Using “D” Messages for Layer 3 • 8-1 •
The H.110 Basic Rate ISDN Board
8.0 Using “D” Messages for
Layer 3
8.1 Q.931 Messages
Call control for ISDN interfaces is handled at the Network Layer, also
known as Layer 3. The specification that defines the message set used for
call control is Q.931. Specifics for the U.S. implementation of the Basic
Rate interface can be found in the Bellcore documents Generic Guidelines
for ISDN Terminal Equipment on Basic Access Interfaces (SR-NWT-
001953) and National ISDN Generic Guidelines for ISDN Terminal
Equipment on Basic Rate Interfaces (SR-NWT-002661). These latter two
documents give details of the messaging on services of the National ISDN-1
specification which is to be supported by most switch and terminal
equipment manufacturers in the U.S.
The message set defined in Q.931 consists of a number of command
messages to establish and break down a call. These include SETUP to
initiate a call, ALERTing, CALL PROCeeding, and PROGress, to
handle the various stages involved in accepting or rejecting a call,
CONNect to complete the connection, and DISConnect, release, and
RELEASE COMplete to break down a call. Additional messages are
provided as acknowledgments, SETUP ACKnowledge and CONNect
ACKnowledge.
Features beyond the basics of making a call are supported by additional
messages. A call may be placed on hold and retrieved using the messages
HOLD and RETrieve. These messages have corresponding
acknowledgment messages, HOLD ACKnowledge and RETRIEVE
ACKnowledge, as well as rejection messages, HOLD REJect and
RETreive REJect. INFOrmation messages can be used for a variety of
purposes such as sending text for display, activating features or indicators,
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