SFP Receive Power Range

[neiren]

Hello

Hope everything is working fine with you.

Could you please do me a favor on below questions?

We shut down the local SFP, and monitor the received power on opposite SFP. The received power is always -40dBM when we using below modules. I think its the value when the port is shutdown.

GLC-LH-SMD

SFP-10G-LR

SFP-25G-SR-S

QSFP-100G-LR4-S

But when we try on GLC-SX-MMD, the received power is -25.2dBM.

Could you please tell me why there is such difference? -25.2 is specific to GLC-SX-MMD when it is down?

The value of Rx dBm when the SFP is shutdown,（-25dBM on GLC-SX-MMD, and -40dBM on other four）is the minimum dBM that the SFP can received? Or it’s just a fake value we use when the connection is down on each SFP.

Attach the log for your reference.

Thanks in advance.

Br/Neil

[Marvin Rhoads · Accepted solution]

The longer reach and/or single mode optics will have a lower "noise floor". That's what we call the level below which no usable signal can be extracted. That's one reason why they cost more - more sensitive receivers that are able to work over a longer distance are a higher precision component.

The "entry level" GLC-SX-MMD won't distinguish any signal below approximately -25 dBm; so even a receive link with no light on it will register as having that level.

[neiren]

Thanks Marvin,

So can I think when GLC-SX-MMD is shutdown, the Rx dBm will shown as -25.2?