RE: XAUI signal detect
Joel,
Here is the background you asked for on this change. Comment 930 was
accepted in principle at the Interim in Irvine last month, meaning the
proposed remedy that "we need to add signal detect line to the XGXS
interface because a PHY end that is a simple retimer is unlikely to want to
generate LF codes" was accepted. The detailed implementation of the change
was left to the editors. This detailed editing was worked out at the open
editors meeting hosted by the 10GEA in San Jose on Jan. 25, where the
editors and other interested parties participated in detailed preparation of
D2.1. The text in this case was leveraged from subclause 39.2.3 and altered
as needed to fit XAUI/XGXS.
Thank you for the thorough description of your concern. I would like to hear
from other circuit designers concerning implementation practicality.
-Dawson
-----Original Message-----
From: Joel Dedrick [mailto:Joel_Dedrick@xxxxxxxxxxxxxx]
Sent: Friday, February 23, 2001 3:33 PM
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 bur!
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