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RE: XAUI signal detect

I think that in Clause  47 (p.278) it is defined as a variable, not
necessarily implemented as a XAUI signal. A better reference for your
assertion may be sub section e in clause 48: " passing of
signal_detect from the PMD to the PCS through the PMA...". I think that the
last sentence implies a specific implementation by additional signal from
the PMD to the PMA, because it means (to my understanding) that the
signal_detect states explicitly detected by the PMD. (However, the PMD-PMA
I/F is not defined as XAUI).


> -----Original Message-----
> From: Joel Dedrick [mailto:Joel_Dedrick@xxxxxxxxxxxxxx]
> Sent: Saturday, February 24, 2001 1:33 AM
> To: 'dawson.w.kesling@xxxxxxxxx'
> Cc: 'stds-802-3-hssg@xxxxxxxx'
> Subject: XAUI signal detect
> Dawson:
> In an editor's note on 278, you indicate that signal detect 
> was added to XAUI as part of a resolution of comment 930.  
> Was this left to editorial license, or was a specific remedy 
> voted on?  We think this is a bad idea, for reasons given 
> below, and will recommend it be reversed.  We'll input a 
> comment, but I wanted to find out where it originated.
> The note indicates it was a response to comment #930, which 
> presumes the existence of a simple retimer in the PMA, which 
> wants to relay a loss-of-light input over XGXS (XAUI) without 
> use of a LF code, presumably by squelching its output.  Since 
> the XGXS is AC coupled, this will result in differential 
> inputs at the DTE end which are biased at their switching 
> point.  The addition of a signal detect function I believe is 
> an attempt to recognize this condition and ensure that the 
> lack of a valid signal is detected at the DCE.
> We think that this new function isn't needed, and that it 
> will have a pretty negative impact on XAUI performance and 
> reliability.  It's not needed because even simple retimers 
> could easily implement a mode which outputs the LF sequence 
> interspersed with idle repeatedly when a signal detect input 
> from the optics is inactivated.  It may be acceptable to not 
> randomize idles in this fault condition.  This method for 
> communicating LF is extraordinarily simple -- there's no 
> reason to define another one.  Moreover, the basic function 
> of the retimer is to reset the jitter budget.  Since this is 
> best done with an implementation which fully decouples the 
> media clock from the XGXS clock, the proffered case of a 
> simple retimer which does not have this capability may be 
> rare.  A full retimer, which would include a clock tolerance 
> FIFO capable of IDLE insertion/removal clearly could 
> obviously generate LF sequences.  Easing implementation of 
> the regenerator-style retimer does not justify b!
> ur!
> !
> dening every XGXS implementation with a significant 
> performance and reliability penalty.
> The more important objection is that implementing an analog 
> signal detect will reduce performance and reliability of all 
> XAUI implementations to support a rare case.  Here's why:
> Typical forward crosstalk for 50 Ohm signals implemented with 
> stripline construction and 9 mil space is about 5%.  This 
> value saturates in only 2 cm of side-by-side run for the 
> risetimes typical of XAUI signals.   5% crosstalk with a 800 
> mV single-ended drive results in 40 mV of single-ended noise 
> coupled to the line, from a single interferer and a coupled 
> length of 2 cm.  For even modest run lengths, and including 
> other noise effects, a minimum of 100mV of effective 
> differential noise would be expected.  This is by no means 
> worst case.  
> In theory, signal detect functionalilty could be implemented 
> either as an analog envelope detector, or by differentially 
> biasing the inputs and then detecting a continuous zero at 
> the input.  But, an envelope detector which can reliably 
> detect a signal smaller than the 200mV XAUI sensitivity but 
> larger than the 100mV expected noise across process, voltage, 
> and temperature is a challenging design, which would 
> significantly complicate the already difficult XAUI receiver. 
>  This receiver is required by the deterministic jitter and 
> ISI requirements to provide gain to a pulse of less than 200 
> ps. duration and 200 mV differential amplitude.  Such a high 
> gain, wide bandwidth amplifier will almost certainly 
> oscillate if its inputs are biased at zero differential 
> voltage, with undriven, AC coupled inputs.  So, if squelched 
> outputs on XAUI lanes are an acceptable way to indicate 
> failure, then offset bias must be used to prevent 
> oscillation.  However, 100mV of differential offset would !
> di!
> !
> rectly subtract from the sensitivity of the receiver, 
> resulting in a severe reduction in reach.  In addition, it 
> would displace received edges in time, adding the equivalent 
> of .1 to .2 UI of deterministic jitter.  This seems like an 
> unacceptable penalty.
> Best Regards,
> Joel