Re: 8b/10b and EMI
Look at Joel Goergen's presentation from the March 2000 meeting --
he has spectral plots of 8b/10b Idle sequences compared to
scrambled -- there is at least a 10dB reduction in power for the
Ron Miller wrote:
> Hi Ed
> All those that I know in fibre channel using GBICs has had problems when
> the idle signal is sent. I do not know yet whether 8B/10B is the culprit,
> idle signal sure seems to have accentuated the problem compared to a random
> data stream.
> Has anyone done a spectral analysis of 46b/66b vs 8b/10b with idle
> characters continuous?
> ron miller
> Edward Chang wrote:
> > 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.
> > Regards,
> > Edward S. Chang
> > NetWorth Technologies, Inc.
> > EChang@NetWorthtech.com
> > Tel: (610)292-2870
> > Fax: (610)292-2872
> > -----Original Message-----
> > From: email@example.com
> > [mailto:firstname.lastname@example.org]On Behalf Of Rick Walker
> > Sent: Thursday, March 16, 2000 11:03 PM
> > To: email@example.com
> > Subject: Re: 8b/10b and EMI
> > Dear Tom,
> > Tom Truman <firstname.lastname@example.org> 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.
> > --
> > Rick Walker
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> (408)487-8017 (' 0-0 ') fax(408)487-8017
> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
> email@example.com, firstname.lastname@example.org
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