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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