VoIP Voice and Fax Signal Processing

16.6 IFP OVER TCP (TCP/IP/IFP)

TPKT/TCP is also considered for the transport of IFP T.38 messages, but it is better suited for T.37 store-and-forward fax. The TPKT header is defined in RFC1006 and will precede the IFP packet in TCP implementations as shown in Fig. 16.1(c). Implementations using TPKT set the T.38 version to “1” or higher. Versions with “0” implementations will not use TPKT. TCP transport is a session-based, confirmed delivery service, which provides full correction. In TCP mode, it is not required to use redundancy and FEC for packet recovery. Packet drop is taken care of in TCP through retransmission. The delays can increase because of retransmissions, and the message collisions or time-outs can occur with TCP. TCP-based fax transmission requires a lower bit rate under reasonably good network conditions. The high-level TPKT/TCP/IP packet structure shown in Fig. 16.1(c) will also use extra network interface headers for actual delivery. In TCP implementation, a TCF training signal for determining the high-speed data rate is generated locally by the receiving gateway. Several gateways support UDPTL-based fax transmission and do not interoperate in the TCP mode of operation.