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 using the local loop. Class-5 switches were slower to convert from circuit switching technologies to time division multiplexing than the other switch classes.
Telephone switch hierarchy
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 USA and Canadian networks.
- Class-2 exchanges were tandem exchanges which interconnected whole 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 the various areas of a city or towns in a region.
- Class-5 exchanges were those to which subscribers and end-users telephone lines would connect.
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. 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.
Function
The fundamental difference between a class-5 and the other classes of exchange 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 dial tone – they simply route calls between other switches, so they are more concerned with efficient switching and signalling.
Typically a class-5 switch covers an area of a city, an individual town, or several villages and could serve from several hundred to 100,000 subscribers.
In the British telephone network, a class-5 switch is known as digital local exchange (DLE).
Since the replacement in the 1980s and 90s of electromechanical exchanges by modern digital ones, the function of a class-5 switch in rural areas is often performed by some form of 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.
Hardware
When the office classification system for DDD was established, the principal designs in use for class-5 in the US were step by step, panel, and crossbar. 5XB crossbar switches were introduced in large numbers in the 1950s and 60s, and 1ESS switches and variants in the 1970s and 80s. Most of the above 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 IP-based technology from various companies.