VoIP Voice and Fax Signal Processing

16.3 T.38 BASIC PAYLOAD BYTES FOR V.27 TER, V.29, V.17, AND V.34

This section is presented on fax machine basic modulation – demodulation bit rates and basic payload byte sizes. The bit rate mentioned in this section does not include any packet headers. Fax tolerates longer round-trip delay up to 3 seconds, which allows higher packetization intervals compared with voice packets. In practical deployments, 40- to 100-ms packet intervals are used with T.38. The packet interval considered in this chapter is 40 ms (0.040 seconds).

Table 16.2. VoIP Fax Pass-Through Packets Bit Rate on an Ethernet Interface

Table 16.3. VoIP G.711 Fax Pass-Through Bit Rate with Redundancy on a DSL Interface with PPPoE

In 0.040 seconds, a fax machine at the 9600 rate generates (9600) (0.040) = 384 bits, which is equivalent to 384/8 = 48 bytes. In Table 16.4, raw payloads for V.27ter, V.29, V.17, and V.34 modules [V27ter (1988), V29 (1988), V17 (1991), V34 (1998)] are given for various supported fax rates. V.27ter supports 2400 and 4800 rates; V.29 supports 7200 and 9600 rates; and V.17 modem supports 7200, 9600, 12,000, and 14,400 rates. The byte size values listed in Table 16.4 are reused in subsequent sections, while calculating the payload along with multiple IP headers. The name “V27” is used instead ...