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

Signaling System No. 7

By Lee Dryburgh and Jeff Hewett

Chapter 5. The Public Switched Telephone Network (PSTN)

The term Public Switched Telephone Network (PSTN) describes the various equipment and interconnecting facilities that provide phone service to the public. The network continues to evolve with the introduction of new technologies. The PSTN began in the United States in 1878 with a manual mechanical switchboard that connected different parties and allowed them to carry on a conversation. Today, the PSTN is a network of computers and other electronic equipment that converts speech into digital data and provides a multitude of sophisticated phone features, data services, and mobile wireless access.

At the core of the PSTN are digital switches. The term "switch" describes the ability to cross-connect a phone line with many other phone lines and switching from one connection to another. The PSTN is well known for providing reliable communications to its subscribers. The phrase "five nines reliability," representing network availability of 99.999 percent for PSTN equipment, has become ubiquitous within the telecommunications industry.

This chapter provides a fundamental view of how the PSTN works, particularly in the areas of signaling and digital switching. SS7 provides control signaling for the PSTN, so you should understand the PSTN infrastructure to fully appreciate how it affects signaling and switching. This chapter is divided into the following sections:

  • Network Topology
  • PSTN Hierarchy
  • Access and Transmission Facilities
  • Network Timing
  • The Central Office
  • Integration of SS7 into the PSTN
  • Evolving the PSTN to the Next Generation

We conclude with a summary of the PTSN infrastructure and its continuing evolution.

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