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When looking at the output of show interfaces transceiver on a Cat 4500X you notice that there is a Tx and a Rx power measurement. My questions about this output are:

  1. Why is each transceiver transmitting at a different power level?
  2. How is the transmission power chosen? I always thought there was some standard defining the transmission power.
  3. When subtracting the RX on one end from the TX of the transceiver on the other end, do I get the actual loss on the fiber? Is this an accurate measurement?

    Switch#show interfaces 
    
    If device is externally calibrated, only calibrated values are printed.
    ++ : high alarm, +  : high warning, -  : low warning, -- : low alarm.
    NA or N/A: not applicable, Tx: transmit, Rx: receive.
    mA: milliamperes, dBm: decibels (milliwatts).
    
                                     Optical   Optical
               Temperature  Voltage  Tx Power  Rx Power
    Port       (Celsius)    (Volts)  (dBm)     (dBm)
    ---------  -----------  -------  --------  --------
    Te1/1/3      27.9       3.19      -2.3      -4.7
    Te1/1/4      30.2       3.32      -1.9      -4.3
    Te2/1/3      32.5       3.30      -1.0      -4.1      
    Te2/1/4      32.1       3.24      -2.4      -5.6
    Te2/1/17     33.4       3.32      -5.4      -8.5
    Te2/1/18     33.3       3.32      -2.4      -2.4
    Te2/1/19     32.4       3.34      -2.1      -1.6
    Te2/1/20     33.2       3.31      -2.2      -3.3
    Te2/1/21     31.5       3.32      -2.2      -4.6
    Te2/1/22     34.2       3.32      -1.9     -11.0
    Te2/1/23     33.3       3.31      -1.9      -2.0
    Te2/1/24     30.5       3.31      -2.3      -2.5
    Te2/1/25     27.3       3.32      -2.4      -6.6
    
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  1. Differences in manufacturing and different types of optics ( SR,LR, ZR.. Brands.. )
  2. It is dependent on the specific optic ( varies per each unique optic ), it is specified for the type of transceiver by the manufacturer in the datasheet. It is not a dynamic value.
  3. This is usually good enough for rough measurements but can not be considered to be as good as a proper attenuation measurement done with calibrated instrument. This measurement is also only at one wavelenght for obvious reasons. I have used optical values from 'show int tran' many times for finding out headroom before CWDM/DWDM filter inserts etc and it has been good enough for that use.
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  • When looking at such data sheets on cisco.com/c/en/us/products/collateral/interfaces-modules/… I notice ranges up to 8 dBm for some transceivers. I cannot really believe they sell those SFPs with such an incredibly large tolerance. See GLC-BX40-D-I. Are you sure the transmission power is a fixed unit dependant property? – bot47 Mar 6 '17 at 5:44
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TX and RX values will always vary slightly, manufacturers have a range they need to stay within, but there are always differences.

This will help you to find out how far optics will actually reach. Keep in mind that the examples here are under perfect conditions.

The first thing we need to know is what wavelength are we working with? For this example, we will use our SFP-10G-ZR which runs at 1550nm. Next up we make a note of the optics minimum output power. In this case, it would be 0dBm. We can find this value from the data sheets on the manufacturer's website. Now we need to know the receiver sensitivity. For SFP-10G-ZR this value is usually -24dBm. The last thing we need to remember is that every connection has a typical loss of 0.75dB.

First up we find out how much power we have to work with:

Minimum Tx Power 0dBm
Receiver Sensitivity -24dBm
Available Power =24dB

Connection loss - 0.75dB

Available Power = 23.25dB

Now we take the available power and divide that by the loss per km.
Windows / Wavelength / Loss
1st wavelength / 850nm / 3dB/km
2nd wavelength / 1310nm / 0.4dB/km
3rd wavelength / 1550nm / 0.25dB/km
4th wavelength / 1625nm / 0.25dB/km

23.25dB / 0.25dB = 93km

So from this, we see that under perfect conditions SFP-10G-ZR should be able to reach 93km. Most manufacturers subtract 3dB from the available power as a safety margin which is why this part is listed at 80km.

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