VoIP Voice and Fax Signal Processing

16.1 OVERVIEW ON T.38 AND G.711 PASS-THROUGH BIT RATE

In the public switched telephone network (PSTN), fax and voice will share the same telephone interface. At the digital hierarchy of PSTN, fax or voice takes 64 kbps. In some VoIP deployments, fax is sent as a pass-through mode using G.711 [PCM μ-law (PCMU) or PCM A-law (PCMA)] or G.726 at 32/40 kbps as the compression. When network conditions are perfect, fax pass-through works similar to a PSTN-based fax. In VoIP deployments, some amount of packet impediments is unavoidable. To cater to packet impediments, RTP [Schulzrinne et al. (2003)] makes use of multiple redundant packets that are sent as per RFC2198 [Perkins et al. (1997)] and forward error correction technique as per RFC2733 [Rosenberg and Schulzrinne (1999)]. A basic G.711, 10-ms packetization-based fax pass-through call takes 126.4 kbps on an Ethernet interface, and at redundancy three (R3), it takes 328.8 kbps. In practical systems, a G.711 fax pass-through is used with 10-ms packets with redundancy one (R1—one extra payload) in reasonably acceptable IP networks. Redundancy with G.711 is limited to R1 mainly to reduce the Internet bit rate requirements and to improve on interoperability conditions. T.38-supported gateways cater to a higher level of redundancy by design. The redundancy implementation differs for RTP and UDPTL as indicated in Fig. 15.1 of Chapter 15. The redundancy scheme based on RFC2198 is applicable to voice packets, fax pass-through, and T.38 IFP ...