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<H2><A NAME="s3">3. Overview</A></H2>
<H2><A NAME="ss3.1">3.1 What is VoIP?</A>
</H2>
<P>VoIP stands for 'V'oice 'o'ver 'I'nternet 'P'rotocol. As the
term says VoIP tries to let go voice (mainly human) through IP packets
and, in definitive through Internet. VoIP can use accelerating hardware
to achieve this purpose and can also be used in a PC environment.
<H2><A NAME="ss3.2">3.2 How does it work? </A>
</H2>
<P>Many years ago we discovered that sending a signal to a remote
destination could have be done also in a digital fashion: before
sending it we have to digitalize it with an ADC (analog to digital
converter), transmit it, and at the end transform it again in analog
format with DAC (digital to analog converter) to use it.
<P>VoIP works like that, digitalizing voice in data packets, sending
them and reconverting them in voice at destination.
<P>Digital format can be better controlled: we can compress it,
route it, convert it to a new better format, and so on; also we saw
that digital signal is more noise tolerant than the analog one (see
GSM vs TACS).
<P>TCP/IP networks are made of IP packets containing a header (to
control communication) and a payload to transport data: VoIP use
it to go across the network and come to destination.
<P>
<PRE>
Voice (source) - - ADC - - - - Internet - - - DAC - - Voice (dest)
</PRE>
<H2><A NAME="ss3.3">3.3 What is the advantages using VoIP rather PSTN?</A>
</H2>
<P>When you are using PSTN line, you typically pay for time used
to a PSTN line manager company: more time you stay at phone and more
you'll pay. In addition you couldn't talk with other that one person
at a time.
<P>In opposite with VoIP mechanism you can talk all the time with
every person you want (the needed is that other person is also connected
to Internet at the same time), as far as you want (money independent)
and, in addition, you can talk with many people at the same time.
<P>
<P>If you're still not persuaded you can consider that, at the same
time, you can exchange data with people are you talking with, sending
images, graphs and videos.
<H2><A NAME="ss3.4">3.4 Then, why everybody doesn't use it yet?</A>
</H2>
<P>Unfortunately we have to report some problem with the integration
between VoIP architecture and Internet. As you can easy imagine,
voice data communication must be a real time stream (you couldn't
speak, wait for many seconds, then hear other side answering): this
is in contrast with the Internet heterogeneous architecture that
can be made of many routers (machines that route packets), about
20-30 or more and can have a very high round trip time (RTT), so
we need to modify something to get it properly working.
<P>In next sections we'll try to understand how to solve this great
problem. In general we know that is very difficult to guarantee a
bandwidth in Internet for VoIP application.
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