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Re: PAM-5, what are your BERs ?


	According to the ITU standards on FEC,  BCH-3 requires 0.9% overhead
	and turns a 10^-7 BER into 10^-17 BER, as Sean described.   A much more
	powerful (255,239) Reed-Solomon code requires 6.8% overhead,
	and turns a 10^-5 BER into 10^-24 BER (one error every 3 million years).
	Further, the Reed-Solomon code can tolerate 1024 bursty bit errors in
	a row!   The FEC capability and coding gain is well over the rate
	overhead penalty.  The implementation complexity is also modest
	(about 100K gates, 700mW in 2.5V CMOS).  The concerns are usually the
	latency, the rate conversion and learning curve.  IMHO, the real 	
	question for the committee is  whether we can do without FEC 
	in a cost effective way for distance objectives we are interested in.
Mark Yu						phone: 732-949-2185
Bell Laboratories, Rm. 4E504			fax:   732-949-9118
101 Crawfords Corner Rd.			yu@xxxxxxxxxxxxxxxxxxxxxx
Holmdel, NJ 07733, USA

> Hi Sean,
> unfortuantely, FEC has additional drawbacks beyond just latency.  the
> primary concern here would be that you need to ratchet-up the signaling 
> rate even faster to include the overhead of sending the FEC bits.  
> Depending on the amount of redundancy in the FEC information,
> you may need to increase the signalling rate as much as 5-to-10%.
> Regards,
> Ed Grivna
> Cypress Semiconductor