V.91: digital to digital¶
V.90 has one digital end and one analogue end. V.91 is for when both modems are on digital lines, for example two D-Modems calling each other over SIP, or through Asterisk. Since neither end has an analogue line, both directions can use the fast digital method, giving about 62000 bit/s each way.
The code is dsp/v91.cpp. For the start-up steps, the retrain rules and
the measured rates, see the technical details.
How it starts¶
Both modems say during V.8 that they can do V.91. If V.91 then fails to
start, they fall back to V.34. On a 4-wire digital leased circuit
(AT&L2+MS=V91, see leased lines) V.91 starts
straight away with no V.8.
Speeds¶
| Network | Speed |
|---|---|
| clean G.711, µ-law or A-law | 62667 bit/s |
| with "robbed bits" (some American trunks) | 61333 bit/s |
| with a 3 dB volume pad | 60000 (µ-law) or 61333 (A-law) bit/s |
| converted between µ-law and A-law on the way | 62667 bit/s |
With error correction on, about 58700 bit/s of real data gets through, compared with about 31400 bit/s for V.34.
If the line degrades, the modems retrain, and error correction resends anything that was lost.