This section provides a brief overview of how the SS7 architecture is applied to the PSTN. Since SS7 has not been presented in great detail, the examples and information are brief and discussed only in the context of the network nodes presented in this section.
The PSTN existed long before SS7. The network’s general structure was already in place, and it represented a substantial investment. The performance requirements mandated by the 800 portability act of 1993 was one of the primary drivers for the initial deployment of SS7 by ILECs in the United States. IXCs embraced SS7 early to cut down on post-dial delay which translated into significant savings on access/egress charges. Federal regulation, cost savings, and the opportunity to provide new revenue generating services created a need to deploy SS7 into the existing PSTN.
SS7 was designed to integrate easily into the existing PSTN, to preserve the investment and provide minimal disruption to the network. During SS7′s initial deployment, additional hardware was added and digital switches received software upgrades to add SS7 capability to existing PSTN nodes. In the SS7 network, a digital switch with SS7 capabilities is referred to as a Service Switching Point (SSP). When looking at the SS7 network topologies in later chapters, it is important to realize that the SSP is not a new node in the network.
Instead, it describes an existing switching node, to which SS7 capabilities have been added. Similarly, SS7 did not introduce new facilities for signaling links, but used timeslots on existing trunk facilities. PSTN diagrams containing End Offices and tandems connected by trunks represent the same physical facilities as those of SS7 diagrams that show SSP nodes with interconnecting links. The introduction of SS7 added new nodes, such as the STP and SCP; however, all of the switching nodes and facilities that existed before SS7 was introduced are still in place.
