VoIP Voice and Fax Signal Processing

16.2 G.711 FAX PASS-THROUGH BIT RATE

In this section, the details on fax pass-through bit rates for redundancies zero to three are given. The G.711 codec operates at a 64-kbps rate. A loss of a G.711 pass-through packet can create impediments to the end fax delivery, and to counter this, redundancies are used. In redundancy, some immediate previous payload bytes are grouped together in a specified format. Some more details on RTP packets and redundancy format are given in Section 16.7. In G.711 pass-through, 10 or 20 ms may be used, but a 10-ms packet interval is preferred and all the calculations in this book are given for a 10-ms packetization period. For a packetization period of 10 ms, a typical G.711 payload is 80 bytes. With redundancy = one, the payload is created as present 80 bytes and previous 80 bytes grouped together as 160 bytes. In Tables 16.2 and 16.3, fax pass-through modes with redundancies zero to three on Ethernet and DSL interfaces are listed. The details on different VoIP packet headers used in Tables 16.2 and 16.3 are given in Chapter 11. From the list presented in tables, fax pass-through takes approximately 128 to 380 kbps, which is higher than the T.38 bit rate. A fax bit rate with 20-ms fax pass-through saves on bit rate, but it is still higher than the T.38 fax bit rate. Fax pass-through is independent of fax modulations. The bit rate calculations for any modulations of G3 or SG3 V.34 fax in G.711 pass-through mode are the same.