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A softswitch is a central device in a telecommunications network which connects telephone calls from one phone line to another, typically via the internet, entirely by means of software running on a computer system. This work was formerly carried out by hardware, with physical switchboards to route the calls.

Although the term softswitch is sometimes used to refer to any such device, it is more commonly applied to a device that handles IP-to-IP phone calls, while the phrase "access server" or "media gateway" is used to refer to devices that either originate or terminate traditional "land line" (hard wired) phone calls. Often times, in practice, such devices can do both. As a practical distinction, a Skype to Skype phone call is entirely IP (internet) based, and so uses a softswitch somewhere in the middle connecting the calling party with the called party. In contrast, access servers might take a mobile call or a call originating from a traditional phone line, convert it to IP traffic, then send it over the internet to another such device which terminates the call by reversing the process and converting the IP call back to ISDN digital or analog and connecting it to a destination phone number.

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.

The Call Agent takes care of functions including billing, call routing, signalling, call services 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 (voice and data) in the call. It may have interfaces to connect to traditional PSTN networks like DS1 or DS3 ports (E1 or STM1 in the case of non-US networks), it may have interfaces to connect to ATM and IP 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.

The softswitch generally resides in a building owned by the telephone company called a central office. The central office will have telephone trunks to carry calls to other offices owned by the telecommunication company and to other telecommunication companies (aka the Public Switched Telephone Network or PSTN).

Looking towards the end users from the switch, the Media Gateway may be connected to several access devices. These access devices can range from small Analog Telephone Adaptors (ATA) which provide just one RJ11 telephone jack to an Integrated Access Device (IAD) or PBX which may provide several hundred telephone connections.

Typically the larger access devices will be located in a building owned by the telecommunication company near to the customers they serve. Each end user can be connected to the IAD by a simple pair of copper wires.

The medium sized devices and PBXs will typically be used in a business premises and the single line devices would probably be found in residential premises.

At the turn of the 21st century with IP Multimedia Subsystem (or IMS), the Softswitch element is represented by the Media Gateway Controller (MGC) element, and the term "Softswitch" is rarely used in the IMS context, rather it is called AGCF (Access Gateway Control Function). more...