H.248

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Not to be confused with Media Gateway Control Protocol, another Protocol for controlling Media Gateways based on Media Gateway Control Protocol Architecture.

Gateway Control Protocol Relationship. H.248/Megaco is used for downward communication between Media Gateway Controller and Media Gateways. Media Gateway Controller talk with each other using SIP, H.323 or ISUP

H.248 or Megaco or Gateway Control Protocol is a recommendation from ITU Telecommunication Standardization Sector (ITU-T) which defines protocols that are used between elements of a physically decomposed multimedia gateway. It is an implementation of the Media Gateway Control Protocol Architecture (RFC 2805). H.248 is also called Megaco in IETF domain. It is now known as Gateway Control Protocol. The current standard published in September 2005 by ITU-T is H.248.1: Gateway control protocol: Version 3.^[2]

H.248/Megaco is standard protocol for controlling the elements of a physically decomposed multimedia gateway, which enables separation of call control from media conversion. 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.^[3]^[4]

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).^[5] Media Gateway Control Protocol (MGCP) in turn was formed by merging Simple Gateway Control Protocol (SGCP) with Internet Protocol Device Control (IPDC).^[6]

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,^[2] 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).^[7] 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.^[7]

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).^[4] From a SIP perspective, the combination of MGC and MGs are treated together as a SIP Gateway.

3GPP / TISPAN IMS Architectural Overview. H.248 messages are used between Media Gateway Control Function (MGCF) control IMS - Media Gateways (IMS-MGW). SIP is used by MGCF to interact with Call Session Control Function (CSCF) and Breakout Gateway Control Function (BGCF)

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.

Physical Terminations represent physical entities that have a semi-permanent existence. For example, a Termination representing ports on the gateway, such as TDM channel or DS0 might exist for as long as it is provisioned in the gateway. Ephemeral Terminations represent Connections or data flows, such as RTP streams, or MP3 streams, and usually exist only for the duration of their use in a particular Context.

Terminations have properties, such as the maximum size of a jitter buffer, which can be inspected and modified by the MGC. A termination is given a name, or Termination ID, by the MG.

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.

The normal, "active" context might have a physical termination (say, one DS0 in a DS3) and one ephemeral one (the RTP stream connecting the gateway to the network). Contexts are created and released by the MG under command of the MGC. A context is created by adding the first termination, and it is released by removing (subtracting) the last termination.

A termination may have more than one stream, and therefore a context may be a multistream context. Audio, video, and data streams may exist in a context among several terminations.

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.^[3]

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.

Gateway Control Evolution. H.248/Megaco is standard as defined in RFC 3525. Current version of H.248 - Gateway control protocol: Version 3 is standard and in force ^[1]

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:

H.248/Megaco	MGCP
A call is represented by Terminations within a call Context	A call is represented by Endpoints within Connections
Call types include any combination of multimedia and conferencing	Call types include point-to-point and multipoint
Syntax is text or binary	Syntax is text
Transport layer is TCP, UDP or SCTP	Transport Layer is UDP
Standard protocol for Media Gateway Control	IETF status - Informational. MGCP does not specify an Internet standard of any kind
Defined by ITU (formerly by IETF and ITU)	Managed by the industry. Many Companies have their own implementation of MGCP

ITU-T Recommendations of the H.323 System

ITU-T H.248 Recommendations

ITU-T Recommendation H.248.1, Gateway control protocol: Version 3.
ITU-T Recommendation H.248.2, Gateway control protocol: Facsimile, text conversation and call discrimination packages.
ITU-T Recommendation H.248.4, Gateway control protocol: Transport over Stream Control Transmission Protocol (SCTP).
ITU-T Recommendation H.248.12, Gateway control protocol: H.248.1 packages for H.323 and H.324 interworking.
ITU-T Recommendation H.248.15, Gateway control protocol: SDP H.248 package attribute.

RFCs

RFC 3015 - Megaco Protocol Version 1.0, November 2000, (Standard Track)
RFC 3525 - Gateway Control Protocol Version 1, June 2003 (Obsoletes: RFC 3015) (Standard)

References

↑ "H.248.1 : Gateway control protocol: Version 3". ITU Telecommunication.
↑ 2.0 2.1 H.248.1 "H.248.1 : Gateway control protocol: Version 3". International Telecommunication Union - Telecommunication. Retrieved 2012-06-13.
↑ 3.0 3.1 "Use of MEGACO vis-à-vis MGCP to build a Gateway Solution".
↑ 4.0 4.1 "H2.48 history". packetizer.com. Retrieved 2012-06-07.
↑ "Toward the PSTN/Internet Inter-Networking - MEDIA DEVICE CONTROL PROTOCOL". IETF. November 1998. Retrieved 8 June 2012.
↑ "Level 3 Communications, Bellcore Announce Merger of Protocol Specifications for Voice Over IPe". Level 3 Communications. Retrieved 8 June 2012.
↑ 7.0 7.1 , The H.248 Gateway Control Protocol version 2

External links

ITU-T H-series Recommendations ITU-T Publication site where the H-series, including the H.248 Sub-series, Recommendations can be downloaded in PDF format free of charge.
H.248.1 Base protocol specification .
H.248 History
MEGACO vs MGCP White Paper from Hughes Software Systems
MEGACO 1st draft, IETF, April 1999]
Protocols of Megaco and MGCP by Doug Allen at Dynamix

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