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The following is an archive of a post made to our 'vox-tech mailing list' by one of its subscribers.

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[vox-tech] Re: [vox] Overheard...
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[vox-tech] Re: [vox] Overheard...



This seemed a bit technical for vox...

On 15 May 2002, Micah Cowan wrote:

> On Wed, 2002-05-15 at 16:55, Ryan wrote:
> > On Wednesday 15 May 2002 03:15 pm, joseph e arruda wrote:
> > > One of my biggest gripes has been the use of the term 'modem' for what is
> > > just a fucking network gateway, amongst a whole litany of complaints of
> > > how DSL companies are dorks.
> > 
> > Modem == Modulator/Demodulator
> > 
> > I'm pretty sure DSL modems modulate and demodulate signals..... And NICs to I 
> > suspect, but I'm not really sure and don't feel like looking it up.......
> 
> I don't think so - at least not what is meant by modulation in the
> context of a modem.  Modems translate pure signals into changes in AC
> frequency and back again, so that they will be interpreted as pitches on
> the telephone (and, in fact, by your ear).  NICs and gateways don't
> actually have to do this.

ADSL modems use CAP (Carrier-less Amplitude/Phase modulation) or DMT
(Discrete Multi-Tone, based on multiple frequencies each Quadrature
Amplitude Modulated).

The modulation is of a sinusoidal electrical signal... not necessarily in
the audible band.  In this case, the frequencies modulated are supposed to
be above the audible frequencies (though my DSL has plenty of audible
interference), with more bands allocated to the CO-to-home direction than
the home-to-CO direction.

Just above the modulation layer, at the raw digital level, they use ATM
(Asynchronous Transfer Mode). I think this is a runner-up for the most
misleading terminology, since it is built out of a variable number of
fixed-bandwidth channels (each having fixed packet sizes sent at fixed
intervals, known as "cells")... the receiver either gets the packets, or
it doesn't, but they keep coming at a fixed rate even if there was no data
to put in them or earlier ones were corrupted.  The ADSL modem has to act
like a protocol bridge ("gateway", if we could agree on what that meant)
between the wild and woolly variable-sized-packets world of Ethernet to
the orderly (and, as it looks to me, "synchronous") world of ATM.  This is
typically implemented by creating a "TCP/IP bridge" between your ADSL
modem's ethernet port and a switch at the ISP premises.  Thus, the ADSL
modem does not need to "route" the packets... it just picks them up on one
side (your ADSL modem's ethernet port or the ISP's switch)  and puts them
down on the other.

(ADSL modems usually do support their own private IP address, through
which they can be configured.  This can be abused remotely if you enable
the TCP "echo" service on your "external NIC", so don't.)

Ref: http://www.adsl.com/aboutdsl/faq.html

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