Technical library

Handshake timing diagram

^{(adapted from 'ITU-T Recommendation V.34', 1996)}

Handshake phase annotations

• Phase 1 signals (V.8 network interaction)
• CI: call indicator (optional, sent using V.21(L), here not present)
• ANSam: amplitude modulated answer tone, with phase reversals (indicating V.8 capability)
• CM: call Menu, sent using V.21(L)
• JM: joint Menu, sent using V.21(H)
• CJ: CM terminator, sent using V.21(L)
• Phase 2 signals (line probing and ranging)
• INFO: 4 sequences sent using 600 bps DPSK
• L1/L2 : 21 tone line probing signals
• A/A' : 2400 Hz tone with phase reversals
• B/B' : 1200 Hz tone with phase reversals
• Phase 3 signals (equalizer and echo canceller training)
• S/S': symbol sequence pattern
• MD: manufacturer defined training sequence for training the echo canceller (optional)
• PP: training sequence for training the equalizer
• TRN: training sequence (scrambled ones)
• J/J': specific 16-bit patterns
• MP: modulation parameter signal
• Phase 4 signals (final training, full duplex mode)
• J/J'/TRN: see phase 3
• MP/MP'
• E: end of MP
• B1: scrambled ones

Analysis of a recorded call

• Phase 1 signals (V.8 network interaction)
• Phase 2 signals (line probing and ranging)
• Phase 3 signals (equalizer and echo canceller training)
• Phase 4 signals (final training, full duplex mode) - not shown

^{Annotated spectral plot of recorded audio (Frequency/Hz against Time/s)}

Modulation

Adopted in 1994, V.34 improves upon V.32bis in a number of ways: in adaptivity, constellation shaping and precoding. Adaptivity is achieved by using a range of carrier frequencies and symbol rates, including some beyond those in V.32bis. Shaping is a technique used to minimize the average signal power for a given constellation, and a technique known as shell mapping is used, yielding a 0.8dB shaping gain. Precoding is equalization applied at the transmitter based on information from the receiver, ensuring better use of the full bandwidth. V.34 uses a larger, 1664-point super-constellation, with all symbols being a subset of this super-constellation. For example, at 28,800 bps, Fc=1800 Hz, Rs=3000 baud, Symbols=1408; at 33,600 bps, Fc=1959 Hz, Rs=3429 baud, Symbols=1664.