EE302 Lesson 20: Transmission of Binary Data in Communication Systems - PowerPoint PPT Presentation


Title: EE302 Lesson 20: Transmission of Binary Data in Communication Systems


1
EE302 Lesson 20Transmission of Binary Data in
Communication Systems
2
Topics Covered in Chapter 11
  • 11-1 Digital Codes
  • 11-2 Principles of Digital Transmission
  • 11-3 Transmission Efficiency
  • 11-4 Basic Modem Concepts
  • 11-5 Wideband Modulation
  • 11-7 Error Detection and Correction

3
11-1 Digital Codes
  • The proliferation of applications that send
    digital data over communication channels has
    resulted in the need for efficient methods of
    transmission, conversion, and reception of
    digital data.
  • Digital codes have evolved as technology has
    advanced.

4
11-1 Digital Codes
  • Early Digital Codes
  • The first digital code was developed by Samual
    Morse.
  • The Morse code was originally designed for wired
    telegraph, but was later adapted for radio
    communication.
  • The Morse code consists of a series of dots and
    dashes that represent letters of the alphabet,
    numbers, and punctuation marks.

Figure 11-1 The Morse Code
5
11-1 Digital Codes
  • Baudot Code
  • The Baudot (baw dough) code was one of the first
    alphanumeric codes developed in the early days of
    teletype machines.
  • The Baudot code is a 5-bit code giving it 25 or
    32 possible values (it actually had 52 symbols
    using a control character).
  • It is obsolete and of historical interest only.

6
11-1 Digital Codes
Baudot Code
7
11-1 Digital Codes
  • ASCII
  • Binary representation of alphanumeric symbols
    (letters, numbers, punctuation, etc.) are given
    by American Standard Code of Information
    Interchange (ASCII) code.
  • Each ASCII codeword is 7-bits long yielding 27 or
    128 possible characters.
  • ASCII has remained the international standard in
    data communications.

8
ASCII
Figure 11-3 The ASCII Code
9
11-1 Digital Codes
  • Modern Binary Codes Extended Binary Coded
    Decimal Interchange Code
  • The Extended Binary Coded Decimal Interchange
    Code (EBCDIC) was developed by IBM.
  • The EBDIC is an 8-bit code allowing a maximum of
    256 characters to be represented.
  • The EBCDIC is used primarily in IBM and
    IBM-compatible computing systems and is not
    widely used as ASCII.

10
11-2 Principles of Digital Transmission
  • Serial Transmission
  • As discussed earlier, data can be transmitted in
    two ways
  • Parallel all bits transmitted simultaneously
  • Serial all bits transmitted one after another
  • Data transfers in long-distance communication
    systems are made serially. Parallel data
    transmission is not practical.
  • The LSB is transmitted first and the MSB is
    transmitted last.
  • Each bit is transmitted for a fixed interval of
    time, t.

11
11-2 Principles of Digital Transmission
Figure 11-4 Serial transmission of the ASCII
letter M.
12
11-2 Principles of Digital Transmission
  • Serial Transmission Expressing the Serial Data
    Rate
  • The speed of data transfer is usually indicated
    as number of bits per second (bps or b/s).
  • The speed in bps is the reciprocal of the bit
    time, t.
  • bps 1/t.
  • Example if bit time is 104.17 µs, bps1/104.17µs
    9600 bps
  • Another term used to express the data speed in
    digital communication systems is baud rate.
  • Baud rate is the number of signaling elements or
    symbols that occur in a given unit of time.
  • A signaling element is simply some change in the
    binary signal transmitted. In many cases it is a
    binary logic voltage level change, either a 1 or
    a 0.

13
11-2 Principles of Digital Transmission
  • Serial Transmission Expressing the Serial Data
    Rate
  • With the new modulation schemes (discussed
    later), multiple bits can be transmitted with one
    symbol.
  • Now, Bit rate baud rate x bits per symbol
  • or
  • Bit rate baud rate x log2S,
  • where S number of states per symbol.
  • These modulation schemes were developed to
    improve transmission rates over bandwidth-limited
    communication channels, such as the telephone
    lines.

14
11-2 Principles of Digital Transmission
  • Asynchronous Transmission
  • In asynchronous transmission each data word is
    accompanied by start and stop bits that indicate
    the beginning and ending of the word.
  • When no information is being transmitted, the
    communication line is usually high, or binary 1.
  • In data communication terminology, this high
    level is referred to as a mark.
  • To signal the beginning of a word, a start bit, a
    binary 0 or space is transmitted.
  • The change from mark to space indicates the
    beginning of a word.

15
11-2 Principles of Digital Transmission
Figure 11-6 Asynchronous transmission with start
and stop bits.
16
11-2 Principles of Digital Transmission
  • Asynchronous Transmission
  • Asynchronous transmissions are extremely
    reliable.
  • Most low-speed digital transmission (the 1200- to
    56,000-bps range) is asynchronous.
  • The primary disadvantage of asynchronous
    communication is that the extra start and stop
    bits effectively slow down data transmission.
  • The extra start and stop bits are called
    overhead and reduce efficiency

17
11-2 Principles of Digital Transmission
  • Synchronous Transmission
  • The technique of transmitting each data word one
    after another without start and stop bits,
    usually in multiword blocks, is referred to as
    synchronous data transmission.
  • To maintain synchronization between transmitter
    and receiver, a group of synchronization bits is
    placed at the beginning and at the end of the
    block.
  • Each block of data can represent hundreds or even
    thousands of 1-byte characters.

18
11-2 Principles of Digital Transmission
  • Synchronous Transmission
  • The special synchronization codes at the
    beginning and end of a block represent a very
    small percentage of the total number of bits
    being transmitted, especially in relation to the
    number of start and stop bits used in
    asynchronous transmission.
  • Synchronous transmission is therefore much faster
    than asynchronous transmission because of the
    lower overhead.
  • An error detection code usually appears at the
    end of the transmission (discussed later).
  • Synchronous transmission uses a precise clock to
    track the individual bits.

19
11-2 Principles of Digital Transmission
Figure 11-8 Synchronous data transmission.
20
11-2 Principles of Digital Transmission
  • Encoding Methods
  • Whether digital signals are being transmitted by
    baseband methods or broadband methods, before the
    data is put on the medium, it is usually encoded
    in some way to make it compatible with the
    medium.

21
11-2 Principles of Digital Transmission
  • Encoding Methods
  • In the nonreturn to zero (NRZ) method of
    encoding, the signal remains at the binary level
    assigned to it for the entire bit time.
  • Normally used at slow speeds, when asynchronous
    transmission is being used.
  • Since there is no voltage change when there are
    long strings of 1s and 0s transmitted, it is
    difficult for the receiver to determine where one
    bit begins and ends.
  • In return to zero (RZ) encoding the voltage level
    assigned to a binary 1 level returns to zero
    during the bit period.
  • Because there is clearly one discernible pulse
    per bit, it is extremely easy to derive the clock
    from the transmitted data.

22
11-2 Principles of Digital Transmission
  • Encoding Methods
  • Manchester encoding, also referred to as biphase
    encoding, is widely used in LANs.
  • In this system a binary 1 is transmitted first as
    a positive pulse, for one half of the bit
    interval, and then as a negative pulse for the
    remaining part of the bit interval.
  • A binary 0 is transmitted first as a negative
    pulse, for one half of the bit interval, and then
    as a positive pulse for the remaining part of the
    bit interval.
  • The choice of an encoding method depends on the
    application

23
11-2 Principles of Digital Transmission
Figure 11-9 Serial binary coding methods
Unipolar NRZ
Bipolar NRZ
Unipolar RZ
Bipolar RZ
Bipolar RZ-AMI
Manchester
24
11-3 Transmission Efficiency
  • Transmission efficiency is the accuracy and speed
    with which information, whether it is voice or
    video, analog or digital, is sent and received
    over communication media.
  • It is the basic subject matter of the field of
    information theory.

25
11-3 Transmission Efficiency
  • Transmission Media and Bandwidth
  • The two most common types of media used in data
    communication are wire cable and radio.
  • The two types of wire cable used
  • Coaxial cable usable bandwidth 200 MHz-3 GHz
    depending on the size. Bandwidth decreases with
    length.
  • Twisted-pair cable usable bandwidth 2 KHz-100
    MHz.
  • Coaxial cable has a center conductor surrounded
    by an insulator over which is a braided shield.
    The entire cable is covered with a plastic
    insulation.
  • A twisted-pair cable is two insulated wires
    twisted together.

26
11-3 Transmission Efficiency
Coaxial Cable
Twisted Pair
Figure 11-10 Types of cable used for digital data
transmission
27
11-3 Transmission Efficiency
  • The radio channel bandwidth must be wide enough
    to pass all harmonics and preserve the waveshape.
  • If the higher harmonics are filtered out, the
    signal will be distorted.
  • Hartleys Law
  • The amount of information that can be sent in a
    given transmission is dependent on the bandwidth
    of the communication channel and the duration of
    transmission.
  • Mathematically, Hartleys law is
  • C 2B
  • Where C is the channel capacity (bps) and B is
    the channel bandwidth (Hz). Assuming there is no
    noise in the system.

28
11-3 Transmission Efficiency
  • Hartleys Law
  • The greater the number of bits transmitted in a
    given time, the greater the amount of information
    that is conveyed.
  • The higher the bit rate, the wider the bandwidth
    needed to pass the signal with minimum
    distortion.
  • Example The maximum theoretical bit capacity for
    a 10 kHz bandwidth channel is
  • C 2B 2(10,000 Hz) 20,000 bps

29
11-3 Transmission Efficiency
  • The encoding method used also effects the
    required bandwidth for a given signal.
  • The bandwidth requirement for an RZ scheme is
    twice that for an NRZ scheme.
  • Multiple Coding Levels
  • Channel capacity can be increased by using
    multiple-level encoding schemes that permit more
    bits per symbol to be transmitted (Section 11-4).

30
11-3 Transmission Efficiency
  • Impact of Noise in the Channel
  • Increasing bandwidth increases the rate of
    transmission but also allows more noise to pass.
  • Shannon-Hartley Theorem determines channel
    capacity in the presence of noise.
  • Shannon-Hartley Theorem
  • C B log2(1 S/N)
  • C Channel capacity, bps
  • B bandwidth, Hz
  • S/N signal-to-noise ratio (power)

31
11-3 Transmission Efficiency
  • Example Problem 1
  • Find the channel capacity for a voice grade
    telephone line with a bandwidth of 3100 Hz and a
    S/N ratio of 30 dB (dB 10 log P)?

32
11-3 Transmission Efficiency
  • This answer conflicts with Hartleys Law
  • C 2B 2(3100 Hz) 6200 bps
  • Shannon-Hartley Theorem determines what is
    theoretically possible. But, multilevel coding is
    required to achieve these higher rates.
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Title: EE302 Lesson 20: Transmission of Binary Data in Communication Systems


1
EE302 Lesson 20Transmission of Binary Data in
Communication Systems
2
Topics Covered in Chapter 11
  • 11-1 Digital Codes
  • 11-2 Principles of Digital Transmission
  • 11-3 Transmission Efficiency
  • 11-4 Basic Modem Concepts
  • 11-5 Wideband Modulation
  • 11-7 Error Detection and Correction

3
11-1 Digital Codes
  • The proliferation of applications that send
    digital data over communication channels has
    resulted in the need for efficient methods of
    transmission, conversion, and reception of
    digital data.
  • Digital codes have evolved as technology has
    advanced.

4
11-1 Digital Codes
  • Early Digital Codes
  • The first digital code was developed by Samual
    Morse.
  • The Morse code was originally designed for wired
    telegraph, but was later adapted for radio
    communication.
  • The Morse code consists of a series of dots and
    dashes that represent letters of the alphabet,
    numbers, and punctuation marks.

Figure 11-1 The Morse Code
5
11-1 Digital Codes
  • Baudot Code
  • The Baudot (baw dough) code was one of the first
    alphanumeric codes developed in the early days of
    teletype machines.
  • The Baudot code is a 5-bit code giving it 25 or
    32 possible values (it actually had 52 symbols
    using a control character).
  • It is obsolete and of historical interest only.

6
11-1 Digital Codes
Baudot Code
7
11-1 Digital Codes
  • ASCII
  • Binary representation of alphanumeric symbols
    (letters, numbers, punctuation, etc.) are given
    by American Standard Code of Information
    Interchange (ASCII) code.
  • Each ASCII codeword is 7-bits long yielding 27 or
    128 possible characters.
  • ASCII has remained the international standard in
    data communications.

8
ASCII
Figure 11-3 The ASCII Code
9
11-1 Digital Codes
  • Modern Binary Codes Extended Binary Coded
    Decimal Interchange Code
  • The Extended Binary Coded Decimal Interchange
    Code (EBCDIC) was developed by IBM.
  • The EBDIC is an 8-bit code allowing a maximum of
    256 characters to be represented.
  • The EBCDIC is used primarily in IBM and
    IBM-compatible computing systems and is not
    widely used as ASCII.

10
11-2 Principles of Digital Transmission
  • Serial Transmission
  • As discussed earlier, data can be transmitted in
    two ways
  • Parallel all bits transmitted simultaneously
  • Serial all bits transmitted one after another
  • Data transfers in long-distance communication
    systems are made serially. Parallel data
    transmission is not practical.
  • The LSB is transmitted first and the MSB is
    transmitted last.
  • Each bit is transmitted for a fixed interval of
    time, t.

11
11-2 Principles of Digital Transmission
Figure 11-4 Serial transmission of the ASCII
letter M.
12
11-2 Principles of Digital Transmission
  • Serial Transmission Expressing the Serial Data
    Rate
  • The speed of data transfer is usually indicated
    as number of bits per second (bps or b/s).
  • The speed in bps is the reciprocal of the bit
    time, t.
  • bps 1/t.
  • Example if bit time is 104.17 µs, bps1/104.17µs
    9600 bps
  • Another term used to express the data speed in
    digital communication systems is baud rate.
  • Baud rate is the number of signaling elements or
    symbols that occur in a given unit of time.
  • A signaling element is simply some change in the
    binary signal transmitted. In many cases it is a
    binary logic voltage level change, either a 1 or
    a 0.

13
11-2 Principles of Digital Transmission
  • Serial Transmission Expressing the Serial Data
    Rate
  • With the new modulation schemes (discussed
    later), multiple bits can be transmitted with one
    symbol.
  • Now, Bit rate baud rate x bits per symbol
  • or
  • Bit rate baud rate x log2S,
  • where S number of states per symbol.
  • These modulation schemes were developed to
    improve transmission rates over bandwidth-limited
    communication channels, such as the telephone
    lines.

14
11-2 Principles of Digital Transmission
  • Asynchronous Transmission
  • In asynchronous transmission each data word is
    accompanied by start and stop bits that indicate
    the beginning and ending of the word.
  • When no information is being transmitted, the
    communication line is usually high, or binary 1.
  • In data communication terminology, this high
    level is referred to as a mark.
  • To signal the beginning of a word, a start bit, a
    binary 0 or space is transmitted.
  • The change from mark to space indicates the
    beginning of a word.

15
11-2 Principles of Digital Transmission
Figure 11-6 Asynchronous transmission with start
and stop bits.
16
11-2 Principles of Digital Transmission
  • Asynchronous Transmission
  • Asynchronous transmissions are extremely
    reliable.
  • Most low-speed digital transmission (the 1200- to
    56,000-bps range) is asynchronous.
  • The primary disadvantage of asynchronous
    communication is that the extra start and stop
    bits effectively slow down data transmission.
  • The extra start and stop bits are called
    overhead and reduce efficiency

17
11-2 Principles of Digital Transmission
  • Synchronous Transmission
  • The technique of transmitting each data word one
    after another without start and stop bits,
    usually in multiword blocks, is referred to as
    synchronous data transmission.
  • To maintain synchronization between transmitter
    and receiver, a group of synchronization bits is
    placed at the beginning and at the end of the
    block.
  • Each block of data can represent hundreds or even
    thousands of 1-byte characters.

18
11-2 Principles of Digital Transmission
  • Synchronous Transmission
  • The special synchronization codes at the
    beginning and end of a block represent a very
    small percentage of the total number of bits
    being transmitted, especially in relation to the
    number of start and stop bits used in
    asynchronous transmission.
  • Synchronous transmission is therefore much faster
    than asynchronous transmission because of the
    lower overhead.
  • An error detection code usually appears at the
    end of the transmission (discussed later).
  • Synchronous transmission uses a precise clock to
    track the individual bits.

19
11-2 Principles of Digital Transmission
Figure 11-8 Synchronous data transmission.
20
11-2 Principles of Digital Transmission
  • Encoding Methods
  • Whether digital signals are being transmitted by
    baseband methods or broadband methods, before the
    data is put on the medium, it is usually encoded
    in some way to make it compatible with the
    medium.

21
11-2 Principles of Digital Transmission
  • Encoding Methods
  • In the nonreturn to zero (NRZ) method of
    encoding, the signal remains at the binary level
    assigned to it for the entire bit time.
  • Normally used at slow speeds, when asynchronous
    transmission is being used.
  • Since there is no voltage change when there are
    long strings of 1s and 0s transmitted, it is
    difficult for the receiver to determine where one
    bit begins and ends.
  • In return to zero (RZ) encoding the voltage level
    assigned to a binary 1 level returns to zero
    during the bit period.
  • Because there is clearly one discernible pulse
    per bit, it is extremely easy to derive the clock
    from the transmitted data.

22
11-2 Principles of Digital Transmission
  • Encoding Methods
  • Manchester encoding, also referred to as biphase
    encoding, is widely used in LANs.
  • In this system a binary 1 is transmitted first as
    a positive pulse, for one half of the bit
    interval, and then as a negative pulse for the
    remaining part of the bit interval.
  • A binary 0 is transmitted first as a negative
    pulse, for one half of the bit interval, and then
    as a positive pulse for the remaining part of the
    bit interval.
  • The choice of an encoding method depends on the
    application

23
11-2 Principles of Digital Transmission
Figure 11-9 Serial binary coding methods
Unipolar NRZ
Bipolar NRZ
Unipolar RZ
Bipolar RZ
Bipolar RZ-AMI
Manchester
24
11-3 Transmission Efficiency
  • Transmission efficiency is the accuracy and speed
    with which information, whether it is voice or
    video, analog or digital, is sent and received
    over communication media.
  • It is the basic subject matter of the field of
    information theory.

25
11-3 Transmission Efficiency
  • Transmission Media and Bandwidth
  • The two most common types of media used in data
    communication are wire cable and radio.
  • The two types of wire cable used
  • Coaxial cable usable bandwidth 200 MHz-3 GHz
    depending on the size. Bandwidth decreases with
    length.
  • Twisted-pair cable usable bandwidth 2 KHz-100
    MHz.
  • Coaxial cable has a center conductor surrounded
    by an insulator over which is a braided shield.
    The entire cable is covered with a plastic
    insulation.
  • A twisted-pair cable is two insulated wires
    twisted together.

26
11-3 Transmission Efficiency
Coaxial Cable
Twisted Pair
Figure 11-10 Types of cable used for digital data
transmission
27
11-3 Transmission Efficiency
  • The radio channel bandwidth must be wide enough
    to pass all harmonics and preserve the waveshape.
  • If the higher harmonics are filtered out, the
    signal will be distorted.
  • Hartleys Law
  • The amount of information that can be sent in a
    given transmission is dependent on the bandwidth
    of the communication channel and the duration of
    transmission.
  • Mathematically, Hartleys law is
  • C 2B
  • Where C is the channel capacity (bps) and B is
    the channel bandwidth (Hz). Assuming there is no
    noise in the system.

28
11-3 Transmission Efficiency
  • Hartleys Law
  • The greater the number of bits transmitted in a
    given time, the greater the amount of information
    that is conveyed.
  • The higher the bit rate, the wider the bandwidth
    needed to pass the signal with minimum
    distortion.
  • Example The maximum theoretical bit capacity for
    a 10 kHz bandwidth channel is
  • C 2B 2(10,000 Hz) 20,000 bps

29
11-3 Transmission Efficiency
  • The encoding method used also effects the
    required bandwidth for a given signal.
  • The bandwidth requirement for an RZ scheme is
    twice that for an NRZ scheme.
  • Multiple Coding Levels
  • Channel capacity can be increased by using
    multiple-level encoding schemes that permit more
    bits per symbol to be transmitted (Section 11-4).

30
11-3 Transmission Efficiency
  • Impact of Noise in the Channel
  • Increasing bandwidth increases the rate of
    transmission but also allows more noise to pass.
  • Shannon-Hartley Theorem determines channel
    capacity in the presence of noise.
  • Shannon-Hartley Theorem
  • C B log2(1 S/N)
  • C Channel capacity, bps
  • B bandwidth, Hz
  • S/N signal-to-noise ratio (power)

31
11-3 Transmission Efficiency
  • Example Problem 1
  • Find the channel capacity for a voice grade
    telephone line with a bandwidth of 3100 Hz and a
    S/N ratio of 30 dB (dB 10 log P)?

32
11-3 Transmission Efficiency
  • This answer conflicts with Hartleys Law
  • C 2B 2(3100 Hz) 6200 bps
  • Shannon-Hartley Theorem determines what is
    theoretically possible. But, multilevel coding is
    required to achieve these higher rates.
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