A class-4, or tandem, telephone switch is a central office telephone exchange used to interconnect local exchange carrier offices for long distancecommunications in the public switched telephone network.
A class-4 switch doesn't connect directly to any telephones; instead, it connects to other class-4 switches and to class-5 telephone switches. The telephones of service subscribers are wired to class-5 switches. When a call is placed to a telephone that is not on the same class-5 switch as the subscriber, the call may be routed through one or more class-4 switches to reach its destination.
a class-4 switch that connects class-5 switches to the long-distance network is called an "access tandem." A class-4 switch that connects class-5 switches to each other, but not to the long-distance network, is called a "local tandem."
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 servingsubscribers. 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.
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).
Softswitches are also subdivided into two classes. Class 4 softswitches and Class 5 softswitches.
Softswitches used for transit VoIP traffic between carriers are usually called class 4 softswitches. Analogous with other Class 4 telephone switches, the main function of the class 4 softswitch is the routing of large volumes of long distance VoIP calls. The most important characteristics of class 4 softswitch are protocol support and conversion, transcoding, calls per second rate, average time of one call routing, number of concurrent calls.
Class 5 softswitches are intended for work with end-users. These softswitches are both for local and long distance telephony services. Class 5 softswitches are characterized by additional services for end-users and corporate clients such as IP PBX features, call center services, calling card platform, types of authorization,QoS,Business Groups and other features similar to other Class 5 telephone switches.