VoIP Voice and Fax Signal Processing

16.7 IFP OVER UDP

UDP is the most popular method for T.38 packet transmission. A T.38 fax can be transmitted over UDP using one of the techniques of IFP data encapsulated in the UDPTL RTP or protocol. UDP has good real-time performance, and routers do not discard the datagram during congestion. However, the downside to UDP is a reduced quality of service. It is possible for packets to get lost, to arrive out of order, or to be duplicated. To overcome these impediments, well-designed redundancy and FEC algorithms are used in UDP mode.

16.7.1 IFP over RTP

The RTP protocol is part of the UDP-based fax transmission and can be used as an alternative to IFP over UDPTL. The RTP protocol is used when both gateways negotiate this capability during call setup. Additional capabilities such as error protection schemes are also negotiated for redundancy [Perkins et al. (1997)] and FEC [Rosenberg and Schulzrinne (1999)]. For the RTP transport, the IFP packets are encapsulated in RTP as shown in Fig. 16.2(a). Within an RTP packet, an IFP packet may be combined optionally with a redundant IFP packet or with a combination of redundancy and FEC packets.

Advantage of Using RTP Protocol. RTP allows improving T.38 to permit faster switching from the VoIP voice call to the FoIP call. The switching from VoIP to FoIP could be accomplished by simply changing payload types “on the fly” rather than by renegotiating, which thus improves the interoperability and switching speed. An RTP-based fax is a recent ...