ECEN4533 Data Communications Lecture

1
ECEN4533 Data CommunicationsLecture 41 27
April 2009Dr. George Scheets

• Problems 2006 Final Exam
• Corrected Tests Due 29 April (Stw)
• 1 week after return (elsewhere)
• DO YOUR OWN WORK!
• Wireshark extra credit due 1 May (Live)
• Week of 4-8 May (Distance Learning)
• Final Exam, Monday, 4 May, 0800-0950

2
ECEN4533 Data CommunicationsLecture 42 29
April 2009Dr. George Scheets

• Problems 2007 Final Exam
• Corrected Tests Today (Stw)
• 1 week after return (elsewhere)
• DO YOUR OWN WORK!
• Wireshark extra credit due 1 May (Live)
• Week of 4-8 May (Distance Learning)
• Final Exam, Monday, 4 May, 0800-0950

3
Digital Communication Systemwith Analog Source
Source Outputs continuous waveform.
Source Coder Converts samples to a bit
stream. May compress.
Sampler Samples waveform.
Low Pass Filter Smoothes replica so it more
closely resembles original source.
Source Decoder Converts bit stream to replica of
original source.
Digital Communication System (see next slide)
4
Digital Communication System
Source Data, Digitized audio or video. Outputs
bits.
Modulator Converts bits to a symbol suitable for
channel.
FEC Adds extra parity bits.
Layer 2-6 IP, TCP, etc.
Optional
FEC Decoder Examines blocks of bits. If possible,
corrects or detects bit errors. Outputs
estimate of bit stream.
Symbol Detector Examines received symbol
outputs 1 (binary) or more (M-Ary) bits.
Channel Attenuates, distorts, adds noise to
symbols.
5
Binary PSK
1.5
1 MHz
0
-1.5
.00001
0
Two different phases are transmitted 10
cycles/.00001 seconds 1 MHz 5 cycles/symbol 200
K symbols/second 200 K bits/second
6
M-Ary Signaling

• One of M possible signals transmitted each symbol
  interval
• Tends to be used where bandwidth is tight SNR
  decent at the receiver.
• Each symbol can represent log2(M) bits
• Example In 16 PSK
• one of 16 possible phase angles is transmitted
  every symbol interval
• each symbol can represent 4 bits
• QAM normally used
• each symbol has different amplitude phase

7
No coding Binary System
Source
Channel Coder
Suppose P(Bit Error) 0.1
Symbol Detector
Channel

• Source Outputs a bit stream
• Channel Coder Maps bit stream into a form
  suitable for channel
• Channel Attenuates, distorts, adds noise
• Symbol Detector Looks at received symbol and
  decides "1?" or "0?"

8
21 FEC Binary System
2R code bps
Source Coder Input 1 bit. Output Input
Parity bit.
Source
Channel Coder
R application bps
R app. bps
2R code bps
Source Decoder Looks at blocks of 2 bits.
Outputs 1 bit.
Symbol Detector
Channel

• Suppose P(Code BE) 0.1
• P(Data BE) also 0.1

9
Example) Two bit code words

• Suppose you now transmit each bit twice, and
  P(Code Bit Error) .1
• Legal Transmitted code words 00, 11
• Possible received code words00, 11 (appears
  legal, 0 or 2 bits decoded in error) 01, 10
  (clearly illegal, 1 bit decoded in error)P(No
  bits in error) .9.9 .81P(One bit in error)
  2.9.1 .18P(Both bits in error) .1.1
  .01
• Decoder takes 2 Code bits at a time outputs 1
  bit of DataIf illegal code word received, it can
  guess 0 or 1.81 18(1/2) 90 of time the
  correct bit is output 1 18(1/2) 10 of
  time the incorrect bit is output
• Same performance as No Coding _at_ twice the bit rate

10
31 FEC Binary System
3R code bps
Source Coder Input 1 bit. Output Input 2
Parity bits
Source
Channel Coder
R application bps
R app. bps
3R code bps
Source Decoder Looks at blocks of 3 bits.
Outputs 1 bit.
Symbol Detector
Channel

• Suppose P(Code BE) 0.1
• P(Data BE) 0.028

11
Example) SSD 3 bit code words

• Transmit each bit thrice, P(Code Bit Error) .1
• Legal Transmitted code words 000, 111
• Possible received code words000, 111 (appears
  legal, 0 or 3 bits in error) 001, 010, 100
  (clearly illegal, 1 or 2 bits in error)011, 101,
  110 (clearly illegal, 1 or 2 bits in error)P(No
  bits in error) .9.9.9 .729P(One bit in
  error) 3.92.1 .243P(Two bits in error)
  3.9.12 .027 P(Three bits in error)
  .1.1.1 .001
• Decoder takes 3 bits at a time outputs 1 bit.
  Majority Rules.72.9 24.3 97.2 of time
  correct bit is output .1 2.7 2.8 of time
  incorrect bit is output
• Improved performance as No Coding _at_ thrice the
  bit rate

12
Final Exam (weighting)
Data Comm
HW
1
2
2-
3
2
1
1-
Readings
Lectures
13
(No Transcript)