[c-nsp] Interpreting DOM outputs

John Brown john at citylinkfiber.com
Sat Dec 31 17:56:53 EST 2011

Hi Rob,

I would also take real power meter readings from each transmitter, at the
Compare that to what the DOM is saying to see how well calibrated your DOM

Also, if you know the TX at the far side and now the RX at the near side
you can approx your loss across the link.

Recently I did a circuit turn up at a west coast carrier/colo and had 9dBm
of loss between the 17th floor and the 3rd floor.
WAY OUTSIDE of what it should have been.  After video scoping (never use a
mechanical scope) the connectors, we found lots of dirty connectors.
Despite people "cleaning them".

As we push more data, more waves, etc down these thin strands of glass it
will be critically important to have the right optical test get to QUICKLY
troubleshoot issues.

We graph our DOM data.

On 12/31/11 11:18 AM, "John Gill" <johgill at cisco.com> wrote:

>Hi Rob,
>-4.9 dBm is indeed stronger than -6.9dBm.
>For example:
>-4.9 dBm is  .32mW
>-6.9 dBm is  .20mW
>These are negative values since 0dBm is the reference point of 1mW and
>you are looking at values below this.
>A greater value indicates higher power.  So, you can say this is usually
>better, except in the extreme cases where you are flooding the receive
>path with too much light, maybe connecting different transceivers over
>short distances for example (LX and SX over 1m).
>Anton made a good point - there can be some variance in these
>measurements from component to component so don't use them as absolute
>fact, but rather look at the data over time which can indicate a fiber
>or even a transceiver problem.
>John Gill
>On 12/31/11 12:10 PM, Robert Hass wrote:
>> On Sat, Dec 31, 2011 at 6:02 PM, Anton Kapela<tkapela at gmail.com>  wrote:
>>> That is, these measurements are best-used as a referential figure, not
>>> absolute -- meaning you ought to start polling&  storing them now for
>>> the most utility to be found in troubleshooting later. ;)
>> Thanks for explanation.
>> But I'm still unsure regarding my questions of understanding:
>> Tx Power '-4.9' better/stronger than '-6.9'
>> Rx Power '-9.6' is better/stronger than '-11.2'
>> My above understanding is correct or incorrect ?
>> Thanks,
>> Rob
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