V.91 digital to digital: technical details¶

This is the technical companion to V.91: digital to digital, the plain-English overview. Start there; this page keeps the full detail: the code's names, the Recommendations' clauses, the numbers and the test lists.

Overview¶

dsp/v91.cpp implements ITU-T V.91 (05/99): a modem on a 4-wire digital connection talking to another digital modem. Typical cases are D-Modem calling D-Modem, or D-Modem calling the Asterisk module over SIP. Both ends send PCM codes (G.711 octets as exact values), and both run the same start-up (8.2.1/V.91).

The V.34 pump (dp_v34.cpp) hands over to V.91 at the end of V.8 when both the CM and the JM have the V.91 bit (b7, V8_PCM_V91) set in their PCM modem availability octet (v8_v91()). The pump only offers V.91 when:

• +MS selects V.90, V.92 or V.91;
• the line runs at 8000 Hz (a digital network's line, not an analogue one);
• DSP_V91 is not set to 0.

If V.91 fails to start, both modems fall back to V.34 (see Escape to V.34 below).

Start-up sequence¶

The terms used below:

• Ucode: a PCM code number from 0 to 127 (the G.711 octet without its sign), as V.90 defines it.
• DIL (digital impairment learning): a sequence of known codes that lets the receiver see what the network does to each code.
• Data frame interval: one of the 6 symbol positions in a data frame.

The steps, as each modem sends them:

1. Ez: 24 symbols of −Ucode 66 (after V.8's silence).
2. INFO and INFO': INFO (Table 6/V.91) is 62 bits, coded differentially into the sign of Ucode 66 and repeated. Once the remote's INFO has arrived, the modem sends INFO' instead (INFO with bit 28 set).
3. We always ask for the default DIL (Table 5/V.91, bit 26 clear).
4. We never ask for transparent mode (bit 40 clear). Through dmodemd's 16-bit linear audio, the µ-law codes +0 and −0 become the same value, so transparent mode cannot work.
5. Bit 39 gives our G.711 law; bit 27 asks for the control channel (see Data mode).
6. Eu, or J + PHIL + Em: the remote decides which DIL it wants from us.
7. Eu (12 zeros, coded as INFO is) means the default DIL follows.
8. J (Table 12/V.90, the same format as V.90's Ja) describes a DIL of the remote's own. It is followed by PHIL (scrambled ones) and Em (12 scrambled zeros). We then send the DIL that J described.
9. DIL (as 8.4.1/V.90). While the remote's DIL comes in, the receiver records, for each of the 6 data frame intervals, the value that each code and sign arrives as.
10. SCR: scrambled ones, until the remote's DIL has been analysed.
11. CP and CP': CP (Table 4/V.91) has the same layout as V.90's CP. It carries the constellations we can receive and the rate. CP' (CP with its acknowledge bit set) follows once the remote's CP has arrived.
12. Es and B1: Es is 12 scrambled zeros; B1 is two data frames of ones.
13. Data mode (below).

What the DIL analysis keeps¶

For each data frame interval, the receiver (V91::choose()) goes through the codes from the loudest down. A code is usable when:

• both of its signs were seen, and each arrived as the same value every time;
• its two signs arrived as different values; and
• neither value is already taken by a louder code that was kept.

Where the network makes several codes arrive as one value (robbed bits, a digital loss pad, or a conversion between µ-law and A-law), only one of them is kept. The codes kept are the constellations the receiver asks for in its CP. In data mode, each received value is decoded as the code whose DIL value is nearest (V91Map::decode()).

The rate¶

K is the number of bits per data frame that go through the modulus encoder. It is the most that the product of the six intervals' constellation sizes can carry. K is at most 41, which gives (41 + 6) × 8000 / 6 = 62667 bit/s. 64000 bit/s would need all 128 codes in every interval, and so transparent mode. A K below 15 means V.91 cannot run.

Data mode¶

The transmitter is the V.90 encoder (5.4/V.90) with Sr = 0 (no spectral shaping) and S = 6: K bits through the modulus encoder and 6 sign bits, coded differentially (V91Enc). The receiver is its inverse (V91Dec).

The control channel (bit 27 of INFO) uses the sign bits of every 64th frame. It is sent only when the remote asks for it. Setting DSP_V91_CC (to any value) makes our INFO ask for it.

The core's DC remover leaves the received codes alone while V.91 runs: the pump reports exact PCM through dp_runtime_pcm().

Retrains¶

A retrain in V.91 means sending INFO again. V.42 carries on across a retrain. The modem starts one when:

• more than 5% of the symbols in a second (400 of 8000) are off the constellation; or
• in data mode, the same value is received 6 ms in a row. That happens with silence, or with the idle code of a digital circuit that has been cut; scrambled data never does it.

The modem also answers the remote's INFO when it arrives in data mode. If a retrain does not reach data mode again within 10 s, the call ends.

The DTE's retrain and rate renegotiation (ATO1, ATO3) are refused with V.91 (Modem::link_req() in core/modem.cpp).

Escape to V.34¶

If there is no INFO' within 1.5 s, or no data mode within 10 s, the modems go to V.34 phase 2, starting from INFO0 (8.3/V.91).

4-wire leased circuits¶

On a 4-wire digital leased circuit (8.8/V.91, AT&L2+MS=V91), the pump starts V.91 at once. There is no V.8 and no Ez: the modem sends INFO until the far end's INFO arrives, then has 10 s to reach data mode. There is no escape to V.34; the call ends instead. See leased lines.

Settings¶

Tests¶

dmodem_test puts a G.711 network between two D-Modems (the V91 suite in dsp/test/dmodem_test.cpp; run them with ctest -R v91):

The tests check a rate of at least 60000 bit/s (62666 for v91_alaw); the exact rates above are what the calls reach. Other cases in the same suite:

• v91_bulk: with the default AT+MS=92, 200 kB of data that does not compress goes across V.42 at about 58700 bit/s. (V.34 at 33600 bit/s manages 31400.)
• ms56: K56flex's AT+MS=56 finds V.91 as well.
• v91_retrain: the caller retrains 3 s into the data; V.42 carries on.
• v91_cc: the control channel, on the robbed-bit network.
• v91_escape: one modem silent in V.91; both escape to V.34 at 33600.

Over SIP, frontend/asterisk/test/sip.sh (dmodemd calling DModem in Asterisk, also with CODEC=alaw and with INCOMING=1) connects at 62666/62666.