25Gbps Communication over 10GBASE-KR...

25Gbps Communication over 10GBASE KR Ch l10GBASE-KR Channels

Will BlissBroadcom Corp.

Jan 12 2011Jan 12, 2011

www.broadcom.com.

Outline• The 10GBASE-KR channel specifications for Insertion Loss to Cross

Talk Ratio (ICR) and Insertion Loss (IL)• Is it possible to send 25Gbps over these channels, which were p p ,

defined for 10Gbps?• Is it practical to send 25Gbps over these channels?

1Will Bliss Broadcom Corporation

A channel specification for 100GBASE-KR4?

G S f• The 10GBASE-KR channel specification already exists – The vast majority of the industry is using the Informative 10GBASE-KR channel

characteristics as a specificationDoesn’t need to be negotiated– Doesn’t need to be negotiated

• So potentially faster standard development• And faster real deployment

• Th 10GBASE KR h l ifi ti i k t b t ith l t b d • The 10GBASE-KR channel specification is known to be met with low cost boards and connectors

– It had been thought very tough to meet the informative channel characteristics, but it has turned out to be an easy to meet specification has turned out to be an easy to meet specification

– Including long reaches, to 38”– Already a large installed base of compliant channels

• So faster and more accurate development of real PHYsSo faster and more accurate development of real PHYs• But does the 10GBASE-KR channel support 25 Gbps?


The 10GBASE-KR channel specification

C ( C ) S f• The Insertion loss to Cross talk Ratio (ICR) is the SNR bound of the channel – Other impairments are self-inflicted circuit and signal processing non-optimal

implementations• The 10GBASE KR ICRmin(f)=23 3 18 7*log10(f / 5GH )• The 10GBASE-KR ICRmin(f)=23.3 -18.7*log10(f / 5GHz)

– For 100 MHz < f < 5.15625 GHz– “Accounts for worst-case differences in characteristics (e.g., amplitude, transition

times) between the victim and aggressor transmitters”times) between the victim and aggressor transmitters– “It also assumes a 3dB signal-to-noise ratio penalty related to insertion loss

deviation”– “It is recommended that ICRfit(f) be greater than or equal to ICRmin(f) …” It is recommended that ICRfit(f) be greater than or equal to ICRmin(f) … – So as normal, some margin is already in the specification

• The 10GBASE-KR Insertion Loss (IL) specification is approximately– A modest 4 3dB/GHz slope for 1 < f < 6 GHzA modest 4.3dB/GHz slope for 1 < f < 6 GHz– A steep 14.5 dB/GHz slope for 6 < f < 15 GHz

• Which is very discouraging for using Baud rates much greater than 14 GBaud/sec


10GBASE-KR Insertion loss to Cross-talk Ratio (ICR) specificationICR 10GbaseKR specification

45

50

55ICR 10GbaseKR specification

30

35

40

dB

10

15

20

25

10-1

100

101

0

5

10

GHz

• The ICR specification is an SNR bound• The 10GBASE-KR ICR specification stops at 5.15.. GHz

GHz

4

• Shannon Capacity C = 52.2 Gbps

Will Bliss Broadcom Corporation

Shannon Capacity of the 10GBASE-KR Channel

S C f f• The Shannon Capacity of a narrow spectral segment of width W Hz in additive Gaussian noise is

– W * log2(1 + Signal/Noise) in bits/sec• Ignoring the non Ga ssian nat re of Cross Talk and integrating o er f• Ignoring the non-Gaussian nature of Cross-Talk, and integrating over f

– Shannon Capacity of 10GBASE-KR ICR spec. is C = 52.2 Gbps– More than double our target of 25Gbps per link

• Thi i i th h it l b d• This is encouraging, even though its only a bound– The Shannon bound gives no overt directions on how to accomplish such capacity

• What ‘modulation’ and/or FEC coding to useM ti l i ti t t ithi 10% f it– Many practical communication systems now operate within 10% of capacity

• But they typically require FECs and modulation systems more complex than has been considered for KR


Salz DFE Bounds of the 10GBASE-KR channel

f ( )• What performance is possible with very simple modulation (line codes) and very simple FEC codes?

– Consider only PAM-M, for NRZ (M=2) up to PAM-8• PAM M is the simplest possible ‘advanced’ modulation choice• PAM-M is the simplest possible ‘advanced’ modulation choice• This isn’t new, as many prior contributors (not listed here) have proposed PAM-

M as the next step for back plane communications– Consider only a simple single error event correcting FEC – Consider only a simple single error event correcting FEC

• Similar to current Fire code performance • The Salz SNR upper bounds the SNR possible with a Decision Feedback Equalizer

(DFE)(DFE)– DFE is one of the simplest possible ‘advanced’ receivers possible

• Patented in the 19th century!– No limit on accuracy (length) of Feed Forward Equalizer (FFE) in boundNo limit on accuracy (length) of Feed Forward Equalizer (FFE) in bound– No limit on length of Feed Back Filter (FBF) in bound– No SNR penalty for decision errors in bound


10GBASE-KR Insertion Loss Specification with 25Gbps PAM-M

I ti L ifi ti 10G BASE KR

• Slope of IL changes at 6GHz

• 4.3 dB/GHz below,-20

0Insertion Loss specification 10G BASE KR

PAM-6 & 10GBASE-KRPAM 4 • 14.5 dB/GHz above

• An increase in slope of 10.2 dB/GHz

• 25Gbps Nyquist IL;-60

-40

dB

PAM-4

PAM 3 25Gbps Nyquist IL;• NRZ = 126.8 dB• PAM-3 = 66.3 dB• PAM-4 = 35.4 dB-100

-80

d PAM-3

• PAM-6 and 10GBASE-KR Nyquist IL = 25.2 dB

0 2 4 6 8 10 12

-120

GHz

NRZ=PAM-2

• The NRZ Nyquist IL of 126.8 dB is not close to feasible• The PAM-3 Nyquist IL of 66.3dB is much more difficult than prior channels, and not feasible

GHz

7

• The PAM-4 Nyquist IL of 35.4dB is more difficult than prior channels, but is feasible


{1+ Signal/Noise} from 10GBASE-KR ICR and IL extrapolation1 + S/N from 10GbaseKR ICR and IL extrapolation

40

45

50

55p

• Black = the 10GBASE-KR ICR specification

• Red = the extrapolation of ICR above 5 15 GHZ

25

30

35

40

dB

ICR above 5.15 GHZ• The ICR equation is

extrapolated, and• The change in IL Slope is

10

15

20

25 The change in IL Slope is applied above 6GHz

• As if the cross talk noise followed the ICR

10-1

100

101

0

5

10

GHz

specification, but the signal followed the IL specification

• Salz SNR_dB = mean(10*log10{1 + Signal(f)/Noise(f)}) • Because minimum mean squared error solutions de-

weight poor SNR ‘bins’ to a minimum of 0dB

8

weight poor SNR bins to a minimum of 0dB


25Gbps PAM-M with 10GBASE-KR ICR and IL Specification

10Gbps

NRZ

25Gbps

NRZ

25Gbps

PAM‐3

25Gbps

PAM‐4

25Gbps

PAM‐5

25Gbps

PAM‐6

25Gbps

PAM‐7

25Gbps

PAM‐8

1 bits/Baud modulation

1 3/2 2 9/4 5/2 11/4 3

10.3125 Baud Freq GH

25.7813 17.1875 12.8906 11.4583 10.3125 9.3750 8.5938GHz

31.1 Salz bound dB

15.8 23.6 29.2 30.3 31.1 31.9 32.6

15.5 Req. SNR for 1e‐15 / i l t FEC

15.5 19.8 22.5 24.5 26.2 27.5 28.7

M i i i i d i h PAM 4

w/ single event FEC

15.6 Margin=Salz – Req’d dB

0.3 3.8 6.7 5.8 5.9 4.5 3.9

• Margin is maximized with PAM-4– It is feasible to build a practical communication systems with this margin– But a different experience than with10G BASE KR, which has high margin

• Note that NRZ and PAM 3 bounds at 25Gbps are not practically feasible

9

• Note that NRZ and PAM-3 bounds at 25Gbps are not practically feasible– The huge loss of signal above 7GHz is impractical with real DFE systems


S l SNR d R i d SNR M f PAM M

Salz SNR and Required SNR vs. Modulation Index M (PAM-M)

30

35Salz SNR and Required SNR vs M of PAM-M

• Red = Salz SNR using 10GBASE-KR ICR and IL specification

25

B

Margin max @ PAM-4• Blue = Required SNR

for 1e-15 error rate with single error event correcting FEC

20

dB correcting FEC • The margin is

maximized with PAM-4• The PAM-3 and NRZ

2 3 4 5 6 7 810

15The PAM 3 and NRZ Salz bounds are not feasible due to the severe Insertion Loss

b 7 GH2 3 4 5 6 7 8M of PAM-M above 7 GHz


Next Steps for 25Gbps over 10GBASE-KR channelsf f G S• Increase library of measured channels that were designed for 10GBASE-KR

– Presumably all were designed for and meet the 10GBASE-KR normative characteristicsCompare the real IL and real ICR to the specification and extrapolation– Compare the real IL and real ICR to the specification and extrapolation

– Calculate real RX performance on the real ICR and real IL for various PAM-M choices

– Consider practical implementation issues for higher PAM M– Consider practical implementation issues for higher PAM-M– Choose the most practical PAM-M

• Consider more powerful FEC (independent of channel model) Code in at least the 25 Gbps streams but best in the100 Gbps stream– Code in at least the 25 Gbps streams, but best in the100 Gbps stream

• Too much extra latency from coding in the virtual 5 Gbps streams– Consider maximum latency acceptable

Consider increased PHY Baud rate to allow increased code redundancy– Consider increased PHY Baud rate to allow increased code redundancy• Transparency to higher Ethernet layers is possible


25Gbps Communication over 10GBASE-KR...

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