SFP output power control

Unanswered Question
Feb 15th, 2010

Hi, I can see from the spec of an SFP module it has a range of output power. How do i control that in the IOS or does it do it automatically based on rx power level?


I know there is 'show interface tranceiver' to show me whats happening but not sure how to set anything.


Any help appreciated.

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Tom Randstrom Mon, 02/15/2010 - 11:02

You cannot set the output power for these SFPs via software.  The published operating range takes into account the manufacturing variabilities as well as output power over its operating temperature range.  If you have too much power at the receive end of the link, you will need to install an optical attenuator to bring it within the receiver's operating input range.


Hope this helps!

CSCO10631105 Thu, 03/11/2010 - 09:51

Thanks for the response.


Too much receive signal is not my problem, I need to ensure the maximum output power is enabled.  I think the optical link is about 8dB @ 1310nm.


Basic sums says this should work....


Unit is quoted with -3 to -9dBm ouput power and a Rx sensitivity of -3 to -19dBm.


Worst case -9 tx with -19 rx minus 8dB loss gives me 2dB headroom.  I'd still prefer to have more.


Is it really true to say the tx can vary between -3 and -9 as this seems a very wide range for modern kit!


Thanks

Tom Randstrom Fri, 03/12/2010 - 08:17

Unfortunately, I don't have the manufacturer's testing reports to look at the actual output ranges being seen across the module's production.  I would suspect the operating ranges are much tighter than the quoted specifications.  However, manufacturer's also must meet the published IEEE minimum standards, which specify the optical parameters; which makes deploying the technology more consistent, interoperable and cost effective.


The other option for your application is to upgrade to a longer reach optical module. This will provide you with additional optical margin, but with an added cost for the link.


Hope this helps.

Marvin Rhoads Sat, 03/13/2010 - 16:48

I agree with baileyshbr. One cannot generally control the SFP output (or sensitivity) via software. Optical platforms (e.g. ONS series) allow this, but that's one of the (many) reasons why they cost so much more than LAN switches.


If field measurements indicate you are outside of accepatable operating ranges, you either need to attenuate (using readily available optical atenuation patch cables) or increase launch power (using an optical amplifier or different launching optics).


You are correct that a given module's launch power should not have a 6 dB variability. It should actually vary within a much smaller range, due primarily to temperature of the components. So get some actual measurements of the ones) you install and then adjust accordingly if necessary.

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