Telecommunications Concepts

1
TelecommunicationsConcepts

• Chapter 1.4
• Communications
• Theory

2
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

3
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

4
Parallel Transmission
Disadvantages Differences in propagation
delay Cost of multiple communication channels
5
Serial Transmission
6
Serial Transmissionwith clock/data multiplexing
7
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

8
Synchronous Transmission
DTE
DTE
Data is carried by the clock signal
Rx clock extracted by DCE
DCE
DCE
Tx clock in DTE or DCE
Modem
Modem
9
Synchronous Transmission
10
Asynchronous Transmission
The DCEs just transmit data bits. Provisions
for Clock synchronization need to be included in
data
DTE
DTE
DCE
DCE
Modem
Modem
11
Start-stop synchronization
clock
Designed for electro-mechanical terminals Still
used in modern electronic terminals !
12
External PC modems
Most external PC modems use an asynchronous link
between the PC and the modem and a synchronous
link between the modems. The modem contains a
microcomputer that buffers the data
Asynchronous links(serial port or USB)
Synchronous link
13
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

14
Digital Data Communications
011001
TX
Analog communication channel
011001
RX
15
Encoding and Decodingdigital signals

• Transmitter (Tx)
• Input stream of binary numbers
• Output stream of analog signals suitable for
  transmission over long distances
• Receiver (Rx)
• Input stream of analog signals
• generated by transmitter
• distorted by transmission channel
• Compares each input signal with all signals which
  could have been transmitted and decides from
  which one the input is a distorted image.
• Output stream of binary numbers, preferably
  identical to the input of the transmitter

16
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

17
Analog Transmission Channel
Characterized by

• Bandwidth
• Difference between highest and lowest frequency
  of sine waves which can be transmitted
• Number of possible state changes per second
• Signal to Noise ratio
• S/N (signal power) / (noise power)
• S/N determines number of distinct states which
  can be distinguished within a given observation
  interval

Received power
Frequency
18
Binary vs. Multi-bit encoding
Noise margin /- 4 V
Noise margin /- 2 V
Modulation rate 1/t (in Baud) Data rate (1/t)
Log 2 n (in b/s)
19
Shannons Theorem
DataRate lt B.Log2(1S/N)
B Channel Bandwidth (in Hertz) S/N Signal to
Noise ratio
20
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

21
Eye Diagrams
1
0
1
t
Clock
The incoming waveforms are displayed on an
oscilloscope, synchronized by the recovered
clock
22
Multi-bit eye diagrams
Modern communication channels use phase and
amplitude shifts, best displayed in polar eye
diagrams
Good signal/noise ratio
Poor signal/noise ratio
23
Communications in degraded mode
Same baud rate Half bit/s rate
24
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

25
Error detection and correction
k r lt LMax k bits r bits, f(inf.mess.) 2 k 2
kr
Length of messages Informative
message Redundancy Messages send
Messages received Hamming Distance (X-Y)
i1
????Xi-Yi
kr
26
Error Detection ExampleBelgian Bank Account
Numbers

• Bank account number structure
• Bank identification 3 digits
• Account number 7 digits
• Error detection 2 digits
• The ten first digits modulo 97 are appended for
  error detection purposes.
• This algorithm allows detection of all single
  digit errors
• Example
• 140-0571659-08. 1400571659 MOD 97 08
• 140-0671659-08. 1400671659 MOD 97 01

27
Error detecting codes
k 1 r 1 red.bit inf.bit.
Single bit errors are detected if hamming
distance between legitimate messages gt 1. No
guessing is possible as erroneous messages are at
equal distances from several correct ones.
28
Error correcting codes
k 1 r 2 red.bits inf.bit.
Hamming distance between legitimate messages gt 2.
This implies that each erroneous message is
closer to one correct message than to any other.
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Error correcting codes
Required Overhead for single bit error
correction kr lt 2r
30
Error correction with a 43 bit code
2
0000000
1111100
1
0001011
1111111
3
4
0010110
1110100
4
3
3
0011101
1101001
4
6
1100010
0100111
0101100
1011000
4
0110001
1010011
7
1001110
0111010
1000101
The three redundant bits are a function of the
four data bits
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Error Correction

• Error detecting codes
• Correction by retransmission of erroneous blocks
• If few errors, very low overhead
• Most common approach to error correction in data
  communications
• Error correcting codes
• Very high overhead with short data blocks
• Longer data blocks can have multiple errors
• Used when retransmission impossible or
  impractical
• Also used when error rate rather high.
• Error correcting codes for long blocks, with
  multiple errors exist and are used (trellis
  encoding)

32
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

33
Error Correction byRetransmission
Time-out
1
2
3
4
4
Data
A
Ack
B
time
34
Error Correction byRetransmission
Inefficient unless round-trip delay ltlt
transmission time of a datablock
2
3
4
1
Data
A
Ack
B
time
35
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
6
7
8
Data
A
Ack
B
1
2
3
4
5
6
7
8
time
36
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
6
7
8
Data
A
Ack
B
time
37
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
6
7
8
Data
A
Ack
B
1
time
38
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
6
7
8
Data
A
Ack
B
1
2
time
39
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
6
7
8
Data
A
Ack
B
1
2
time
40
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
Time-out
6
7
8
1
2
3
4
5
Data
A
Ack
B
1
2
4
time
41
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
Time-out
6
7
8
1
2
3
4
5
Data
A
Ack
B
1
2
4
5
time
42
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
6
3
4
5
Data
A
Go Back n window management
Ack
B
1
2
4
5
time
43
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
6
3
4
5
Data
A
Ack
B
3
4
1
2
4
5
time
44
Error Correctionwith sliding window
Data blocks in sliding window can be
transmitted without waiting for an
acknowledgment. Receiving acknowledgments pushes
window forward.
1
2
3
4
5
8
3
6
7
Data
A
Buffering required in receiver
Ack
B
1
2
4
5
time
45
Contents

• Data transmission fundamentals
• Parallel vs. serial transmission
• Synchronous vs. asynchronous communications
• Analog vs. digital communications
• Shannons theorem
• Eye diagrams
• Transmission error correction
• Redundant encoding
• Sliding window error correction
• Encoding and modulation

46
Characterization of random signals
Autocorrelation function
Fourier Spectrum
Students with inadequate mathematical
background may skip this slide
47
Straight Binary Code

• Frequency spectrum
• Maximum at f 0
• important DC component due to voltage asymetry
• No energy at clock frequency
• Amplitude of maxima decreases as 1/f

48
Manchester Code
v
0
0
0
0
1
1
1
1
1
t

• Frequency spectrum
• Nothing at f 0
• High energy at clock frequency
• Amplitude of maxima decreases as 1/f

49
Asymptotic Behavior of Spectra
Both studied codes have energy spectra decreasing
as 1/f2 , meaning that the voltage or current
spectra decrease as 1/f.
This is a consequence of the instantaneous state
transitions
1
å
w
t
n
sin
n
,
5
,
3
,
1
50
Asymptotic Behavior of Spectra
The smoother the waveforms are, the lesser energy
will be found in the spectrum at higher
frequencies
In actual transmission systems, rounded
waveforms, such as parts of sine waves will be
used.
51
Modulation Techniques
52
Introduced concepts

• Parallel vs. Serial transmission systems
• Transmission channel
• characterized by bandwidth signal to noise
  ratio
• puts upper limit on the information throughput
• Error correction by using redundant coding of
  information
• with error correcting codes
• with error detecting codes and retransmission
• Throughput close to the upper limit requires
  specific coding of the information
  (modulation/demodulation)

53
BibliographyTo know More about Communication
Theory

• I.A.Glover
• P.M.Grant
• Digital Communications,
• Prentice Hall 1998
• ISBN 0 - 13 - 565391 - 6

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