What is Voice Over IP (VOIP) ?
IP telephony uses the Internet protocol to transmit voice
as packets over an IP network. This means that IP telephony
can be achieved, at least in principle, on any data network
that uses IP, such as the internet or any of a growing number
of intranets and local area networks (LANs). This is in contrast
to traditional circuit switched telephony, where an end-to-end
circuit is set up between two telephones. A circuit switched
connection is established for the duration of every telephone
call, with a fixed bandwidth (64 kbit/s) reserved even during
In an IP telephony connection, the voice signal is digitized,
compressed and converted into IP packets, which are transmitted
over the IP network and shared with other IP traffic. A normal
telephone call is connected through an end-to-end circuit
with a fixed bandwidth.
With IP telephony, a packet-based network is used where a
number of calls share the same network link. The data networking
technology moves information at a much lower cost by making
better use of the network capacity. Not only is a packet-
shared network more efficient than a fixed 64 kbit/s circuit
switched connection, but it also compresses the voice signal.
Typically IP telephony is handled by a voice gateway that
is placed between the PSTN and the IP network. The Voice Gateway
provides the physical interface between the telephone network
and the IP network. The Gateway handles the signaling to and
from the telephone network, reception of telephone numbers,
conversion between telephone numbers and IP addressing in
the IP network, as well as voice processing.
The voice processing implies reception of the voice signal,
compression and packetization, echo cancellation, silence
suppression, etc. The gateway compresses the voice signal
for two reasons: to reduce the amount of bandwidth required
in order to reduce cost and to reduce the delay impact from