Determining Real-time Optical Power Loss using "show interface transceiver" command

[ramal]

Reading through the various topics here, I haven't seen this question directly addressed, so here goes.

For the sake of discussion, I have two Cisco switches, Switch1 and Switch2.  I  run the "show interface transceiver" command at both and get the following:

Switch1 Port	Optical Tx Pwr (dBm)	Optical Rx Pwr (dBm)	Switch2 Port	Optical Tx Pwr (dBm)	Optical Rx Pwr (dBm)
Te1/1/9	-1.3	-2.1	Te1/0/1	-1.9	-1.5

In this example, Switch1's Te1/1/9 is connected to Switch2's Te1/0/1.

Assuming the measured dBm values provided by each switch's SFP are accurate, can you calculate the real-time loss for the fiber link as follows:

Switch1->Switch2 Loss (dB) = Switch1 TxPwr - Switch2 RxPwr

   and

Switch2->Switch1 Loss (dB) = Switch2 TxPwr - Switch1 RxPwr

Of course, this results in a loss of < 0 for Switch1->Switch2.

So ... am I comparing apples to aardvarks here?

v/r

[Joseph W. Doherty]

Correct, i.e.: Loss (dB) = TxPwr dBM - Switch1 RxPwr dBM

"Of course, this results in a loss of < 0 for Switch1->Switch2."

Eh?
Sw1>Sw2: -1.3dBm - -1.5dBm = -.2dB
Sw2>Sw1: -1.9dBm - -2.1dBm = -.2dB
NB: actual subtractions' value is .2, but loss is always negative
Same loss in both directions - what one might expect, more or less.

[ramal]

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@Joseph W. Doherty wrote:
Correct, i.e.: Loss (dB) = TxPwr dBM - Switch1 RxPwr dBM

"Of course, this results in a loss of < 0 for Switch1->Switch2."

Eh?
Sw1>Sw2: -1.3dBm - -1.5dBm = -.2dB
Sw2>Sw1: -1.9dBm - -2.1dBm = -.2dB
NB: actual subtractions' value is .2, but loss is always negative
Same loss in both directions - what one might expect, more or less.

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Sorry. I didn't have enough coffee when I wrote that. I have no idea what I was thinking at the time.
Anyway, I wanted to make sure that you could make reasonably accurate loss measurements (sans using an actual calibrated meter) using what the IOS provided.
Thanks!