RE: modems

What, no mention of constellations?

When you talk of baud or symbols per second, it helps to keep in mind that 
a symbol is a shift from one analog 'waveform' to another.  These analog 
waves have three characteristics:  frequency, phase, and amplitude.  The 
earlier modems simply shifted between two frequencies (e.g. Bell 103, if 
memory serves).  The later V.xx standards typically used a combination of 
amplitudes and phases.  If you plotted these on an 2-D surface it appears 
like a grid of dots.  Depending on the number of dots, you get many 
different possible states to jump between.  For example, two amplitudes and 
4 phases would produce 8 states, which are numbered 000-111.  Thus a shift 
to a new state represents an output of three bits.
You can only take this so far as it becomes harder and harder to "squeeze" 
more dots in and still distinguish one state from another.  To understand 
where this limit exists, study Shannon's Theorem.  After that 
pre-compression algorithms come into play.
Mike


At 06:26 PM 6/12/2002 +0200, TOMSON ERIC wrote:
> 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?