Re: Modems

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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?
> >
> >
> > __________________________________________________
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> >
>
>


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