OnlyMe said:
I am using dialup and my modem is configured to run at 115200 bps,
it is a Agere Systems PCI Soft Modem on (COM3) this is a config
setup by the driver disk,and was concerned that it is so high,would it be
better to run at a lower
bps speed I have enabled the hardware flow control & modem error control
but have not enabled the modem compression. __________thanks to all
With Kind Regards from WillofAustralia.
There is a big difference, between an external modem connected via
RS232, and a soft modem (which connects to a PCI slot).
This would be an external modem. With the external modem, the "115.2K"
number is a real physical quantity. The UART in this example, transfers
data from the modem, over an RS232 cable, at 115200 baud. The RS-232 cable
is a real, physical thing.
46K 115.2K
Internet ------------------- external_modem ------------- UART -- PCI -- Processor
(Dialup (Variable RS-232
Modem performance) cable
Pool)
This is with a soft modem. A soft modem converts analog from the phone
line, into digital samples at a fixed rate. The samples are transferred
to the computer, via the PCI bus the soft modem card is plugged into.
The main CPU inside the computer, runs a driver code, which processes
the digital samples and converts them into the original 1's and 0's.
46K 133MB/sec
Internet ------------------- soft_modem --------------- System Memory,
(Dialup (Variable (PCI slot) PCI Bus Processor etc.
Modem performance) (driver does DSP
Pool) to decode data)
In this case, the reported "115.2K" is inside the "cloud" in the last step
on the right. It has no physical significance. The number is bogus and
meaningless. It is not a bottleneck.
(I have purposely not shown the sampling rate of the soft_modem.
It does analog to digital conversion samples fast enough to meet
the Nyquist criterion. DSP processing does the rest, converting
information in the frequency domain, back into user data.)
What you want, is to give a dialup modem text string to the modem,
which causes it to report the "line rate". That is the "46K" value
on the left. That is the value that affects your download rate.
On the soft modem, the "115.2K" number doesn't physically exist.
Setting the string to report that value, isn't a good use of
the reporting function.
*******
If you want to prove whether a dialup modem has a bottleneck in
performance somewhere...
1) Investigate the name of the protocols used (i.e. V42bis). Each protocol has
a maximum compression ratio. Some are 8:1, some 4:1. (V44 here is
listed as a 6:1 compression ratio.)
http://en.wikipedia.org/wiki/Modem#Using_compression_to_exceed_56k
2) Enable that compression on the line. It requires adding a Hayes
Command Set option for compression.
3) Find a file on the Internet which is "all zeros". In other words,
any file which is known to be highly compressible by modem
protocols. Any repeating character would do, and it doesn't
have to be zeros. If you have storage on the Internet somewhere,
you can upload your own file full of zeros, as a test file for
later download.
4) Using a browser, download the file. Make the file big enough,
that the transfer takes 30 seconds or so. View the bandwidth
recorded in the browser download dialog box.
If the line rate was 46K, and a 6:1 compression protocol was used,
then the rate data would show up in the browser would be close
to 46Kbits/sec * 6 = 276000 bits/sec. Divided by 8 bits per byte,
that would be 34500 bytes per second.
Normal files download at around 5KB/sec, and so you'd notice
if the download rate suddenly became 34.5KB/sec. By
doing such evaluations, you can check for any intervening
bottlenecks. For example, on the external dialup modem, the
115200 might be the limiting factor. On a soft modem, you
should see no such limit, and should see the fantastic 34.5KB/sec
download rate. (Real files don't compress that well, which is
why the special "file full of zeros" is needed to show the
impressive download rate.)
Enabling compression protocol on the modem, helps in cases where
the data is not already compressed. Many downloads, such as a 1GB
game demo, are already compressed. In such a situation, enabling
compression doesn't make sense. In this case, you'd enable
compression long enough to do the experiment, and then
disable it later.
People who use dialup modems, do experiments like this (compare
compressed to uncompressed operation), as they try to get a bit
more than 5KB/sec from the modem. A modem user has to learn a
lot about the protocols involved, while doing the experiments.
After a while, you'll have memorized the entire Hayes Command Set
Paul
