Deployment of voice-over-IP endpoints, such as Session Initiation Protocol phones and H.323 IP PBXs, within corporations has presented IT with new interoperability challenges. A new breed of network equipment called session controllers offer H.323/SIP interworking to solve the complexities of connecting a diverse set of VoIP and other collaborative applications.

Although H.323 and SIP address similar requirements, the mechanics of how they perform call setup, media negotiation and call tear-down makes them incompatible and prevents direct connectivity between SIP and H.323 endpoints.

That's where a session controller comes in. A session controller provides services between H.323 and SIP endpoints. In effect, this hardware/software combination operates simultaneously as an H.323 gatekeeper and SIP proxy server. In addition, it provides the SIP/H.323 interworking function that enables any-to-any connectivity between endpoints.

The H.323 gatekeeper provides address translation, and controls access to the network for H.323 endpoints. The SIP proxy provides the primary capabilities required for call-session management in a VoIP net and processes SIP requests and responses.

H.323 endpoints provide real-time, two-way communications. An H.323 endpoint can offer speech only; speech and data; speech and video; or speech, data and video. A SIP user agent is equivalent to an H.323 endpoint.

H.245 is the ITU-T recommendation that describes how H.323 endpoints perform mode-switching and exchange capabilities such as codec support via the terminal capability set. SIP Session Description Protocol (SDP) is the SIP equivalent to H.245.

When calls are placed between an H.323 endpoint and an SIP user agent, logically, the session controller views a call as two call legs - an ingress leg terminating on the session controller and an egress leg that the session controller generates. The protocol used for the egress call leg is determined dynamically and is triggered by the protocol type provisioned for the remote destination.

The session controller's interworking function must support all mandatory features of SIP and H.323 and user addressing (that is, phone numbers) must be protocol-independent with common registration paradigms adhered to.