When transit network selection is supported, the user identifies the selected transit network(s) in the SETUP message. One Transit network selection information element is used to convey a single network identification.
The user may specify more than one transit network. Each identification is placed in a separate information element. The call would then be routed through the specified transit networks in the order listed in the SETUP. For example, a user lists networks A and B, in that order,in two Transit network selection information elements within a SETUP message.The call is first routed to network A (either directly or indirectly), and then to network B (either directly or indirectly), before being delivered.
As the call is delivered to each selected network, the corresponding transit selection may be stripped from the call establishment signalling, in accordance with the relevant internetwork signalling arrangement. The Transit network selection information element(s)is/are not delivered to the destination user.
No more than four Transit network selection information elements may be used in a single SETUP message.
When a network cannot route the call because the route is busy, the network shall initiate call clearing in accordance with5.3 with cause No. 34, no circuit/channel available.
If a network does not recognize the specified transit network, the network shall initiate call clearing in accordance with 5.3, with cause No. 2, no route to specified transit network. The diagnostic field shall contain a copy of the contents of the Transit network selection information element identifying the unreachable network.
A network may screen all remaining Transit network. selection information elements to
a) avoid routing loops; or
b) ensure an appropriate business relationship exists between selected networks; or
c) ensure compliance with national and local regulations.
If the transit network selection is of an incorrect format, or fails to meet criteria a), b) or c), the network shall initiate call clearing in accordance with 5.3, with cause No. 91, invalid transit network. selection.
When a user includes the Transit network selection information element, pre-subscribed default Transit network selection information (if any) is overridden.