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.