Thread Links Date Links
Thread Prev Thread Next Thread Index Date Prev Date Next Date Index

Re: 64/66 control code mapping

Dear Kamran,

Some comments about 64b/66b.

> >e) SLP frames provide no error detection or containment capabilities for
> >transported data thereby increasing the undetected error rate of the link.  
> 64b/66b does not provide data error detection capability. How come ethernet 
> works well with 64b/66b that does not support error detection?

Each 64b/66b frame is prefaced with two complementary sync bits.  These
periodic patterns can be used to improve burst error detection.  Any
burst longer than 66 bits will cause a sync violation.  This protection
is in addition to the normal CRC32 burst protection. 

The 3% overhead of 64b/66b thereby provides a bit of ongoing sanity
checking.  The frames must all contain valid sync bits and must proceed
in a correct frame-type order. 

> 64b/66b does not work for XAUI.  The encoding does not allow mapping
> things li ke ------------------------------ D D D T I I I S, which you
> can observe on a lane of HARI.  Therefore 64b/66b is not a candidate. 
> And I don't recall Rick Walker or Rich Dugan ever saying that it could
> be used in HARI. 

This question was asked in Kauai.

Our answer was that we didn't see any compelling need to redefine HARI. 
The 3.125G data rate seemed feasible, and the code was well known and
low risk.  However, we had considered the possibility of using a
modified 64b/66b code over the four HARI lanes, and would consider
describing such a system if there was a clear interest from the 10GbE

You are right that the serial code that we have described will not work
directly over XAUI.  However, there are several ways that 64b/66b could
be adapted to work over XAUI.  In such a system, it would be possible
to redefine the PMD to do a relatively simple 4:1 multiplexing.

This might be a good feature, but I'm not sure how the committee will weigh
this desire with other conflicting requirements.

If it appears that 8B/10B is no longer desirable due to EMI and skin-loss
limitations on the PCB, we will be quite happy to present our ideas w.r.t.
using 64b/66b on the 4 XAUI lanes.

Rick Walker