AT&T 7300 series, 7500 series, 7100 series, 2500 series manual Other Common Standards, Ccitt-Iso

COMMUNICATIONS PROTOCOLS D-9

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Application
Presentation	End-to-end			CCITT-ISO
	user			OSI-related protocols
Session	signaling
Transport
Network	Call control	X.25	Further			X.25
	I.451	Packet level	( study )			Packet level
Data link		LAP-D (I.441)				X.25
						LAP-B
Physical			Layer 1 (I.430, I.431)
	Signal	Packet	Telemetry	Circuit	Leased	Packet
				switching	circuit	switching

D Channel

B Channel

Adapted from: Data and Computer Communications, by William Stallings, Macmillan Publishing Co., N.Y., 1988

Figure D-2.ISDN Protocols

Other Common Standards

In addition to standards formulated by the CCITT, many other standards are common. Most notable among these non-CCITT standards are some of the most popular protocols at layers 1 and 2.

Layer 1 protocols include:

∙RS-232C — A common physical interface used to connect DTE devices to voice-grade modems for use in the public network.

∙RS-449 — A replacement specification for the RS-232C specification. It was devised to overcome the RS-232C distance restrictions and the lack of modem control that RS-232C procedures afford.

∙RS-366A — A physical interface between a DTE and automatic calling equipment for data communications.

Layer 2 protocols include:

∙SDLC (Synchronous data link control) — An IBM protocol that has a 1 byte addressing field and is capable of transmitting messages in multiple frames.

∙HDLC (High-level data link control) — A very common bit-oriented standard issued by the ISO. It features a multi-byte addressing field, but otherwise, is quite similar to SDLC, from which it was derived.

∙BSC (Binary synchronous communications) — An early IBM character-oriented, half-duplex protocol that transmits messages consisting of strings of characters. Control information is provided by special non-printing characters.