N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 11 - The PSTN Evolution and Convergence                                                                               Slide 3 of 45                                                                                                    ________________

         The Public Switched Telephone System (PSTN)
Switching Hieararchy

 
                                            Class 5 End Office

                                            Class 4 Toll center

                                            Class 3 Primary Center

                                            Class 2 Sectional Center

                                            Class 1 Regional Center

                                                                                                   ________________

A PSTN is a complex switching network including hierarchical switching structure. The above diagram shows pre-1984 Bell Systems switching architecture. The network switching architecture is a pyramid.

The North American switching system includes regional offices (10 in the United States and 2 in Canada) and many sectional, Primary, Toll and class 5 switches. All customer's local-loops terminate in a Class 5 office. Toll offices complete calls between class 5 switches and do not connect to subscribers. Primary offices switch Toll office traffic while a Sectional office switches traffic between Primary offices.