Re: 64/66 control code mapping
Hi Joel,
please see my embedded coments below.
> Kamran /Rick/All
>
> I know I still have more lab verification, and I know 8B10B is
> well known/used, but we have got to do something to slow the
> XAUI/HARI portion down a hair.
By "slow down" I'm assuming you mean slow down the rate at
which implementaions are being developed around this.
> Aside from concerns we have all been addressing, there were two
> more brought to my attention yesterday in a customer meeting.
> The first is if we are worried about being like Infiniband, and
> I am not saying we should be and I am not saying we should not be,
> but 8B10B at 3.125 in a PC is 'nuts because most PCs barely
> pass emissions as it is'. The addition of the spectral content
> of 8B10B could be a disater for that interface. We should be
You are correct that some PCs are built poorly and have EMI problems.
But this is not true of all PCs.
You also need to keep in mind that most PCs are built toward a
completely different cost structure than what is being considered
here. Then you also need to consider that the PC enclosure design
that most companies use has changed little in the last 20 years.
At that time PCs ran with a processor clock rate of 4.77 MHz. Now
the same processor internal clocks have passed 1 GHz.
I don't think you can blame the EMI problems of a PC on the 8B/10B
code. For example, I've seen the radiated emissions of a PC
REDUCED when the internal IDE drives are replaced with a Fibre Channel
card. Why? Simple, the IDE bus in PCs is an UNTERMINATED transmission
line. It has generates tons of EMI, but is still used because its
CHEAP.
Yes, you CAN build lousy cards that will radiate something fierce.
But you can also apply good, sound, engineering practices to build
that same card such that it will be very quiet. The same is true for
optical modules. There are GBICs out there that are made with
full metal shells to keep the EMI down. Others are built with
plastic shells. Some of the plastic ones are much worse than the
metal ones. Some plastic ones are BETTER than the metal ones.
The differnce? The engineering INSIDE the module.
> careful, if that is one of our resaons, about trying to be similar
> to an interface that most likely will have to change.
Hmn. Most likely will have to change? Where did this come from?
Do you know something that the rest of us don't? Unless you
can back this statement up with some facts, I will consider this
to be an unqualified opinion.
> The second actually bothered me more because I never thought about
> it and have to do some more verification to determine validity,
> but the customer pointed out that only fr-4 was readily available
> in his country, where as some of the materials I have been using
I'm missing something here. One of the target goals of XAUI was
to operated on FR4. So whats the issue? AMP/Tyco has presented
data showing even raw 10 GBd signalling will work on FR4 (not very
well, but it works) so long as you don't add any connectors.
Mike Jenkins of LSI Logic has presented data showing >2.5GBd signaling,
in CMOS, over 24" of FR4, with an eye opening at the RX end
big enough to drive a truck through. And oh by the way, this was all
done with 8B/10B coded data streams.
> was very difficult for them to find. That being said, with respect
> to skin effect of the geometry and the erratic loss tangency
> of fr-4 ( the real part isn't pretty either ), the slower speed
> can make a difference.
Absolutely. Thats why XAUI splits the data into 4 streams in the
first place.
> > If it appears that 8B/10B is no longer desirable due to EMI and
> > skin-loss limitations on the PCB, we will be quite happy to
> > present our ideas w.r.t. using 64b/66b on the 4 XAUI lanes.
I don't believe that anyone has shown any data to this effect.
Until some one does, this is all speculation. Unfortunatly we
can't design based on speculation, we must limit ourselves to the
available facts, or perform the necessary resarch to validate
(or invalidate) alternate hypothesis, when can then be presented
as new facts.
If you are signing up for this research effort, I believe you will
find everyone in support of your efforts. None of us are afraid
of new knowledge. But we must all keep our guard up to ensure
that what we are basing decisions on are facts, and not just
speculation.
Regards,
Ed Grivna
Cypress Semiconductor
> >
> > --
> > Rick Walker
>
> Joel Goergen
>