Trunk Devices

This chapter discusses the gateway protocols that Cisco CallManager uses to communicate with Cisco gateway and trunk devices.

The primary purpose of station devices is to provide telephony services to individuals, and the primary purpose of gateway devices is to connect telephony networks to each other. CallManager uses gateway devices to allow IP telephony users to connect to non-IP-enabled telephony networks such as the Public Switched Telephone Network (PSTN) or on legacy circuit-switched Private Branch Exchanges (PBX).

CallManager also uses IP-to-IP trunking to connect members in one CallManager cluster to other voice over IP (VoIP) networks. In addition, IP trunking is used to connect one CallManager cluster to another CallManager cluster when an enterprise deploys multiple clusters for scalability, geographical, or administrative reasons.

Because CallManager supports a variety of circuit-switched and VoIP protocols, one function it can serve in your network is as a protocol translator, to connect gateways that support different call signaling protocols.

This chapter includes the following sections:

• "Architectural Overview of Trunk Devices" presents the general structure whereby CallManager supports voice gateways and IP trunks.
• "Overview of Circuit-Switched Interfaces" discusses the analog and digital protocols that traditional telephony networks use to communicate with gateways associated with CallManager. This section also discusses a variant of Q.931 called QSIG, which is designed to foster feature transparency between PBXs.
• "VoIP Gateway Security" briefly addresses the techniques you can use to provide for authenticated, authorized, and private communication from Cisco gateways.
• "H.323 Gateways" discusses CallManager's use of the H.323 ITU-T protocol for control of gateways. It describes the basic components of an H.323 network, the individual protocols that comprise H.323, and the use of CallManager intercluster trunks, which run a variant of H.323, to connect CallManager clusters together, either directly or via an H.323 gatekeeper. This section also provides a detailed breakdown of the fields in H.323 messages that CallManager supports.
• "MGCP Gateways" discusses CallManager's use of MGCP for control of VoIP-to-circuit-switched gateways.
• "SIP" discusses CallManager's support for the Session Initiation Protocol (SIP), which currently permits CallManager to connect in a basic way to SIP networks.

Appendix C, "Protocol Details," provides detailed information for H.323, QSIG, and SIP signaling protocols. After reviewing this chapter, refer to Appendix C for additional, related information.