Media Gateway Control Protocol (MGCP)http://publib.boulder.ibm.com/infocenter/sametime/v8r0/index.jsp?topic=/com.ibm.help.sametime.telephony.doc/03-Technologien3.html

MGCP OverviewWith the introduction of Voice over IP (VoIP), communication between IP-based and classic PBX environments had to be ensured. For this purpose, so-called media gateways were implemented to bridge the networks. The MGCP protocol can be used to control such gateways by a so-called call agent.

The following figure provides an overview of the relevant components in an MGCP-based network.

The central network components in terms of MGCP usage are:

• The media gateway

• The Call Agent - also called Media Gateway Controller MGC.

Let us take a closer look at the tasks of these two components.

The media gateway

The media gateway operates as an interface between a VoIP and a classic telephone network.

Its task is to

• terminate ISDN channels and to generate IP-based media streams from the voice or video information transmitted in them. In the opposite direction, the reverse occurs: conversion of IP packets to classic voice or video signals.

• set up, in the direction of the IP network, a direct RTP connection to the target system of a communication connection. This target system can be another gateway or an IP-based terminal device.

The call agent

The call agent controls and manages the IP-based communication connections in a VoIP network. It provides the media gateways and IP-based terminal devices of an MGCP-based network with all the information they need to set up a direct RTP connection.

If several call agents are used in a network, they communicate with each other, for example, via the SIP or H.323 protocol.

Using MGCP in the Media Server/Media Gateway

Note: MGCP is in the Media Server/Media Gatewau only used.

The following figure shows how the MGCP protocol is used in the Media Server/Media Gateway.

MGCP serves in a PBX environment to control the Media Server by the PBX. In this way, the Telephony Control Server is able to inform the Media Server which announcement it should send to which terminal device. Only after this instruction phase the announcement thus defined can be directly transmitted by the Media Server to the telephone terminal device via the RTP protocol.

The Media Server assumes the role of the media gateway, while the PBX executes the call agent function.

Media Gateway Control Protocol (MGCP)http://searchnetworking.techtarget.com/definition/Media-Gateway-Control-Protocol

Media Gateway Control Protocol (MGCP), also known as H.248, is a standard protocol for handling the signaling and session management needed during a multimedia conference. The protocol defines a means of communication between a media gateway, which converts data from the format required for a circuit-switched network to that required for a packet-switched network and the media gateway controller. MGCP can be used to set up, maintain, and terminate calls between multiple endpoints.

The standard is endorsed by the International Telecommunications Union (ITU) as H.248. H.323, an earlier protocol, was used for local area networks (LANs), but was not capable of scaling to larger public networks. MCGP/H.248 model removes the signaling control from the gateway and puts it in a media gateway controller, which can then control multiple gateways.

The MGCP specifies a protocol at the Application layer level that uses a master-slave model, in which the media gateway controller is the master. MGCP makes it possible

for the controller to determine the location of each communication endpoint and its media capabilities so that a level of service can be chosen that will be possible for all participants. The H.248 version of MGCP supports more ports per gateway, as well as multiple gateways, and support for time-division multiplexing (TDM) and asynchronous transfer mode (ATM) communication.

MGCP uses the Session Description Protocol (SDP) for specifying and negotiating the media streams to be transmitted in a call session and the Real-time Transport Protocol (RTP) for framing of the media streams.

Media Gateway Control Protocol (MGCP)http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/9_0_1/ccmsys/CUCM_BK_CD2F83FA_00_system-guide_chapter_0100111.html

&

http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a00801da84e.shtml

MGCP provides Cisco Unified Communications Manager with a powerful, flexible and scalable resource for call control. Cisco Unified Communications Manager uses MGCP to control media on the telephony interfaces of a remote gateway and also uses MGCP to deliver messages from a remote gateway to appropriate devices.

MGCP enables a call agent (media gateway controller) to remotely control and manage voice and data communication devices at the edge of multiservice IP packet networks. Because of its centralized architecture, MGCP simplifies the configuration and administration of voice gateways and supports multiple (redundant) call agents in a network. MGCP does not provide security mechanisms such as message encryption or authentication.

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.[2][3]

Understanding MGCP Interactions with Cisco CallManagerhttp://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/9_0_1/ccmsys/CUCM_BK_CD2F83FA_00_system-guide_chapter_0100111.html

&

http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a00801da84e.shtml

Media Gateway Control Protocol (MGCP) is a plain-text protocol used by call-control devices to manage IP Telephony gateways.

MGCP (defined under RFC 2705 ) is a master/slave protocol that allows a call control device (such as Cisco CallManager) to take control of a specific port on a gateway. This has the advantage of centralized gateway administration and provides for largely scalable IP Telephony solutions. With this protocol, the Cisco CallManager knows and controls the state of each individual port on the gateway. It allows complete control of the dial plan from Cisco CallManager, and gives CallManager per-port control of connections to the public switched telephone network (PSTN), legacy PBX, voice mail systems, plain old telephone service (POTS) phones, and so forth. This is implemented with the use of a series of plain-text commands sent over User Datagram Protocol (UDP) port 2427 between the Cisco CallManager and the gateway.

Components Used

The information in this document is based on these software and hardware versions:

Cisco CallManager 3.2c

Cisco 7960 IP Phone

Cisco VG200 Voice Gateway

Endpoints

Endpoints are any of the voice ports on the designated gateway. These voice ports provide connectivity to both analog ports (such as Foreign Exchange Office (FXO)/Foreign Exchange Station (FXS)) and digital trunks (such as a T1 or E1) to the PSTN. Ports on gateways are identified by endpoints in very specific ways. It is important to note that gateways can have multiple endpoints dependent on the number of ports it contains, and that the endpoints are case insensitive. This is a sample endpoint and an analysis of each portion of it:

AALN—Analog Access Line eNdpoints. This name is used to designate that the type of endpoint is analog. This means that either FXO or FXS voice interface cards (VICs) are in use. This value changes dependent on what type of endpoint is in use. For example, if a DS3 interface is used, this value would be "ds3". More on the digital endpoint specification is given later in this document.

S1—Slot 1. This is the slot number on the chassis that holds the voice network module.

SU0—Subunit 0. This is the slot number on the voice network module that holds VICs and voice/WAN interface cards VWICs.

0—This is the voice port number on a specific VIC or the VWIC.

av-vg200-1.cisco.com—This is the hostname of the sample endpoint. If the gateway has been configured with a domain name, it is appended to the hostname as seen in this example.

In this endpoint, the voice port 1/0/0 on a gateway with a hostname of av-vg200-1 and a domain name of cisco.com is described. AALN describes this to be an analog port, S1 describes that the network module is in slot 1, and SU0/0 indicates the interface card and port number on the network module.

Here is an example of an MGCP endpoint identifier for T1 PRI. Note the only difference is the trunk type and the B-channel. The trunk type designates what type of trunk the endpoint describes. Some examples of valid trunk types are ds1, ds3, e1, and e3. The B-channel specifies which B-channel on the trunk this endpoint is associated with.

Call Agents

Call agents are external control devices in a voice system. In MGCP, the call agent in this case Cisco CallManager is the device that has complete control of the gateway. This is a very efficient system as all the administration is performed by the call agent. There is very little setup required on the end of the gateway, as all route patterns and dial-plans are configured on the Cisco CallManager.

It is important to remember that this protocol is used for control purposes only. No voice data is transmitted through the MGCP protocol itself. All the voice data transfer occurs directly between the phone and the gateway. This diagram explains these relationships:

The Cisco 7960 IP phones in this example use the Skinny Call Control Protocol (SCCP) to communicate with the Cisco CallManager. The actual voice data is transferred through Real-

time Transport Protocol (RTP) directly between the two devices. MGCP is used by the Cisco CallManager only to control the gateway.

Registration and Endpoint Initialization

This diagram describes how Cisco CallManager registers voice gateways in its database with use of MGCP. The acknowledgment (ACK) commands are standard TCP acknowledgements of the received command:

MGCP Commands MGCP is implemented by a set of commands and responses between the call agent and the gateway transmitted in plain text. These commands are transmitted and received across UDP port 2427. There are eight different types of MGCP commands. This table defines them:

Link:http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/9_0_1/ccmsys/CUCM_BK_CD2F83FA_00_system-guide_chapter_0100111.html

&

http://www.cisco.com/en/US/tech/tk1077/technologies_tech_note09186a00801da84e.shtml