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15.3 CONNECTIONLESS SCCP 481SCCP-D receives the message from MTP-D and passes it to TCAP-D in anN-unitdata indication, which includes the calling address PC ¼ a, SSN ¼ q.Finally, TCAP-D delivers the message to R800-D.Response Message. R800-D uses the received CGA (PC ¼ a, SSN ¼ q) as theCDA for its response message. SCCP-D passes the message to MTP-D, in aMTP-transfer request that includes DPC ¼ a. The transfer of the message toSCCP-A is done by MTPs exclusively (no GT translation needed). At exchangeA, subsystem number SSN ¼ q in the CDA is used to deliver CGA and therouting number for the call (in the subsystem data field) to Q800-A. Exchange Athen routes the call to its destination.The transaction between Q800-A and R800-D requires just two messages. This isthe case for most transactions. However, after the second message, both R800-D andQ800-A know each other’s PC þ SSN address. If a transaction requires additionalmessages, the called addresses in these messages are always PC þ SSN addressesand can be transferred by MTPs along the signaling route (no GT translationrequired).15.3.5 Final and Intermediate GT TranslationsIn the above example, SCCP-E makes a final GT translation, which yields thePC þ SSN address of destination R800-D. This is possible because a SCCP in asignal transfer point has the necessary data to perform final translations for all destinationsin its network, and we have assumed that all entities in Fig. 15.3-4 are in thesame network.When the originating and destination ASEs are in different networks, the SCCPsin the originating network have no data to do final GT translations. In this case, themessage transfer involves one or more intermediate GT translations to routethe message to a STP in the destination network, and a final GT translation by theSCCP at that STP.Figure 15.3-5 shows an example. We consider long-distance networks incountries 1 and 2 and explore the transfer of an initial message by originatingASE-A to destination. ASE-E.Each national network has international STPs (STPI), whose SCCPs can do finalGT translations for destinations in their respective countries, and intermediate GTtranslations to route a message to a STPI in the destination country.In the figure, the point codes of signaling points in countries 1 and 2 are denotedby la, lb, 1c, 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 internationalcalling-card verification transaction. An operator at exchange A has received acall from a caller who has a calling card issued in country 2 and wants the call tobe charged to the card. We denote the calling card number by 2n. Before extendingthe call setup, the operator needs to verify the validity of the calling card. Informationabout the validity of 2n is available only in country 2 (at ASE-E).