Re: 56K Modems - Do I really want mine? (fwd)

Karl Denninger (karl@Mcs.Net)
Fri, 22 Aug 1997 08:13:57 -0500

On Fri, Aug 22, 1997 at 02:20:00AM -0700, MegaZone wrote:
> Once upon a time Stephen Fisher shaped the electrons to say...
> >Does anyone have some numbers comparing compression of V.42 vs. STAC on an
> >analog modem 33.6k / 56k connection? I'd rather not shell out $500 and
> >get everyone to make these changes just to find out it isn't as good =)
>
> V.42bis is generally better. And it is already there for anyone with
> modems.
>
> On top of that, the PM-3 is smart enough not to let someone do BOTH.
> So unless the user is bright enough to turn off V.42bis in their modem
> they will be denied Stac.
>
> It is really worth it for ISDN users, but I don't see it as being really
> important with modems.
>
> -MZ

On a K56flex connection, with a 115.2kbps port speed, the PM3 w/STAC (or
V.42bis) is easily fast enough to overrun the port speed. In fact, when
downloading news and/or group lists (ie: Free Agent, etc) you can even get
close enough to start worrying about this on a 33.6 modem in the PM3!

There is *NO FIX FOR THIS* without the customer buying hardware (ie: a
230kbps serial card).

If you shut off V.42bis and run STAC instead, you get around the problem.
Now the compression is done in the Host CPU.

Now, if you want the lowest LATENCY as well, then the best path is to
shut off V.42 *ALSO*, allow PPP to do error control (which it does quite
well, thank you), and run STAC for compression. This will cut your ping
times on a K56Flex connection to under 100ms.

This is a bad move if you are CPU starved at the customer end. But with
today's machines (ie: Pentium 90s and better) its a net win.

We've tested this.

I'll post some numbers here later if I get the chance.

--

-- 
Karl Denninger (karl@MCS.Net)| MCSNet - Serving Chicagoland and Wisconsin
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