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Voice over IP (VoIP) uses the Internet Protocol (IP) to convey voice as packets over an IP network. Using VoIP protocols, voice communications can be achieved on whichever IP network anyway it is Internet, Intranets or Local Area Networks (LAN). In a VoIP enabled network, the voice signal is digitized, compressed and converted to IP packets and then transmitted over the IP network. VoIP signaling protocols are used to system and tear down calls, carry information required to locate users and negotiate capabilities. The key benefits of Internet telephony (voice over IP) are the very squat cost, the integration of data, voice and video on one network, the new services created on the converged network and simplified management of end user and terminals.
There are a few VoIP protocol stacks which are derived from a variety of standard bodies and vendors, namely H.323, SIP, MEGACO and MGCP.
H.323 is the ITU-T's customary, which was originally developed for multimedia conferencing on LANs, but was later extended to cover Voice over IP. The standard encompasses both point to point communications and multipoint conferences. H.323 defines four logical components: Terminals, Gateways, Gatekeepers and Multipoint Control Units (MCUs). Terminals, gateways and MCUs are known as endpoints.
Session Initiation Protocol (SIP) is the IETF's regular for establishing VoIP connections. SIP is an application layer control protocol for creating, modifying and terminating sessions with one or more participants. The architecture of SIP is similar to that of HTTP (client-server protocol). Requests are generated by the client and sent to the server. The server processes the requests and then sends a response to the client. A request and the responses for that request make a operation.
Media Gateway Control Protocol (MGCP) is a Cisco and Telcordia proposed VoIP protocol that defines communication between call control elements (Call Agents or Media Gateway) and telephony gateways. MGCP is a control protocol, allowing a central coordinator to observe events in IP phones and gateways and instructs them to send media to specific addresses. In the MGCP architecture, The call control intelligence is located outside the gateways and is handled by the call control elements (the Call Agent). Also the call control elements (Call Agents) will bring into line with each other to send sound commands to the gateways under their control.
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