The term soft switch is not defined in a standard… meaning that marketing departments at different equipment and software manufacturers use the same term to describe different things.
A switch, in its simplest form, is a device that causes communications to happen from one point to one other particular point, often when there are multiple “other” points to choose from.
A traditional Central Office (CO) telephone switch might be called a “hard” switch, since it has physical line cards that terminate loops. The switching software running on the computer which is the CO switch directs traffic between a line card and a trunk or between two line cards during a phone call.
The term soft switch is used to mean a computer running switching software that does not have telephone line cards – the communications are instead directed to the correct destination by routers routing packets, a software function.
As there are different approaches for the architecture – centralized vs. distributed switching, for example – and many, many vendors of products, there are many implementation variations and a plethora of jargon and buzzwords in this area. Soft switches are deployed by carriers to provide network-based services, and by end-user business customers to do it themselves.
Different standards for call setup define terms: the dominant Session Initiation Protocol (SIP) uses the terms proxy and back-to-back user agent; the older H.323 defines a gatekeeper. Terms used by product manufacturers for their products that might implement these and other functions include call manager, call server, VoIP switch, communication server and hosted PBX.
Regardless of what it is called, there are in general two main functions that may be performed by a soft switch: terminal control and call control.
Terminal control includes registration, admission and status. Registration means authenticating a telephone (or telephone client software) and associating the telephone and its IP address with a user in a directory. Admission means controlling whether that telephone is permitted to make or receive calls. Status is keeping track of the current status of the telephone, client software and/or user as an input to processing call requests.
Call control can involve a number of different functions, including address resolution, call routing, call signaling and call accounting. Address resolution means determining the numeric IP address of the called telephone. Call routing in an IP telephone network boils down to determining the IP address of the next hop for the phone call – usually a router – based on the destination address and the available networks and services. Call signaling includes initiating and terminating the phone call by exchanging control messages with a far-end device. This often includes negotiation of the multimedia attributes and coding protocol for the communications. The soft switch may also use the signaling and other information to generate Call Detail Records as an input to a call accounting system.
In addition to these basic functions, specific products may include hundreds of other functions such as those needed for call centers.
Source: Teracom’s Course 130: Understanding Voice over IP, pages 1.03 and 1.04