topic 1 basic concept of data communication

TOPIC 1:

BASIC CONCEPT OF DATA COMMUNICATION

DEPARTMENT OF ELECTRICAL

ENGINEERING

1

At the end of the topic, student should be able

to explain the:

Importance of data communication.

Application of communication codes.

Basic data communication system.

Data encoding.

Data transmitting.

Error encoding.

2

Data Communication

Main purpose of an electronic communications system is to transfer information from one place to another.

Electronic communications can be viewed as the transmission, reception and processing of information

between two or more locations using electronic

circuit/device.

Basic communication models shows the communication flows between 2 points.

data : number, alphabet or symbol processed by computer.(raw facts before processing).

computer data: binary digit (0s and 1s binary). information: data, voice, image, character and code has been processed in a form that can be use and understand by

receiver.

code : message that can be read and has a meaning that can be understood by the end user (machine or human).

3

Exchangeable of digital data

coding between two devices via some form of transmission

medium.

Definition of Data Communication

The system consists of

group up the data, processing

the data and transmit the data using a specified communication

channel

Data Communication

4

IMPORTANCES OF DATA COMMUNICATION:

1) Electronic communication: - email, video teleconferencing, etc. 2) Internet access: - email, chat, download 3) ATM card: - money draw from bank that has link 4) Shopping privilege: - order through television or radio 5) Public access

- Jabatan Pendaftaran Negara, Jabatan Pengangkutan Jalan & many others.

5

ADVANTAGES OF DATA COMMUNICATION: 1) Safety: digital system is much safer because it can be

encode to a code that is only knew by the sender and receiver.

2) Small error: digital system has smaller error compare to analogue

system. 3) Low cost: digital system has low cost compares to analogue

system, for example in a process of frequency division.

4) Small interruption: interruption did not affect data transmission because

digital data can be regenerate at each repeater station. 5) Easy to interface: digital circuit is easier to interface because data digital

only consists of two levels, which are 1 and 0 bit.

6

The evolution of telecommunication technologies with the development of computer = Data

Communication.

HISTORY OF DATA COMMUNICATION

7

In 1837, Samuel Morse's invention of the telegraph began the history of data communication.

In 1876, Alexander Graham Bell improved the telegraph with the introduction of the telephone.

1910:Howard Krum developed Start/Stop Synchronization. 1930: Development of ASCII Transmission Code

HISTORY OF DATA COMMUNICATION cont.

The first generation of computers started in 1940s to be used for World War II.

1945: Allied Governments develop the First Large Computer.

1950: IBM releases its first computer IBM 710.

1960: IBM releases the First Commercial Computer IBM 360.

In 1970s mainframe computes were used and people connected to it with unintelligent terminals.

This was the first kind of computer network and several persons could use the computer simultaneously.

8

When the computer became cheaper and smaller people tried to maintain large amounts of data in one computer.

Then the database management concept immerged.

One high-end computer called a server was used to maintain the database and others could connect to the server from their PCs.

HISTORY OF DATA COMMUNICATION cont.

Electronic Mail (e-mail or Email) replaces snail mail. E-mail is the forwarding of electronic files to an electronic post office for the recipient to pick up.

Scheduling Programs allow people across the network to schedule appointments directly by calling up their fellow worker's schedule and selecting a time.

Videotext is the capability of having a two-way transmission of picture and sound. Games like Red Alert, distance education lectures, etc. use video text.

9

1) E-mail: send and receive mail by electronically. 2) Teleconferencing: attend meeting or discussion without

present to the real location. 3) Fax: send or receive fax. 4) Banking: involving transfer of finance data, especially

which consumes the ATM machine. 5) Internet: surf internet to get information and others. 6) Electronic government: including many government sectors, for

example JPN, JPJ and many others.

APPPLICATION OF DATA COMMUNICATION

10

Communication Codes

Definition coding is a representative set of symbols using

that used before being processed.

Coding is rule for converting a piece of information into another form or representation in order to make it the system could read the

information.

COMMUNICATION

CODES

1.Morse code

2.Boudot code 3.EBCDIC

code

4.ASCII code

11

Morse Code

Morse code consist dot (.) and dashes (-)

Dot (.) short beep = 1 unit time base.

Dash (-) long beep = 3 unit time base.

The space between dot and dash = 1 unit time base

The space between letters = 3 unit times

Communication Codes cont...

The earliest code established.

The simplest code, just transmit characters for telegraphic process.

Is a method of transmitting textual information of on-off

tones,light or click.

1890 began extensively use for early radio communication

before it was possible to transmit voice.

12

Morse

Code

Table


13


14

Exercise:

1. Convert English code back into Morse and how many times required for data transmission of

SWEET 17.

2. Convert Morse code back into English


15


16


17

Baudot code

The first code is created for computer. Using the number 0 and 1 to represent the character. Each character contains 5 bits.

1875 Thomas Murray named the code after Emily Baudot

First fixed-length character code develop for machines.

1.2 .1 Communication Codes cont...

18

Example:

Convert English code into Baudot code for;

2 BUS

Solution:

Shift to upper case column 2 : 11011 10011

Space : 00100

Shift to lower case column B : 11111 11001

Shift to lower case column U : 11111 00111

Shift to lower case column S : 11111 00101


20

21

Exercise:

Translate those characters using Boudot code;

DIS 13


22

Solution:

Shift to upper case column : 11011 10001

Shift to lower case column D : 11111 01001

Shift to lower case column I : 11111 00110

Shift to lower case column S : 11111 00101

Space : 00100

Shift to lower case column 1 : 11011 10111

Shift to lower case column 3 : 11011 00001

Shift to upper case column : 11011 10001


Extended Binary coded Decimal Characters Information.

8 bit characters created by IBM.

there are 256 different combination.

Often used in IBM.

EBCDIC Code


23

Example:

Translate those characters using EBCDIC code;

a) B

b) 5

Solution:

a) B: 1100 0010

b) 5: 1111 0101


25

Exercise


a) A

b) 7

c) a

d) SYN


26

Solution:


a) A : 1100 0001

b) 7 : 1111 0111

c) a : 1000 0001

d) SYN : 0011 0010


27

ASCII was established to achieve compatibility between various types of data processing equipment.

The standard ASCII character set consists of 128 decimal numbers ranging from zero through 127 assigned to letters, numbers, punctuation marks, and the most common special characters.

American Standard Character Information Interchange

Consists of 7 bit character.

Has 128 different character combination.

ASCII Code


28

29

ASCII Code Table

Example:

Translate those characters using ASCII code;

a) 7

b) SYN

Solution:

Translate those characters using ASCII code;

a) 7 : 0110111

b) SYN : 0010110


30

Exercise

Translate those phrases using ASCII code of

1 SeRaH


31

Solution:

Translate those phrases using ASCII code of

1 SeRaH

: 010 0010

1 : 011 0001

S : 101 0011

E : 110 0101

R : 101 0010

A : 110 0001

H : 100 1000

: 010 0010


32

APPLICATION OF COMMUNICATION CODES

amateur radio operators

identification of navigational radio beacon and

land mobile transmitters, plus some military

communication, including flashing-light

semaphore communications between ships in

some naval services

Morse

code

Application for low speed teletype equipment

such as TWX/Telex system, radio teletype.

used extensively in telegraph systems

Boudot

code

used mainly on IBM mainframe and IBM

midrange computer operating systems.

used for data communication, processing and

storage

EBCDIC

code

33

most popular code for serial data

communications today

used as the data code for keyboards in

computers

ASCII code

APPLICATION OF COMMUNICATION CODES cont.

34

35

BASIC DATA COMMUNICATION SYSTEM

Terminal Modem

Telecommunication

Network Modem Terminal

DTE DTE DCE DCE

36

BASIC DATA COMMUNICATION SYSTEM cont.

Transmitter /source/ sender

A part of system where the information signal is being produce, process and transmit.

The device that sends the data message.

Example of sender: computer, workstation,

telephone handset, video camera and so on.

Example of information signal;

Audio signal Video signal

Printed signal Coding signal.

Transmission Medium

The physical path by which a message travels from sender to receiver.

Example of medium transmission;

Coaxial cable Twisted pair cable

Fiber optic Copper cable

Microwave

37

Repeater

A device to regenerate

back the signal.

Receiver /sink

The device that receives the message.

a device to detect electrical signal and translate back to the original signal.

BASIC DATA COMMUNICATION SYSTEM cont.

Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)

DTE

A subscriber equipment or users device for data communications.

Consists of a source of data or receiving data or both.

These tools may include an error control, synchronization and identification capabilities of the station.

Examples of DTE is the computers, logical control, visual display units and work station.

DCE

Provided by authorities or by client communication network itself.

DCE is capable of implementing, operating and terminate a data communication, exchanging signals and coding needed to make the relationship between the DTE and data circuits.

Internal or external parts of a computer.

Example: a modem or data set.

38

Information Capacity, Bits, Bit Rate and Baud

39

Information Capacity (I), unit: bps

Information capacity is a measure of how much information

can be propagated through a communications system and is

a function of bandwidth and transmission time.

Information capacity represents the number of independent

symbol that can be carried through a system in a given unit of

time. Usually expressed as a bit rate.

BIT and BIT RATE

40

Bit :

A Bit is a digit in the binary number system. It can have two values, 0 or 1 (basic digital symbol)

Bit Rate :

The number of bits transmitted in one second and expressed in bits per second (bps).

The rate of change of a digital signal which usually binary.

Sometimes is written bit rate or data rate.

Information Capacity, Bits, Bit Rate and Baud cont....

41

Baud Rate :

The number of symbols transmitted during one second and is

expressed in symbols per second.

The rate of change of a signal on the transmission medium

after encoding and modulation have occurred.

Sometimes is written transmission rate, modulation rate or

symbol rate.

Bandwidth (BW), unit: Hz

(1) the range of frequencies contained in a composite

signal of frequency spectrum.

(2) the difference between the highest and lowest

frequencies contained in the information.

Indicates the capacity of data.

BAUD RATE and BANDWIDTH


Bit Rate vs Baud Rate

Bit rate Baud rate

Is the number of bits per second.

Is the number of signal units per second that are require to represent those bit

42


43

Bit rate and baud rate are not always the same. The bit rate is the

number of bits transmitted per second, whereas, the baud rate is the number

of signal units transmitted per second. Therefore, baud rate is always less

than or equal to the bit rate but never greater.

Example:

What is the bit rate and baud rate for an analogue signal that carries 3

bits in each signal unit if 2000 signal units are sent per second?

Solution:

Baud rate = 2000 baud per second

Bit rate = 2000 x 3 = 6000 bps

What is the baud rate for an analogue signal if the bit rate of the

signal is 2000 and each signal unit carries 4 bits?

Solution:

Baud rate = 2000 / 4 = 500 baud


44

Exercise:

a) An analog signal carries 4 bits in each signal unit. If

1000 signal units are sent per second, find the baud rate

and the bit rate.

b) The bit rate of a signal is 3000. If each signal unit carries

6 bits, what is the baud rate?


45

Solution:

a) Baud rate = 1000 bauds per second (baud/s)

Bit rate = 1000 x 4 = 4000 bps

b) Baud rate = 3000 / 6 = 500 baud/s


46

Based on this law, the information capacity of any communication

channel is related to its bandwidth and the signal-to-noise ratio.

The higher the signal-to-noise ratio, the better the performance and

the higher the information capacity.

Mathematically stated, the Shannon limit for information capacity is;

N

S B .I

or

N

SB I

1log323

1log

10

2 where;

I = information capacity (bits per second)

B = bandwidth (Hz)

S/N = signal to noise power ratio (unitless)

Shannons Limit


47

EXAMPLE:

For a standard telephone circuit with a signal-to-noise power ratio of 1000W (30dB) and a bandwidth of 2.7kHz, the Shannon limit for information capacity is,

kbps.I

))(.(I

N

S B .I

926

10001log2700323

1log323

10

10


48

Encoding Techniques

1. Digital data Digital signal.

2. Digital data Analog signal.

3. Analog data Digital signal.

4. Analog data Analog signal.

DATA ENCODING

Four possible combinations :

Digital data-to-digital signal:

Reason: equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to-analog conversion.

49

Digital data-to- analog signal:

Reason: Some transmission media, such as optical fiber and the unguided media, will only propagate analog signals.

Analog data-to- digital signal:

Reason: Conversion of analog data to digital form permits the use of modern digital transmission and switching equipment.

Analog data-to-analog signal:

Reason: Analog data in electrical form can be transmitted as baseband signals easily and cheaply.

DATA ENCODING cont

50

There are several ways for encoding digital data to digital signals:

DATA

ENCODING

Non-return to Zero (NRZ)

Return to Zero (RZ)

Manchester

High Density Bipolar 3 Zero (HDB3)

AMI (Alternate Mark Inversion)

51

Non Return to Zero (NRZ)

Traditionally, a unipolar scheme was design as a NRZ: 0 = Low voltage level (0V)

1 = High voltage level (+V volts)

Unipolar

Digital to Digital Encoding

52

Non Return to Zero (NRZ) cont

Another scheme is Polar. Non Return to Zero Level (NRZ-L)

0 = Low voltage level (+V volts)

1 = High voltage level (-V volts)

Non Return to Zero Invert (NRZ-I) 0 = No changes voltage level

1 = Changes voltage level

Digital to Digital Encoding cont.....

53

Non Return to Zero (NRZ) cont

The main problem with NRZ encoding occurs when the sender and receiver clocks are not synchronized.

The receiver does not know when one bit has ended

and the next bit is starting.

One solution is return-to-zero (RZ) scheme.

Digital to Digital Encoding cont.....

54

Return to Zero (RZ)

RZ uses there value: positive, negative and zero. The signal changes not between bits but during the bit.

0 = Transition from high to low in the middle of a bit (-ve in 1st half and 0 in 2nd half).

1= Transition from low to high in the middle of a bit (+ve in 1st half and 0 in 2nd half).

Digital to Digital Encoding cont...

55

Return to Zero (RZ) cont

The problem of RZ are:

That it requires two signal changes to encode a bit. A sudden change of polarity resulting in all 0s interpreted as 1s and all 1s interpreted as 0s but no DC component problem.

Use three level of voltage which is more complex to create and discern.

RZ has been replaced by better performing Manchester and Differential Manchester schemes.


56

Also known as Biphase Encoding.

The duration of the bit is divided into two halves. The voltage remains at one level during the first half and moves to the other level during the second half.

0 = Transition from high to low in the middle of a bit (-ve in 1st half and +ve in 2nd half).

1= Transition from low to high in the middle of a bit (+ve in 1st half and -ve in 2nd half).

Since both 0 and 1 have mid-bit transitions, there is less/ no DC content. More importantly, this code is self-clocking (or self-synchronizing) code since a receiver can extract the clock information from the incoming codes by looking at the ever-present middle transitions.

Manchester Code


57

1 0 1 0 1 1 1 1 1 0

Note: There is always a transition at the

centre of bit duration.

Manchester Code cont

Digital to Digital

Encoding cont...

58

Bipolar AMI (Alternate Mark Inversion)

0 = Neural zero (0 volts) 1 = Alternate Positive (+) and Negative (-) voltages for successive

1s

This code is used in long distance. This code reduces/ no the DC(Direct Current) content from the line; the

1s will have positive voltage followed by negative voltage, in other words, the voltages go up and down.

This code has a problem. A long stream of 0s can cause a receiver to go out of synchronization (lose the bit boundaries) since 0s have no voltage.

The commonly used cures are B8ZS and HDB3.


59

Bipolar AMI(Alternate Mark Inversion) cont


60

Commonly used outside of North America. The HDB3 code is a bipolar signaling technique (i.e. relies on the transmission of both positive and negative pulses).

Four consecutive zero-level voltages are replaced with a sequence of 000V or B00V.

The reason for two different substitutions is to maintain the even number of nonzero pulses after each substitution.

The two rules states as follows: a) If the number of nonzero pulses after the last substitution is

odd, the substitution pattern will be 000V, which makes the

total number of nonzero pulses even.

b) If the number of nonzero pulses after the last substitution is

even, the substitution pattern will be B00V, which makes the

total number of nonzero pulses even.

High Density Bipolar Order 3 Encoding (HDB3)


61

High Density Bipolar Order 3 Encoding (HDB3) cont


62

high density bipolar of order 3 (HDB3) code replaces any instance of 4 consecutive 0 bits with one of the

patterns "000V" or "B00V".

Number of Bipolar

Pulses (Bit 1) since Last

Substitution

Polarity of preceding pulse

Odd Even

- 000- +00+

+ 000+ -00-

Example:

The pattern of bits

1 1 0 0 0 0 1 1 0 0 0 0 0 0

+ - 0 0 0 0+ - 0 0 0 0 0 0 (AMI)

Encoded in HDB3 is:

+ - B 0 0 V - + B 0 0 V 0 0,

which is:

+ - +0 0 + - + - 0 0 - 0 0



63

Exercise:

Encoded the data below to AMI and HDB3:

a) " 1 0 0 0 0 1 1 0 "

b) " 1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 "



64

High Density Bipolar Order 3 Encoding (HDB3) cont Solution:

a) The pattern of bits

" 1 0 0 0 0 1 1 0 "

the corresponding encoding using AMI is;

" + 0 0 0 0 - + 0 "

encoded in HDB3 is:

" + 0 0 0 V - + 0 "

b) The pattern of bits

" 1 0 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0 0 0 0 0 0 "

the corresponding encoding using AMI is:

" + 0 - 0 0 0 0 0 + - 0 0 0 0 - + 0 0 0 0 0 0 "

encoded in HDB3 is:

" + 0 - 0 0 0 V 0 + - B 0 0 V - + B 0 0 V 0 0 "

which is:

" + 0 - 0 0 0 - 0 + - + 0 0 + - + - 0 0 - 0 0 "


65

NRZ Non Return to Zero

(0 0, 1 +ve )

NRZ Non Return to Zero

(0 -ve, 1 +ve )

NZ Non Return to Zero

(0-ve in 1st half and 0 in 2nd half, 1 +ve in 1st half and 0 in 2nd half)

Manchester

(0-ve in 1st half and +ve in 2nd half, 1 +ve in 1st half and -ve in 2nd half)

DATA ENCODING WAVEFORM (NRZ, RZ and Manchester)


66

Information Signal is in digital waveform. While Carrier signal is in

analog waveform.

There are four basic technique for digital modulation .

Digital to Analogue Encoding

67

Amplitude Shift Keying (ASK) - the amplitude (V) of the carrier is

varied proportional to the information signal.

Frequency Shift Keying (FSK) - the frequency (f) of the carrier is

varied proportional to the information signal.

Phase Shift Keying (PSK) - the phase () of the carrier is varied proportional to the information signal.

Quadrature Amplitude Modulation (QAM) - both amplitude (V)

and phase () are varied proportional to the information signal.

Digital to Analogue Encoding cont....

68

Digital to Analogue Encoding cont...

69

Amplitude Shift Keying (ASK)

ASK is simplest digital modulation techniques.

ASK is a process where the binary information signal directly

modulates the amplitude of an analog carrier.

The carrier is transmitted when the modulating data is one and the carrier is rejected from transmission when the data is zero

When the data is bit 1, the carrier signal has the amplitude, when the data is bit 0, the amplitude of carrier signal is 0.

Digital to Analogue Encoding cont...

70


Frequency Shift Keying (FSK)

In FSK, the frequency of the carrier signal is varied to represent data. Amplitude and phase of the carrier signal remain the same.

As the binary input signal changes from a logic 0 to a logic 1 and vice versa, the output frequency shifts between two frequencies: a mark, or logic 1 : high frequency (fm) and a space, or logic 0 : low frequency (fs).

mark (fm) = logic 1 frequency space (fs) = logic 0 frequency

71

Phase Shift Keying (PSK)

The phase of the carrier is varied to represent two or more different signal elements. Both peak amplitude and frequency remain constant.

The input is a binary digital signal and there are a limited number of output phase possible.

The input binary information is encoded into groups of bits before modulating the carrier.

The simplest form of PSK is binary shift keying (BPSK), which have only 2 signal elements ; (phase of 0 & phase of 180). These two phases will represent a logic 1 and logic 0.

As the input digital signal changes (i.e. changes from a 1 to a 0 or from a 0 to a 1, the phase of the output carrier shifts between two angles that are separated by 180).


72

Changes from a 1 to a 0 or from a 0 to a 1:

The phase of the output carrier shifts between

two angles that are separated by 180

Phase Shift Keying (PSK) cont


73

Modulation has been defined as the process of combining an input signal m (t) and a carrier frequency fc to produce a signal s (t) whose bandwith is usually centered on fc.

Ex. Voice is represented by electromagnetic signal with same frequency components and transmitted on voice grade line

Can also produce a new analog signal at higher frequency.

Analogue to Analogue Encoding

74

Techniques used to modulate include

AM Amplitude Modulation

FM Frequency Modulation

PM Phase Modulation

Analogue to Analogue Encoding cont...

75

AM also as ASK, means changing the height of the wave to encode data.

Figure shows a simple case of amplitude modulation in which one bit is encoded for each carrier wave change.

The frequency and phase of the carrier remain the same, only the amplitude changes.

Amplitude Modulation (AM)

A high amplitude means

a bit value of 1.

Zero amplitude means

a bit value of 0.


76

Sending Multiple Bits Symbol

Each modification of the carrier wave to encode information is called a symbol.

By using a more complicated information coding system, it is possible to encode more than 1 bit/symbol.

Figure (b) gives an example of amplitude modulation using 4 amplitude levels, corresponding to 2 bits/symbol.

Increasing the possible number of symbols from 4 to 8 corresponds with encoding 3 bits/symbol, 16 levels to 4 bits, and so on.

Fig (b) : Two-bit amplitude

modulation

Amplitude Modulation (AM) cont


77

FM as FSK, means changing the frequency of the carrier wave to encode data.

The peak amplitude and phase of the signal remain constant.

Figure (c) shows a simple case of frequency modulation in which one bit is encoded for each carrier wave change.

Frequency Modulation (FM),

Changing the carrier wave to a higher frequency encodes

a bit value of 1.

No change in the carrier wave

frequency means a bit value

of 0.


78

PM as PSK means changing the carrier waves phase to carry data.

Figure (d) shows a simple case of phase modulation in which one bit is encoded for each carrier wave change. A 180o phase shift corresponds to change of bit either

from 1 to 0 or from 0 to 1. The normal carrier wave would follow the broken line, but instead the phase suddenly shifts and heads off in another direction.

No phase shift means the bit value remain the same. Two bits per symbol could be encoded using phase

modulation using 4 phase shifts such as 0o, 90o, 180o and 270o.

Phase Modulation (PM)


79

A digital signal is superior to an analog signal.

The tendency today is to change an analog signal to digital data.

In this section we describe pulse code modulation techniques.

Pulse Code Modulation (PCM)

The PCM is a technique to convert the analog signal to digital signal.

PCM also essentially analog-to-digital conversion of a special type where the information contained in the instantaneous samples of an

analog signal is represented by digital words in a serial bit stream.

PCM consists of three steps to digitize an analog signal: i. Sampling ii. Quantization iii. Encoding

Analogue to Digital Encoding

80

The Sampling process is sometimes referred to as a flat-top pulse amplitude modulation signal (PAM)

Sampling

The analog signal is sampled every Ts s, discrete in time.

Quantization

Makes the signal discrete in amplitude.

Encode

Maps the quantized values to digital words that are bits long.


Sampling

Pulse Code Modulation (PCM) cont...

81

Three different sampling methods for PCM



82


Components of a PCM decoder


83



84

In binary coding:

Data bit 1 has waveform 1

Data bit 0 has waveform 2

Data rate = bit rate = symbol rate

In M-ary coding, take M bits at a time (M = 2k) and create a waveform (or symbol).

00 waveform (symbol) 1




Symbol rate = bit rate/k

M-ary Coding

85

M-ary is a term derived from the word binary.

M = represents a digit that corresponds to the number of conditions or levels or combinations possible for a given number of binary variables (n).

For example, a digital signal with 4 possible conditions (either voltage, levels, frequencies, phases and so on) is an M-ary system where M = 4.

The number of bits that necessary to produce a given number of conditions (M) is expressed mathematically as;

Where; n = number of bits

M = number of conditions, levels or combinations possible with n bits

Mn 2log

M-ary Coding cont

86

Equation above can be simplified and rearranged to express the number of conditions possible, M with n bits.

For example, with n = 1 bit, only 21 = 2 conditions are possible. With two bits, 22 = 4 conditions are possible. With three bits, 23 = 8 conditions are possible, and so on.

Mn 2

M-ary Signaling

M-ary Coding cont

87

Advantages:

Required transmission rate is low (bit rate/M).

Low bandwidth.

Disadvantages:

Low signal to noise ratio (due to multiple amplitude pulses).

M-ary Coding cont

88

Quadrature Phase Shift Keying

(QPSK)

A phase modulation technique that transmits two bits in four

modulation states.

00 phase 0 10 phase 180 01 - phase 90 11 phase 270


89

QPSK is a form of PSK in which two bits are modulated at once, selecting one of four possible carrier phase shifts

0,90,180,270 or 0, 45, 135, .

QPSK perform by changing the phase of the In-phase (I) carrier from 0 to 180 and the Quadrature-phase (Q) carrier between 90 and 270.

PSK/QPSK

QPSK digital data is represented by 4 points of a 2-bit

binary code around a circle

which correspond to 4 phases

of the carrier signal. These

points are called symbols.


90

PSK/QPSK cont


91

Constellation diagram

example for BPSK.

Constellation diagram

for QPSK with Gray

coding. Each adjacent

symbol only differs by

one bit.

PSK/QPSK cont Digital to Analogue Encoding cont....

92

The 4-QAM (Quadrature Amplitude Modulation)

QAM is a combination of ASK and PSK so that a maximum contrast between each signal unit (bit, dibit, tribit, and so on) is achieved.

QAM technique that widely used to transmit digital signals such as digital cable TV and cable Internet service, QAM also used as the modulation technique in orthogonal frequency division multiplexing .

The "quadrature" comes from the fact that the phase modulation states are 90 degrees apart from each other.


93

The QAM cont


94

4 QAM and 8 -QAM constellations The QAM cont


95

Comparison between QAM and QPSK

QAM QPSK

Have amplitude levels Using phases for representation of messages.

Depending on type. i.e 16-QAM,64-QAM,256-QAM. How many amplitude levels to be used accordingly i.e 16,64,256

2 bits per symbol is used with four different phases.


96

Execise:

a) Compute the bit rate for a 1000-baud 16-QAM signal.

b) Compute the baud rate for a 72,000-bps 64-QAM signal.

The QAM cont


97

Solution:

a) A 16-QAM signal has 4 bits per signal unit since

log216 = 4.

Thus,

(1000)(4) = 4000 bps

b) A 64-QAM signal has 6 bits per signal unit since

log2 64 = 6.

Thus,

72000 / 6 = 12,000 baud

The QAM cont


98

Data can be received appropriately without any error.

For example, if the sender sends data at the rate of 100Mbps speed, but the receiver can only process data at a rate of 1Mbps, the data transmission will overload and most of the transmitted data will be lost.

The importance of timing and framing

Data Transmitting

99

The data needs to pack bits into frames, so that each frame is

distinguishable from another.

A frame in a character-oriented protocol

The importance of timing and framing cont

FRAMING (KERANGKA)

Form a character or a complete block of characters that is sent in each transmission.

The process of inserting additional bits along with the actual data.

Bits may represent a station codes, error detection, start/stop control bit either in front or end of the data, or both.

10

0

FRAME STRUCTURE

HEADER - consists of physical address (physical

address) sender and receiver. - 48-bit address (6 bytes). - consists of the codes related to the

station, start bit, SYN character , STX and others. DATA - data

TRAILER - consists of error detection (Frame Check

Sequence), which includes error detection and correction, ETX character and others.


10

1

Signal timing repetition signal (clock) used to control timing operations.

Sender and receiver - should have the same timing bit so that sampling process can be done appropriately (preferably in the middle of the bit period) to determine exactly the level of the data , either bit 0 or bit 1.

TIMING (PEMASAAN)


10

2

Data Transmitting

10

3

In this all the bits of a byte are transmitted simultaneously on separate wires.

Suitable for transmission over short distance. e.g.- Computer to Printer, Communication within the Computer

Parallel and Serial Transmission

Parallel Transmission

Data Transmitting

10

4

Parallel and Serial Transmission cont

Serial Transmission

Bits are transmitted one after the other

Usually the Least Significant Bit (LSB) has

been transmitted first

Suitable for Transmission over Long distance.

Data Transmitting cont

105

Asynchronous and Synchronous Transmission

Timing problems require a mechanism to synchronize the transmitter and receiver:

timing (rate, duration, spacing) of the data bits must be the same at transmitter & receiver

receiver samples stream of data bits at bit intervals.

if clocks not aligned and drifting, the receiver will sample at wrong time after sufficient bits are sent.

Example: for 1Mbps data stream, one bit will be transmitted every 1s. With 1% clock drift at the receiver (faster or slower than transmitter), then wrong sampling will occur after 50 bit (50*0.01s=0.5 s).

Two solutions to synchronizing clocks:

asynchronous transmission

synchronous transmission


106

Avoid timing problem by not sending long stream of bits.

Data is transmitted one character at a time, where each character is five or eight bits in length

Receiver can synchronize at the beginning of each new character

idle state: no transmission,

NRZ-L signalling is common for asynchronous transmission.

The beginning of the character is signalled by a start bit .

This is followed by a character of 5 or 8 bits long as data.

The bits of the character are transmitted beginning with the least significant bit

A parity bit is then added for the purpose of error detection.

The end of the character is a stop bit element.

Asynchronous Transmission


107

Asynchronous Transmission cont

Data Transmitting cont.

10

8

We send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the

end of each byte.

There may be a gap between each byte.

It is asynchronous at the byte level, bits are still synchronized; their durations are the same.



109

Example: The figure below shows the effects of a timing error of sufficient magnitude to cause error in reception. In this example, we assume a data

rate of 10Kbps; therefore each bit is 100s duration. Assume that the receiver is fast by 6%, or 6s per bit time. Thus, the receiver samples the incoming character every 94s. As we can see, the last sample is erroneous.

Effect of timing error in asynchronous transmission



110

Synchronous Transmission

Block of data bits are transmitted as a frame.

Clocks must be synchronized:

can use separate clock line between transmitter & receiver

one side send one short pulse and the other side uses this pulse for clocking; problem with long distances

or embed the clocking information in the data signal

Manchester encoding for digital signals

carrier frequency for analog transmission

Need to indicate start and end of block of data

use preamble (8bit flag) and postamble (8bit flag)

Control fields contain data link control protocol information.

More efficient (lower overhead) than asynchronous.

Synchronous Frame format


111

We send bits one after another without start or stop bits or gaps.

It is the responsibility of the receiver to group the bits.

Synchronous Transmission cont


113

Error and Error Coding

11

4

Error and Error Coding cont

115

(a) Single bit Error


116

(b) Multiple bit and Burst Error

The condition when more than one bit is in error in a given number of bits.

In case of burst error, if two or more bits from a data unit such as byte change from 1 to 0 or from 0 to 1 then burst errors are said to have occurred. the length of burst is measured from the first corrupted bit to last corrupted bit.


117

(b) Multiple-bit and Burst Error cont


118

Multiple-bit and Burst Error cont


11

9

Error control / Detection

Despite the best prevention techniques, errors may still happen.

To detect an error, something extra has to be added to the data/signal. This extra is an error detection code.

Lets examine two basic techniques for detecting errors:

Error Control

Parity checking Redundancy checking ~ VRC(Vertical Redundancy Check)

~ LRC(Longitudinal Redundancy) / BCC (Block Character

Checking)

~ Cyclic Redundancy Checksum (CRC).

120

Error control / Detection cont

Error Control cont

121

One of the simplest error-detection schemes

It refers to the use of parity bits to check that data has been transmitted accurately.

Add an extra bit to a code to ensure an even or odd number of 1s.

1-bit error detection with parity.

Every code word has an even or odd number of 1s.

Parity checking has limitations.

It cannot detect an error when an even number of bits change in the same data unit

Error Control cont


122


123

(a) Parity Checks

What happens if the character 10010101 (parity bit is the last bit)

and the first two 0s accidentally become two 1s?

Thus, the following character is received: 11110101.

Will there be a parity error?

Problem: Simple parity only detects odd numbers of bits in

error (50%)


124

(b) Cyclic Redundancy Checksum (CRC)

One of the simplest error-detection schemes

The CRC error detection method treats the packet of data

to be transmitted as a large polynomial.

The quotient is discarded but the remainder is attached to the end of the message (remainder (mod) arithmetic).

The transmitter takes the message polynomial and using

polynomial arithmetic, divides it by a given generating

polynomial.


125

Polynomials

CRC generator(divisor) is most often represented not as a string of 1s and 0s, but as an algebraic polynomial.

A polynomial representing a divisor



12

6

EXAMPLE: Data, M(x): 1001000 Generator, P(x): 1101


127

1000

1001

1000

1011

1100

1101

1101

1101

1101

1101

101

1101

1101

k + 1 bit check

sequence c,

equivalent to a

degree-k

polynomial

Remainder

m mod c

10011010000 Message plus k

zeros

Result:

Replace the added data bit

0 with the remainder data

bit.

Transmit message followed

by remainder:

10011010101

C(x) = x3 x2 1 = 1101 Generator

M(x) = x7 x4 x3 x = 10011010 Message

CRC Example Encoding

11111001

128

CRC Example Decoding No Errors

11111001

Result:

CRC test is past.

129

CRC Example Decoding with Errors

Result:

CRC test is failed.

13

1

REFERENCES:

Main: Forouzan, B.A. (2012). Data Communications and Networking (5th edision). Mc Graw Hill. (ISBN: 978-0-07-131586-9) Additional: William Stallings. (2011). Data And Computer Communication (9th edition). Prentice Hall. (ISBN-10: 0131392050)

topic 1 basic concept of data communication

Documents