[SI-LIST] Re: Fibre channel interconnect margins

Hey Chris,
When you started that thread I didn't answer, since it seemed pretty 
obvious that "errors are bad", and I didn't know better.

In fact, I think that companies DO ship products that toss a random 
error approximately every 10e-xx or so. Why? Because the statistical 
theory behind those errors is that random/Gaussian noise is, by 
definition, unbounded - errors are a fact of life, even if the error 
rate is very low. Eventually you have to an edge that is outside the 
jitter spec. A single unrepeatable error out of billions of bits simply 
has to be expected.

What matters is that the system (specifically the higher layers of the 
stack) respond to these random errors in such a way that they are not 
necessarily catastrophic. Most serial protocols will simply resend the 
packet. If it's truly a random error, the next transmission (or the 
theirs) will be sent correctly. As well, many protocols also have error 
detection and correction built in, and thus can recover in that fashion.

So sure, you should expect an error every day or so. Your system must 
simply be able to handle that eventuality. What concerns me is that 
these protocols are capable of masking serious bit error problems, which 
don't become apparent until someone notices the system is really 
lagging, or their video is starting to stutter.

Keep in mind that we're talking about random errors. If you get an error 
that happens every time you send a particular packet, you have a faulty 
product. Repeating that pattern will increase the bit error rate above 
the spec, and allow you to diagnose and fix the problem.

As a friend of mine once said: "Randomness is too important to be left 
to chance".

Alan


Chris Cheng wrote On 06/30/06 21:15,:

>A while ago I've started a long thread about "do you really ship a product at 
>bert 10e-xx?"
>You seem to imply that a 10e-12 will be the benchmark for acceptance.
>Does CISCO really ship a product that will take an error every few days as 
>acceptable ?
> 
>________________________________
>
>From: si-list-bounce@xxxxxxxxxxxxx on behalf of Mcgrath, Christopher
>Sent: Fri 6/30/2006 1:03 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Fibre channel interconnect margins
>
>
>
>Some stuff that I have done in the past to stress test FC links:
>1. Max out the cable length defined by the FC-PI spec.
>2. Put the product under test into a thermal chamber while varying all
>voltages associated with the FC link (ASIC, SERDES, PHY, etc.) across
>all corners.  (i.e. 2 voltages across hot/cold corners =3D 8 test cases)
>3. User random data and not just the idle characters on the link.
>
>Using the BER as the benchmark for acceptance (something like 10e-12),
>these three things were the things we did to beat the hell out of links
>before officially qualifying the physical link.
>
>Our philosophy was to not use things like pre-emphasis or techniques
>like that to stress the link but to tune the link for optimal
>performance and reliability (best BER).  Once we established that after
>our standard tests (#1-3 in my list) was sufficient to pass
>interoperability standards with good margin (>2 orders of magnitude of
>BER), we elected not to mess around with emphasis or amplitude.  The
>only thing that we had to tune was the RX termination in the ASIC to
>best match the board trace impedance, but this tuning was a separate
>effort.
>
>In summary:
>1. Tune the link for optimal eye under lab conditions with random data
>patterns.
>2. Max out the cable length while testing all thermal and voltage
>corners with random data patterns.
>
>If you meet your BER requirement across several platforms, ship it!  If
>not, you may have to tune the eye based on the failing corner cases.
>
>Oh yeah- and if you are using pluggable optics then repeat the
>qualification for all pluggable optic model numbers that you are
>intending to ship with the unit (or put on your interoperability table)
>as well. =20
>
>-Chris
>
>
>
>
>  
>
>>-----Original Message-----
>>From: si-list-bounce@xxxxxxxxxxxxx
>>    
>>
>[mailto:si-list-bounce@xxxxxxxxxxxxx] On
>  
>
>>Behalf Of Babid A
>>Sent: Friday, June 30, 2006 11:20 AM
>>To: si-list@xxxxxxxxxxxxx
>>Subject: [SI-LIST] Fibre channel interconnect margins
>>
>>Hello,
>>
>> we have a fibre channal interfaces(@2.125Gbps) running on our boards
>>    
>>
>and
>  
>
>>looking for ways to stress the links. Right now, I am playing with
>>    
>>
>chip
>  
>
>>settings like pre-emphasis, equalization,amplitude, longer cables  etc.
>>
>> I really apprecaite any other suggestions/ideas to stress the links
>>    
>>
>to
>  
>
>>find out the margins. What is the right way of quantifying the signal
>>margins?
>>
>> -Thanks,
>>
>>
>>
>>
>>---------------------------------
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-- 
Alan Hilton-Nickel
Signal Integrity Engineer
Sun Microsystems Inc.
Netra Systems and Networking
Newark, CA



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