Media (MGCP) Technology

MGCP - Media Gateway Control Protocol - is the most important protocol in next- generation networks because it is responsible for implementing the migration from PSTN to IP telephony at large enterprises, ISPs, and carriers by converting today's TDM circuits into tomorrow's voice packets.

Media Gateway Controller Protocol (MGCP) is a device control protocol developed by IETF and destined to control devices, like Media Gateways and Integrated Access Devices (IADs), by using text format messages to set up, manage, and terminate multimedia communication sessions in a centralized communications system. The difference between MGCP and other multimedia control protocol systems is that MGCP allows the endpoints in the network to control the communication session.

MGCP is a protocol that operates between a Media Gateway (MG) Media Gateway Controller (MGC) Call Agents ), allowing the Media Gateway Controller to control the Media Gateway. MGCP enforces the Media Gateway as the fundamental component of multipoint, next generation, converged networks. MGCP was developed as part of the convergence movement, which brings voice and data together on the packet-switched Internet.



MGCP provides a general description of the Media Gateway/Media Gateway Controller model. It describes an architecture in which call control intelligence is outside the Media Gateways and handled by Media Gateway Controllers. These elements synchronize with one another to send coherent commands to the Media Gateways under their control. A control protocol is used to control VoIP gateways from the external call agents.

The Media Gateway (MG) is a basic device that terminates PSTN switched circuits (trunks and local loops) and converts from pulse code modulated information to packetized information and vice versa. It also handles RTP media streams across the IP network.

Examples of gateways:

 

Trunking gateways - interface between the telephone network and a VoIP network. Such gateways typically manage a large number of digital circuits.

Voice over ATM gateways - operate much the same way as VoIP trunking gateways, except that they interface to an ATM network.

Residential gateways - provide a traditional analog interface to a VoIP network.

Access gateways - provide a traditional analog or digital PBX interface to a VoIP network.

Business gateways

Network Access Servers - can attach a modem to a telephone circuit and provide data access to the Internet.

- can offer a control interface to an external call control element.

 

handles registration, management, and control functionality of resources in the Media Gateway. It performs protocol conversion between PSTN signaling protocols and IP telephony. It gathers information about IP and circuit flows and provides that information to billing and management systems.

The MGCP model consists of endpoints and connections:

 

are sources or sinks of data and could be physical or virtual. An example of a virtual endpoint is an audio source in an audio-content server. Creation of physical endpoints requires hardware installation, while creation of virtual endpoints can be done by software

is an association between endpoints over which data is transmitted. Point-to-point and multipoint connections are possible. Connections may exists over IP networks, ATM networks, or internal connections such as TDM backplanes or gateway backplanes. For point-to- point connections, the endpoint of a connection could be in separate gateways or in the same gateway

A basic network architecture is shown below:

 

 

 

Basic network architecture.

This architecture allows for specialization of function and economies of scale and is expected to become the architecture of choice in next generation converged voice/ data IP networks.



In a classic call scenario between two endpoints, the call agent(s) controlling the endpoints will establish two connections: C1 and C2. Each connection will be designated locally by a connection identifier, and will be characterized by connection attributes.

Assume that User A wants to call User B. User A is located within the IP network, served from a residential gateway and User B is located off-net via the PSTN. When User A picks up the phone, a is sent from the residential gateway to the Call Agent. The Call Agent asks the gateway to create a connection on the endpoint that went off hook by sending a create connection command. The gateway acknowledges to the Call agent the create connection command plus provides a session description. The session description contains information required by a third party, in this case the trunking gateway (G6), to send packets toward the newly created connection. The Session Description Protocol (SDP) is used for this and contains such things as User A's IP address, the UDP port to identify the session, packetization parameters such as compression techniques, and a media type such as RTP audio (voice). The trunking gateway responds to the Call Agent providing its own session description.

The Call Agent uses a modify connection to provide the session description from the trunking gateway to the residential gateway. A two-way full duplex communication can now be set up between the residential gateway (IAD, MTA) and the trunking gateway (G6). When a connection is set up between endpoints, RTP (Real-time Transport Protocol) is used. RTP is an IETF standard that provides end- to-end network transport functions for real time applications such as voice, video, and multimedia. RTP runs on top of UDP because it has multiplexing capabilities, and acknowledgement of packet delivery is not required.

When two endpoints are located on gateways that are managed by the same call agent, the creation is done via the following steps:
 

The Call Agent asks the first gateway (MG 1) to create a connection on the first endpoint. The gateway allocates resources to that connection, and respond to the command by providing a session description that contains IP address, UDP port, etc.

The Call Agent asks the second gateway to create a connection on the second endpoint. The command carries the session description provided by the first gateway. The gateway allocates resources to that connection, and respond to the command by providing its own session description.

The Call Agent uses a ModifyConnection command to provide this second session description to the first endpoint. Once this is done, communication can proceed in both directions.

The Call Agent removes a connection by sending to the gateway a DeleteConnection command. The gateway may also, under some circumstances, inform a gateway that a connection could not be sustained.

 

 

 

MGCP call setup.





Currently, MGCP and Session Initiation Protocol (SIP) are the two carrier-class interoperability protocols with the most promise of becoming industry standards. The inherent simplicity of these protocols makes them easy to deploy in networks, and numerous industry vendors already are implementing MGCP and SIP into Voice-over-Internet Protocol (VoIP) solutions.

MGCP is central to VoIP solutions and may be integrated into products such as:

 

Central Office Switches

Gateways



Cable Modems

PBXs, etc., in order to develop a convergent voice and data solution

There are several advantages of using MGCP and IP-based communications systems over traditional telephony engineering models:

 

Provides simplicity and reliability.

Programming difficulties are concentrated on MGCs and not on the protocol.

Service providers can develop reliable and cheap local access system.

Provides synchronization through MGC.

There are carrier class MGCP/Megaco media servers available today and deployed in the field. SIP lags MGCP/ Megaco in this respect.

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.

Softswitches already use MGCP/ Megaco and event packages for Media Gateway control, and can reuse much of this functionality for media server control.