414 APPENDIX D: CONNE XTIONS H.323 GATEWAYThe H.323 protocol addresses these main areas:■ Negotiated Connections■ Negotiated Voice Compression■ Standard Extensions■ Remote Internet Device ConnectionsNegotiatedConnectionsThe H.323 protocol adds negotiated call setup and tear-down capabilitiesto Internet Protocol (IP) connections. It exists because Internet protocolswere designed to deliver text messages and computer files in datapackets. IP networks were not originally concerned about involving aperson in a real-time conversation as a telephone does.H.323 provides call setup capability to negotiate the readiness of twoparties to exchange information and how they do it. It then keeps theconnection alive until one of the parties ends the connection. A calltear-down signal indicates to the network, and to the other party, when acall ends. On standard telephone networks, the telephone company usesthis signal to determine when to start and stop charging for long distancecalls, but long distance charges do not normally apply to H.323 calls.Other reasons for call setup and tear-down signals are to indicate whenan IP network can release bandwidth to support other calls, and toinform other devices, such as voice mail systems, when to stop theirconversation-related activities.Negotiated VoiceCompressionIP networks can carry a lot of traffic, creating competition for theavailable bandwidth. Devices have the best access, and the least delay,when they communicate messages by using fewer and smaller packets.This also means lower cost.Voice compression offers a way to reduce the number and size of thedata packets needed to carry each second of a voice conversation.However, voice compression needs high-speed processors to remove theredundancies that are inherent in the way standard voice is represented.The international standard for representing voice (G.711) requires 64 Kbfor each second of conversation. NBX Business and Basic Telephonescontain a digital signal processor (DSP) that transforms spoken voice intothis form. An Ethernet interface, also within each telephone, breaks upthe 64 Kbps stream into frames, adds addressing and error checking, anddumps the voice-data frames (now 83 Kbps) onto a 10 Mbps LAN.
