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Unusually low Rx power values ? WS-X6704-10GE (10Gbase-SR)


The Optical Transmit and Receive Specifications for this transceiver (XENPAK-10GB-SR) indicate :
Tx Power (dBm) : Max -1.2, Min -7.3
Rx Power (dBm) : Max -1.0, Min -9.9

We are seeing readings on one of these modules that are clearly well outside these ranges.
Tx Power (dBm) : -18.6
Rx Power (dBm) : -40.0

No interface errors or anomolous behaviour of any kind is indicated on this interface at this time.

My question.
Without getting into the detail of what could be the potential cause of this particlar interface showing these readings (as all of the other interfaces are reading within the specified range), could I get an opinion as to what impacts, if any, we can expect to see on this interface as a result of these observed values ?

Thank you for your inputs.

Everyone's tags (2)
Cisco Employee

Re: Unusually low Rx power values ? WS-X6704-10GE (10Gbase-SR)

Tx power is measured from this XENPAK and must be inside the specified range. -18dBm is around 32 times less than expected. I would consider this XENPAK as broken and replace it as such strong power difference can cause errors on a receive side.

Rx power range is what is expected from the remote end. Measuring Rx power makes sense when there is something transmitting light to us and we want to confirm an incoming power does not exceed our Rx power range.


Re: Unusually low Rx power values ? WS-X6704-10GE (10Gbase-SR)

I would suspect that the transceiver you displayed the readings for is improperly calibrated.  If you actually had a reading of -40 dbm on the receiver, the link would not be functioning. Is this just a lab test you are doing, looping back the fiber?  If not, what does the transceiver read at the far end of the link? This should tell us if this transceiver is really operating at these levels.

If a fiber link is operated just below its Min. Rx operating range [in this case: Rx Power (dBm) Min -9.9)], then you may start to see errors on the link, which can cause FCS/CRC errors and upper layer errors (TCP retransmits).  As the Rx power drops further, errors get worse until the receiver can no longer distiguish the digits from the noise and you loose your link.

If the fiber link is operated above its Max. Rx operating range [Rx Power (dBm) : Max -1.0], similar results may occur; such as increased bit errors and loss of the ability for the receiver to decode the bits.  If you operated above the Receiver's Threshhold, you can damage the receiver so that it will no longer function.


Community Member

Re: Unusually low Rx power values ? WS-X6704-10GE (10Gbase-SR)

Tom, it would indeed appear that improper calibration is the most likely answer here.

Although the readings are from production interfaces, the other end of the interface in question is showing normal values, i.e. Tx of -2.0 and Rx of -2.5

As you have indicated, if the values I am seeing were valid, some serious errors would be expected.

Both ends of the link are completely error free, with a continuous rate of approx 300 to 500 Mbit/sec across it.

Thank you both for the feedback you have provided to this question.

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