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A goal for SIP was to provide a superset of the call processing functions and features present in the public switched telephone network ( PSTN). As such, features that permit familiar telephone-like operations are present: dialing a number, causing a phone to ring, hearing ringback tones or a busy signal. Implementation and terminology are different.
SIP also implements many of the more advanced call processing features present in Signalling System 7 (SS7), though the two protocols themselves could hardly be more different. SS7 is a highly centralized protocol, characterized by highly complex central network architecture and dumb endpoints (traditional telephone handsets). SIP is a peer-to-peer protocol. As such it requires only a very simple (and thus highly scalable) core network with intelligence distributed to the network edge, embedded in endpoints (terminating devices built in either hardware or software). Many SIP features are implemented in the communicating endpoints as opposed to traditional SS7 features, which are implemented in the network.
Although many other VoIP signaling protocols exist, SIP is characterized by its roots in the IP community rather than the telecom industry. SIP is being standardized and governed by the IETF while older, more complex VoIP protocols were proposed by the ITU.
SIP works in concert with several other protocols and is only involved in the signaling portion of a communication session. SIP acts as a carrier for the Session Description Protocol (SDP), which describes the media content of the session, e.g. what IP ports to use, the codec being used etc. In typical use, SIP "sessions" are simply packet streams of the Real Time Transport Protocol ( RTP). RTP is the carrier for the actual voice or video content itself.
The first proposed standard version (SIP 2.0) was defined in RFC 2543. The protocol was further clarified in RFC 3261, although many implementations are still using interim draft versions. Note that the version number remains 2.0.
SIP is similar to HTTP and shares some of its design principles: it is human readable, very simple and request-response. However, some would counter that while SIP originally had a goal of simplicity, in its current state it has become as complex as H.323. It shares many HTTP status codes, such as the familiar '404 not found'. Much of the promise of SIP is that the rapid innovation and application development that has characterized the Web will now mark the telephony industry too. SIP is not limited to voice but can mediate any kind of communication session from voice to video to future, unrealized applications.
Hardware endpoints, devices with the look, feel, and shape of a traditional telephone, but that use SIP and RTP for communication, are commercially available from several vendors. Some of these can use Electronic Numbering (ENUM) to translate existing phone numbers to SIP addresses using DNS, so calls to other SIP users can bypass the telephone network, even though your service provider might normally act as a gateway to the PSTN network for traditional phone numbers (and charge you for it).
Today, software SIP endpoints are more common. Microsoft Windows MessengerWindows Messenger is an instant messaging client from Microsoft that can communicate both via SIP and. NET Messenger Service. It is pre-installed in Windows XP, and versions 5 and higher are available for download for Windows 2000. It should not be confus uses SIP and in June, 2003, Apple ComputerApple Computer, Inc. is a Silicon Valley company based in Cupertino, California, whose main business is computer technologies. Best known for its range of Macintosh computers and, more recently, its iPod personal audio ( MP3 and otherwise) player and iTun announced, and released in public betaIn software engineering, development stage terminology expresses how far through the development sequence things have progressed and how much further development a product may require. Alpha An alpha version or alpha release represents the first version o, iChat AViChat is a third party AOL Instant Messenger client written by Apple Computer that runs only in Mac OS X and which is fully endorsed by AOL. It bears a brushed metal interface and uses speech bubbles and pictures that some advocates claim personify the on, a new version of their AOL Instant Messenger compatible client that supports audio and video chat through SIP.
SIP also requires proxy and registrar network elements to work as a practical service. Although two SIP endpoints can communicate without any intervening SIP infrastructure (which is why the protocol is described as peer-to-peer), this approach is impractical for a public service. There are various softswitch implementations (by Nortel, Sonus and many more) which can act as proxy and registrar. Other companies, led by Ubiquity Sotware and Dynamicsoft have implemented products based on the proposed standards, building on the Java JAIN specification.