VoIP Voice and Fax Signal Processing

14.4 PSTN FAX CALL PHASES

The PSTN fax call is divided into five phases of A, B, C1, C2, D, and E [ITU-T-T.30 (2005), McConnell et al (1999), URL (Audiocodes-fax), URL (SPRA073), URL (Sage-fax)] as shown in Fig. 14.3. Fax call messages for multiple pages of transmission are illustrated in Fig. 14.4. In this section, an overview of fax call flow messages is given. Complete details, several messages, various options, and errors are given in the T.30 recommendation Appendix A.

Phase A is the call establishment, and this is achieved either manually or automatically.

Phase B is the premessage procedure for call control and capabilities exchange. The major steps of this phase are as follows:

Figure 14.3. PSTN fax call phases.

Figure 14.4. Basic fax call phases in relation to messages, pages, and multiple page extensions.

• The calling and answering fax devices identify themselves and exchange capabilities
• Transmission parameters selection

Phase C1 is the in-message procedure for message transmission and controls, and it completes signaling for in-message procedures, namely in-message synchronization, error detection, correction, and line supervision:

• The calling unit sends a test pattern to determine the maximum data rate
• The answering unit either accepts the data rate or requests a lower ...