Figure D-2. Isdn Protocols - AT&T DEFINITY 2500 series Network Manual

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Application
Presentation
End-to-end
user
signaling
Session
Transport
Call control
Network
I.451
Data link
Physical
Signal
Adapted from: Data and Computer Communications, by William Stallings, Macmillan Publishing Co., N.Y., 1988
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.
(
X.25 Further
Packet level study
LAP-D (I.441)
Layer 1 (I.430, I.431)
Packet Telemetry
D Channel

Figure D-2. ISDN Protocols

COMMUNICATIONS PROTOCOLS
CCITT-ISO
OSI-related protocols
)
Circuit
switching
B Channel
D-9
X.25
Packet level
X.25
LAP-B
Leased Packet
circuit switching