The Anatomy of a Fax Call

Whether there's a fax modem or fax machine on either side (or both sides) of the line, all fax calls go through the same five-step process:

1. Connection : The calling fax uses the standard V.21 fax protocol to establish a physical connection with the receiving fax at 300 bps. The receiving station sends a STID (station ID) to the calling fax.

2. Phasing and training : Still at 300 bps, the calling station sends a single HDLC (high-level data link control) frame to the receiving station. During a 200-millisecond "window," both sides negotiate a maximum sending speed (depending on line conditions). The calling station then sends null characters to the receiving station at t he negotiated speed. If the receiving station receives characters, it sends confirmation and control is returned to the calling station.

3. Unidirectional transfer : The calling station sends data to the receiving station in a one-direction, nonprotocol transfer (no error correction). At the end of each page, the calling station sends an EOM (end of message) and the speed returns to 300 bps. If no further pages are to be sent, the calling station goes to step 5. If more pages are to be sent, the calling station goes to step 4.

4. Negotiation : Before another page is sent, both of the stations go through the same negotiation as found in step 2 to detect if the line conditions have changed.

5. Disconnect : At the end of the transfer, both systems disconnectfrom the telephone line.