VoIP Voice and Fax Signal Processing

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FAX OVER IP PAYLOAD FORMATS AND BIT RATE CALCULATIONS

In VoIP fax transmission, T.38 and G.711 pass-through are the two real-time fax transmission methods. A. T.38-based fax call makes use of Transport Control Protocol (TCP)- and User Datagram Protocol (UDP)-based transport protocols to deliver packets on the Internet Protocol (IP) network [ITU-T-G.711 (1988), ITU-T-T.38 (2005)]. On UDP, Internet facsimile protocol (IFP) packets are sent using either Real-Time Transport Protocol (RTP) or UDP transport Layer (UDPTL). T.38 fax over UDP with UDPTL is the most popular and well-established method in the deployments, and many VoIP gateways support UDPTL-based T.38 fax and G.711 fax pass-through. G.711 fax pass-through is similar to a VoIP voice calls with RTP packets. UDPTL makes use of redundancy and forward error correction (FEC) techniques as defined in T.38 to improve performance even with IP impediments. Network bandwidth/bit rate calculations and tables are provided in this chapter for multiple fax-machine data rates. Example calculations are presented for a packet redundancies scheme with UDPTL-based transport. The fax over IP network bit rate results are compared with G.711 fax pass-through on Ethernet and digital subscriber line (DSL) interfaces. Store-and-forward T.37 fax transmission can use similar packetization to T.38-based fax transmission, but T.37 may use the TCP method because of the non-real-time nature of fax delivery.

In this chapter, the keyword “bit rate” is ...