HDLC | High Level Data Link Control Protocol and Frame Format

HDLC (High Level Data Link Control Protocol ) Protocol and Frame Format HDLC is a data link layer (Layer-2) protocol. The job of data link layer is to deliver error-free message to the intended destination. HDLC is defined by ISO. Leased lines provide physical layer services only so we need some data link layer protocol and HDLC is used for that. Standard HDLC Frame given by ISO has following fields. Flag field : Specific bit pattern indicating the arrival of new frame Address field: shows the destination address Control field: define different formats depending on the type of frame Data field: The packet sent by network layer FCS (Frame Check Sequence) : Used to detect errors during transmission between nodes Cisco HDLC Frame: In addition to all the fields given by ISO, there is TYPE field as well. Type field: Type of packet encapsulated in the frame (It offers multiprotocol support) Characteristics of HDLC Protocol: HDLC is a bit-oriented protocol which means bits instead of bytes or characters are used for control information HDLC supports point-to-point as well as point-to-multipoint connection HDLC is synchronous data link protocol: It means both the transmitter and receiver are synchronized by a clock pulse Cisco uses HDLC as the default encapsulation method on synchronous serial lines #HDLC #HDLCFrameFormat