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. _______________________________________________ cisco-nsp mailing list cisco-nsp@puck.nether.net https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/