Hi, guys. Here is my contribution to this surprising debate about modems. :)
To the point of view of an application running above the Application layer 7,
the Data Link layer 2 receives BITS from the Physical layer 1 and organizes
them into FRAMES, before transmitting its contents (the payload) to the Network
layer 3, and so on (actually, at each layer, the payload is extracted and
transmitted to the upper layer, where it becomes that upper layer's Protocol
Data Unit).
To the point of view of a transmission medium (copper cable, fiber optics,
radio waves, etc.), BITS come from the Data Link layer 2 into the Physical
layer 1, where they are converted into a specific signal that can be
transmitted on the given medium. In case of copper cabling, it will be an
electric signal. In case of fiber optics, it will be light pulses, etc.
Now, talking about modems. Modems were invented to transmit digital data over
an analog line. It means that the data coming from a computer is digital (i.e.
a non-continuous signal, carrying a limited number of values/levels) and needs
to be converted into an analog signal (i.e. a continuous signal, carrying an
infinite number of values/levels), using frequencies limited to the range
accepted by the telephone network.
The range of frequencies accepted by the telephone network (i.e. the BANDWIDTH)
is theoretically between 0Hz and 4000Hz - more practically between 300Hz and
3500Hz. Why not higher frequencies? Because high frequencies are more
sensitive, more fragile, and are corrupted first during the transmission,
corrupting then the whole signal - then the whole telephone conversation. So,
this filtering guarantees a minimum quality of a telephone conversation.
Why that range of frequencies instead of another one? Because it's the range
used by human voice - what we actually want to transmit over a telephone
network, plus some harmonics necessary to ensure a minimum quality of speech
and to allow the speaker to be recognized and then identified.
As the telephone network was the only omnipresent, global, ubiquitous network
available at that time (the 50's), it was obvious that it should be used to
interconnect computers through long distances. But computers didn't use analog
signals (a fortiori since Von Neumann strongly recommended a digital
architecture for computers during the 40's).
So BELL LABs developed the MODEM to convert digital signals into analog signals
to be able to transport data through the telephone network, and then convert
back analog signals into digital signals on the other end (for the destination
computer to understand the data transmitted through the telephone network).
Remember : the analog lines were low-pass-filtered and then limited to a
maximum of 3500Hz (then a range of frequencies that can be heard by human ear -
compare to the range of frequencies supported by an ordinary Hi-Fi system,
usually covering 20Hz to 20000Hz). So, when MODULATING the incoming digital
signal, the modem created an analog signal in the range of frequencies between
300Hz and 3500Hz - then a signal that can be HEARD, hence the noise generated
by a modem.
Now, why do we hear noise only at the beginning of the transmission? Because
modems are configured to let users hear noise only at the beginning of the
transmission to have the audible confirmation that it's working. After some
seconds, the internal modem's speaker is turned off to prevent annoying the
users.
What about the baudrate, the bitrate and "modulation"? Well, modems will
communicate through the telephone network by exchanging an analog, audible
signal. How to transport bits and bytes with such a noisy signal? By modulating
one or several of the characteristics of this noisy, analog signal, which are :
the frequency, the amplitude, the phase. Note that, by combining several
modulation techniques, you increase the number of bits that can be represented
- then transported. That's why, today, with a baudrate of 2400 bauds per
second, we can transmit 33600 bits per second (because we transport 14 bits per
baud), while some years ago we transported 1 bit per baud at a baudrate of 300
bauds per second, achieving a bitrate of 300 bits per second.
Why can't we hear Gigabit Ethernet? 1.In case of copper cabling (IEEE 802.3ab),
it's a pure baseband technology, using pure digital signalling, using
frequencies of 80MHz (80 millions of Hertz!!!) on Category 5 UTP copper
cabling, and that's far, far higher than the highest audible frequency. 2.In
case of fiber optics (IEEE 802.3z), it's a pure light transmission, using pure
light signalling on fiber optics, and that's not at all audible - only visible.
:)
Last word : computers don't communicate by screeching or talking or whatever :
they communicate by exchanging a specific signal through a physical medium.
Depending on this medium, this signal will be either electrical (copper
cabling) or optical (fiber optics) or radio or infra-red or micro-wave (etc.).
I hope I could combine the engineer and academic points of view in a
comprehensible, yet proper way. ;)
P.S.: if one of you detects a mistake or an error, please let me know - I'm
always learning. Every single day of my life.
-----Original Message-----
From: Bill Cunningham [mailto:billcu(_at_)citynet(_dot_)net]
I know modems communicate on the physical layer by electrical pulses or
binaries sent on copper wires. Is that screeching you hear electrical
communication? Computers don't communicate by screeching...or do they?