ISDN messages and information elements for usage allocation

Last Updated : Oct 30, 2012 |
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Using Call-by-Call Service Selection, the system can specify service types on a call-by-call basis. To specify service types, you assign incoming calls to an ISDN Call-By-Call trunk group based on the number of the called party.
You can also specify service types in a SETUP message. This message indicates that the originating system intends to use the specified service or facility to initiate a call. The SETUP message can contain units called Information Elements (IE) that specify call-related information. Call-by-Call Service Selection uses the following IEs:

Network-Specific Facility (NSF)

Network-Specific Facility (NSF) indicates which facilities or services are used to complete the call. NSF is usually not used outside the US and Canada.
The system also checks all incoming ISDN trunk calls for the presence of an NSF IE. If an NSF IE is present, the system ensures that the requested service is compatible with the trunk administration before the system accepts the call.
For an outgoing CBC trunk group, the system uses the service or the feature that is specified on the selected route pattern for the call to construct the NSF IE.
If an associated NSF does not exist for the specified service or feature, the system does not send an NSF IE. For example, SETUP messages for incoming and outgoing calls that are classified only by a called-party number do not contain an NSF IE.

Transit Network Selection

Transit Network Selection indicates which interexchange carrier (IXC) the system uses on an inter-LATA call.
If a call requires both the service or the feature and the IXC to be specified, the system sends the IXC information in the NSF IE, instead of in the Transit Network Selection IE.
Related links
Detailed description of Call-by-Call Service Selection