Explain the HDLC Operational and Non-Operational Modes

A mode in HDLC is the realigning between two machines contained in a network. The mode describes who controls the link. HDLC determines three types of stations such as primary, secondary, and combined.

A primary station is a controlling station on a connection. It controls the secondary stations on the connection and handles data flow and error recovery.

A secondary station as existing on a connection where there is a primary station. The secondary station is contained by the primary station and has no explicit authority for maintaining the connection.

A combined station is a combination of primary and secondary stations.

HDLC Operational Modes

There are three different modes of operations which are as follows −

Normal Response Mode (NRM)

This is the mode in which the primary station launch transfers to the secondary station. The secondary station should get proper permission from the primary station to change a response. After taking permission from the primary station, the secondary station opens its transmission. The secondary station should stay once again for specific permission to message all from the primary station.

Asynchronous Response Mode (ARM)

In this mode, the secondary station does not have to wait to accept specific permission from the primary station to remove any frames. Because this mode is asynchronous, the secondary station should wait until it can transfer any frames. This is when the ARM link is performing at half-duplex.

Asynchronous Balanced Mode (ABM)

In this mode, all stations are the same and hence combined stations linked in point-to-point. There is no requirement for permission on the method of any station in this mode. This is due to combined stations not being set of information to execute any function on the connection.

HDLC Non-Operational Modes

There are three non-operational modes which are as follows −

• Normal Disconnected Mode (NDM)

• Asynchronous Disconnected Mode (ADM)

• Initialization Mode (IM)

The first two-mode differs from operational mode because in this one station is logically disconnected from the link. In the IM mode, one of the communicating stations needs regeneration or requires an exchange of parameters.