In this topic, the basics on fax machine, coding techniques for fax data compression, fax call messages, timing, fax data modulations, HDLC framing, and deframing are given. The PSTN fax will be successful in most situations. Because of interoperability of fax machines and analog telephone lines, fax calls in PSTN may fail. Several mechanisms are built into fax coding, messages, timing, and supporting error correction modes for data that helps smooth fax transmission. In this topic, the main interest is VoIP fax or fax over IP. Fax over IP systems mainly deals with fax modulations, messages, and timing. Certain additional techniques are incorporated into fax over IP systems specific to packet network impediments that are presented in topic 15 . End-to-end fax over IP systems are made to work like PSTN, but some of these IP network techniques and features are different from the PSTN system.

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