ISP Information:
Acronym: Fear, Uncertainty, and Doubt. A common motivational strategy.
ISP Glossary:
FUD - The problem is, we are asking the wrong question. The real question isnot 'What it the most data we can transmit over a wire between twopoints?' It is 'What is the most data we can transmit over the dial uptelephone network?'We can transmit a lot of data between two points. Certainly a few Mbsover a high speed line. Much more if we are prepared to pay for adedicated line. But the ability to dial up anyone in the world, or evenanyone in North America is something else.The real reason is very simple: the dial up system ( Called the PublicSwitched Telephone System or PSTS) is designed to make voiceconnections, person to person, on demand. Voice is analog andhistorically about 4 kHz bandwidth was enough. As the world wentdigital, only the connection from your house to the nearest telephoneoffice remained analog, all the conneections between offices, and allthe switching which routed your connection to the right destinationbecame digital, was almost univerally standardized on 8000 samples persecond (to allow nearly 4 kHz analog signals to be represented) and 8bit samples to provide sufficient signal to noise. So the connectionbetween telephone offices is basically 64 kbs. In fact some of thetransmission equipment uses one bit per sample for other purposes, sothe most you can rely on is 56 kbs of clear channel. Thats all the bitsthere are, you can't do better unless you redesign the whole PSTS.Technically, your ISP can send a 56 kb signal to you. He sends it as adigital stream to the telephone company, they forward it to an exchangenear you, they convert it to analog there and send the result to youjust as if it were voice, and your 56 kbs modem can turn it back intodata stream for you. Turns out it doesnt quite work, because the analogsignal created at the last leg is powerful enough at certain frequenciesto create interference with neighboring phones, so it had to be reducedfrom 56kbs to 53kbs to eliminate this interference.Even that you can only do if the sending end (your ISP) has a digitallink to the phone company. For ordinary consumer connections, there isan analog link at each end, which introduces enough extra distortionthat you can't reliably transmit better than 33 kbs. Its actuallyamazing you can do that well.But, bottom line, there is a hard limit of 56 kbs over the publicswitched telephone network, and practical limits rather lower. Its not aregulatory decision, and its not basically about signal to noise ratiosover the line to your house, its fixed because that is how the PSTSworks.
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