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SubjectRe: Winmodem support, some performance tradeoff estimates

You also need to factor in that at least 90% of the systems are idle at
least 90% of the time.

Further more, you are talking about two potentially differnt things.. A
soft modem uses the modem hardware as nothing more then a sound card. A
win modem might be that, or it might impliment the DSP stuff in hardware
while moving the protocall handling into software.

All of the winmodem's I've seen are the second type and require very
little extra cpu. This second type has the potential of being FASTER then
a normal modem because you could potentiall combine the low level
protocall stuff with the PPP stuff and reduce latency..

The first type uses TONS of cpu and is not currently viable... the second
type wouldn't be all that bad if specs were available..

On Sun, 16 Aug 1998, Adam J. Richter wrote:

> Executive summary: I was very wrong in my estimation of the
> CPU power that a WebTV has available to run its soft modem in the
> background. I apologize for the misinformation. Nevertheless, I
> think winmodems would be economically optimal were there Linux support
> for them now, and that this balance will probably shift more in favor
> of winmodems in the future.
>
> I wrote:
> > The latest revision of WebTV runs on a 56k soft modem in the
> >background on an 8MHz MIPS R4000 processor.
>
> ...and then argued that this meant that the CPU performance
> cost of a soft modem was about $6.
>
> Well, I was really wrong about this, and should have double
> checked before posting. Thanks to Alan Cox for questioning this
> number.
>
> The WebTVPlus, which has the 56kbps soft modem as a background
> task, actually uses a 167MHz R4640. So, my approximate upper bound on
> CPU cost of a soft modem at $.75/Mhz by my previous argument would
> then be $125.25 significantly more than the cost of a hardware modem
> (correcting my previous estimatation of 8MHz x $.75/MHz = $6
> estimate).
>
> It might be worth finding out what is the slowest CPU on
> which 56kbps winmodems currently run, as a better rough upper bound.
> In the meantime, however, Alan has kindly included another basis for a
> reasonable estimate of the CPU cost of a soft modem:
>
> >Digitals own figures for a 200Mhz strongarm SA110 and their commercial
> >software mode is 25% of the resources of the chip.
>
> If 1 strongarm MHz = 1 Pentium MHz = $.75, then this would be
> about $37.50 of CPU cost, or only a little savings from the cost
> premium of a hardware modem, or maybe negative savings if the strongarm
> is significantly faster per megahertz than the pentium.
>
> On the other hand, the Cyrix cost per Pentium MHz appeared to
> be something like $.25, which could bring that soft modem CPU cost
> down to $12.50 ($37.50/3), and I think CPU price/performance ratios
> will improve faster than those of hard modems, and you can use the
> soft modem CPU power for other things when your modem is off, and I
> think you CPU will likely continue in some active use for longer than
> your modem. Also, if your usage pattern is that your CPU is basically
> idle when you are downloading, your CPU cost is really only the CPU
> cost of maintaining the software modem in an idle but connected state.
>
> Anyhow, I will have to revise my claim that the CPU costs of a
> winmodem are almost always vastly less than the price premium of a
> hardware modem. It looks like it really depends on what level of CPU
> you are using. For high end systems, with higher $/MIPS ratios,
> WinModems may not yet be economically optimal. For most other systems
> (Celeron, Cyrix), they might save you money already if there were
> Linux support.
>
> It also appears that the estimated 7.25 million Linux users of
> the world are not going to collectively waste over US$100 million on
> hardware modems, but I do think that that figure is probably still in
> the tens of millions of US dollars as those that would save money with
> winmodems find themselves unable to do so.
>
> --
> Adam J. Richter __ ______________ 4880 Stevens Creek Blvd, Suite 205
> adam@yggdrasil.com \ / San Jose, California 95129-1034
> +1 408 261-6630 | g g d r a s i l United States of America
> fax +1 408 261-6631 "Free Software For The Rest Of Us."
>
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