energy efficient bandwidth for electrical backplanes and
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Energy Efficient Bandwidth for Electrical Backplanes and Copper Interconnects
Dr. Jeffrey H. Sinsky
Optical Subsystems and Advanced Photonics Department, Bell Labs, Alcatel-Lucent
jeffrey.sinsky@alcatel-lucent.com
2 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Contributors
David Neilson, Bell Labs, Alcatel-Lucent
Andrew Adamiecki, Bell Labs, Alcatel-Lucent
3 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Overview
The limitations of copper – how far can we go with copper… when do we need optics
Understanding the fundamental challenges of electrical backplanes and copper interconnects
Thinking about electrical transmission lines from a “Shannon” perspective
Using higher order modulation formats – the good and the bad
Examples of multilevel signaling performance over backplanes and cables
Current state of the industry
Where we need to go
Conclusion
4 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
The Limitations of Copper – Experimental Findings To Date
D.T.Neilson, D. Stiliadis, and P. Bernasconi, “Ultra-High Capacity…" ECOC 2005, Glasgow, UK, September 2005
5 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Applications that will require optical interconnects
- Distance bandwidth products of over 100 Gb/s ⋅m are possible candidates
- Distance bandwidth products of over 1Tb/s ⋅m will almost certainly be optical!
EXAMPLES -distance bandwidth products between 100 Gb/s ⋅m and 1Tb/s ⋅m
Between Racks: 100m+ @ 1Gb/s-10Gb/s rates Backplanes: 1m @ 100Gb/s-1Tb/s rates Linecards: 0.3m @ 300Gb/s-3Tb/s rates On chip: 0.03m @ 3Tb/s-30Tb/s rates
Rates are per waveguide!
D.T.Neilson, Challenges for Optical and Electrical Interconnects,” WTM 2010 IEEE Photonics Society Winter Topical Meeting on Photonics for Routing and Interconnects, Majorca, Spain, January 12 , 2010.
Understanding the fundamental challenges of electrical backplanes and copper interconnects
7 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Electrical Transmission Line Losses
impedance sticcharacteri widthtraceW
resistance surface2
where
lengthdB/unit 686.8
0
0
0
==
==
=
Z
R
WZR
s
sc
σωμ
α
tangentloss tan
h wavelengtguidedconstant dielectric effective
constant dielectric relative
lengthdB/unit tan11686.8
=
===
⎟⎟⎠
⎞⎜⎜⎝
⎛−−
=
δ
λεε
λδ
εε
εεπα
g
re
r
gre
r
r
red
loss leakagel
lossradiation r
loss dielectricd
lossconductor
=
=
=
=
+++=
α
α
α
α
ααααα
c
lrdc
Jia-sheng Hong, M.J. Lancaster, Microwave Filters for RF/Microwave Applications, Wiley, New York, pg. 83.
f∝ f∝
8 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Backplane Design Challenges at High Speed
Backplane Via
Backplane Trace
Backplane PCB
ConnectorLine Card Via
Line Card Trace
Tx/Rx ASIC (includes via)
AirMaxTM
steep roll offfrom lossy trace
narrow resonancefrom stub effect
0 dB
-20 dB
-40 dB
-60 dB
12.5 GHz
-33 dB
~ 14 GHz
severeattenuation
FCI Airmax Demo24” Trace
Transfer Function
•Controlled impedance must be maintained•Cross-talk between pins is difficult to reduce
•The viahole looks like an inductor•Unless removed, the unterminated part of the viahole acts as a resonant stub
9 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
The impact of backplane connectors and dielectric loss
Jri Lee, Ming-Shuan Chen, and Huai-De Wang, “Design and Comparison of 20-Gb/s Backplane Transceivers for Duobinary, PAM4, and NRZ Data, IEEE Journal of Solid-State Circuits, Vol. 43, No. 9, September 2008.
10 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Backplane Losses – Examples
FR4 Backplanes : 5dB/GHz/m
10Gb/s data rate (f=5GHz) 0.8m length : 20dB loss
Improved Backplane materials : 1dB/GHz/m
50Gb/s data rate (f=25GHz) 0.8m length : 20dB loss
Coax Low loss dielectric : 0.05dB/GHz/m
40Gb/s data rate (f=20GHz) 20m length : 20dB loss
Note: Losses calculated at Nyquist frequency (Bitrate/2 for NRZ)
D.T.Neilson, Challenges for Optical and Electrical Interconnects,” WTM 2010 IEEE Photonics Society Winter Topical Meeting on Photonics for Routing and Interconnects, Majorca, Spain, January 12 , 2010.
11 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Cable Design Challenges at High Speed
The cable dilemna•Larger diameter less loss, worse high frequency performance•Smaller diameter better high frequency performance, more loss
The cable dilemna•Larger diameter less loss, worse high frequency performance•Smaller diameter better high frequency performance, more loss
{ }
( )
2 42 1 (1/ 6)( / 2 ) (7 /120)( / 2 ) ... ***
where/ 2
c m m m
m o i
o i
r t r t r
r r rt r r
λ π= − − −
= +
= −***Green, H., “Determination of the Cutoff of the First Higher Order Mode in a Coaxial Line by the Transverse Resonance Technique,” IEEE MTT Trans., Vol. 37, No.10, Oct. 1989, pp. 1652-1653.
( )11.811
whereinner radius in inchesouter radius in inches
GHzcr o i
i
o
fr r
rr
π ε≅
+
==
ro
ri
A non-TEM mode will start to propagate at λc, or fcGHz
A non-TEM mode will start to propagate at λc, or fcGHz
12 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Materials
For cables
Lower loss dielectric materials
Clever use of metals that try to equalize the frequency response roll off
Example: Gore “eye opener” cable
For Backplanes
Lower loss dielectric materials – must be cheap and applicable to multilayer assembly
Remove woven nature of the dielectric material
Example: Dupont is developing dielectric materials that do not exhibit the periodic mismatch of a typical FR4 which results from its woven structure
Aramid reinforcement -More Uniform and Homogenous
106 glass
1080 glass
Woven-Glass-Non Uniform
Thinking about electrical transmission lines from a “Shannon” perspective
14 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Interconnect Channels: Shannon C: Channel capacity (bits/s)
B: Channel bandwidth (Hz)
SNR: Signal energy / noise energy
Shannon formula assumes
optimum constellation and optimum coding of data
the best you can do
C = B log2 (1 + SNR)
Shannon capacity:
SNR (dB)
spec
tral
eff
icie
ncy
(bit
s/s/
Hz)
-5 0 5 10 15 20 25 300123456789
10
Shannon
’s lim
it
“Error-fr
ee”
achievab
le“Erro
r-free”
not achievab
le
15 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Spectral Efficiency (b/s/Hz)
Electrical InterconnectsUsing Higher order Formats
Possible to use higher order signaling to reduce max frequency but requires higher SNR (e.g. 2b/s/Hz requires 5dB improved SNR but only half frequency so lower losses)
If we take the FR4 example above 5dB/GHz/m for 0.8m backplane
Look at required SNR at launch: this accounts for losses and ShannonAbove 2.5Gb/s optimum format is not NRZ
Operation at 20Gb/s and 40Gb/s may be
possible
NRZ D.T.Neilson, Challenges for Optical and Electrical Interconnects,” WTM 2010 IEEE Photonics Society Winter Topical Meeting on Photonics for Routing and Interconnects, Majorca, Spain, January 12 , 2010.
backchllossa =
D. T. Neilson, "Photonics for switching and routing," IEEE J. Sel. Topics Quantum Electron. 12, 669-678 (2006).
16 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Choosing a Signaling Format for Energy Efficient Transmission - Tradeoffs
“Fitting” signal bandwidth into optimal portion of the channel response
Intelligent transmission of signal energy
Sensitivity of format to “spectral anomalies”
Impact on power requirements
Increased SNR requirement for higher order constellations
Efficiency of required electronics
Implementation complexity
Cost
17 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Signal Bandwidth Compression – Spectrally Efficient Formats for Cables and Backplanes
Formats
Duobinary
PAM-4
polybinary
PAM-X
Pattern dependence issues must be kept in mind
Multilevel signaling relocates signal energy to the frequency range with the lowest insertion loss
This makes sense for electrical backplanes and cables!
•H.-J. Goetz, J.H. Sinsky, “The Duobinary Format: A New Application for an Idea Published Long Ago,” DesignCon 2006, Santa Clara, CA, invited talk.
18 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Considerations for Optimal Transmission
distortion function
Certain formats are more sensitive to the fine structure found in the preferred region.
19 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Compare Signaling Architecture Hardware Complexity
NRZ
Duobinary
Jri Lee, Ming-Shuan Chen, and Huai-De Wang, “Design and Comparison of 20-Gb/s Backplane Transceivers for Duobinary, PAM4, and NRZ Data, IEEE Journal of Solid-State Circuits, Vol. 43, No. 9, September 2008.
20 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Compare Signaling Architecture Hardware Complexity (continued)
PAM-4
Jri Lee, Ming-Shuan Chen, and Huai-De Wang, “Design and Comparison of 20-Gb/s Backplane Transceivers for Duobinary, PAM4, and NRZ Data, IEEE Journal of Solid-State Circuits, Vol. 43, No. 9, September 2008.
21 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Power Consumption of a Typical 90nm CMOS Buffer as a Function of Bandwidth
Jri Lee, Ming-Shuan Chen, and Huai-De Wang, “Design and Comparison of 20-Gb/s Backplane Transceivers for Duobinary, PAM4, and NRZ Data, IEEE Journal of Solid-State Circuits, Vol. 43, No. 9, September 2008.
It is important that the hardware required to transmit and receive the hardware is not too complex!!
(simulation)
22 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Using Better Materials and Connectors Makes Sense Megtron 6 vs. Taconic Substrates
Backplane thickness – 250 milsDaughter card thickness - 150 mils
Channel bounds – OIF CEI-25G-LR
•1 dB/GHz/m
2 dB/GHz/m------ Taconic------ Megtron6
STRADA Whisper
Daughtercards have 10” of a 6-9-6 pair and backplane has16 total inches of 6-9-6 diff pair. Each daughtercard interface has a STRADA Whisper mated connector pair. (total length = 1 meter)
(simulation)
Courtesy of Tyco Electronics
Examples of Backplane and Copper Interconnect Signaling Techniques
24 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Examples
PAM-4 backplane (5 Gb/s)
Duobinary backplane (25 Gb/s)
Duobinary cable (40 Gb/s)
NRZ with equalization (25 Gb/s)
25 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
PAM-4 Example
Stonick, J.T.; Gu-Yeon Wei; Sonntag, J.L.; Weinlader, D.K.; , "An adaptive PAM-4 5-Gb/s backplane transceiver in 0.25-μm CMOS," Solid-State Circuits, IEEE Journal of , vol.38, no.3, pp. 436- 443, Mar 2003.
26 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Electrical Duobinary over Backplanes and Cables
How it works
We take advantage of the natural spectral roll off response of the backplane channel to convert standard NRZ data to duobinary data, effectively using the channel as part of the transmitter.
At the receiver, a quasi-digital duobinary-to-binary converter is used to regenerate NRZ data.
We were the first to demonstrate GHz-rate duobinary data transmission over FR4 backplanes and the first to demonstrate 25 Gb/s data transmission over an FR4 backplane
)()()()( ωωωω duoEQChFIR HHHH =⋅⋅
25 Gb/s Example
Avoids resonances
Sinsky, J.H.; Duelk, M.; Adamiecki, A., "High-speed electrical backplane transmission using duobinarysignaling," Microwave Theory and Techniques, IEEE Transactions on , vol.53, no.1pp. 152- 160, Jan. 2005.
27 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
10 Gb/s Backplane Transmission Results
Tyco Quadroute Backplane (34” trace)
Pre-emphasized Signal Into the backplane
34” backplanechannel
10 Gb/s Duobinary backplane outputPRBS 23 Measurement
4,6,4 (mils)
Trace Geometry (width, space, width)
HM-ZD4000-6Quadroute
Connector TypeNelco DielectricBoard Name
Duobinarydecision thresholds
BER <10-13
Xaui Reference Backplane
First demonstration of GHz-
speed duobinarysignaling over a
backplane
J.H. Sinsky, A. Adamiecki, M. Duelk, “10-Gb/s Electrical Backplane Transmission using DuobinarySignaling,” IMS-2004, Fort Worth, TX, June 2004.
28 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
25 Gb/s Backplane Transmission Results (4 x 25 Gb/s)
2
20
Layers
AirMax VS®7.5cm, 25cm 50cm, 75cm
Nelco 4000-6
AIRMAX Backplane
AirMax VS®5cm eachRogers RO4350
2 Line Cards
Connector Type
Line Lengths
PCB Material
Board Name
Line Card
0 5 10 15 20 25
-50
-40
-30
-20
-10
0 35cm (14'') 60cm (24'')
S21
Mag
nitu
de [d
B]
Frequency [GHz]
25 Gb/s Duobinary backplane outputPRBS 7 Measurement
10ps/DIV65mV/DIV
Pre-emphasized Signal Into the backplane
24” backplanechannel
BER <10-13
0.2V/DIV 10ps/DIV
PRBS 7
First demonstration of 25 Gb/s data
transmission over an FR4 backplane
Adamiecki, A.; Duelk, M.; Sinsky, J.H., "25 Gbit/s electrical duobinarytransmission over FR-4 backplanes," Electronics Letters , vol.41, no.14pp. 826- 827, 7 July 2005.
29 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
40 Gb/s over an 24.4 meters of SMA Coaxial CablePre-emphasized Signal Into the cable
40 Gb/s Duobinary cable outputPRBS 7 Measurement
24.4 meter coaxcable channelDC-24 GHz
DC-18 GHz
Operating Bandwidth
MegaPhase
MIDISCO
Manufacturer
Micro-porousPTFE Tape
Air/PTFE
Dielectric
40 feet0.200”
0.205”
Outer Diameter
40 feet
Length
BER <10-14
First demonstration
of 40 Gb/sduobinary data transmission over a 24.4 m
coax cable
J.H. Sinsky, A. Konczykowska, A. Adamiecki, F. Jorge, M. Duelk, “39.4 Gb/s Duobinary Transmission over 24.4 meters of Coaxial Cable using a Custom Indium Phosphide Duobinary-to-Binary Converter Chip,” accept IEEE Trans. On Microwave Theory and Techniques, Dec. 2008 or Jan. 2009.
30 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
NRZ with Equalization Example (25 Gb/s) – Tyco Designcon 2010
18” of Megtron 6 (including daughtercards) with Avago silicon
Both daughtercards use Tyco’s STRADA Whisper connectors on them
•25 Gb/s
Courtesy of Tyco Electronics
31 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Current State of the Industry
OIF
Looking at 25 Gb/s Long Reach over backplane (CEI-25G-LR)
Have been exploring NRZ signaling with pre-emphasis and equalization
IEEE 802.3ba - 40Gb/s and 100Gb/s Ethernet Task Force
Provide Physical Layer specifications which support 40 Gb/s operation over
at least 10km on SMF
at least 100m on OM3 MMF
at least 7m over a copper cable assemblyat least 1m over a backplane
Provide Physical Layer specifications which support 100 Gb/s operation over:
at least 40km on SMFat least 10km on SMFat least 100m on OM3 MMFat least 7m over a copper cable assembly
32 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Where we need to go
Need to focus more on energy efficiency
Intelligent use of channel
Use better channels! They exist!
Lower loss dielectrics
Tighter more random weaves
Improve metallization roughness spec
Once materials are improved, backplane connectors will become the bottleneck!
Innovative designs are needed – very challenging
Consider “low” frequency equalization carefully for multi-level formats
There may be more sensitivity to gain and phase variations
33 | Energy Efficient Backplanes | April 2010 |J. Sinsky All Rights Reserved © Alcatel-Lucent 2010
Conclusion
Electrical backplanes and interconnects are not dead yet!!
Electrical backplane transmission at 25 Gb/s should be possible up to 1 meter
An optimal combination of multi-level formats, improved dielectric material, and improved connectors should pave the way for progress
For coaxial cables, rates to 40 Gb/s should be possible in a commercial setting over many meters
An exciting challenge would be to push backplane line rates to 40 Gb/s
Would require innovation in connectors and connection techniques between backplane layers
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