ISDN SIGNALING MESSAGES
Many of the VoIP Gateway operations entail the processing of ISDN signaling messages. These messages and their functions are defined in the ITU-T Q.931 Recommendation.
The ISDN Q.931 messages are used to manage ISDN connections on the B channels. These messages are also used (with modifications) by Frame Relay and ATM for setting up calls on demand at a UNI, and for provisioning services between networks at a NNI. Table B-1 lists these messages, and a short explanation is provided here about the functions of the more significant messages:
ALERTING: This message is sent to indicate that the called user party has been "alerted" and the call is being processed. This message is sent in response to an incoming SETUP message, ...
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