Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Softswitch refers to an architecture for a device that supports the integration of IP telephony and the PSTN. In the NPN, the traditional circuit-switched voice network will slowly give way to a packet-oriented voice and data network based on Internet technology. See "NPN (New Public Network)."

Softswitches are an alternative form of Class 5 switch. A Class 5 switch is a big expensive telephony switch, located in central offices all over the world. It accepts dial-up telephone calls from users and creates circuits across a hierarchy of telephone switches, some local, and some regional, national, or international. Call setup and management is handled by SS7 (Signaling System 7), which runs as an out-of-band signaling protocol to control PSTN switching equipment. See "Telecommunications and Telephone Systems" for more details about this system.

Proponents of the convergence of voice and data on an all-packet network point out the superiority of the packet model. Bandwidth and QoS to support voice are quickly becoming a reality in the Internet. However, the PSTN and the Internet will coexist for some time and there will be a need for integration. For example, until convergence is complete, IP telephone users will no doubt want to connect with a PSTN telephone user, and vice versa. This means that Internet protocol devices will need to talk to SS7 devices, and vice versa. That calls for a gateway.

Proponents of convergence also point out the inflexibility of the traditional telephone switch. Wrapped up in one big device are switching, call setup and management features, and application-level calling features like caller ID and call waiting. This model has made it very difficult for the telephone company to add new features. The convergence proponents replace the Class 5 switch with a "softswitch architecture" that has these components:

• Media gateway    The media gateway is just the switch portion of the Class 5 switch. Switching technology has advanced tremendously over the past few years, and vendors can easily produce inexpensive media gateways that can handle many more calls in a smaller size than Class 5 switches.


• Media gateway controller    This component runs the call control and application-level calling features in a server that supports easy software upgrades and expansion of features. The media gateway translates between circuit-switched voice traffic and packet-based traffic. It is also called the "call agent" and it controls the media gateway.

The media gateway is a relatively inexpensive "dumb box" that translates packets into circuits and circuits into packets. As such, the media gateway has become a commodity item and is appearing in rack-mounted systems at carrier POPs that support co-location. In contrast, the media gateway controller holds all the intelligence. It supports the integration of SS7 and Internet protocols, and maintains information about traffic flows that can be used for billing purposes.

Since the media gateway and the media gateway controller are separated, a protocol is needed that allows the media gateway controller to control the media gateway. In 1999, the IETF and the ITU formally agreed to work on a single protocol, which is known as Megaco/H.248. The ITU has largely taken over this development as H.248. This is discussed further under "Voice over IP (VoIP)." A related protocol is SIP (Session Initiation Protocol), an application layer control protocol for setting up, maintaining, and terminating voice and videoconferencing sessions. It allows different media gateway controllers to communicate and allows end users to request services from media gateway controllers. See "SIP (Session Initiation Protocol)."

An example softswitch is the Alcatel 1000, which is designed to support the gradual migration from voice-centric to data-centric environments. It provides the brains for a converged voice and data network, handles call setup and establishes control paths, controls the trunking gateways that convert TDM signals (PSTN voice calls) to voice over IP, and supports all services in the existing PSTN IN (Intelligent Network).

The Softswitch Consortium Web site has a list of product vendors. This topic continues under "Voice over IP (VoIP)." A full set of Web links is also located under that topic. Also see "Voice/Data Networks."

The IETF PINT Working Group has addressed the arrangement through which Internet applications can request PSTN services. The IETF SPIRITS (Service in the PSTN/IN Requesting InTernet Service) Working Group has addressed the opposite arrangement in which PSTN users request services that require an interaction between the PSTN and the Internet. Some examples covered by SPIRITS include Internet call waiting, Internet caller-ID delivery, and Internet call forwarding. See "SPIRITS (Service in the PSTN/IN Requesting InTernet Service)" and "PINT (PSTN and Internet Internetworking)" for more information.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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