Voice over IP (VOIP) refers to using a group of technologies to transmit voice, as well as video signals, as packets over an IP network. VOIP is replacing the traditional PBX and PSTN technologies to become the main stream in corporate and public telecommunications. There are a few VOIP architectures, derived by various standard bodies and vendors that are based on different signaling protocol stacks, namely H.323, SIP, and MGCP/MEGACO/H.248. The following table outlines the main characters of the three VOIP architectures:

H.323 is the ITU-T’s standard, 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 standard 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 transaction.

The Media Gateway Control protocol (Megaco) is a result of joint efforts of the IETF and the ITU-T (ITU-T Recommendation H.248). Megaco/H.248, based on the Media Gateway Control Protocol (MGCP), is designed to provide an architecture where call control and services could be centrally added to a VoIP network. Megaco/H.248 allows companies or service providers to build large-scale networks that are scalable, resilient, and redundant. It provides mechanisms for interconnecting with other VoIP networks and for adding intelligence and features to the call agent or media gateway controller. In the Megaco/H.248 architecture, the call control intelligence is located outside the gateways and is handled by the media gateway controller(or Call Agent). Also, the call agent will synchronize with each other to send coherent commands to the gateways under their control. CableLab has adopted the MGCP for its PacketCable embed clients in VOIP applications and the resulted protocol is called Network Based Signaling Protocol (NCS). Megaco/H.248 is essentially quite similar to MGCP from an architectural standpoint and the controller-to-gateway relationship but Megaco/H.248 supports a broader range of networks.

VOIP Architecture

Related Terms: VOIP Architecture, VOIP Technologies, SIP, H.323, MGCP, H.248, Megaco, VOIP, PSTN, PBX

Reference Links: http://www.javvin.com/protocolVOIP.html: VOIP Protocols