Avaya 38DHB0002UKDD manual Voice Over IP Basics, Gateway

Appendix B: Concepts Voice Over IP Basics - Page 123

Voice Over IP Basics

The diagram below shows the components in a H323 network and shows the

IPNC and the INDeX platform offering the components of a VoIP network.

IPNC

VoIP Extension

Gatekeeper

IP Network

Gateway

INDeX System

INDeX

PC software Phones

Line Cards

DT Handsets

PSTN Network

Gateway

The gateways are the devices that communicate between the telephone signals and the IP endpoint. The gateways usually perform the following six functions:

•Search function

•When an IP gateway is used to place a call across an IP network, it receives a called party phone number. It converts it into the IP address of the far end gateway, possibly through a table lookup in the originating gateway or in a centralized directory server.

•Connection Function

•The originating gateway establishes a connection to the destination gateway, exchanges call setup, compatibility information and performs any option negotiation and security handshake.

•Digitising function

•Analogue telephone signals coming into a trunk on the gateway are digitised by the gateway into a format useful to the gateway. This requires the gateway to interface to a variety of Telephone-signalling conventions.

•Demodulation functions

•With some gateways the gateway trunk can accept only a voice signal or a fax signal but not both. But sophisticated gateways handle both. When the signal is a fax, it is demodulated by the DSP back into the original 2.4-14.4 kbps digital format. This is then put into the IP packets for transmission. The demodulated information is remodulated back to the original analog fax signal by the remote gateway, for delivery to the remote fax machine.

•Compression functions

•When the signal is determined to be voice, it is usually compressed by a DSP from 64K PCM to a 6.4 Kbps signal, which is the G.723.1 standard.

•Decompression and Remodulation functions.

At the same time that the gateway performs steps 1-5, it is also receiving packets. Hence this function is required

INDeX IPNC Cassette Administration Manual	Appendix B: Concepts - Page 123
38DHB0002UKDD – Issue 7 (22/11/02)	Voice Over IP Basics