VoIP Protocols - What are they, and what are they used for?
There are a number of VoIP protocols that have been developed to establish, maintain and terminate multimedia sessions such as telephony, video-conference and data exchange. As carriers seek new sources of revenue, they are forced to adopt more flexible business models that include peering with other carriers and direct IP connectivity to the enterprise. Within this environment, carriers must meet several technical challenges to fulfill their business objectives. Specifically, while the use of VoIP technologies reduces operating costs and provides new revenue bearing applications, it also creates new issues of network security, signaling interworking and multivendor interoperability. The two principal VoIP protocols, however, and those that you will hear referred to regularly, are H.323 and SIP (Session Initiation Protocol). The proliferation of VoIP endpoints within the enterprise has forced the carrier to confront VoIP traffic on the access network. Likewise, carriers must adapt to IP when peering with the early adopters of VoIP that use H.323 technologies for the trunking or backhaul of their voice traffic. Because SIP and H.323 messages carry end point addresses within the payload, these carriers are forced to expose the elements of their core network to other carriers and enterprises. This presents a threat to the security and service availability of the carrier’s network. Of the VoIP call setup protocols, SIP and H.323 have gained prominence. Here we take a slightly closer look at both of these protocols, their uses, and how they are shaping the future of Voice Over IP.
H.323 is a protocol that was developed by the International Telecommunications Union Telecoms Standardisation Sector (ITU-T). Its original use was for transmitting audio, video and data across an IP network, such as the Internet. Any two user devices may communicate with each other, as long as they comply with H.323.
The various components of H.323 address call signaling and control as well as multimedia transport and control and bandwidth control for point to point and multipoint conferences. H.323 was originally designed with the purpose of transporting multimedia applications across a Local Area Network (LAN), but more recently it has evolved to address the needs of a VoIP network. The standard was designed with the following objectives : - To be based on the existing standards - To add some of the advantages that packet switching networks offer to transport real-time data - To solve the problems of real-time data on packet switching networks One of the advantages of H.323 is that the designers realised that over time requirements for communication change, and therefore allowed that the equipment manufacturers could add their own specifications and features to H.323 whilst still allowing these devices to communicate with each other. As early adopters of VoIP, many carriers tended to implement networks using a single H.323 vendor such as Cisco, Clarent or VocalTec. Given that each vendor had differentiating features that eased their deployment, interoperability issues arose with other H.323 implementations. As these carriers migrate to next generation infrastructure, their fiscal constraints require them to preserve their existing infrastructure investment and find a way to consolidate these islands of H.323 into a single operational network.
However, more recently H.323 is being substituted with another of the VoIP protocols, more widely being used in VoIP networks – SIP.
SIP is one of the application layer signaling VoIP protocols, which is used to establish, maintain and terminate multimedia sessions. SIP invitations can be used to establish a session, and also to carry session descriptions. It is a text based VoIP protocol that is part of the overall Internet Engineering Task Force (IETF) multimedia architecture. SIP is used more and more as the protocol for Voice Over IP, and in 2000 it was also chosen as the 3G protocol used by Mobile Network Operators (MNO’s) within the IP Multimedia Sub-system (IMS). All SIP messages are either requests or responses to a request. The general format is: - A start line
A motivating goal in the creation of SIP as another of the VoIP protocols was to provide a signaling and call set-up VoIP protocol for IP-based communications that can support the call-processing functions of the PSTN. Ie it would make the interaction of the PSTN and IP-based networks more feasible. Hardware endpoints (SIP phones) which have the look and feel of a traditional phone, are widely available from several vendors. They use Electronic Numbering to translate phone numbers to SIP addresses so calls to other SIP users bypass the traditional telephone network. Examples of SIP applications today are Windows Messenger and AOL’s Instant Messenger. SIP based networks are emerging as service providers are deploying SIP based infrastructure to prepare themselves for the flexible and collaborative applications being developed for SIP. Microsoft XP provides a great impetus for new applications to take advantage of this converged next generation network infrastructure. As a result, carriers must have a technology strategy for both VoIP protocols (H.323 to SIP) interworking which ensures maximum service reach to any endpoint and also merges their H.323 and SIP networks into a single network for greater operational efficiencies.
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