Types of Stations (Computers), and Data Transfer Modes
Synchronous Data Link Control (SDLC) was originally designed to connect one computer with multiple peripherals. The original "normal response mode" is a master-slave 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 also a strong distinction between commands sent by the primary to a secondary, and responses sent by a secondary to the primary. Commands and responses are in fact indistinguishable; the only difference is the direction in which they are transmitted.
Normal response mode allows operation over half-duplex communication links, as long as the primary is aware that it may not transmit when it has given permission to a secondary.
Asynchronous response mode is an HDLC addition for use over full-duplex links. While retaining the primary/secondary distinction, it allows the secondary to transmit at any time.
Asynchronous balanced mode added the concept of a combined terminal which can act as both a primary and a secondary. There are some subtleties about this mode of operation; while many features of the protocol do not care whether they are in a command or response frame, some do, 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).
Some HDLC variants extend the address field to include both source and destination addresses, or an explicit command/response bit.
Read more about this topic: High-Level Data Link Control
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