SS7 can employ different types of signaling network structures. The choice between these different structures can be influenced by factors such as administrative aspects and the structure of the telecommunication network to be served by the signaling system.
The worldwide signaling network has two functionally independent levels:
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International
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National
This structure makes possible a clear division of responsibility for signaling network management. It also lets numbering plans of SS7 nodes belonging to the international network and the different national networks be independent of one another.
SS7 network nodes are called signaling points (SPs). Each SP is addressed by an integer called a point code (PC). The international network uses a 14-bit PC. The national networks also use a 14-bit PC—except North America and China, which use an incompatible 24-bit PC, and Japan, which uses a 16-bit PC. The national PC is unique only within a particular operator’s national network. International PCs are unique only within the international network. Other operator networks (if they exist) within a country also could have the same PC and also might share the same PC as that used on the international network. Therefore, additional routing information is provided so that the PC can be interpreted correctly—that is, as an international network, as its own national network, or as another operator’s national network. The structure of point codes is described in Chapter 7, “Message Transfer Part 3 (MTP3).”
Signaling Links and Linksets
SPs are connected to each other by signaling links over which signaling takes place. The bandwidth of a signaling link is normally 64 kilobits per second (kbps). Because of legacy reasons, however, some links in North America might have an effective rate of 56 kbps. In recent years, high-speed links have been introduced that use an entire 1.544 Mbps T1 carrier for signaling. Links are typically engineered to carry only 25 to 40 percent of their capacity so that in case of a failure, one link can carry the load of two.
To provide more bandwidth and/or for redundancy, up to 16 links between two SPs can be used. Links between two SPs are logically grouped for administrative and load-sharing reasons. A logical group of links between two SP is called a linkset. Figure 4-2 shows four links in a linkset.
A number of linksets that may be used to reach a particular destination can be grouped logically to form a combined linkset. For each combined linkset that an individual linkset is a member of, it may be assigned different priority levels relative to other linksets in each combined linkset.
A group of links within a linkset that have the same characteristics (data rate, terrestrial/satellite, and so on) are called a link group. Normally the links in a linkset have the same characteristics, so the term link group can be synonymous with linkset.
Routes and Routesets
SS7 routes are statically provisioned at each SP. There are no mechanisms for route discovery. A route is defined as a preprovisioned path between source and destination for a particular relation. Figure 4-3 shows a route from SP A to SP C.
Tags: C7, network, signaling, SS7, telecommunication
