[j-nsp] optical transceivers recommended "Rx power" levels and signal measurement intervals

[Martin T]

Hi,

> I've found them reliable. But it's very coarse metric, you know you're
> getting enough light but it does not say anything about actually your
> ability to find the symbols in the light.

yes, one can for example connect 10GBASE-LR X2 transceiver with
1000BASE-LX10 SFP and SFP receives -3dBm of optical power while they
have different PHY and such link would never come up. Probably it just
reports that photodiode receives light, but as you said, it doesn't
care if the photon pulses make sense even to PHY protocols.


regards,
Martin


2013/6/16, Saku Ytti <saku at ytti.fi>:
> On (2013-06-16 20:40 +0300), Martin T wrote:
>
> Hi Martin,
>
>> 1) As I understand, XFP vendors store the recommended "Rx power" levels
>> in
>> transceiver non-volatile memory during the manufacturing process and
>> those
>> are read by switch/router over I2C bus. Are those values usually
>> reliable?
>
> I've found them reliable. But it's very coarse metric, you know you're
> getting enough light but it does not say anything about actually your
> ability to find the symbols in the light.
> It's like measuring quality of music by dB, hmmm, that's bad analogy,
> that's what we actually do.
>
>> 2) At least in case of DPC-R-40GE-SFP line-card the "show interfaces
>> diagnostics optics" displays Rx average optical power for SFP's. What is
>> the measurement interval? I mean is it 10s average? 60s average?
>
> The SFP specification which document DDM is publicly and freely
> downloadable, I didn't verify, but I cannot recall it making any
> requirements on poll rates.
> There are probably quite few things which affect this, the HW in the optic,
> the microcontroller reading the HW and populating the EPROM, frequency of
> your vendor linecard reading the data off the I2C.
>
> So my guess is, answer is 'it depends'.
>
> --
>   ++ytti
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