network protocols handbook.pdf

119Protocols GuideVoice over IP and VOIP ProtocolsDescriptionVoice over IP (VOIP) uses the Internet Protocol (IP) to transmit voice aspackets over an IP network. Using VOIP protocols, voice communicationscan be achieved on any IP network regardless whether it is Internet, Intranetor Local Area Networks (LAN). In a VOIP enabled network, the voice signalis digitized, compressed and converted to IP packets and then transmittedover the IP network. VOIP signaling protocols are used to set up and teardown calls, carry information required to locate users and negotiate capabilities.The key benefits of Internet telephony (Voice over IP) are the verylow cost, the integration of data, voice and video on one network, the newservices created on the converged network and simplified management ofend user and terminals.There are a few VOIP protocol stacks which are derived by various standardbodies and vendors, namely H.323, SIP, MEGACO and MGCP.H.323 is the ITU-T’s standard, which was originally developed for multimediaconferencing on LANs, but was later extended to cover Voice over IP. Thestandard encompasses both point to point communications and multipointconferences. H.323 defines four logical components: Terminals, Gateways,Gatekeepers and Multipoint Control Units (MCUs). Terminals, gateways andMCUs are known as endpoints.Session Initiation Protocol (SIP) is the IETF’s standard for establishing VOIPconnections. SIP is an application layer control protocol for creating, modifyingand terminating sessions with one or more participants. The architectureof SIP is similar to that of HTTP (client-server protocol). Requests are generatedby the client and sent to the server. The server processes the requestsand then sends a response to the client. A request and the responses for thatrequest make a transaction.Media Gateway Control Protocol (MGCP) is a Cisco and Telcordia proposedVOIP protocol that defines communication between call control elements(Call Agents or Media Gateway) and telephony gateways. MGCP is a controlprotocol, allowing a central coordinator to monitor events in IP phones andgateways and instructs them to send media to specific addresses. In theMGCP architecture, The call control intelligence is located outside the gatewaysand is handled by the call control elements (the Call Agent). Also thecall control elements (Call Agents) will synchronize with each other to sendcoherent commands to the gateways under their control.The Media Gateway Control Protocol (Megaco) is a result of joint efforts ofthe IETF and the ITU-T (ITU-T Recommendation H.248). Megaco/H.248 is aprotocol for the control of elements in a physically decomposed multimediagateway, which enables separation of call control from media conversion.Megaco/H.248 addresses the relationship between the Media Gateway(MG), which converts circuit-switched voice to packet-based traffic, and theMedia Gateway Controller, which dictates the service logic of that traffic.Megaco/H.248 instructs an MG to connect streams coming from outside a