Sdlc was the first link layer protocol based synchronous

C H A P T E R	16

Synchronous Data Link Control and Derivatives

SDLC supports a variety of link types and topologies. It can be used with point-to-point and multipoint links, bounded and unbounded media, half-duplex and full-duplex transmission facilities, and circuit-switched and packet-switched networks.

SDLC identifies two types of network nodes: primary and secondary. Primary nodes control the operation of other stations, called secondaries. The primary polls the secondaries in a predetermined order, and secondaries can then transmit if they have outgoing data. The primary also sets up and tears down links and manages the link while it is operational. Secondary nodes are controlled by a primary, which means that secondaries can send information to the primary only if the primary grants permission.

			0

	Poll final	Function code	1

	–
	–	Unnumbered (U) frame—Supports control purposes and is not sequenced. A U frame can be used to initialize secondaries. Depending on the function of the U frame, its control field is 1 or 2 bytes. Some U frames have an information field.
	Data—Contains a path information unit (PIU) or exchange identification (XID) information.

Fig

Derivative Pr

Th
HD

•

SDLC link

Terminals

HD LC shares the frame format of SDLC, and HDLC fields provide the same functionality as those in SDLC. Also, as in SDLC, HDLC supports synchronous, full-duplex operation.

HD LC differs from SDLC in several minor ways, however. First, HDLC has an option for a 32-bit checksum. Also, unlike SDLC, HDLC does not support the loop or hub go-ahead configurations.

A—HDLC, which supports three transfer modes, while SDLC supports only one; LAPB, which is restricted to the ABM transfer mode and is appropriate only for combined stations; IEEE 802.2, which is often referred to as LLC and has three types; and QLLC, which provides the data-link control capabilities that are required to transport SNA data across X.25 networks.

Internetworking Technologies Handbook