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Asynchronous balanced mode (ABM) is a balanced configuration.Reduces overhead as no frames need to be sent to allow secondary nodes to transmit.It has a single primary station and multiple secondary stations. Asynchronous Response Mode (ARM) is an unbalanced configuration.It is similar to NRM except that the secondaries can initiate transmissions without direct polling from the primary station.It is an unbalanced configuration and good for multi-point links. It is used for both point-to-point and multipoint communications. HDLC communications session can use one of the following connection modes, which determine how the primary and secondary stations interact.Ī secondary station can only transmit when specifically instructed by the primary station in response to polling. It does not rely on others for sending data.Acts as both primary and secondary stations.The primary station maintains a separate logical link with each secondary station.Frames issued by a secondary station are called responses.It acts as a slave and operates under the control of the primary station.Frames issued by the primary station are called commands.It acts as a master and controls the operation.Types of stations for HDLC Protocol: Primary station: ISO/IEC 13239:2002, the current standard, replaced all of these specifications.
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ISO 6256-1981 – Balanced Classes of Procedure ISO 6159-1980 – Unbalanced Classes of Procedure Original ISO standards for HDLC Protocol: