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1-2 Introduction
Any of the 24 DS-0 channels in a DS-1 trunk can be a D-channel, but
convention designates the DS-0 in the 24th position as the D-channel.
Q.931 signaling protocol
The standardized, all-digital characteristics of PRI support Q.931. PRI
combined with Q.931 makes up a common channel signaling system using a
protocol that fits into the OSI model. Q.931 is a message-oriented,
out-of-band signaling protocol that is capable of providing telephony, data,
and supplementary services. Q.931 defines the content of messages used to
control calls over ISDN interfaces.
Protocol overview
A set of protocols governs the format, timing, and sequencing between two
network terminal stations. Figure 1-1 is a simplified view of the role that
protocol plays.
Figure 1-1
Protocol view
The architecture used for ISDN protocol consists of four modular levels,
based on the seven-layer open systems interconnection (OSI) reference
model. (This model, developed by the International Standards Organization
in 1983, was adopted by CCITT.) The OSI defines consistent languages and
communication boundaries for protocols so that systems conforming to its
rules are "open" to each other. Figure 1-2 illustrates the hierarchy of
functional levels defined within the OSI reference model. Each successive
layer represents a higher level of system supervision.
OSI protocol layers
Each of the seven OSI layers uses a series of services provided by the layer
below it. Each layer builds on those services to perform communication
functions and furnish a series of services to the layer above it. This layered
approach splits the complex communication requirements of a wide variety
of systems into a series of derivative blocks. Although connected, each
block exists independent of the others.
297-2521-106 Standard 23.01 March 2001
Terminal
station
Protocols
Communications
network
Terminal
station
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