Digital Transmission

Digital Transmission 1

Line Coding

Some Characteristics Line Coding Schemes Some Other Schemes 2

Line coding

3

DC component

4

Lack of synchronization

5

Line coding schemes

6

Note: Unipolar encoding uses only one voltage level.

7

Unipolar encoding

8

Note: Polar encoding uses two voltage levels (positive and negative).

9

Types of polar encoding

10

Note: A good encoded digital signal must contain a provision for synchronization.

11

Manchester encoding

12

Note: In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation.

13

Differential Manchester encoding

14

Note: In differential Manchester encoding, the transition at the middle of the bit is used only for synchronization. The bit representation is defined by the inversion or noninversion at the beginning of the bit. 15

Transmission Media

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©The McGraw-Hill Companies, Inc., 2000

Transmission medium and physical layer

17

Classes of transmission media

18

Guided Media Twisted-Pair Cable Coaxial Cable Fiber-Optic Cable 19

Twisted Pair 



Consists of two insulated copper wires: one for carrying signal the other for ground reference Twisted together to decrease the crosstalk interference between adjacent pairs in a cable

20

Unshielded versus Shielded Twisted-Pair Cable 

Unshielded Twisted-Pair (UTP)  



Most commonly used Cheaper than STP

Shielded Twisted-Pair Cable (STP) 

Has a metal foil or braided-mesh covering that encases each pair of insulated conductors



Bulkier and more expensive

21

Categories of unshielded twisted-pair cables Category

Bandwidth

Data Rate

Digital/Analog

Use

1

very low

< 100 kbps

Analog

Telephone

2

< 2 MHz

2 Mbps

Analog/digital

T-1 lines

3

16 MHz

10 Mbps

Digital

LANs

4

20 MHz

20 Mbps

Digital

LANs

5

100 MHz

100 Mbps

Digital

LANs

6 (draft)

200 MHz

200 Mbps

Digital

LANs

7 (draft)

600 MHz

600 Mbps

Digital

LANs

22

UTP connector

23

UTP performance

24

TWISTED PAIR – Transmission Characteristics 

Analog 



Digital  

   

Amplifiers every 5km to 6km Use either analog or digital signals repeater every 2km or 3km

Limited distance Limited bandwidth (1MHz) Limited data rate (100Mbps) Susceptible to interference and noise 25

TWISTED PAIR - Applications

 

Most common medium Often used in buildings for LAN and PBX station connections 



For local area networks (LAN) 



Also used in telco outside plant (local loops) 10Mbps or 100Mbps

Can carry both voice and data

26

Telephone channel bandwidth

27

Coaxial Cable

28

BNC connectors

•Terminator is used to prevent the reflection

29

Coaxial cable performance

30

Transmission Characteristics 

Analog   



Digital  



Amplifiers every few km Closer if higher frequency Up to 500MHz Repeater every 1km Closer for higher data rates

Less susceptible to interference and crosstalk than twisted pair 31

Application

 

Most versatile medium Television distribution  



Long distance telephone transmission  

 

Ariel to TV Cable TV Can carry 10,000 voice calls simultaneously Being replaced by fiber optic

Short distance computer systems links Local area networks

32

Bending of light ray

33

Optical fiber

34

Fiber construction

35

Propagation modes

36

Modes

37

Fiber types Type

Core

Cladding

Mode

50/125

50

125

Multimode, graded-index

62.5/125

62.5

125

Multimode, graded-index

100/125

100

125

Multimode, graded-index

7

125

Single-mode

7/125

38

Fiber-optic cable connectors

39

Optical fiber performance

40

Advantages and disadvantages 

Advantages:      



Higher bandwidth Less signal attenuation Less interference Resistance to corrosive materials Light weight More immune to tapping

Disadvantages 

Expensive to install 41

Application

     

Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs Other  

Backbone networks TV distribution

42

Guided transmission media summary Type

Advantage

Disadvantage

Twisted Pair Wire

Very inexpensive Easy to install Already installed in many locations

Doesn’t pass high frequencies well

Coaxial cable

Shielded Fairly inexpensive Moderately high bandwidth

Bulky and somewhat inflexible

Fiber optic cable

Transmission Expensive to install unaffected by noise Very high bandwidth Great repeater spacing 43

The Data Link Layer

Data Link Layer Design Issues •

• • •

Services Provided to the Network Layer Framing Error Control Flow Control

45

Functions of the Data Link Layer •

• •

Provide service interface to the network layer Dealing with transmission errors Regulating data flow •

Slow receivers not swamped by fast senders

46

Functions of the Data Link Layer (2) Relationship between packets and frames.

47

Services Provided to Network Layer

48

Services Provided to Network Layer (2) Placement of the data link protocol.

49

Framing A character stream. (a) Without errors. (b) With one error.

50

Framing (2) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

51

Framing (3) Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing.

52

Error Detection and Correction Error-Correcting Codes • Error-Detecting Codes •

53

CRC generator and checker

54

Binary division in a CRC generator

55

Binary division in CRC checker

56

10.10

A polynomial

57

10.11

A polynomial representing a divisor

58

Standard polynomials Name

Polynomial

Application

CRC-8

x8 + x2 + x + 1

ATM header

CRC-10

x10 + x9 + x5 + x4 + x 2 + 1

ATM AAL

ITU-16

x16 + x12 + x5 + 1

HDLC

ITU-32

x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1

LANs

59

Example It is obvious that we cannot choose x (binary 10) or x2 + x (binary 110) as the polynomial because both are divisible by x. However, we can choose x + 1 (binary 11) because it is not divisible by x, but is divisible by x + 1. We can also choose x2 + 1 (binary 101) because it is divisible by x + 1 (binary division).

60

Example The CRC-12

x12 + x11 + x3 + x + 1 which has a degree of 12, will detect all burst errors affecting an odd number of bits, will detect all burst errors with a length less than or equal to 12, and will detect, 99.97 percent of the time, burst errors with a length of 12 or more.

61

Checksum

62

Data unit and checksum

63

Note: The sender follows these steps: •The unit is divided into k sections, each of n bits. •All sections are added using one’s complement to get the sum. •The sum is complemented and becomes the checksum. •The checksum is sent with the data. 64

Note: The receiver follows these steps: •The unit is divided into k sections, each of n bits. •All sections are added using one’s complement to get the sum. •The sum is complemented. •If the result is zero, the data are accepted: otherwise, rejected. 65

Example Suppose the following block of 16 bits is to be sent using a checksum of 8 bits. 10101001 00111001 The numbers are added using one’s complement 10101001

Sum

00111001 -----------11100010

Checksum

00011101

The pattern sent is

10101001 00111001 00011101 66

Example Now suppose the receiver receives the pattern sent in previous Example and there is no error. 10101001 00111001 00011101 When the receiver adds the three sections, it will get all 1s, which, after complementing, is all 0s and shows that there is no error. 10101001 00111001 00011101 Sum

11111111

Complement

00000000 means that the pattern is OK. 67

Example Now suppose there is a burst error of length 5 that affects 4 bits. 10101111 11111001 00011101 When the receiver adds the three sections, it gets 10101111 11111001 00011101 Partial Sum Carry

1 11000101 1

Sum

11000110

Complement

00111001

the pattern is corrupted.

68

Correction

Retransmission Forward Error Correction Burst Error Correction 69

Data and redundancy bits Number of data bits m

Number of redundancy bits r

Total bits m+r

1

2

3

2

3

5

3

3

6

4

3

7

5

4

9

6

4

10

7

4

11

70

Positions of redundancy bits in Hamming code

71

Redundancy bits calculation

72

Example of redundancy bit calculation

73

Error detection using Hamming code

74

Burst error correction example

75