February 2001

BY DAVID FRIDLEY

Why use MGCP when there's already SS7, SIP, H.323, and MEGACO? There is sometimes confusion over the application of MGCP versus SIP, H.323, or SS7 and there is also competition between MGCP and MEGACO. I hope to dispel some of the misunderstanding surrounding the application of MGCP, especially in connection with other established protocols that appear to conflict with MGCP.

You may ask, why would you use MGCP in addition to SIP or H.323 or SS7? Why would you want to add another layer? First, we'll look at what MGCP has in common with these standards and how it differs from them. Then, we'll investigate how MGCP works through decomposing the gateway. Finally, we'll examine the future of MGCP and the decomposed gateway model.

What does MGCP have in common with SS7, SIP, and H.323, and how does it differ? MGCP is a protocol designed for the Internet. That is, like H.323 and SIP, it is carried over TCP/IP. In addition, these seemingly competing protocols have the Real-time Transport Protocol and the Real-time Transport Control Protocol (RTP/RTCP) in common -- the basic Internet standards for carrying live voice and video data over IP. This is really good news. Although H.323 and SIP are competing protocols, even they agree on how to carry voice over the Internet. We'll see why this is so important in a little bit.

We can differentiate between SS7, SIP, and H.323 and MGCP in this way: SS7, SIP, and H.323 negotiate calls between terminals and telephony systems. MGCP is used within a telephony system to allow the system to be decomposed into physically separate components. MGCP is a protocol that allows you to build an IP-based telecommunications system out of components that are connected over IP networks.

How Does MGCP Work?
Media Gateway Control Protocol, or MGCP, is an ideal protocol to use for building large, scalable systems. The International Engineering Task Force (IETF) designated MGCP, otherwise known as RFC 2705, a real albeit developing standard.

With MGCP, a telephony server called an "agent" or sometimes a "Media Gateway Controller" sets up calls between MGCP media gateways. Consider that MGCP

is not a symmetrical call setup and teardown protocol, like SIP or H.323. With H.323 or SIP every device is a peer: One device can initiate a call to another device, and vice-versa. With MGCP, gateways are slaves and agents are masters.

With H.323, for example, when a call is placed into a gateway, the H.323 gateway gathers the digits of the dialed number, and then figures out where to route the call. Then the H.323 gateway contacts the far-end H.323 gateway and exchanges capabilities. The capabilities exchange includes things like voice, video, compression algorithms, packet size, and other things. After the two H.323 gateways have done this, they establish RTP/RTCP sessions with each other that carry the voice or video data between the two ends.

Where H.323 and SIP, and SS7 for that matter, conflict is on the language of setting up and controlling the call. MGCP acts as a "peacekeeper," since this protocol is strictly about establishing RTP and PSTN sessions and leaves the setup and control issues to the agent. Therefore, it is possible to build an agent that accepts both H.323 and SIP call setup requests and then uses MGCP to establish calls between MGCP media gateways and H.323 and SIP gateways or endpoints.
A basic MGCP-based telephony system involves one or more media gateways and at least one agent, sometimes called a media gateway controller (Figure 1). The agent is signaled by all the media gateways that serve it of events such as incoming call, off-hook, and hang-up. The agent, in turn, sends commands to all of these media gateways to ring phones, dial numbers, and establish RTP/RTCP connections.

There are only eight commands and three signals (Table 1 and Table 2) defined in MGCP. The real power behind the benefits of MGCP is in the protocol's ability to define "packages," which are collections of command and signal parameters that can be used to support specific devices. For example, MGCP supports a DTMF package that allows keypad tone digits to be dialed and/or collected and an Announcement package that allows voice prompts to be played. With these packages, the NotificationRequest and ModifyConnection commands, and the Notify signal, Interactive Voice Response (IVR) can be implemented.

The Future Of MGCP
In most cases you will want to use SIP and/or H.323 and/or SS7 to place calls between different telecommunication systems, because these are the standards for interoperability with the rest of the world. How do you decide when to use one and when to use the other? A telecommunications carrier may have one agent using MGCP within a region. The agent controls all calls within that region, and MGCP is used to communicate between the physically separate endpoint components, and the agent. Calls between regions, especially calls with other carriers would be negotiated using SS7/SIP/H.323 by the agents controlling each region. You might ask, how big is a region? That would depend on the carrier, the application, and the number of endpoints involved.

As it stands now, MEGACO/H.248 is an incompatible next-generation protocol to MGCP. The IETF (Internet Engineering Task Force) group that developed MGCP has joined forces with the ITU (International Telecommunications Union) and developed MEGACO/H.248 that is standardized by both agencies. Most companies that are currently deploying MGCP products plan to move to MEGACO. However, with MGCP-based systems currently in use, MGCP remains an important standard for the immediate future.

Products using MGCP are operating in the field right now: Those products are being used in H.323, SIP and even SS7 signaling environments. The current trend of decomposing the gateway through MGCP has a future and the adoption of the joint IETF/ITU standard for MEGACO will only strengthen the position the decomposed model now holds in the market. 

David Fridley is director of marketing for Anatel Communications. Anatel is building the foundation on which the next generation of "killer" Internet Telecommunications applications and features will run, working with OEM customers and partners to drive this development in "Internet time." 

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