Media gateway control protocol architecture
The media gateway control protocol architecture is a methodology of providing telecommunication services using decomposed multimedia gateways for transmitting telephone calls between an Internet Protocol network and traditional analog facilities of the public switched telephone network (PSTN). The architecture was originally defined in RFC 2805 and has been used in several prominent voice over IP (VoIP) protocol implementations, such as the Media Gateway Control Protocol (MGCP) and Megaco (H.248), both successors to the obsolete Simple Gateway Control Protocol (SGCP).
The architecture divides the functions required for the integration of traditional telecommunication networks and modern packet networks into several physical and logical components, notably the media gateway, the media gateway controller, and signaling gateways. The interaction between the media gateway and its controller is defined in the media gateway control protocol.
Media gateway protocols were developed based on the Internet model of networking, the Internet Protocol Suite, and are referred to as device control protocols. A media gateway is a device that offers an IP interface and a legacy telephone interface and that converts media, such as audio and video streams, between them. The legacy telephone interface may be complex, such as an interface to a PSTN switch, or may be a simple interface to a traditional telephone. Depending on the size and purpose of the gateway, it may allow IP-originated calls to terminate to the PSTN or vice versa, or may simply provide a means to connect a telephone to a telecommunication system via an IP network.
Originally, gateways were viewed as monolithic devices that had call control, using protocols such as H.323 and the Session Initiation Protocol, and hardware required to control the PSTN interface. In 1998, the idea of splitting the gateway into two logical parts was proposed: one part, which contains the call control logic, is called the media gateway controller (MGC) or call agent (CA), and the other part, which interfaces with the PSTN, is called the media gateway (MG). With this functional split, a new interface existed between the MGC and the MG, requiring a framework for communication between the elements, resulting in the media gateway control protocol architecture.
SIP and H.323 are signaling protocols, while media gateway control protocols are device control protocols. The architectural difference between SIP and H.323, and the media gateway control protocols is that the relationships between entities in SIP and H.323 are peer-to-peer, while the relationships between entities in media gateway control protocols use the master/slave (technology) model. SIP and H.323 handle call setup, connection, management, and tear-down of calls between like interfaces, whereas media gateway control protocols define the mechanisms of setup of media paths and streams between IP and other networks.
Several implementations of the media gateway control protocol are in common use. The names of the best-known protocols are abbreviations of the protocol group:
- The Media Gateway Control Protocol (MGCP) was first described in RFC 2705, and revised in RFC 3435.
- Megaco, or H.248, or Megaco/H.248, was first described in RFC 3525, which was revised and extended in various specifications, leading to its obsolescence as explained in RFC 5125.
Although similar in architecture, MGCP and H.248/Megaco are distinctly different protocols and are not interoperable. H.248/Megaco and MGCP protocols are complementary to H.323 and SIP, which both may be referred to as intelligent endpoint protocols. H.248/Megaco and MGCP may be referred to as device control protocols.
Other media gateway control protocols include the predecessors of MGCP, namely the Simple Gateway Control Protocol (SGCP) and the Internet Protocol Device Control (IPDC). A proprietary protocol using a similar architecture is the Cisco Skinny Call Control Protocol (SCCP).
A media gateway is a device that converts media streams in the form of digital data or analog signals in telecommunication for services such as voice, video, and fax applications between two, usually dissimilar, interfaces using different technologies. One of the technologies usually is a packet, frame, or cell network. For example, it may convert voice telephone calls between a traditional analog telephone to a digital format for transmission over an Internet Protocol (IP) network, to facilitate voice over IP communications.
Media gateway controller
A media gateway controller (MGC) controls the media gateways. The MGC receives signalling information (like dialed digits) from the Media Gateway and can instruct it to alert the called party, to send and receive voice data etc. There are several protocols which can be used between MGC and Media Gateway: SGCP, IPDC, MGCP and Megaco (also known as H.248). The MGC also receives the term Call Agent when referred in the context of MGCP.
A VoIP architecture using an MGC is sometimes called a softswitch architecture. Advantages of a softswitch architecture are that the Media Gateways are easy to maintain and that the softswitch operator retains full control. On the downside, softswitch architectures tend to be inflexible. Highly available MGCs are expensive to implement, as an MGC needs to be stateful.
The device control protocols evolved through several versions. MGCP emerged from a group now called the International SoftSwitch Consortium. This group started early with Level 3 Communications (through its acquisition of Xcom) and Telcordia (BellCore).
In July 1998, Telcordia (Bellcore) and Cisco Systems created a protocol called Simple Gateway Control Protocol (SGCP) for controlling Telephony Gateways from external call control elements. Meanwhile in mid-1998, Level 3 created a Technical Advisory Council (TAC), composed of a dozen leading communications equipment manufacturers. The TAC proposed a device protocol called Internet Protocol Device Control (IPDC) in August 1998. IPDC was intended to used between a media gateway and a media gateway controller. Media gateway were capable of acting as a voice over IP gateway, voice over ATM gateway, dialup modem media gateway, circuit switch, or cross- connect. In October 1998, Simple Gateway Control Protocol (SGCP) was combined with Internet Protocol Device Control (IPDC), resulting in MGCP.
MGCP was submitted to the IETF’s MeGaCo working group in October 1998. In November 1998, Lucent Technologies submitted a draft for third device protocol, called Media Device Control Protocol (MDCP) to used by media gateways and their controllers. IETF merged MGCP and MDCP and proposed a new and improved protocol named MeGaCo protocol (also known as H.248) in April 1999.
The first "official" version of MGCP is defined in RFC 2705 as informational. RFC 3435 obsoleted RFC 2705. MGCP currently is purely informational rather than a standard-track protocol, although it includes protocol specification. Even while MGCP was still an Internet Draft, many companies developed included MGCP with their own development rather than wait for astandardized protocol. Therefore, the decision was made to release MGCP as an informational RFC in October 1999. Specification on MGCP has stopped, although companies continue to implement MGCP within their systems Collins, Daniel (September 22, 2000). "Chapter 6: Media Gateway Control and the Softswitch Architecture". Big Compilation Book With Many Chapters and distinct chapter authors. Book Publishers. pp. 239–240. ISBN 0071363262.
Further standardization of MGCP effort was pursued in the IETF, in the MEGACO working group, and also in the ITU-T/SG16, under the code name H.GCP. RFC 3015 standard tracks the MEGACO protocol (also H.248) and
The drive behind the development of Megaco/H.248 was the need to provide various requirements that were not addressed properly by MGCP. Megaco/H.248 is an evolution of MGCP. Megaco/H.248 is combination of MGCP and MDCP and was published as Standard in RFC 3015 in November 2000. The implementation of H.248/Megaco and MGCP is different and they are not directly compatible.
Currently H.248 (H.248.1 Gateway Control Protocol version 3 is published by International Telecommunication Union Telecommunication (ITU-T) as standard protocol. The ITU-T has published three versions of H.248.1. Media Gateway Control Protocol (MGCP) is defined Internet Engineering Task Force as informational by RFC 3525.
Both H.248 and MGCP are protocol for controlling media gateways using a media gateway controller or called call agents. In a VoIP system, H.248 and MGCP are used with SIP or H.323. SIP or H.323 will provide the call control functionality and MGCP can be used to manage media establishment in media gateways.
- RFC 2805 - Media Gateway Control Protocol Architecture and Requirements, April 2000 (Informational)
- RFC 2705 Media Gateway Control Protocol (MGCP) Version 1.0 October 1999 (Informational)
- RFC 3435 - Media Gateway Control Protocol (MGCP) Version 1.0 (this supersedes RFC 2705) (Informational)
- RFC 3015 - Megaco Protocol Version 1.0, November 2000, (Standard Track)
- RFC 3525 - Gateway Control Protocol Version 1, June 2003 (Obsoletes: RFC 3015) (Standard)
- RFC 2805, Media Gateway Control Protocol Architecture and Requirements, N. Greene, M. Ramalho, B. Rosen, IETF, April 2000
- "Understanding VoIP Protocols". packetizer.com. Retrieved 2012-06-07.
- RFC 3435, Media Gateway Control Protocol (MGCP) Version 1.0, F. Andreasen, B. Foster, The Internet Society (January 2003)
- RFC 3525, Gateway Control Protocol Version 1, C. Groves, M. Pantaleo, T. Anderson, T. Taylor (editors), The Internet Society (June 2003)
- RFC 5125, Reclassification of RFC 3525 to Historic, T. Taylor, The IETF Trust (February 2008)
- title=Use of MEGACO vis-à-vis MGCP to build a Gateway Solution
- "SIP core working group charter h2.48 history". packetizer.comg. Retrieved 2012-06-07.
- "Simple Gateway Control Protocol (SGCP". IETF. 30 July 1998. Retrieved 8 June 2012.
- "IPDC - Connection Control Protocol". IETF. August 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.
- "Media Gateway Control Protocol (MGCP)". IETF. 27 October 1998. Retrieved 8 June 2012.
- "Toward the PSTN/Internet Inter-Networking - MEDIA DEVICE CONTROL PROTOCOL". IETF. November 1998. Retrieved 8 June 2012.
- "MEGACO Protocol". IETF. 16 April 1999. Retrieved 8 June 2012.
- "H.248.1 : Gateway control protocol: Version 3". ITU Telecommunication.
- "Media Gateway Control Protocol". telecomspace.com. Retrieved 2012-06-07.
- Simple Gateway Control Protocol 1st Draft, IETF, 30 July 1998
- Connection Control Protocol 1st Draft, IETF, August 1998
- Media Gateway Control Protocol (MGCP)1st Draft, IETF 27 October 1998
- MEDIA DEVICE CONTROL PROTOCOL (MDCP) 1st Draft, November 1998
- MEGACO 1st Draft, IETF, April 1999
- 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 , ITU-T
- MGCP Information Site Information related to MGCP.
- H.248 Information Site Information related to H.248/Megaco, including pointers to standards and draft specifications.
- Implementing Media Gateway Control Protocols - A RADVISION White Paper
- A Description of MGCP and SIP Michael Lamy, ADTRAN Enterprise Networks Division