Editing High-Level Data Link Control (section)

==Link configurations==

Link configurations can be categorized as being either:
* ''Unbalanced'', which consists of one primary terminal, and one or more secondary terminals.
* ''Balanced'', which consists of two peer terminals.

The three link configurations are:
* ''Normal Response Mode'' (NRM) is an unbalanced configuration in which only the primary terminal may initiate data transfer.  The secondary terminals transmit data only in response to commands from the primary terminal.  The primary terminal polls each secondary terminal to give it an opportunity to transmit any data it has.
* ''Asynchronous Response Mode'' (ARM) is an unbalanced configuration in which secondary terminals may transmit without permission from the primary terminal.  However, there is still a distinguished primary terminal which retains responsibility for line initialization, error recovery, and logical disconnect.
* ''Asynchronous Balanced Mode'' (ABM) is a balanced configuration in which either station may initialize, supervise, recover from errors, and send frames at any time. There is no master/slave relationship. The DTE ([[data terminal equipment]]) and DCE ([[data circuit-terminating equipment]]) are treated as equals. The initiator for Asynchronous Balanced Mode sends an SABM.

An additional link configuration is ''Disconnected mode''.  This is the mode that a secondary station is in before it is initialized by the primary, or when it is explicitly disconnected.  In this mode, the secondary responds to almost every frame other than a mode set command with a "Disconnected mode" response.  The purpose of this mode is to allow the primary to reliably detect a secondary being powered off or otherwise reset.