Explain the HDLC Operational and Non-Operational Modes

A mode in HDLC defines the relationship between two machines in a network and describes who controls the link. HDLC determines three types of stations: primary, secondary, and combined.

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

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

A combined station combines the functions of both primary and secondary stations, operating as an equal peer in the communication.

HDLC Operational Modes

There are three operational modes that define how stations communicate:

Normal Response Mode (NRM)

In this mode, the primary station initiates transfers to the secondary station. The secondary station must receive explicit permission from the primary station before it can send a response. After receiving permission, the secondary station can transmit its data, but must wait again for specific permission from the primary station for subsequent transmissions.

Asynchronous Response Mode (ARM)

In this mode, the secondary station does not need to wait for explicit permission from the primary station to send frames. However, since this mode operates asynchronously, the secondary station must wait until the medium is available before it can transmit frames. This typically occurs when the ARM link is operating in half-duplex mode.

Asynchronous Balanced Mode (ABM)

In this mode, all stations are equal and act as combined stations in a point-to-point configuration. No station requires permission to transmit data. This is because combined stations have equal authority to perform any function on the connection, making it ideal for peer-to-peer communication.

HDLC Non-Operational Modes

There are three non-operational modes:

• Normal Disconnected Mode (NDM) − Station is logically disconnected from the link but can respond to specific commands

• Asynchronous Disconnected Mode (ADM) − Similar to NDM but allows asynchronous responses to certain commands

• Initialization Mode (IM) − Used when a station needs to exchange parameters or perform initialization procedures

The first two modes differ from operational modes because one station is logically disconnected from the link. In Initialization Mode, one of the communicating stations requires parameter exchange or system regeneration before normal operation can begin.

Conclusion

HDLC operational modes define the communication control between primary, secondary, and combined stations, while non-operational modes handle disconnected states and initialization procedures. The choice of mode depends on the network topology and required level of control between communicating stations.