Final and Intermediate GT Translations
In the above example, SCCP-E makes a final GT translation, which yields the PC + SSN address of destination R800-D. This is possible because an SCCP in a signal transfer point has the necessary data to perform final translations for all destinations in its network, and we have assumed that all entities in Fig. 14.3-4 are in the same network.
When the originating and destination ASEs are in different networks, the SCCPs in the originating network have no data to do final GT translations. In this case, the message transfer involves one or more intermediate GT translations to route the message to an STP in the destination network, and a final GT translation by the SCCP at that STP.
Figure 14.3-5 shows an example. We consider long-distance networks in countries 1 and 2, and explore the transfer of an initial message by originating ASE-A to destination ASE-E.
Each national network has international STPs (STPI), whose SCCPs can do final GT translations for destinations in their respective countries, and intermediate GT translations to route a message to an STPI in the destination country.
In the figure, the point codes of signaling points in countries 1 and 2 are denoted by la, lb, lc, and 2d, 2e. The STPIs also have an international point code (ic, id).
We now examine the routing of the first unitdata message of an international calling-card verification transaction. An operator at exchange A has received a call from a caller who has a calling card issued in country 2, and wants the call to be charged to the card. We denote the calling card number by 2n. Before extending the call set-up, the operator needs to verify the validity of the calling card. Information about the validity of 2n is available only in country 2 (at ASE-E).
By entering the country code (cc2) of country 2 and In, the operator causes ASE-A to start the query. The ASE generates the global title address GTA = cc2-2n and translation type TT = It (indicating international calling card verification). SCCP-A sets RTI to 0, and routes the message to STP B.
Country 1 Country 2
Exchange A STP B STPIC STPID Database E
PC = la PC = lb PC = lc/PC = ic PC = id/PC = 2d PC = 2e
Exchange A STP B STPIC STPID Database E
PC = la PC = lb PC = lc/PC = ic PC = id/PC = 2d PC = 2e
Figure 14.3-5 Address information in the first SCCP unitdata message of an international transaction.
Post a comment