Softswitch
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A switch is a hardware device used to break or open an electric circuit or to divert electric current from one conductor to another.
In the context of a telephone system, a switch is the device which connects calls from one phone line to another. Now this work which used to be carried out by hardware can be done by software running on a computer system. This behaviour achieved by software is called a softswitch.
A Softswitch is typically used to control connections at the junction point between circuit and packet networks. A single device containing both the switching logic and the switching fabric can be used for this purpose; however, modern technology has led to a preference for decomposing this device into a Call Agent and a Media Gateway. Well known manufacturers of Softswitches include Lucent, Nortel, Siemens, Sonus, Alcatel, Santera, Data Connection and Cisco.
The Call Agent takes care of functions like billing, call routing, signalling and so on and is the 'brains' of the outfit. A Call Agent may control several different Media Gateways in geographically dispersed areas over a TCP/IP link.
The Media Gateway connects different types of digital media stream together to create an end-to-end path for the media (sounds) in the call. It may have interfaces to connect to traditional PSTN networks including DS1 or DS3 ports, it may have interfaces to connect to ATM networks and in the modern system will have ethernet interfaces to connect VoIP calls. The call agent will instruct the media gateway to connect media streams between these interfaces to connect the call - all transparently to the end-users.