Optical transceiver question

Olivier Benghozi olivier.benghozi at wifirst.fr
Wed Sep 7 21:49:28 UTC 2016

It's a bit like car fuel efficiency values, even with reputable brands :)

In this industry, the number of kms for such optics is a best case approximation of the combination of (most notably) those elements:
worst case power budget, capability to deal with chromatic scattering on this length without compensation, with perfect fiber attenuation values without connectors/splicings (by the way, per km attenuation depends of the type of fiber ; so it's really possible to have less than 0.4dB/km even near 1310nm).

I confirm it's normal for several optics of the same model to have a large panel of Tx values as soon as they are within the guaranteed Tx range.
The actual value of Tx is only guaranteed to be somewhere within the Tx range shown in the specs (and it will vary/lower during the life of the optic). So the power budget of an optic is the value in the worst case.

If you cannot OTDR the link (or just put a known source at one side and a power meter at the other side), then you have to estimate: take the length and the attenuation per km for the wavelength, add attenuation values for connectors (crossconnects, patches) and current splicings and so on, take a margin (at least 3dB let's say ?) for later splicings or small defects, and then you obtain the minimum power budget.

10 years later, this page is still relevant:
http://www.cisco.com/c/en/us/support/docs/optical-networking/ons-15454-sonet-multiservice-provisioning-platform-mspp/27042-max-att-27042.html <http://www.cisco.com/c/en/us/support/docs/optical-networking/ons-15454-sonet-multiservice-provisioning-platform-mspp/27042-max-att-27042.html>

Anyway, you must not use the km value for anything else than sorting the answers before looking at the specs :)

> Le 7 sept. 2016 à 22:23, Frank Bulk <frnkblk at iname.com> a écrit :
> We recently purchased some generic optics from a reputable reseller that
> were marketed to reach 60 km.
> But what we found, based on the spec sheets, is that it could only reach
> that distance if the optics were transmitting on the high side of the
> transmit power range. 
> For example, if the TX range was 0 to +5 dBm and minimum RX power was -20
> dB, the designed optical budget should be no more than 20 dB (0 - -20).
> Based on the wavelength the appropriate loss would be 0.4 dB/km and results
> in only 50 km, not 60 km.  To get 60 km it would need 24 dB of link margin,
> and that would only be attainable if it was transmitting on the high side,
> at +4 dBm.
> Is it an industry practice to market distance based on the hot optics, not
> on the worst case, which is minimum TX power?
> Frank

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