The Gateway Control Protocol (H.248, Megaco) is an implementation of the media gateway control protocol architecture for providing telecommunication services across a converged internetwork consisting of the traditional public switched telephone network (PSTN) and modern packet networks, such as the Internet. H.248 is the designation of the recommendations developed by the ITU Telecommunication Standardization Sector (ITU-T) and Megaco is an abbreviation used primarily in the Internet Engineering Task Force (IETF). The current standard published in March 2013 by ITU-T is H.248.1: Gateway control protocol: Version 3.
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The protocol is used between elements of a physically decomposed multimedia gateway, the media gateway and one or more media gateway controllers, and architecture that separates call control from media conversion between dissimilar networks. H.248/Megaco is a master/slave protocol used to separate the call control logic from the media processing logic in a gateway.
H.248/Megaco follows the guidelines of the API Media Gateway Control Protocol Architecture and Requirements in RFC 2805 (April 2000). Though H.248 performs the same functions as other Media Gateway control protocol namely MGCP, it uses different syntax, commands and processes and supports a broader range of networks. H.248 and MGCP protocols are complementary to H.323 and SIP protocols.
The protocol was the result of collaboration of the MEGACO working group of the Internet Engineering Task Force (IETF) and International Telecommunication Union Telecommunication Study Group 16. The IETF originally published the standard as RFC 3015, which was later replaced by RFC 3525. The term Megaco is the IETF designation. Megaco combines Media Gateway Control Protocol (MGCP) and Media Device Control Protocol (MDCP). Media Gateway Control Protocol (MGCP) in turn was formed by merging Simple Gateway Control Protocol (SGCP) with Internet Protocol Device Control (IPDC).
The ITU later took ownership of the protocol and IETF's version has been reclassified as historic by RFC 5125. The ITU has published three versions of H.248, the most recent in September 2005. H.248 encompasses not only the base protocol specification in H.248.1, but many extensions defined throughout the H.248 Sub-series.
Another implementation of the Media Gateway Control Protocol architecture is Media Gateway Control Protocol. This is used over the same interface and similar in application and service functionality, however, it is a different protocol and the underlying differences make them incompatible.
Protocol Overview
H.248/Megaco exploded gatekeeper model of H.323 and put signaling control in a Media Gateway Controller (MGC) thereby unbundling call intelligence from media. H.248 is meant to address the relationship between the Media Gateways (MGs), which converts circuit-switched voice to packet-based traffic, and the Media Gateway Controller (MGC). Media Gateway Controller (MGC) using H.248 instructs one or more Media Gateways (MG)s to connect streams coming from outside a packet or cell data network onto a packet or cell stream such as the Real-Time Transport Protocol.
H.248/Megaco due to its master - slave nature does not describe the establishment of calls across domains or across Media Gateway Controllers. H.248/Megaco is used for communication downward, to the media gateways and does not constitute a complete system. The architecture requires a Peer-to-Peer for communication between Media Gateway Controllers.
The device that handles the call control function is referred to as an intelligent Media Gateway Controller (MGC) and the device that handles the media is referred to as a relatively unintelligent Media Gateway (MG). H.248 defines the protocol for Media Gateway Controllers to control Media Gateways for the support of multimedia streams across IP networks and the Public Switched Telephone network (PSTN). It is typically used for providing Voice over Internet Protocol (VoIP) services like voice and fax between IP networks and the PSTN), or entirely within IP networks.
Because of the types of devices targeted for control by H.248/Megaco and the low level of its control structure, H.248 is generally viewed as complementary to H.323 and SIP. While a Media Gateway Controller (MGC) will use H.248/Megaco to manage media establishment and control with a number of Media Gateways (MG), other VoIP protocol such as SIP and H.323 are used for one Media Gateway Controller (MGC) to communicate with another Media Gateway Controller (MGC). From a SIP perspective, the combination of MGC and MGs are treated together as a SIP Gateway.
The H.248/Megaco model describes a connection model that contains the logical entities, or objects, within the Media Gateways (MGs) that can be controlled by the Media Gateway Controller. The main entities are Contexts and Terminations.
Terminations These source or sink one or more media streams or control streams. Terminations may be physical or ephemeral. Contexts These are star connections created by associating multiple terminations. A Context is a logical entity on an MG that is an association between a collection of Terminations. A NULL context contains all non-associated terminations. A Context is a logical entity on an MG that is an association between a collection of Terminations. A ContextID identifies a Context.In IP Multimedia Subsystem (IMS), Media Gateway Control Function (MGCF) control Media Gateways (MGW)s to send and receive call to / from the PSTN circuit switched (CS) networks using. H.248. The MGCF uses SIP messages to interact with Call Session Control Function (CSCF) and Breakout Gateway Control Function (BGCF).
Although the modeling of the Media Gateway differs in H.248/Megaco when compared to MGCP, there is a similarity between the semantics of the commands in the two specifications. There is almost a one-to-one mapping between the commands of MEGACO and MGCP. For example the Create connection command in MGCP has an equivalent ADD termination command in MEGACO, the Modify connection command in MGCP equates to the MODIFY termination command of MEGACO and the Delete connection command equates to the SUBTRACT termination command of MEGACO.
Comparison with MGCP
The H.248/Megaco model is a much more complex than the Media Gateway Control Protocol (MGCP) model and it provides far greater flexibility when defining media control. For example, in Media Gateway Control Protocol (MGCP) you can set a mode such as “conference” to manage the stream mixing, but it cannot achieve the fine grain control that H.248/Megaco has, such as how to manage the media streams.
The H.248/Megaco model considerably simplifies connection setup within the MG and to entities outside the MG. It simplifies the mechanism by which the Media Gateway Controller (MGC) can specify associated media streams as well as specify the direction of media flow. H.248/Megaco is therefore able to provide greater application level support than Media Gateway Control Protocol (MGCP). For example, setting up a multi-party conference using H.248/Megaco merely involves adding several terminations to a context. In case of Media Gateway Control Protocol (MGCP) however, the Media Gateway Controller (MGC) needs to establish several connections to a special type of endpoint called the conference bridge.
Following are the main differences between Megaco/H.248 and MGCP: