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Re: [HSSG] BER Objective



Jugnu,
 
So the issue with bit errors is their impact on the performance of protocol like TCP trying to get high throughput over the connection. Here is a brief simplified description of what happens for something like TCP when a packet is dropped:
 
TCP discovers the packet is dropped either because it is using the fast retransmit option or because there was a timeout. Probably a high-throughput user of TCP would be have implemented fast retransmit because if they hadn't the connection would be stalled when it got to the end of the ack window until the timeout occurred (the timeout can easily be over a second as an RFC recommends a minimum time out of 1 second). So, lets say that fast retransmit is enabled which means that the sender will realize the packet got lost because the receiver will send acks with a sequence number acking everything up to the drop as it gets packets after the drop. The sender takes getting three duplicate acks (ie the original ack plus 3 more) as a sign that a drop occurs.
 
What happens then? It starts resending. That may include resending more data then was actually lost because TCP doesn't know if only one packet or more got dropped. Once it gets an ack that acknowledges the data it sent, then it knows it has finished filling the hole. During this period throughput is lost not only because of resending the actual dropped packet but also because of possibly sending extra data that wasn't lost and because of more conservative sending behavior during the retransmit. If SACK, selective  acknowledge, is used, this is improved somewhat.
 
One also often loses throughput during recovery because the sender is using an exception processing mode rather than its optimized fastest paths.
 
Next, TCP assumes that a packet drop is caused by congestion. It goes to a more conservative sending behavior and ramps back up to normal behavior as it receives acks for successful transmission.
 
Because recovery depends on getting acks, on long links even very low bit error rates cause significant throughput loss. Unfortuantely I don't have a number for how low BER has to be, but my recollection is that 10^-9 or 10^-10 will keep a connection from being able to get a throughput 10 Gig over a WAN link. 
 
Regards,
Pat


From: OJHA,JUGNU [mailto:jugnu.ojha@xxxxxxxxxxxxx]
Sent: Tuesday, August 29, 2006 1:14 PM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: Re: [HSSG] BER Objective

Steven,

 

I was trying to understand whether 1 error per second is a lot worse from a performance point of view than 1 error every ten seconds.  Your point about the extra test time is valid for the foreseeable future, where these interfaces are only used for high-end applications.  

 

Regards,

Jugnu

 


From: Trowbridge, Stephen J (Steve) [mailto:sjtrowbridge@xxxxxxxxxx]
Sent: Tuesday, August 29, 2006 1:11 PM
To: OJHA,JUGNU; STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: RE: [HSSG] BER Objective

 

Jungu,

I'm confused. I see a proposal to use a BER requirement of 10^-10 and these words about errors being "few and far between", yet 100 Gbit/s is 10^11 bits/s. An average of 10 errors per second doesn't meet my idea of "few and far between", and I doubt this would be acceptable to most users who will pay the kind of money this kind of interface is likely to cost.

 

I am curious whether a really high end interface like this makes a lengthier test more feasible (i.e., you couldn't afford to do a 10 minute test on an interface you expected to sell for $10. But for an infrastructure interface like this one, perhaps some extra testing time wouldn't be a significant portion of the interface cost)

Regards,

Steve

 


From: OJHA,JUGNU [mailto:jugnu.ojha@xxxxxxxxxxxxx]
Sent: Tuesday, August 29, 2006 11:48 AM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: Re: [HSSG] BER Objective

Roger, 

 

I understand that test time is the issue.  The point I’m getting at (and which I’ve always wondered about) is, if the errors are so few and far between that it takes so long to find them, how much impact can they really be having on the system/network performance?  I.e., are we being too demanding with the BER requirements.  

 

Jugnu

 


From: Roger Merel [mailto:roger@xxxxxxxxxxx]
Sent: Tuesday, August 29, 2006 10:44 AM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: Re: [HSSG] BER Objective

 

It’s not hard to measure, just time consuming.  If one wants to keep optics affordable, one need manufacturing test to be <minutes, not >10 minutes.

 

Although my position is that 1E-15 BER is not required; but only 1E-13 at most.

 


From: OJHA,JUGNU [mailto:jugnu.ojha@xxxxxxxxxxxxx]
Sent: Tuesday, August 29, 2006 10:37 AM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: Re: [HSSG] BER Objective

 

All of this raises the following question:  If this is so hard to measure, how much impact can it really have in the real world?  Why not back the BER requirement off to 10e-10? 

 

Regards,

Jugnu

 


From: Petar Pepeljugoski [mailto:petarp@xxxxxxxxxx]
Sent: Monday, August 28, 2006 8:03 PM
To: STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject: Re: [HSSG] BER Objective

 


I agree with Howard. It is impractical and expensive to test for very low BERs - the specs should be such that the power budget is capable of achieving BER =1e-15, yet the testing can be some kind of accelerated BER at lower value that is derived from the curve interpolation.

However, the as with any extrapolation of testing results one has to be careful, so in this case it will be manufacturers' responsibility to guarantee the BER=1e-15.  

Regards,

Petar Pepeljugoski
IBM Research
P.O.Box 218 (mail)
1101 Kitchawan Road, Rte. 134 (shipping)
Yorktown Heights, NY 10598

e-mail: petarp@xxxxxxxxxx
phone: (914)-945-3761
fax:        (914)-945-4134

Howard Frazier <hfrazier@xxxxxxxxxxxx>

08/28/2006 05:39 PM

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Re: [HSSG] BER Objective

 

 

 




 
For the 100 Mbps EFM fiber optic links (100BASE-LX10 and 100BASE-BX10)
we specified a BER requirement of 1E-12, consistent with the BER requirement
for gigabit links. We recognized that this would be impractical to test in a
production environment, so we defined a means to extrapolate a BER of 1E-12
by testing to a BER of 1E-10 with an additional 1 dB of attenuation.  See
58.3.2 and 58.4.2.
 
Howard Frazier
Broadcom Corporation


From: Roger Merel [mailto:roger@xxxxxxxxxxx]
Sent:
Monday, August 28, 2006 1:54 PM
To:
STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject:
Re: [HSSG] BER Objective


David,
 
Prior to 10G, the BER standard (for optical communications) was set at 1E-10 (155M-2.5G).  At 10G, the BER standard was revised to 1E-12.  For unamplified links, the difference between 1E-12 and 1E-15 is only a difference of 1dB in power delivered to the PD.  However, the larger issue is one of margin and testability (as the duration required to reliably verify 1E-15 for 10G is impractical as a factory test on every unit) especially since we’d want to spec worst case product distribution at worst case path loss (cable+connector loss) and at EOL with margin.  Thus in reality, all products ship at BOL from the factory with a BER of 1E-15 and in fact nearly all will continue to deliver 1E-15 for their entire life under their actual operating conditions and with their actual cable losses.
 
Thus, if by “design target”, you mean a worst case-worst case with margin to be assured at EOL on every factory unit, then this is overkill.  I might be willing to entertain a 1E-13 BER as this would imply that same number of errors per second (on an absolute basis; irrespective of the number of bits being passed; this takes the same time in the factory as verifying 1E-12 at 10G although this is in fact a real cost burden which adversely product economics); however, this would not substantially change the reality of the link budget.  It would make for a sensible policy for the continued future of bit error rate specs (should their be future “Still-Higher-Speed” SG’s).
 
-Roger
 
 


 



From: Martin, David (CAR:Q840)
Sent:
Friday, August 25, 2006 12:22 PM
To:
STDS-802-3-HSSG@xxxxxxxxxxxxxxxxx
Subject:
BER Objective

 
During the discussion on Reach Objectives there didn’t appear to be any mention of corresponding BER.
 
Recall the comments from the floor during the July meeting CFI, regarding how 10GigE has been used more as infrastructure rather than as typical end user NICs. And that the application expectation for 100GigE would be similar.
 
Based on that view, I’d suggest a BER design target of (at least) 1E-15. That has been the defacto expectation from most carriers since the introduction of OC-192 systems.
 
The need for strong FEC (e.g., G.709 RS), lighter FEC (e.g., BCH-3), or none at all would then depend on various factors, like the optical technology chosen for each of the target link lengths.

...Dave

David W. Martin
Nortel Networks

dwmartin@xxxxxxxxxx
+1 613 765 2901 (esn 395)
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