VoIP Voice and Fax Signal Processing

14.13 V.21 HDLC FRAMING AND DEFRAMING

The main purpose of the V.21 FSK modem is to exchange control information between fax modems. This exchange of control information between two fax modems is established using the HDLC protocol. The HDLC protocol specifies a packetization standard for serial links. The basic HDLC structure consists of several frames, each of which is subdivided into many fields. It provides frame labeling, error checking, and confirmation of correctly received information. A basic HDLC frame structure is given below [ITU-T-T.30 (2005), URL (HDLC)].

Flag field: The 8-bit HDLC flag sequence is used to denote the beginning and end of the frame. For the facsimile procedure, the flag sequence is used to establish bit and frame synchronization. The trailing flag of one frame may be the leading flag of the next frame. Continued transmission of the flag sequence may be used to signal to the distant terminal that the terminal remains on line but is not currently prepared to proceed with the facsimile procedure. The 8-bit flag sequence is represented in the format “0111 1110” in binary or “0x7E” in hexadecimal format. In the idle state or in between HDLC frames, fill flags may be transmitted with the flag identifier as “11111111 – byte value 0xFF” or 0x7E. The fill bytes are optional and vary with the implementation.

Figure 14.13. HDLC packet format.

Address field: The ...