Wednesday, July 21, 2010

Common Electrical Interface for 25/28G - Possibility or Pipe Dream?

by Lisa Huff

Monday, I sat through a workshop hosted by the Optical Interconnecting Forum (OIF) on its “Common Electrical Interface (CEI) project for 28G Very Short Reach (VSR).” What quickly became clear to me was that I was in a room of VERY optimistic engineers.

I sat through presentations that were characterized as “Needs of the Industry,” which consisted of content from the leaders of IEEE 802.3ba (40/100G Ethernet), T11.2 (Fibre Channel) and InfiniBand standards groups. Yet all of these representatives made sure they carefully stated that what they were presenting was their own opinions and not that of their standards groups, which I found odd since most of what they showed was directly from the standards. Legalities I guess. I also noticed that they never really sited any kind of independent market research or analysis of what the “needs of the industry” were. For instance, one speaker said that InfiniBand has a need for 26AWG, 3-meter copper cable assemblies for 4x25G operation in order to keep the cost down within a cabinet. Yet, he did not present any data or even mention what customers are asking for this. Maybe this exists, but to me unless it is presented, the case for it is weak. I do have evidence directly from some clustering folks that they are moving away from copper in favor of fiber for many reasons – lower power consumption, weight of cabling, higher density, and room in cabinets, pathways and spaces.

Today, data center managers are really still just starting to realize the benefits of deploying 10G, which has yet to reach its market potential. I understand that standards groups must work on future data rates ahead of broad market demands, but this seems extremely premature. None of the current implementations for 40/100G that use 10G electrical signaling have even been deployed yet (except for maybe a few InfiniBand ones). And, from what I understand from at least one chip manufacturer who sells a lot of 10G repeaters to OEMs for their backplanes, it is difficult enough to push 10G across a backplane or PCB. Why wouldn’t the backplane and PCB experts solve this issue that is here today before they move onto trying to solve a “problem” that doesn’t even exist yet?

Maybe they need to revisit optical backplanes for 25G? It seems to me that 25G really won't be needed any time soon and that their time would be better spent on developing something that would appear to have relevancy beyond 25G. To me, designing some exotic DSP chip that would allow 25G signals to be transmitted over four-to-12 inches of PCB and maybe 3m of copper cable for one generation of equipment may not be very productive. Maybe this is simpler than I anticipate, but then again, there was a similar but a little more complicated problem with 10GBASE-T and look how that turned out...

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