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Feature Article
March 2002


Device Control Protocol Alternatives For Media Servers


As the newest component of next generation voice architectures, media servers have been receiving a lot of attention recently. While there is strong consensus about what a media server is and how it fits into packet voice networks, there is some debate about how to best control the features, or �service building blocks,� offered by media servers. The goal of this article is to examine, in detail, the various options currently available for media server control. The main control protocols contrasted are MGCP/Megaco and SIP. In addition, we will look at the differences in tightly coupled and loosely coupled interactions between a media server and its control agent.

The media server control interface can be viewed as allowing a softswitch or application server, called a control agent, to perform two main functions, namely bearer control and media control.

Bearer control enables the establishment of media streams between the media server and far-end devices such as media gateways and packet-attached terminals (e.g., SIP or H.323 terminals). Session Description Protocol (SDP) is used to describe these media streams. SDP descriptions are exchanged between far-end devices and the media server, through the control agent, and are used to negotiate the characteristics of media streams. Once these characteristics are negotiated, the media server and far-end device set up the agreed-on media streams.

In contrast, media control allows the control agent to tell the media server exactly how to process media streams.

Protocols such as MGCP, Megaco, and SIP fundamentally provide bearer control, but can also provide media control, when augmented with additional mechanisms such as signals and events (tightly coupled interactions) or scripts (loosely coupled interactions). Both signals/events and scripts can be used with MGCP/Megaco or SIP, although, as will be described below, MGCP/Megaco is commonly associated with tightly coupled media control and SIP is commonly associated with loosely coupled media control.

Tightly Coupled Media Control
In tightly coupled interaction, detailed, real-time, asynchronous messages are sent back and forth between the control agent and the media server. To use the MGCP/Megaco terms, messages downward (from the control agent to the media server are signals,) and message upward (in the reverse direction are events).

Each signal message provides specific instructions to the media server on actions to perform on media and/or what events to look for. Each event message provides complete information on an event condition that the media server has detected. 

A good example of tightly coupled operation is a multi-level IVR prompt menu, where the control agent and media server exchange signals before (what announcements to play and digits to detect) and events after (what digits were detected) each menu level.

Loosely Coupled Media Control
In loosely coupled operation, messages are still exchanged between control agent and media server. There are fewer such messages, however, because the control agent sends the media server a script with a potentially complex set of instructions. Only after the script has completed executing will the media server return results to the control agent. The control agent then processes the results and can, if necessary, send the media server a new script to be executed. In the example above with a multi-level IVR prompt system, the entire menu tree would be described completely in the script passed to the media server.

Until now the discussion around the media server control interface has been theoretical: Bearer control versus media control, and tight coupling versus loose coupling. We will now examine specific protocols that carry out these operations in today�s media servers.

MGCP and Megaco (H.248)
MGCP and Megaco are the most common media server control protocols, and they provide essentially similar and interchangeable functionality. They began as media gateway control protocols and have been evolved to also support control of media servers and simple terminals. As a bearer control protocol, MGCP/Megaco by itself allows the setup and tear down of single (e.g., IVR) or multiple (e.g., conferencing) stream sessions. For media control, more than 50 event packages have been defined for MGCP/Megaco. The majority of these have been defined in the IETF/ITU for Megaco and not for media servers, although two that are especially relevant to media servers have been defined for MGCP in the PacketCable project of CableLabs.

These latter two are called Basic Audio Server (BAU) and Advanced Audio Server (AAU) and are described in the PacketCable Audio Server Specification (PASS). BAU and AAU provide rich and mature functionality for announcements, IVR, and play and record. Additional event packages are currently being defined for ASR and TTS.

MGCP and Megaco are usually associated with tightly -coupled media control but an event package has been defined to describe how they invoke scripts such as VoiceXML. Thus MGCP and Megaco can also support loosely coupled operation.

With rich and mature standards and support of key bodies such as the Softswitch Consortium, CableLabs PacketCable, and IPCableCom, MGCP has become the most widely adopted control interface for media servers. Megaco, although not currently deployed widely for media servers, is endorsed by the Internet Engineering Task Force (IETF), the International Telecommunications Union (ITU), the Multiservices Switching Forum (MSF), and the Third Generation Partnership Project (3GPP).

SIP is a newcomer to the world of media server control protocols and while not as mature, offers some powerful capabilities. Although SIP is a peer-to-peer protocol designed, and used, for end-to-end signaling, it can easily be used in a client-server or master-slave manner, when coupled with third-party call control by the control agent. Control SIP is not a new protocol variant, however, just a way of using SIP, essentially a profile of SIP.

There are only two differences in this profile:

  • The media server waits passively for service requests (SIP INVITE messages) and never sends service requests to the control agent.
  • There is a special syntax defined for the service request (for the Request-URI) to specify what task the media server is to perform.

As a bearer control protocol, SIP by itself, just like MGCP/Megaco, allows the setup and tear down of single or multiple stream sessions. SIP service requests to the media server can also, however, play announcements and invoke scripts.

In contrast with MGCP/Megaco, SIP is usually thought of today in association with loose coupling and scripted operation, typically VoiceXML (in combinations with HTTP). The Speech Application Language Tags (SALT) scripting language has been proposed by Microsoft, Cisco, and others as an alternative to VoiceXML and may be a strong contender once it is fully defined by mid-year. In addition the industry is now starting to look at how to support tight coupling in SIP. SIP already has in place an events mechanism (also called SUBSCRIBE/NOTIFY) that provides a framework for event packages. It is likely that this mechanism will be used for tight coupling in SIP.

Tight versus Loose Coupling
Because both MGCP/Megaco and SIP have very similar bearer control capabilities, a key decision factor in media server control protocols is the type of media control, tight coupling versus loose coupling.

The basic conclusions that can be drawn are:

  • Plain SIP can support both simple conferencing and announcements.
  • Plain MGCP/Megaco is used, and is always combined with event packages even in the simplest implementations.)
  • Scripting allows all tasks except business conferencing and fax to be supported, for both bearer control protocols.
  • Event packages allow all tasks to be supported, for both bearer control protocols.

Thus loose coupling alone does have limitations compared to tightly coupled media control, for example it cannot provide the complex interaction required by business conferencing or facsimile.

SIP versus MGCP/Megaco
As indicated above, both MGCP/Megaco and SIP provide very similar bearer control capabilities. Where they differ, though, mainly because of their unique histories, is in their degree of maturity and the degree of maturity of their event packages.

The following are some key factors that favor SIP:

  1. The SIP protocol is better specified than MGCP. Work on the MGCP protocol was distracted by the introduction of Megaco, and the MGCP specification is consequently not as solid as it might be. But although Megaco itself is better specified than MGCP, its media server events packages lag MGCP�s.
  2. As a result of SIP�s popularity as an end-to-end signaling protocol, there are more powerful development tools available for SIP. This results in shorter development times and less expensive development.
  3. Application servers already have need to support SIP for signaling and HTTP for Web interfaces, and so don�t need to add additional protocols in order to support SIP media servers.
  4. Some applications need only basic media processing such as simple announcements and simple conferencing, and for them, plain SIP is a good fit.
  5. Some applications do additionally need IVR but want to, and are able to, entirely delegate the IVR handling to the media server through the use of scripts. For this, plain SIP augmented with scripting is a good fit.

MGCP/Megaco, however, still have some strong advantages today:

  1. There are carrier class MGCP/Megaco media servers available today and deployed in the field. SIP lags MGCP/Megaco in this respect.
  2. MGCP/Megaco is the only alternative possible today for tasks requiring signals and events, such as business conferencing or facsimile or more complex features. MGCP/Megaco�s event packages are mature, tested, and deployed, whereas SIP�s event packages have not yet been defined.
  3. Softswitches already use MGCP/Megaco and event packages for media gateway control, and can reuse much of this functionality for media server control.

Our analysis has shown that overall there is no clear winner today between MGCP/Megaco and SIP. In the absence of a simple answer, the prospective user of media servers must carefully consider their own technical and timing requirements in order to select the protocol that best meets their needs.

Finally, a related factor to consider is multi-protocol support. Media servers are infrastructure components that will likely be communicating with multiple application servers and softswitches, and there is therefore a strong case for requiring media servers to concurrently support multiple device control protocols and coupling types, i.e., both MGCP/Megaco and SIP as well as loose and tight coupling. Only when such capabilities are available will service providers be able to fully realize the operational and capital savings afforded by a multi-service, shared resource such as a media server. c

Garland Sharratt is chief architect and vice president of business development at Convedia, a leading supplier of carrier class, next-generation media servers. For more information, please visit the company�s Web site at www.convedia.com.

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