Fibre Channel (SAN)

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Occasional Contributor
Posts: 11
Registered: ‎04-21-2010

What are acceptable Power Levels when looking at a sfpShow output?

The sfpShow command will provide power levels for both TX and RX on a particular port.  What are typical acceptable level ranges for them?  As sfp's and cables age these values change and weak signals may start to cause various error recovery and have been known to cause errors.  I am just trying to understand the normal limits. Thanks

Valued Contributor
Posts: 551
Registered: ‎03-20-2011

Re: What are acceptable Power Levels when looking at a sfpShow output?

you can try looking for the specs published by sfp vendor.
i also believe that sfpshow should give you the acceptable low watermarks for each value.
my own feeling (for the regular 8gb ports) is that anything higher than -3 is perfect, -3 to -4 is ok, and lower than -4 is a good reason to check/clean/re-seat the cable and/or the sfp.
i might be wrong, hope that others can correct me.
Valued Contributor
Posts: 761
Registered: ‎06-11-2010

Re: What are acceptable Power Levels when looking at a sfpShow output?

Hi there,

 

In my experience I consider that a received signal below 100 uW (-10 dBm) is problematic. A higher Rx signal usually does not generate errors, but it is also true that as SFP speeds increase this threshold of mine gotta raise, maybe to 200uW (-7 dBm) in the case of 8G/16G links.

 

Rgds,

Feilpon

Valued Contributor
Posts: 551
Registered: ‎03-20-2011

Re: What are acceptable Power Levels when looking at a sfpShow output?

so it seems i'm a perfectionist :)
Contributor
Posts: 26
Registered: ‎09-11-2012

Re: What are acceptable Power Levels when looking at a sfpShow output?

TX Power is dependant upon type. A Short Wave will typically transmit in the ~300-400 mWatt range. A Long Wave will be much higher.

 

RX Power is dependant upon distance. A 30 meter cable should be fairly close to the TX Power. I don't know the formula for drop, but a much longer distance cable will have it. (Best way to see this is to look at a long distance SFP. ~1200 TX Power and ~250 RX Power. Seems off, but distance does affect this)

Contributor
Posts: 26
Registered: ‎09-11-2012

Re: What are acceptable Power Levels when looking at a sfpShow output?

(mWatt = microWatt) Can't do the odd shaped u here! lol

Frequent Contributor
Posts: 130
Registered: ‎02-05-2014

Re: What are acceptable Power Levels when looking at a sfpShow output?

I wrote a short article on this over here > http://erwinvanlonden.net/2012/03/the-importance-of-clean-fibre-optics/

 

There is a huge difference between te SFP's type especially when you start comparing long-wave ones.

 

Lookk at the SFP specs over here : http://www.brocade.com/products/all/transceivers/product-details/transceiver-modules/specifications.page

 

 

Kind regards,
Erwin van Londen
Brocade Distinguished Architect
http://www.erwinvanlonden.net The Fibre Channel blog


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