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NE40E V800R010C00 Feature Description - WAN Access 01

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HDLC Operation Modes

HDLC Operation Modes


Nodes on a network running HDLC are called stations. HDLC specifies three types of stations: primary, secondary, and combined.

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

A secondary station is present on a link where there is a primary station. The secondary station is controlled by the primary station, and has no direct responsibility for controlling the link. Under normal circumstances, a secondary station will transfer frames only when requested to do so by the primary station, and will respond only to the primary station.

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

Frames transferred by a primary station to a secondary station are called commands, and frames transferred by a secondary station to a primary station are called responses.

On a point to multipoint (P2MP) link, there is a primary station and several secondary stations. The primary station polls the secondary stations to determine whether they have data to transmit, and then selects one to transmit its data. On a point to point (P2P) link, both ends can be combined stations. If a node is connected to multiple links, the node can be the primary station for some links and a secondary station for the other links.

HDLC Operation Modes

HDLC can run in three separate modes:

  • Normal Response Mode

    In Normal Response Mode (NRM), the primary station on an HDLC link initiates information transfers with secondary stations. A secondary station will respond only after receiving a command from the primary station. The secondary station can respond with one or more frames, and must indicate which frame is the last frame in the transmission.

    The primary station manages the entire link and is responsible for timeout, retransmission, and error recovery.

    NRM is generally used for terminal-oriented P2P links and P2MP links.

  • Asynchronous Response Mode

    In Asynchronous Response Mode (ARM), the secondary station can transmit frames to the primary station without first receiving a command from the primary station. The secondary station is responsible for timeout and retransmission.

    This mode is necessary for multi-node links that use polling.

  • Asynchronous Balanced Mode

    In Asynchronous Balance Mode (ABM), all stations are combined stations. All stations can transmit information without permission from any other station, as well as transmit and receive commands, send responses, and correct errors. This mode improves link transmission efficiency.

Updated: 2018-07-04

Document ID: EDOC1100027168

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