Media Gateway Control Protocol (MGCP) Technol…

Overview

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) and a Media Gateway Controller (MGC) - (also known as Call Agents or Soft Switches), 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 Architecture

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 - provide a traditional digital PBX interface or an integrated "soft PBX" interface to a VoIP network.
  
  
• Network Access Servers - can attach a modem to a telephone circuit and provide data access to the Internet.
  
  
• Circuit switches (packet switches) - can offer a control interface to an external call control element.
  
  

The Media Gateway Controller (MGC) 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:

• Endpoints 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
  
  
• A connection 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:

Figure 1. 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.

MGCP Call Setup

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.

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