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This article glances at, then misses the fundamental technical reason why modems topped out at ~ 56kbps: that’s really all the bandwidth that is available on a
by marcus0x62 3y ago
This article glances at, then misses the fundamental technical reason why modems topped out at ~ 56kbps: that’s really all the bandwidth that is available on a POTS line, due to the bandpass filter, the ADC on the other end, companding, and engineering practices that led the Bells to maintain their lines to the standards of an analog voice conversation.
The robbed-bit signaling thing is really a red herring: it was a problem, but not THE problem. At the time analog 56kbps modems were popular, it was already possible to negotiate 64kbps/8-bit clean DS0 channels over T1 — you just needed a PRI or an inter-machine trunk to do so.
In fact, if you were an ISP at the time, you generally needed those modems connected to T1 PRIs or IMTs anyway to support your ISDN customers economically. (My memory may be getting fuzzy on this, but I also remember that certain access concentrators with k56/v.90 digital modems would only negotiate the higher analog speeds on a PRI or IMT bearer channel.)
What there was absolutely no appetite to do, on the part of the voice carriers, was beef up the bandwidth available on POTS lines by developing new ADCs, getting rid of companding, or engaging in labor-intensive “facilities grooming”[0] for a POTS subscriber, in the way they routinely did for T1[1] circuits.
0 - Telcos call their lines and equipment “facilities.” Grooming, in the context of outside plant (telco terminology for the physical cabling that goes to subscribers) means to select wire pairs that exhibit good noise and echo characteristics and to remove elements, like poor splices and bridge taps, that impede performance.
1 - T1s, unless delivered to a customer via an optical transport network, use the same copper lines as analog POTS lines. Originally, T1s required two pairs (four wires). This eventually was replaced with HDSL4 (two pairs, but better distance and repeater characteristics,) then HDSL2 (single pair.) As the names suggest, the encoding used was a DSL variant.
- Sesse__ 3y agoAnd even for 56kbps, you need to have the ISP end talk digitally to the telco, so that you don't get the noise (including from the DAC) of the local loop on the ISP end. That's really what pushes the SNR far enough down to make it work (and why the uplink is only 33.6kbps; I guess they didn't want to try to make a separate modulation for that direction).
- marcus0x62 3y agoV.92 supported up to 48kbps uplinks, but that standard caught on about as well as you might imagine an analog modem standard from the year 2000/2001 to. https://en.m.wikipedia.org/wiki/ITU-T_V.92 https://en.m.wikipedia.org/wiki/ITU-T_V.92
- Sembiance 3y agoThe article mentioned using an 8bit ADC. Could a 16bit ADC be used on BOTH sides and achieve 112kbps?
- dreamcompiler 3y agoNo because you still have to modulate the signal to get it to/from a house, and modulation schemes are where you squeeze maximum bits per second into available analog bandwidth within the limits imposed by Claude Shannon. The article didn't talk about modulation schemes and I wish it had.
- marcus0x62 3y agoSure, with other changes. You would still have to: * Select (or repair) “good” pairs that didn’t have poor echo, impedance, noise, etc. characteristics. * Not compand the resulting PCM sample with mu/alaw companding. * Provide an atomic bearer channel from the telephone switch to the ISP that was at least 112kbps. * Develop a modem encoding scheme that could take advantage of the extra bandwidth. At this point, you’ve reinvented a really crappy version of DSL that could be user-signaled to call different networks (i.e., phone numbers.) The other option, the one that people selected, was to use the copper facilities but bypass the phone switch and its associated audio and in-band signaling shenanigans. That was/is xDSL.
- rasz 3y agohttps://en.wikipedia.org/wiki/Noisy-channel_coding_theorem https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem going to https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theorem https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore... Bumping ADC to 16bit will remove some of quantization noise, but still leave you with inherent noise of the transmission medium and power limit (FCC/AT&T).
- 13of40 3y agoWhen I was in highschool I lived in a rural town of about 7000 people that had an analog telephone exchange. I read about the bandwidth limitations, and a friend of mine and I decided to test them over a local and long distance call. The long distance one cut out pretty much exactly at the frequency the spec said it should, but the local one could transmit and receive much higher frequencies, presumably because it had its own circuit and wasn't multiplexed with other ones. (That led to a lot of unrealized fantasies about creating a homemade super modem for local connections.)
- marcus0x62 3y agoAssuming the local call wasn’t on a party line, your lines were probably served by an analog switch or maybe on the same line card of something like a DMS-100. What you described wouldn’t work on “modern” digital switch.
- deleted 3y ago[deleted]