INFRASTRUCTURE PROTOCOLS AND APPLICATIONS

H.323

H.323 defines packet standards for terminal equipment and services for multimedia communications over local and wide area networks communicating with systems connected to telephony networks such as ISDN. The initial version of this standard came from the International Telecommunications Union (ITU) in June 1996.

It defines communication over IP-based local area networks (LANs). A later version (v2), adopted in January 1998, extended it over wide area use and general-purpose IP networks. Several subprotocols are included under H.323 relating to call setup and signaling.

Four components for a multimedia communication system include terminals, gateways, gatekeepers, and multipoint control units (MCU). Gateways and gatekeepers are used in negotiation for PSTN connections, whereas MCUs enable multiparty audio and videoconferences. One drawback of H.323 is that it is somewhat complex and inflexible. However, it is ISDN-based and relatively easy to build applications across it. For many applications, H.323 is satisfactory, but falls short for more advanced implementations and solutions.

All things considered, the most likely scenario is that multiple protocols will be used with H.323, such as SIP for exchange between soft switches and gateways and MGCP for call setup, because H.323 is too complex and time consuming to set up a call.

MGCP/MEGACO

The Media Gateway Control Protocol (MGCP) specifies communication between call control elements and telephony gateways. It is a text-based protocol. Media gateways are telephony gateways that convert circuit-switched voice signals to data packets for multiservice packet networks. The Internet Engineering Task Force (IETF) created MGCP to address some of the perceived shortcomings of H.323.

The main purpose of MGCP is to place control of call signaling and processing intelligence in call agents or media gateway controllers. (Call agents and media gateway controllers are synonymous with and similar to the gatekeeper functions in H.323 and are also called soft switches.) A new version of MGCP, released in August 2000, is called Megaco or H.248.

Although Megaco was created for the same purpose, Voiceover-IP, it differs from MGCP because it supports a broader range of networks and devices such as ATM, Remote Access Servers, Multi-Protocol Label Switching routers (MPLS), Digital Subscriber Line Access Multiplexers (DSLAMs), and more.

Because Megaco is very new, interoperability testing is ongoing. It appears to answer many of the deficiencies of H.323 and will become very important as we move toward Voice-over-IP networks. It is somewhat unclear at this time if Megaco will replace MGCP or just supplement it. Megaco is more suited for media applications than MGCP, but MGCP may be a better choice for nonmedia-centric applications, such as MPLS-based session control.

SESSION INITIATION PROTOCOL (SIP)

Session Initiation Protocol (SIP) is an application-layer control protocol that can establish, modify, and terminate multimedia sessions or calls. Like MGCP, SIP is text-based. SIP came out of the Internet Engineering Task Force (IETF) in 1998 as an RFC. It has rapidly gained widespread support, including Microsoft’s announcement that SIP will be supported in the next generation Windows XP product.

SIP uses a “request-response” model like that used in Hypertext Transfer Protocol (HTTP). There is one major difference between MGCP and SIP—a call agent is not necessary to mediate between clients.

The usefulness of SIP for multimedia is almost limitless. Sessions can be unicast or multicast and include multimedia conferences, distance learning, VoIP, or similar applications. Some examples of multicast protocols include email, news groups, Web pages, and the like. SIP also supports the ISDN and Intelligent Network telephony subscriber services for personal mobility, which is important for the Wireless Internet.

SIP is reliable, scalable, and can be used with other protocols. Development is fast because it is very similar to HTTP, thus making the addition of feature-rich applications very quick to implement. Initially H.323 and MGCP may be the protocols of choice for tomorrows’ media gateways and soft switches but SIP and Megaco will be strong contenders as they mature.

No matter which protocol or protocols become the favorite, soft switches will be flexible enough to adapt. This adaptability makes network service providers very happy. Unlike old legacy switches, this new breed of switches will be quick to accept added features or changed services without waiting months for a manufacturer to modify the switch design.