[various snipas] On Wed, 12 Jun 2002, TOMSON ERIC wrote: > 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). well, there's passing error checking/correction, and segmentation and reassembly as well. > Now, talking about modems. Modems were invented to transmit digital > data over an analog line. modulation and demodulation predates this use. > 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? Kudos for using the term 'bandwidth' correctly instead of for link capacity of bits/second... > As the telephone network was the only omnipresent, global, > ubiquitous network available at that time (the 50's), Er, the telegraph? (See 'The Victorian Internet' by Tom Standage.) Neither the phone network nor the telegraph were really ubiquitous in the 50s imo. > 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). 20Hz-20kHz is an odd meme, paricularly if you look at the frequency responses at each end of that range. > 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. a baud is a symbol per second (not a bit per second, as is often believed). So bauds per second would be the rate of change of baud during negotiation/adjusting rate due to noise... > 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!!!) Hertz will let me reserve a car, but not a cycle. Odd, that. Also odd that Hertz is first, not second. You'd think that what with being cycles per second... (I know, appalling jokes. I resolve to try harder.) > 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. :) It's not always visible. (and just because you can't see it doesn't mean it can't burn your retina out.) > Last word : computers don't communicate by screeching or talking or > whatever : they communicate by exchanging a specific signal through > a physical medium. If you read the trade mags you'd think computers only ever communicated via XML... > 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.). this brings up that old question: what is the physical medium of a radio signal? L. massive confusion of ether and ethertypes, here we come! > > 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@citynet.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? > <http://www.ee.surrey.ac.uk/Personal/L.Wood/><L.Wood@surrey.ac.uk>