[191317] in North American Network Operators' Group
Re: Optical transceiver question
daemon@ATHENA.MIT.EDU (Olivier Benghozi)
Wed Sep 7 17:51:34 2016
X-Original-To: nanog@nanog.org
From: Olivier Benghozi <olivier.benghozi@wifirst.fr>
Date: Wed, 7 Sep 2016 23:49:28 +0200
To: nanog@nanog.org
In-Reply-To: <001001d20945$b2dc9680$1895c380$@iname.com>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
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-son=
et-multiservice-provisioning-platform-mspp/27042-max-att-27042.html =
<http://www.cisco.com/c/en/us/support/docs/optical-networking/ons-15454-so=
net-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 =C3=A0 22:23, Frank Bulk <frnkblk@iname.com> a =C3=A9cri=
t :
>=20
> We recently purchased some generic optics from a reputable reseller =
that
> were marketed to reach 60 km.
>=20
> 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.=20
>=20
> 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.
>=20
> Is it an industry practice to market distance based on the hot optics, =
not
> on the worst case, which is minimum TX power?
>=20
> Frank
>=20
