Class 5 telephone switches
From Wikipedia, the free encyclopedia
A Class 5 switch, in United States telephony jargon refers to a telephone switch or exchange 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.
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[edit] 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 a hierarchy containing five levels (or 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 centres within 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 in great part by changes in the power of switches and the relative cost of transmission, both of which tended to flatten the switch hierarchy.
[edit] Class 5 switch
The fundamental difference between a Class 5 and the other classes of exchange is that a Class 5 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 will cover an area of a city, an individual town, or several villages and could serve from several hundred to 100,000 subscribers.
In the UK network, a Class 5 switch would be known as Digital Local Exchanges (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.
[edit] Hardware
When the office classification system for DDD was set up, the principal designs in use for Class 5 in the USA 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 the above were removed in the late 20th Century, mostly replaced in North America by DMS-100 and 5ESS switches in the Bell operating territories and the GTD-5 EAX in the GTE operating areas. Principal European products include Siemens EWSD and Ericsson AXE telephone exchange.
