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Asoftswitch (software switch) is a call-switching node in atelecommunications network, based not on the specialized switching hardware of the traditionaltelephone exchange, but implemented in software running on a general-purpose computing platform. Like its traditional counterparts it connectstelephone calls between subscribers or other switching systems across a telecommunication network. Often a softswitch is implemented to switch calls usingvoice over IP (VoIP) technologies,[1] but hybrid systems exist.
Although the termsoftswitch technically refers to any such device, it is conventionally applied to a device that handles IP-to-IP phone calls, while the phraseaccess server or "media gateway" is used to refer to devices that either originate or terminate traditionalland line phone calls. In practice, such devices can often do both. An access server might take a mobile call or a call originating from a traditional telephone line, convert it to IP traffic, then send it over an IP network to another such device, which terminates the call by reversing the process and converting thevoice over IP call to circuit-switched digital systems using traditional digitaltime-division multiplexing (TDM) or analogPOTS protocols.
The call agents are the software switching elements of the softswitch. Other components handle functions for billing, directory services, network signaling. The network elements that convert voice streams between VoIP links and traditional media technologies, such as analog telephone lines, pair-gain devices, carrier systems, are called media gateways. A call agent may control many different media gateways in geographically dispersed areas via an IP network.
The softswitch generally resides in a building owned by atelephone company, called atelephone exchange or central office, or in adata center. Such locations have high capacity connections to carry telephone calls or digital communication to other switching centers.
Access devices to the services of a softswitch range from large media gateways with high port density tointegrated access devices (IAD) at office locations, to small analog telephone adaptors (ATA) which provide just oneRJ11 telephone jack to a residence. Embedded multimedia terminal adapters (eMTAs) are also built into cable television modems.
A softswitch routes telephone calls using theSignalling System No. 7 (SS7) network. SS7 modules may be implemented directly in the softswitch, or accessed from standalone signaling servers.
At the turn of the 21st century withIP Multimedia Subsystem (IMS), the softswitch element is represented by themedia gateway controller (MGC) element, while the termsoftswitch is rarely used in the IMS context, where it is called anaccess gateway control function (AGCF).
VoIP softswitches are subdivided into Class 4 and Class 5 systems, in analogy to the traditional functions in thepublic switched telephone network.
Softswitches used for transit VoIP traffic between carriers are usually called Class 4 softswitches. Analogous with otherClass 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 ofconcurrent calls.
Class 5 softswitches are intended to serve subscribers. Class 5 softswitches are characterized by additional services for end-users and corporate clients such asIP PBX features, call center services,calling card platform, types of authorization, QoS, Business Groups and other features similar to otherClass 5 telephone switches.
