VoIP Voice and Fax Signal Processing

15.4 T.38 FAX RELAY

The end-to-end functional representation of a real-time FoIP call is shown in Fig. 15.2. The calling G3 or SG3 facsimile equipment is connected to an emitting gateway, and the called G3 facsimile terminal is connected to a receiving gateway through the RJ-11 connector. The emitting gateway is connected to a receiving gateway through an IP network, which makes a VoIP call to the called gateway. Once the VoIP call is established at both ends, the two G3 facsimile terminals are linked virtually. An alternative option is available of connecting an Internet-aware fax device directly to the IP network. At the calling facsimile terminal, the optically scanned fax image data in the form of run lengths of black-and-white picture elements are coded according to the one-dimensional code book and optionally use the two-dimensional coding techniques as defined in T.4 [ITU-T-T.4 (2003)].

The coded fax data are then modulated to one of the transmission bit rates of 33,600, 28,800, 14,400, 12,000, 9600, 7200, 4800, and 2400 bps as defined in the standards ITU V.17, V.27ter, V.29, and V.34. ITU V.21 is defined as the low-speed 300 bps fax modulation used to exchange the binary procedural data. V.17, V.27ter, V.29, and V.34 are defined as the high-speed fax modulation protocol for T.4 and T.6 [ITU-T-T.6 (1988)] based fax equipment. The T.38 fax relay resides in the VoIP gateway that consists of the fax data pump, T.38, and VoIP signaling [URL (Cisco-fax1), URL (Cisco-fax2), URL ...