Using “D” Messages for Layer 3 • 8-3 •
The H.110 Basic Rate ISDN Board
provide a simpler interface through the Layer 3 support level for those
instances when only simple call control is needed for equipment that
conforms to NI-1.
The Layer 3 support level is achieved using “D” channel commands and
responses. These messages consist of an initial “D”, a second character
indicating the Q.931 message type, for example an “A” for an ALERTing
message or a “C” for a CONNect message, a two digit number indicating
the B channel, and for some messages additional arguments. Each of these
additional arguments are typically encoded as a single character or as a pair
of hexadecimal digits in the case of the Cause element or Call references.
The intent is to keep the messages short and simple with only the necessary
information and allow the on board firmware to fill in the details of the
actual Q.931 messages sent. The “D” messages are also similar in their
organization to other XDS messages such as the “C” and “S” messages
used to control the B-channel connections on the H.110 BRI Board and
ports on other XDS line boards. This simplifies the overall task of the
application.
“D” channel messages and the corresponding Q.931 messages are given in
the following list:
DAxx ALERTing
DCxx CONNect
DCxxA CONNect ACKnowledge
DDxx DISConnect
DFxx Feature activation or indication using an
INFOrmation message
DGxx RETrieve
DGxxA RETrieve ACKnowledge
DGxxR RETrieve REJect
DHxx HOLD
DHxxA HOLD ACKnowledge
DHxxR HOLD REJect
DIxx INFOrmation
DKxx Keypad element INFOrmation message
DNxx NOTIFY
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