[c-nsp] Nexus 5000 / Nexus 2000 SFP+ with LRM

[Nick Hilliard]

On 10/05/2010 08:34, Marian Ďurkovič wrote:
> LRM SFP+ is just part of the stuff you need. For LRM to work, the switch
> linecard must have appropriate EDC functionality. If it's not there, it simply
> won't work.

To give some back-ground on this, LRM is long-reach multimode.  As it's
multimode, modal dispersion comes into play pretty quickly, and even over
relatively short distances, it causes severe signal distortion - this is
one of the primary distance limiting factors of multimode.

On xenpaks, x2 and xfp, the dispersion compensation is performed on the
transceiver (by the EDC), and you end up with a fully digital signal being
transmitted from the transceiver's electrical interface to the line card.
However as the SFP+ form factor is really tiny, there isn't enough room to
house various components such as an EDC or a CDR (clock / data recovery).
For SFP+, these components are housed on the line card, if at all, and in
many cases the line card simply won't have EDC.  Perhaps the n5k main board
doesn't have EDC processors, which would make it unsuitable for LRM.

> (One more "thanks" to all people who thought that analog interface between SFP+
> and linecard is a good idea...)

Fibre and transceiver deployments are all about choosing the appropriate
technology.  If you need to run fibre over longer distances, doing this
over MMF probably isn't the best idea.  I appreciate that lots of
organisation have cartloads of legacy 62.5µ MMF and that they tend to be
unhappy about the prospect of changing longer runs to use SMF, but 62.5µ
wasn't designed for longer runs at very high speeds.

In some senses, you might as well complain that SFP+ isn't physically large
enough to house enough lasers for LX4.  10G standards like LX4 and LRM were
only created to try to deal with legacy plant deployments which weren't
really designed for anything more than 100M-FX.  Anyone sensible MMF
deployment done over the past couple of years will have been OM3, where you
can use SR transceivers instead of LRM or LX4.

If you need distances longer than 200m, LR + SMF is a better choice of
technology to use.

Nick