XAUI jitter tolerance
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
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.