Long Lines network [2], and a fiber map with no provenance of 'today' [3] ... it's widely similar, but there's some routes that pop out at me as not being well connected in the 1960: Salt Lake City to Seattle; Amarillo, TX to Dallas, TX. On the other hand, Long Lines was radio, and radio is faster than fiber, so that makes up a little.

There may have also been more cases where routing was simplified to be more manageable at the expense of delay... AT&T wouldn't have run long distance trunks from everywhere to everywhere, you might need to do hierarchical routing --- if you're on the east coast, route to New York, and from there to the big city near your destination, and then to destination, etc. There's still some of that in internet routing, but it's a lot less. BGP and internal routing protocols make it a lot easier to manage more direct connections and let the software figure things out.

But there's no need to add a large buffer for voice data for POTS, and it was very expensive to do it, so it wasn't done. There is a need to do it for VoIP, and it's no longer expensive, so it is done.

[1] https://www.youtube.com/watch?v=saRir95iIWk

[2] http://www.long-lines.net/places-routes/maps/MW6003.jpg from http://www.long-lines.net/places-routes/maps/MW6003.jpg

[3] https://i.insider.com/56e03df7918a0f7333e350f3?width=1190

[4] https://telephoneworld.org/telephone-switching-systems/step-...

[seagreen]

That video is wonderful. And thanks for all the other information as well, it's very helpful.