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I have a modem over a phone line. Why can't it ever connect at more than 56 kbps?

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possible duplicate of Why is dial up so slow? – sleske Jan 23 '12 at 9:21
up vote 8 down vote accepted

Modems are designed to work over the classic phone network, which only ever supported 8kHz samples of 8-bits each. This provides an absolute maximum of 64,000 bits per second.

In order to support actual modem speeds higher then 33.6 (aka 56kflex, x2, v.90/v.92), one end of the connection must be digital, and there can only be a single analog to digital conversion in the communication path.

Given that most consumers have a modem connected to a standard POTS line, this is the first and only analog part of the link. On the service provider's end, there is a PRI, BRI, or channelized T1 providing the digital side of the link.

DSL tech, even though it runs over the same copper, uses very different technology in frequencies outside the normal voice ranges, and thus can obtain higher performance then plain old modems.

Because modems have to operate within a certain set of frequencies to be properly supported over analog POTS lines, this is part of what creates the 56kbit ceiling. The FCC, due to power and signal issues, further limits the connection to 53kbit.

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Good answer! +1 – skub Jan 23 '12 at 4:38
This answer is not correct; there are far more details than provided here. – Ignacio Vazquez-Abrams Jan 23 '12 at 4:48
56kbit is the maximum. The rest is used for in-band signalling to fill out your 64kbit channel – Fiasco Labs Jan 23 '12 at 5:21
@FiascoLabs: in-band signalling is used in the US only, and there the maximum is actually 53.3 kbit/s. The rest of the world uses out-of-band signalling. (E-carrier series). The 64-56 difference is actually due to the Error-Correction Codes needed to deal with analog noise. – MSalters Jan 23 '12 at 8:45

The FCC only allows a modem to connect at 53.3k. Even if the modem itself reports 56 or 54 or something higher than 53.3, it won't be higher than that in actuality. Additionally, most often the connect speed you see, if it is accurate, is only the initial speed, actual average speed can often also be lower throughout the duration of the connection due to line quality, connection quality, etc etc.

So really in the end it isn't so much a limitation of the technology itself, but of the FCC and the quality of the carrier.

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I was going to say , it wont connect faster than 56k because it is a 56k modem , but these answers are much better. I think we got about 46-49 actual download speeds on a good day with them. – Psycogeek Jan 23 '12 at 5:25

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