Class-5 telephone switch

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A Class-5 telephone switch is a telephone switch or telephone exchange in the public switched telephone network located at the local telephone company's central office, directly serving subscribers. Class-5 switch services include basic dial-tone, calling features, and additional digital and data services to subscribers connected to a local loop.

Telephone switching hierarchy[edit]

In order to organize direct distance dialing (DDD) American Telephone & Telegraph divided the various switches in the U.S. public switched telephone network (PSTN) into an "office classification" hierarchy containing five levels (classes).

  • Class-1 exchanges were international gateways, handing off and receiving traffic from outside the US and Canadian networks.
  • Class-2 exchanges were tandem exchanges which interconnected regions of the AT&T network.
  • Class-3 exchanges were tandem exchanges connecting major population centers within a particular region of the AT&T network.
  • Class-4 exchanges were tandem exchanges connecting various areas of a city or towns in a region.
  • Class-5 exchanges provide telephone line to subscribers.

In modern times, only the terms Class-4 and Class-5 are much used, as any tandem office is referred to as a Class 4 system. This change was prompted by changes in the power of switches and the relative cost of transmission, both of which tended to flatten the switch hierarchy.


The fundamental difference between Class 5 and the other classes is that a Class-5 switch provides telephone service to customers, and as such is concerned with "subscriber type" activities: generation of dial-tone and other "comfort noises"; handling of network services such as advice of duration and charge etc. Specifically, a class-5 switch provides dial tone, local switching and access to the rest of the network. Class-4 switches do not provide subscriber lines, their role is to route calls between other switches.

Typically a Class-5 switch serves an area of a city, an individual town, or several villages and could serve from several hundred to 100,000 subscribers.

Since the replacement of electromechanical exchanges by modern digital ones, the function of a Class-5 switch in rural areas is often performed by a remote switch or Remote Digital Terminal installed at the original switch site to handle local switching or concentration, respectively. The Class-5 switching infrastructure is then physically located in a larger population center. Urban areas with extensive underground plant tend to keep the classic Class-5 office architecture.


When the office classification system for DDD was established, the principal designs in use for Class-5 in the US were Strowger-type step-by-step systems, Panel switches, and crossbar systems. 5XB crossbar switches were installed in large numbers in the 1950s and 60s, and 1ESS switches and variants starting in the 1960s. Most of these systems were removed in the late 20th century, primarily replaced in North America by DMS-10, DMS-100 and 5ESS switches in the Bell operating territories and the GTD-5 EAX in the GTE operating areas. Principal European products include Ericsson AXE telephone exchange, Siemens EWSD and Alcatel-Lucent S12 and E10.

By the turn of the century, US and European service providers continued to upgrade their networks, replacing older DMS-10, DMS-100, 5ESS, GTD-5 and EWSD switches with Voice over IP technology.

See also[edit]