This looks like the ISO OSI model. I'm more familiar with TCP/IP. ----- Original Message ----- From: "Michel Py" <michel@arneill-py.sacramento.ca.us> To: "Bill Cunningham" <billcu@citynet.net>; "Craig S. Williams" <wcraig72@yahoo.com>; <ietf@IETF.ORG> Sent: Wednesday, June 12, 2002 5:32 PM Subject: RE: Modems > Frame is the PDU name for layer 2. > > Layer Name PDU > 7 Application message > 6 Presentation message > 5 Session message > 4 Transport Segment > 3 Network Packet/Datagram > 2 Data Link Frame > 1 Physical Bit > > Michel. > > -----Original Message----- > From: Bill Cunningham [mailto:billcu@citynet.net] > Sent: Wednesday, June 12, 2002 1:54 PM > To: Craig S. Williams; ietf@IETF.ORG > Subject: Re: Modems > > When you say FRAMES I guess you mean PPP frames. > ----- Original Message ----- > From: "Craig S. Williams" <wcraig72@yahoo.com> > To: <ietf@ietf.org> > Sent: Wednesday, June 12, 2002 1:08 PM > Subject: Re: Modems > > > > This is one to hold on to! Thanks!! Verrrry good > > stuff!! > > > > Craig Williams > > ONI Systems > > > > > > -----Original Message----- > > From: owner-ietf@ietf.org > > [mailto:owner-ietf@ietf.org] On Behalf Of TOMSON ERIC > > Sent: Wednesday, June 12, 2002 12:27 PM > > To: 'Bill Cunningham'; 'Valdis.Kletnieks@vt.edu'; > > 'Pankaj Bhandari'; 'David Frascone'; 'Pete Resnick'; > > 'Bill Strahm'; 'Lloyd Wood'; 'Nepple, Bruce' > > Cc: ietf@ietf.org > > Subject: RE: modems > > > > 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@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? > > > > > > __________________________________________________ > > Do You Yahoo!? > > Yahoo! - Official partner of 2002 FIFA World Cup > > http://fifaworldcup.yahoo.com > > > >