Internet Protocol (IP) telephony differs from digital telephony in that it transmits digitised voice data via networks originally built for data transmission, not for voice transmission. At its basic level, IP is a set of rules that allows computer networks to communicate with other, with IP telephony building on these rules. IP is based on internet protocols, not telephony protocols. This means that IP networks use the same rules that govern the World Wide Web, not those that govern standard circuit-switched PABX telephony.
SIP is an extension of the IP protocol. Essentially, SIP is the future standard for communications of all kinds via a data network : SIP sets up, handles and ends “sessions” over IP networks. A session could be a simple two-way telephone call or it could be a collaborative multi-media conference session. The ability to establish these sessions means that a host of innovative services become possible, such as voice-enriched e-commerce, web page click-to-dial and Instant Messaging.
Over the last couple of years, the Voice over IP community has adopted SIP as its protocol of choice for signalling and SIP is now a specification of the International Engineering Task Force (IETF). .
Like the Internet, SIP is easy to understand, extend and implement. In telephony this means that all communication is managed by the network, from a simple phone call through to voice-enriched eCommerce, web page click-to-dial and instant messaging. SIP users may locate and contact one another-regardless of media content or number of participants. SIP extends the open-standards spirit of the Internet to messaging, enabling disparate computers, phones, televisions and software to communicate with each other.