VoIP Voice and Fax Signal Processing

16.5 T.38 IFP PACKETS

IFP or the name “IFP packets” refers to T.38 packets even though the name “IFP packet” is more correct. The IFP packet contains IFP header bytes, raw payload bytes based on the packet interval, and selected fax modulation. Viewing IP packets on a network, the IFP packet is a payload that forms IP packets using RTP, UDPTL, or Transport protocol data unit packet (TPKT) headers. In the usage, the fax payload is referred to by the name IFP packet because of the inclusion of the IFP header with raw fax payload bytes. Voice packets do not have any corresponding headers that are similar to the IFP header.

Every message of the analog fax maps to unique bytes in T.38. At the source, bytes are extracted from the fax-sampled signals. At the destination, bytes are used to regenerate the signals to a fax machine. Two broad classes of T.38 packets exist—namely, the T.30 indicator packets for tones, no signal, preamble flags, and modulation indications and the T.30 data packets for commands, messages, training, and actual fax pages.

16.5.1 T.30 Indicator Packets

The T.30 indicator type is used by the gateways to indicate the detection of signals such as calling (CNG), CED tones; high-level data link control (HDLC) preamble flags, no signal indications, and modulation training selection. The basic IFP indicator packet of two bytes is marked in Fig. 16.1(a), which contains length of packet as first byte and type of message or indicator as second byte. The indicator payload ...