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

HARI: Question regarding byte (column?) versus word striping


Should be be considering the scalability of the two proposals?  It would be
nice if the protocol (including idle insertion, etc) could extend easily in
terms of the number of lanes, as well as frequency of operation, for future
versions of Ethernet or for Infiniband.

I could be wrong, as I haven't been following every detail of the debate,
but my first instinct tells me that the word-striped proposal would more
easily scale with the number of lanes.  It is also possible that it is not
customary (or taboo!) to discuss future scalability when defining an IEEE
standard, although I think that it is reasonable to at least consider the
ramifications.  For example, an early Hari proposal mentioned that since
packets must start on lane 0, odd length packets (i.e. 65 bytes) would have
a larger than normal IPG.  What happens if, for 100Gig, we move to 20 lanes
at 6.25GHz?  Not only does the IPG get extended even further, but it would
be extended for "normal" length packets such as 64 bytes.

The word-striped proposal works by putting one "word" of data (the word
width given at the XGMII interface) on each lane.  The number of lanes
required is solved by dividing the total data rate (10Gbps) by the rate of
each line (2.5Gbps).

Byte-striping, in contrast, works by putting one "byte" (fixed at 8 bits) of
data on each lane.  The number of lanes required is solved, again, by
dividing the total data rate by the rate of each line.  In this case,
through, the XGMII interface width is dependant on the number of lanes,
fixed to (8 bits)*(number_of_lanes).  Parts of the protocol (for example,
IPG extension) are dependant on number of lanes.  This seems to be slightly
less flexible.

Basically, my point is that if byte- and word-striping are quite comparable
in terms of complexity at 10Gbps, should we examine the future scalability
of both proposals to help us select one?

 Simon L. Sabato                ph (408) 765-7814
 Level One Communications
 An Intel Company