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Re: DSL "modem" [7:79175] posted 11/18/2003
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Marc Thach Xuan Ky wrote:
> When I was a kid someone told me that modulation was when you
> varied a
> carrier frequency with the signal to be transmitted (as in AM
> and FM
> radio). 

Yes, that's a good definition of modulation per classic data comm books,
those books that Chuck claims to have packed in his garage. Obviously, they
are still packed and haven't been cracked in a while. :-)

> Now I don't know whether DSL uses a carrier or not but
> if it
> doesn't then I guess it doesn't use a modem in the purest sense.

I think the analog voice part of DSL does modulation, but modem means
modulation/demodulation. Demodulation isn't necessary. The voice part just
goes to your old analog telephone. (The other messages in this discussion
thread were helpful with regards to how this works.)

> Now just for a laugh, consider this.  When I was a bit older
> somebody
> told me that broadband was a term used when a medium had
> several signals
> frequency-division-mulitiplexed as opposed to baseband, which
> was when
> the signal was applied directly to the medium without
> modulation.

That's right. Those are good classic definitions of broadband and baseband.
As an example, IEEE 10Broad36 used CATV to transmit Ethernet across a
maximum of 3600 meters, using analog signaling, transmitting a modulated RF
data signal over the carrier. FDM was used so that the coaxial medium could
also be used for other services, such as Cable TV. It's the reason that we
have to say BASE for every other Ethernet variety. We have to be sure nobody
thinks we're talking about 10Broad36! :-)

10BaseT sends a single digital signal using Manchester encoding. 100BaseT is
similar (although the encoding is MLT encoding to avoid so many changes from
high to low and low to high 100 million times per second.)

>  So
> tell me, is DSL really broadband?

The other messages said that yes, DSL is broadband because it carries 2
signals, voice and data. They use 2 sets of frequencies. The voice is analog
and presumably modulates a carrier signal. The data is a digital signal. I
think the data signal probably fits with a basic definition of baseband
where the signal is transferred directly onto the transmission medium
without modulation.

However, the other criterion for declaring DSL baseband doesn't fit. You
can't say that the entire bandwidth of the transmission medium is used to
transmit a single digital signal because some of the bandwidth is used by
analog voice.


> rgds
> Marc
> Chuck Whose Road is Ever Shorter wrote:
> > 
> > ""Zsombor Papp""  wrote in message
> > news:200311140304.hAE34Zep007173@xxxxxxxxxxxxxxxxx
> > > Jonathan V Hays wrote:
> > > > with your DSL "modem" (which is an erroneous but popular
> term)
> > >
> > > Btw, while we are at defining terms and such, why is "DSL
> modem" not a
> > > correct term, and how would you call it?
> > 
> > MOdulator DEModulator of analogue signalling to digial
> signalling and back.
> > Classic telco data comm. dates from the days when we took our
> standard Bell
> > telephone handset and set it onto an appropriately shaped
> receptable which
> > in turn was connect to a computer or other device via RS232.
> At 300 baud. if
> > we were lucky.
> > 
> > while the files were downloading we walked to and from school
> ten miles in
> > the snow uphill both ways.
> > 
> > Let's see - there are DSL routers, DSL bridges - that's the
> limit of my own
> > exposure to the beasts.
> > 
> > kids today......  no sense of history
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