[c-nsp] Cheap 10G between 7600 and Procurve 5406zl

[Marian Ďurkovič]

On Sun, 14 Mar 2010, Lincoln Dale wrote:
> SFP+ is one of the newest transceiver formats and has a lot more of the 'stuff' 
> that used to be inside the transceiver on the switch PCB itself.  one of the 
> things that has been moved is a component called the EDC (electronic dispersion 
> compensation).  different transceiver types have different requirements as far 
> as EDC parameters and this has been one case where it shows that not all 
> transceivers are created equal.
> 
> with non-optimal or incorrect EDC values you may still get "link up" but you 
> may have such an excessive error-rate that its practically unusable.  or you 
> may get cases where link comes up but randomly drops out.  or doesn't drop out 
> when the link partner goes away.
> 
> point being here is that while its a commonly held belief that "all 
> transceivers are created equal" we have seen this to not be the case with SFP+ 
> -- probably because its the "newest" transceiver format for 10G.

Well, SFP+ design has violated the long-established practice to have decent
and deterministic host-to-module interface - so it's no surprise that
real-life experience is so bad. Yes, SFP+ works for trivial cases like SR
and LR, but anything more complex is either plain impossible (DWDM) or
requires too much hassle just to get it working. 

A decent pluggable compensates all fiber/coax impairments inside and presents
decent digital signal (zeroes/ones) to the switch. Thus all EDC stuff is the
sole responsibility of transceiver manufacturer, which could fine-tune EDC
characteristics differently for each module type (LRM/DWDM/coax). This way,
indeed "all modules are equal" from the switch point of view, as all of them
produce the same digital signal.

On the other hand, linear SFP+ presents "analog" signal to the switch,
meaning it's no longer zeroes/ones, but the actual signal coming from the
fiber/coax. This signal is further distorted by the PCB traces in the switch
and each module type might require different EDC characteristics. So what
was before the job of transceiver manufacturer, must now be compensated
and fine-tuned in the switch, which is basically a nightmare (ready to
upgrade IOS to get rid of bit errors on some specific module type?)

Thus, the massive rush towards SFP+ might at the end of the day turn out 
to be a serious flaw, since the interop issues, lack of long reach / DWDM
versions and vendor-locking games might turn it into a dead end in
comparision with XFP, 10GBase-T and possible upcoming technologies.