Why V.32bis modems are 14400 bps rather than 19200 bps (was Re: vt100 terminfo with padding for an actual vt100?)

[Eric Smith]

On Tue, Sep 6, 2016 at 5:18 PM, Eric Christopherson
<echristopherson at gmail.com> wrote:
> Is this why modems went to 14400 instead of 19200?

No, that was because the V.32 modulation was easily extended to 12000
and 14400 bps by slightly expanding the QAM signal constellation,
requiring a little better SNR on the line, but keeping other
parameters the same and increasing the constellation size enough to
get 19200 bps was not expected to work within available SNR on most
POTS lines.  That didn't stop some vendors from offering 19200 as a
proprietary extension to V.32bis. It's known unofficially as V.32ter,
but was never actually ratified as an ITU-T V-series recommendation.

V.32 at 9600bps uses 1800Hz carrier, 2400 baud, and 32 carrier states
(constellation points) using one trellis coding bit for 4 data bits,
so effectively 4 data bits per baud, and 2400 * 4 = 9600 bps.

V.32bis at 14400bps uses the same carrier and baud rate, and  128
carrier states (constellation points), with slightly more complex
trellis coding, so effectively 6 data bits per baud, and 2400 * 6 =
14400 bps.  (There's also a fallback to 12000 bps)

V.32 and V.32bis are synchronous modulation, and when used with V.42
error control and a normal serial port configuration of 8N1, the modem
effectively removes the start and stop bits (20% overhead on the
serial port), though framing is added so that the throughput doesn't
go up by that full amount.  This is noticeable when running such a
modem with a higher serial port baud rate (requiring flow control).
For instance, a V.32 9600bps modem without V.42 would be able to
transfer 960 characters per second, but with V.42 and a higher serial
port rate with flow control, can exceed that.

V.42bis compression can further improve the throughput provided that
the data is compressible.

To go beyond 14400 bps with conventional modulation and typical POTS
line SNR requires more complex techniques, used in V.34 for up to
33600 bps.

Abandoning conventional modulation and introducing a direct dependency
on the PCM line code used within the PSTN allows up to 56000 bps PCM
downstream and 33600 bps using V.34 modulation upstream (V.90), or
56000 bps PCM downstream and 48000 bps PCM upstream (V.92)