Session controllers join H.323 and SIP

Deployment of voice-over-Internet Protocol (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/ Session Initiation Protocol (SIP) interworking to solve the complexities of connecting a diverse set of voice over IP (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.

In addition, relevant H.245 terminal-capability-set parameters from the H.323 endpoint must be mapped to SDP destined for the SIP user agent and vice versa so that the endpoints can convey their capabilities to each other. In this way, the session controller translates the messages between the two protocol sets.

For more-advanced capabilities such as call hold, call transfer and dual-tone multifrequency (DTMF) processing, session controllers must translate messages and also bridge between the protocol sets to provide total seamlessness.

Low bit-rate codecs, such as G.723.1, render DTMF tones unintelligible and require the use of specially marked Real-time Transport Protocol (RTP) packets such as Remote Function Call (RFC) 2833 or out-of-band methods to carry DTMF information.

RFC 2833 uses the RTP stream to carry DTMF data and is the transport method of choice if the SIP user agent and H.323 endpoint support it.

By resolving the signaling conflict between H.323 and SIP using session controllers, companies successfully can deploy cost-effective, H.323-based IP PBXs while building toward an SIP-based applications architecture. Session controllers provide IT managers with an off-the-shelf solution that provides H.323/SIP interworking functions.

- Ramachandran is co-founder and Chief Technology Officer of NexTone Communications. He can be reached at

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