Copryright SS8 Networks - 2002
Before we get deeply into the subject of Global Title, it might help to get a very broad view of exactly what Global Title is. Simply defined, it is an address. But it is not an address of a node in the SS7 network (DPC, SSN). Instead, it is an alias for such an address that needs to be translated into an SS7 network address. With that definition out of the way, let’s quickly review what we know about SS7 addressing in general. To begin with, each User Part approaches addressing in a different way. The MTP (Message Transfer Part) has a job that is limited to reliably transferring messages over the links in a link set. That is, MTP only cares about the address of the node at the other end of the links it is tending. Therefore the only addressing the MTP requires is the SPC (Signalling Point Code) of the node at the end of its links. MTP sees this address as the Destination Point Code (DPC) of all messages it sends over the links. The only concern MTP has for any other location in the network is to be able to make use of the final destination of the message to help it pick out one link set from all the available linksets as the best one to use for sending the message. This is what MTP routing is all about. ISUP addressing is different. In normal Call Control use, ISUP addresses a switch at the other end of its trunk connections. For the SS7, this too means using a Destination Point Code (DPC). But the switch ISUP talks to (which is the next switch in a circuit being set up or torn down) is not necessarily (and really not likely) to be located at the other end of its own SS7 links. The job of addressing all other locations falls to the SCCP. Actually SCCP could be used to address the same switching locations as ISUP. If it were used in this way, SCCP could be used for end-to-end signal routing in conjunction with ISUP. But since the usual switch-to-switch routing of ISUP provides complete circuit information for all switches along the voice path, SCCP is rarely used in this way. Like the other User Parts, SCCP can, and does, make use of DPC. This address alone can be used to get a message to any node in the global SS7 network in the same way that a telephone number can be used to address any telephone in the global telephone network. But SCCP addressing needs to go beyond this method of addressing. The reason is that at each DPC there is a “system” operating. That system may be a Call Control application or a database or some other program of some type.
The problem is that within that system there may be multiple applications running. Thus a Signalling Point Code (which is addressed as a Destination Point Code) may be the home of both a Credit Card Database and an 800 Number Database. Using the DPC as the SS7 address will get the message delivered to the “system” but it won’t get the message delivered to the appropriate database application. For this purpose, a separate identifier of a system within the system is required. That identifier is the Subsystem Number (SSN). It may be tempting to think of SSN as a database identifier. And, indeed, an SSN will be applied to a database even when only one database application is available at a specified DPC. But, avoid the temptation. The truth is that SSN is also used to sub-address any location at which multiple applications are running. For example, a switch offering several features may use SSN to separately identify each feature. Think of SSN as simply an application identifier. That brings us to what you came for. The third addressing mechanism employed by SCCP is the Global Title, You might say this is the address used when the location requiring information doesn’t know the address. A Global Title implies the need for translation. Before you become too confused, let’s try an example. If you dial an 800 number, the switch to which you are connected becomes more confused than you are right now. The reason is...