Download Unit 3 - NIST NACOL

DATA COMMUNICATION
AND
COMPUTER NETWORKS
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Data Communication and Computer Networks
Contents
1
Communication ............................................................................................................................... 4
1.1
There are mainly two types of communication: ..................................................................... 4
Voice Communication: .................................................................................................................... 4
Data Communication: ..................................................................................................................... 4
1.2
2
3
1.2.1
Source: ............................................................................................................................ 4
1.2.2
Transmitter: .................................................................................................................... 4
1.2.3
Transmission system: ...................................................................................................... 4
1.2.4
Receiver:.......................................................................................................................... 4
1.2.5
Destination: ..................................................................................................................... 4
Importance of Networking.............................................................................................................. 4
2.1.1
Resource Sharing: ........................................................................................................... 4
2.1.2
User communication: ...................................................................................................... 5
2.1.3
Increases Span of Management:..................................................................................... 5
2.1.4
Reduces Barriers between Departments: ....................................................................... 5
2.1.5
Network gaming: ............................................................................................................. 5
2.1.6
Voice over IP (VoIP):........................................................................................................ 5
Computer Network ......................................................................................................................... 5
3.1
4
5
6
2
Components of Data communication: .................................................................................... 4
Type of Network ..................................................................................................................... 5
3.1.1
Peer – to Peer architecture ............................................................................................. 6
3.1.2
Client Server Network Architecture ................................................................................ 6
Network On the Basis of Geographical Area .................................................................................. 7
4.1
Local Area Network ................................................................................................................. 7
4.2
Metropolitan Area Network ................................................................................................... 8
4.3
Wide Area Network................................................................................................................. 9
LAN Topologies ............................................................................................................................... 9
5.1
Bus Topology ........................................................................................................................... 9
5.2
Star Topology ........................................................................................................................ 10
5.3
Ring Topology........................................................................................................................ 11
5.4
Mesh Topology...................................................................................................................... 11
5.5
Tree Topology ....................................................................................................................... 12
5.6
Hybrid Topology .................................................................................................................... 12
Transmission Media ...................................................................................................................... 12
6.1
Twisted Pair Cable ................................................................................................................. 13
6.2
Coaxial Cable (COAX) ............................................................................................................ 14
Data Communication and Computer Networks
7
6.3
Fiber Optics Cable ................................................................................................................. 15
6.4
Radio Wave ........................................................................................................................... 16
6.5
Microwave System ................................................................................................................ 16
6.6
Satellite Communication ....................................................................................................... 17
Data Transmission and Data Networking ..................................................................................... 17
7.1
8
9
7.1.1
Switching Circuit............................................................................................................ 17
7.1.2
Message Switching ........................................................................................................ 18
7.1.3
Packet Switching ........................................................................................................... 19
Modes of Data Transmission ........................................................................................................ 19
8.1
Simplex Mode ....................................................................................................................... 19
8.2
Half Duplex Mode ................................................................................................................. 20
8.3
Full Duplex Mode .................................................................................................................. 20
Wireless Networking ..................................................................................................................... 21
9.1
3
Data Transmission and Data Networking ............................................................................. 17
Two kinds of wireless networks: ........................................................................................... 21
9.1.1
Ad-hoc / peer-to-peer wireless network consists of .................................................... 21
9.1.2
Infrastructure Wireless Network: ................................................................................. 21
Data Communication and Computer Networks
1 Communication
Communications is a term used to describe the process of exchanging information or data from one
computer to another computer or device. Communication describes the system or process in which
one computer transfer data, instruction and information to another computer. It is the means of
transfer of information such as thoughts and messages between two entities. Communication requires
a device that initiates the transfer (sending devices) a communication device (such as modem) that
converts the sent material into signal capable of being carried by communication channel.
1.1 There are mainly two types of communication:
Voice Communication: Voice communication refers to the very common form of telephonic
conversation that we have with our family members or friends, colleagues and other business people.
Data Communication: Rather than lying new communication networks for carrying computer data,
the technologies decided to reuse the existing telephone networks to carry computer data. This means
that some telephone networks can carry not only voice but also computer data. Data communication
is the transfer to data or information between a source and receiver.
1.2 Components of Data communication:
1.2.1 Source:
The components that generates the message to be transmitted. The source can be a person in front
of a microphone or a computer itself sending a file.
1.2.2 Transmitter:
Usually, data generated by a source system is not transmitted directly in the form in which they are
generated. The component which transform and encodes the data into transmittable signal that can
be transmitted across some transmission media. E.g. Amplifier, Repeater, Modem etc .
1.2.3 Transmission system:
It is the medium which connects source to destination. It can be guided media or unguided media. It
both cases communication is in the form of electromagnetic waves.
1.2.4 Receiver:
The receiver amplifies the received signals removes any unwanted signals (noise) introduced by the
communication channel during propagation of signal and feeds to the destiny. It converts received
signal into data.
1.2.5 Destination:
The user at the other end finally receives the message through the data terminal equipment stationed
at the other side.
2 Importance of Networking
Computer networks help users on the network to share the resources and in communication. It is
difficult to imagine a world now without emails, online newspapers, Wogs, chat and the other services
offered by the internet? The following are the important uses and benefits of a computer network. –
2.1.1 Resource Sharing:
Resource can anything like file, hardware or software. Networking of computers helps the network
users to share data files. It also enables users to share devices such as printers, scanners, CD-ROM
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Data Communication and Computer Networks
drives, hard drives etc. Without computer networks, device sharing is not possible. In addition,
applications can also be shared over the network.
2.1.2 User communication:
Networks allow users to communicate using e-mail, newsgroups, and video conferencing etc. This
enables easy and cost effective communication among staffs of an organization as well as between
organization and its customers and business partners.
2.1.3 Increases Span of Management:
Networks allows managers to control employees and organizational environment easily. A single
manager can now control more departments and employees. This can be done attaching CCTV in
different areas of organization and attaching them to some computer through networking.
2.1.4 Reduces Barriers between Departments:
With the help of networking it is easy to exchange files and information between departments even if
they are located at distant locations.
2.1.5 Network gaming:
A lot of network games are available, which allow multi-users to play from different locations and
hence entertains users.
2.1.6 Voice over IP (VoIP):
Voice over Internet Protocol (IP) is a revolutionary change in telecommunication which allows to send
telephone calls (voice data) using standard Internet Protocol (IP) rather than by traditional PSTN.
3 Computer Network
A network is a collection of computers, servers, mainframes, network devices, peripherals, or other
devices connected to one another allowing for data to be shared and used. A great example of a
network is the Internet, connecting millions of people all over the world together. Below is an example
image of what a home network with multiple computers and other network devices all connected to
each other and the Internet.
3.1 Type of Network
Depending upon the network architecture, computer network can be classified into:
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Data Communication and Computer Networks


Peer – to Peer architecture
Client/Server architecture
3.1.1 Peer – to Peer architecture
It is simple and inexpensive architecture than client server architecture.
It is suitable for small sized private owned networks such as personal
network in home, school, and cyber cafe. Each workstation or computer
connected in the network has unique address for its identification.
When a message is to be sent from one computer to another computer
in the network, the address of source computer and destination
computer are sent along with the message.
Suppose computer A wants to send some message to computer D, then Figure 1 Peer to Peer Network
first the message is sent to computer B and computer 13 compares the
destination address of the message with its address. If both the addresses are same then the message
is accepted otherwise it is retransmitted to the next adjacent computer C and the process continues,
finally computer D receives the message. Similarly acknowledge message is transmitted from receiver
computer D to the sender computer A.
Advantages
• It has simple architecture and easy to install.
• It doesn't require additional servers and network devices so it is inexpensive architecture.
• Each node has equal privileges to access the resources of its own and other nodes connected to
the network.
• It is suitable for small sized network which has maximum 10 nodes.
Disadvantages
• Its performance becomes very poor for large size network (more than 10 nodes).
• There is no central administration and monitoring mechanism for the network.
• Network security problems due to equal privileges of the nodes.
• There is no central backup and recovery mechanism, each node has responsibility to maintain its
own backup.
3.1.2 Client Server Network Architecture
In this architecture, there are two types of computers, one
is server and the other is client Server is the main computer
in a network that controls, manages and provides various
services to the clients such as print server, file server,
database server, mail server and web server. Server is a
very high performance computer and it is expensive too. A
client is normal workstation that is connected to server. A
client requests for services to the server and the server
responses for the respective services. Network resources Figure 2Client Server Architecture
are centralized to the server so all the network activities
such as data storage, data processing, data transfer etc. are carried out via server. It is based on
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Data Communication and Computer Networks
request response mechanism. As it is based on broadcast architecture, so message is transmuted from
one client to others via server. Server is responsible (or broadcasting the messages to the entire
network.
Suppose client 1 wants to send some message to Client 4, then firstly message is sent to the server.
The server broadcasts the message along with source II' address and destination IP address to all the
clients which are connected to the server. After receiving the message, all the clients check the
destination IP address of the message. If the destination IP address of the message and II' address of
the client is same. Then the message is accepted otherwise the message is discarded. Finally Client 4
receives the message. Similarly acknowledge message is transmitted from receiver client 4 to the
sender client 1.
Advantages
• It is easier for centralized administration, controlling and monitoring the entire network from
server.
• It provides centralized backup and recovery features so it is considered as more secure and reliable
architecture.
• It is easier for managing the large size network (more than 10 computers).
Disadvantages
• Resources are centralized to the server, so if any problem occurs in the server the entire network
will down.
• There may be maximum data traffic at server so there may be chances of data collision.
• It is more expensive due to dedicated server and additional network utilities and network
operating system such as MS Windows Server, UNIX or Linux etc.
4 Network On the Basis of Geographical Area
On the basis of size or geographical area that means how much area it covers, computer networks are
classified into three categories: local area network (LAN), metropolitan area network (MAN) and wide
area network (WAN).
4.1 Local Area Network
A local area network (LAN) is privately owned small size
network. It spans only in small geographical area such as
within a room, office, buildings or up to few kilometers
(2KM or 3KM). It connects the network resources such as
computers, faxes, printers, and various networking devices.
Moreover, user also can share the resources such as data,
file, software, etc in the network. Common example of LAN
is network in our college or office. LAN is simpler, cheaper
and highly secured network. Generally, LAN uses only one
type of transmission.
Figure 3LAN
Local Area Network medium such as twisted pair
cable/coaxial cables/optical fiber cable etc. Two common types of LAN protocols are Ethernet and
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Data Communication and Computer Networks
Token Passing. The most common topologic for LAN are bus, ring and star. It has high speed data
transfer rate 100Mbps or 1000Mbps (Megabits Per Second) and low amount of error rate 1000 times
lower than WAN.
Features of LAN
• LAN is small privately owned network that covers only few kilometers (up to 3KM).
• It has higher speed data transfer rate maximum up to 1000Mbps i.e. 1Gbps.
• It is highly secured network and it has least error rate than others.
• It is least expensive type of network because it requires least expensive devices such as NIC, I lub,
Bridge, Modem and Switches, etc. It uses same type of transmission media such as twisted pair
cable, coaxial cable, or wireless.
• The most common type of LAN protocols am Ethernet or Token passing.
• Common LAN topologies are bus, ring, star, tree, mesh, and hybrid.
4.2 Metropolitan Area Network
A metropolitan area network (MAN) can be either public or
privately owned network. Its size is bigger than LAN and smaller
than WAN. It spans within one metropolitan city or larger
geographical area. It can connect large number of computers
and heterogeneous multiple LANs within a city maximum up to
100KM. Branch offices are connected to head office through
MANs. Common examples of MAN are Cable TV network,
Internet Service Provider (ISP) in a city etc.
Figure 4MAN
MAN may use wired or wireless transmission media such as
coaxial cable, optical fiber or microware. It is more expensive, more complex and less secured network
than LAN. It uses different set of protocols such as Frame Relay, ATM. X.25, etc. It has lower data
transfer rate 10Mbps (Megabits Per Second) than LAN and low amount of error rate 100 times lower
than WAN.
Features of MAN
• MAN is complex and heterogeneous type of network and it may be either public or privately own.
• The size of MAN is bigger than LAN and it covers maximum up to 100KM.
• It is less secured network and higher error rate than LAN.
• It is more expensive than IAN because it requires repeater, router, gateway, and microwave station
etc.
• It uses different types of media such as coaxial cable, optical fiber, microwave etc.
• The most common MAN protocols are ATM, Frame Relay, X25.
• It uses mesh and hybrid topologies.
• It can connect 100s of LANs and 1000s of individual computers.
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Data Communication and Computer Networks
4.3 Wide Area Network
A wide area network (WAN) is basically public type
heterogeneous network. It is the largest sized network
and connects millions of computers, thousands of
LANs, hundreds of MANs around the counties,
continents and even the whole world. Example of WAN
is Internet where we can share information around the
world. Communication is done through public
channels such as telephone fine, optical fiber and
microwave, satellite and VSAT (Very Small Aperture
Terminal). It is the most expensive and most complex
type of network that transfers text, graphics, audio, video, etc around the world. It uses hundreds of
different protocols such as ECP/ IP, ATM, Frame Relay, X.25 etc. It is capable to connect different types
of computers, transmission media, topologies, etc. It has least speed data transfer rate 64Kbps
(Kilobits Per Second) to 10Mbps or higher and very high amount of error rate 1000 times higher than
that in LAN.
Features of WAN
• WAN is basically public network that covers millions of kilometers around the world.
• It has low speed data transfer 64Kbps to 10Mbps or more.
• It is less secured network and it has highest error rate.
• It is the most expensive type of network because it requires large amount of expensive devices
such as router, gateway, microwave station, satellite, etc.
• It uses various types of transmission media such as telephone line, optical fiber, radio wave,
microwave, satellite, etc.
• It uses hundreds of different protocols such as ICP/ IP, FT? Imp, ATM, Frame Relay, X.25 etc.
• It uses mesh and hybrid topologies.
• It can connect unlimited LANs and MANs.
5 LAN Topologies
Network topology refers to the physical layout of the network. It shows the geometrical
representation of all the links and linking devices, also called nodes. Simply, it reflects the computers
in the network and how they are interconnected to each other. The main objective of the network
topology is to find out the most economical and efficient way of transmission channel. The most
common LAN topologies are bus, star, ring, tree, mesh and hybrid.
5.1 Bus Topology
Computers are connected to a single continuous
cable that is called 'bus'. It acts as backbone. A bus
must be terminated on both sides to prevent
signal bounce and computers are connected
to the bus with the help of drop line and Tconnecter (tap). A bus runs throughout the
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Data Communication and Computer Networks
office to which all the computers are connected. Message originating from a computer is
transmitted in both directions on the bus. It is based on client server network architecture.
Advantages
• It is simple and easy to setup and extend the network.
• It is inexpensive topology because it requires Is amount of cable and no additional network
devices.
• If any computer in the network downs, then it does not affect other computers.
• It is more flexible because we can easily connect and disconnect any number of computers in the
bus.
Disadvantages
• Data traffic is very high in bus so there may be chances of data collision.
• The length of bus should be small otherwise the performance of the network goes down.
• If there is problem in bus then the entire network goes down.
• It is very difficult to find out the fault in the bus.
5.2 Star Topology
Computers in the network are connected to each other with
the help of central connecting device hub or switch or server.
It is based on client server architecture. The communication is
done through the central server with the help of hub or switch
in the entire network. The hub controls traffic on the network.
It is the most popular and widely used topology for LAN.
Advantages
• It is simple, reliable and easy to set up and re-configuration.
• It is flexible to connect new computer and remove existing computer in the network.
• It is very easy to find out fault because of the use of hub or switch.
• If any computer in the network goes down, then other computers can continue their functions.
Disadvantages
• It requires very large amount of cables.
• It is expensive topology because of cables and networking device hub or switch.
• If there is any problem in central device hub or switch then the entire network will be down.
• The data traffic is high in central device hub so there may be chances of data collision.
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Data Communication and Computer Networks
5.3 Ring Topology
Computers are interconnected to each other by making a
closed circular structure that means each computer is
connected to other two adjacent computers in either side.
It is based on peer to peer network architecture.
Communication is done in single direction only. When one
computer receives the message, then it compares the
destination address of the message and its own address, if
it matches then it accepts the message otherwise the
message is retransmitted to the next computer in the
network.
Advantages
• It is simple and inexpensive topology.
• There is less chance of data collision because of unidirectional data transmission.
• There is no server so each computer has equal access facilities to the resources.
• Its performance is better than bus topology for small size network.
Disadvantages
• It is not flexible topology so it is difficult for adding and removing new nodes.
• It is not suitable for large size network (not more than 10 nodes).
• If there is problem in any computer or connection then the entire network goes down.
• It is very difficult to find out the errors in the network.
5.4 Mesh Topology
Every computer in the network has point to point connection to all other
computers by using multi-port connecter. There are multiple connections
between two computers which provide the alternate paths for data
transfer. The communication is done in both directions. It is also based on
peer to peer architecture. For n numbers of computer, we require n-1 ports for each computer and n
(n-1)/ 2 connections.
Advantages
• It is the fastest and robust type of topology.
• It is considered as the most reliable topology because of multiple connections between any two
nodes. • Failure in any computer or transmission media does not affect the rest of the network.
• There is less amount of data traffic due to multiple paths.
Disadvantages
• It is very much complex and most expensive topology.
• It is difficuli.to find an error in the network.
• It requires maximum amount of cables and multiport connecters.
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Data Communication and Computer Networks
• It is difficult to add and remove nodes in the network so it is not flexible.
5.5 Tree Topology
Tree topology is the extension of bus or star topology. So it
extends the branches of bus or star topology allowing more
stations to access the bus. As shown in the figure, the
computers are arranged in a distinct hierarchical structure.
Data can flow from top to bottom and vice versa. A break in
the bus is very difficult to locate.
Advantages
• It is easy to manage network as per our needs because of many sub networks or units.
• It is very flexible so we can add and remove any number of nodes.
• It is easier to find the fault nodes or hubs in the network.
• It makes easier to isolate and assign priorities into different level of units.
Disadvantages
• The failure of root node will cause the failure of entire network.
• It is expensive because of large amount of cables and network devices- hubs or switch.
• The data traffic is high at root node so there may be chances of data collision.
5.6 Hybrid Topology
If two or more topologies are combined together
then it is called hybrid topology. So it is very difficult
to design and to implement the hybrid topology. It is
expensive too. Because we need to install additional
network devices such as router or gateway.
6 Transmission Media
Transmission media refers to the wires, cables and other means through which data travels from its
source to destination. Transmission media are often called as communication media. The basic
requirements for working of a communication system are: A sender (source), which creates the
message to be transmitted. A medium, which carries the messages from sender to receiver. A receiver
(s Bounded media (also known as guided media) contains wires or cables such as:
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Data Communication and Computer Networks
TRANSMISSION MEDIA
Unbound
Media
Bounded
Twisted Pair
Cable
Coaxial
Cable
Radio Wave
Fiber Optics
Cable
Microwave
Satellite
Bounded Media
i.
ii.
iii.
Twisted pair cable
Coaxial cable and
Fiber optics cable
Unbounded media (also known as unguided media) includes:
i.
Microwave system
ii.
Satellite communication
iii.
Infrared technology
6.1 Twisted Pair Cable
Twisted pair cable is the oldest and still most common transmission media. It contains a pair of cables,
twisted with each other. Tighter the twisting cables, greater the transmission rate, and less chances
of error in transmission. The most common application of the twisted pair is in the telephone system
and in LAN. Nearly all telephones are connected to the telephone office by a twisted pair cable.
Twisted pair can only run few meters without amplification, but for longer distance, repeaters are
needed. Twisted-pair offers some significant benefits. It is cheaper, lighter, thinner, more flexible, and
easier to install than coax or fiber-optic cable. It is therefore ideal in offices and small network. There
are five categories of twisted pair specified by EIA/TIA
Category 1: Voice only waft data rate less than 100Kbps
Category 2: Data up to 2 Mbps
Category 3: Data up to 10 Mbps
Category 4: Data up to 20 Mbps
Category 5: Data up to 100 Mbps
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Data Communication and Computer Networks
Category 6: Data up to 200 Mbps
The twisted pair cable is connected with the help of
RJ-45 connector (for LAN) or RJ-11 (for telephone). RJ
stands for Registered Jack. Twisted pair cable is of two
types: UTP (Unshielded Twisted Pair) and STP
(Shielded twisted Pair). ink) which receives the
message. Transmission media is a medium used for
transmitting data.
Advantages
i. It is light and thin, so it more flexible to fit in a LAN.
ii. It is cheaper than other cables.
iii. It can transmit data at a higher bandwidth for short distance.
Disadvantages
i. It cannot be used for long distance transmission.
ii. It is slower for data transmission than other cables.
iii. It can be affected by electrical and magnetic field.
6.2 Coaxial Cable (COAX)
Coaxial cable (in short coax) carries
data signal of higher frequency and at
faster speed than twisted pair cable.
Its common use is for the
transmission of signals in cable
television system and broadband
transmission. Broadband and cable
TV take advantage of coax ability to
transmit many signals at the same
time. Each signal is called a channel.
Each channel travels along at a
differed frequency, so it does not
interfere with other channels. Coax has a large bandwidth, which means it can handle plenty of traffic
at high speeds. Other advantages include its ability to carry signals over a significant distance.
However, it is difficult install and bend as compared to twisted pair. Coaxial cable has a single copper
conductor at its center. A plastic layer provides insulation between the center conductor and a braided
metal shield and finally surrounded by an external insulate layer. The metal shield helps to block any
outside interference from fluorescent light and of computers. Coaxial cable is of two types:


Thin coax (also known as thinnet) has the speed of 10 base 2 i.e. 10mbps speed up to 200
meters.
Thick coax (also known as thicknet) has the speed of 10 base 5 i.e. 10mbps speed up to meters.
Advantages
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Data Communication and Computer Networks
i. It provides faster data transmission than the twisted pair cable.
ii. It can transmit data for medium range distance. iii. It is easier to install, modify and manage the
network.
Disadvantages
i. It is expensive to install.
ii. It is not appropriate for long distance transmission.
iii. It is rarely used at present for computer network.
6.3 Fiber Optics Cable
(Optical Fiber Cable) A fiber-optic cable is made of glass
or plastic and transmits signals in the form of light. It
contains single or multiple hairs like thin filaments of glass
fibers wrapped by a protective jacket. These glass
filaments can transmit data in the form of light at a
transmission speed of around 2 billion bits per second. So,
it is about 10 times faster than coaxial cable and 200
times faster than twisted pair cable. Fiber-optic can carry
voice, video, and data. It has enormous bandwidth and
can carry signals for extremely long distances. As it uses
light pulses to carry data, it is immune to electromagnetic
interference and is extremely faster. It is also more
secured than other cables. Despite its many advantages,
fiber optics implementation in the LAN is difficult to install
and costlier. Optical fibers cabling consists glass core at the center, which is surrounded by several
layers of protective materials. Fiber optics cable itself is a core fiber surrounded by cladding. A
protective covering surrounds both. LEDs or light emitting diodes send the signals down the cable. A
detector receives the signals and converts them back to the electrical impulses that computers can
understand and use. It is used for services like telecommunication, video conference and interactive
services. This cable is connected with the help of S-T connector.
Advantages
i. It provides fastest data transmission than other transmission media.
ii. It can be used for both short to long distance transmission.
iii. It provides error free and highly secured transmission.
iv. It can transmit multiple data at a same time due to the use of multiple fibers in a cable.
Disadvantages
i. It is highly expensive and difficult to install.
ii. It is not appropriate for short distance transmission due to its thickness and difficult to bend. iii.
Skilled technical manpower is required to install and are rare.
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Data Communication and Computer Networks
iv. It is not prefer to use hillside area; like up and down field connection
among stations.
6.4 Radio Wave
Radio Wave are a type of electromagnetic radiation with wavelengths in
the electromagnetic spectrum longer than infrared light. Radio waves
have frequencies from 300 GHz to as low as 3 kHz, and corresponding
wavelengths ranging from 1 millimeter (0.039 in) to 100 kilometers (62
mi). Radio waves have penetration power and can penetrate through
wall like obstacles. Thus we say that, line sight communication is not needed in radio waves. Radio
waves are used in AM and FM radio, cordless Phones, paging, Wi-Fi etc.
Disadvantage
If you have a large amount of radio waves people say it can cause disorders like cancer, leukemia. They
say that if you have power cabled above, or near your home it can affect your health.
6.5 Microwave System
Microwave system uses very high frequency radio signals
to transmit data through air. Transmitter and receiver of a
microwave system should be in line-of-sight because the
radio signal cannot bend. For this reason, transmitting and
receiving antenna must be placed on tall buildings towers,
hills and mountain peaks.
With microwave very long distance transmission is not
possible. In order to overcome the problem of line of sight
and shorter distance, repeaters are used at intervals of 25
to 30 kilometers apart. When one repeating tower receives signal, it amplifies the signal and sends it
to next tower. The frequency at which microwave radio signals are transmitted is commonly called
Radio frequency (RF). Radio frequencies that are used for commercial microwave transmission are
2,4,6,11,18 and 23 GHz.
Advantages:



No cables needed
Multiple channels available
Wide bandwidth
Disadvantages:



16
Line-of-sight will be disrupted if any obstacle, such as new buildings, are in the way
Signal absorption by the atmosphere. Microwaves suffer from attenuation due to atmospheric
conditions.
Towers are expensive to build
Data Communication and Computer Networks
6.6 Satellite Communication
Satellite communication is the most common worldwide
communication system at present. It uses a satellite as a
repeater. The satellite is the repeater that orbits in space.
Satellite communication systems have receivers and
transmitters located in ground stations. In this
communication, microwave signal is transmitted from a
transmitter on earth to the satellite at space. The satellite
amplifies the weak signal and transmits it back to the
receiver. To make communication possible in even longer
distance, multiple satellites are used.
Advantages:



High bandwidth
Coverage over a large geographical area
Can be cheaper over long distances
Disadvantages:



Huge initial cost
Noise and interference
Propagation delay
7 Data Transmission and Data Networking
Switching is process to forward packets coming in from one port to a port leading towards the
destination. When data comes on a port it is called ingress, and when data leaves a port or goes out it
is called egress. A communication system may include number of switches and node. Basically, there
are three switching techniques: Circuit switching, Message switching, and Packet switching.
7.1 Data Transmission and Data Networking
Switching is process to forward packets coming in from one port to a port leading towards the
destination. When data comes on a port it is called ingress, and when data leaves a port or goes out it
is called egress. A communication system may include number of switches and nodes. Basically, there
are three switching techniques: Circuit Switching, Message Switching, and Packet
7.1.1 Switching Circuit
Switching when two nodes
communicate with each other
over
a
dedicated
communication path, it is called
circuit switching. There is a
need of pre-specified route
from which data will travels and
no other data is permitted. In
circuit switching, to transfer the
data, circuit must be established so that the data transfer can take place. Applications which use circuit
switching may have to go through three phases: Establish a circuit, Transfer the data, Disconnect the
circuit. Circuit switching was designed for voice applications. Telephone is the best suitable example
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Data Communication and Computer Networks
of circuit switching. Before a user can make a call, a virtual path between callers and called is
established over the network.



It is the telecommunication technology by which two networks nodes establish a dedicated
communications channel.
The circuit remains connected for the duration of the communication session.
Each circuit cannot be used by other callers until the circuit is released and a new connection
is set up.
Advantage

It provides continuous transfer without the overhead associated with network packets making
maximum use of available bandwidth for that communication.

Time delay is minimum
Disadvantage
•
Connection and the bandwidth associated with it are reserved and unavailable for other uses.
•
Long call setup delay or difficult.
7.1.2 Message Switching
This technique was somewhere in middle
of circuit switching and packet switching.
In message switching, the whole message
is treated as a data unit and is transferred
in its entirety. A switch working on
message switching, first receives the
whole message and buffers it until there
are resources available to transfer it to the
next hop. If the next hop is not having
enough resource to accommodate large
size message, the message is stored and
switch waits.
This technique was considered substitute to circuit switching. Unlike circuit switching, here whole path
is not dedicated foe two entities. E-mail is example of a message switching system. Message switching
has the following drawbacks:






Every switch in transit path needs enough storage to accommodate entire message
Because of store-and-forward technique and waits included until resources are available,
message switching is very slow
Message switching systems are nowadays mostly implemented over packet switched or circuit
switched data.
Each Message contains addressing information, and at each switch this information is read
and the transfer path to the next switch is decided.
Email is a common application for Message switching.
No physical path is established in advance in between sender and receiver.
Advantage
•
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Data channels are shared among communication devices improving the use of bandwidth.
Data Communication and Computer Networks
•
Message can be stored temporarily at message switches.
•
Priorities may be used to manage network traffic.
•
Broadcast addressing used bandwidth more efficiently because messages are delivered to
multiple destination.
Disadvantage
•
High Delay (Processing delay + queuing delay)
•
Not suitable for integrated service (data + voice)
7.1.3 Packet Switching
Shortcomings of message switching gave
birth to an idea of packet switching. The
entire message is broken down into smaller
chunks called packets. The switching
information is added in the header of each
packet and transmitted independently. It is
easier for intermediate networking devices
to store small size packets and they do not
take much resource either on carrier path or in the internal memory of switches. Packet switching
enhances line efficiency as packets from multiple applications can be multiplexed over the carrier. The
internet uses packet switching technique. Packets are stored and forwarded according to their priority
to provide quality of service.
Advantage:
•
Efficient use of transmission resources.
•
Suitable for burst data traffic.
Disadvantage:
•
Longer delays in receiving messages due to the time required to package and route packets.
•
Potential for network security risks due to the use of shared physical links.
8 Modes of Data Transmission
Modes of data transmission can be divided into three categories:
i.
ii.
iii.
Simplex mode
Half Duplex mode
Full Duplex mode
8.1 Simplex Mode
A simplex mode is a connection in which the data flows in only one direction, from the transmitter to
the receiver. This type of connection is useful if the data do not need to flow in both directions. Data
transmission from computer to the printer is an example where simplex mode is suitable. Other
examples are data transmission from mouse to computer, data transmission from cable operator to
television etc.
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Data Communication and Computer Networks
8.2 Half Duplex Mode
Half-duplex connection is sometimes also referred to as alternating connection or semi-duplex
connection. In this mode data flows in one direction or the other, but not both at the same time. With
this type of connection, each end of the connection transmits data in turn. This type of connection
makes it possible to have bidirectional communications using the full capacity of the line. Data
transmission from RAM to CPU is an example of full duplex transmission. Other examples are data
transmission from disk to RAM, data transmission in telephone systems etc.
8.3 Full Duplex Mode
A full-duplex connection is a connection in which the data flow in both directions simultaneously. Each
end of the line can thus transmit and receive at the same time, which means that the total bandwidth
is divided in two for each direction of data transmission if the same transmission medium is used for
both directions of transmission. Data transmission in internet is an example of full duplex
transmission.
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Data Communication and Computer Networks
9 Wireless Networking
A wireless network is any type of computer
network that uses wireless data connections for
connecting network nodes. It is a method by
which homes, telecom networks and enterprise
(business) installations avoid the costly process
of introducing cables into a building, or as a
connection between various equipment
locations. A wireless network enables people to
communicate and access applications and
information without wires. This provides
freedom of movement and the ability to extend
applications to different parts of a building, city,
or nearly anywhere in the world. Wireless networks allow people to interact with e-mail or browse
the Internet from a location that they prefer.
9.1 Two kinds of wireless networks:
9.1.1 Ad-hoc / peer-to-peer wireless network consists of



A number of computers each equipped with a wireless
networking interface card.
Each computer can communicate directly with all of the other
wireless enabled computers.
They can share files and printers, but may not be able to
access wired LAN resources, unless one of the computers acts
as a bridge to the wi red LAN using special software. (This is
called "bridging")
9.1.2
Infrastructure Wireless Network:
A wireless network can also use an access point, or base station.

It can connect (or "bridge") the wireless LAN to a wired
LAN, allowing wireless computer access to LAN resources.
Advantages
Convenience
The wireless nature of such networks allows users to access network resources from nearly any
convenient location within their primary networking environment (a home or office). With the
increasing saturation of laptop-style computers, this is particularly relevant.
Mobility
With the emergence of public wireless networks, users can access the internet even outside their
normal work environment. Most chain coffee shops, for example, offer their customers a wireless
connection to the internet at little or no cost.
Productivity
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Data Communication and Computer Networks
Users connected to a wireless network can maintain a nearly constant affiliation with their desired
network as they move from place to place. For a business, this implies that an employee can
potentially be more productive as his or her work can be accomplished from any convenient location.
Deployment
Initial setup of an infrastructure-based wireless network requires little more than a single access point.
Wired networks, on the other hand, have the additional cost and complexity of actual physical cables
being run to numerous locations (which can even be impossible for hard-to-reach locations within a
building).
Expandability
Wireless networks can serve a suddenly-increased number of clients with the existing equipment. In
a wired network, additional clients would require additional wiring.
Cost
Wireless networking hardware is at worst a modest increase from wired counterparts. This potentially
increased cost is almost always more than outweighed by the savings in cost and labor associated to
running physical cables.
Disadvantage:
Security
To combat this consideration, wireless networks may choose to utilize some of the various encryption
technologies available. Some of the more commonly utilized encryption methods, however, are
known to have weaknesses that a dedicated adversary can compromise.
Range
The typical range of a common 802.11g network with standard equipment is on the order of tens of
meters. While sufficient for a typical home, it will be insufficient in a larger structure. To obtain
additional range, repeaters or additional access points will have to be purchased. Costs for these items
can add up quickly.
Reliability
Like any radio frequency transmission, wireless networking signals are subject to a wide variety of
interference, as well as complex propagation effects that are beyond the control of the network
administrator.
Speed
The speed on most wireless networks (typically 1-54 Mbps) is far slower than even the slowest
common wired networks (100Mbps up to several Gbps). However, in specialized environments, the
throughput of a wired network might be necessary.
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Data Communication and Computer Networks