Session Initiation Protocol (SIP) is a signaling protocol used to establish session between pairs. It allows to exchange information about session’s description through the Session Description Protocol (SDP).
The signaling protocol manages the user’s registration and his location. It is also a mechanism to establish and destroy connections end-to-end by some request message. It also routes response from destination to the source in order to ask to the caller.
SIP vs SS7
If we compare the two protocols they are designed for different kind of networks: SIP works on the IP network whereas SS7 in the PSTN networks. Because of that it is not possible to physically replace SS7 with SIP.
The two main applications of SIP are VoIP phones such as softphones and mobile VoIP with DECT protocols. However, the kill application is VoIP, which is the best-performing industry of the past decade and hard VoIP users top 100 million.
H.323 was created to enable videoconferencing capabilities over a LAN. It was quickly adopted by the industry as a means of transiting voice communication over a variety of IP networks. It was perceived as the biggest SIP competitor, but nowadays the market is divided: H.323 is used in enterprises for video conferencing systems, while SIP is used for Internet telephony voice services.
H.325 is a new multimedia system with significant improvement over legacy SIP and H.323 in the diversity of applications that will enable. Nowadays H.323 is a work in progress but we can predict an important impact of this protocol on the market.
XMPP is an open-standard protocol based on XML language. Nowadays users of Facebook, Google Talk and Windows Live Messenger use this protocol, which means than over 1200 million users are using XMPP. Because of that, SIP is a potentially great competitor of SIP.
Skype is the most popular commercial VoIP softphone which uses a private protocol: the code is obfuscated and some parts are encrypted. Nowadays it has more than 663 million users and several platforms and probably in the next few years that won’t change.