Question about modems...

[squee]

Just some questions that are implied/asked by some stuff I read recently...

Just wondering, is the "speed limit" we've got with modems nowadays dependent upon the frequency limitations on copper phone line? Ie, without a greater frequency bandwidth you can't transmit stuff? I suppose I could find a modem handbook but I doubt it would explain this...hope someone can help...



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Non sibi sed patriae
Non nobis Domine, Domine! ...Sed nomine Tuo da gloriam!

[skalie]

Not really sure, but always willing to try and help.

Always remember that modems are rated in Kilobites per second, it makes them sound faster. A 56 Kbps is little more than a 7 kilo byte per second modem. (/8)

Modems are slowed down by factors such as initial connection times, intervening devices on the way, and 'line noise'.

Even with a fast server and a good ISP with a 56Kbps modem it is unlikely that you will get that speed. 56Kbps operate at 33.6Kbps over analog phone lines, if the ISP has digital lines there is a chance that you could get full speed, but uploads will always stay at 33.6Kbps.

Did that sound nerdy to you?

[squee]

Well, here's the thing.
A standard voice communication line has a bandwidth of what, 4khz? So let's say optimistically that you get the upper end of the audible range, so your signal takes up the band from 16-20 khz. The keying speed (bauds per second) of the modem depends on the frequencies of the audible analog signal generated by the modem (the digital to analog modulator).

So I am wondering, is it the FCC frequency limitation that causes the slowdown, or is it that we don't have hardware that can modulate fast enough, or what?

Nerd onward!

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Non sibi sed patriae
Non nobis Domine, Domine! ...Sed nomine Tuo da gloriam!

[skalie]

I'll be polite enough to let Rogue answer first this time.

[squee]

Ok, I don't care about signal degradation and all that, I'm just wondering about a perfect-world system, no loss, no delays other than the modulate/demodulate one, et cetera: what factors lead to one modem having a transmission speed of 300 bps versus 1200 bps? What forms of modulation do they use? What kind of bandwidth do they have? Standard voice grade channels? What? Curiouser and curiouser...


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Non sibi sed patriae
Non nobis Domine, Domine! ...Sed nomine Tuo da gloriam!

[squee]

*calls the phone company to speak to a line technician* http://www.thehypertribe.net/ubb/biggrin.gif

Rogue, you understand my interest, because you know what I'm studying http://www.thehypertribe.net/ubb/smile.gif

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Non sibi sed patriae
Non nobis Domine, Domine! ...Sed nomine Tuo da gloriam!

[Back Tick]

heres the thing.

the FCC rated that a frequency higher than a certain given limit being carried over a line can cause degradation of the line.

It's not that difficult.

In Analog systems (like old phone lines) it's been determined that in order to reliably transmit information (as bits) over the line, we need a certain pulse "width". If we go below this width, then signals seem to be garbled in a diminished returns sort of way. (we wind up having to retransmit so much that its not worth going to a pulse width lower than it) This pulse width is rated in time... We then usually double this so we can have a burst of information followed by an equal period of no-signal-transmission. It acts as a buffer so the information is more reliable.

Bandwidth is in effect 1 divided by this time (2T)

So on our analog phone lines with a limit of 56000 bits per second, the minimum pulse width (by a simple calculation) would be

1/((56000)/2)

3.57e-5

0.0000357 seconds for the pulse.

So we send a bit during every 2 of these quantities of time... which when you think about it, is a lot to ask for on these cruddy old analog lines.


I could be wrong, but thats my basic understanding.

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Just because some of us can read and write and do a little math, that doesn't mean we deserve to conquer the Universe.

[squee]

Eh, it has all been made clear (Backtick's explanation helped). Thanks everyone.

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Non sibi sed patriae
Non nobis Domine, Domine! ...Sed nomine Tuo da gloriam!