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Re: XAUI jitter tolerance


Nothing in the presently proposed jitter spec (1.8 MHz) prohibits
any manufacturer from setting the receiver bandwidth as high as
he wants.  It only prohibits bandwidths below that value.  We don't
need to increase the "break" frequency.  The only thing that would
do is legalize transmitters with worse low frequency jitter.

FWIW, here's the rationale used in Fibre Channel for the Fc/1667
"break" frequency spec:  It's based on a ref clock's low frequency 
jitter and "wander".  This is effectively limited by the frequency 
tolerance spec (say 100 ppm).  The worst case is that the frequency 
slam back and forth between these extremes:

        F = Fref * (1 +/- 0.0001)               cycles/sec

The resulting phase error (the integral of frequency error) is:

        (Tm/2) * 0.0001 * Fref          cycles of the REFCLK

where Tm = 1/Fm, and Fm is the rate at which the REFCLK frequency
slams back and forth between the 100 ppm limits.

If the REFCLK is 1/N of the bit rate (N is generally 10 or 20), 
then this peak-peak jitter as a percent of the bit period is

        (Tm/2) * 0.0001 * Fref * N      cycles of the bit-rate clock

For example, at a modulation rate that is 1/1667 of the bit rate
(1.875 MHz for 3.125 Gbps), the jitter is 0.083 of the 320 pS baud.
At higher modulation rates, the jitter is proportionally smaller,
and at lower modulation rates, the jitter is proportionally larger.


Allan Liu wrote:
> Hi,
> A jitter specification has been proposed by Ali Ghasi of Broadcom for
> XAUI.  At the December Jitter Meeting in Austin, Texas, Agilent made a
> proposal for an improvement.  Those presentations, as well as a summary
> of the meeting by Anthony Sanders(facilitator of the XAUI Jitter Ad-Hoc)
> are available at:
> Our proposal is to increase the "break" frequency of the jitter
> tolerance mask.  Currently, the proposal puts that "break" frequency at
> 1.8MHz.  We are proposing to push it up higher to 3-5MHz.
> There are four reasons why we believe the time is ripe for making this
> change:
> 1) The current "break" frequency was derived from Fibre Channel which
> derived it from Sonet; it's equal to the fundamental frequency divided
> by 1667.  I have looked long and hard and I have not found any
> documentation as to why this number was picked.  If anybody has an
> explanation, there is massive group of people waiting to hear the
> answer, including myself.  In the meanwhile, let me just repeat what I
> have heard from different people; this is in line with what Larry Devito
> of Analog Devices has posted to the reflector before.  Current wisdom
> has it that this number had to do with old SAW filter technology that
> was used at the time Sonet was created.  In addition, Sonet had to
> contend with the inherent problems with using regenerators in the system
> and thus had to make their jitter specs more stringent.  And to be
> compatible with older systems, today's Sonet systems are designed to the
> same old spec.  Fibre Channel comes along and copies this spec from
> Sonet.  Infiniband comes along and copies it from Fibre Channel.  And
> now, XAUI comes along and also wants to copy it from Fibre Channel.  And
> nobody knows why! XAUI is brand new and does not carry any old baggage.
> We have a chance to do it right and to write the specification to
> reflect current technologies and current implementations.
> 2) Today's Fibre Channel systems use receivers with a much higher
> bandwidth.  My measurements show that they are in the 3-5MHz range.
> During the December meeting, Jeff Cain of Cisco said their 1G Ethernet
> systems are using SerDes with bandwidths in the same range.  And all
> these systems are working perfectly.  So why do we continue to limit
> ourselves to a legacy specification that everybody exceeds? We should
> write the XAUI spec to reflect what people are implementing and what
> makes sense.  And from my understanding of how receivers work, a higher
> bandwidth equals better performance.
> 3) Other technologies do not necessarily have the same ambitious cost
> structure as Ethernet historically has.  XAUI is suppose to be a low
> cost interface to connect the optics to the MAC.  At 3.125G, it is not
> easy to build a functional system.  Increasing the "break" frequency
> means that the receiver is able to track more jitter.  This will make it
> easier for everybody to meet specification and produce a fuctional
> system.  And of course, this will drive down the cost of each port.  But
> the crucial point is not cost, but that increasing the break frequency
> will NOT impact system performance in any negative way.
> 4) Increasing the "break" frequency will also make it easier and cheaper
> for the integration of XAUI into bigger chips, like MACs.  Integration
> can mean 1 channel of XAUI or 100 channels of XAUI.  Obviously the
> current generation of XAUI will be discrete, but from the days of HARI,
> integration has been the goal.  We must not forget this goal as we move
> forward with the specification.  And again, this goes back to the point
> about XAUI being a low cost interface.  Increasing the bandwidth means
> the ability to use a smaller filtering cap in the PLL which means a
> higher level of integration can be achieved.
> I believe the result from the straw poll during the December meeting is
> short-sighted.  There is nothing scary or dangerous about this change.
> It is certainly a logical and much needed change to reflect what our
> technology can do and what the market is producing.  There is no need to
> copy other standards, especially if those standards have no technical
> basis for their jitter tolerance numbers.  However, there are clear
> reasons, as listed above, as to why these numbers need to be changed.
> Any claims that XAUI can leverage from this standard and that standard
> are unfounded.  There are no standards out there that has the same
> goals as XAUI or have the same fundamental frequency as XAUI.  And
> why should XAUI continue to carry the old baggage as the other
> standards do? XAUI was suppose to be easy.  After all, 3.125G is only
> only 625MHz from 2.5G.  Wrong! XAUI chips have yet to be abundantly
> available.  This suggests that they are much harder to implement than
> people previously thought.  Any thoughts of XAUI being easy because it's
> "similar" to other standards are simply preposterous.
> This message is to open up this discussion to a wider audience and to
> get people thinking about this issue as we approach the next meeting.
> Any and all input on this matter is appreciated.
> Regards,
> -Allan

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