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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.
VoIP Extensio
n
PC software Phones
IP Network
DT Handsets
INDeX
Line Cards
PSTN Network
INDeX System
IPNC
Gatekeeper
Gateway
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