Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services

Signaling System No. 7

By Lee Dryburgh and Jeff Hewett

Integration of SS7 into the PSTN

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. Figure 5-12 shows a simple view of the PSTN, overlaid with SS7-associated signaling capabilities.


Figure 5-12 SS7 Overlaid onto the PSTN

View a in the previous figure shows that trunk facilities provide the path for voice and in-band signaling. View b shows the SS7 topology using simple associated signaling for all nodes. View c shows the actual SS7-enabled PSTN topology. The existing switching nodes and facilities are enhanced to provide basic SS7 call processing functionality. Although this associated signaling architecture is still quite common in Europe, the United States primarily uses a quasi-associated signaling architecture.

SS7 Link Interface

The most common method for deploying SS7 links is for each link to occupy a timeslot, such as a T1 or E1, on a digital trunk. As shown in Figure 5-12, the signaling links actually travel on the digital trunk transmission medium throughout the network. At each node, the SS7 interface equipment must extract the link timeslot from the digital trunk for processing. This process is typically performed using a channel bank, or a Digital Access and Cross-Connect (DAC), which demultiplexes the TDM timeslot from the digital trunk. The channel bank, or DAC, can extract each of the timeslots from the digital stream, allowing them to be processed individually. The individual SS7 link provides the SS7 messages to the digital switch for processing. While implementations vary, dedicated peripheral processors usually process the lower levels of the SS7 protocol (Level 1, Level 2, and possibly a portion of Level 3); call- and service-related information is passed on to the central processor, or to other peripheral processors that are designed for handling call processing–related messages. Of course, this process varies based on the actual equipment vendor.

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