Understanding TX RX light level

[1230639am]

Hi,

I hope someone could please help clarify TX and RX light level.

This is the information i got from the CLI of cisco router:

                                         Optical   Optical
         Temperature  Voltage  Current   Tx Power  Rx Power
Port     (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
-------  -----------  -------  --------  --------  --------
Te4/1      26.9       0.00      29.7 --   -0.6      -2.0

Now I don’t understand the TX Power being -0.6 and RX Power being -2.0 is that indicating there is an issue with the fiber cable?

From what i have understood if an interface is shutdown then the TX Power level is -40.00dBm, which means the port is not sending any single.

If you could please explain the TX RX level that will be most appreciated.

Thanks in advance.

[PKH1994]

Hello,

Do i need to take action upon following output of the reading ? Thank you

[Tom Randstrom]

Just based on the threshold data provide, the values are within operating range. This is only one end of the link, is some type of performance error being reported? What transceiver is being used? 

[davidjbradley]

Why do we expect the Tx to be a negative value?

[Tom Randstrom]

Not all optical transceivers have transmitters with negative output power levels. Long reach optics will have positive output power levels. 

[Marvin Rhoads]

@davidjbradley, in addition to what @Tom Randstrom correctly noted, we generally expect short range optical Tx levels to be between -3.0 and -9.0 dBm because that is the nominal operating range (i.e., between the high and low power warning levels) as specified and shown in the sample output below.

[Tom Randstrom · Accepted solution]

As a follow-on to Marvin's comments.  

To determine if an optical transceiver (transmitter and receiver pair) is operating at the appropriate signal levels, the data sheets for the appropriate transceiver, typically posted by link speed, should be referenced.  These documents provide critical information such as link reach (distance), fiber type (singlemode or multimode), transmitter output power range and optical receive power range.

When you are reading the CLI output for a transceiver, the Optical Tx Power is the signal level leaving that device, and it should fall within the transmitter output power range shown in the data sheet. The optical receive power is the incoming signal level being received from the far end device, and should fall within the data sheets specified optical receive power range.

In most cases, the actual value shown in the output of the CLI is "generally" not as important as that it falls within the transceiver's Tx and Rx operating ranges shown in the data sheet.

Can be a confusing subject.  To get some general fiber optic information, you may want to reference www.thefoa.org.  Good overviews.

Good luck!         

[1230639am]

Thank you for providing a clear explanation.

[jigojar]

Hi Genius,

 Can anyone pls help me why the optical transmit power is varying? why optics cant transmit fixed power? 

the transmit power is varying port by port even same optics and same hardware, so i like to understand why this variation? 

[jigojar]

Hi Genius,

 Can anyone pls help me why the optical transmit power is varying? why optics cant transmit fixed power? 

the transmit power is varying port by port even same optics and same hardware, so i like to understand why this variation? 

[Tom Randstrom]

What type of optical transceiver are you using (model number)?  Are you indicating that on an individual fiber link the optical power is fluctuating? If so, how much is it changing over what period of time?

The optical transceivers, as seen in the transceiver data sheet, has a transmit operating range.  Each transceiver manufactured will have some power output variance that will fall within the range. Once turned up, the laser's output is fairly stable, potentially degrading slightly over the life of the component.      

[Marvin Rhoads]

There's variation due to the lack of need to manufacture most commercial optics to a more precise transmit power level. As long as they fall within the acceptable range, the manufacturers don't need to increase their cost to make them more precise. Some very high end optics such as those used in DWDM networks will have more precise power levels - and cost a lot more.