I'm currently looking at 40Gbase-LR and am curious about the advertised power budget. Here are the specifications:

Transmit max    2.30dBm 
Transmit min   -7.00dBm 
Receive max     2.30dBm 
Receive min   -11.50dBm 
Power Budget    4.50dB 

The attenuation test on the cable I want to use indicates a ~6dB drop, mostly due to insertion loss. The indicated power budget is not enough to manage that. This appears to be calculated based on Transmit min and Receive Min (-7 - -11.5=4.5), but I'm wondering if I have more flex here since the maximum transmit power is 2.3dBm (2.3 - -11.5 = 13.8).

What I don't get is why the power budget is listed as 4.5dB when the specs appear to indicate it can go to 11.8dB.

  • 1
    You may generally guess that the intended transmit power is somewhat below the guaranteed maximum and above the guaranteed minimum, likely trending a bit above half-way between. But for system design purposes, one has to look at the worst-case, and that is only what is guaranteed, not what you might get. Might want to clean and inspect the connections on that cable really well... – Ecnerwal Feb 8 '16 at 17:11

4.5 dB is the maximum loss where it's still guaranteed to work. Of course, it will likely be better, but as they say, your mileage may vary.

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