The Winning that is Digital Optical Monitoring (DOM)

A number of Cisco Optical Interfaces implement Digital Optical Monitoring (DOM) which enables the monitoring of some interesting status values on the interface with the most useful values being the optical receive and transmit powers.

By being able to monitor transmit and receive power levels of optical interfaces you are able to characterise the fibre loss and isolate any unidirectional connectivity issues.

The following link is useful for identifying which optical interfaces support DOM – http://www.cisco.com/en/US/docs/interfaces_modules/transceiver_modules/compatibility/matrix/OL_8031.html

If your IOS Version, Platform and Interface support DOM you are able to look up the current status with the following command:

Router# show interface (IF) transceiver

An example of the command output,

R1#sh int gig 0/0/0 transceiver
Transceiver is externally calibrated.

Transceiver monitoring is disabled for all interfaces.

++ : 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  Current   Tx Power  Rx Power
Port        (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
----------  -----------  -------  --------  --------  --------
Gi0/0/0       44.9       3.29     1605.4      -4.9      -9.7

If you had two DOM capable SFPs on either end of the fibre you would be able to compare the Tx Power to the Rx Power to determine the optical loss of the fibre medium.

The Power is measured in dBm which are Decibels Milliwatt which means the decibel (dB) value is with reference to 1 milliwatt (1 mW is a thousandth of a Watt). A decibel measurement reflects a logarithmic measure of power, simply  3dB represents double the power and conversely -3dB is half of the power.  More about dBs can be found at  http://en.wikipedia.org/wiki/Decibel

To understand the loss of the fibre system;

If the Tx Power is -6 dBm and the Rx Power of the receiving SFPs Rx Power is -9 dB that would mean that the loss on the fibre is 3 dB, this means that half of the optical signal is lost in transmission. It is important to know the Transmit Power Limits and the Receive Power Limits, this will determine the maximum loss of the fibre system which is generally the length of the fibre path. This information can be found at

http://www.cisco.com/en/US/prod/collateral/modules/ps5455/ps6577/product_data_sheet0900aecd8033f885.html

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