Wireless Communication Engineering Data Communications

1
Wireless Communication Engineering - Data
Communications

• Li-Der Jeng
• Department of Electronic Engineering
• Chung-Yuan Christian University
• Chung-Li, Taiwan, ROC
• TEL [phone]
• E-mail lider_at_cycu.edu.tw

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Contents

• Introduction to Data Communications
• Information Encoding
• Analog and Digital Transmission Methods
• Transmission Media

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Introduction to Data Communications
4
Fundamental concepts

• Communication can be defined as exchange of
  information between two humans.
• Data communications can be defined as the
  exchange of information between two computers.
• In its simplest form, the data communications can
  be shown as in the following figure.

5
Data Communication
Source
Destination
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Real-life Data Communication Systems
Demultiplexer
Multiplexer
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Data communications

• In the simplest form, data communications
  involves the exchange of data between two
  computers.
• Computers work with a binary language consisting
  of zero and one.
• Therefore, a computer generates a stream of zeros
  and ones and sends it to another computer to
  which it is connected in some fashion.
• The connection can be either a simple wire or it
  can be through wireless media.

8
Data communications (I)

• For enabling data communications, a combination
  of hardware and software is essential. In any
  data communications system, three characteristics
  are described
• Correct delivery When a sender transmits data
  for an intended recipient, the data must reach
  only the intended recipient and not someone else.
• Accurate delivery The data sent must be received
  in the same form as the one in which it was sent.
  There must not be any sort of alternations to it
  in transit.
• Timely delivery The data must travel from the
  sender to the receiver in a finite amount of
  time. The term finite is quite vague, and would
  depend on the reasons why the data communication
  is taking place.

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Data communications (II)

• Two key aspects of data communication systems
  need a good amount of understanding.
• Transmission media the physical path over which
  data travels from the sender to the receiver. Ex
  twisted-pair of copper wires, coaxial cable,
  optical fiber or wireless media such as radio
  waves.
• Protocol a set of rules and conventions. Ex The
  sender and the receiver, the two key parties in
  data communication must agree on a common set of
  rules, i.e. protocols before they can communicate
  with each other.

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Protocols

• A protocol defines the following  
• Syntax (What is to be communicated) The syntax
  defines the structure or format of data. This
  means that the order in which it is to be sent is
  decided. For instance, a protocol could define
  that the first 16 bits of a data transmission
  must always contain the receivers address. 
• Semantics (How it is to be communicated) The
  semantics define the interpretation of the data
  that is being sent. For example, the semantics
  could define that if the last two bits of the
  receivers address field contain a 00, it means
  that the sender and the receiver are on the same
  network.
• Timing (When it should be communicated) This
  refers to an agreement between the sender and the
  receiver about the data transmission rates and
  duration. For instance, a protocol could demand
  that the sender must send 1000 bytes and then
  wait for an acknowledgement from the receiver
  before sending any more data.

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Standards

• Standards are necessary in every walk of life.
  For instance, when you want to replace a light
  bulb in your home because it has been damaged,
  you expect the new bulb to fit in the holder
  straightaway and work like the old bulb did. What
  is the use if the bulb does not fit in the
  holder, or if it fits in the holder but does not
  illuminate because it requires a different
  voltage level? Consequently, everything that we
  use in our daily life has some common features,
  some standards that every manufacturer must abide
  by. In the absence of standards, every
  manufacturer can theoretically manufacture a set
  of goods or services that are incompatible with
  other manufacturers.
• To avoid such anomalies, a set of standards is
  established, which governs the rules that
  manufacturers must obey. In exactly the same
  fashion, standards for data communications have
  been set. Consequently, a lot of incompatibility
  issues have no place in data communications,
  which is highly desirable.

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Bandwidth of a signal and a medium

• The term bandwidth is very commonly used in data
  communication. The basic idea behind bandwidth
  can be understood quite easily with a simple
  example of pipes carrying water to our homes.
  What is the maximum amount of water a pipe can
  carry at any given time? The maximum capacity of
  the pipe at a given instance is its bandwidth.

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Analog and digital signals

• Any signal can be classified into one of the two
  types analog and digital.
• An analog signal is a continuously varying
  signal, similar to a sinusoidal waveform.
• A digital signal takes the form of pulses, where
  we have something or nothing.

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Analog Signal
Digital Signal
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Amplitude, period, frequency, phase

• Amplitude the signal has maximum value
• Period the time taken for the completion of one
  cycle
• Frequency the number of cycles or revolutions
  that our particle would make in one second
• Phase the phase of a signal is related to the
  position of a waveform relative to time zero

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Fourier analysis and the concept of bandwidth of
a signal

• Ex A periodic signal has been decomposed using
  Fourier analysis to yield four sine waves of
  frequencies 100, 400, 600 and 800 Hz. What is the
  bandwidth of the resulting periodic signal?
  (800-100700)
• Ex A signal has a bandwidth of 20Hz and its
  highest frequency is 60Hz. What is the lowest
  frequency? (60-2040)

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A Digital Signal With Infinite Bandwidth


0 1 0 0 0
1 0



(a)

Pulses before transmission Bit rate 2000 bits
per second






(b)

Bandwidth 500 Hz





(c)

Bandwidth 1500 Hz






(d)

Bandwidth 2000 Hz





(e)

Bandwidth 2500 Hz





(f)

Bandwidth
3000 Hz






Bandwidth 5000 Hz

(g)





(h)


Bandwidth
Hz





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A Digital Signal of 1Hz
0 1 0
1 0 1
0 1 0
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A Sinusoidal Wave with Frequency 10 Hz
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A Medium and Channels
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Information Encoding
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Introduction

• How computers store data?
• Can a computer understand English?
• Does it store data in some other language?
• If a computer cannot understand English, how can
  we codify the data to be stored in a fashion that
  the computer will be able to understand?
• Why a computer uses binary language?

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The BCD Equivalent of Decimal Digits
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The BCD code

• Binary Coded Decimal (BCD) code
• Decimal 2 5
• Binary 0010 0101
• The BCD number for a decimal number 25 is
  00100101
• Decimal 1 0
• Binary 0001 0000
• The BCD number for a decimal number 10 is 00010000

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Portion of the ASCII Table
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Portion of the EBCDIC Table
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Multimedia

• These days, computers can also be used for the
  following
• Drawing, storing and viewing pictures
• Storing sounds and playing them back
• Storing videos and playing them back
• However, pictures, videos and sounds are not made
  up of alphabets and numbers. How can a computer
  recognize and store them? How can we codify
  information about these so that a computer can
  store them?
• To solve this problem of codification of
  pictures, videos and sounds, the concept of
  multimedia came into being. As the name says,
  multimedia means multiple media.

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Picture/Images

• Now let us imagine that we divide the picture by
  a number of horizontal and vertical lines to form
  a grid. Each rectangle is called picture element
  or pixel.
• As the number of pixels increases (also called
  higher resolution), the pixel size decreases.
• Ex We choose the resolution such that each pixel
  has either a dot or a blank.

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Pixels Being Mapped to Zeroes and Ones
Computers Memory
Screen Output
00000000 01100000 00000000
01100000 00001111 11110000 00010000
00001000 00100000 00000100
01000000 00000010 01000000
00000010 01111111 11111110 00100111
01110100
. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . .
Graphics Hardware / Software
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Picture/Image

• In practice, it is common to have 8, 16 or 24
  bits to represent one pixel on the screen.
• It is well known that any color can be derived by
  adding or mixing various intensities of three
  basic colors Red, Green, Blue thus the name
  RGB.
• Many systems that use 24 bits to describe the
  color of one pixel use 8 bits each to describe
  the intensities of Red, Green, Blue.

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Video

• The basic principle behind video is the
  technology animation. The idea behind animation
  is very simple and is used in cartoons and films.
• If a set of pictures is shown rapidly, the human
  eye can be fooled into believing that the
  pictures are in motion.
• It has been proved that if 24 pictures are shown
  in succession in one second, our eyes sense it as
  a continuous motion.

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Sound

• A sound wave (i.e. an audio signal) is continuous
  in nature.
• The continuity is in two respects the strength
  of wave/signal (called amplitude) and time.
• In contrast, a computer works only with binary
  values 0 and 1.
• If we want to translate an audio signal so that
  it can be mapped to the computer-recognizable
  data of 0 and 1, it should be clear that we must
  somehow map an analog signal as a digital signal.

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Audio Signal in the Analog Form
Amplitude
Time
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Signal Representing Only Binary Values (0 and 1)
Amplitude Time
1
0
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Sound

• Sampling measuring the audio signal at fixed
  intervals of time is called sampling. Thus, if we
  decide that the audio signal would be measured
  say 60 times a second, the sampling rate would be
  60.
• Quantizing having determined how many times the
  signal should be measured, the next step is to
  assess the range of amplitudes.

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5 4 3 2 1 0
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Analog and Digital Transmission Methods
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Introduction

• We have studied that the two major types of
  signals are analog and digital. However, the
  manner in which these two types of signals can be
  transmitted are also of the same types, that is
  analog and digital.
• We have four possible combinations
• Analog Signal, Analog Transmission
• Digital Signal, Digital Transmission
• Digital Signal, Analog Transmission
• Analog Signal, Digital Transmission

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Analog Signal, Analog Transmission
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Analog signal, analog transmission

• The term analog is very common and used for
  decades in the field of telephony.
• The human voice generates an analog(i.e.
  continuously varying) signal, which is
  transmitted as an analog signal over the medium.
• On the way, the signal suffers attenuation.
• Amplifiers are used to overcome this problem, but
  then amplifiers amplify noise along with the
  original signal, too.
• The problem with this type of combination is that
  if the signal gets distorted, it cannot be
  reconstructed at all!
• This is the reason why this type is not used
  where a high level of accuracy is desired.

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Digital signal, digital transmission

• We know that the information coming out of a
  computer is the form of digital signals.
• We also know that a digital signal has an
  infinite bandwidth, whereas any medium has only a
  limited bandwidth.
• Therefore, as the signal is generated and enters
  the medium, the signal is distorted.
• The hardware equipment called regenerative
  repeater or repeater is used to regenerate the
  signal.

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Regenerative Repeaters
A
C
B
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Digital signal, digital transmission

• The input to the regenerative repeater is a
  signal, which looks like a digital signal.
  Therefore, the repeater measures the signal
  values at regular intervals to recognize the 0s
  and 1s in the signal and regenerate them.
  Therefore, there is no loss of information.
• However, only one repeater will not do. You will
  require many such repeaters. The distance between
  the repeaters is very crucial. We may like to
  increase that distance as much as possible to
  reduce the cost but then there is also a
  disadvantage to this. (it may be difficult to
  differentiate 0 and 1.)
• Any line with repeaters placed at the appropriate
  distance is called a digital line.
• ATT put such repeaters on the wire pairs used
  for telephonic conversations, separated by a
  distance of only 6000 feet. This digital line is
  called a T1 line, which can carry a data rate of
  1,54,400 bits per second (1.544 Mbps).

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A T1 Line Contains Many Repeaters
Destination
Repeater
Repeater
Repeater
Digital line
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Digital signal, analog transmission

• The designers had two choices for data
  communications between two computers.One was to
  create a new digital network with repeaters etc,
  or use the existing telephone network.
• When computers were invented, the telephone
  network was already in existence. However,
  telephones use analog signals and analog
  circuits. The problem how to send a digital
  signals over an analog network?
• We use a modem for this purpose. The modem is
  derived from two components a modulator and a
  demodulator.

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Use of Modem for Sending Digital Data Over Analog
Lines
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Digital signal, analog transmission

• As the above figure shown, the digital signals
  originating from the computer go through the
  modem where they are converted (i.e., codified or
  modulated) into analog signals whose bandwidth is
  lt 4000 Hz. This is because the channel for
  telephone conversation requires a bandwidth of
  4000 Hz.

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Modulation techniques

• Amplitude Shift Keying ASK
• Frequency Shift Keying FSK
• Phase Shift Keying PSK
• Quadrate Amplitude Modulation QAM
• The main limitation of PSK is the inability of
  the hardware equipment to distinguish small
  differences in terms of phase changes. This puts
  a limitation on its data rate.
• Combine ASK and PSK, makes higher data rates
  possible (since the bandwidth of the transmission
  medium is a major limitation, we cannot combine
  FSK with anything else)

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Amplitude Shift Keying (ASK)
The frequency is between 0 and 4000 Hz
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Frequency Shift Keying (FSK)
f1 f2 f2 f1 f2
f 1 f2 f1
f1 and f2 are between 0 and 4000 Hz
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Phase Shift Keying (PSK)
1 0 0 1 1 0
1 0 0 1 1
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Baud rate and bits per second

• Many people confuse baud rate and bit rate or
  bits per second (bps). There is a difference
  between them.
• The baud rate is the number of times the signal
  level changes in a channel per second. This
  signal level could be amplitude, frequency of
  phase.
• The bandwidth of a transmission medium is finite,
  how can we achieve higher data rates?
• By associating more than one bit for each signal
  level, one can achieve a higher data rate. That
  is, the bit rate will be higher that the baud
  rate in such a case. All this has to be built
  into the modem.

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Single Bit Transmission by Using FSK
bit rate baud rate
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Double Bit Transmission by Using FSK
bit rate 2baud rate
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Analog signal, digital (storage and ) transmission

• This type of transmission is becoming very
  popular due to many reasons that we will discuss
  later.
• The idea is somehow to represent an analog signal
  into digital bits and then transmit it as a
  digital signal.
• There are several techniques that are possible to
  achieve this and we have discussed the general
  overview of the basic idea, but Pulse Code
  Modulation (PCM) is the most popular.

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The basic steps in PCM

• At source
• Sample the analog signal at regular interval say
  t. (Sampling)
• Convert the analog signal into some discrete
  values. (Quantization)
• Convert these values into binary numbers by
  assigning a fixed number of bits for each value.
  (Encoding)
• Convert the binary numbers as a digital signal by
  concatenating all these binary numbers.

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The basic steps in PCM

• At destination
• Convert the digital signal into binary numbers.
• Separate out the discrete values of signals by
  using the number of bits for each discrete value.
• Reconstruct the original analog signal
• We require an equipment called codec
  (Coder/Decoder) at both the source and
  destination to perform these functions. We can
  call it also as A/D (Analog to Digital) converter
  and D/A (Digital to Analog) converter.

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Pulse Code Modulation (PCM)
1.2
1.1
1.07
1. 04
1.0
1.0
?
1.1
?
0.9
0. 83
0.8
0. 8
?
0.7
0.78
0.8
0.6
?
0.5
?
0.68
0.70
0.49
0.4

0.5
?
0.3
1.0.1.1.1.1.1.1.
0.21
0.2
?
0.2
0.1
t
t
Time
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Pulse Code Modulation (PCM)

• The A/D process is called quantization.
• The problem quantization error.
• In the whole process, we have saved a lot in the
  number of bits that we needed to send. Thus,
  there is a trade-off between accuracy and cost or
  speed. The aim of any good PCM strategy would be
  to reduce the quantization noise to a negligible
  level without increasing the load on the network
  significantly.
• The current PCM standard assumes eight
  bits/sample.

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Nyquist theorem

• An interesting question is How do we choose the
  time interval for sampling or slicing the analog
  signal?
• At higher speeds of sampling, the signal is more
  likely to be reproduced faithfully than if the
  speeds are low
• In fact, the sampling speed is related to the
  highest frequency in a signal. Nyquist showed
  that the sampling speed should be 2fmax where
  fmax represents the highest frequency in that
  signal resulting out of Fourier analysis. This is
  called Nyquist theorem.

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Sampling Speeds for Different Frequencies
(a) Low frequency
Amplitude
0 t 2t 3t
4t 5t
Time t
x
y
z
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Nyquist theorem

• Ex if we want to sample a telephone voice with a
  maximum frequency of 4000 Hz, we must have a
  sampling rate of 8000 samples per second. This is
  exactly what is used in PCM standard.
• Video signals have a far higher bandwidth with
  signals at very high frequencies than voice
  signals. This is obvious from the data contents
  and rates of VCD and DVD players (for video disk)
  which are far higher as compared to that of a CD
  (Compact Disk) for music.

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Nyquist theorem

• The human voice has various frequency components
  in the range of 0 to 20000 Hz. However, out of
  this range, the frequency range of 300-3300 Hz is
  sufficient to recognize the voice in a telephone
  conversation.
• The frequencies 0-300 Hz and 3300-4000 Hz act as
  guard bands, so that multiplexing of many signals
  in a single wire is possible. Therefore, the
  maximum frequency in this case is 4000 Hz.
• Using Nyquist theorem, we can conclude that to
  transmit human voice over digital telephone line,
  we must have a sampling rate of 400028000
  samples per second. Moreover, if each sample
  consists of 8 bits, we can have the following
  equation for the bandwidth required of the
  telephone lines to carry digitized human voice

Highest frequency of human voice 2 Number of
bits in each such sample 4000 2
8 64,000 bits per second 64 Kbps
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Transmission Media
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Introduction

• Transmission media are the physical
  infrastructure components that carry data from
  one computer to another.
• Examples
• Telephone wires that connect telephones to the
  central office
• Coaxial cables that carry the cable television
  transmission to homes
• Transmission media need not always in the form of
  a physical wire they can be invisible as well.

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Categories of Transmission Media
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(No Transcript)
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(No Transcript)
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Unshielded Twisted Pair (UTP)

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Categories of UTP
71
Shielded Twisted Pair (STP)
Metal shield
Copper
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Coaxial Cable
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Optical Fiber

• Optical Fiber Structure
• Optical fibers use light instead of electrical
  signals as a means of signal propagation. They
  are made of glass fibers that enclosed in a
  plastic jacket. This allows the fibers to bend
  and not break.
• A transmitter at the senders end of the optical
  fiber sends a light emitting diode (LED) or laser
  to send pulse of light across the fiber.
• A receiver at the other end makes use of a
  light-sensitive transistor to detect the absence
  or presence of light to indicate 0 or 1.

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Optical Fiber
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Propagation Modes
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Multimode Step Index Fiber
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Multimode Graded Index Fiber
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Single Mode Fiber
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Advantages/Disadvantages of Optical fiber

• Advantages
• Resistance to noise
• Huge bandwidth
• Higher signal carrying capacity
• Disadvantages
• Fragility
• Cost
• Maintenance overhead

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Unguided Media

• Unguided media, also called as wireless
  communication, transport electromagnetic waves
  without using a physical conductor.
• The signals propagate through air (or sometimes
  water).
• The communication band for unguided media is as
  shown in the following figure.

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Radio Communications Band
Radio communication
3 KHz
300 GHz
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
3 KHz
300 KHz
30 MHz
3 GHz
300 GHz
30 KHz
3 MHz
300 MHz
30 GHz
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Terrestrial Microwave Communication
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Satellite Microwave Communication
Satellite
A
B
Ground stations
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Satellite Communication

• Problem
• If the earth along with its ground stations is
  revolving and the satellite is stationary, the
  sending and receiving earth stations and
  satellite can be out of sync over time.
  Therefore, normally Geosynchronous satellite are
  used, which move at the same Revolutions Per
  Minute (RPM) as that of the earth in the same
  direction, exactly like the earth.
• Frequency SHF, 3 GHz to 30 GHz
• Two frequency bands
• From the earth to the satellite (called uplink)
• From the satellite to the earth (called downlink)

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Three Satellites to Cover the Planet
86
Access Methods

• There are three methods for communication using
  satellite. These three methods use principles
  that are similar in concept to normal wired
  communication. Like the wired world, satellite
  communication is also based on modulation
  techniques. The three primary modulation
  techniques are
• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division Multiple Access (CDMA)
• Multiple Access This simply means that more than
  one user (multiple) can use (access) each cell.

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  Frequency Division Multiple Access (FDMA)
This is the most popular method for communication
using satellites.
88
Time Division Multiple Access (TDMA)
This is the second most popular mechanism for
communication using satellites.
89
Code Division Multiple Access (CDMA)
90
Cellular (Mobile) Telephones

• First mobile telephone
• As early as 1946.
• The city of St. Louis in USA
• Half-duplex system, known as push-to-talk-system,
  was installed in the big cities in 1950s.
• Even today, taxi, CB-radio etc. use the same
  technology
• The second development took place in 1960s.
• Improved Mobile Telephone System (IMTS)
• Full-duplex system two frequencies are used
• 23 channels
• In IMTS, users had to wait for a long time to get
  a dial tone

91
Cellular (Mobile) Telephones

• The third step Advanced Mobile Phone System
  (AMPS) (TACS in England, MCS-L1 in Japan)
• Cellular phones (Cell radius 012 miles)
• The cells are actually circular, they are shown
  as hexagonal for conceptual clarity.
• Each cell has an antenna and a cell office to
  control that call.
• A Mobile Telephone Switching Office (MTSO)
  control various cell offices and coordinates the
  communication between them and Telephone Central
  Office (TCO) or a telephone exchange.
• TCO a part of the wired land telephone system
• The computer at MTSO is responsible for not only
  the connections but also for the information and
  billing of the calls.

92
Cellular Phone System
Mobile Telephone Switching Office (MTSO)
Cell Office
Cell Office
Cell Office
Telephone Central Office (TCO)
To land telephone system
93
Bands in Cellular Telephony

• Classically, analog transmission is used for
  cellular telephony.
• Frequency modulation is used for communication
  between the mobile phone and the cell office.
  Normally, two frequency bands are allocated for
  this purpose.
• For preventing interference, adjacent channels
  are rarely allocated.
• Some channels are also required for control
  purposes.
• The number of channel 40 in USA
• In USA, two bands 824-849 MHz and 869-894 MHz

94
Frequency Reuse
95
Calls Using Mobile Phones

• A call is made from the mobile phone
• Entering a 7-, 8- or 10-digit phone number
• Mobile phone ? cell office ? MTSO ? TCO. If the
  party is available, CTO lets MTSO know. At this
  juncture, MTSO allocates an empty voice channel
  to the cell to establish the connection.
• When a land phone places a call to a mobile phone
• TCO ? MTSO ? all cell (paging). The cell where
  the mobile phone is currently located responds to
  the MTSO. The MTSO then transmit the incoming
  call signal to the mobile phone, and when the
  mobile phone is answered, the MTSO assigns a
  voice channel to the call, thus enabling the
  conversation.

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Handoff Part 1 - A Unit Becomes Weak in Cell A
97
Handoff Part 2 MTSO Enquires to See If Anybody
Can Take up Unit 50
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99
Transmission Media Characteristics
100
New Developments

• 1. Digital cellular telephone.
• 2. The integration of cellular phones with
  satellite communication. This will enable a
  person to have a unique but same telephone number
  throughout the world, unlike today, when we have
  different numbers for land phones and mobile
  phones.
• 3. The integration of mobile telephony with the
  PC. The scheme is called Mobile Personal
  Communication.