MGCP was created for a centralized architecture, where most of the configuration and call-control intelligence resides on a call agent, such as Cisco CallManager. The traditional role of an MGCP gateway is media translation. PSTN connections, such as Foreign Exchange Office (FXO), Foreign Exchange Station (FXS), and PRI lines, typically terminate in the gateway. The gateway then translates between the PSTN and the IP network. MGCP gateways are sometimes divided into two categories:
This is somewhat of an artificial distinction, because MGCP gateways can contain both analog and trunk interfacesthus functioning as both types of gateways, in addition to IP WAN interfaces.
An MGCP gateway routes calls in response to instructions from the call agent (in this case the Cisco CallManager). These calls could be to or from a telephone on the PSTN, or across a WAN to an IP or analog phone at a remote site. The gateway does not make call routing decisions. It needs to be able to reach a CallManager before it can handle calls. You can specify multiple CallManagers for the gateway to use. CallManager also controls other aspects of the call, such as the use of Resource Reservation Protocol (RSVP). Caller ID is available on digital lines but not on analog lines.
MGCP is an extensible protocol. Added functionalities are described as packages. In Chapter 8, "Connecting to an IP WAN," the section on WAN security describes the Secure Real-Time Protocol (SRTP) package that you can use to encrypt voice traffic. Other packages include capabilities for Real-Time Protocol (RTP), trunks, dual tone multifrequency (DTMF) tones, and announcement servers. You can see the full list in the output of the global configuration command mgcp package-capability ?.
When you are using an MGCP gateway, all the dial plan knowledge resides on the CallManager. You do not need to configure dial peers, unlike H.323 and SIP. However, this leaves the gateway unable to route calls if it cannot reach a CallManager. To remedy this, the MGCP Fallback feature was developed to allow gateways to fall back to using H.323 when a CallManager is unavailable. Fallback configuration enables the voice ports that were registered to the Cisco CallManager using MGCP to be available for routing calls to the PSTN. This is frequently used in conjunction with Survivable Remote Site Telephony (SRST) on the MGCP gateway. When IP phones register with the SRST router, they can use these voice ports.
MGCP is the protocol of choice when you need any of the following features:
As with any protocol, MGCP has its pros and cons.
MGCP pros are as follows:
MGCP cons are as follows:
Part I: Voice Gateways and Gatekeepers
Gateways and Gatekeepers
Part II: Gateways
Media Gateway Control Protocol
Session Initiation Protocol
Connecting to the PSTN
Connecting to PBXs
Connecting to an IP WAN
Influencing Path Selection
Configuring Class of Restrictions
SRST and MGCP Gateway Fallback
Using Tcl Scripts and VoiceXML
Part III: Gatekeepers
Part IV: IP-to-IP Gateways
Cisco Multiservice IP-to-IP Gateway
Appendix A. Answers to Chapter-Ending Review Questions