What we're actually referring to as 'digital' are 'modems' (better:
modem emulators) that do the incoming analog signal processing not by
analog circuitry (with analog PLL's and such stuff), but by _simulating_
such a modem with mathematical algorithms on a Digital Signal Processor
If your POP has a digital telco connection, the callers modem doesn't
need to be converted to analog and back to digital using a modem chipset
(i.e. Rockwell), but the whole signal path on your side is digital.
Some of the advantages are:
1. The noise level (or signal-to-noise ratio) is much better
2. less power consumption/heat dissipation -> better reliability
3. Easy upgradability if new modem standards arrive - the processing is
not fixed into a modem chip, so no hardware replacement is necessary
4. You can have 1 huntgroup for both digital and analog callers
5. It is the _only_ technology today that supports 56.2 k analog
To the best of my knowledge, the USR total control and the Livingston
PM3 are the only devices that have those 'True Digital' modems today.
However, at least Cisco, 3Com and Ascend have announced DSP-based modems
for their next product revisions. Up to now they're really using analog
modem chips on a digital backplane.
[...lots of disclaimers...]
>From: Mitch Wagers [SMTP:email@example.com]
>Sent: Thursday, April 17, 1997 2:08 AM
>Subject: Re: New equipment.
>I don't mean to be picky here, but I can't stand seeing it, since we fell
>into the same trap...
>The term "Digital Modems" isn't anything new, modems have been digital in
>nature since the turn from acoustical units. By buying something like a
>USR Total Control unit or Cisco's or Ascend's, you aren't physically
>getting anything more than standard (usually Rockwell chipsets) modems that
>are "digitally" integrated into a board. A Cisco 2511 with 16 Cardinal
>modems serves the same purpose, costs a lot less money, and is more
>versatile AND we have gotten better performance from them than our
>as5200's(costing over 3 times as much). I think the attractiveness of a
>pre-boxed unit like a 5200 is the size and only one power plug, instead of
>17. Maybe I threw that digital stuff out there, but I think it is
>important because of this:
>When we bought our equipment(unto my knowledge), we were told the Cisco
>as5200 has 48 Digital Modems. OOOOOOO, ahhhhhhhh, said my boss, I'll take
>it! It's a marketing scheme, because modems are Digital-to-Analog and
>Analog-to-Digital Converters and you can't change that fact because the
>user calling in is sending out ANALOG signals. This is the point of going
>with ISDN, ADSL, FRAD's and whatever else there is, to go to a
>complete(voice and data) digital signal, which will not happen in the next
>10 years. My apologies if I have offended anyone or spoke out of line, but
>it is important for this to be known.
>Director, Technical Service
> RadiusNT Mailing List firstname.lastname@example.org