Open Call Control Layer

Call-control, in a nutshell, is the process of making a routing decision about where a call needs to go and somehow making the call happen. In the PSTN today, these decisions are carried out by SS7 and are made by Service Control Points (SCPs). Chapter 8, "VoIP: An In-Depth Analysis," discusses how the different VoIP protocols work and how they solve different network design issues.

In this new model of separating the bearers (RTP streams) from the call-control layer and separating the call-control layer from the services, it is necessary to make sure that standards-based protocols are used. Data networking is unique in the fact that multiple protocols can co-exist in a network and you can tailor them to the particular needs of the network.

Many different IP routing protocols exist, for example, and each is specifically designed for a certain type of network. Some of these include the Router Information Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediary System to Intermediary System (IS-IS), Open Shortest Path First (OSPF), and Border Gateway Protocol (BGP). Each protocol solves a similar problem—routing updates. Each routing problem is slightly different, however, and requires a different tool. In this case, the tool is a routing protocol, which solves each problem.

You can say the same of VoIP call-control protocols. They all solve a similar problem—phone numbering to IP address translation; however, they might all be used for slightly different purposes.

For instance, currently H.323 is the most widely deployed VoIP call-control protocol. H.323, however, is not widely seen as a protocol that is robust enough for PSTN networks. For these networks, other protocols such as Media Gateway Control Protocol (MGCP) and Session Initiation Protocol (SIP) are being developed.

Many VoIP call-control protocols are being developed, so it is possible that different protocols will be deployed throughout the coming years. Each protocol will be developed to fix a certain problem and serve a particular purpose. A leader will emerge from the mud, but only if there must be a winner. For the short term, at least, many protocols will be used, and there will be no need for a single call-control protocol.

0 0

Post a comment