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COMPUTER NETWORKS
Ajit K Nayak, Ph.D.
Department of Computer Science &Information Technology,
School of Computer Science a...
Computer Networking / Module I / AKN / 2
Out Line of Module I
 Overview of Data Communications and Networking
 Physical ...
Computer Networking / Module I / AKN / 3
Overview of Data Communications
and Networking
Lecture I
• Data Communication
• N...
Computer Networking / Module I / AKN / 4
Data Communication
 Sharing of information is “Data Communication”
 Sharing can...
Computer Networking / Module I / AKN / 5
Characteristics of Data Communication
 Delivery: system must deliver data to cor...
Computer Networking / Module I / AKN / 6
Components of Data Communication
 Message: It is the Information (data) to be
co...
Computer Networking / Module I / AKN / 7
Direction of Data Flow
 Communication can be simplex, Half-duplex, or
full-duple...
Computer Networking / Module I / AKN / 8
Networks & Distributed processing
 Interconnection of „Intelligent devices‟ is c...
Computer Networking / Module I / AKN / 9
Network Criteria
 Reliability depends on
 Frequency of failure: all networks fa...
Computer Networking / Module I / AKN / 10
Physical Structure
 It refers to the way two or more devices are
attached to a ...
Computer Networking / Module I / AKN / 11
Topology
 Topology of a network is the geometric
representation of the links an...
Computer Networking / Module I / AKN / 12
Mesh Topology
 Every device has a dedicated point-to-
point link to every other...
Computer Networking / Module I / AKN / 13
Star Topology
 Each computer has a point-point
link only to a central controlle...
Computer Networking / Module I / AKN / 14
Bus Topology
 Multi-point
 One long cable acts as a
backbone to link all the
d...
Computer Networking / Module I / AKN / 15
Ring Topology
 Point-to-point
 Each device is linked only
to its immediate
nei...
Computer Networking / Module I / AKN / 16
Category of networks
 The networks may be categorized according to
its size, ow...
Computer Networking / Module I / AKN / 17
Local Area Network(LAN)
 LAN is a privately owned
networks within a single
buil...
Computer Networking / Module I / AKN / 18
Figure 1.13 LAN (Continued)
Example: LAN of an organisation
Computer Networking / Module I / AKN / 19
Metropolitan Area Network(MAN)
 MAN is designed to extend
over an entire city
...
Computer Networking / Module I / AKN / 20
Metropolitan Area Network(MAN)
 It utilizes public, leased or private communica...
Computer Networking / Module I / AKN / 21
A metropolitan area network based on cable TV.
Computer Networking / Module I / AKN / 22
The Internet
 It is a specific world wide network (i.e. A network of networks)
...
Computer Networking / Module I / AKN / 23
Internet Today
 It is difficult to give an accurate representation of the Inter...
Computer Networking / Module I / AKN / 24
Internet today
History of Internet
- read yourself
(page 15, sec 1.3)
Computer Networking / Module I / AKN / 25
Services provided by Internet
 The www including browsing & internet commence
...
Computer Networking / Module I / AKN / 26
Protocol !!!
 What is a Protocol?
 What does a protocol do?
 How would you re...
Computer Networking / Module I / AKN / 27
Protocol
 First you offer a
greeting (Hi )
 The typical response to
a Hi is a ...
Computer Networking / Module I / AKN / 28
Protocol
 But what happens when a different response comes to
the initial Hi li...
Computer Networking / Module I / AKN / 29
Protocol
 If people run different protocols! Say
 If one person has manners an...
Computer Networking / Module I / AKN / 30
A Network Protocol
 Visiting a Web site
 Type in the URL in Web
browser
 Firs...
Computer Networking / Module I / AKN / 31
Defining A Protocol
A Protocol defines the format and the order
of messages exch...
Computer Networking / Module I / AKN / 32
Protocols contd.
 A protocol defines what is communicated, How
it is communicat...
Computer Networking / Module I / AKN / 33
Standards
 The standard provides a model for development that
makes it possible...
Computer Networking / Module I / AKN / 34
Standards Organizations
 Standards Creation Committees
 International Standard...
Computer Networking / Module I / AKN / 35
Layered Tasks
 The service that we expect from a Computer Network are much more...
Computer Networking / Module I / AKN / 36
Example: Sending a letter
Computer Networking / Module I / AKN / 37
Example: The philosopher-translator-secretary architecture.
Computer Networking / Module I / AKN / 38
The Internet model
 The layered protocol stack that is used in
practice is a fi...
Computer Networking / Module I / AKN / 39
Peer-to-peer processes
Computer Networking / Module I / AKN / 40
An exchange using the Internet model
Computer Networking / Module I / AKN / 41
Physical layer
 The responsibility of physical layer is to coordinate the funct...
Computer Networking / Module I / AKN / 42
Data link layer
 is responsible for transmitting frames from
one node to the ne...
Computer Networking / Module I / AKN / 43
Datalink layer contd.
 Physical addressing and hop-hop delivery can
be done in ...
Computer Networking / Module I / AKN / 44
Network Layer
 The network layer is responsible for the delivery of packets
fro...
Computer Networking / Module I / AKN / 45
Source-to-Destination
Computer Networking / Module I / AKN / 46
An Example
sending from a node with network
address A and physical address 10 to...
Computer Networking / Module I / AKN / 47
Transport layer
 The transport layer is responsible for delivery of a message
f...
Computer Networking / Module I / AKN / 48
Transport layer contd.
 Connection control
 Two types of connection service is...
Computer Networking / Module I / AKN / 49
Application layer
 The application layer is responsible for providing
services ...
Computer Networking / Module I / AKN / 50
Summary of duties
Computer Networking / Module I / AKN / 51
OSI model
 Session Layer is the network dialog controller, It
establishes maint...
Computer Networking / Module I / AKN / 52
The Physical Layer
Lecture II
• Signals
• Digital Transmission
• Analog Transmis...
Computer Networking / Module I / AKN / 53
Position of the physical layer
Computer Networking / Module I / AKN / 54
Signals
 Information is transmitted in the form of
electromagnetic signals
 Si...
Computer Networking / Module I / AKN / 55
Periodic / Aperiodic Signals
Periodic Signal: A signal completes a pattern withi...
Computer Networking / Module I / AKN / 56
Analog Signals
 The sine wave is the most fundamental form of a
periodic signal...
Computer Networking / Module I / AKN / 57
Amplitude Period and frequency
Computer Networking / Module I / AKN / 58
Time and frequency domains
A signal can also be represented in frequency domain
Computer Networking / Module I / AKN / 59
Composite signals
 A single-frequency sine wave is not useful in
data communica...
Computer Networking / Module I / AKN / 60
Example : A Square wave
 According to Fourier analysis, this signal can be
deco...
Computer Networking / Module I / AKN / 61
Three harmonics
Computer Networking / Module I / AKN / 62
Frequency spectrum
The Signal using the
frequency domain and
containing all its
...
Computer Networking / Module I / AKN / 63
Example
A signal has a spectrum with frequencies between 1000 and
2000 Hz (bandw...
Computer Networking / Module I / AKN / 64
Digital Signals
 Digital signals can be better described by two terms
 Bit int...
Computer Networking / Module I / AKN / 65
Analog vs Digital
• Channels or links are of two types
• low-pass: lower limit i...
Computer Networking / Module I / AKN / 66
Data rate limits
 Data rate depends on
 The BW available
 The levels of signa...
Computer Networking / Module I / AKN / 67
Signal to noise ratio
 SNR=Avg. Signal Power/Avg. Noise Power
 SNRdb = 10 log1...
Computer Networking / Module I / AKN / 68
Example
We have a channel with a 1 MHz bandwidth. The SNR for this
channel is 63...
Computer Networking / Module I / AKN / 69
Signal
 Data rate depends on
 The BW available
 The levels of signal that can...
Computer Networking / Module I / AKN / 70
Transmission Impairment
 In practice the signal sent at sending end using
a tra...
Computer Networking / Module I / AKN / 71
Distortion
 Signal changes its forms at the receiving end
 It is normally happ...
Computer Networking / Module I / AKN / 72
Noise
 Several types of noise such as
 thermal noise: random motion of electro...
Computer Networking / Module I / AKN / 73
More terminologies
 Throughput: number of
bits passed per second at a
given poi...
Computer Networking / Module I / AKN / 74
Digital Transmission
Line coding
Block Coding
Sampling
Transmission Mode
Computer Networking / Module I / AKN / 75
What is Line Coding?
 Is the process of converting binary data (a
sequence of b...
Computer Networking / Module I / AKN / 76
Signal Level versus Data Level
 No of values allowed in a signal
 No of values...
Computer Networking / Module I / AKN / 77
DC Component
 A component having zero frequency
 Can‟t be passed through a tra...
Computer Networking / Module I / AKN / 78
Pulse Rate versus Bit Rate
 No of pulses per second
 Minimum amount of time re...
Computer Networking / Module I / AKN / 79
 No Synchronization: if receivers clock is faster
 A Signal that includes timi...
Computer Networking / Module I / AKN / 80
Example
In a digital transmission, the receiver clock is
0.1 percent faster than...
Computer Networking / Module I / AKN / 81
Line Coding Schemes
Computer Networking / Module I / AKN / 82
Unipolar encoding uses only one voltage
level.
Note:
UniPolar Encoding
Computer Networking / Module I / AKN / 83
Unipolar Encoding
 One is coded as +ve voltage
 Zero is coded as –ve voltage
Computer Networking / Module I / AKN / 84
Polar encoding uses two voltage levels
(positive and negative).
Note:
Polar Enco...
Computer Networking / Module I / AKN / 85
Polar Encoding
 Avarage voltage level is decreased
 DC component problem is av...
Computer Networking / Module I / AKN / 86
In NRZ-L the level of the signal is
dependent upon the state of the bit.
Note:
N...
Computer Networking / Module I / AKN / 87
In NRZ-I the signal is inverted if a 1 is
encountered.
Note:
NRZ-I Encoding
Computer Networking / Module I / AKN / 88
NRZ Encoding
 Loss of synchronization incase of continuous
ones or zeros
Computer Networking / Module I / AKN / 89
RZ uses three values i.e. +ve, zero & -ve
Signal change occurs during each bit
N...
Computer Networking / Module I / AKN / 90
RZ Encoding
 A +ve voltage means 1 and –ve voltage means
zero.
 But signal ret...
Computer Networking / Module I / AKN / 91
RZ is a good encoded digital signal that contain
a provision for synchronization...
Computer Networking / Module I / AKN / 92
In Manchester encoding, the transition at
the middle of the bit is used for both...
Computer Networking / Module I / AKN / 93
Manchester Encoding
 It achieves the synchronization but with two levels of
amp...
Computer Networking / Module I / AKN / 94
In differential Manchester encoding, the
transition at the middle of the bit is ...
Computer Networking / Module I / AKN / 95
Diff-Manchester Encoding
 Manchester Encoding used for 802.3 base band
– CSMA/C...
Computer Networking / Module I / AKN / 96
In bipolar encoding, we use three levels:
positive, zero,
and negative.
Note:
Bi...
Computer Networking / Module I / AKN / 97
Bipolar Encoding
Computer Networking / Module I / AKN / 98
2B1Q Encoding
 Two Binary One Quaternary
 Each pulse represents 2 bits
-1
-3
Computer Networking / Module I / AKN / 99
MLT-3 Encoding
 Multi transmission, three level (MLT-3)
 The signal transition...
Computer Networking / Module I / AKN / 100
 To ensure synchronization some
redundant bits may be introduced
Steps in Tran...
Computer Networking / Module I / AKN / 101
Block Coding
Computer Networking / Module I / AKN / 102
Substitution
Computer Networking / Module I / AKN / 103
4B/5B Encoding
 Each 4-bit 'nibble' of received data has an extra
5th bit adde...
Computer Networking / Module I / AKN / 104
Data Code Data Code
0000 11110 1000 10010
0001 01001 1001 10011
0010 10100 1010...
Computer Networking / Module I / AKN / 105
Example 8B/6T
 sends 8 data bits as six ternary (one of three voltage
levels i...
Computer Networking / Module I / AKN / 106
Pulse Amplitude Modulation
 Generates a series of pulses by sampling a
given a...
Computer Networking / Module I / AKN / 107
Pulse amplitude modulation has some
applications, but it is not used by itself ...
Computer Networking / Module I / AKN / 108
PCM: Quantization
 It is a method of assigning integral values in a
specific r...
Computer Networking / Module I / AKN / 109
Binary encoding
 Each quantized value is translated into a 7bit
binary equival...
Computer Networking / Module I / AKN / 110
Line coding
 The binary digits are transformed to a digital
signal by using on...
Computer Networking / Module I / AKN / 111
Analog to PCM Digital Code
Computer Networking / Module I / AKN / 112
According to the Nyquist theorem, the
sampling rate must be at least 2 times th...
Computer Networking / Module I / AKN / 113
Nyquist Theorem
Computer Networking / Module I / AKN / 114
Example
What sampling rate is needed for a signal with a
bandwidth of 10,000 Hz...
Computer Networking / Module I / AKN / 115
Example
A signal is sampled. Each sample requires at least
12 levels of precisi...
Computer Networking / Module I / AKN / 116
Example
We want to digitize the human voice. What is the bit
rate, assuming 8 b...
Computer Networking / Module I / AKN / 117
Transmission mode
Computer Networking / Module I / AKN / 118
 Information is organized into group of bits
 All bits of one group are trans...
Computer Networking / Module I / AKN / 119
Serial Transmission
 One bit follows another using same line
 Reduced cost (b...
Computer Networking / Module I / AKN / 120
In asynchronous transmission, we send 1
start bit (0) at the beginning and 1 or...
Computer Networking / Module I / AKN / 121
Asynchronous Transmission
 Insertion of extra bits & a gap makes it slower
 B...
Computer Networking / Module I / AKN / 122
Asynchronous here means “asynchronous
at the byte level,” but the bits are stil...
Computer Networking / Module I / AKN / 123
In synchronous transmission,
we send bits one after another without
start/stop ...
Computer Networking / Module I / AKN / 124
Synchronous Transmission
 More speed
 Synchronization is necessary
 Accuracy...
Computer Networking / Module I / AKN / 125
Modulation of Digital Data
Digital-to-Analog Conversion
Amplitude Shift Keying ...
Computer Networking / Module I / AKN / 126
Digital to analog modulation
It is Needed if the transmission line is analog bu...
Computer Networking / Module I / AKN / 127
Bit rate is the number of bits per second. Baud
rate is the number of signal un...
Computer Networking / Module I / AKN / 128
Example
An analog signal carries 4 bits in each signal unit. If
1000 signal uni...
Computer Networking / Module I / AKN / 129
Amplitude Shift Keying
• The intensity of the signal is
varied to represent bin...
Computer Networking / Module I / AKN / 130
Example
Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz), draw
the full-duple...
Computer Networking / Module I / AKN / 131
Frequency Shift Keying
 Frequency of carrier signal
varies to represent a bina...
Computer Networking / Module I / AKN / 132
Example
Find the maximum bit rates for an FSK signal if the
bandwidth of the me...
Computer Networking / Module I / AKN / 133
Phase Shift Keying
 Phase of carrier signal varies
to represent a binary 1
(18...
Computer Networking / Module I / AKN / 134
Other variations of PSK
4-PSK / Q-PSK, 2 bits per baud
8-PSK, 3 bits per baud
i...
Computer Networking / Module I / AKN / 135
QAM is a combination of ASK and PSK
so that a maximum contrast between each
sig...
Computer Networking / Module I / AKN / 136
4-QAM & 8-QAM Constellation
Computer Networking / Module I / AKN / 137
16-QAM constellations
QAM is less susceptible to noise than ASK?
Bandwidth requ...
Computer Networking / Module I / AKN / 138
Bit/Baud Comparison
Computer Networking / Module I / AKN / 139
A telephone line has a bandwidth of almost 2400 Hz for data
transmission.
Modem...
Computer Networking / Module I / AKN / 140
Modulation/Demodulation
 A modulator creates a band-pass
signal from binary da...
Computer Networking / Module I / AKN / 141
V series modems
V.32 constellation & BW
• published by ITU-T
• it uses a techni...
Computer Networking / Module I / AKN / 142
V.32bis constellation & BW
 Uses 128-QAM (7 bits/ baud
with 1 bit for error co...
Computer Networking / Module I / AKN / 143
Traditional modems
56 K Modems
• Sampled, digitized and
at telephone comp
• The...
Computer Networking / Module I / AKN / 144
Modulation of Analog Signals
Methods:
Amplitude Modulation (AM)
Frequency Modul...
Computer Networking / Module I / AKN / 145
Amplitude modulation
• The carrier signal is modulated so that
its amplitude va...
Computer Networking / Module I / AKN / 146
Frequency modulation
• The carrier signal is modulated so
that its frequency va...
Computer Networking / Module I / AKN / 147
The Physical Layer contd.
Lecture III
• Multiplexing
• Transmission Media
• Swi...
Computer Networking / Module I / AKN / 148
Multiplexing
 It is not practical to have a separate line for each other devic...
Computer Networking / Module I / AKN / 149
Categories of multiplexing
Computer Networking / Module I / AKN / 150
Frequency Division Multiplexing
 FDM is an analog
multiplexing technique
that ...
Computer Networking / Module I / AKN / 151
FDM
• Carrier frequencies are separated by sufficient BW to
accommodate modulat...
Computer Networking / Module I / AKN / 152
Example 1
Assume that a voice channel occupies a bandwidth of 4 KHz.
We need to...
Computer Networking / Module I / AKN / 153
Example
Five channels, each with a 100-KHz bandwidth, are to be
multiplexed tog...
Computer Networking / Module I / AKN / 154
Example
Four data channels (digital), each transmitting at 1 Mbps, use
a satell...
Computer Networking / Module I / AKN / 155
Analog hierarchy
Computer Networking / Module I / AKN / 156
Wave Division Multiplexing
 Very narrow bands of light
from different sources ...
Computer Networking / Module I / AKN / 157
Time division Multiplexing
 Each shared connection occupies a portion of time ...
Computer Networking / Module I / AKN / 158
Time division Multiplexing contd.
 If the data rate of a link is 3 times the d...
Computer Networking / Module I / AKN / 159
Example
Four 1-Kbps connections are multiplexed together. A unit is
1 bit. Find...
Computer Networking / Module I / AKN / 160
Example
Four channels are multiplexed using TDM. If each channel
sends 100 byte...
Computer Networking / Module I / AKN / 161
Example
A multiplexer combines four 100-Kbps channels using a time
slot of 2 bi...
Computer Networking / Module I / AKN / 162
Synchronization
• Synchronization between multiplexer and demultiplexer is
impo...
Computer Networking / Module I / AKN / 163
Example
We have four sources, each
creating 250 characters per
second. If the i...
Computer Networking / Module I / AKN / 164
Bit Padding
 If one or more devices are faster than other
devices than faster ...
Computer Networking / Module I / AKN / 165
Example 9
Two channels, one with a bit rate of 100 Kbps and
another with a bit ...
Computer Networking / Module I / AKN / 166
DS hierarchy
Telephone companies implement TDM through hierarchy of
digital sig...
Computer Networking / Module I / AKN / 167
T-1 line for multiplexing telephone lines
o Digital Signal services are impleme...
Computer Networking / Module I / AKN / 168
T-1 frame structure
• The frame used on a T-1 line is usually 193 bits divided ...
Computer Networking / Module I / AKN / 169
E Line
Rate
(Mbps)
Voice
Channels
E-1 2.048 30
E-2 8.448 120
E-3 34.368 480
E-4...
Computer Networking / Module I / AKN / 170
Multiplexing and inverse multiplexing
• Inverse multiplexing takes data from hi...
Computer Networking / Module I / AKN / 171
Transmission Media
 Signals in the form of electromagnetic energy is
propagate...
Computer Networking / Module I / AKN / 172
Classes of transmission media
Computer Networking / Module I / AKN / 173
Guided Media
 Provides a conduit from one device to another,
includes
 Twiste...
Computer Networking / Module I / AKN / 174
Unshielded vs Shielded Twisted-Pair Cable
 STP has a metal foil or braided-mes...
Computer Networking / Module I / AKN / 175
Categories of Unshielded Twisted-Pair cables
Category Bandwidth Data Rate Digit...
Computer Networking / Module I / AKN / 176
UTP Contd.
 RJ-45 (Registered-Jack)is
used for 4-pair UTP cable
 UTP can pass...
Computer Networking / Module I / AKN / 177
Coaxial Cable
 It can carry higher frequency ranges
than UTP
 The outer metal...
Computer Networking / Module I / AKN / 178
Coaxial Cable contd.
 BNC connectors are
used(Bayone-Neill-Concelman)
 BNC co...
Computer Networking / Module I / AKN / 179
Fiber-Optic cables
 Transmits signals in the form of visible light
 It uses t...
Computer Networking / Module I / AKN / 180
Propagation modes
Current technology allows two modes of propagating
light alon...
Computer Networking / Module I / AKN / 181
Mechanism
 Distortion is less as compared to step-index as distance traveled i...
Computer Networking / Module I / AKN / 182
Fiber Optics contd. Type Core
Clad
ding
Mode
50/125 50 125
Multimode,
graded-in...
Computer Networking / Module I / AKN / 183
Fiber Optics contd.
 It uses three different types of
connectors
 Subscriber ...
Computer Networking / Module I / AKN / 184
Advantages and Disadvantages
Adavntages
 Higher Bandwidth
 BW is not limited ...
Computer Networking / Module I / AKN / 185
Unguided Media
 It transports electromagnetic waves without using a
physical c...
Computer Networking / Module I / AKN / 186
Radio and microwaves of Electromagnetic spectrum is divided into 8 ranges
Band ...
Computer Networking / Module I / AKN / 187
Wireless transmission waves
 Wireless transmission is broadly divided into thr...
Computer Networking / Module I / AKN / 188
Antennas
Unidirectional Antenna
Radiation and reception of electromagnetic wav...
Computer Networking / Module I / AKN / 189
Switching
 To connect multiple devices over a distance we
adopt a method calle...
Computer Networking / Module I / AKN / 190
Circuit Switching
 It creates a direct physical connection between two
devices...
Computer Networking / Module I / AKN / 191
Crossbar switch
 It connects n I/Ps and m O/Ps in a grid
 Each cross point co...
Computer Networking / Module I / AKN / 192
Multistage switch
 Uses crossbar switches in several stages
 The design of mu...
Computer Networking / Module I / AKN / 193
Time Division Switches
 It uses time division multiplexing to achieve switchin...
Computer Networking / Module I / AKN / 194
TDM Bus
 In this case the I/P and
O/P are connected to a
high speed bus throug...
Computer Networking / Module I / AKN / 195
TDM Bus
 Space division switches have no delay and time division
switches requ...
Computer Networking / Module I / AKN / 196
Telephone Network
 Telephone network is made of three major components: local
...
Computer Networking / Module I / AKN / 197
Making a Connection
 Accessing the switching station at the end offices is
acc...
Computer Networking / Module I / AKN / 198
Voice communication used analog signals
in the past, but is now moving to digit...
Computer Networking / Module I / AKN / 199
Packet Switching
 Circuit switching are best suited for voice
communication, a...
Computer Networking / Module I / AKN / 200
Datagram Approach
 In this approach each packet treated independently called
d...
Computer Networking / Module I / AKN / 201
Virtual Circuit Approach
 In this approach the relationship between all packet...
Computer Networking / Module I / AKN / 202
A Comparison for data traffic
 A circuit switch connection creates a physical ...
Computer Networking / Module I / AKN / 203
Effect of Packet Size
 Virtual circuit from x to y
 a and b are intermediate ...
Computer Networking / Module I / AKN / 204
Packet Size contd.
 Case I
 packet is first transmitted from X to a. when the...
Computer Networking / Module I / AKN / 205
One more comparison
 Performance
 Propagation delay
 Time it takes a signal ...
Computer Networking / Module I / AKN / 206
Circuit Switching Datagram Virtual-Circuit
Dedicated transmission path No dedic...
Computer Networking / Module I / AKN / 207
Network Performance
 Throughput
 Is a measure of the actual transmission of d...
Computer Networking / Module I / AKN / 208
End of Module I
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Computer Networks Module I

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Data Communication
Networks & Internet
Protocols & Standards
Layered Tasks
Internet Model
OSI Model
Digital Transmission
Analog Transmission
Multiplexing
Transmission Media
Circuit switching and Telephone Network
Signals
Digital Transmission
Analog Transmission
Multiplexing
Transmission Media

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Computer Networks Module I

  1. 1. COMPUTER NETWORKS Ajit K Nayak, Ph.D. Department of Computer Science &Information Technology, School of Computer Science and Engineering, ITER, SOA University. Lecture Notes Module I
  2. 2. Computer Networking / Module I / AKN / 2 Out Line of Module I  Overview of Data Communications and Networking  Physical Layer  Digital Transmission  Analog Transmission  Multiplexing  Transmission Media  Circuit switching and Telephone Network Readings: “Data Communications and Networking” Behrouz A Forouzan, Chapter 1 - Chapter 7
  3. 3. Computer Networking / Module I / AKN / 3 Overview of Data Communications and Networking Lecture I • Data Communication • Networks & Internet • Protocols & Standards • Layered Tasks • Internet Model • OSI Model
  4. 4. Computer Networking / Module I / AKN / 4 Data Communication  Sharing of information is “Data Communication”  Sharing can be local (face to face)  Remote (over a distance)  “Data” refers to facts, concepts and / or instructions  In the context of computers, data represented in the form of 0‟s and 1‟s  “Data Communication” is “Exchange of data between two/more devices via a transmission medium.
  5. 5. Computer Networking / Module I / AKN / 5 Characteristics of Data Communication  Delivery: system must deliver data to correct destination  Accuracy: Accurate data should be delivered  Timeliness: Data delivered late are useless
  6. 6. Computer Networking / Module I / AKN / 6 Components of Data Communication  Message: It is the Information (data) to be communicated (shared) with others  Sender: The device that sends the message  Receiver: The device that receives the message  Medium: Physical path by which a message travels from sender to receiver  Protocol: A set of rules that governs the data communication
  7. 7. Computer Networking / Module I / AKN / 7 Direction of Data Flow  Communication can be simplex, Half-duplex, or full-duplex.  Simplex: communication is unidirectional  Half-duplex: bi-directional but not at the same time  Full-duplex: bi-directional and simultaneously. Any real life examples?
  8. 8. Computer Networking / Module I / AKN / 8 Networks & Distributed processing  Interconnection of „Intelligent devices‟ is called a „computer network‟  In „Distributed processing‟ a task is divided and submitted among multiple computers using network  Network Criteria: to design an effective and efficient network the most important criteria are  „Performance‟ depends on  No of users: large no of users may slow down the „response time‟ due to heavy traffic  Type of transmission medium: defines the speed at which the data can travel (speed of light is the upper bound)  Hardware: A high-speed computer with greater storage provides better performance  Software: efficient mechanisms to transform raw data into transmittable signal, to route the signals, to ensure error-free delivery etc.
  9. 9. Computer Networking / Module I / AKN / 9 Network Criteria  Reliability depends on  Frequency of failure: all networks fail occasionally  Recovery time: how long does it takes to restore the service  Catastrophe: networks should be protected from fire, earthquake, theft, etc.  Security depends on  Unauthorized access should be prevented  Should be protected from viruses, spywares, adwares, malwares etc.
  10. 10. Computer Networking / Module I / AKN / 10 Physical Structure  It refers to the way two or more devices are attached to a link  Point-to-Point: provides a dedicated link between two devices. i.e. entire capacity of the link is reserved for transmission between those two devices  Multi-point: In this configuration more than two devices share the same link  If several devices can use the link simultaneously then called „spatially shared connection‟  If devices take turns then it is a time-shared connection (temporally)
  11. 11. Computer Networking / Module I / AKN / 11 Topology  Topology of a network is the geometric representation of the links and nodes of a physical network. ETC.
  12. 12. Computer Networking / Module I / AKN / 12 Mesh Topology  Every device has a dedicated point-to- point link to every other device  A fully connected mesh network has n(n-1)/2 links  Every device required to have at least n-1 I/O ports  Eliminates traffic problem as links are not shared  It is robust as breaking one link couldn't defunct the network completely  Privacy/security is maintained  Installation and reconfiguration is difficult due to complicated connections  Expensive in terms of cost and space  Not Difficult to add/remove a device
  13. 13. Computer Networking / Module I / AKN / 13 Star Topology  Each computer has a point-point link only to a central controller called the HUB  HUB acts as an exchange to send data from one device to another  Less expensive than mesh  It is robust as one link failure causes that device to go out of the network and it does not affect others  Easy fault finding  when one device sending data to another device, all other devices have to be idle  however, a switch in place of hub can eliminate this problem
  14. 14. Computer Networking / Module I / AKN / 14 Bus Topology  Multi-point  One long cable acts as a backbone to link all the devices  There is a limit on the no of drop lines (tapes) as in each tape some energy is lost  Installation is easy  It uses less cabling than star or mesh  difficult reconnection and fault finding  Adding new device may require modification/replacement of the backbone otherwise the performance will be degraded  Fault in bus stops all transmission, the damaged area reflects signal back in the direction of origin, creating noise in both directions
  15. 15. Computer Networking / Module I / AKN / 15 Ring Topology  Point-to-point  Each device is linked only to its immediate neighbours  To add or remove a device requires moving two connections only  Each device in the ring incorporates a repeater to regenerate a signal before passing to neighbour.  Easy to install and reconfiguration  Maximum ring length and no of devices are fixed  failure of one device causes network failure if not bypassed  unidirectional data traffic
  16. 16. Computer Networking / Module I / AKN / 16 Category of networks  The networks may be categorized according to its size, ownership, distance it covers and its physical architecture.
  17. 17. Computer Networking / Module I / AKN / 17 Local Area Network(LAN)  LAN is a privately owned networks within a single building or campus  Size is restricted? (10m-1KM)  Common LAN topologies are bus, ring, star  Speed is high (100Mbps – 1 Gbps)  These are designed to share resources (hardware/software) between personal computers or workstations  the size is restricted as the H/w will not work correctly over wires that exceed the bound as electrical signal becomes weaker over distance due to resistance.  Also the delay increases as the distance, but LANs are designed for specific delays?
  18. 18. Computer Networking / Module I / AKN / 18 Figure 1.13 LAN (Continued) Example: LAN of an organisation
  19. 19. Computer Networking / Module I / AKN / 19 Metropolitan Area Network(MAN)  MAN is designed to extend over an entire city  It may be either private(cable TV, Bank ATMs), or public (Telephone)  May be a single network like cable TV or may be a means of connecting a number of LANs into a larger network so that the resources may be shared  It forms the basic long distance connection in a large network & technologies that provide high speed digital access to individual homes & business  Also sometimes called the access network, as it provides access to various services, say cable TV, Internet etc.
  20. 20. Computer Networking / Module I / AKN / 20 Metropolitan Area Network(MAN)  It utilizes public, leased or private communication devices  The end systems are connected to subnets, which are intelligent entities and contains communication channels and routers  A WAN wholly owned by a single company is called an „enterprise network „  speed is less than LANs  WAN provides long distance transmission of data, voice, image, and video information over large geographical areas that may comprise a country, a continent or even the whole world
  21. 21. Computer Networking / Module I / AKN / 21 A metropolitan area network based on cable TV.
  22. 22. Computer Networking / Module I / AKN / 22 The Internet  It is a specific world wide network (i.e. A network of networks) that interconnects millions of computing devices throughout the world  Computing devices include  PCs, UNIX based workstations, servers(?)  PDAs, TVs, Mobile computers, automobiles, Toaters, …  End systems are connected either directly by „communication links‟ or indirectly by intermediate switching devices called „switches/Routers‟  Communication links include  Coaxial cable, copper wire, fiber optics, radio spectrum  Different communication links can transmit data at different speeds. The link transmission rate is called „bandwidth‟  Switches/Routers receives a chunk of information (called a packet) and forwards it towards destination
  23. 23. Computer Networking / Module I / AKN / 23 Internet Today  It is difficult to give an accurate representation of the Internet as it is continuously changing  It is represented in form of hierarchy of Service providers  International Service Providers  That connect nations together  National Service Providers  Are backbone networks created and maintained by specialized companies like SprintLink, PSINet, etc  Theses networks are connected by complex switching stations called Network Access Points (NAPs)  Regional Service Providers  Are smaller ISPs that are connected to one or more NSPs  Local Service Providers  Provide direct service to end users, may be connected to regional ISPs or directly to NSPs
  24. 24. Computer Networking / Module I / AKN / 24 Internet today History of Internet - read yourself (page 15, sec 1.3)
  25. 25. Computer Networking / Module I / AKN / 25 Services provided by Internet  The www including browsing & internet commence  E-mail including attachment  Instant messages  Peer-to-peer file sharing  VOIP  Online Games  Tele Conferencing  Video-on-demand  Remote Login (SSH client, Telnet) etc…  Remote file transfer  . . .
  26. 26. Computer Networking / Module I / AKN / 26 Protocol !!!  What is a Protocol?  What does a protocol do?  How would you recognize a protocol if you met one? A Human Analogy  What you do when you want to ask some one for the time of day?
  27. 27. Computer Networking / Module I / AKN / 27 Protocol  First you offer a greeting (Hi )  The typical response to a Hi is a returned Hi  This response is an indication that you can proceed and ask for the time  And the conversation continues . . .
  28. 28. Computer Networking / Module I / AKN / 28 Protocol  But what happens when a different response comes to the initial Hi like  Don’t bother me! OR  I don’t speak English OR  Some unprintable reply! OR  No response at all !!!  Then human protocol would be not to ask for the time of day  In our human protocol, there are specific messages we send, and specific actions we take in response to the received reply messages
  29. 29. Computer Networking / Module I / AKN / 29 Protocol  If people run different protocols! Say  If one person has manners and other does not  If one understands concept of time other does not  Then protocols do not interoperate and no useful work can be accomplished.  The same is true in networking – It takes two (or more) communicating entities running the same protocol in order to accomplish a task  But the exception is that the entities exchanging messages and taking action are Hardware and/or Software components of some device
  30. 30. Computer Networking / Module I / AKN / 30 A Network Protocol  Visiting a Web site  Type in the URL in Web browser  First your computer will send a connection request message to the Web Server  Web Server will respond by returning a connection reply message  Your computer then sends the name of the web page  Finally the server returns the page to you.
  31. 31. Computer Networking / Module I / AKN / 31 Defining A Protocol A Protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message of other event. . . . J. F. Kurose
  32. 32. Computer Networking / Module I / AKN / 32 Protocols contd.  A protocol defines what is communicated, How it is communicated, when it is communicated  The key elements of a protocol are  Syntax: refers to structure or format of data, i.e. the order in which they are presented Example: a date  Semantics: refers to structure meaning of each section  Timing: refers to two characteristics. i. When data should be sent. ii. How fast they can be sent  Depends on link availability, and speed of receiver day Yearmonth 8 8 16
  33. 33. Computer Networking / Module I / AKN / 33 Standards  The standard provides a model for development that makes it possible for a product to work regardless of the individual manufacturer  Example: A steering wheel of a car from one make may not feet into other make  Standards are essential in creating and maintaining an open and competitive market and guarantees international inter-operability  Two categories of standards  De Facto: that have just happened without any formal plan  De Jure: are formal, legal standards adopted by some authorized or officially recognized body
  34. 34. Computer Networking / Module I / AKN / 34 Standards Organizations  Standards Creation Committees  International Standards Organization (ISO)  International Telecommunications Union-Telecommunication standards (ITU-T)  American National Standards Institute (ANSI)  Institute of Electrical and Electronics Engineers (IEEE)  Electronic Industries Association (EIA)  Forums  The forums work with universities and users to test, evaluate and the conclusion is presented to standard bodies to standardize new technologies  Regulatory Agencies  Govt. agencies responsible for protecting the public interest.  Internet Standards  Internet draft is a working document with no official status and a 6 month life time.  If recommended by IETF then a draft may be published as a Request for Comment (RFC)
  35. 35. Computer Networking / Module I / AKN / 35 Layered Tasks  The service that we expect from a Computer Network are much more complex than just sending a signal from one device to another.  To solve a complex problem we apply the strategy “Divide and Rule”. i.e. the main problem is divided into some small tasks/ levels of reduced complexity and then handled individually.  In other words Each level is responsible to solve a more focused problem of the original problem is a called layer in network terminology.  Each layer observes a different level of abstraction and performs some well defined functions.  Each layer uses the service of the layer below below it and each layer provides service to its upper layer.  There exists an interface between each pair of adjacent layers that defines the information and services a layer must provide to the adjacent layer.
  36. 36. Computer Networking / Module I / AKN / 36 Example: Sending a letter
  37. 37. Computer Networking / Module I / AKN / 37 Example: The philosopher-translator-secretary architecture.
  38. 38. Computer Networking / Module I / AKN / 38 The Internet model  The layered protocol stack that is used in practice is a five ordered layer Internet model, also called TCP/IP protocol suite  The responsibility of each layer is well defined and focused  Each end user device engaged in communication must have these layers in it (in form of HW or SW)  An intermediate device may not have all the layers but at least first three layers  Layer x on one device communicates with layer x of other device.  The processes on each machine that communicate at a given layer are called peer-to-peer processes.
  39. 39. Computer Networking / Module I / AKN / 39 Peer-to-peer processes
  40. 40. Computer Networking / Module I / AKN / 40 An exchange using the Internet model
  41. 41. Computer Networking / Module I / AKN / 41 Physical layer  The responsibility of physical layer is to coordinate the functions required to transmit a bit stream over a physical medium  The duties are  Defines the characteristics of the interface between devices and transmission medium  Type of transmission medium, topology, etc…  Representation of bits  Encoding, voltage level, duration etc…  Data rate  Synchronization of bits  Sender‟s and receiver‟s clock shynchronization
  42. 42. Computer Networking / Module I / AKN / 42 Data link layer  is responsible for transmitting frames from one node to the next  The duties are  Framing  Stream of bits received from upper layer is divided into manageable data units(?) called frame  Physical addressing  Adds the address of sender and receiver in the header  Flow control  This mechanism helps to prevents overflow at receiving side  Error control  Mechanism to detect/correct errors in transmission  Access Control  Which device has the control over the link at a given time
  43. 43. Computer Networking / Module I / AKN / 43 Datalink layer contd.  Physical addressing and hop-hop delivery can be done in one network only  If the message is to be passed across the network then network layer functionality is required.
  44. 44. Computer Networking / Module I / AKN / 44 Network Layer  The network layer is responsible for the delivery of packets from the original source to the final destination possibly across multiple networks.  The Duties are  Logical addressing  It adds logical addresses into the packet header  Routing  Forwarding the packet towards the destination
  45. 45. Computer Networking / Module I / AKN / 45 Source-to-Destination
  46. 46. Computer Networking / Module I / AKN / 46 An Example sending from a node with network address A and physical address 10 to a node with a network address P and physical address 95 Because the two devices are located on different networks, we cannot use physical addresses only;as the physical addresses only have local jurisdiction. What we need here are universal addresses that can pass through the LAN boundaries. The network (logical) addresses have this characteristic.
  47. 47. Computer Networking / Module I / AKN / 47 Transport layer  The transport layer is responsible for delivery of a message from one process to another.  The Duties  Port addressing  Actual transmission occurs from a specific process on one device to a process of another.  Port address (an integer) defines the process/application in a device  Segmentation and reassembly  Message received from application layer is divided in to transmittable segments containing sequence nos
  48. 48. Computer Networking / Module I / AKN / 48 Transport layer contd.  Connection control  Two types of connection service is allowed  Connection oriented: establish the connection, use the connection, release the connection. (guarantee of delivery)  Example: telephone  Connection less: each message carries the destination address and routed through the system  Example: postal service  Flow Control  Responsible for end-to-end flow control as well as intermediate flow control (congestion)  Error Control  End-to-end error control
  49. 49. Computer Networking / Module I / AKN / 49 Application layer  The application layer is responsible for providing services to the user.  It provides user interfaces and support services such as email, remote file transfer, remote logins etc…
  50. 50. Computer Networking / Module I / AKN / 50 Summary of duties
  51. 51. Computer Networking / Module I / AKN / 51 OSI model  Session Layer is the network dialog controller, It establishes maintains and synchronizes the interaction between communicating systems  Duties are  Dialog control  Synchronization at data level  Presentation layer is concerned with syntax and semantics of the information exchanged between two systems  Duties are  Translation: converting to bit streams  Encryption: to ensure privacy  Compression: increases virtual BW
  52. 52. Computer Networking / Module I / AKN / 52 The Physical Layer Lecture II • Signals • Digital Transmission • Analog Transmission • Multiplexing • Transmission Media
  53. 53. Computer Networking / Module I / AKN / 53 Position of the physical layer
  54. 54. Computer Networking / Module I / AKN / 54 Signals  Information is transmitted in the form of electromagnetic signals  Signals are of two types  Analog Signal is a continuous signal in which the signal intensity varies smoothly over time  Digital Signal is a discrete signal in which the signal intensity maintains a constant level for some period and then changes to another constant level.  Analog Data: human voice, Digital data: data stored in a computer
  55. 55. Computer Networking / Module I / AKN / 55 Periodic / Aperiodic Signals Periodic Signal: A signal completes a pattern within a measurable time frame (period) The completion of one full pattern is called a cycle. The period is constant for any given periodic signal Aperiodic Signal: Changes without exhibiting a pattern In data communication, we commonly use periodic and analog signals and aperiodic digital signals Aperiodic Signal Periodic Signal
  56. 56. Computer Networking / Module I / AKN / 56 Analog Signals  The sine wave is the most fundamental form of a periodic signal  Represented as s(t)=Asin(2ft+)  Characterstics  Amplitude: intensity of signal at any given time  Frequency: no of cycles/periods in one second, measured in Hz  Frequency = 1/Period  Phase: describes the position of the waveform relative to time zero  A complete cycle is 360o = 2  Wavelength:The distance a signal can travel in one period   = c/f, c: speed of light
  57. 57. Computer Networking / Module I / AKN / 57 Amplitude Period and frequency
  58. 58. Computer Networking / Module I / AKN / 58 Time and frequency domains A signal can also be represented in frequency domain
  59. 59. Computer Networking / Module I / AKN / 59 Composite signals  A single-frequency sine wave is not useful in data communications; we need to change one or more of its characteristics to make it useful.  When we change one or more characteristics of a single-frequency signal, it becomes a composite signal made of many frequencies.  A composite signal is composed of multiple sine waves called harmonics
  60. 60. Computer Networking / Module I / AKN / 60 Example : A Square wave  According to Fourier analysis, this signal can be decomposed in to a series of sine waves i.e.  f is called fundamental frequency  3f is third harmonic, and 5f 5th harmonic  To recreate the complete square wave requires all the odd harmonics upto infinity ...])5(2sin[ 5 4 ])3(2sin[ 3 4 2sin 4 )(  tf A tf A ft A ts      
  61. 61. Computer Networking / Module I / AKN / 61 Three harmonics
  62. 62. Computer Networking / Module I / AKN / 62 Frequency spectrum The Signal using the frequency domain and containing all its components is called the frequency spectrum of that signal  The range of frequencies that a medium can pass is called its Bandwidth  The bandwidth is a property of a medium: It is the difference between the highest and the lowest frequencies that the medium can satisfactorily pass.
  63. 63. Computer Networking / Module I / AKN / 63 Example A signal has a spectrum with frequencies between 1000 and 2000 Hz (bandwidth of 1000 Hz). A medium can pass frequencies from 3000 to 4000 Hz (a bandwidth of 1000 Hz). Can this signal faithfully pass through this medium? Solution The answer is definitely no. Although the signal can have the same bandwidth (1000 Hz), the range does not overlap. The medium can only pass the frequencies between 3000 and 4000 Hz; the signal is totally lost.
  64. 64. Computer Networking / Module I / AKN / 64 Digital Signals  Digital signals can be better described by two terms  Bit interval: time required to send a single bit  Bit rate: number of bit intervals in one second  A digital signal is a composite signal having an infinite number of frequencies i.e. infinite bandwidth  The digital BW is bits per sec (bps)
  65. 65. Computer Networking / Module I / AKN / 65 Analog vs Digital • Channels or links are of two types • low-pass: lower limit is zero and upper limit is any frequency () • band-pass: has a band width with frequencies f1and f2  A digital signal theoretically needs a BW between o and   if the upper limit will be relaxed than digital transmission can use a low-pass channel  An analog signal has a narrower BW with frequencies f1and f2  Also BW of analog signal can be shifted, i.e. f1and f2 can be shifted to f3 and f4 Analog signal can use a band-pass channel
  66. 66. Computer Networking / Module I / AKN / 66 Data rate limits  Data rate depends on  The BW available  The levels of signal that can be used  The quality of channel (i.e. the level of noise)  Nyquist Bit rate: noise less channel  Bit rate= 2  BW  lg L  For a noise less channel the nyquist bit rate defines the theoretical maximum bit rate  BW: band width of channel, L: no of signal levels used to represent data  Shannon Capacity: noisy channel  Capacity = BW  lg (1+SNR)  The signal-to-noise ratio is the statistical ratio of power of the signal to the power of the noise
  67. 67. Computer Networking / Module I / AKN / 67 Signal to noise ratio  SNR=Avg. Signal Power/Avg. Noise Power  SNRdb = 10 log10 SNR  Example:  SNRdb=36, BW=2MHz, Find C  SNR=10SNRdb/10  C = B log2 (1+SNR) = 24Mbps
  68. 68. Computer Networking / Module I / AKN / 68 Example We have a channel with a 1 MHz bandwidth. The SNR for this channel is 63; what is the appropriate bit rate and signal level? Solution C = B log2 (1 + SNR) = 106 log2 (1 + 63) = 106 log2 (64) = 6 Mbps Then we use the Nyquist formula to find the number of signal levels. 6 Mbps = 2  1 MHz  log2 L  L = 8 First, we use the Shannon formula to find our upper limit.
  69. 69. Computer Networking / Module I / AKN / 69 Signal  Data rate depends on  The BW available  The levels of signal that can be used  The quality of channel (i.e. the level of noise)  Nyquist Bit rate: noise less channel  Bit rate= 2  BW  lg L  For a noise less channel the nyquist bit rate defines the theoretical maximum bit rate  BW: band width of channel, L: no of signal levels used to represent data  Shannon Capacity: noisy channel  Capacity = BW  lg (1+SNR)  The signal-to-noise ratio is the statistical ratio of power of the signal to the power of the noise
  70. 70. Computer Networking / Module I / AKN / 70 Transmission Impairment  In practice the signal sent at sending end using a transmission medium is not exactly same at receiving end due to some impairments  Attenuation: loss of energy  Decibel: is the unit to measure the relative strength of two signals  dB = 10 log (P1/P2)  It is negative if attenuated and +ve if amplified
  71. 71. Computer Networking / Module I / AKN / 71 Distortion  Signal changes its forms at the receiving end  It is normally happens in case of composite signals  As each signal component has its own propagation speed thus received out of phase
  72. 72. Computer Networking / Module I / AKN / 72 Noise  Several types of noise such as  thermal noise: random motion of electrons in a wire  induced noise: sources such as motors and elecrical appliances  cross talk: effect of one wire over the other  impulse noise: is a spike may corrupt the original signal that comes from power lines and lightning
  73. 73. Computer Networking / Module I / AKN / 73 More terminologies  Throughput: number of bits passed per second at a given point  Propagation Delay: the time required for a bit to travel from one point to another  Wavelength: is the distance a signal can travel in  = c / f
  74. 74. Computer Networking / Module I / AKN / 74 Digital Transmission Line coding Block Coding Sampling Transmission Mode
  75. 75. Computer Networking / Module I / AKN / 75 What is Line Coding?  Is the process of converting binary data (a sequence of bits) to a digital signal
  76. 76. Computer Networking / Module I / AKN / 76 Signal Level versus Data Level  No of values allowed in a signal  No of values used to represent data
  77. 77. Computer Networking / Module I / AKN / 77 DC Component  A component having zero frequency  Can‟t be passed through a transformer  Energy consumed is useless
  78. 78. Computer Networking / Module I / AKN / 78 Pulse Rate versus Bit Rate  No of pulses per second  Minimum amount of time required to transmit a symbol  No of Bits per second  If a pulse carries one bit then pulse rate and bit rate are same Example A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows:Pulse Rate = 1/ 10-3= 1000 pulses/s Bit Rate = Pulse Rate x log2 L = 1000 x log2 2 = 1000 bps
  79. 79. Computer Networking / Module I / AKN / 79  No Synchronization: if receivers clock is faster  A Signal that includes timing information along with data is called a self-synchronizing signal  i.e. transitions in the signal alerts the receiver to reset the clock Self Synchronization
  80. 80. Computer Networking / Module I / AKN / 80 Example In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps? How many if the data rate is 1 Mbps? Solution At 1 Kbps: 1000 bits sent 1001 bits received1 extra bps At 1 Mbps: 1,000,000 bits sent 1,001,000 bits received1000 extra bps
  81. 81. Computer Networking / Module I / AKN / 81 Line Coding Schemes
  82. 82. Computer Networking / Module I / AKN / 82 Unipolar encoding uses only one voltage level. Note: UniPolar Encoding
  83. 83. Computer Networking / Module I / AKN / 83 Unipolar Encoding  One is coded as +ve voltage  Zero is coded as –ve voltage
  84. 84. Computer Networking / Module I / AKN / 84 Polar encoding uses two voltage levels (positive and negative). Note: Polar Encoding
  85. 85. Computer Networking / Module I / AKN / 85 Polar Encoding  Avarage voltage level is decreased  DC component problem is avoided  Four Important type of polar encoding are: There are many others also!
  86. 86. Computer Networking / Module I / AKN / 86 In NRZ-L the level of the signal is dependent upon the state of the bit. Note: NRZ-L Encoding
  87. 87. Computer Networking / Module I / AKN / 87 In NRZ-I the signal is inverted if a 1 is encountered. Note: NRZ-I Encoding
  88. 88. Computer Networking / Module I / AKN / 88 NRZ Encoding  Loss of synchronization incase of continuous ones or zeros
  89. 89. Computer Networking / Module I / AKN / 89 RZ uses three values i.e. +ve, zero & -ve Signal change occurs during each bit Note: RZ Encoding
  90. 90. Computer Networking / Module I / AKN / 90 RZ Encoding  A +ve voltage means 1 and –ve voltage means zero.  But signal returns to zero at mid of the bit interval
  91. 91. Computer Networking / Module I / AKN / 91 RZ is a good encoded digital signal that contain a provision for synchronization. But it requires two signal changes to encode 1 bit  more bandwidth! Note: RZ Encoding
  92. 92. Computer Networking / Module I / AKN / 92 In Manchester encoding, the transition at the middle of the bit is used for both synchronization and bit representation. Note: Manchester Encoding
  93. 93. Computer Networking / Module I / AKN / 93 Manchester Encoding  It achieves the synchronization but with two levels of amplitude  Datarate(R) = 1/tb , tb: bit duration in seconds  Modulation rate (D) = R/b, b: no of bits per signal element
  94. 94. Computer Networking / Module I / AKN / 94 In differential Manchester encoding, the transition at the middle of the bit is used only for synchronization. The bit representation is defined by the inversion or noninversion at the beginning of the bit. Note: Diff-Manchester Encoding
  95. 95. Computer Networking / Module I / AKN / 95 Diff-Manchester Encoding  Manchester Encoding used for 802.3 base band – CSMA/CD Lans  Diff-Manchester is used foe 802.5 token ring LAn
  96. 96. Computer Networking / Module I / AKN / 96 In bipolar encoding, we use three levels: positive, zero, and negative. Note: Bipolar Encoding
  97. 97. Computer Networking / Module I / AKN / 97 Bipolar Encoding
  98. 98. Computer Networking / Module I / AKN / 98 2B1Q Encoding  Two Binary One Quaternary  Each pulse represents 2 bits -1 -3
  99. 99. Computer Networking / Module I / AKN / 99 MLT-3 Encoding  Multi transmission, three level (MLT-3)  The signal transition from one level to the next at the beginning of a 1 bit
  100. 100. Computer Networking / Module I / AKN / 100  To ensure synchronization some redundant bits may be introduced Steps in Transformation  Division  Substitution  Line Coding Block Coding
  101. 101. Computer Networking / Module I / AKN / 101 Block Coding
  102. 102. Computer Networking / Module I / AKN / 102 Substitution
  103. 103. Computer Networking / Module I / AKN / 103 4B/5B Encoding  Each 4-bit 'nibble' of received data has an extra 5th bit added.  If input data is dealt with in 4-bit nibbles there are 24 = 16 different bit patterns. With 5-bit 'packets' there are 25 = 32 different bit patterns.  As a result, the 5-bit patterns can always have two '1's in them even if the data is all '0's a translation.  This enables clock synchronizations required for reliable data transfer.
  104. 104. Computer Networking / Module I / AKN / 104 Data Code Data Code 0000 11110 1000 10010 0001 01001 1001 10011 0010 10100 1010 10110 0011 10101 1011 10111 0100 01010 1100 11010 0101 01011 1101 11011 0110 01110 1110 11100 0111 01111 1111 11101 4B/5B encoding
  105. 105. Computer Networking / Module I / AKN / 105 Example 8B/6T  sends 8 data bits as six ternary (one of three voltage levels i.e. +, 0, -) signals.  Each bit block of 8-bit group with a six symbol code  i.e. 8 bit  28 & six symbol 36 possibilities  i.e. the carrier just needs to be running at 3/4 of the speed of the data rate.  Helps to maintain synchronization and error checking
  106. 106. Computer Networking / Module I / AKN / 106 Pulse Amplitude Modulation  Generates a series of pulses by sampling a given analog signal  Sampling is measuring amplitude in equal intervals
  107. 107. Computer Networking / Module I / AKN / 107 Pulse amplitude modulation has some applications, but it is not used by itself in data communication. However, it is the first step in another very popular conversion method called pulse code modulation. Note: PAM
  108. 108. Computer Networking / Module I / AKN / 108 PCM: Quantization  It is a method of assigning integral values in a specific range to sampled instances
  109. 109. Computer Networking / Module I / AKN / 109 Binary encoding  Each quantized value is translated into a 7bit binary equivalent.  The eighth bit indicates the sign
  110. 110. Computer Networking / Module I / AKN / 110 Line coding  The binary digits are transformed to a digital signal by using one of the line coding techniques.
  111. 111. Computer Networking / Module I / AKN / 111 Analog to PCM Digital Code
  112. 112. Computer Networking / Module I / AKN / 112 According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency. Note: Sampling rate  Accuracy of reproduction depend on the no of samples taken  What should be the sampling rate?
  113. 113. Computer Networking / Module I / AKN / 113 Nyquist Theorem
  114. 114. Computer Networking / Module I / AKN / 114 Example What sampling rate is needed for a signal with a bandwidth of 10,000 Hz (1000 to 11,000 Hz)? Solution The sampling rate must be twice the highest frequency in the signal: Sampling rate = 2 x (11,000) = 22,000 samples/s
  115. 115. Computer Networking / Module I / AKN / 115 Example A signal is sampled. Each sample requires at least 12 levels of precision (+0 to +5 and -0 to -5). How many bits should be sent for each sample? Solution We need 4 bits; 1 bit for the sign and 3 bits for the value. A 3-bit value can represent 23 = 8 levels (000 to 111), which is more than what we need. A 2-bit value is not enough since 22 = 4. A 4-bit value is too much because 24 = 16.
  116. 116. Computer Networking / Module I / AKN / 116 Example We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample? Solution The human voice normally contains frequencies from 0 to 4000 Hz. Sampling rate = 4000 x 2 = 8000 samples/s Bit rate = sampling rate x number of bits per sample = 8000 x 8 = 64,000 bps = 64 Kbps
  117. 117. Computer Networking / Module I / AKN / 117 Transmission mode
  118. 118. Computer Networking / Module I / AKN / 118  Information is organized into group of bits  All bits of one group are transmitted with each clock tick from one device to other  More speed  Cost is high restricted to short distance Parallel Transmission
  119. 119. Computer Networking / Module I / AKN / 119 Serial Transmission  One bit follows another using same line  Reduced cost (by a factor n)  Parallel/serial converter required  May used for large distance
  120. 120. Computer Networking / Module I / AKN / 120 In asynchronous transmission, 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. Note: Asynchronous Transmission  Serial transmission occurs in one of the two ways
  121. 121. Computer Networking / Module I / AKN / 121 Asynchronous Transmission  Insertion of extra bits & a gap makes it slower  But cheap and effective  Suitable for low speed communication like KB to computer. i.e. typing is done one character at a time and unpredictable gap between characters.
  122. 122. Computer Networking / Module I / AKN / 122 Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same. Note: Asynchronous Transmission  When receiver detects a start bit, it starts a timer and begins counting  After receiving a stop bit it ignores all pulses till next start bit arrives and resets the timer
  123. 123. Computer Networking / Module I / AKN / 123 In synchronous transmission, we send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits. Note: Synchronous Transmission
  124. 124. Computer Networking / Module I / AKN / 124 Synchronous Transmission  More speed  Synchronization is necessary  Accuracy is completely dependent on the ability of the receiving device to keep an accurate count of the bits as they come in  Byte synchronization is done in datalink layer
  125. 125. Computer Networking / Module I / AKN / 125 Modulation of Digital Data Digital-to-Analog Conversion Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) Quadrature Amplitude Modulation Bit/Baud Comparison Analog Transmission
  126. 126. Computer Networking / Module I / AKN / 126 Digital to analog modulation It is Needed if the transmission line is analog but the data produced is binary. Example: sending data from a computer via a public access telephone line
  127. 127. Computer Networking / Module I / AKN / 127 Bit rate is the number of bits per second. Baud rate is the number of signal units per second. Baud rate is less than or equal to the bit rate. Note: Bit rate / Baud rate The sending device produces a signal that acts as a basis of information signal called carrier signal or carrier frequency The digital information is then modulates the carrier signal by modifying one or more of its characteristics.
  128. 128. Computer Networking / Module I / AKN / 128 Example 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 Solution Baud rate = 1000 bauds per second (baud/s) Bit rate = 1000 x 4 = 4000 bps Example The bit rate of a signal is 3000. If each signal unit carries 6 bits, what is the baud rate? Solution Baud rate = 3000 / 6 = 500 baud/s
  129. 129. Computer Networking / Module I / AKN / 129 Amplitude Shift Keying • The intensity of the signal is varied to represent binary one or zero • ASK is highly susceptible to noise interference, i.e a zero may be changed to 1 or vice versa • If one of the bit values is represented by no voltage then it is called on/off keying (OOK). It results in reduction of energy transmitted. • ASK modulated signal contains many simple frequencies • band width is given by BW=(1+d) Nbaud • Where Nbaud is the baud rate and d is a factor of modulation with minimum value=0
  130. 130. Computer Networking / Module I / AKN / 130 Example Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz), draw the full-duplex ASK diagram of the system. Find the carriers and the bandwidths in each direction. Assume there is no gap between the bands in the two directions. Solution For full-duplex ASK, the bandwidth for each direction is BW = 10000 / 2 = 5000 Hz The carrier frequencies can be chosen at the middle of each band fc (forward) = 1000 + 5000/2 = 3500 Hz fc (backward) = 11000 – 5000/2 = 8500 Hz
  131. 131. Computer Networking / Module I / AKN / 131 Frequency Shift Keying  Frequency of carrier signal varies to represent a binary 1 or 0  Effect of noise is less, receiving device ignores spikes but more Bandwidth is required  Although there are two carrier frequencies, the process of modulation produces a composite signal  Bandwidth = fc1 – fc0 + Nbaud
  132. 132. Computer Networking / Module I / AKN / 132 Example Find the maximum bit rates for an FSK signal if the bandwidth of the medium is 12,000 Hz and the difference between the two carriers is 2000 Hz. Transmission is in full-duplex mode. Solution Because the transmission is full duplex, only 6000 Hz is allocated for each direction. BW = baud rate + fc1 - fc0 Baud rate = BW - (fc1 - fc0 ) = 6000 - 2000 = 4000 But because the baud rate is the same as the bit rate, the bit rate is 4000 bps.
  133. 133. Computer Networking / Module I / AKN / 133 Phase Shift Keying  Phase of carrier signal varies to represent a binary 1 (180o)or 0 (0o) also called 2- PSK or binary PSK  Avoids problems of noise and bandwidth  Can be represented in a constallation diagram or phase-state diagram  BW=same as of ASK  More variations in phase may be added to represent more than one bit
  134. 134. Computer Networking / Module I / AKN / 134 Other variations of PSK 4-PSK / Q-PSK, 2 bits per baud 8-PSK, 3 bits per baud i. The bit rate increases as compared to baud rate ii. But needs sophisticated devices to distinguish small difference in phase
  135. 135. Computer Networking / Module I / AKN / 135 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. Note: Quadrature Amplitude Modulation
  136. 136. Computer Networking / Module I / AKN / 136 4-QAM & 8-QAM Constellation
  137. 137. Computer Networking / Module I / AKN / 137 16-QAM constellations QAM is less susceptible to noise than ASK? Bandwidth required for QAM is same as PSK and ASK
  138. 138. Computer Networking / Module I / AKN / 138 Bit/Baud Comparison
  139. 139. Computer Networking / Module I / AKN / 139 A telephone line has a bandwidth of almost 2400 Hz for data transmission. Modem Standards Modem stands for modulator/demodulator.
  140. 140. Computer Networking / Module I / AKN / 140 Modulation/Demodulation  A modulator creates a band-pass signal from binary data.  A demodulator recovers the binary data from the modulated signal
  141. 141. Computer Networking / Module I / AKN / 141 V series modems V.32 constellation & BW • published by ITU-T • it uses a technique called trellis coded modulation I.e. QAM plus one redundant bit • 32 QAM with a baud rate of 2400 and datarate is 2400*4=9600kbps (1 bit redundant)
  142. 142. Computer Networking / Module I / AKN / 142 V.32bis constellation & BW  Uses 128-QAM (7 bits/ baud with 1 bit for error control)  datarate (2400*6)=14400 bps V.90  Asymetric modems, i.e. downloading speed is 56 kbps and uploading speed is 33.6 kbps  This is possible if one party is using digital signaling V.92  can adjust their speed I.e. if noise allows than it can upload at a rate of 48 Kbps  Additional features like modem can interrupt internet connection for a incoming phone call etc.
  143. 143. Computer Networking / Module I / AKN / 143 Traditional modems 56 K Modems • Sampled, digitized and at telephone comp • The quantization noise introduced thus data rate is limited according to shannon capacity i.e. 33.6k • signal not affected by quantization noise and not limited by shannon capacity • sampling is done at a rate of 8000 samples/sec with 8 bits per sample. • One bit is used for control thus speed becomes 8000*7=56 kbps
  144. 144. Computer Networking / Module I / AKN / 144 Modulation of Analog Signals Methods: Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM) • Representation of analog information by an analog signal • i.e. shifting the center frequency of baseband signal up to the radio carrier • It is needed because • To reduce Antenna length (length  1/f) • helps in frequency division multiplexing • To support medium characteristics
  145. 145. Computer Networking / Module I / AKN / 145 Amplitude modulation • The carrier signal is modulated so that its amplitude varies with the changing amplitude of modulating signal • Phase and frequency remains the same • The modulating signal becomes an envelope to the carrier • The bandwidth of an AM signal is twice the bandwidth of the modulating signal • BWt = 2  BWm • BWt is total bandwidth • BWm is bandwidth of modulating signal
  146. 146. Computer Networking / Module I / AKN / 146 Frequency modulation • The carrier signal is modulated so that its frequency varies with the changing amplitude of modulating signal • Phase and peak amplitde remains the same •The bandwidth of an AM signal is ten times the bandwidth of the modulating signal • BWt = 10  BWm • BWt is total bandwidth • BWm is bandwidth of modulating signal
  147. 147. Computer Networking / Module I / AKN / 147 The Physical Layer contd. Lecture III • Multiplexing • Transmission Media • Switching
  148. 148. Computer Networking / Module I / AKN / 148 Multiplexing  It is not practical to have a separate line for each other device we want to communicate  Therefore, it is better to share communication medium  The technique used to share a link by more than one device is called multiplexing  Multiplexing needs that the BW of the link should be greater than the total individual BW of the devices connected.  In a multiplexed system one link may contain more than one channel
  149. 149. Computer Networking / Module I / AKN / 149 Categories of multiplexing
  150. 150. Computer Networking / Module I / AKN / 150 Frequency Division Multiplexing  FDM is an analog multiplexing technique that combines signals  Signals generated by each device modulate different carrier frequencies  These modulated signals are combined to form a composite signal  Demultiplexer uses a series of filters to decompose the signal into its component signals
  151. 151. Computer Networking / Module I / AKN / 151 FDM • Carrier frequencies are separated by sufficient BW to accommodate modulated signal •These BW ranges are channels through which the various signal travel • Channels must be separated by strips of unused BWs (called Guard Bands) to prevent signals from overlapping • Carrier frequencies must not interfere with the original signals f t
  152. 152. Computer Networking / Module I / AKN / 152 Example 1 Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency domain without the use of guard bands. Solution Shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure
  153. 153. Computer Networking / Module I / AKN / 153 Example Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 x 100 + 4 x 10 = 540 KHz as shown in Figure
  154. 154. Computer Networking / Module I / AKN / 154 Example Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration using FDMSolution • The satellite channel is analog. We divide it into four channels, each channel having a 250-KHz bandwidth. • Each digital channel of 1 Mbps is modulated such that each 4 bits are modulated to 1 Hz. • One solution is 16- QAM modulation. • Figure shows one possible configuration.
  155. 155. Computer Networking / Module I / AKN / 155 Analog hierarchy
  156. 156. Computer Networking / Module I / AKN / 156 Wave Division Multiplexing  Very narrow bands of light from different sources are combined to make a wider band of light  A prism is used to bend a beam of light based on the angle of incidence and frequency and acts like a multiplexer  Another prism may be used to reverse the process and acts like a demultiplexer
  157. 157. Computer Networking / Module I / AKN / 157 Time division Multiplexing  Each shared connection occupies a portion of time but uses full BW f t  The data flow of each connection is divided into units  For n input connections, a frame is organised into a minimum of n units  Each slot carrying one unit from each section  Data rate of the link has to be n times the data rate of one unit
  158. 158. Computer Networking / Module I / AKN / 158 Time division Multiplexing contd.  If the data rate of a link is 3 times the data rate of a connection  then the duration of a unit on a connection will be 3 times that of a time slot
  159. 159. Computer Networking / Module I / AKN / 159 Example Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find (1) the duration of 1 bit before multiplexing, (2) the transmission rate of the link, (3) the duration of a time slot, and (4) the duration of a frame? Solution 1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms). 2. The rate of the link is 4 times the rate of connection, i.e. 4 Kbps. 3. The duration of each time slot is 1/4 th of the bit duration before multiplexing i.e. 1/4 ms or 250 ms. or inverse of data rate i.e. 1/4 Kbps = 250 ms. 4. The duration of a frame is same as duration of each unit, i.e. 1 ms. or 4 times the bit duration i.e. 4 * 250 ms = 1ms
  160. 160. Computer Networking / Module I / AKN / 160 Example Four channels are multiplexed using TDM. If each channel sends 100 bytes/s and we multiplex 1 byte per channel, show the frame traveling on the link, the size of the frame, the duration of a frame, the frame rate, and the bit rate for the link. Solution
  161. 161. Computer Networking / Module I / AKN / 161 Example A multiplexer combines four 100-Kbps channels using a time slot of 2 bits. Show the output with four arbitrary inputs. What is the frame rate? What is the frame duration? What is the bit rate? What is the bit duration? Solution
  162. 162. Computer Networking / Module I / AKN / 162 Synchronization • Synchronization between multiplexer and demultiplexer is important otherwise a bit of one channel may be received by other channel • To avoid this one or more synchronization bits may be added called Framing bits
  163. 163. Computer Networking / Module I / AKN / 163 Example We have four sources, each creating 250 characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (1) the data rate of each source, (2) the duration of each character in each source, (3) the frame rate, (4) the duration of each frame, (5) the number of bits in each frame, and Solution 1. The data rate of each source is 2508=2000 bps 2. The duration of a character is 1/250 s, or 4 ms. 3. The link needs to send 250 frames per second. 4. The duration of each frame is 1/250 s, or 4 ms. 5. Each frame is 4 x 8 + 1 = 33 bits. 6. The data rate of the link is 250 x 33, or 8250 bps.
  164. 164. Computer Networking / Module I / AKN / 164 Bit Padding  If one or more devices are faster than other devices than faster devices are given more time slots than others  e.g. we can accommodate a device 5 times faster than others by giving time slots as 5:1  When speeds are not integer multiples of each other then bit padding is used  In bit padding the multiplexer adds extra bits to device‟s source stream to force the speed relationships as integer multiples
  165. 165. Computer Networking / Module I / AKN / 165 Example 9 Two channels, one with a bit rate of 100 Kbps and another with a bit rate of 200 Kbps, are to be multiplexed. How this can be achieved? What is the frame rate? What is the frame duration? What is the bit rate of the link?Solution We can allocate one slot to the first channel and two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second because it carries 1 bit from the first channel. The frame duration is 1/100,000 s, or 10 ms. The bit rate is 100,000 frames/s x 3 bits/frame, or 300 Kbps.
  166. 166. Computer Networking / Module I / AKN / 166 DS hierarchy Telephone companies implement TDM through hierarchy of digital signals called Digital Signal service
  167. 167. Computer Networking / Module I / AKN / 167 T-1 line for multiplexing telephone lines o Digital Signal services are implemented by T Lines (T-1 to T-4) o T Lines are digital lines designed for transmission of digital data, audio or video
  168. 168. Computer Networking / Module I / AKN / 168 T-1 frame structure • The frame used on a T-1 line is usually 193 bits divided into 24 slots of 8 bits each plus 1 extra bit for synchronization (24*8 + 1) • If a T-1 line carries 8000 frames then data rate = 193*8000 = 1.544 Kbps
  169. 169. Computer Networking / Module I / AKN / 169 E Line Rate (Mbps) Voice Channels E-1 2.048 30 E-2 8.448 120 E-3 34.368 480 E-4 139.264 1920 • Europeans use E Lines in place T Lines. Both are conceptually same only capacity differs
  170. 170. Computer Networking / Module I / AKN / 170 Multiplexing and inverse multiplexing • Inverse multiplexing takes data from high speed line and breaks it into portions that can be sent across several lower speed lines • If an organisation wants to send data, audio and video, each requires a different bandwidth • using an agreement called Bandwidth on Demand • The organisation can use any of the channel whenever and however it needs them
  171. 171. Computer Networking / Module I / AKN / 171 Transmission Media  Signals in the form of electromagnetic energy is propagated through transmission media from one device to another device  A selected portion of electromagnetic spectrum are currently usable for telecommunication like Power, radio waves, infrared, visible light, ultra- violate, and X, gamma and cosmic rays etc.
  172. 172. Computer Networking / Module I / AKN / 172 Classes of transmission media
  173. 173. Computer Networking / Module I / AKN / 173 Guided Media  Provides a conduit from one device to another, includes  Twisted-Pair Cable  Consists of two conductors, each with its own plastic insulation, twisted together  Due to twists, the noise interference and crosstalk affects both wires equally thus cancels each other  i.e. no of twists per unit length determines the quality of the cable; more twists mean better quality
  174. 174. Computer Networking / Module I / AKN / 174 Unshielded vs Shielded Twisted-Pair Cable  STP has a metal foil or braided-mesh covering that encases each pair of insulated conductor  Metal casing improves mechanical strength, prevents penetration of noise or cross talk but is bulkier and more expensive  STP is produced by IBM and seldom used else where.  EIA developed standards for UTP in 7 categories
  175. 175. Computer Networking / Module I / AKN / 175 Categories of Unshielded Twisted-Pair cables Category Bandwidth Data Rate Digital/Analog Use 1 very low < 100 kbps Analog Telephone 2 < 2 MHz 2 Mbps Analog/digital T-1 lines 3 16 MHz 10 Mbps Digital LANs 4 20 MHz 20 Mbps Digital LANs 5 100 MHz 100 Mbps Digital LANs 6 200 MHz 200 Mbps Digital LANs 7 (draft) 600 MHz 600 Mbps Digital LANs
  176. 176. Computer Networking / Module I / AKN / 176 UTP Contd.  RJ-45 (Registered-Jack)is used for 4-pair UTP cable  UTP can pass a wide range of frequencies  Performance is measured as attenuation versus frequency and distance  Attenuation is measured as decibels per mile and is increased sharply after 100KHz
  177. 177. Computer Networking / Module I / AKN / 177 Coaxial Cable  It can carry higher frequency ranges than UTP  The outer metallic wrapping serves both as a shield against noise and as the second conductor  These cables are categorized by their radio government (RG) ratings  These are categorized according to gauge of wire, thickness and type of insulation, construction of the shield and size of type of outer casing Category Impedan ce Use RG-59 75 W Cable TV RG-58 50 W Thin Ethernet RG-11 50 W Thick Ethernet
  178. 178. Computer Networking / Module I / AKN / 178 Coaxial Cable contd.  BNC connectors are used(Bayone-Neill-Concelman)  BNC connector is used to connect end of the cable to a device  BNC-T is used in ethernet  BNC terminator is used at the end of the cable  Attenuation is much higher than the UTP  Frequent use of repeaters is needed to avoid attenuation
  179. 179. Computer Networking / Module I / AKN / 179 Fiber-Optic cables  Transmits signals in the form of visible light  It uses the refraction property of light for transmission  i.e. light travels in a straight line in an uniform medium and changes the direction when passes from one medium to another having different density Core: glass or plastic, cladding: covering with less dense glass or plastic
  180. 180. Computer Networking / Module I / AKN / 180 Propagation modes Current technology allows two modes of propagating light along optical channels Multimode: multiple beams Single mode: single focused beam
  181. 181. Computer Networking / Module I / AKN / 181 Mechanism  Distortion is less as compared to step-index as distance traveled is less and received time variation is less  Single Mode:  Uses focused source of light and step-index fiber having small diameter  Propagation of beams is almost horizontal  Multimode step index:  The density of core remains constant from core center to edges.  Light moves in a straight line and reflects back from edge  Distortion is more as various rays received at different times  Multimode graded index:  The density of core varies (decreases) from core center to edges.  Light undergoes a series of refraction
  182. 182. Computer Networking / Module I / AKN / 182 Fiber Optics contd. Type Core Clad ding Mode 50/125 50 125 Multimode, graded-index 62.5/125 62.5 125 Multimode, graded-index 100/125 100 125 Multimode, graded-index 7/125 7 125 Single-mode  Optical fibers are defined by the ratio of their diameter of their core to cladding  Cable composition  Outer jacket is made of either PVC or teflon  Inside the jacket are Kevlar strands to strengthen the cable  Below the Kevlar another plastic coating is there  The fiber is at the center of the cable, and it consists of cladding and the core
  183. 183. Computer Networking / Module I / AKN / 183 Fiber Optics contd.  It uses three different types of connectors  Subscriber channel(SC) connector used in cable TV with a push/pull locking system  Straight Tip (ST) connector is used for connecting cable to networking devices with a bayonet locking system  MT-RJ is a new connector with same size as RJ-45  Attenuation is flatter than TP and coax thus less no of repeaters are needed to transmit(10 times less)
  184. 184. Computer Networking / Module I / AKN / 184 Advantages and Disadvantages Adavntages  Higher Bandwidth  BW is not limited by medium but by signal generation and reception  Less Signal Attenuation  Can run 50 KM without regeneration  No electromagnetic interference  Resistance to corrosive materials  Light weight  Tapping is difficult Disadvantages  Installation and Maintenance  Unidirectional (two fibers needed to make it bi-directional)  Cost
  185. 185. Computer Networking / Module I / AKN / 185 Unguided Media  It transports electromagnetic waves without using a physical conductor called Wireless Communication  Unguided signals can travel from source to destination in several ways
  186. 186. Computer Networking / Module I / AKN / 186 Radio and microwaves of Electromagnetic spectrum is divided into 8 ranges Band Range Propagation Application VLF 3–30 KHz Ground Long-range radio navigation LF 30–300 KHz Ground Radio beacons and navigational locators MF 300 KHz–3 MHz Sky AM radio HF 3–30 MHz Sky Citizens band (CB), ship/aircraft communication VHF 30–300 MHz Sky and line-of-sight VHF TV, FM radio UHF 300 MHz–3 GHz Line-of-sight UHF TV, cellular phones, paging, satellite SHF 3–30 GHz Line-of-sight Satellite communication EHF 30–300 GHz Line-of-sight Long-range radio navigation
  187. 187. Computer Networking / Module I / AKN / 187 Wireless transmission waves  Wireless transmission is broadly divided into three groups  Radio Wave: Between 3KHz to 1GHz, omni directional, can travel long distance thus making suitable for log-distance broadcasting like AM radio, FM radio, TV, cordless phones etc.  Low and medium frequencies can penetrate walls, uses omni directional antennas, high interference  Microwave: Ranging from 1 and 300GHz, unidirectional, low interference uses unidirectional antennas with line-of-Sight (LOS) propagation  Very high frequency microwaves cannot penetrate walls, used for long distance transmission, cellular phones, wireless LANs, two types: terrestrial microwave and satellite microwave  Infrared: frequencies from 300GHz to 400THz, can be used for very short range communication, cannot penetrate walls, confined to one room only(remote control of TV), no licensing required  May be used to communicate between devices such as keyboards, mice, PCs, printers, handset, PDAs etc.
  188. 188. Computer Networking / Module I / AKN / 188 Antennas Unidirectional Antenna Radiation and reception of electromagnetic waves Coupling of wires to space for radio transmission It works as an adapter between a guided and unguided media
  189. 189. Computer Networking / Module I / AKN / 189 Switching  To connect multiple devices over a distance we adopt a method called switching  Switches are hardware and/or software devices capable of creating temporary connections as per requirements  A switched network consists of a series of interlinked switches  Switching Methods  Circuit switching  Packet switching
  190. 190. Computer Networking / Module I / AKN / 190 Circuit Switching  It creates a direct physical connection between two devices i.e. it establishes a physical circuit before transmission  It uses a device with n I/P s and m O/Ps  Circuit Switching Techniques  Space Division Switches  Crossbar switch, multistage switch  Time division switches  Time Slot Interchange, TDM Bus
  191. 191. Computer Networking / Module I / AKN / 191 Crossbar switch  It connects n I/Ps and m O/Ps in a grid  Each cross point consists of a electronic switch  The order of switch required is huge O(nm)  It is impractical because of the size of the crossbar  It is also inefficient because in practice 25% of the switches are used at a given time
  192. 192. Computer Networking / Module I / AKN / 192 Multistage switch  Uses crossbar switches in several stages  The design of multistage switch depends on the no of stages and the no of switches required in each stage  Number of outputs in one stage=number of switches in the next stage  The number of cross points required is much less than a crossbar switch  The reduction in the number of cross points results in blocking. i.e. one input is blocked to connect to a output due to unavailability of a path
  193. 193. Computer Networking / Module I / AKN / 193 Time Division Switches  It uses time division multiplexing to achieve switching  Time Slot Interchange(TSI)  It changes ordering of slots based on desired connections  It consists of RAM with several memory location  Size of each location is same as size of time slot  TSI fills up incoming data inorder of reception  Slots are sent out in an order based on the decission of control unit
  194. 194. Computer Networking / Module I / AKN / 194 TDM Bus  In this case the I/P and O/P are connected to a high speed bus through input output gates  Each input gate is closed during the time slots and only one output gate is closed.  The controlling unit decided which switches are to be closed
  195. 195. Computer Networking / Module I / AKN / 195 TDM Bus  Space division switches have no delay and time division switches requires cross points  Combining both technologies will result in switches that are optimised both in physically (no of components) and temporally (delay)  It can be designed as TST, TSST, STTS, etc.
  196. 196. Computer Networking / Module I / AKN / 196 Telephone Network  Telephone network is made of three major components: local loops, trunks, and switching offices  Local loop: that connects the subscriber telephone to the nearest end office or local central office  Trunk: transmission media that handle the communication between offices, normally handles hundreds or thousands of connections through multiplexing  Switching Office: A switch connects several local loops or trunks and allows a connection between different subscribers.
  197. 197. Computer Networking / Module I / AKN / 197 Making a Connection  Accessing the switching station at the end offices is accomplished through dialing  In case of rotary dialing a digital signal is sent to the end office  In case of touch-tone technique two analog signals are sent to the end office, depending on the row and column of the switch position.  e.g. for 8, the signals 852Hz and 1336Hz are sent
  198. 198. Computer Networking / Module I / AKN / 198 Voice communication used analog signals in the past, but is now moving to digital signals. On the other hand, dialing started with digital signals (rotary) and is now moving to analog signals (touch-tone). Note:
  199. 199. Computer Networking / Module I / AKN / 199 Packet Switching  Circuit switching are best suited for voice communication, as data communication are bursty in nature i.e. data transmitted in blocks with gaps between them  A circuit switched link assumes a single data rate for both devices  In Circuit switching all transmissions are equal, priority base communication is not allowed  In Packet switching data transmitted in discrete units called packets  There are two approaches for packet switching  Datagram approach, and Virtual Circuit approach
  200. 200. Computer Networking / Module I / AKN / 200 Datagram Approach  In this approach each packet treated independently called datagrams  Each datagram contains appropriate information about the destinations and the network carries the datagrams towards destination  Datagrams may reach at destination out of order  The links joining each pair of nodes may contain multiple channels. Each of these channels is capable of carrying datagrams from several sources or from a single source
  201. 201. Computer Networking / Module I / AKN / 201 Virtual Circuit Approach  In this approach the relationship between all packets belonging to a message is preserved  A single route is chosen between sender and receiver at the beginning of session  All packets now travel one after another along the same route  It is implemented in two formats  Switched Virtual Circuit (SVC), and Permanent Virtual Circuit (PVC)  Switched Virtual Circuit  A Virtual Circuit is created whenever it is needed (e.g. TCP‟s three way handshake) and exists for the duration of the specific exchange  Each time a device makes a connection to another device, the route may be same or may differ in response to varying network conditions  Permanent Virtual Circuit  The same virtual circuit is provided between two users on a contineous basis. The circuit is dedicated to specific users without making a connection establishment or release
  202. 202. Computer Networking / Module I / AKN / 202 A Comparison for data traffic  A circuit switch connection creates a physical path between two points where as a virtual circuit creates a route between two points  The Network resources (link and switches) that make a path are dedicated but that make a route can be shared by other connections  The line efficiency is greater in Packet switching as a single link can be shared by many packets over time  A packet switching network can perform data-rate conversion. i.e. two stations having different data rates can exchange packets but it is not possible in circuit switching  In a typical user/host data connection, much of the time line is idle thus making circuit switching inefficient  When traffic becomes heavy on a circuit switching network, some calls are blocked, but in packet switching network
  203. 203. Computer Networking / Module I / AKN / 203 Effect of Packet Size  Virtual circuit from x to y  a and b are intermediate switches  Message of size 40 octets  Packet header 3 octets (control information)  Case I: entire message sent as one packet  Case II: entire message sent as two packets  Case III: entire message sent as five packets  Case IV: entire message sent as ten packets
  204. 204. Computer Networking / Module I / AKN / 204 Packet Size contd.  Case I  packet is first transmitted from X to a. when the entire packet is received by a, it can then be transmitted to b.  Ignoring switching time, total transmission time is 433=129 octet time  Case II  Node a can begin transmitting the first packet as soon it has arrived from X, without waiting for the second packet. Overlapping in transmission time!  Total transmission time is 234=92 octet time  Case III  packets are transmitted still faster due to more number of overlapping  Total transmission time is 117=77 octet time  Case IV  Total transmission time is 712=84 octet time  Time is increased as fixed header becomes an overhead. i.e. 3 10=30 octets of header information for 40 octets of data!
  205. 205. Computer Networking / Module I / AKN / 205 One more comparison  Performance  Propagation delay  Time it takes a signal to propagate from one node to another  Transmission Time  Time it takes for a transmitter to push a block of data to the medium  Propagation delay  Time it takes for a node to perform the necessary processing as it switches data
  206. 206. Computer Networking / Module I / AKN / 206 Circuit Switching Datagram Virtual-Circuit Dedicated transmission path No dedicated path No dedicated path Continuous transmission of data Transmission of packet Transmission of packet Fast enough for interactive Fast enough for interactive Fast enough for interactive Messages are not stored Packets may be stored until transmitted Packets may be stored until delivered The path is established for entire conversation Route established for each packet Route established for entire conversation Call set-up delay, transmission delay Packet transmission delay Call setup delay, packet transmission delay Busy signal if called party busy Sender may be notified if packet not delivered Sender notified of connection denial Overload may block call setup; no delay for established calls Overload increases packet delay Overload may block call set-up; increases packet delay Usually no speed or code conversion Speed and code conversion Speed and code conversion Fixed Bandwidth Dynamic use of bandwidth Dynamic use of bandwidth No overhead bits after call setup Overhead bits in each packet Overhead bits in each packet
  207. 207. Computer Networking / Module I / AKN / 207 Network Performance  Throughput  Is a measure of the actual transmission of data in a network per unit time.  Latency  Propagation time + Transmission Time + Queuing Time + Processing Delay  Propagation Time = Distance/Propagation speed  Transmission Time= Message size/Bandwidth  Bandwidth Delay Product  BDP defines the number of bits that can fill the link
  208. 208. Computer Networking / Module I / AKN / 208 End of Module I

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