Why doesn't 56k modem maintain initial download speed?

[Doc]

I find that when downloading a file, the initial kbps speed shown
typically starts out much faster than it ends up. It might show a
speed of 15 or 20 kbps and then eventually winds down to typically
around 5 kbps.

During those initial seconds when the speed is higher, it might d/l
300 or 400 kb really quick, at a much faster pace than it does for the
rest of the download.

It seems that the modem is capable of taking the data much more
rapidly than the speed at which it maintains the download, why doesn't
it maintain the higher speed?

 

[Craig Coope]

I find that when downloading a file, the initial kbps speed shown
typically starts out much faster than it ends up. It might show a
speed of 15 or 20 kbps and then eventually winds down to typically
around 5 kbps.

During those initial seconds when the speed is higher, it might d/l
300 or 400 kb really quick, at a much faster pace than it does for the
rest of the download.

It seems that the modem is capable of taking the data much more
rapidly than the speed at which it maintains the download, why doesn't
it maintain the higher speed?

I don't know if this is true but someone once told me this...

When you get the prompt to download and save a file it is already
downloading the file whilst u select its save destination. Because of
this, when Windows calculates how much of the file it has already it
gives a false reading as it assumes it has 200K etc in a couple of
seconds...

I had dial-up for years and only once saw a true download speed that
was higher than 5k...I managed to get 12K download constantly using a
peer-peer app once...

I may be completely wrong about the abov though.....

 

[Paul]

I find that when downloading a file, the initial kbps speed shown
typically starts out much faster than it ends up. It might show a
speed of 15 or 20 kbps and then eventually winds down to typically
around 5 kbps.

During those initial seconds when the speed is higher, it might d/l
300 or 400 kb really quick, at a much faster pace than it does for the
rest of the download.

It seems that the modem is capable of taking the data much more
rapidly than the speed at which it maintains the download, why doesn't
it maintain the higher speed?

I've noticed that in Firefox, that the download dialog doesn't take into
account stuff that started to download before the dialog started
to register the data. The initial transfer rate ends up overestimated,
and declines with time.

What I just tried, is I went to Control Panels:Administrative_Toolserformance.
(This is in Win2K, may be slightly different in other OS versions.)
In there is a charting tool. It starts out empty, and shows no statistics.
(BTW - this puzzled me for the longest while when I first tried to use it.)
I started clicking on stuff in frustration, and eventually discovered that
if I right clicked in the lower right area, I could "Add counters". A box
with categories shows up, and one category is "Network Interface". I was
able to select my Ethernet interface (as it connects to a router and ADSL
modem). In "Select Counters", I picked "Bytes Received/sec".

Then, I went to the Intel site, and downloaded a technical document.

With the Performance graph in view, and the Firefox Download box also
in view, I started a transfer from the Intel site. The Intel site
usually has a pretty good transfer rate, so I wasn't expecting any
trouble from the site.

For the most part, the Performance graph was a flat line at about
124KB/sec during the transfer. Firefox, on the other hand, reported
about 300KB/sec at the beginning of the transfer, and that declined
during the duration of the transfer, and was getting close to the
number the other tool reported near the end.

So, what you need, is a tool with better reporting capabilities.
The Performance tool looks a bit more reasonable.

Paul

 

[Aaron Leonard]

~ I find that when downloading a file, the initial kbps speed shown
~ typically starts out much faster than it ends up. It might show a
~ speed of 15 or 20 kbps and then eventually winds down to typically
~ around 5 kbps.
~
~ During those initial seconds when the speed is higher, it might d/l
~ 300 or 400 kb really quick, at a much faster pace than it does for the
~ rest of the download.
~
~ It seems that the modem is capable of taking the data much more
~ rapidly than the speed at which it maintains the download, why doesn't
~ it maintain the higher speed?

My guess is that what you are calling "5 kbps" is really more like 40+ kbps
(i.e. 40 thousand bits per second)

Hint: PC people think in terms of bytes; networking people think in terms of bits.

PC people think in terms of powers of 2 (e.g. "K" is 2^10=1024); networking
people think in terms of powers of 10 (e.g. "k" is 10^3=1000)

So when your modem trains with a download carrier of say "44k", that means
44kbps i.e. 44 * 1000 BITS per second i.e. 44000 bits/sec.

I have resuscitated my old "how fast is my modem connection" page:

http://www.employees.org/~aaron/ftp/modems/how-fast.htm

How do I tell how fast my modem connection really is?
http://www.employees.org/~aaron/ftp/modems/what-speed.htm

which you can now read if you want to learn more.

As far as why you started with an apparent speed of "15 to 20 kbps" - my guess is
that your assumption is correct - the modem really CAN buffer data sent from
the PC a lot faster than it can dequeue it on the modem link. Because, if
by "15 to 20 kbps" you really mean something like 140,000 bits/sec, then indeed,
no voiceband modem *can* transmit data at that rate (unless the data is being
compressed.)

Aaron

 

[Unknown]

It is constantly calculating an average speed as it is running. If for
example, one block of data has a transmission error
it must be retransmitted changing the average speed.

 

[Plato]

It is constantly calculating an average speed as it is running. If for
example, one block of data has a transmission error
it must be retransmitted changing the average speed.

Due to the speed of the connection and the server you are downloading
from.

 

[Franc Zabkar]

I had dial-up for years and only once saw a true download speed that
was higher than 5k...I managed to get 12K download constantly using a
peer-peer app once...

Compressed files will download at about 5KB/s but compressible files
will exceed that. In fact my internal "hard" modem managed 22KB/s for
an "infinitely" compressible file. "Soft" or controllerless modems
will probably do better than that.

To test the limits of your dialup connection, create a 1MB file
consisting of 1 million repetitions of the letter "A", and then email
it to yourself. Or use ftp to upload and download it to and from your
web space.

- Franc Zabkar