RE: 8b/10b and EMI
Tom, Rick and all:
If the only reason to scramble the 8B/10B code is to minimize the
probability of EMI emission caused by the occasional, repetitive IDLE
signal, we may have to ask ourselves a question: have we done enough home
work to prove it is required? Even a simple circuit, it is not free.
So far, in the real industry-wide installations, no one has the 8B/10B IDLE
EMI problem. Furthermore, no one has proved that 8B/10B IDLE signals will
cause EMI problem for 10 GbE in an enclosed environment.
Edward S. Chang
NetWorth Technologies, Inc.
[mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Rick Walker
Sent: Thursday, March 16, 2000 11:03 PM
Subject: Re: 8b/10b and EMI
Tom Truman <truman@xxxxxxxxxx> writes:
> If 8b/10b were to be scrambled, then it would appear to me that all it
> is providing at the XAUI interface is packet delineation and some
> error monitoring capability. I imagine that each lane would need a
> separate scrambler/descrambler, initialized to different states so
> that the transitions across the lanes are uncorrelated. Synchronizing
> these scramblers, and deskewing the lanes would require some thought
> -- it isn't difficult, but it isn't as straightforward as the
> "alignment column" proposed for HARI.
It's not as bad as you think.
The scrambling is done *prior* to 8b/10b encoding, so that the full
run-length and DC-balance properties are preserved.
The scramblers would be randomized by the data itself, and no special
effort would be required to de-correlate them.
> At that point, the 25% overhead of the 8b/10b scheme seems to be a
> staggering price to pay for delineation and error monitoring -- why
> not start with scrambling, at a lower baud rate, and make the overall
> design problems simpler?
Because the data is scrambled *prior* to coding, the benefits of 8b/10b
are not lost. The net result is that the spectral properties are improved
at the cost of some added circuitry.