[SI-LIST] Re: purpose of 8b/10b encoding
- From: david_instone@xxxxxxxxxxxxxx (David Instone)
- To: zabinski.patrick@xxxxxxxx
- Date: Tue, 11 Sep 2001 13:09:24 +0100
Seems no-one from IBM (who hold the patent on it) are going to answer.
So-
First advantage- it encodes the 256 possible 8 bit characters into 10
bits (4096 combinations) so there is redundancy and it is possible to
tell if a 10bit byte is valid to a 'high' degree of confidence. This
also leaves room for certain special characters that can be used to
detect byte and word synchronisation.
2nd advantage- the code is basically DC balanced. There are never
more than 5 consecutive 1's or 0's and over, (I think 40 bits) the
difference between the number of 1's and the number of 0's is never more
than 2.
This latter means that the effects of interconnect phase dispersal
(read ISI) are limited
Zabinski, Patrick J. wrote:
>
> What's the purpose of 8b/10b encoding?
>
> I've heard reasons ranging from "to provide DC-Free data
> streams" to "to reduce the effective spectral content of
> the link". This is all fine-and-dandy, but the efficiency-
> hit 8b/10b (80%) has on a link is a hefty price to pay.
> I have a difficult time believing the benefits outweigh
> the cost.
>
> For links such as InfiniBand (and others), does anyone know
> why 8b/10b encoding was used? What are folks trying to accomplish
> when they add this requirement to the spec(s)?
>
> I'm sure there's something I'm missing here, and I'd appreciate
> it if someone could clue me in.
>
> Thanks,
> Pat Zabinski
>
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--
Regards
Dave Instone. Compliance Engineer
Storage Systems Development, MP24/22
Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
Tel: +44 (0)23-92-496862 (direct line)
Fax: +44 (0)23-92-496014
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