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Message structure and function
This chapter describes the signaling messages that PRI call processing
supports, and their associated information elements. Also included are the
purpose for each information element and the role of each message in call
processing. For more information on call control procedures during call
processing, see Chapter 5, "Call control procedures." For more information
on datafill, see the DMS-250 Data Schema Reference Manual.
Message structure
Signaling messages contain information elements that hold information for
call processing. An information element may be sent from the user to the
network, from the network to the user, or in both directions. Information
elements ignored by the DMS-250 switch are acceptable if received in a
Q.931 message, but are never sent by the DMS-250 switch.
Mandatory information elements
Every message contains mandatory information elements and, if required,
other information elements specific to the message. The maximum message
length is 256 octets.
The three items listed below are common to all the messages and must
always be present. (Other information elements are specific to a message
type and may or may not be required.)
These mandatory information elements are described in the following
information.
Protocol discriminator
The Protocol Discriminator information element distinguishes messages for
call control and maintenance (or service) from other messages.
Protocol Discriminator
Call Reference
Message Type
Digital Switching Systems DMS-250 ISDN Reference Manual MWC14 (CSP14)
4-1
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