Faster x2 Download speed versus upload speed / signal level control, impact on new v.pcm standard

Faster x2 Download speed versus upload speed / signal level control, impact on new v.pcm standard

Post by Richard Adam » Thu, 08 Jan 1998 04:00:00


A few days ago a thread here took a wrong turn when readers
began to question whether the speed of downloading for x2
could be improved by lowering the signal level of the back
channel.  At least one reader inquired if there was a way
to reduce that signal level and gain FASTER DOWNLOADS.

Definition: Back channel is the data path that is used to
send data from your computer back to the ISP and beyond.

Having discussed and considered the design and implementation
of x2 with knowledgable people, here are the real facts.
When x2 is active, the modem automatically reduces the back
channel to half the signal amplitude normally used for non-x2
communication.  This was determined to provide the optimum
combination of upload and download performance for the typical
user where moderate error rates in the back channel are
acceptable while download speed is maximized.

In actuality, the reduction in back channel level primarily
helps to improve download speeds that are already fairly good.
Only those line / modem combinations where the download speeds
are already 44000 or higher realize some improvement by the
reduced back channel level.  That is to say that those persons
whose line/modem aren't showing 44000 are not seeing any benefit
in download speed because the modem halved the signal level.
A further reduction to a level lower than the 1/2 amplitude
wouldn't produce any significant improvement in download speed.

BOTTOM LINE:
Not only is there no option to select a further reduction in
the back channel level in x2; none is needed or desireable.

Its NOT that the modem manufacturer is keeping the speed from you.
There's just is no benefit to lower the level further.

MORE:
On the other hand, certain modem/line situations might benefit
if there was a method to throttle the back channel level higher
even when x2 was being used.  Examples of this include conditions
where the user wanted a faster and / or more error free back
channel, and download speed wasn't as critical or wasn't realizing
much benefit from the lower level.  I sure hope the manufacturers
consider this option in upcoming firmware releases since the
ability to do this is currently not provided for.

As the v.pcm standard draws toward finalization, ir combines the best
of all strategies to get the most out of analog modems (together
with 50 tons of politics of course).  I note that the ability to
adjust the backchannel level would probably not conflict with the new
standard so long as the signal level was below the maximum specified by
the jurisdiction i.e. FCC in USA.

Richard

 
 
 

Faster x2 Download speed versus upload speed / signal level control, impact on new v.pcm standard

Post by dopgan.. » Fri, 09 Jan 1998 04:00:00



> A few days ago a thread here took a wrong turn when readers
> began to question whether the speed of downloading for x2
> could be improved by lowering the signal level of the back
> channel.  At least one reader inquired if there was a way
> to reduce that signal level and gain FASTER DOWNLOADS.

There used to be, in the UK. I can remember tweaking S36 (I think?) to get
better connects. This was the register the controls the outbound signal
level. I tweaked it until the tx connect was around 24000, and got the best
connects (it was around 2-3 clicks higher than otherwise, as I recall). I
tweaked it further, bottoming out around 14400; after a while, it started
degrading the connection in the rx direction, too. This was confirmed by
others in Europe. Unfortunately, some of the US x2 beta people got annoyed
at the Europeans going off in their own directions and locked S36 to a
maximum value of 18, IIRC. Note that simply setting a higher connect value
or disabling symbol rates isn't enough, you need to alter the outbound TX
level.

Quote:> In actuality, the reduction in back channel level primarily
> helps to improve download speeds that are already fairly good.
> Only those line / modem combinations where the download speeds
> are already 44000 or higher realize some improvement by the
> reduced back channel level.

That's about what I was seeing (though the bottom level for me was 42666, I
think).

Quote:> A further reduction to a level lower than the 1/2 amplitude
> wouldn't produce any significant improvement in download speed.

I dunno. I'm sure there's more to it than that. I've got a sneaking
suspicion that there was some mild crosstalk going on *somewhere* that was
affecting it, but I could never pin it down. Playing about with the
impedance didn't help, either.

Quote:> MORE:
> On the other hand, certain modem/line situations might benefit
> if there was a method to throttle the back channel level higher
> even when x2 was being used.  Examples of this include conditions
> where the user wanted a faster and / or more error free back
> channel, and download speed wasn't as critical or wasn't realizing
> much benefit from the lower level.  I sure hope the manufacturers
> consider this option in upcoming firmware releases since the
> ability to do this is currently not provided for.

Oh, it's in there for some European countries. I vaguely remember it was
locked in other countries, probably because you could get the modem to blast
a signal out that was way out of spec. It wasn't in the Sportster Flash or
Voice, from what I remember.

Quote:> As the v.pcm standard draws toward finalization, ir combines the best
> of all strategies to get the most out of analog modems (together
> with 50 tons of politics of course).  I note that the ability to
> adjust the backchannel level would probably not conflict with the new
> standard so long as the signal level was below the maximum specified by
> the jurisdiction i.e. FCC in USA.

One can live in hope. I suppose you could always solder up a little
bodge-box to run the line through to drop the tx level...

Cheers... Dop.

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