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:
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: