80 lines
3.4 KiB
HTML
80 lines
3.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<HEAD>
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<META NAME="GENERATOR" CONTENT="SGML-Tools 1.0.9">
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<TITLE>VoIP Howto: Overview</TITLE>
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<LINK HREF="VoIP-HOWTO-4.html" REL=next>
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<LINK HREF="VoIP-HOWTO-2.html" REL=previous>
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<LINK HREF="VoIP-HOWTO.html#toc3" REL=contents>
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</HEAD>
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<BODY>
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<A HREF="VoIP-HOWTO-4.html">Next</A>
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<A HREF="VoIP-HOWTO-2.html">Previous</A>
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<A HREF="VoIP-HOWTO.html#toc3">Contents</A>
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<HR>
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<H2><A NAME="s3">3. Overview</A></H2>
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<H2><A NAME="ss3.1">3.1 What is VoIP?</A>
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</H2>
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<P>VoIP stands for 'V'oice 'o'ver 'I'nternet 'P'rotocol. As the
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term says VoIP tries to let go voice (mainly human) through IP packets
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and, in definitive through Internet. VoIP can use accelerating hardware
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to achieve this purpose and can also be used in a PC environment.
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<H2><A NAME="ss3.2">3.2 How does it work? </A>
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</H2>
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<P>Many years ago we discovered that sending a signal to a remote
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destination could have be done also in a digital fashion: before
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sending it we have to digitalize it with an ADC (analog to digital
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converter), transmit it, and at the end transform it again in analog
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format with DAC (digital to analog converter) to use it.
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<P>VoIP works like that, digitalizing voice in data packets, sending
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them and reconverting them in voice at destination.
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<P>Digital format can be better controlled: we can compress it,
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route it, convert it to a new better format, and so on; also we saw
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that digital signal is more noise tolerant than the analog one (see
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GSM vs TACS).
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<P>TCP/IP networks are made of IP packets containing a header (to
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control communication) and a payload to transport data: VoIP use
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it to go across the network and come to destination.
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<P>
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<PRE>
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Voice (source) - - ADC - - - - Internet - - - DAC - - Voice (dest)
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</PRE>
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<H2><A NAME="ss3.3">3.3 What is the advantages using VoIP rather PSTN?</A>
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</H2>
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<P>When you are using PSTN line, you typically pay for time used
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to a PSTN line manager company: more time you stay at phone and more
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you'll pay. In addition you couldn't talk with other that one person
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at a time.
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<P>In opposite with VoIP mechanism you can talk all the time with
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every person you want (the needed is that other person is also connected
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to Internet at the same time), as far as you want (money independent)
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and, in addition, you can talk with many people at the same time.
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<P>
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<P>If you're still not persuaded you can consider that, at the same
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time, you can exchange data with people are you talking with, sending
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images, graphs and videos.
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<H2><A NAME="ss3.4">3.4 Then, why everybody doesn't use it yet?</A>
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</H2>
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<P>Unfortunately we have to report some problem with the integration
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between VoIP architecture and Internet. As you can easy imagine,
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voice data communication must be a real time stream (you couldn't
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speak, wait for many seconds, then hear other side answering): this
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is in contrast with the Internet heterogeneous architecture that
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can be made of many routers (machines that route packets), about
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20-30 or more and can have a very high round trip time (RTT), so
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we need to modify something to get it properly working.
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<P>In next sections we'll try to understand how to solve this great
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problem. In general we know that is very difficult to guarantee a
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bandwidth in Internet for VoIP application.
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<HR>
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<A HREF="VoIP-HOWTO-4.html">Next</A>
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<A HREF="VoIP-HOWTO-2.html">Previous</A>
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<A HREF="VoIP-HOWTO.html#toc3">Contents</A>
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</BODY>
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</HTML>
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