RE: XAUI signal detect
Joel,
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: "...direct 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).
Boaz
> -----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
>