Entry level WDM gear? follow-up

Chuck Anderson cra at WPI.EDU
Sat May 18 03:29:53 UTC 2013


On Fri, May 17, 2013 at 10:18:34PM -0400, Jeff Kell wrote:
> On 5/10/2013 9:56 AM, Jerimiah Cole wrote:
> > On 05/08/2013 09:21 PM, Jeff Kell wrote:
> >> Ciena/Cyan/etc are way over our non-existant budget...  what is the
> >> going recommendation to throw say 4-8 lambdas over a dark pair without
> >> breaking the bank?  :)
> > I've used http://www.omnitron-systems.com/ media converters and found
> > them reliable.  They've got the filters to do an 8 channel system.
> 
> Thanks for this and other responses.  Cumulatively I have some more
> information, but also more questions :)
> 
> We have an existing fiber pair to location "A" where it is
> cross-connected to location "B" and terminated.  It's currently a ~35km
> link running 10G-ER optics (1550nm).  We're getting a little less than
> -7dBm receive over the link now with standard 10G-ER optics.
> 
> We need to connect to another provider at location "A" (also 10G), so
> thinking of xWDM from campus to location "A".  Would like to handoff one
> lambda on to location "B" to maintain that circuit, and the
> new/additional ones would terminate at location "A".

Typically you would use an Optical Add/Drop Multiplexer at each
intermediate site and a regular Optical Mux at the endpoint sites, but
you should be able to simplify this to just two OMUXes, one at "A" and
the other at campus, sending the lambda for "B" through the
cross-connect as long as you don't need more than one lambda at "B".

> CWDM is obviously cheaper and supports the 1550nm current band (but do
> we need to replace existing optics with "tuned" ones to keep things
> honest?).

Should work fine with your existing 1550 ER optics, as long as you
have enough optical budget to account for the additional loss of the
CWDM passives.  You should even be able to use one of the wavelengths
of DWDM C-band optics within the 1550nm 20nm-wide channel of a CWDM
system.  I know somemone who did this to "future-proof" their optics
for an eventual upgrade to a DWDM system.

> Cisco lists no CWDM 10G optics at all in any form factor, only DWDM, and
> they're "really proud" of them based on the list price.

Transition Networks and Integra Networks should both have 10G SFP+ and
XFP optics in CWDM wavelengths.  Integra can also do the CWDM passives
including custom arrangements in various form factors.

> The "tuned" optics have no SR/LR/ER/ZR attributes... so what are their
> real distance characteristics?  In particular, can we cross-connect one
> of the outputs to the existing location "B" and have the dBm budget to
> get there?

Distance specs are always "approximate" or "nominal" with no guarantee
that you will reach that far since it depends on lots of different
factors and in some cases you can even go farther.  You should be able
to tell definitively by the optical specs, specifically output power
in dBm & receiver sensitivity in dBm (subtract the two to get the link
budget in dB) or the optical budget may be given directly in dB or you
may be able to infer by the distance spec (different vendors' 40km,
80km, 120km optics I've seen all have similar optical power
specs/budgets--but these may be different 1gig vs. 10gig so only
compare distances of the same speed optics to infer optical budgets
and keep this in mind when upgrading a link from 1g to 10g).

Once you know the budget for each pair of optics, you need to add up
the loss of all components between the two endpoints of each pair,
using the losses given in the CWDM passives spec sheet for add/drop
loss, pass-thru loss, etc. as well as connector, splice, and distance
losses.  In my experience, so-called 80km 10gig optics were necessary
to go even 2km (two km) in a CWDM system with several add/drops in
between the endpoints (including some leftover budget for expansion to
more add/drops), while so-called 40km 1gig optics were fine under the
same conditions.



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