Editing High-Level Data Link Control (section)

== Types of stations (computers) and data transfer modes ==
[[Synchronous Data Link Control]] ([[Synchronous Data Link Control|SDLC]]) was originally designed to connect one computer with multiple peripherals via a [[multidrop bus]].  The original "normal response mode" is a primary-secondary mode where the computer (or '''primary terminal''') gives each peripheral ('''secondary terminal''') permission to speak in turn.  Because all communication is either to or from the primary terminal, frames include only one address, that of the secondary terminal; the primary terminal is not assigned an address.  There is a distinction between '''commands''' sent by the primary to a secondary, and '''responses''' sent by a secondary to the primary, but this is not reflected in the encoding; commands and responses are indistinguishable except for the difference in the direction in which they are transmitted.

'''Normal response mode''' allows the secondary-to-primary link to be shared without [[Resource contention|contention]], because it has the primary give the secondaries permission to transmit one at a time.  It also allows operation over [[half-duplex]] communication links, as long as the primary is aware that it may not transmit when it has permitted a secondary to do so.

'''Asynchronous response mode''' is an HDLC addition<ref>{{harv|Friend|Fike|Baker|Bellamy|1988|p=191}}</ref> for use over [[full-duplex]] links.  While retaining the primary/secondary distinction, it allows the secondary to transmit at any time.  Thus, there must be some other mechanism to ensure that multiple secondaries do not try to transmit at the same time (or only one secondary).

'''Asynchronous balanced mode''' adds the concept of a ''combined terminal'' which can act as both a primary and a secondary.  Unfortunately, this mode of operation has some implementation subtleties.  While the most common frames sent do not care whether they are in a command or response frame, some essential ones do (notably most unnumbered frames, and any frame with the P/F bit set), and the address field of a received frame must be examined to determine whether it contains a command (the address received is ours) or a response (the address received is that of the other terminal).

This means that the address field is not optional, even on point-to-point links where it is not needed to disambiguate the peer being talked to.  Some HDLC variants extend the address field to include both source and destination addresses, or an explicit command/response bit.