13 HDLC CONFIGURATIONWhen configuring HDLC, go to these sections for information you are interestedin:■ “Introduction to HDLC” on page 281■ “Configuring HDLC” on page 282Introduction to HDLCHDLC Overview High-level data link control (HDLC) is a bit-oriented link layer protocol. Its mostprominent feature is that it can transmit any types of bit stream transparently.■ HDLC supports point-to-point link only and does not supportpoint-to-multipoint link.■ HDLC supports neither IP address negotiation nor authentication. It useskeepalive messages to check link status.■ HDLC can only be encapsulated on synchronous link. A synchronous/asynchronous interface can also apply HDLC provided that it works insynchronous mode. Currently, this protocol is applied on the Serial interfaceand POS interface that work in synchronous mode.HDLC Frame Format andFrame TypeThere are three types of HDLC frames: information frame (I frame), supervisionframe (S frame) and unnumbered frame (U frame).■ Information frame is responsible for transmitting useful data or information.■ Supervision frame is responsible for error control and flow control.■ Unnumbered frame is responsible for the link establishment, teardown, and soon.An HDLC frame is composed of flag field, address field, control field, informationfield and checksum field.■ The flag field, 0111111, marks the beginning and end of an HDLC frame. Eachframe begins with F and ends with F.■ The address field is eight bits; it identifies the source or destination where theframe is sent or received.■ The control field is eight bits; it identifies the control type and defines the frametype (control or data).■ The information field can be an arbitrary binary bit set. The minimum lengthcan be zero and the maximum length is decided by the FCS field or the buffersize of the communicating node. Generally, the maximum length is between1000 and 2000 bits.
