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Re: Interface reality check

Dear Roy,

> True 64B/66B supports all of the "Ethernet packet signalling semantics"
> BEFORE it encodes the 802.3 frames.  But quite simply, after it is encoded,
> it is no longer octet aligned.  The WAN compatible PHY is octet aligned.
> ANSI T1X1 and ITU standards are also octet aligned.  It has nothing to do
> with control codes or any other issue.  For example your 1500 octet 802.3
> Ethernet MAC frame becomes 1546.875 octets after it is encoded with 64B/66B.
> This type of octet alignment failure may cause other problems in octet
> aligned signaling schemes.  I noticed this octet alignment failure in
> looking at the possibility of using IPG compression with 64B/66B to help
> reduce the additional ~3% transfer bandwidth loss.

It is well known to logic designers how to build what is colloquially known
as a "gearbox".  Even though the code is defined on 66 bit boundaries,
it is easy to actually transport it as octets.  As long as SONET can
reliably transport octets, then the RX gearbox reconstructs the 66bit
codewords with no problem. 

You can see such a structure converting 33 bit words into 16 bit words in
the last 64b/66b presentation slides.  It should be trivial to see how
a similiar structure can convert from 66 bit words to a sequence of 8 bit

So, in summary, we only need to use SONET as a stripped down octet transport
mechanism.  It never needs to know about the underlying 66-bit structure.

Best regards,
Rick Walker