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Re: [10GMMF] 62MMF Worst Case Connector Offset Calculations

Hi all,

Couple of thoughts on the notes sent earlier on this topic:

1. I thought that the 1 percentile case applies to one single parameter at its worst case. However, when you have multiple connectors, since they are independent, you have to treat them as multiple parameters, and therefore have to use smaller percentiles. Let say you have transmitter lens (with one loss) and receiver lens with another loss. If you relax the parameters on these, their worst case values have to be at 0.5 percentile each. We can apply the same to the connectors, since each has its own impact.  So, you need to tighten the connector specification to achieve the overall 1 perceintile loss. Since we can't do this - the connectors are already installed and individually specified at 0.75dB worst case (3sigma), the rest of the link parameters need to be tightened to compensate.
2. I believe that in the link connector loss calculations for Ethernet we used an average value and standard deviation. So, the typical link had 4 connectors, for which we added the mean values linearly and the std deviations as square root of sum of the squares.

3. Connector loss degrades with number of matings. Since many of the legacy installations are old, it is reasonable to expect the mean and std. dev of the connector loss to have shifted upwards.  

4. I believe that the OFL connector loss measurements do not take into account the mismatch in size of the optical fibers.



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

phone: (914)-945-3761
fax:        (914)-945-4134

David Cunningham <david_cunningham@AGILENT.COM>
Sent by:

08/09/2004 10:25 AM
Please respond to
"IEEE 802.3aq 10GBASE-LRM"

[10GMMF] 62MMF Worst Case Connector Offset Calculations

Dear All,

I have attached a presentation which analyzes the connector offset based on the legacy Ethernet and building cabling standards maximum connector loss specifications and worst-case link configurations.  

Both Ethernet and the building cabling standards limit the maximum per connector loss with overfilled launch (OFL) to 0.75 dB. Furthermore, the worst-case cable model for Gigabit Ethernet is one with two 0.75 dB connectors and, for LX, offset launch jumper cables at each end (in place of normal jumper cables).

I have therefore calculated the loss versus offset for 62 MMF with OFL.  The allowable worst-case offset is then the offset which  produces 0.75 dB of loss - this occurs at approximately 6 um.

The main conclusions are:

1) For 62 MMF, if all the loss is attributed to lateral offset, the worst case connector offset is approximately 6 um for a loss of 0.75 dB(OFL).
2) For 62 MMF, the worst case link for modal noise or channel variance measurements is one with two connectors each having 6 um offset equivalent to 0.75 dB (OFL) loss.  This is because modal variation effects are larger when there is a small number of higher loss components in the optical path.
3) If three connectors are used the maximum loss (OFL) per connector must be reduced to 0.5 dB which is equivalent to a 4 um lateral offset per connection.

Interestingly, if we use the Rayleigh distribution with a mean of 3.58 um, as proposed by Joerg, the probability of a 6 um offset is about 0.1 and the probability of two 6 um offset connectors is about 0.01. If the Rayleigh distribution is correct this means that we are investigating the 1 percentile case which is consistent with the Ethernet methodology. This does not mean that I have confirmed that the proposed distributions of connector offsets is correct - rather I'm just noting that for the 62 MMF case there does seem to be agreement between the two methods on this point.

I also have placed the presentation on the FTP site per Jonathan King's e-mail.

[attachment "Worst Connector Offset Calculations.pdf" deleted by Petar Pepeljugoski/Watson/IBM]