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New Contributor III

RX & TX power value different.

Hi Guys


Buliding-01|Access Switch --------LIU---------CoreSwitch|Building-02


In above, We have two switches.. One in Building01 and second in Building02 and the switches are connected each other via LIU ports of rack.


So, The ports that are connected to the LIU are getting down intermediately & as checked with TAC, The issue is with the RX and TX power difference. It must be same.


Below is the value of the rx and tx power of the switch ports:

tx-power: -2.054 dBm

rx-power: -15.087 dBm


Is anyone aware why this value is different. We have tested with different SFP modules and fiber cable also. 



Unless whoever the TAC person you talked to is new to this industry, virtually anyone knows those TX and RX power are almost impossible to be the same at least on one side. I think he/she meant to say RX power on one side needs not to be much lower than TX power on the other side.

Wherever the light goes over the connection point between two fibers attenuation happens. So if the TX power is -2.0 dBm on the opposite side the RX power might be -3.2 dBm (higher negative number means weaker).

First you need to check the TX power on the opposite side if it's 2 - 4 dBm. But -15 dBm is closer to almost no light. Since changing the SFP doesn't change much, the problem is either the transmit side or more likely in between.



Contributor III

The transmit power (TX) and receive power (RX) in a fiber optic link are usually different. This is because:

- The TX power is the strength of the signal when it leaves the transmitter (in this case, the SFP module in your switch).
- The RX power is the strength of the signal when it arrives at the receiver (the SFP module in the switch at the other end of the link).

The difference between the TX and RX power can be due to a number of factors, including:

- **Fiber optic cable length**: The longer the fiber optic cable, the more signal loss (attenuation) there will be. This is normal and expected in fiber optic networks.
- **Cable quality**: If the fiber optic cable is damaged or of poor quality, it can cause additional signal loss.
- **Connectors and splices**: Each connector or splice in the fiber optic link can cause some signal loss.
- **Wavelength**: Different wavelengths of light can experience different levels of signal loss in the same fiber optic cable.

In your case, a TX power of -2.054 dBm and RX power of -15.087 dBm is a significant difference, suggesting that there may be substantial signal loss in the link. This could be due to one or more of the factors mentioned above.

There are a few steps you can take to troubleshoot this issue:

1. **Check the fiber optic cable**: Inspect the fiber optic cable for damage and replace it if necessary. Also, ensure that the cable is not bent or coiled too tightly, as this can cause signal loss.
2. **Clean the connectors**: Dust or dirt on the fiber optic connectors can cause signal loss. Use a fiber optic cleaning kit to clean the connectors.
3. **Check the SFP modules**: Ensure that the SFP modules are compatible with your switches and with each other. Also, check that they are properly seated in the switches.
4. **Reduce the number of splices and connectors**: Each splice or connector in the link can cause signal loss. If possible, reduce the number of splices and connectors.
5. **Use an optical time-domain reflectometer (OTDR)**: An OTDR can measure the amount of signal loss at each point along the fiber optic link, helping you to identify where the signal loss is occurring.

If you've tried all of these steps and are still experiencing issues, it may be best to seek professional help. Fiber optic networks can be complex, and a professional network technician or engineer should be able to help you diagnose and fix the problem.

Contributor II

TX is the TX power which will be measured as RX on the distant end of the cable. If you transmit at -2 and the distant end receives -15 you have a lot of loss from patching or coupling.

Your SFP should be fine. Loss normally happens at the connectors. The more fiber patches or inline couplers you have the greater the loss. It could also be faulty or out of tolerance connectors if you terminated your own cables. Also, you need to match the type of fiber optics to the SFP.

The SFP data sheet will tell you if it requires multimode, singlemode, or one of the OM variants. Some SFP can work with multimode fiber and for short distances you will use a mode conditioning cable at each end. Again, this all depends on the specs and datasheet of your SFP.

Also you might be using SFP as a generic term. You have SFP, SFP+, QSFP+, and QSFP28 all which may require a different type of fiber, patch panels, connectors, and couplers.


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