RE: 20 / 100 ppm
Folks,
Although I'm not an expert on this issue, I can propose some reasoning
behind some of the cost statements being made. Presumably, clock recovery
should be easier with a lower-jitter signal (20 ppm), since the signal is
effectively cleaner (i.e., you should need a lower-Q resonator - another way
of saying the resonator doesn't need to be as robust against jitter).
Basically, a cleaner signal relaxes the requirements on the receiver. Now,
this should only matter much if it makes the difference between having an
integrated resonator or a discrete one - the latter would increase costs
associated with packaging, yield loss, etc.
Regards,
Jugnu
Jugnu J. Ojha, Ph.D.
Technical Advisor, Optical Networking
Caspian Networks Inc.
(408) 382-5213
fax: (408) 382-5588
jojha@xxxxxxxxxxxxxxxxxxx
-----Original Message-----
From: pat_thaler@xxxxxxxxxxx [mailto:pat_thaler@xxxxxxxxxxx]
Sent: Tuesday, May 08, 2001 3:01 PM
To: benny.christensen@xxxxxxxxx; stds-802-3-hssg@xxxxxxxx
Subject: RE: 20 / 100 ppm
Benny,
The pointer adjustment mechanism can adjust for rate differences of over 100
ppm. Generally, an SDH/SONET network has an alarm threshold that sets an
alarm off if adjustment is happenning that often. The alarm threshold is
normally settable and can be set to not go off at 100 ppm, but the usual
default would go off. The arguments I have heard against 100 ppm are:
Since the SDH/SONET/OTN specs only require tolerance of 20 ppm,
implementations may not tolerate 100 ppm even though the basic mechanism
does.
100 ppm will make alarms go off.
Clock recovery cost for 20 ppm is cheaper than for 100 ppm and 20 ppm clocks
cost the virtually the same as 100 ppm clocks. It is not obvious to me why
this would be the case, but it is an assertion some people are making. Those
who favor 100 ppm make the opposite assertion.
Regards,
Pat
-----Original Message-----
From: Christensen, Benny [mailto:benny.christensen@xxxxxxxxx]
Sent: Tuesday, May 08, 2001 1:04 AM
To: stds-802-3-hssg@xxxxxxxx
Subject: RE: 20 / 100 ppm
Hi all
Just a comment on the 20 / 100 ppm clock accuracy discussion.
As far as I recall from my mind, the 20 ppm (SDH/SONET) spec, is due to the
limited amount of pointer justification. As I understand, the SDH framer can
only adjust +/-8 bit (a byte) pr. two frames (maximum) (the pointer
adjustment is indicated by inverting the even or odd bits of the pointer,
giving a error prone majority decision), so this sets the limitation on
maximum clock difference that the framer can handle before experiencing a
buffer under/overrun.
The situation for 10GE is a little different as one has the idle characters
to insert or delete in order to account for a slight difference in clock
frequencies. THis may be regarded as the equivalent to pointer processing in
SDH framers.
There is normally not any problems with the ref.clk at the RX side. At
least, the GIGA 10 Gb/s CDR devices have a 500 ppm (or 2000 ppm) auto-lock
range detection circuit, which ensures a reliable lock conditions.
Experimentally, I found that the PLL acquisition/capture range is up to
about 20 MHz. This is mainly set by the PLL bandwidth of the CDR, which is
minimum 4 MHz (set by the jitter tolerance criteria). Even when the closed
loop PLL gain drops to 0.1 (at 40 MHz frequency offset) there is still
sufficient gain to pull the VCO into lock.
Benny
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GIGA, an Intel company
Benny Christensen, M.Sc.E.E, Ph.D.
Mileparken 22, DK-2740 Skovlunde, Denmark
Tel: +45 7010 1062, Fax: +45 7010 1063
e-mail: benny.christensen@xxxxxxxxx, http://www.giga.dk
> I have yet to see an explanation as to why a 100 PPM clock
> recovery system
> is more expensive than a 20 PPM clock recovery system. If
> you use the
> argument set by the legacy SONET standards body, the
> receivers for 10GbE
> would be less expensive if the clock were specified at 20
> PPM. I have yet
> to hear from any of the component vendors to support that
> argument. All of
> the component vendors seem to believe that the SONET/SDH version is
> slightly higher in cost.
>