dial up modems-why the slow speed?

[araaraara]

Why is it that most modems seem to only download at 10% of their rated speed? A 56k modem does ~5-6kb/sec, 33.6 ~1-3kb.sec, and so on. I've hear of rare exceptions, but those seem to be under ideal conditions. What causes this? Is it a limit of the phone lines? I thought that phone lines could do 56kb/sec (limited to 53kb/sec by law in Canada) and can't go faster because of frequency limitations. Are we being ripped off?

[DVNT1]

I believe your speed units comparison is off by a unit of abuot 10.

There's some more information relating to this at http://www.speedguide.net/Cable_modems/bandwidth.shtml

[Andy_L]

To put it simply, your modem is connected at 56 Kilobits (Kb) max, and the download is measured in Kilobytes (KB). There are 8 bits in a Kbyte (KB)so 56 Kb = 7 KB.

[Warthog]

Yah, was about to say that about the bits and bytes.

The gov here limits something about 56k modems....too late, can't remember.

53 kb/sec in Canada on phone lines? What about DSL? That's on phone lines.

Warthog

[araaraara]

Oh, so it's kilobytes vs kilobits! Now I get it!So my modem really has been maxing out a few times http://www.sysopt.com/forum/biggrin.gif

I'm not sure how or why the governemnt limits the max bandwidth on phone lines, amybe they're trying to prevent the entire bandwidth of the lines from being completly tied up os that it can be used during and emergency or something. I don't think it applies to dsl because it uses different, digital frequencies. I'll I know is that there is some thing which limits the available frequencies to ---hz.

[DanU]

AFAIK, there is no restriction on the bandwidth of a signal transmitted on a phone wire. But there IS a gov't mandated restriction on the power transmitted. This is to prevent crosstalk between phone circuits. Trying to transmit the full 56K using the V.92 modulation scheme would violate the power restrictions.

DSL must also adhere to the power restriction. This doesn't pose a problem since DSL transmits a low-power, high frequency signal.

[Graham]

Here in the UK, and I guess its similar elsewhere, the telephone system was designed before digital anything was a twinkle in anyones eye. It developed to provide 'adequate' voice transmission, the important bit being adequate. Historically the practical bandwidth of the system overall, was ony in the order of a few hundred Hz to a few Khz, when the systems became more complex (1950's and 60's), this bandwidth was in fact fixed to be 300-3400 Hz so that multiplexing systems (PCM) could employed.
The new up to date digital systems still suffer the limitations of the 'local loop' and so still limit to about the same bandwidth, (local loops are generally copper to the local distribution, fibre only being used between exchanges, or to provide high speed (2.048 [1.54? in the US]Mb/s and over, links), ADSL and the like often use fibre to get to the local distribution (cabinet or poletop box).

So we are trying to 'get a quart into a pint pot'.

We have to use modems (MODulators/DEModulators)to convert from the digital signal to analogue, because the true square wave digital signals degrade too fast and need special 'repeaters' frequently along the route.
I remember, about 25 years ago, being shown, at the BT research centre, the first full duplex 2wire 9K6 modem, it took two 19" shelves !!, things have improved a bit.


G

[n8782474]

bandwidth was limited to 300-3400 because that's what the human ear can hear. (You're only leasing a voice line)
http://808hi.com/56k/index.htm

Above link will explain.

[socalgal]

http://www.sysopt.com/forum/Forum1/HTML/013987.html

Moved to Networking. On behalf of all members and staff, thanks for your support. http://www.sysopt.com/forum/wink.gif