A SCCP called party address (Par.l) can contain various combinations of PC, SSN and/or GT. We now explore the reasons for global titles and GT translations.
A subsystem in a network is uniquely identified by the combination of the point code PC of its signaling point, and its SSN at that point. If an SCCP message has a PC + SSN called party address, the MTPs along the message path then use PC to route the message to its destination signaling point, and the SCCP at this point delivers it to the subsystem specified by SSN. For example, in Fig. 14.3-1, a SCCP message with called party address PC = c, SSN = r, is delivered to subsystem R800 in database C.
A global title is a "functional" address of a subsystem in an exchange or database. The reason for these functional addresses is illustrated with the example of Fig. 14.3-1.
Suppose that the set-up of a call with a called 800 number, say 800-1234567, has reached exchange A. Subsystem Q800-A then has to send a query message to an R800 subsystem at a database that stores the translation for this number—in this example: databases C and D.
In principle, exchange A could translate the received 800 number into a PC 4- SSN called party address of an R800 in the appropriate database. However, this would require "800 number" translation tables in all exchanges, and would entail a large effort when an item in the tables has to be added or removed.
A better arrangement is to let the exchanges use the received 800 number as a global title (GT) address, which is the "functional" address of an R800 at a database with information on the 800 number, and install the GT translation capability in the SCCPs of the signal transfer points (STP) of the network. The SCCP at the originating exchange then routes the query message to a directly connected STP, whose SCCP translates GT into the PC + SSN address of the destination. From this point on, the message can be routed by MTP to its destination.
The number of exchanges in a network greatly exceeds the number of STPs, and placing the GT translation capability in the SCCPs at the STPs instead of in the exchanges greatly reduces the effort to update the translation data.
A GT consists of two parts, known as the GT address (GTA), which is a digit string received from a calling subscriber, and the translation type (TT), which indicates the desired translation (Fig. 14.2-2). For example, if GTA is the national number of a subscriber (S), one value of TT could indicate that GTA is to be translated into the PC + SSN address of an ASE in the maintenance center that covers S; another TT value could require a translation that yields the PC + SSN address of an ASE in the revenue accounting center that covers S, and so on.
We now examine the transfer of unitdata messages for an 800 number query— response transaction—see Fig. 14.3-1. We assume that all entities in the figure are part of the same telecommunication network.
Query Message. Suppose that the set-up of an 800 call has reached exchange A. The Q800-A then launches a query to a database, to obtain the routing number for the call. We assume that the call-routing information for this particular 800 number is stored in R800-C and R800-D (at databases C and D).
Figure 14.3-4 shows the MTP and SCCP address parameters in the primitives at signaling points A, E and D, and in the message signal unit (MSU) that carries the first unitdata message.
TCAP-A has received the calling and called address (CDA, CGA) from Q800-A, and includes them in the N-unitdata request to SCCP-A. The CDA is a GT in which the translation type = i, and the address = n (the called 800 number). The translation type indicates that n has to be translated into the SP + SSN address of an R800 at a database with information on that number.
SCCP-A enters GT in the CDA field of the unitdata message, PC = a, SSN = q in the CGA field, and sets the routing indicator (Fig. 14.2-1) to RTI = 0, indicating that a GT translation is needed. Since SCCP-A has received a GT called address, and knows that the SCCP-E can perform GT translations, it
Exchange A STP E STP H Database D
Exchange A STP E STP H Database D
includes the MTP address of signaling point E (DPC = e) when passing the message. MTP-A then forms the MSU, and transfers it to STP-E.
SCCP-E translates the GT, and obtains the addresses PC= c, SSN = r, and PC = d, SSN = r, of the R800 units at databases C and D. Assuming that SCCP-E selects database D, it enters SSN = r in the CDA of its outgoing message, and passes it to its MTP-E, in a MTP-transfer request which includes destination point code DPC = d. It also sets the routing indicator to RTI = 1. The MTP-E routes the MSU to MTP-H, which routes it to database D.
SCCP-D receives the message from MTP-D, and passes it to TCAP-D in a N-unitdata indication, which includes the calling address PC = a, SSN = q. Finally, TCAP-D delivers the message to R800-D.
Response Message. R800-D uses the received CGA (PC = a, SSN = q) as the CDA for its response message. SCCP-D passes the message to MTP-D, in a
MTP-transfer request that includes DPC = a. The transfer of the message to SCCP-A is done by MTPs exclusively (no GT translation needed). At exchange A, subsystem number SSN = q in the CDA is used to deliver CGA and the routing number for the call (in the subsystem data field) to Q800-A. Exchange A then routes the call to its destination.
The transaction between Q800-A and R-800-D requires just two messages. This is the case for most transactions. However, after the second message, both R800-D and Q800-A know each other's PC + SSN address. If a transaction requires additional messages, the called addresses in these messages are always PC + SSN addresses, and can be transferred by MTPs along the signaling route (no GT translation required).
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