I know that before you use the fiber optic cable you should calculate the power budget and make sure that the total power should be below the Transceiver power budget margin to be sure that the system will work as it designed. If I'm using a 1G, it will run without any problem smoothly.

1) But my question is, let us say that we are using a 1G GBIC card, and after calculating the power budget for my system it shows that it is above the margin. what will happen ? will it works but with lower speed, or it will not work at all since its beyond the margin and the receiver can't receive it ?

ex: using a 1000BASE-LX cisco card that have a 10.5 dbm power margin and my system is having a 12dB. will it work but with lower speed than 1G ? or it will not work at all ?

2)If it will work with lower speed,then can I use a card and I exceed the budget margin and use it with lower speed, rather than buying a new card ? or it will damage the system ? Because I may don't need that high speed.

Sorry for my lot of question. but I need to understand that power budget better.

Thanks alot

1 Answer 1


If the attenuation/loss of the link is higher than the power budget on the optic an insufficient amount of light will make it to the receiving optic and the link will (probably) not function as desired. You can see a complete lack of link, receive errors, intermittent link or have a link that functions just fine but is very, very close to failing. If the link does work you are susceptible to future failures from anything causes a change to the fiber you are using (even moving the fiber cable can cause an outage with that tight of a link budget).

Basically the power budget is the engineering guideline in which the optics are designed to work. If you exceed the guidelines then you are outside the expected environment and all problems are yours to troubleshoot. You can move up to a higher power optic (EX/ZX/etc.) with a larger link budget, and if necessary use attenuators to make sure you don't exceed the rated receive power and cause saturation/eventually burn out the receive sensor.

If you are using the newer gigabit Cisco optics (with the D suffix) or 10G optics and appropriate code, you should be able to check light levels with the show int gi#/# transceiver detail command.

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