On Sat, Jun 05, 1999 at 03:49:01PM +0200, Mohammed Akram Ali Mehkri wrote:
You have misrepresented the quotation, a grave breach of etiquette..
I will rearrange the quotes here below, beginning with yours:
> whether u download or upload a file has nothing to do with the speed
> it all depends on the speed both ur machine and ur server ( the client
> and the server in the ppp paradigm ) have which ever is lower is taken
> for eg if u have a 33.6 modem but conf it to recv only at 1200 then u will
> get 1200 bks and not 33.6 kpbs even if ur server was conf'ed to provide a
> better speed and the same is applicable the other way around
>
> i had a 33.6 download with 33.6 modems both sides downloading gzip files
The QUESTIONS were:
My link-speed is 31200 bps, is 3600 Bps the best it
can do ?
Why does not the PPP link-level compression increase
the speed of download when the data in question is
already compressed ?
On Link Speed:
Yes, it is very much the best that link speed can do.
Calculating all places where the bits are lost/gained is not
quite trivial, but trivially calculating that link speed means
that you are getting on average 8.67 bits per byte, which is
*better*, than fastest "s8n1" async format - 10 bits per byte!
Clearly your MODEM does consolidate async bytes into larger
HDLC frames sent over the physical link.
This is also why at both ends the ASYNC connection speed to
the modem must be faster, than the link speed, or you won't
gain anything. (38.4k serial port speed gives you at most
3.840kB/s async byte speed. That is close, but should not
limit the speed -- unless the modem limits the link speed
to 31200 because of it ?)
Try increasing your ASYNC port speed from 38k4 to 57k6 or
even higher, and your modem speed might go up.
Fast V.90 modems usually need 115k2 bps link speed at the
serial ports, or they won't be able to transfer data at
their maximum modulation speeds.
Also, the async-PPP framing takes some extra bytes (even
varying number per transferred data contents!), as well
as IP, and TCP headers consume something out of the link..
For each 1460 bytes of TCP data some 40+(circa)18 overhead
bytes are transferred over the link -- about 4 %
If the TCP flow operates with smaller MTU, then the overhead
share will increase, of course. MTU of 576 bytes turns that
same absolute amount of overhead per frame into about 10 % !
On Compression effectiveness:
The answer in simple terms is that compressing maximum entropy
data (compression result) is not possible! If it were, the used
first step compression algorithm would not be good at all, and
its result should not be called "compressed".
Also modems often try to do compression at the link, and they
have exactly the same problem. If your modems *do* compress,
you should not waste cycles at running PPP link-level compression.
Just remember to run the async speed from your machine to the
modem must be high enough that in the event the link compression
does supply there speedup of factor 3, you will get all those
bytes thru the serial port!
Running PPP link-level compression will allow the speedup benefit
inside the kernel, or course, not limited to serial port hardware
speed. But whatever you do, if the data to be sent over is highly
compressible and batch style, then doing PPP link-level compression
*might* be worth of the effort, but in general it doesn't feel
quite right for interactive use where lots of keystrokes are sent
over the link.
/Matti Aarnio <[EMAIL PROTECTED]>
> Mohammed Akram Ali Mehkri
> System Administrator Saehan India
> Webmaster www.prizeclub.com
> On Fri, Jun 04, 1999 at 04:04:29PM -0300, Thomas T. Soares wrote:
> > Hi there...
> >
> > Could anyone tell me what should be the transfer rate if I am
> > downloading
> > a compressed file (gzip -9) is I am connected as 31200 bps?
> >
> > I am getting about 3.6Kb/s but not sure if this is the best I can do
> > and the ppp_deflate seems do no difference at all...
> >
> > Thank you.
> > --
> > |Porto Alegre| Thomas Tschoepke Soares | // Mate do
> > | RS - Brasil| [EMAIL PROTECTED] |(~~~//'~) estrivo
>
> -------------------------------
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