Basic Features of HDLC Protocol
HDLC is a bit-oriented protocol where each frame contains up to six fields. It helps in activating the link. HDLC protocol also exchanges the data and deactivates the link between two successive nodes.
HDLC Frame Format:
HDLC uses asynchronous transmission. All the transmissions are in the form of frames. The flag, address and control fields that precede the information field constitute the header part. The FCS and flag field following the data field constitutes the trailer part.
Flag: Flag fields delimit the frame at both ends with a unique pattern. A single flag may use for closing a frame as well as for opening the next frame. Bit stuffing uses for avoiding the pattern appearing inside the data part of the frame.
Address: It identifies the secondary that transmits or is to receive the frame. This field does not require the point-to-point link but it is always used for the sake of maintaining uniformity. This field is usually 8 bits long.
Control: It is 1 or 2 bytes containing flow and error control information.
Payload: This carries the data from the network layer. Its length may vary from one network to another.
FCS: It is a 2-byte or 4 bytes frame check sequence for error detection. The standard code used in CRC (cyclic redundancy code).