topic 1 basic concept of data communication
DESCRIPTION
Basic Concept Of Data CommunicationTRANSCRIPT
-
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
Communication Codes cont...
13
-
Communication Codes cont...
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
Communication Codes cont...
15
-
Communication Codes cont...
16
-
Communication Codes cont...
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
-
19
-
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
Communication Codes cont...
20
-
21
Exercise:
Translate those characters using Boudot code;
DIS 13
Communication Codes cont...
-
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
Communication Codes cont...
-
Extended Binary coded Decimal Characters Information.
8 bit characters created by IBM.
there are 256 different combination.
Often used in IBM.
EBCDIC Code
Communication Codes cont...
23
-
24
-
Example:
Translate those characters using EBCDIC code;
a) B
b) 5
Solution:
a) B: 1100 0010
b) 5: 1111 0101
Communication Codes cont...
25
-
Exercise
Translate those characters using EBCDIC code;
a) A
b) 7
c) a
d) SYN
Communication Codes cont...
26
-
Solution:
Translate those characters using EBCDIC code;
a) A : 1100 0001
b) 7 : 1111 0111
c) a : 1000 0001
d) SYN : 0011 0010
Communication Codes cont...
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
Communication Codes cont...
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
Communication Codes cont...
30
-
Exercise
Translate those phrases using ASCII code of
1 SeRaH
Communication Codes cont...
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
Communication Codes cont...
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
Information Capacity, Bits, Bit Rate and Baud cont....
-
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
Information Capacity, Bits, Bit Rate and Baud cont....
-
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
Information Capacity, Bits, Bit Rate and Baud cont....
-
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?
Information Capacity, Bits, Bit Rate and Baud cont....
-
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
Information Capacity, Bits, Bit Rate and Baud cont....
-
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
Information Capacity, Bits, Bit Rate and Baud cont....
-
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
Information Capacity, Bits, Bit Rate and Baud cont....
-
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.
Digital to Digital Encoding cont...
-
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
Digital to Digital Encoding cont...
-
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.
Digital to Digital Encoding cont...
-
59
Bipolar AMI(Alternate Mark Inversion) cont
Digital to Digital Encoding 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)
Digital to Digital Encoding cont...
-
61
High Density Bipolar Order 3 Encoding (HDB3) cont
Digital to Digital Encoding 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
High Density Bipolar Order 3 Encoding (HDB3) cont
Digital to Digital Encoding cont...
-
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 "
High Density Bipolar Order 3 Encoding (HDB3) cont
Digital to Digital Encoding cont...
-
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 "
Digital to Digital Encoding cont...
-
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)
Digital to Digital Encoding cont...
-
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
Digital to Analogue Encoding cont....
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).
Digital to Analogue Encoding cont....
-
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
Digital to Analogue Encoding 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.
Analogue to Analogue Encoding cont...
-
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
Analogue to Analogue Encoding 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.
Analogue to Analogue Encoding cont...
-
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)
Analogue to Analogue Encoding cont...
-
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.
Analogue to Analogue Encoding cont...
Sampling
Pulse Code Modulation (PCM) cont...
-
81
Three different sampling methods for PCM
Analogue to Analogue Encoding cont...
Pulse Code Modulation (PCM) cont...
-
82
Analogue to Analogue Encoding cont...
Components of a PCM decoder
Pulse Code Modulation (PCM) cont...
-
83
Analogue to Analogue Encoding cont...
Pulse Code Modulation (PCM) cont...
-
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
01 waveform (symbol) 2
10 waveform (symbol) 3
11 waveform (symbol) 4
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
Digital to Analogue Encoding cont....
-
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.
Digital to Analogue Encoding cont....
-
90
PSK/QPSK cont
Digital to Analogue Encoding 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.
Digital to Analogue Encoding cont....
-
93
The QAM cont
Digital to Analogue Encoding cont....
-
94
4 QAM and 8 -QAM constellations The QAM cont
Digital to Analogue Encoding 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.
Digital to Analogue Encoding cont....
-
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
Digital to Analogue Encoding 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
Digital to Analogue Encoding 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.
The importance of timing and framing cont
-
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)
The importance of timing and framing cont
-
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
Data Transmitting cont
-
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
Data Transmitting cont
-
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.
Asynchronous Transmission cont
Data Transmitting cont
-
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
Asynchronous Transmission cont
Data Transmitting cont
-
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
Data Transmitting cont
-
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
Data Transmitting cont
-
11
2
-
113
Error and Error Coding
-
11
4
Error and Error Coding cont
-
115
(a) Single bit Error
Error and Error Coding cont
-
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.
Error and Error Coding cont
-
117
(b) Multiple-bit and Burst Error cont
Error and Error Coding cont
-
118
Multiple-bit and Burst Error cont
Error and Error Coding 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
Error control / Detection cont
-
122
Error control / Detection cont
-
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%)
Error control / Detection cont
-
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.
Error control / Detection cont
-
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
(b) Cyclic Redundancy Checksum (CRC)
Error control / Detection cont
-
12
6
EXAMPLE: Data, M(x): 1001000 Generator, P(x): 1101
(b) Cyclic Redundancy Checksum (CRC)
-
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.
-
130
-
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)