56.6k speeds required a digital POTS, it transmitted PCM through the digital portions of the network only to become analog at the last moment, from the nearest pole switch to your house. In other words, the modem you were calling had to be 'all digital' and plugged into the telecom network. There wasn't a lot of connection left to 'audio' for these modems, other than their baseband frequency range (0-4khz).
You are mistaken. 56K baud was the 'last' of the POTS modems, and it worked over an unimproved POTS line. Work beyond 56K was not pursued because ISDN was 64K and "imminent" (only to be DSLAMed :-) by ADSL.)
The distinction between 56k and slower speeds was that 56k could not be transmitted over the digital part of the POTS network in its modem audio format. Telephone systems are generally analogue between the house and the exchange (or road-side cabinet), and then digital between that and the main telephone network. The digital encoding was 8000 samples per second, at eight bits per sample, with a logarithmic transfer function in order to reduce noise levels when the line is quiet. So, the digital part of a telephone line ran at 64kb/s.
In order to support 56k speeds, the exchange (or road-side box) had to be upgraded. Normal exchanges used a simple 8-bit DAC and ADC, but when a 56k modem was attached it would have to switch to a different mode. Effectively, the remote modem was moved from the ISP to the exchange, with the digital part of the POTS network transmitting de-modem-ed data, rather than a digital representation of an analogue signal encoding digital data, which is what happens with slower speeds.
So yes, the 56k signal was transmitted over an analogue line, but no the 56k signal was not transmitted over the main POTS network. Work beyond 56k was not pursued because the underlying digital telephone network could not actually transmit data at a higher rate.
This is also where the 64kb/s speed of an ADSL line comes from by the way. That just extends the digital part of the POTS network to the house. ADSL allows a slightly higher data rate (64kb/s rather than 56kb/s) because it has a side channel that carries the control signals such as dialling and flow control, whereas 56k modems had to send flow control and error correction in-band.
Thanks for the link, want me to send you a Rockwell chip that negotiates 56K over a POTS line? I think I've got about a dozen left but don't have my line simulator anymore but when the company I was with was shipping our business gateway the modem chipset was qualified and homologized for 15 or so different markets. The line analyzer didn't have a digital bone in its body so I doubt the chip would have been able to negotiate anything other than an analog connection through it.
Of course the chip could lie about connecting at 56K I suppose.
I think your memory might be faulty, the consumer end was POTS, but the head-end needed upgrades from the phone company. If the rest of the links in the phone company were all digital then you could get 56k, if not you were stuck with 33.6 at best.
At the ISP I worked at back in the mid-late 90s, I definitely remember us having to upgrade the lines and buy a bunch of fancy new rack mounted modem equipment to support 56k. IIR, the equipment and lines could also handle single and dual channel ISDN connections, which we charged a pretty penny for. But the customer had to have a digital connection at their end. Also, IIR, the actual limit was 53k for some regulatory reason.
Of course I might be wrong, I've forgotten almost everything about the ISP business.
fun note: Before that we had built several racks of custom dial-in hardware out of unboxed external USR 33.6 modems, racked and fan cooled, with long power strips along both sides for power.
Troubleshooting was accomplished by turning up the volume on a suspect modem and waiting for somebody to dial in and listening to a couple fails (we couldn't dial into it directly due to how the incoming telephone calls were routed). A trip down to the local computer shop to buy another modem, crack the case open and voila, fixed.
We eventually were forced to sell the company with the advent of DSL, which, even if we could afford the head-end equipment (which we couldn't) it wouldn't put us in practical competition with the phone companies who were better positioned to offer the service.
The line from your customer to the central office could NOT be digital, because if it were, it meant you are most likely connected to a SLC (aka remote terminal).
The goal of a SLC was to serve a remote area at low cost, by avoiding the need to deploy tons of copper. Most SLCs achieved this by taking a T1 (traditionally 24 POTS lines) and performing an analog-to-digital conversion AND compression (instead of giving you 64KB of bandwidth per channel, you got 16KB) to increase the number of voice lines that could be served.
So instead of deploying 96 pairs of copper, for 10 miles, at a cost of $$$$$ per mile, you deployed 2-4 pairs of copper for 10 miles at $ per mile plus the fixed cost of $$ for the purchase/lease of a SLC.
(The downside is the occasional line break caused by a car accident or hungry squirrel, would take down an entire neighborhood instead of just one customer)
On the provider side, you needed a provider that would deliver you PRIs with echo cancellation turned OFF on your channels.
Echo cancellation helped telecom providers, again, by saving bandwidth. With the advent of packet-switched networks, your voice calls were converted to digital audio, and sliced into chunks of 64KB, and delivered as packets to the remote switch.
Silence suppression was a common feature of packet-switched networks -- instead of delivering 64KB packets of silence, you delivered nothing, which allowed providers to reduce the aggregate amount of bandwidth they needed to service all the calls transiting their network.
With echo cancellation, it further increased the amount of packets the provider could drop to the floor which reduced their overall bandwidth needs. Your provider most likely marketed this new technology as "better quality voice calls" (remember the AT&T commercials inviting you to test the quality of their 'new digital network' for yourself, by calling 10-10-288-something and listening to a clip of Whitney Houston singing?)
The problem is, echo cancellation generates false positives on modem calls, and the subsequent silence suppression would cause 56K calls to retrain down to a level that would eventually not be effected by the echo cans, resulting in a 31.2Kbps or 33.6Kbps call.
This was a highly annoying discovery for providers who invested in brand new 56K-capable modem equipment.
Normally, you could just ask your provider to turn off the echo cans, but after the Telecom Act of 1996, a flood of new providers entered the market who were merely just reselling other larger carriers. This was a problem as your reseller probably didn't know what echo cans were, let alone had the "pull" to ask the underlying provider to turn it off on your behalf.
As an ISP, if you couldn't get echo cans turned off, then your only option was to cancel and find a new provider.
mbell, please don't take this the wrong way, but you seem to be a young couch researcher. I used to think I was the fountain of all knowledge with my exemplary Google searching skills but cursory searching and skim reading doesn't reveal the wealth of knowledge and facts that are still locked up in engineers heads and dusty old text books.
Only downstream was 56.6k PCM, upstream was 33.6k analog-modulated, so two consumer modems hooked together would only be capable of 33.6k.
There was a later V.92 standard that improved upstream speed, but is was a bit wonky as it sacrificed download speed to do it, I'm not sure how much it was really used.
