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 >