Link Loss budget - Max SFP distance? Max TX vs Min TX?

[james]

People say you should use the MinTX when calculating loss budget, if so, what is the Max TX used for?

The QSFP-40G-ER4 has a MaxTX of 4.5, but a MinTX of -2.7. 

Can I use the MaxTX of 4.5 & MaxRX of -21.2 to create a loss budget of -25.7? or is this breaking the rules?

[Tom Randstrom]

When designing an optical link, you want to make sure that it is operational across the entire operating characteristics of the specified transceiver; which allows you to swap a failed module (or similar type) and know that the link will be operational.  If you were to use the Max-TX in your initial calculations, and a replacement transceiver was operating at closer to the Min-Tx specification, your Min-Rx value would fall outside the operating range; resulting in possible bit errors and outage.  Also, transceiver output powers can vary during the lifetime of the product.