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1

Principles of Electronic
Communication Systems

Third Edition

Louis E. Frenzel, Jr.

© 2008 The McGraw-Hill Companies

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Chapter 11

The Transmission of Binary Data
in Communication Systems


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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
 11-6: Broadband Modem Techniques
 11-7: Error Detection and Correction
 11-8: Protocols


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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.


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11-1: Digital Codes
 Data processed and stored by computers can be
numerical or text.

 The signals used to represent computerized data are
digital.

 Even before the advent of computers, digital codes
were used to represent data.


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11-1: Digital Codes
Early Digital Codes
 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.
 The Baudot code was used in the early teletype
machine, a device for sending and receiving coded
signals over a communication link.


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11-1: Digital Codes
Modern Binary Codes
 For modern data communication, information is
transmitted using a system in which the numbers and
letters to be represented are coded, usually by way of a
keyboard, and the binary word representing each
character is stored in a computer memory.


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11-1: Digital Codes
Modern Binary Codes: American Standard Code for
Information Interchange
 The most widely used data communication code is the
7-bit binary code known as the American Standard
Code for Information Interchange (ASCII).
 ASCII code can represent 128 numbers, letters,
punctuation marks, and other symbols.
 ASCII code combinations are available to represent
both uppercase and lowercase letters of the alphabet.
 Several ASCII codes have two- and three-letter
designations which initiate operations or provide
responses for inquiries.


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11-1: Digital Codes
Modern Binary Codes: Hexadecimal Values
 Binary codes are often expressed using their
hexadecimal, rather than decimal values.
 To convert a binary code to its hexadecimal equivalent,
first divide the code into 4-bit groups.
 Start at the least significant bit on the right and work to
the left. (Assume a leading zero on each of the codes.)


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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.


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11-2: Principles of Digital
Transmission
Serial Transmission
 Data can be transmitted in two ways:
1. Parallel
2. Serial
 Data transfers in long-distance communication
systems are made serially.
 In a serial transmission, each bit of a word is
transmitted one after another.
 Parallel data transmission is not practical for long-
distance communication.


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Transmission

Figure 11-4: Serial transmission of the ASCII letter M.

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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).
 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.


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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.


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Transmission
Asynchronous Transmission
 Most low-speed digital transmission (the 1200- to
56,000-bps range) is asynchronous.
 Asynchronous transmissions are extremely reliable.
 The primary disadvantage of asynchronous
communication is that the extra start and stop bits
effectively slow down data transmission.


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Transmission

Figure 11-6: Asynchronous transmission with start and stop bits.

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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.


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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.


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Transmission

Figure 11-8: Synchronous data transmission.

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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
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.
 In return to zero (RZ) encoding the voltage level
assigned to a binary 1 level returns to zero during the
bit period.
 Manchester encoding, also referred to as biphase
encoding, is widely used in LANs. In this system a
binary 1 us 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.


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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.


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Hartley’s 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, Hartley’s law is
C = 2B
Where C is the channel capacity (bps) and B is the
channel bandwidth.


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Hartley’s 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.


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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 are coaxial and
twisted pair.
 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.


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Figure 11-10: Types of cable used for digital data transmission. (a) Coaxial cable.
(b) Twisted-pair cable, unshielded (UTP).

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 Twisted-pair is available as unshielded (UTP) or
shielded.
 Coaxial cable and shielded twisted-pair cables are
usually preferred, as they provide some protection from
noise and cross talk.
 Cross talk is the undesired transfer of signals from
one unshielded cable to another adjacent one
caused by inductive or capacitive coupling.


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 The bandwidth of any cable is determined by its
physical characteristics.
 All wire cables act as low-pass filters because they are
made up of wire that has inductance, capacitance, and
resistance.


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Multiple Coding Levels
 Channel capacity can be modified by using multiple-
level encoding schemes that permit more bits per
symbol to be transmitted.
 It is possible to transmit data using more than just two
binary voltage levels or symbols.
 Multiple voltage levels can be used to increase channel
capacity.
 Other methods, such as using different phase shifts for
each symbol, are used.


30

Impact of Noise in the Channel
 An important aspect of information theory is the impact
of noise on a signal.
 Increasing bandwidth increases the rate of transmission
but also allows more noise to pass.
 Typical communication systems limit the channel
capacity to one-third to one-half the maximum to ensure
more reliable transmission in the presence of noise.


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11-4: Basic Modem Concepts
 Digital data are transmitted over the telephone and
cable television networks by using broadband
communication techniques involving modulation,
which are implemented by a modem, a device
containing both a modulator and a demodulator.
 Modems convert binary signals to analog signals
capable of being transmitted over telephone and cable
TV lines and by radio, and then demodulate such
analog signals, reconstructing the equivalent binary
output.


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 There are four widely used modem types:
1. Conventional analog dial-up modems.
2. Digital subscriber line (DSL) modems.
3. Cable TV modems.
4. Wireless modems.


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Figure 11-12: How modems permit digital data transmission on the telephone network.

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Modulation for Data Communication
 The four main types of modulation used in modern
modems are:
1. Frequency-shift keying (FSK)
2. Phase-shift keying (PSK)
3. Quadrature amplitude modulation (QAM)
4. Orthogonal frequency division multiplexing (OFDM)


35

Modulation for Data Communication: Frequency-Shift
Keying (FSK)
 Frequency-shift keying (FSK) is the oldest and
simplest form of modulation used in modems.
 In FSK, two sine-wave frequencies are used to
represent binary 0s and 1s.
 A binary 0, usually called a space, has a frequency of
1070 Hz.
 A binary 1, referred to as a mark, is 1270 Hz.
 These two frequencies are alternately transmitted to
create the serial binary data.

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Figure 11-13: Frequency-shift keying. (a) Binary signal. (b) FSK signal.

37

Modulation for Data Communication: Phase-Shift
Keying
 In phase-shift keying (PSK), the binary signal to be
transmitted changes the phase shift of a sine-wave
character depending upon whether a binary 0 or binary
1 is to be transmitted.
 A phase shift of 180°, the maximum phase difference
that can occur, is known as a phase reversal, or phase
inversion.
 During the time that a binary 0 occurs, the carrier is
transmitted with one phase; when a binary 1 occurs, the
carrier is transmitted with a 180° phase shift.


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Figure 11-18: Binary phase-shift keying.

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Modulation for Data Communication: QPSK
 One way to increase the binary data rate while not
increasing the bandwidth required for the signal
transmission is to encode more than 1 bit per phase
change.
 In the system known as quadrature, quarternary, or
quadra phase PSK (QPSK or 4-PSK), more bits per
baud are encoded, the bit rate of data transfer can be
higher than the baud rate, yet the signal will not take up
additional bandwidth.
 In QPSK, each pair of successive digital bits in the
transmitted word is assigned a particular phase.
 Each pair of serial bits, called a dibit, is represented by
a specific phase.

40


Figure 11-24: Quadrature PSK modulation. (a) Phase angle of carrier for different
pairs of bits. (b) Phasor representation of carrier sine wave. (c) Constellation diagram
of QPSK.

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Modulation for Data Communication: QPSK
 The QPSK modulator consists of a 2-bit shift register
implemented with flip-flops, commonly known as a bit
splitter.
 The serial binary data train is shifted through the
register.
 The bits from the flip-flops are applied to balanced
modulators.
 The carrier oscillator is applied to one balanced
modulator and through a 90° phase shifter to another
balanced modulator.
 The outputs of the balanced modulators are linearly
mixed to produce the QPSK signal.

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Figure 11-25: A QPSK modulator.

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Modulation for Data Communication: QAM
 One of the most popular modulation techniques used in
modems for increasing the number of bits per baud is
quadrature amplitude modulation (QAM).
 QAM uses both amplitude and phase modulation of a
carrier.
 In 8-QAM, there are four possible phase shifts and two
different carrier amplitudes.
 Eight different states can be transmitted.
 With eight states, 3 bits can be encoded for each baud
or symbol transmitted.
 Each 3-bit binary word transmitted uses a different
phase-amplitude combination.


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Figure 11-29: A constellation diagram of a QAM signal.

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Spectral Efficiency and Noise
 Spectral efficiency is a measure of how fast data can be
transmitted in a given bandwidth (bps/Hz).
 Different modulation methods give different efficiencies.

Modulation Spectral efficiency, bps/Hz
FSK <1
GMSK 1.35
BPSK 1
QPSK 2
8-PSK 3
16-QAM 4

46

Spectral Efficiency and Noise
 The signal-to-noise (S/N) ratio clearly influences the
spectral efficiency.
 The greater the noise, the greater the number of bit
errors.
 The number of errors that occur in a given time is called
the bit error rate (BER).
 The BER is the ratio of the number of errors that occur
to the number of bits that occur in a one second
interval.


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11-5: Wideband Modulation
 While most modulation methods are designed to be
spectrally efficient, there is another class of
modulation methods that does just the opposite.
 These methods are designed to use more bandwidth.
The transmitted signal occupies a bandwidth many
times greater than the information bandwidth.
 The two most widely used wideband modulation
methods are spread spectrum and orthogonal
frequency-division multiplexing.


48

Spread Spectrum
 Spread spectrum (SS) is a modulation and
multiplexing technique that distributes a signal and its
sidebands over a very wide bandwidth.
 After World War II, spread spectrum was developed by
the military because it is a secure communication
technique essentially immune to jamming.
 Currently, unlicensed operation is permitted in the 902-
to 928-MHz, 2.4- to 2.483-GHz, and 5.725- to 5.85-GHz
ranges, with 1 W of power.


49

Spread Spectrum
 Spread spectrum on these frequencies is being widely
incorporated into a variety of commercial
communication systems, particularly wireless data
communication.
 Numerous LANs and portable personal computer
modems use SS techniques, as does a class of
cordless telephones.
 The most widespread use of SS is in cellular
telephones. It is referred to as code-division multiple
access (CDMA).


50

Spread Spectrum
 There are two basic types of spread spectrum:
frequency-hopping (FH) and direct-sequence (DS).
 In frequency-hopping SS, the frequency of the carrier
of the transmitter is changed according to a
predetermined sequence, called pseudorandom, at a
rate higher than that of the serial binary data modulating
the carrier.
 In direct-sequence SS, the serial binary data is mixed
with a higher-frequency pseudorandom binary code at a
faster rate, and the result is used to phase-modulate a
carrier.

51

Frequency-Hopping Spread Spectrum
 In a frequency-hopping SS transmitter, the serial
binary data to be transmitted is applied to a
conventional two-tone FSK modulator.
 The modulator output is applied to a mixer.
 Also driving the mixer is a frequency synthesizer.
 The output signal from the bandpass filter after the
mixer is the difference between one of the two FSK sine
waves and the frequency of the frequency synthesizer.
 The synthesizer is driven by a pseudorandom code
generator, which is either a special digital circuit or the
output of a microprocessor.


52


Figure 11-33: A frequency-hopping SS transmitter.

53


Figure 11-34: A typical PSN code generator.

54

 In a frequency-hopping SS system, the rate of
synthesizer frequency change is higher than the data
rate.
 This means that although the data bit and the FSK tone
it produces remain constant for one data interval, the
frequency synthesizer switches frequencies many times
during this period.


55

 The time that the synthesizer remains on a single
frequency is called the dwell time.
 The frequency synthesizer puts out a random sine wave
frequency to the mixer, and the mixer creates a new
carrier frequency for each dwell interval.
 The resulting signal, whose frequency rapidly jumps
around, effectively scatters pieces of the signal all over
the band.


56

Direct-Sequence Spread Spectrum
 In a direct-sequence SS (DSSS) transmitter, the
serial binary data is applied to an X-OR gate along with
a serial pseudorandom code that occurs faster than the
binary data.
 One bit time for the pseudorandom code is called a
chip, and the rate of the code is called the chipping
rate. The chipping rate is faster than the data rate.
 The signal developed at the output of the X-OR gate is
then applied to a PSK modulator, typically a BPSK
device.


57

 The carrier phase is switched between 0 and 180° by
the 1s and 0s of the X-OR output.
 The PSK modulator is generally some form of balanced
modulator.
 The signal phase modulating the carrier, being much
higher in frequency than the data signal, causes the
modulator to produce multiple, widely spaced sidebands
whose strength is such that the complete signal takes
up a great deal of the spectrum. Thus the resulting
signal is spread.
 Because of its randomness, the signal looks like
wideband noise to a conventional narrowband receiver.

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Figure 11-38: A direct-sequence SS transmitter.

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Figure 11-39: Data signals in direct-sequence SS.

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 Direct-sequence SS is also called code-division
multiple access (CDMA), or SS multiple access.
 The term multiple access applies to any technique that
is used for multiplexing many signals on a single
communication channel.
 CDMA is used in satellite systems so that many signals
can use the same transponder.
 It is also widely used in cellular telephone systems. It
permits more users to occupy a given band than other
methods.


61

Benefits of Spread Spectrum
 Spread spectrum is being used increasingly in data
communication as its benefits are discovered and as
new components and equipment become available to
implement it.
 Security: SS prevents unauthorized listening.
 Resistance to jamming and interference: Jamming signals
are typically restricted to a single frequency, and jamming
one frequency does not interfere with an SS signal.


62

Benefits of Spread Spectrum
 Band sharing: Many users can share a single band with
little or no interference.
 Resistance to fading and multipath propagation: SS
virtually eliminates wide variations of signal strength due
to reflections and other phenomena during propagation.
 Precise timing: Use of the pseudorandom code in SS
provides a way to precisely determine the start and end of
a transmission, making it a superior method for radar and
other applications that rely on accurate knowledge of
transmission time to determine distance.


63


Orthogonal Frequency-Division Multiplexing (OFDM)
 A wideband modulation method called OFDM is
growing in popularity.
 OFDM is also known as multicarrier modulation
(MCM).
 Although OFDM is known as a modulation method, the
term frequency-division multiplexing is appropriate
because the method transmits data by simultaneously
modulating segments of the high-speed serial bit
stream onto multiple carriers spaced throughout the
channel bandwidth.


64

Orthogonal Frequency-Division Multiplexing (OFDM)
 The carriers are frequency-multiplexed in the channel.
 The data rate on each channel is very low, making the
symbol time much longer than predicted transmission
delays.
 This technique spreads the signals over a wide
bandwidth, making them less sensitive to the noise,
fading, reflections, and multipath transmission effects
common in microwave communication.


65


Figure 11-42: Concept of OFDM.

66


Figure 11-44: Simplified processing scheme for OFDM in DSP.

67
11-6: Broadband Modem
Techniques
Analog Telephone Modem
 The most commonly used modem is one that
connects personal computers to the telephone line.
 A typical dial-up modem consists of both transmitter
and receiver sections.
 Most modern modems are implemented using digital
signal processing (DSP) techniques.


68
Techniques
Analog Telephone Modem
 Modems are packaged on a single small printed
circuit board and are designed to plug into the PC
bus.
 Most analog modems today are single chip DSPs
mounted on the PC motherboard.
 The modem takes its power from the PC power
supply.
 An RJ-11 modular connector attaches the modem to
the telephone line.


69
Techniques
Analog Telephone Modem: Modem Operation
 During transmission operations:
1. The data to be transmitted is stored in the computer’s
RAM.
2. It is formatted there by the communication software
installed with the computer.
3. It is then sent 1 byte at a time to the modem.
4. The modem’s first job is to convert parallel data to serial
data. This is done with shift registers. It is usually
carried out by a universal asynchronous
receiver/transmitter (UART), a digital IC that performs
parallel-to-serial conversion for transmission and serial-
to-parallel conversion for reception.


70
Techniques
5. The serial data from the UART is passed through a
scrambler circuit to ensure that the data is random.
6. The random serial data is sent to the modulator.
7. The output of the modulator is filtered to band-limit it
and then fed to an equalizer circuit.


71
Techniques
 During receive operations:
1. The signal is picked off the telephone line.
2. It is passed through the interface circuits.
3. Then it is fed to the receiver section.
4. It first encounters an adaptive equalizer. The adaptive
equalizer adjusts itself automatically to compensate for
the amplitude attenuation and distortion of the signal.
5. The signal is then demodulated, resulting in an NRZ
serial digital signal.


72
Techniques
6. This is passed through a descrambler, which produces
the opposite effect of the transmit scrambler.
7. The descrambler output is the original serial data signal.
This is sent to the UART, where it is translated to a
parallel byte that the computer can store and use.
 Data compression and decompression circuits are now
being used in some modems.
 All the newer modem types incorporate circuitry that
can detect bit transmission errors and correct them as
they occur.


73
Techniques
Analog Telephone Modem: Modem Classification and
Standards
 The International Telecommunications Union (ITU)
sponsors, negotiates, and maintains modem and other
communication standards.
 Modem standards are designated by a special V.xx
symbol.
 Modems are usually capable of operating in several
different V.xx modes.


74
Techniques
Analog Telephone Modem: Modem Classification and
Standards
 The modem will automatically adjust itself to the highest
speed possible but will drop back to a lower speed or
different mode if the receiving modem cannot handle
the highest speed.
 Most modems is use today are the V.90 or V.92 type
and are capable of speeds up to 56 kbps.


75
Techniques
xDSL Modems
 The digital subscriber line (DSL) describes a set of
standards set by the International Telecommunications
Union that greatly extend the speed potential of the
common twisted-pair telephone lines.
 In the term xDSL, the x designates one of several
letters that define a specific DSL standard.
 The most widely used form of DSL is called
asymmetric digital subscriber line (ADSL), which
permits downstream data rates up to 8 Mbps and
upstream rates up to 640 kbps using the existing
telephone lines.

76
Techniques
xDSL Modems
 The modulation scheme used with ADSL modems is
called discrete multitone (DMT), another name for
OFDM.
 It divides the upper frequency spectrum of the
telephone line into 256 channels, each 4 kHz wide.
 Each channel, called a bin, is designed to transmit at
speeds up to 15 kbps/Bd or 60 kbps.


77
Techniques
xDSL Modems
 Each channel contains a carrier that is simultaneously
phase-amplitude-modulated (QAM) by some of the bits
to be transmitted.
 The serial data stream is divided up so that each carrier
transmits some of the bits. All bits are transmitted
simultaneously.
 All the carriers are frequency-multiplexed into the line
bandwidth above the normal voice telephone channel
 The system is complex and is implemented with a
digital signal processor.


78
Techniques

Figure 11-47: Spectrum of telephone line used by ADSL.

79
Techniques

Figure 11-48: ADSL modem—block diagram.

80
Techniques
Cable Modems
 Many cable TV systems are set up to handle high-
speed digital data transmission.
 The digital data is used to modulate a high-frequency
carrier that is frequency-multiplexed onto the cable that
also carries the TV signal.
 Cable modems provide significantly higher data rates
than can be achieved over the standard telephone
system.


81
Techniques
Cable Modems
 Television channels extend from 50 MHz (Channel 2)
up to 550 MHz. In this 500 MHz of bandwidth, up to 83
channels of 6 MHz can be accommodated.
 The spectrum above the TV channels, from 550 to 850
MHz, is available for digital data transmission. Standard
6-MHz channels are used.
 Cable modems use 64-QAM for downstream data.
 Standard QPSK is used in the upstream channels.


82
Techniques

Figure 11-49: Cable TV spectrum showing upstream and downstream data channels.

83
Techniques
Cable Modems
 A typical cable modem is a VHF/UHF receiver
connected to the cable for downloads and a
modulator/transmitter for uploads.
 The signal from the cable passes through the diplexer,
which is a filter circuit that permits simultaneous
transmit and receive operations.
 The signal is amplified and mixed with a local oscillator
signal from the frequency synthesizer to produce an IF
signal.
 The frequency synthesizer selects the cable channel.
The IF signal is demodulated to recover the data.

84
Techniques
Cable Modems
 Reed Solomon error detection circuitry finds and
corrects any bit errors.
 The digital data then goes to an Ethernet interface to
the PC.
 For transmission, the data from the computer is passed
through the interface, where it is encoded for error
detection.
 The data then modulates a carrier that is up-converted
by the mixer to the selected upstream channel before
being amplified and passed through the diplexer to the
cable.

85
Techniques

Figure 11-50: Cable modem block diagram

86
11-7: Error Detection
and Correction
 When high-speed binary data is transmitted over a
communication link, whether it is a cable or radio,
errors will occur.
 These errors are changes in the bit pattern caused by
interference, noise, or equipment malfunctions.
 Such errors will cause incorrect data to be received.
 The number of bit errors that occur for a given number
of bits transmitted is referred to as the bit error rate
(BER).


87
and Correction
 The process of error detection and correction involves
adding extra bits to the data characters to be
transmitted. This process is generally referred to as
channel encoding.
 The data to be transmitted is processed in a way that
creates the extra bits and adds them to the original
data. At the receiver, these extra bits help in
identifying any errors that occur in transmission
caused by noise or other channel effects.


88
and Correction
 A key point about channel encoding is that it takes
more time to transmit the data because of the extra
bits. These extra bits are called overhead in that they
extend the time of transmission.
 Channel encoding methods fall into to two separate
categories, error detection codes and error
correction codes.


89
and Correction
Error Detection
 Many different methods have been used to ensure
reliable error detection:
 Redundancy is a method that ensures error-free
transmission by sending each character or message
multiple times until it is properly received.
 Encoding schemes like the RZ-AMI are used whereby
successive binary 1 bits in the bit stream are transmitted
with alternating polarity.


90
and Correction
Error Detection
 One of the most widely used systems is known as
parity, in which each character transmitted contains
one additional bit, known as a parity bit.
 The cyclical redundancy check (CRC) is a
mathematical technique used in synchronous data
transmission that effectively catches 99.9 percent or
more of transmission errors.


91
and Correction
Error Correction
 A number of efficient error-correction schemes have
been devised to complement error detection methods.
 The process of detecting and correcting errors at the
receiver so that retransmission is not necessary is
called forward error correction (FEC).
 There are two basic types of FEC: block codes and
convolutional codes.


92
and Correction
Error Correction: Block-Check Character
 The block check character (BCC) is also known as a
horizontal or longitudinal redundancy check (LRC).
 It is the process of logically adding, by exclusive-ORing,
all the characters in a specific block of transmitted data.
 The final bit value for each horizontal row becomes one
bit in a character known as the block-check character
(BCC), or the block-check sequence (BCS).


93
and Correction
Error Correction: Block-Check Character
 The most popular FEC codes are the Hamming and
Reed Solomon codes.
 These codes add extra parity bits to a transmitted word,
process them using unique algorithms, and detect and
correct bit errors.
 Interleaving is a method used in wireless systems to
reduce the effects of burst errors.


94
and Correction
Error Correction: Convolutional Codes
 Convolutional encoding creates additional bits from the
data as do Hamming and Reed Solomon codes, but the
encoded output is a function of not only the current data
bits but also previously occurring data bits.
 Convolutional codes pass the data to be transmitted
through a special shift register.
 As the serial data is shifted through the shift register
flip-flops, some of the flip-flop outputs are XORed
together to form two outputs.


95
and Correction
Error Correction: Convolutional Codes
 These two outputs are the convolutional code, and this
is what is transmitted.
 The original data itself is not transmitted.
 Instead, two separate streams of continuously encoded
data are sent.
 Since each output code is different, the original data
can more likely be recovered at the receiver by an
inverse process.


96
and Correction

Figure 11-56: Convolutional encoding uses a shift register with exclusive-OR gates to
create the output.

97

11-8: Protocols
 Protocols are rules and procedures used to ensure
compatibility between the sender and receiver of
digital data regardless of the hardware and software
being used.
 Protocols are used to identify the start and end of a
message, identify the sender and receiver, state the
number of bytes to be transmitted, state the method of
error detection, and for other functions.
 Various protocols, and various levels of protocols, are
used in data communication.


98

11-8: Protocols
Asynchronous Protocols
 Three popular protocols for asynchronous ASCII-coded
data transmission between personal computers, via
modem are:
 Xmodem
 Kermit
 MPN.


99

11-8: Protocols
Asynchronous Protocols: Xmodem
 In Xmodem, the data transmission procedure begins
with the receiving computer transmitting a negative
acknowledge (NAK) character to the transmitter.
 NAK is a 7-bit ASCII character that is transmitted
serially back to the transmitter every 10 seconds until
the transmitter recognizes it.
 Once the transmitter recognizes the NAK character, it
begins sending a 128-byte block of data, known as a
frame (packet) of information.


100

11-8: Protocols

Figure 11-60: Xmodem protocol frame.

101

11-8: Protocols
Asynchronous Protocols: Kermit
 The Kermit protocol transmission begins with a start-
of-header (SOH) character followed by a length (LEN)
character, which tells how long the block of data is.
 Next is a packet sequence number (SEQ).
 There can be up to 63 blocks, and these are given a
sequence number so that both transmitter and receiver
can keep track of long messages.
 Kermit is reliable because it requires every packet sent
be acknowledged by the receiver as being read
correctly.


102

11-8: Protocols
Asynchronous Protocols: MNP
 Microcom Networking Protocols (MNPs) are a series
of protocols developed by the manufacturer Microcom
to be used with asynchronous modems.
 They specify ways to handle error detection and
correction and how to specify whether or not data
compression is used.
 There are 10 classes of protocols.
 MNPs are easy to implement because they can be
programmed into the control microcomputer used in
most modems.


103

11-8: Protocols
Synchronous Protocols
 Protocols used for synchronous data communication
are more complex than asynchronous protocols.

 Like asynchronous systems, they use various control
characters for signaling purposes at the beginning and
ending of the block of data to be transmitted.


104

11-8: Protocols
Synchronous Protocols: Bisync
 IBM’s Bisync protocol, which is widely used in
computer communication, usually begins with the
transmission of two or more ASCII sync (SYN)
characters.
 These characters signal the beginning of the
transmission and are also used to initialize the clock
timing circuits in the receiving modem.
 This ensures proper synchronization of the data
transmitted a bit at a time.


105

11-8: Protocols

Figure 11-62: Bisync synchronous protocol.

106

11-8: Protocols
Synchronous Protocols: SDLC
 One of the most flexible and widely used synchronous
protocols is the synchronous data link control
(SDLC) protocol.
 SDLC is used in networks that are interconnections of
multiple computers.


107

11-8: Protocols

Figure 11-63: The SDLC and HDLC frame formats.

108

11-8: Protocols
Open Systems Interconnection Model
 The International Organization for Standardization
(ISO) has attempted to standardize data communication
procedures.
 The ISO has come up with a framework, or hierarchy,
that defines how data can be communicated.
 This hierarchy, known as the open systems
interconnection (OSI) model, is designed to establish
general interoperability guidelines for developers of
communication systems and protocols.


109

11-8: Protocols
 The OSI hierarchy is made up of seven levels, or layers.
 Each layer is defined by software (or, in one case,
hardware) and is clearly distinct from the other layers.
 These layers are not protocols, but they provide a way
to define and partition protocols to make data transfers
in a standardized way.


110

11-8: Protocols
 The layers are:
 Layer 1: Physical layer: The physical connections
and electrical standards for the communication
system are defined here.
 Layer 2: Data link: This layer defines the framing
information for the block of data.
 Layer 3: Network: This layer determines network
configuration and the route the transmission can
take.


111

11-8: Protocols
 Layer 4: Transport: Included in this layer are
multiplexing, error recovery, partitioning of data, and
addressing and flow control operations.
 Layer 5: Session: This layer handles such things as
management and synchronization of the data
transmission.
 Layer 6: Presentation: This layer deals with the form
and syntax of the message.
 Layer 7: Applications: This layer is the overall general
manager of the network or the communication
process.


112

11-8: Protocols

Figure 11-64: The seven OSI layers.

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