33.6k modem handshake (V.34)

Line probing, explained

V.34 (1994, revised 1998) was the first modem standard that measured the line before choosing how to use it. After the V.8 menus, each modem sends a short INFO0 burst: 600 bit/s DPSK listing which symbol rates and carriers it supports (Table 14/V.34), protected by a 16-bit CRC.

Then the probing. L1 is a periodic signal made of 21 cosines, every 150 Hz from 150 to 3750 Hz, with 900, 1200, 1800 and 2400 Hz omitted (those frequencies are used by the tones around it). Each cosine has a fixed starting phase from Table 17, and L1 lasts exactly 160 ms at 6 dB above normal. L2 repeats it at normal level until the far end has heard enough. The receiver compares the 21 tones it gets with the 21 it expected and computes the line's frequency response and noise. That picks the symbol rate (2400 to 3429 baud) and carrier for each direction, reported in INFO1.

Play L1 from the answerer Play L1 from the caller Play INFO1a: the decision

Ranging: timing the line with two tones

Between the INFO bursts and the chord, the answerer sends tone A (2400 Hz) and the caller tone B (1200 Hz). Each side flips its tone's phase exactly 40 ms after hearing the other's flip, so the round-trip delay drops out of the arithmetic (§11.2). The echo canceller needs that number.

Training and the rate exchange

Phase 3 is S, PP and TRN, the hiss, at the chosen symbol rate. Phase 4 is J (asking for a 16-point constellation), TRN again, then MP: the maximum rate each way, trellis code and precoder taps (Table 21). Our line presets land on 33 600 (clean), 31 200 (typical), 26 400 (noisy) or 24 000 bit/s (long distance). Try them and listen to the symbol rate change.

Play MP

Approximate: data after CONNECT (no shell mapping, trellis coding, precoding or pre-emphasis).