Bits and Bytes

Ok, there is 8 bits in a byte so 128kbit ADSL does 16KB/s.

But a 56Kbit modem only does 5.6KB/s not 7KB/s.

The reason is that the 56K modem uses start bits and parity bits right? So how come ADSL doesn't need these extra bits too?

Maybe they accounted for that?

Im pretty sure its because of the better through put that adsl has, and maybe a bit todo with how adsl transmission works.

Basically the better the connection, the less errors there are going to be, so you dont have to do as much checking that the data is arriving in order. I think adsl will still have some extra bits, but will rely more on Transport/Netowrk layers ( eg TCP/IP or UDP) to keep things in order...

Really should have paid more attention in my data communications paper last year.

When you get towards ADSL speeds, the limiting factor in download speed could be the server, or a network link between you and the server. With modems, it's usually the modem. ADSL could well have those error checking bits, and we might never notice.

What Circus said about doing error checking at the TCP layer is a good point too. The reliability of ADSL might be such that you don't need to check every single byte that goes thu, just checking each packet might be enough. When you get into fibre, the error rate is so low as to be almost irrelevent (0.00001 or lower) so you'd be silly to check each byte.

Yep Circus got it about right... modem connections are very dodgy... there's so much interference/line noise etc. that it requires pretty heavy error checking stuff, which basically means it's an overhead on the transmission.

I agree with commando he seems to be on the right path.
It seems that with 56k, it is that as it is limited by the speed of the modem whereas adsl is 8mbit, but it is server which controls the flow so maybe the servers put through just over 128k?