Codificacion 8B/10B

Páginas: 6 (1291 palabras) Publicado: 4 de noviembre de 2012
8B/10B Coding 64B/66B Coding
1. 2. 3. 4. Transmission Systems 8B/10B Coding 64B/66B Coding CIP Demonstrator Test Setup
PeterJ Slide 1

Sep 4, 2008

Transmission system General
Data
D Q

Flip Flop Clock
C

@ 1 Gbps = 1 ns = 20 cm
Sep 4, 2008 PeterJ

Slide 2

Transmission system Propagation Delay
Data
D Q

Flip Flop Clock
C

Unequal propagation delay 10 cm @ 1 Gbps

??

?

?

Sep 4, 2008

PeterJ

Slide 3

Transmission system Clock Data Recovery
Data CDR Clock Combine Clock and Data!
PLL

D

Q

Flip Flop
C

If there are enough edges in the data then the clock can be recovered from the data using a PLL

PLL

Sep 4, 2008

PeterJ

Slide 4

Code Properties
1. Provide enough edges in the data to enable Clock Recovery

Sep 4,2008

PeterJ

Slide 5

Transmission system Receiver Threshold
Data CDR Clock
C D Q

Flip Flop

Receiver Threshold refers to “Ground” which must be the same potential as “Ground” at the transmitter!

Jitter!
Sep 4, 2008 PeterJ Slide 6

Transmission
Data Use Differential signalling! CDR Clock Icm
C D Q

Flip Flop

Receiver Threshold is halfway positive and negative signalCommon mode voltage Capacitors… Use AC Coupling difference between transmitter and termination at the receiver can Need DC Balance! result in excessive currents

Sep 4, 2008

PeterJ

Slide 7

DC Balance

1 0 1 0 1

1 0 1 0 1

1 1 1 1 1

1 1 1 ? 0

Define a maximum Run Length Sent equal amount of „1‟s and „0‟s (Running Disparity)
Sep 4, 2008 PeterJ Slide 8

Receiver AutomaticGain Control

Gain

Time

Again: maximum Run Length
Sep 4, 2008

Gain

Time
PeterJ Slide 9

Code Properties
1. Provide enough edges in the data to enable Clock Recovery 2. Maximum Run Length and DC Balance

Sep 4, 2008

PeterJ

Slide 10

History of 8B/10B coding

Sep 4, 2008

PeterJ

Slide 11

8B/10B coding
• Maximum run length = 5 • Running Disparity => DCBalance
• 8 bits => 256 different values • 10 bits => 1024 different values • 8B10B code is built out of:
(5 to 6 bit code) + (3 to 4 bit code)

Sep 4, 2008

PeterJ

Slide 12

8B/10B coding
• Not all 1024 values are useful.
– For example “0100000011” has run length 6

• For most of the 256 (8B) values a positive and a negative 10B value is selected depending on the “Current RunningDisparity”
– For example:
D7.0 = 1110001011 (Current RD-) D7.0 = 0001110100 (Current RD+)

• So about 512 useful values are selected from 1024… • There are still a few codes left!
Sep 4, 2008 PeterJ Slide 13

8B/10B coding table
• 256 “Data” Characters:

Sep 4, 2008

PeterJ

Slide 14

8B/10B coding table
• There were still a few codes left! • 12 “Special” K Characters:

CommaCharacters
“The only patterns that have 5 consecutive ‘1’s or ‘0’s

Sep 4, 2008

PeterJ

Slide 15

8B/10B code “K” Characters
010101001101100111000001010110111010010100100111010011001001110100111001 ??.? K28.5 D16.2 D31.3 D11.3 D0.0

• Comma characters K28.1/K28.5/K28.7 are used word alignment • Create “ordered sets”
– For example Fibre Channel Start Of Frame (SOF) =K28.5/D21.5/D23.0/D23.0 – K30.7 = Error Propagate – K28.3 = Carrier Extend
Sep 4, 2008 PeterJ Slide 16

8B/10B code properties
1. Provide enough edges in the data to enable Clock Recovery 2. Maximum Run Length and DC Balance 3. Add special characters
– Comma for word alignment – Control characters

Sep 4, 2008

PeterJ

Slide 17

64B/66B Coding

Sep 4, 2008

PeterJ

Slide 18

64B/66B
(IEEE std802.3ae-2002, Clause 49)
• • 20% of the bandwidth for 8B/10B is overhead For 64B/66B overhead is 3%
2 bit Sync Header 64 bit (Scrambled)

2 bit Sync Header 00 Code Error 01 64 bit = Data field 10 64 bit = Mixed Data / Control field 11 Code Error



Once every 66 bits there is at least a “01” or a “10” transition
=> Receiver Block Sync (it may take a while…)
PeterJ Slide 19

Sep 4,...
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