You can deploy gateways with many different kinds of call control agents, and they might perform different roles with different ones. The following sections examine some types of call control agents and some gateway considerations for each type.
Cisco CallManager is a call processing application that runs on a server. It is a part of the Cisco Unified Communications solution. You can install CallManager software on multiple clustered servers to achieve high availability and scalability. CallManager can manage both local and remote phones and gateways. For increased scalability, you can deploy multiple clusters and achieve end-to-end IP voice and video communication. CallManagers handle address resolution and call admission control for their registered phones, but they can also interact with H.323 gatekeepers for those functions. Cisco CallManagers can interact with gateways using MGCP, SCCP, SIP, and H.323.
Cisco CallManager Express
CME is an application on a Cisco IOS router that provides standalone call control for small to medium sites. The number of phones supported varies with the type of router, up to a maximum of 240. With CME, the same router can perform typical gateway functions, such as PSTN connectivity, and also call processing functions. Although CME does not provide all the features of a full CallManager installation, it meets the needs of smaller sites. In addition, you can use extensible markup language (XML) applications to provide users with enhanced features. CME can interoperate with gateways using SCCP, H.323, and SIP.
CME can also integrate with a CallManager network. You can use H.323 trunks between CallManager and a CME router to carry calls over an IP WAN. If the site grows, you can expand call capacity by adding the site to a CallManager cluster and using the router solely as a gateway.
SIP Proxy Server
The Cisco SIP proxy server application runs on a server, providing call routing and control services to SIP endpoints, such as phones or PC applications. It combines the functions of a standard SIP proxy server and a SIP registrar. Endpoints register with the proxy server, which builds a database. When the server receives a SIP call setup message, it attempts to resolve the address with its local information, before forwarding a request to remote servers. It can also authenticate and encrypt SIP messages. You can use redundant servers to provide high availability and load balancing.
Cisco Enterprise Gateway
The Enterprise Gateway (EGW) is an application that runs on the same type of server as CallManager. It acts as a transition point for VoIP and legacy TDM networks, providing call control, signaling, and routing for calls between the two networks. It eases the migration to a CallManager VoIP network by allowing legacy PBXs to communicate with CallManager-controlled phones and Unity voice-mail servers. It also facilitates deploying toll bypass with legacy PBXs. When using an EGW, voice-enabled routers connect to the PBX or the PSTN, acting as media gateways. The router backhauls call control information to the EGW and does the actual media conversion between the TDM and packet networks. The EGW uses MGCP to control the media gateway, SIP, to communicate with a Unity server, and H.323 to communicate to devices in the CallManager network.
PBX with Toll Bypass
A call agent does not have to reside on the VOIP networka PBX can perform that function for a gateway. A company that wants to retain its existing legacy telephone structure can leverage its IP WAN for intrasite calls. You would need to connect a voice gateway to the PBX using a telephony interface, such as an E&M or PRI. You also need to provide access to the WAN. When an analog phone calls another company site, the PBX can route it to the voice gateway rather than across the PSTN. The PBX can also perform call admission control, rerouting calls to the PSTN when the link between it and the gateway is busy. This is called toll bypass. The voice gateway translates between the legacy voice and the IP voice signaling and media.
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