The HDLC Address Field

The first byte(s) of a frame transmitted using the High Level Link Control (HDLC) Protocol areused to carry an address field. This field is typically a single byte, but extension is possible allowing a number of bytes to be used. The address format is shown in the figure below.

Format of the Address Byte(s) in HDLC

The address consists of three parts:

• A Service Access Point (SAP) which is usually set to zero, but used in some variants of HDLC to identify one of a number of data link protocol entities.
• A Command/Response bit to indicate whether the frame relates to information frames (I-frames) being sent from the node or received by the node.
• An address extension bit which is usually set to true to indicate that the address is of length one byte. When set to false it indicates an additional byte follows.

The address field is mainly used when HDLC is used in a mode which provides reliable data transfer using numbered control frames.