Talk:56 kbit/s line

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The figure is derived from the bandwidth of 4 kHz allocated for such a channel and the 16-bit encoding (4000 times 16 = 64000) used to change analogue signals to digital, minus the 8000 bit/s used for signalling and supervision.

I'm not sure how the 56 kbps figure is actually derived, but I do know (and the DS0 article agrees with me) that POTS voice signals are sampled at 8 bits/sample and an 8 kHz sample rate, rather than 16 bits and 4 kHz as this article says.

I'm not sure, but I believe an argument could be made that 4 kHz is indeed the bandwidth (Nyquist, right?), but that would mess up my math, and I am at least pretty darned sure about the 8 bits/sample thing. :-)

[edit] robbed bit signaling

This definition should also include a reference to robbed-bit-signaling (RBS) as it too plays a part in the bandwidth limit.

[edit] Article rewritten

I have now substantially rewritten the article. My understanding is that 56k lines only used the most significant 7 bits of each sample period to avoid interference from robbed-bit signaling in the least-significant bit of every sixth sample. A more modern system might have tried to phase-lock to the RBS, or to have used error-correcting codes to mitigate its effects (as well as fixing any residual BER on the line), and thus would have been able to achieve a rate of 64000 * 47/48 = 62666.6+ bps. However, that was way too complex a solution to the problem, given the 1960s-era engineering of the time 56k lines were created, and dropping the bottom bit was simply easier. -- The Anome 10:32, 12 October 2005 (UTC)

[edit] Why Don't You Actually Get 56K a Second?

I used to have a 56k modem hooked up to the internet (in England) ages ago, but I never got transfer speeds close to 56k a second. At best it was like 3 or 4 k. From what I've seen of other peoples' 56 modems, they get similar speeds. Does anyone know why this is? Is it just that they were always busy or something? If so, why didn't it ever get above 3 or 4 k? Thanks. Doom jester 12:03, 24 January 2007 (UTC)

When you download something, the site typically gives you a speed meaured in kiloBYTES per second. The k in 56k refers to kiloBITS per seconds.. Since 1 BYTE == 8 BITS, a 56k modem is really connecting at a speed of ~7 kiloBYTES per second. Your download can't use all of that, as your computer must still maintain normal network traffic (letting the site you are downloading from know that you are still connected to them, any background programs that talk to the internet, etc.) This reduces your download speed to around the 4 to 5 kiloBYTES normally seen. ZydecoRogue 16:32, 9 February 2007 (UTC)

Dude! Thanks, I always wondered that! Maybe there should be something in the article about it.. I bet a lot of people often wonder that. People who aren't particularly expert at computers I mean, like me.Doom jester 12:09, 30 March 2007 (UTC)