Energy-Efficient Solutions for 10Gbps Ethernet

Yury Audzevich, Alan Mujumdar,Philip Watts, Andrew W. Moore

MSN 2012 workshopFriday, July 13th, 2012

Introduction

Energy-efficiency of transmission systems is one of the key priorities with respect to the next generation of networking equipment.

Open questions:

What is the power contribution of the ‘lower layer’ transmission protocols?

What is the power impact of the encoding blocks?Servers vs Network for a Google cluster (fat trees topology)

(Energy proportional datacenter networks, Abts et al. ISCA 2010)

The focus of the research

Which effect DC-balanced codes do have on the optical transmission system?

- the effect on optical power requirement? - front-end power contribution, like PMA and PMD?- the power consumption of line coding block itself?

…and in particular: what is

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10Gb/s optical link simulations

Optical link – transmission system:• 219 bits PRBS is used as an input, the baud rate is adjusted after encoding

Optical link parameters: • 100m Single Mode Fibre with parameters satisfying requirements for 10Gbps

Ethernet over SMF

Optical link – receiving system:• Optical receiver with direct detector and AC coupling achieved using High Pass Filter• BER is calculated using the complementary error function

10Gb/s optical link simulations (cont.)

• The transmission system is relatively insensitive to the DC-balanced codec choice • Taking 100MHz HPF cut-off frequency and assuming 20dB link budget, the laser

power requirement is lower for encoded sequences (0.3mW of savings) in comparison to PRBS

better better

bett

er bett

er

Physical Coding Sublayer – 8B10B (and 64B66B)

8B10B line code:1) Encoder/Decoder – implemented 3B4B and 5B6B codes plus disparity control check for DC-balance

64B66B line code:1) Encoder/Decoder – decoding from XGMII into 10GBASE-R format2) Scrambler/Descrambler – mixing of data to avoid long sequences of 0s/1s

• Codecs were implemented in Verilog HDL and Synthesized using 90nm and 45nm technical process libraries

• Industry standard estimation tools were used for power measurements

Early-days results – 8B10B PCS power

10Gb/s link results: obtained for 30 microseconds simulation periods, with a symbol clock frequency of 625MHz for both 45nm and 90nm tech. process

• IDLE sequences costs MORE to encode

• Low leakage 45nm library provides decrease in power by a factor of 2

Inverse ofenergy-proportionality

Power estimates – 64B66B PCS

10Gb/s link results: Identical pattern sent for 30 microseconds of simulation periods, with a symbol clock frequency of 156.25 MHz

• Power consumption of 64B66B codec is actually more data proportional

• Scrambling & gearbox modules have a fixed power cost (~1.5-2.5 times larger power dissipation than the combination of encoder & decoder power)

64B66B10GBASE-SR / 10GBASE-LR

(commonly used)

8B10B10GBASE-LX4 (less common)

Physical Medium Attachment – 8B10B and 64B66B

PMA components are built using both CMOS and MCML logic families

• CMOS designs were synthesized using standard cell libraries

• MCML designs were built, optimized and analysed using HSPICE tools

PMA power – 8B10B and 64B66B

• MCML power is independent of the operating frequency but is strongly related to the optimization criteria

• At high clock frequencies MCML designs become more power efficient than their CMOS counterparts

• Even well power-optimized PMA designs may require 5x-10x times higher power than the corresponding PCS blocks!

64B66B

Implications

•Our recent analysis of realistic trace data(10Gbps) showed average link utilization of only 8.79% - in concordance with [1].

•The majority of the networks are overprovisioned to sustain peak loads and underutilized most of the time

•Current implementations of Ethernet standards require continuous transmission of IDLE code words (even in the absence of MAC traffic)

So…

May be, we need a system that has good energy-proportionality and can quickly restart

Sounds like we need a new MAC…

[1] T. Benson, A. Akella, and D. A. Maltz. Network traffic characteristics of data centers in the wild. In Proceedings of ACM IMC '10, pp. 267-280, New York, USA, 2010.

Remember this one? Ethernet – CSMA/CD

Many features/ideas we don’t want, but one we do:

Or an old energy-efficient MAC

• Preambles give clocks valuable re-sync. time and allow photonic systems to turn back on

Energy-efficient MAC

Where do we get energy-savings from: Powering down the codecs when no data is present Using a synchronization preamble prior to data transmission for fast CDR

With avg. Ethernet frame size of 1150bytes and 64bits of preamble, the effective energy-saving is ~93%

Is the protocol going to make a difference?

YES, It m

akes

the difference93%

93%

89%

87%

Key take-aways

Optimal laser power is independent of the DC-balanced codec chosen Codec power consumption is not always data-proportional Serialization/deserialization power dominates over all the other power

groups New MACs (off when idle) do save significant power

How do we test, build, trial?regular NICs don’t help

Need something programmable but FAST…

Thank you!

QUESTIONS?