November 14, 2012
11:03 AM EST
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400GbE versus Terabit Ethernet: First steps toward an answer
One of the seemingly ever-present debates in the telecom industry is the question of what the “next rate” will be for data transport. It happened previously as the industry deliberated the upgrade from 10GbE to corresponding 40GbE and 100GbE rates. For that jump, 100GbE seems to have generally won out, with 40GbE being seen as a short-term step.
That same debate is now in full swing for the next step after 100GbE, and one of the people actively engaged in this debate is Ciena’s Pete Anslow. Pete has over 30 years experience in the optical space and is one of the industry's most involved experts in optical standards development. He is the Associate Rapporteur of ITU-T Q.6/15, is the Liaison Rapporteur for ITU-T SG15 to IEEE 802.3, was the Chair of the Optical Sub-task Force of IEEE 802.3ba, the Editor of IEEE 802.3bg and the Editor for the IEEE Bandwidth Assessment (BWA) Ad Hoc.
The last time I talked with Pete Anslow was in March of last year when he gave some of his insights on the 100GbE market and standards (see 100GbE standards Q&A with Ciena expert Pete Anslow).
Since that discussion, the focus has moved to 400GbE versus Terabit Ethernet. The first step in that effort came at a recent meeting of the IEEE 802.3 Industry Connections Higher Speed Ethernet Consensus group in Geneva, which was just officially formed in August to help build consensus on this “next rate” question.
Pete was part of that meeting representing Ciena, and also supported the view that he and several other contributors of the group shared that 400GbE was the next logical data rate for Ethernet. I recently talked with Pete about that meeting, the choice between 400GbE and Terabit Ethernet, and his views on the industry direction.
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Q: What happened at the Higher Speed Ethernet consensus meeting?
A: The presentations in the Geneva meeting had an uncommonly consistent view that Terabit Ethernet is too big of a step to take and that 400GbE should be the next rate. This view was tested in a straw poll on the study group formation proposal called a Call For Interest (CFI). The result was that everyone in the room (> 60 people) supported a 400Gb/s specific CFI with the exception of one vote for the study group being left to decide the rate.
Q: What did that straw poll mean? Has anything been decided?
A: Nothing has been decided officially. The straw poll was only designed to provide a snapshot of the group’s early opinions. The next step is to pull together a consensus presentation in support of a 400GbE specific CFI with a target date of March 2013.
Q: What is your view on moving to 400GbE versus directly to Terabit Ethernet?
A: There is a considerable amount of industry churn associated with each successive step in the Ethernet bit rate, so it is not surprising that there are those who think that going straight to Terabit is the best approach. However, adoption of a new rate depends on its cost-per-bit being supportable for any particular application. 400GbE looks to be realizable using an evolutionary approach (16 lanes of 25G being an obvious possibility) which should allow 400GbE client modules to be introduced at a similar cost-per-bit to future evolved 100GbE modules. Terabit Ethernet, on the other hand, will require a revolutionary approach to make it practical (for example a complex modulation format) which will need a huge industry investment before it will achieve cost-per-bit parity with evolved 100GbE modules.
Q: Ciena has shown our early ability to achieve Terabit transport in the labs. Why would the jump to Terabit Ethernet be any more difficult?
A: The line side has already made the leap to phase modulation and coherent optics driven by the economics of sharing the same long haul optically amplified fiber over higher and higher aggregate bit rates. Extending this to use more complex constellations and multiple carriers with increased symbol rate on each gives a clear route to terabit line side rates. For the client side, the costs are dominated by the optical modules alone. The amount of investment required from the industry to develop modules that have adequate cost, power and density is very much greater for Terabit Ethernet than it would be for 400GbE.
Q: Do you see growth in bandwidth demand that requires a jump to 400GbE or Terabit on the horizon?
A: As recently confirmed by the IEEE BWA Ad Hoc, core networking (line side) requirements have doubled every 18 months, and server I/O (client side) requirements have doubled every 24 months. At that rate of increase, we can easily see the need for speeds above 100GbE for the aggregation of multiple 100GbE feeds.
Q: Do you think 400GbE will be a short-term step between 100GbE and Terabit Ethernet the way many view 40GbE now?
A: 40GbE was justified within IEEE as being for server I/O starting in around 2015, while 100GbE was for aggregation. With an IEEE project just starting to look at the possibility of 40GbE over twisted pair to address the emerging server I/O market, perceptions of the long term role for 40GbE may well change.
In contrast to 40GbE, 400GbE is being proposed as a cost effective speed increase in an aggregation role only. Also, in addition to the high level of investment required to make Terabit Ethernet client modules, Terabit signals on the line side aren’t likely to improve the aggregate capacity of the amplified fiber compared to using 400 Gbit/s signals. These factors make it very difficult to predict how long lived a 400GbE standard would be.
Q: Going back to 100GbE which you were also very involved in, how do you see the adoption of 100GbE services going? Is it progressing slower than you had expected?
A: Not really. The 10GbE standard was approved in June 2002 (compared to June 2010 for 100G). The adoption of 10GbE two and a half years after approval was similar to 100GbE adoption today. The choice of 25 Gbit/s lane rate for the singlemode interfaces meant that the industry had to make a significant investment in processes / equipment etc. which has had to be recouped – the same issue that many module makers fear would inhibit the adoption of a Terabit module in the near term. Ethernet history suggests that 100GbE adoption rates will take off when 10GbE ports become commonplace in servers.
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