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nz_liam
13-11-2002, 04:11 PM
Can someone please explain to me why my 56K modem seems to have a maximum (sustained) download speed of about 7 kbps, surely when I dial my ISP and get a 46.6 kbps speed I should be able to download at something comparable to the connection speed??

(Have always wondered how they worked out the speeds of modems)


Cheers

Liam
http://www.chatf1.net.nz
http://www.desktop.net.nz

-=JM=-
13-11-2002, 04:25 PM
You connected at 46.6Kbps (Kilobits per second). That is what you originally connected to the ISP at, once connected that figure means nothing and it was always just a rough guess anyway.

You're downloading at 7KBps (KiloBytes per second). There are 8bits in a byte then there is a start/stop bit and other bits for parity.

So

46.6/8 = 5.825 KBps.

So I'd say that you're going along quite nicely there.

Graham L
13-11-2002, 04:28 PM
It's not a mystery. :D It's magic. The telephone line has a rated speed of about 3200 baud. With various incantations, and spells (correctly spelt) it is possible to get 56000 bits/sec download, and 33600 bits/sec upstream. Using 10 bits/byte (because of the start and stop bits used in asynchronous communications) that gives you a theoretical rate of 5600 bytes/sec download. If your line is not perfect and noise-free, the modems negotiate a lower rate.

That is for "binary"/"uncompressible" data being transferred. Many files can be compressed and this is done transparently in the modems. So your average transfer rate will be higher than the theoretical rate, but it will be reduced by the overhead of the protocol being used. That is, the addresses etc contained in the TCP/IP header of each packet.

About all you can say is that sometimes you get faster transfers than at other times. Sometimes it's slower.

But remember that there is a difference between a bit and a byte (or octet). The abbreviations are bit="b", byte="B". Also "m" = milli, "M" = mega. The difference is a factor of a thousand million :D People seem to always mix them up. I am amused when I hear of a 90 milliwatt power station. The abbreviation for "kilo" (one thousand or sometimes 2^10, the "binary k") is "k". "K" is the kelvin temperature unit.

-=JM=-
13-11-2002, 04:30 PM
Also there is compression that happens which is why you can get 7KBps and greater download speeds.

I really hate the way that there is no standard to the whole B=Byte, b=bit thing. Some people must get ripped off by things like that.

It's one of my pet hates is the way that happens. Also happens with harddrives some manufacturers use these "Metric" measurements instead.

Graham L
13-11-2002, 04:41 PM
There is a standard. It's called the SI. (Systeme Internationale). People are ignorant. My other peeve is dimensions. Some newspapers use kmh for km/h. There is a difference. They could use "kph" as " mph" was used for mi/hr.

Terry Porritt
13-11-2002, 04:50 PM
Even worse Graham, is that some car dealers dont know how many kilo-metres ( note the pronunciation, it is NOT ..lometres) their cars have done. Some ads miss out the thousands, so the cars have apparently only done 56km, or 95km for example.
Wonder the fair trading inspectors dont get onto them about it.

Terry Porritt
13-11-2002, 05:00 PM
Incidently whilst we're on pronunciation, what is the difference between a micrometer and a micrometre?
One is a measing instrument, the other (micro-metre) is a micron or 0.001mm. Then of course in Germany a metre is spelt meter :)

Jen C
13-11-2002, 05:39 PM
> what is the difference between a micrometer and a micrometre?
> One is a measing instrument, the other (micro-metre)
> is a micron or 0.001mm.

I got tripped up on this one recently. I thought I would change the spelling of micrometer to micrometre believing this was more correct!!!

BTW, micron is the common usage name instead of micro-metre.

Terry Porritt
13-11-2002, 05:55 PM
I can't spell 'measuring' now! Micrometer, a handheld instrument, eg made by Moore & Wright, Browne & Sharpe, Mitutoyo etc. One psychological reason that Europeans eg the Germans or Swiss thought (or think) they could work more accurately than the British is that their micrometers are graduated in 0.001mm divisions, whereas Imperial micrometers are graduated in 0.001 inches, some 2-1/2 times larger.

Jen C
13-11-2002, 06:06 PM
I use a Ernst Leitz Wetzlar micrometer (definitely not hand held!) as I measure objects from 2 microns upwards. The micrometer is as old as the hills but you can't beat the German accuracy and quality (at least not cheaply).

Graham L
13-11-2002, 06:08 PM
Except that most people could estimate to "tenths". (0.0001) Or really, to .0002 or .0005.

Except ... I seem to remember that metric micrometers usually have a resolution of more like 10 microns. The finest useful screw thread would be around .5 mm and that would need 50 divisions on the drum(Common Imperial mikes used 40 tpi (.025" ... roughly 0.6 mm) with 25 divisions on the drum).

Terry Porritt
13-11-2002, 07:26 PM
Right Graham, 0.01mm divisions, not 0.001 for a metric mike, slip of the finger.
The old part of Wetzlar is very attractive, most of it survived wartime bombing. Leitz have an interesting collection of all the cameras they have ever made at the factory there.

Kahawai_Chaser
13-11-2002, 07:50 PM
Hi...

Use a bench type Digical micrometer almost daily for measuring paper & board thickness and I think it shines a light on a scale that is actually set in micron steps; - real accuracy - throughout a wide range of 250 um to 5000 um. Though to achieve this, calibration to ISO standards require 4 or 5 different levels...