High Level Data Link Control (HDLC)

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 HDLC was developed by ISO and has become the most widely accepted data link protocol. It offers a high level of flexibility, adaptability, reliability and efficiency of operation for today as well as tomorrow's synchronous data communication needs. ADCCP developed by ANSI is almost similar to HDLC, IBM'S SDLC is a proper subset of HDLC and level 2 of X-25 is a permissible option of HDLC.
In this chapter, we shall study the basic features and operation of HDLC protocol. Certain liberties have been taken in the level of completeness of description so as not to cloud the overall picture with the details.
GENERAL FEATURES of hdlc  
HDLC is a bit oriented data link control protocol which satisfies wide variety of data link control requirements including:
·         Point-to-point and point-to-multipoint links.
·         Two way simultaneous communication over full duplex circuits.
·         Two way alternate operation over half duplex or full duplex circuits.
·         Synchronous and asynchronous communication.
·         Communication between primary stations and between primary and secondary stations.
·         Full data transparency.
Types of Stations
To make HDLC protocol applicable in various possible network configurations, three types of stations have been defined.
·         Primary station

·         Secondary station
·         Combined station

Communication can be between a primary station and one or more secondary stations . The primary station has the responsibility of link management, i.e. activating and disconnecting the communication link. The secondary stations operate under the control of the primary station. The frames sent by a primary station are called commands and the frames sent by secondary station are called responses. as showing in fig.


A combined station can act as a primary as well as secondary station, i.e. it is capable of link management function, sending and receiving both commands and responses. Such a communication situation occurs when it is between two logical equal stations. 

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