PDF of this tutorialWith the many media interface protocols vying for ascendancy on the packet telephony battlefield, service providers can become the casualties of war in their quest to leverage next-generation technologies to provide voice service. Will H.323 win out, or will upstarts like SIP, MGCP, and media gateway controller (MEGACO) rise to challenge the old guard? More important, because not all voice traffic will be carried on packet networks right away, the next-generation network must still retain the ability to speak to the PSTN using signaling protocols like signaling system 7 (SS7), integrated services digital network (ISDN), and ISDN user part (ISUP)/transactional capabilities application part (TCAP).
To be safe, service providers should aim not to take sides in the protocol war. In order to do that, they must deploy a solution that is also indifferent to which one wins. In other words, the next-generation platform must have the ability to interwork all of the next-generation protocols. This sounds obvious and simplistic, but the implementation is not. Indeed, many softswitch vendors have chosen to include only one or two of the protocols, plus SS7, and hailed their product as the heart of the next-generation network. They tread on dangerous ground.
A last issue is one of end-devices, not protocols. Just because Vendor A’s product talks SIP and Vendor B’s product talks SIP does not mean that the two can communicate. This is because SIP is a highly flexible, extensible protocol, but Vendor A may extend it differently than Vendor B, creating the underlying problem. Therefore, a next-generation softswitch platform must communicate and interwork not only between protocols but also between the different vendor-specific implementations of these protocols.
Below is a brief description of the essential characteristics of each of the major next-generation protocols (Source: IP Telephony Magazine):
- H.323—Often criticized for being inflexible, H.323 was nevertheless the earliest of the next-generation protocols. As such, H.323 has a large installed base and is implemented in many of the new VoIP networks. H.323 defines four major components for a network-based communication system: terminals, gateways, gatekeepers, and multipoint control units (MCUs). Traditional telephony providers and vendors have supported H.323 because they are familiar with the concept and the architecture, as compared to SIP. Many have characterized the H.323–SIP rivalry as the butting of the Bellheads and the dataheads. H.323 was developed by the International Telecommunications Union (ITU).
- SIP—SIP was developed by the Internet Engineering Task Force (IETF) with H.323’s shortcomings in mind. Often hailed as more flexible than H.323, SIP is an application layer control protocol that can establish, modify, and terminate sessions or calls. SIP is text-based and lightweight, and it uses a simple invitation-acceptance message structure. Other benefits of SIP over H.323 include scalability, service richness, lower latency, faster speed, and ability to distribute for carrier-grade reliability.
- MGCP—MGCP is essentially the merger of the IETF's Internet protocol device control (IPDC) and Bellcore's simple gateway control protocol (SGCP). MGCP is prevalent among such devices as media gateways, ATM routers, cable modems, and set-top boxes. Unlike SIP, where the intelligence is in the endpoint, MGCP assumes the intelligences reside in the core of the network. Some have argued that this feature contains strong benefits for the carrier market, as it enables the delivery of basic telephony services over low-cost end points.
- MEGACO—Also known as H.248, MEGACO is being developed by the ITU. It is still in early stages.
