[Beowulf] /. Terabit Ethernet is Dead, for Now

Eugen Leitl eugen at leitl.org
Thu Sep 27 05:51:57 PDT 2012


Terabit Ethernet is Dead, for Now

by Mark Hachman | September 26, 2012

A straw poll of the IEEE's high-speed Ethernet group finds that 400-Gbits/s
is almost unanimously preferred.

Sorry, everybody: terabit Ethernet looks like it will have to wait a while

The IEEE 802.3 Industry Connections Higher Speed Ethernet Consensus group met
this week in Geneva, Switzerland, with attendees concluding—almost to a
man—that 400 Gbits/s should be the next step in the evolution of Ethernet. A
straw poll at its conclusion found that 61 of the 62 attendees that voted
supported 400 Gbits/s as the basis for the near term “call for interest,” or

The bandwidth call to arms was sounded by a July report by the IEEE, which
concluded that, if current trends continue, networks will need to support
capacity requirements of 1 terabit per second in 2015 and 10 terabits per
second by 2020. In 2015 there will be nearly 15 billion fixed and
mobile-networked devices and machine-to-machine connections.

The report goes on to predict that, from 2010 to 2015, global IP traffic will
experience a fourfold increase from 20 exabytes per month in 2010 to 81
exabytes per month in 2015, a 32 percent CAGR. Storage growth is expected to
grow to 7910 exabytes in 2015, with over half of it accessed via Ethernet. Of
course, one of the first places the new, faster Ethernet links will occur
will be in the data center.

With that in mind, the IEEE 802.3 group began formulating a response.
However, virtually all attendees seemed to be in agreement before the meeting
opened, as only one presentation focused on the feasibility of one-terabit
Ethernet, eventually concluding that 400 Gbits/s made more sense in the near

Kai Cui and Peter Stassar from Huawei Technologies suggested that the most
cost-effective method for developing a 1-terabit Physical Medium Dependent
(PMD) would be to leverage today’s 100-Gbit technology, which isn’t yet in
high volume, and therefore not cost-optimized. “[The] cost target for 1Tb/s
needs to be at or below 100G cost/bit*sec and required R&D investments should
be modest,” they wrote as part of their presentation.

“100GbE technology based architecture would imply 40 lanes at 25G, which
clearly would imply impractically big packages and large amount of interface
signals,” Cui and Stassar added, which would need to reduce the number of
electrical and optical interface lanes to enable a reasonable package size.
While alternative modulation formats could be used (5λx200G DP-16QAM, 4
bits/symbol, 25G) “neither the multi-level nor the phase modulation format
based technologies have been demonstrated to be sufficiently mature to
justify usage in client PMDs towards 100Gb/s to 1Tb/s applications.”

They concluded: “1Tb/s does seem a ‘bridge too far’ at least for the coming 3
to 4 years.”

Chris Cole of optical components maker Finisar presented the case for a
400-Gbit CFI, with backing from Brocade, Cisco, HP, IBM, Intel, Juniper, and
Verizon, among others.

Like Huawei’s Cui and Stassar, Cole indicated that 400-Gbit Ethernet can
reuse 100 GbE building blocks, and fits within the existing dense 100 GbE
roadmap. Faster data rates require “exotic” implementations, with higher R&D
investments required and a longer time to market. “Data rates beyond 400Gb/s
require an increasingly impractical number of lanes if 100GbE technology is
reused,” he said.

400 Gbit/s also makes more sense than a 4×100 Gb/s link aggregation, Cole
added, as fewer items promotes management efficiency. Individual link
congestion is also a concern: “Without faster links, [the] link count grows
exponentially, therefore management pain grows exponentially.”

Cole suggested that a potential 400 Gb/s MAC/PCS ASIC could be fabricated in
either 20- or 28-nm CMOS, using a 400-bit wide bus and a 1 GHz clock rate.
“There is a strong desire to reuse 802.3ba, 802.3bj, and 802.3bm technology
building blocks,” he said.

That’s not to say that terabit Ethernet won’t be needed, Cole concluded, or
1.6 terabit Ethernet, at that. The timeframes for those followon CFIs could
be between 3 to 6 years, he said.

The CFI hasn’t formally occurred; until it does, nothing has been decided. So
far, the most likely dates for formalizing the CFI will take place in either
November or next month. But at this point, it looks like terabit Ethernet is
a dead duck, at least for the near future.

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