VoIP Voice and Fax Signal Processing

14.8 V.21 FAX MODEM

V.21 is the ITU-T recommendation for sending the data and messages at a 300 baud rate. Each baud is of 1 bit, which is the slowest rate used by the fax machines and data modems. All G3FE devices must incorporate this standard for exchanging the initial capabilities of two fax machines. V.21 uses a modulation type called FSK with 1 bit for modulation. V.21 and a summary of FSK specifications are given below. One frequency is used for logic “1,” and another frequency is used for logic “0.” Modems that use full-duplex communication use two channels with four different frequencies. Channel-1 is used for the modem that originally made the call, whereas channel-2 is used by the modem receiving the call. Group-3 fax machines communicate in half-duplex mode (i.e., one direction at a time) using channel-2 for both directions. Both G3 fax machines use the same frequencies with bit value “0” at 1850 Hz and bit value “1” at 1650 Hz. The key parameters of the V.21 recommendation are listed here:

Baud rate and modulation: 300 and FSK with capability for full duplex

Channel-1 frequencies: Center carrier 1080 Hz, bit-0 is 1180 Hz, bit-1 is 980 Hz

Channel-2 frequencies: Center carrier 1750 Hz, bit-0 is 1850 Hz, bit-1 is 1650 Hz (carrier is the mean of bit-0, bit-1 frequencies)

Power levels: Power output should not exceed 1 mW, and nominal level −13 dBm as per V.2 [ITU-T-V.2 (1988)]

Signal detection: Bit or signal is detected if signal level is of >−43 dBm

In full-duplex mode, ...