January 12, 2001, 3:46 PM — Last time, we introduced media gateways and the role they play between the PSTN and a voice-over-IP network. Yet we would be incomplete in our review if we didn't mention a critical standard that's part of the conversion process: ITU-T Recommendation H.323, Packet-based multimedia communications systems. (This recommendation has several companion standards, such as ITU-T H.225, H.245 and H.320; we'll tackle these in future newsletters.)
To complete a voice session, H.323 specifies five phases for call completion:
- Endpoint capability exchanged.
- Establish the call.
- Call services (such as bandwidth utilization).
- Call termination.
The H.323 standard, often synonymous with "voice over IP," describes the roles that must be played for successful interoperability of the gateway, terminal devices, gatekeeper and multipoint control unit. We discussed gateway functions in our last newsletter; the roles of the remaining three elements are equally important.
The terminal device is responsible for digitizing and packetizing the call according to ITU-T G.711 and, optionally, G.723.1 and G.729. The multipoint control unit provides control capability when three or more endpoints are required, such as in a conference call.
The gatekeeper provides for:
Address control, as defined by the registration, admission and status (RAS) standards.
- Address translation, for example from an E-164 telephone address to an IP address.
- Bandwidth control, using endpoint RAS messages.
- Bandwidth management.
- Call authorization.
- Call control signaling.
- Call management.
While H.323 has been widely accepted as the standard protocol for voice-over-IP networking, another protocol called Session Initiation Protocol, or SIP, has been gaining popularity as an alternative. SIP, a client-server protocol, was originally created to control multimedia calls over the Internet and is similar to HTTP, which is widely used on the Web.
For a 30-minute tutorial "Voice Signaling in a VoIP World" from Webtorials.Com, click here.