HDLC Protocol Operation

The two basic functions in the protocol

• Link Management
• Data transfer (which includes error and flow control).

1. Link Management - Prior to any kind of transmission (either between two stations connected by a point to point link or between a primary and secondary station a multidrop link) a logical connection between the two communication parties must be established.
2. Data Transfer - In NRM all data (information frames) is transferred under the control of the primary station. The unnumbered poll frame with the P bit set to 1 is normally used by the primary to poll a secondary. If the secondary has no data to transmit, it returns an RNR frame with the F bit set. If data is waiting, it transmits the data, typically as a sequence of information frames. The two most important aspects associated with the data transfer phase are error control and flow control. Essentially, error control uses a continues RQ procedure with either a selective repeat or a go back N transmission strategy, while flow controls bases on a window mechanism.

Some standard subsets are listed in the following table.

HDLC Subset		Used for
802.2	Logical Link Control	FDDI, Token Ring, and some Ethernet LAN's
LAP	Link Access Procedure	Early X.25 implementations
LAPB	Link Access Procedure Balanced	Current X.25 implementations
LAPD	Link Access Procedure for the ISDN D Channel	ISDN D channel and frame relay
LAPM	Link Access Procedure for Modems	Error-correcting modems (specified as part of V.42