[e2e] Random losses on very high speed networks
Cannara
cannara at attglobal.net
Wed Jul 23 09:49:54 PDT 2003
I'll make one clarifying comment here on Shannon's theorem, as Lloyd states
it. C is the theoretical channel capacity. It is not actual. This is
extremely important for anyone in the business to understand. The way C is
approached is by choice of symbols to send. For instance, FEC is a way in
which C can be approached under some noise constraints. So is the way in
which 'simple' modem protocols have been developed over the years, to deal
with the 4kHz local-loop channel provided by telcos.
Shannon did not fully specify how symbols were to be designed to allow C to be
approached, nor did he mean that bits were being sent as bits. Binary
representation of the data was mathematically convenient. In real, physical
links, signalling symbols can be complex, multilevel, self-correcting, etc.
The external digital interfaces may send/receive bits, but the heavy lifting
is not done that way on the link, except where signalling media are still
primitive, as in optical laser transceivers.
Alex
Lloyd Wood wrote:
>
> On Tue, 22 Jul 2003, Constantine Dovrolis wrote:
>
> > Besides, the context in which the term is used is so different
> > for Hz vs bps that probably it never causes confusion.
>
> Apart from something like Shannon's law, which uses both bandwidth and
> capacity, where the context requires that both be used correctly.
>
> > And at the end of the day, the two "bandwidth" concepts are not
> > totally irrelevant after all - see Shannon's law.
>
> That's rather the whole point.
>
> C = W log2(1+S/N)
>
> where C is the channel capacity in bits/second, and W is the bandwidth
> in Hz. Notice how C != W. Shannon was very very clear on the
> distinction between bandwidth and available capacity.
>
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