[c-nsp] How to calculate transceiver power of SFP on 7200 series?

[Rob Seastrom]

"Justin M. Streiner" <streiner at cluebyfour.org> writes:

> On Tue, 8 Jul 2014, PlaWanSai RMUTT CPE IX wrote:
>
>>                TGN#sho int g0/1 tra
>>
>>  TX power 3888800 nW / 5 dBm (+/- 3dBm)
>>
>> What is power in dBm?
>
> Assuming nW is nanowatts (1 billionth of a watt):
> 3888800 nW = .0038888 watts
>
> .0038888 watts = approx 5.898 dBm.
>
> If that "/5 dBm" means the result from above should be divided by 5,
> then the actual transmit power would be a bit under +1.2 dBm.

It means that 3888800 nW is approximately 5 dBm; note the tolerance
immediately afterward of +/- 3 dBm.  Clearly the number of significant
digits one would expect from the first measurement is a bit of a fib.

(Nanowatts?  That's right up there with the n00bs who talk about
nanofarads instead of picofarads and fractional microfarads.  Just
because an SI unit exists doesn't mean it's sane to express a value in
terms of it when the industry standard is something markedly
different.  But I digress...)

> Without knowing what type of transceiver you have, I don't know if
> that's in the expected range.

In practical terms, with just about all optics available in the normal
channels of trade, +5 is "a little hot, but might work".  The wisdom
of giving it a shot if you don't have an attenuator handy is dependent
on how hard it will be to add an attenuator in the future.
Basement/office vs. datacenter vs. datacenter-on-another-continent =
entirely different calculus.

At gigabit (the interface at hand), ZX optics want 0 to +5, EX wants
-1 to +3, everything else seems happy with an upper limit of about -3.
My recollection is that 10ge is similar.  Note that these are spec
values; just as I've seen gigabit ethernet on copper working great at
distances of over 500 feet, it's entirely possible that you can "get
away with" +5.

Is the interface throwing errors?  Getting the speeds you expected?

> Note: Google found this for me:
> http://www.rapidtables.com/convert/power/Watt_to_dBm.htm

dBm is decibels (log scale) referenced to 1 milliwatt.  0 dBm is 1
milliwatt.  +3 dB refers to a doubling of power.

Note that the fundamental unit is the "bel", not the "decibel", and
one should be very careful when looking at equations to make sure what
the expected input is.  Yes, I have gotten wildly incorrect results
from RF path loss calculations by overlooking this minor detail.  :)

-r