7 May
2014
7 May
'14
2:01 a.m.
On Wed, May 07, 2014 at 12:55:45AM -0600, Eric Smith wrote:
[...]
Cool project! I'm interested in hearing how well
V.32bis QAM modulation (14.4
Kbps) works over VOIP, since a lot of VOIP applies compression schemes
intended specifically for voice. On uncompressed PCM, or PCM with only basic
companding (mu-law or A-law) it should work fine, since that's what has most
commonly been used by the phone companies for carrying domestic POTS calls.
International voice calls often used more compression, which at the very
least could be expected to increase the error rate.
My experiments on this front involved CSD calls to my VoIP number, which was
delivered over a 2M/256k ADSL to an ATA into which I'd plugged my old USR
modem.
(CSD is https://en.wikipedia.org/wiki/Circuit_Switched_Data, which is basically
dialup over mobile. It was reasonably popular for mobile Internet access until
the telcos offered cheap(er) packet-switched services.)
I never did get it to work reliably even though it should have been a shoo-in
for a solid connection. After all, the only analogue hop was the couple of feet
of phone cable between the ATA and the modem. I suspect I was being nobbled by
jitter and/or packet loss and the ATA wasn't re-clocking the signal properly.
Yes, I was using the G.711a "codec" so it wasn't compression that was
killing
it.
Curiously, the telco's modem bank tried to make V.34 connections even though
CSD can only do 9.6kb/s. Turning off the advanced modulation schemes in my
modem got me a passable connection at 7.2kb/s with periodic retrains.
I'd try this again now I'm on 17M/1M ADSL, but I'm not sure I could assemble
the technology any more. My old phones have packed in, and I'm unconvinced that
modern handsets implement this somewhat obsolete standard. (And also, my telco
doesn't have any GSM infrastructure, although it's not as if I can't get hold
of a SIM for one that does.)