Signal quality and link stability

I really think I should stop with fillword discussions but here is one more. What happens even if you have set the correct fillword, have made sure all hardware is in tip-top shape and still the encoding errors fly around like a swarm of hornets. Then the problem of ISI might be more problematic.

The main issue still is that the receiving side is unable to distinguish between a 0 and 1. The so called eye-pattern is too narrow or too distorted in such a way the receiver is just seeing gibberish.

Inter Symbol Interference

ISI stands for Inter-Symbol-Interference. In the IT world were more or less used to the fact that everything is digital but when you look on the hardware side on this spectrum it is all still analogue. This means that electrical signals travel through circuits just like they did 100 years ago when AG Bell figured out how to use this on a normal telephone. (Yes young readers, we had analogue non-smart phones back in the days) Since electrical signals have the tendency to influence each-other they may distort other signals or have a negative effect on subsequent signals on the same cable/circuit.

I mentioned before that adjusting pre-emphasis levels may help however this is not always the case. Another technique is DFE or Decision Feedback Equalization. This nifty method is build along the way of adjusting the regular voltage levels in a positive or negative manner depending on the input. This is then fed back into an equaliser to adjust the signal levels after which it can be sent on.

Now, I'm not nearly smart enough to explain this in detail but my friends at Agilent do know how this works and provide some very good videos.

The firs one describes what ISI is and how DFE can help in mitigating the problem.

Decision Feedback Equalisation

The second one goes into detail with a tutorial on how to calculate the correct amount of adjustment of DFE to obtain the proper signal levels. it also shows you the limitations.

DFE Tutorial

In FOS 7.1.2 there is a new command called "portCfgNonDfe". This is only active on the Condor 3 chipsets who connect on 8Gb.

As shown this command normally should only be used where attached devices are unable to use the proper fillword. As you may have seen the condor 3 platforms have abolished the "portcfgfillword" command and therefore rely on DFE to actively adjust signal levels.

New command in FOS 7.1.2

Support disabling the Decision Feedback Equalization (DFE) mode on selected ports for 8Gb FC ports on Gen5 platforms.
1. Supported through the new CLI command portCfgNonDfe only.
2. Disabling DFE mode is only required to support attachment of select LTO tape devices to Brocade Gen 5 platforms when the tape devices do not comply with FC standards for fill word behaviour and  the fill word behaviour cannot be changed on the tape device.

So if yo have a device which is unable to connect on 8G to a Condor 3 port (all 16G platforms) and encoding errors keep ramping up you might want to give this command a try.

Regards,

Erwin van Londen

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About Erwin van Londen

Master Technical Analyst at Hitachi Data Systems
Brocade, Config Guide, Fibre Channel, Storage Networking, Troubleshooting , , , , ,