Re: XAUI Related Issues
All -
The FC method averages out the jitter terms which is why it was adopted and why I offered it.
Since all system-level signal integrity requirements are differential, they inherently include any effects of skew. The only reason to keep skew as a separate spec would be for isolated control over boards - where a board may be tested without an actual transmitter. In that case, both skew limits and methods are needed. It seems that keeping such info available as informative is a good idea.
FC uses long copper differential cables, up to >30 meters, from independent suppliers. So in FC, skew is important, separately measurable and manageable.
Tom
Tord Haulin wrote:
> Ali,
> Differential skew can be measured in presence of both DCD and jitter.
> The Fibre Channel method that Tom Lindsay reported seems to be
> robust. My concerns with the differential skew are more concerning
> consistency of the standard. It is the only one out of a whole
> range of signal balance quality numbers that are otherwise left
> outside the standard. Rather than adding all that, I suggest we
> remove the differential skew spec.
>
> Best Regards
> Tord.
>
> -----Original Message-----
> From: Ali Ghiasi [mailto:aghiasi@xxxxxxxxxxxx]
> Sent: Wednesday, February 28, 2001 01:19
> To: Kesling, Dawson W
> Cc: Serial PMD reflector (E-mail); Anthony Sanders (E-mail); Ali Ghiasi
> Subject: XAUI Related Issues
>
> Hi Dawson
>
> I was looking at D2.1 and I found several potential issue:
>
> - In Tampa meeting we agreed the RX amplitude will be fixed to
> 1600 mV and TX need to be determined. Here you need some margin
> and as allowing some reflection. Probably maximum TX need to be
> lowered.
>
> - Return loss originally was intended only for receiver (10 dB
> diff, and 5 dB common mode). It would very difficult to meet
> to meet transmit return loss. In the IB we added Zdiff,
> ZSingle_Ended, 10% matching, but not the return loss.
>
> - Differential Skew can't be measured separately from DJ even if
> you are sending 1010, you still may have DCD. The total DJ
> already include the Diff_Skew so I would propose removing it
> since it can't be measured.
>
> - We need to have better definition of how much skew you can
> have lane to lane. Chapter 48 says you can have 1 UI allocated
> to the PMA, but we need to allocated like 0.5 UI to the
> electrical
> XAUI portion.
>
> - It seems to me the connector loss is lumped as part of
> interconnect
> loss resulting in 7.5 dB of loss. Under some cases where the
> 7.5 dB
> is just ISI some implementation may not work. We should try to
> separate
> ISI loss from connector+misc+croasstalk loss.
>
> - Fig 47-4 assuming measured with an ideal load does not include
> 4% cross talk which equates to 63 mV under worst case scenario
> of
> 1600 mV and 200 mV. Loss allocation in the table 47-6 specifies
> 4.5 dB
> for others but according to eye diagram Fig 47-4 some one may
> introduce
> more loss. The 4% crosstalk results in 31% additional eye
> closure with
> an actual XAUI receiver. Return loss penalty associated with a
> XAUI
> receiver gets absorbed in to the SerDes effective sensitivity,
> but the
> crosstalk may not. As the biggest source of the crosstalk is
> the
> connector.
>
> Thanks,
>
> --
> Ali Ghiasi
> Broadcom Corp.