Softswitch
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A softswitch is a central device in a telephone network which connects calls from one phone line to another, 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.
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 like 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 telephone company and to other telephone 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 telephone 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.
In more recent times (i.e., the 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.
[edit] Feature server as a part of softswitch
The feature server, often built into a call agent/softswitch, is the functional component that provides call-related features. Capabilities such as call forwarding, call waiting, and last call return, if implemented in the network, are implemented in the feature server. The feature server works closely with the call agent, and may call upon the media server to provide these services. These features do not require the subscriber to explicitly request them but tend to be triggered within the call handling logic.
An example of a feature service is last call return, in which the user picks up the phone, dials *69, and hears, “The number that last called you was xxx-xxx-xxxx. Press 1 to return this call.” When the call agent sees the dial string *69, it triggers an invocation of the feature server function. The feature server examines its database, finds the user and the caller identification of the last call, then asks the media server to play the announcement and collect a digit. When the media server returns a “1”, the feature server instructs the call agent to establish a call between the user and the party that last called that user.
