Answer Posted / swarna sekhar dhar
The basic principle of Voip is very simple. It's the same
technology you have probably used already to listen to
music over the Internet. Voice sounds are picked up by a
microphone and digitized by the sound card. The sounds are
then converted to a compressed form, compact enough to be
sent in real time over the Internet, using a software
driver called a codec. The term codec is short
for "encoder/decoder". The sounds are encoded at the
sending end, sent over the Internet and then decoded at the
receiving end, where they are played back over the
speakers. The only requirements are a connection between
the two computers of an adequate speed, and matching codecs
at each end.
To be usable, a Voip system also needs a method for
establishing and managing a connection, for example,
calling the other computer, finding out if they accept the
call, and closing the connection when a user hangs up.
Because Voip allows two way communication, and even
conference calls, it's a lot more complicated than simple
audio streaming. How calls are managed is the area in which
Voip systems fundamentally differ, and two Voip users must
be using the same system (or compatible ones) in order to
be able to call each other.
Because most Internet users don't have a permanent Internet
address (IP address, a number like 212.44.88.17 that
uniquely identifies that computer, at that moment), Voip
systems don't generally work by calling another computer
direct although that may be an option for those who do
have a permanent address. Instead, each user of the service
registers with an intermediate server, which maintains a
record of their IP address all the time they are connected.
An example of a Voip application that works this way is
Picophone. The small size of the PicoPhone application file
(about 64Kb, barely larger than Windows Notepad)
demonstrates clearly that the basic principles of Voip are
not complicated to implement.
Another reason for using an intermediate server is that it
eases the problem of getting Voip to work through the
firewalls that everyone uses these days. Many firewalls
block any data from the Internet that is not sent in
response to a specific request. This makes it impossible to
call another computer direct. Because the called computer
did not request any data from the caller, the call request
would be blocked. By establishing a connection with a
server, the Voip software opens a channel of communication
through which other computers can call it. Communication
may continue using the server, or information may be passed
via the server that allows the two computers to open a
direct connection between them and continue using that.
source : http://www.tech-pro.net/voice-over-ip.html
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