Fibre Channel (SAN)

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Rav
New Member
Posts: 1
Registered: ‎05-16-2011

SFPSHOW and linkbudget

Hi all

is it possible to have the linkbudget(loss) with the sfpshow command ?

Super Contributor
Posts: 635
Registered: ‎04-12-2010

Re: SFPSHOW and linkbudget

Hello Rav,

you need to know the senders transmitter power to calculate the link loss. The sfpshow can not display this information.

The SFP is only aware about its own values but not of these of its counterpart.

I hope this helps,

Andreas

Frequent Contributor
Posts: 76
Registered: ‎04-17-2010

Re: SFPSHOW and linkbudget

That's actually incorrect, you can very well calculate the loss of a link using `sfpShow` and by looking at the Rx/Tx power values of two adjacent links.

In newer FabOS releases the dBm is already calculated for you and printed out right next to the Rx/Tx power, like so :

admin> sfpshow 4
Identifier:  3    SFP
Connector:   7    LC
Transceiver: 1501101002000000 100,200,400_MB/s SM lw Long_dist
Encoding:    1    8B10B
Baud Rate:   43   (units 100 megabaud)
Length 9u:   40   (units km)
(...)
Temperature: 39 Centigrade
Current:     25.472 mAmps
Voltage:     3300.0 mVolts
RX  Power:    -1.6  dBm (699.0 uWatts)     <<<<

TX  Power:    2.2  dBm (1643.2 uWatts)     <<<<

If you're running an earlier FabOS release, you can still calculate this using the following formula :

10 * log (uW / 1000)

The difference between Rx and Tx yields the cable attenuation of the link.

Frequent Contributor
Posts: 76
Registered: ‎04-17-2010

Re: SFPSHOW and linkbudget

to be more precise: the difference between switchA Rx and switchB Tx gives the attenuation in one direction and the other way round.

Super Contributor
Posts: 635
Registered: ‎04-12-2010

Re: SFPSHOW and linkbudget

Christophe,

good too see that you correct your own mistake.

The SFPSHOW command only displays the TX and RX power of the SFP! This give no indication of the linkbudget.

Andreas

Frequent Contributor
Posts: 76
Registered: ‎04-17-2010

Re: SFPSHOW and linkbudget

I fail to see my "own mistake" here, but I'm open for suggestions

The loss budget is defined (for instance in the FC-PI specification) and depends on many factors, like distance, link speed, quality of the fiber. Obviously other factors like amount of patch panels (i.e. each connector / splice adds insertion loss) will also contribute to loss budget. The actual attenuation is one of the variables you need though and the aforementioned formulas are easy ways to do so (rather than having to measure the optical return loss).

Also, for each SFP type it is defined what a critical low (or high if using LWL modules on short distance or repeaters) power values are, i.e. what is still to be considered a valid signal and what not.

Obviously things like cable bending, dirty cable ends, badly aligned splicing etc. can also add attenuation.

When using active or passive equipment on the ISL (think multiplexers for example), the values shown in the `sfpShow` output are obviously useless, since they don't reflect the end-to-end loss of the link.

Super Contributor
Posts: 635
Registered: ‎04-12-2010

Re: SFPSHOW and linkbudget

Please see the original question:

Is it possible to have the linkbudget (loos) with the sfpshow command?

And I wrote that the sfpshow command does only know its own values and you need the counter part vaules as well. This is so far correct to calculate the linkbudge.

Your snipset point to RX and TX vaules.

But this information is from a single SFP and this is not enough to calculate the linkbudget.

Andreas

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