DP> in my life to complete this assignment I have for university. I have to say 
DP> that I do not really know much about how information travels through a 
DP> telephone line across the world.

You do not want to know the full answer to that question, because there are
so many layers in the system that it's very, very, very complicated. Taking
the next-from-simplest case where you are using two (modern, V.34) modems
on one telephone network, your modem takes an input data stream (eg the
ASCII characters for the word "Fred"), and turns it into an
analog waveform for transmission over an audio link (phone system). There
is a complication here :
the absolute maximum number of information units (symbols) per second which
can travel down a "standard" phone line is about 3500. Your modem
wants to send 28,800 bits per second (or more) down the line. So it has to
use a special-purpose chip called a DSP (Digital Signal Processor) to
implement a system of shifting mixed waveforms to encode multiple bits into
each symbol. Most modems use an internationally-agreed set of standards
devised by an organization formerly called the CCITT, now called the ITU-T
(the latter is the former name translated from French to English, roughly).
The ITU-T standard for 28800bps data communications is called V.34. V.34
uses a modulation system called Trellis Code Modulation, which is the most
complex system currently in use for voiceband comms.

Anyway, take it as read your data is now in a form the telephone network
can transmit. Your phone line is analog, so your signal goes down to the
nearest telephone exchange as an analog waveform. During this process it
typically picks up some noise. When it hits the exchange, the first thing
the exchange does (if it's reasonably modern, e.g. an Alcatel System 12) is
digitize the signal. Australian exchanges use an 8kHz sampling rate. If you
have a sound card in your PC, and some sound recording software, grab it,
set the software up to record at 8000Hz, 8 bit sampling, and record
yourself saying something. Save the file. That file contains the type of
data your exchange is working with. The exchange will compress the data
going through it to squeeze as many calls as possible onto the one channel
- PKZIP the file you just saved and you'll see what I mean.

Your call data now gets bundled onto a very high-speed fiber optic digital
link to the exchange servicing the other modem you're talking to. You're in
Sydney, I'm in Melbourne... Telstra has so much cable between our two
cities that every man, woman and child in each city could be holding about
two and a half conversations to the other city, at once. Something like 80%
of the system's capacity is unused even at peak times.

And then when it gets to the other end, the reverse steps occur. The remote
exchange decompresses the received data stream (PKUNZIP), and converts it
back into an analog waveform. It sends it down an analog line to the other
modem, which converts the complex "chords" back into a digital
data stream and sends them into the remote host.

That's not even beginning to take into account things like error correction
and data compression, both in the modems and in the PSTN, and the various
high-level transfer protocols involved in getting your echomail message to
(say) my system.

Like I say, you don't want to know the answer.
-- Lewin A.R.W. Edwards [Team OS/2]  Tel 0419320415 * 0412809805 * 0414927056
@EOT:

--- MsgedSQ/2 3.35