Data Communication and Computer Networks
Chapter Two
Introduction to Computer Networks
2.1 What is Networking?
A computer network is a collection of computers and other devices that communicate to share data,
hardware, and software.
Copying files onto floppy disks and giving them to others to copy onto their computers was
sometimes referred to as the "sneakernet." This early form of computer networking is one that
many of us have used and perhaps still use today.
Generally a computer network is a system in which a number of independent computers are linked
together to share data and peripherals, such as files and printers. In the modern world, computer
networks have become almost indispensable. All major businesses, and governmental and
educational institutions make use of computer networks to such an extent that it is now difficult to
imagine a world without them.
Note:
•
Connecting together of computers and other devices is called a network, and the concept of
connected computers sharing resources is called networking
2.2 Elements of Computer Networking
Figure 1: Basic elements of computer networks
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2.3 Uses (Application) of Computer Networking
Networks increase efficiency and reduce costs. Computer networks achieve these goals in the
following primary ways:
1. Sharing Information (Or Data)-- like, Company Data Base
2. Sharing Hardware(Peripheral Devices) Printers, Faxes, FDD,HDD,CD-ROM-DRIVE etc
3. Sharing Internet Connection /communication by e-mail, voice and video conferencing.
4. Sharing Programs/Software - Expensive Professional Software
5. Centralizing Administration And Support
i.
Centralized Software Update: like Antivirus
ii.
Centralized Storage and Backup
iii.
Centralized Processing and management of data, such as Airline reservation.
The following figure illustrates the benefit of computer networking
Figure 2: Benefit of computer networks
Sharing Information/data
The reason for the popularity of computer networks is that they offer many advantages. Information
such as important files, video and audio, and email can be easily shared between users. Users in a
certain network environment have the liberty of sharing data and information across the network.
Data sharing enables different users to work on a certain file concurrently. Few examples of data
sharing are:
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• Database: - databases are often managed centrally and several users can have access to the
database at same time. For example, in a networked banking system, different bank
branches can have access to the central account database. This enables bank clients to carry
their transactions on any branch bank office.
• Email: email communication can be achieved over the network enabling networked users in
the company to communicate messages across the network using email.
Sharing Hardware (Peripheral devices)
Computer networks enable us to share expensive hardware resource among several computers.
Peripherals such as printers and modems can be shared over the network. For example, a company
may prefer to acquire one expensive printer and connect it to the network to provide high quality
printing to users. This avoids the need to have separate printer for each computer in the office.
For example, Figure 1 shows a printer being used in a stand-alone environment and in a networked
environment. By connecting many computers to a print server any of them may make use of the
printer directly, instead of the single computer in the stand-alone environment.
Figure 3: Printer in a stand-alone environment (left) and in a networked environment (right)
Central Disk storage can also be shared among users through networks. Network system provide
the possibility of using a dedicated file server to store all the company data in one location. Users
will be able to access their data over the network from their workstations. Using central disk
storage system facilitates data backup operations from a central location guarantying complete data
recovery in case of system failures.
Sharing Programs/Software
Application (software) sharing occurs when several users at different locations run application
programs that are installed centrally in one location (application server). Software (applications)
that are installed centrally can be managed and upgraded centrally. Such systems are mostly
common in financial system where the main application is stored centrally and users in different
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sections of the finance department are able to run the system from the network. It is effective for
several reasons
• Less disk storage space is required because the program is stored only once on the server,
instead of being stored on the hard disk s of multiple stand alone computers
• When a new version of the software is released, it is easier to update on copy of the program
on the server than to update many copies stored on stand –alone computer.
• Purchasing a software license for a network can be less expensive than purchasing a singleuser license for every workstation on the network
In short sharing programs on a network Saves disk space, Reduces maintenance, and Reduce
licensing cost
Centralizing Administration And Support
The login (gain access to) Process: Even if your computer is physically connected to a network,
you cannot typically use network resources until you log into the network. When you log in, you
formally identify yourself to the network by providing your ID and password.
Your user ID and password are the basis for your user account. A user account provides access to
network resources and accumulates information about your network use by tracking when you log
in and log out.
A network administrator/supervisor is the person who is responsible for setting up user accounts
and maintain a network. The network administrator provides each new user with a user ID and
starter password. In this way centralized administration and support can be achieved.
Note: Network administration tasks include:
•
Managing users and security.
•
Making resources available.
•
Maintaining applications and data.
•
Installing and upgrading application and operating system software.
2.4 Disadvantage of Networking
However, with these advantages come a number of potential disadvantages. Some of the
disadvantages and challenges of networking are discussed as follow:
Security/Privacy
Making important and sensitive information available to every user of the network is not normally
desirable. For example, information about employees’ salaries should not be freely available for
anybody to look at. Data security is therefore an important concern in a networked environment.
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Viruses
If a virus is introduced to the network, either intentionally or unintentionally, it will rapidly spread
around all of the workstations and could start to cause havoc to peoples' files or to the efficient
working of the network.
Network failure
If the file server fails then no-one on the network can access any files or folders. This means
that nobody can do any work. For an organization, this would be extremely costly and disruptive. If
a cable, hub or switch on the network fails, this would mean that any computers connected to that
part of the network couldn't be used to access network resources. They could still be used as
individual, stand-alone machines.
Slow service
As more users log onto the network and request files, send things to be printed and open more
software applications, the network can start to slow down. There is only a limited amount of
bandwidth and the more data that is travelling around the network, the slower things become.
Figure 4: Summary of computer network uses
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2.5 Computer Networking Terminologies
• Stand-Alone Computer: A computer that is not connected to a network
• Peers: are Computers which use and provide network resources.
• Clients: are Computers that uses but do not provide network resources
• Servers: are Powerful computers that provide network resources. Server computer has
powerful processors, large size memory (RAM), hard disk with High storage capacity
(RAID) and more powerful network adapter/network interface card (NIC)
• Media: A communication channel (bounded/unbounded) that make physical connections.
• Network Resources: any service or device, such as files, printers, or other items, made
available for use by members of the network.
Example: Shared data, Shared printers and other peripherals, & shared applications
• Node: each device in the network
2.6 Network Types
Geographically a network may be spanned across your table, among Bluetooth enabled
devices, ranging not more than few meters. It may be spanned across a whole building, including
intermediate devices to connect all floors. Moreover may be spanned across a whole city or across
multiple cities or provinces. It may also be one network covering whole world.
Generally network can be grouped in three major categories depending on its size and the
geographical scope they cover.
1. Local Area Network (LAN)
2. Metropolitan Area Network (MAN)
3. Wide Area Network (WAN)
Local Area Network (LAN)
A local area network (LAN) is the basic building block of any computer network. A LAN can range
from simple (two computers connected by a cable) to complex (hundreds of connected computers
and peripherals throughout a major corporation). (See Figure 5). The distinguishing feature of a
LAN is that it is confined to a limited geographic area usually within a building or a campus. LANs
are found in most organizations, businesses, government offices, educational institutions, and in
home computing environment.
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Figure 5: A Local Area Network (LAN)
LANs are primarily designed to allow resources to be shared between personal computers or
workstations. The resources to be shared can include hardware (e.g., a printer), software (e.g., an
application program), or data. In addition to size, LANs are distinguished from other types of
networks by their transmission media and topology. In general, a given LAN will use only one
type of transmission medium. The most common LAN topologies are bus, ring, and star.
Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range. Today, however,
speeds are normally 100 or 1000 Mbps. Wireless LANs are the newest evolution in LAN
technology.
Wide Area Network (WAN)
Wide area networks are networks that cover very large geographical area such as a country or the
whole world. WAN has no geographical limit (see Figure 6). Because a WAN has no geographical
limitations, it can connect computers and other devices in separate cities or on opposite sides of the
world. A WAN is made up of a number of interconnected LANs. A typical WANs are the telephone
networks operated by most long distance telephone companies. Large companies having offices
scattered around the world use WAN to connect their computers. Perhaps the ultimate WAN is the
Internet.
WAN unlike LAN usually use combination of several types of communication media such as
cables, satellites, Microwaves, fiber-optics.
Figure 6: A Wide Area Network (WAN)
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Typical examples of WAN:
Internet: It is a global collection of networks, both big and small. It is a giant network.
Intranet: Are similar to World Wide Web (WWW) where centrally stored hypertext
documents can be accessed using the web. Unlike the WWW, intranets are available only to
user within the company network system. Intranet is a very common service in large
networked organizations.
Extranet: Although similar to intranet, extranet provides selected users from outside the
organization to access data from the internal network. Extranets are commonly used by
suppliers to provide data to company clients.
Metropolitan Area Network (MAN)
A metropolitan area network (MAN) is a network with a size between a LAN and a WAN. It is a
collection of LANs linked together with high performance hardware within a university or college
campus. It normally covers the area inside a town or a city. MAN is designed for customers who
need a high-speed connectivity, normally to the Internet, and have endpoints spread over a city or
part of city.
A good example of a MAN is the part of the telephone company network that can provide a highspeed DSL line to the customer (we will discuss DSL in later chapters). Another example is the
cable TV network that originally was designed for cable TV, but today can also be used for highspeed data connection to the Internet.
MAN can be in the form of Ethernet, Token-ring, ATM, or Fiber Distributed Data Interface
(FDDI). Metro Ethernet is a service which is provided by ISPs. This service enables its users to
expand their Local Area Networks. For example, MAN can help an organization to connect all of its
offices in a city. The backbone of MAN is high-capacity and high-speed fiber optics. MAN works
in between Local Area Network and Wide Area Network. It provides uplink for LANs to WANs or
internet
Figure 7: A Metropolitan Area Network (MAN)
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LAN Vs. WAN
LANs typically have much higher transmission rates than WANS. Most LANs are able to transmit
data at around 100Mbps (million bits per second), whereas WANs generally transmit at less than
10Mbps. Another difference is the error rates in transmission: the likely number of errors in data
transmission is higher for a WAN than for a LAN.
This distinction between LANs and WANs is made because of the locality principle. The locality
principle in computer networking states that computers are much more likely to want to
communicate with other computers that are geographically close, than with those that are distant.
For example, if you want to request a printout from your PC, it makes much more sense to use the
printer in the next room rather than one that is hundreds of kilometers away. Because of the locality
principle network designers tend to use higher performance hardware within a LAN compared to
the connections between different LANs that form a WAN.
As a general we can distinguish the difference of network type, in addition to geographical span
coverage, based on the following key points
Type of media they used
Data transmission rate
Type of service they provide
Cost
2.7 Type of Connection
As discussed in previous section a network is two or more devices connected through links. A link
is a communications pathway that transfers data from one device to another. For visualization
purposes, it is simplest to imagine any link as a line drawn between two points. For communication
to occur, two devices must be connected in some way to the same link at the same time. There are
two possible types of connections:
Point-to-point
Multipoint.
Point-to-point
A point-to-point connection provides a dedicated link between two devices.
The entire capacity of the link is reserved for transmission between those two devices.
Most point-to-point connections use an actual length of wire or cable to connect the two ends,
but other options, such as microwave or satellite links, are also possible.
When you change television channels by infrared remote control, you are establishing a
point-to-point connection between the remote control and the television's control system
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Multipoint
A multipoint (also called multidrop) connection is one in which more than two specific
devices share a single link.
In a multipoint environment, the capacity of the channel is shared, either spatially or
temporally.
If several devices can use the link simultaneously, it is a spatially shared connection.
If users must take turns, it is a timeshared connection.
2.8 Network Topologies
The term topology, or more specifically, network topology, refers to the arrangement of computers,
cables, and other components on a network. It is a map of the physical network. "Topology" is the
standard term that most network professionals use when they refer to the network's basic design. In
addition to the term "topology," you will find several other terms that are used to define a network's
design:
Physical layout
Design
Diagram
Map
The topology of a network is the geometric representation of the relationship of all the links and
linking devices (usually called nodes) to one another.
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The type of topology you use affects the type and capabilities. In general the choice of one topology
over another will have an impact on the:
Type of equipment that the network needs
Capabilities of the network
Growth of the network
Way the network is managed
Developing a sense of how to use the different topologies is a key to understanding the capabilities
of the different types of networks.
Before computers can share resources or perform other communication tasks they must be
connected. Most networks use cable to connect one computer to another. However, it is not as
simple as just plugging a computer into a cable connecting to other computers. Different types of
cable combined with different network cards, network operating systems, and other components
require different types of arrangements. To work well, a network topology takes planning. For
example, a particular topology can determine not only the type of cable used but also how the
cabling runs through floors, ceilings, and walls. Topology can also determine how computers
communicate on the network. Different topologies require different communication methods, and
these methods have a great influence on the network.
The topology is both physical and logical:
Physical topology describes how the physical components on a network are connected
Logical topology describes the way network data flows through the physical components
The most common types of standard physical topologies, which we are going to analyze, are:
Bus Topology
Star/Hub Topology
Ring Topology
Mesh Topology
In addition to the basic network topology, the following also used:
Hybrid Topology
Tree Topology
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2.8.1) Bus Topology
Bus Topology is the simplest of network topologies and often referred to as a "linear bus" because
the computers are connected in a straight line. In this type of topology, all the nodes (computers as
well as servers) are connected to the single cable (called bus/trunk or also called backbone or
segment) that carries the data, by the help of interface connectors. This central cable is the backbone
of the network and is known as Bus (thus the name). Every workstation communicates with the
other device through this Bus.
A signal/traffic generated from the source is broadcasted and it travels to all workstations
connected to bus cable. Although the message is broadcasted but only the intended recipient, whose
MAC address or IP address matches, accepts it. If the MAC /IP address of machine doesn’t match
with the intended address, machine discards the signal.
Because only one computer at a time can send data on a bus network, the number of computers
attached to the bus will affect network performance. The more computers there are on a bus, so will
the network traffic and the more computers will be waiting to put data on the bus. This can greatly
decrease the performance and available bandwidth of your network and, consequently, the slower
the network will be. This works well in a small network of 2-5 computers. Bus topology is fairly old
news and you probably won't be seeing much of these around in any modern office or home.
Computers on a bus either transmit data to other computers on the network or listen for data from
other computers on the network. They are not responsible for moving data from one computer to the
next.
Signal bounce:- Because the data, or electronic signal, is sent to the entire network, it travels from
one end of the cable to the other. If the signal is allowed to continue uninterrupted, it will keep
bouncing back and forth along the cable and prevent other computers from sending signals.
Therefore, the signal must be stopped after it has had a chance to reach the proper destination
address.
Terminator:- To stop the signal from bouncing, a component called a terminator is placed/added at
each ends of the central cable to absorb free signals and to prevent bouncing of signals. Absorbing
the signal clears the cable so that other computers can send data.
In a bus topology, if a break in the cable occurs the two ends of the cable at the break will not have
terminators, so the signal will bounce, and all network activity will stop. This is one of several
possible reasons why a network will go "down." The computers on the network will still be able to
function as stand-alone computers; however, as long as the segment is broken, they will not be able
to communicate with each other or otherwise access shared resources.
A barrel connector can be used to extend it. Figure 8 shows a typical bus topology.
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Figure 8: The bus topology
As you can see in the above example, all computers are attached to a continuous cable, which
connects them in a straight line. The arrows clearly indicate that the packet generated by Node 1 is
transmitted to all computers on the network, regardless the destination of this packet.
Also, because of the way the electrical signals are transmitted over this cable, its ends must be
terminated by special terminators that work as "shock absorbers", absorbing the signal so it won't
reflect back to where it came from. The value of 50Ohms has been selected after carefully taking in
consideration all the electrical characteristics of the cable used, the voltage that the signal which
runs through the cables, the maximum and minimum length of the bus and a few more.
Advantages of Bus Topology
It is cost effective.
It is easy to install and understand.
Cable required is least compared to other network topology.
Easy to add extra workstations or expand joining two cables together
Used in small networks. Good for LAN and best choice for temporary networks
Disadvantages of Bus Topology
There is a limit on central cable length and number of nodes that can be connected.
Dependency on central cable. If the main cable (i.e. bus ) encounters some problem or
broken, whole network breaks down or whole network fails.
Proper termination is required to dump signals. Use of terminators is must.
It is difficult to detect and troubleshoot fault at individual station.
Maintenance costs can get higher with time.
It is not suitable for networks with heavy traffic.
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If network traffic is heavy or nodes are more the performance of the network decreases and
data can travel slowly.
Data collisions can happen as the network becomes busy
Security is very low because all the computers receive the sent signal from the source.
2.8.2) Star Topology
In the star topology, all the components of network are connected to the central device called “hub”
which may be a hub, a router or a switch by a cable segments. Unlike Bus topology (discussed
earlier), where nodes were connected to central cable, here all the workstations are connected to
central device with a point-to-point connection. So it can be said that every computer is indirectly
connected to every other node by the help of “hub”.
Figure 9 shows eight computers and a hub connected in a star topology. All Signals are transmitted
from the sending computer on the star topology passes through the central device before reaching
the intended destination. Hub acts as a junction to connect different nodes present in Star Network,
and at the same time it manages and controls whole of the network. Depending on which central
device is used, “hub” can act as repeater or signal booster. Central device can also communicate
with other hubs of different network. Unshielded Twisted Pair (UTP) Ethernet cable is used to
connect workstations to central node.
Figure 9: The star topology
Because each computer is connected to a central point, this topology requires a great deal of cable
in a large network installation. Also, if the central point fails, the entire network goes down. If one
computer or the cable that connects it to the hub fails on a star network, only the failed computer
will not be able to send or receive network data. The rest of the network continues to function
normally.
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The Star or Hub topology is one of the most common network topologies found in most offices and
home networks. It has become very popular in contrast to the bus type (which we just spoke about),
because of the cost and the ease of troubleshooting.
Advantages of Star Topology
Fast performance with few nodes and low network traffic.
Hub can be upgraded easily.
Easy to setup and modify as well as to troubleshoot.
Easy to expand. In star topology new nodes can be added easily without affecting rest of the
network. Similarly components can also be removed easily.
Failure of one node or link doesn’t affect the rest of network. At the same time it's easy to
detect the failure and troubleshoot it.
Star networks are very reliable. If one connection fails, it does not affect other users
Very few data collisions as each workstation has its own cable to the server
Centralized management. It helps in monitoring the network.
Good security - no workstation can interact with another without going through the server
first
Disadvantages of Star Topology
Cost of installation is high and Expensive to use. Because the use of hub, a router or a
switch as central device increases the overall cost of the network..
Too much dependency on central device has its own drawbacks. If it fails whole network
goes down.
Performance and as well number of nodes which can be added in such topology is depended
on capacity of central device.
The most expensive network layout to install because of the amount of cables needed
Installing the network usually needs experts to set it up
Extra hardware such as hubs and switches may be needed
If the server crashes or stops working then nobody will be able to access their files or use the
network.
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2.8.3) Ring Topology
In the ring topology, computers are connected on a single circle of cable forming a closed loop.
Unlike the bus topology, there are no terminated ends. The signals travel around the loop in one
direction and pass through each computer, which acts as a repeater to boost the signal and send it to
the next computer. On a larger scale, multiple LANs can be connected to each other in a ring
topology by using Thicknet coaxial or fiber-optic cable. Figure 10 shows a typical ring topology
with six workstations.
Messages in a ring network pass in one direction, from node to node. As a message travels around
the ring, each node examines the destination address attached to the message. If the address is the
same as the address assigned to the node, the node accepts the message; otherwise, it regenerates
the signal and passes the message along to the next node in the circle. Such regeneration allows a
ring network to cover larger distances than star and bus networks. It can also be designed to bypass
any malfunctioning or failed node. The method by which the data is transmitted around the ring is
called token passing. A token is a special series of bits that contains control information. Possession
of the token allows a network device to transmit data to the network. Each network has only one
token.
Token Passing (in brief): Token contains a piece of information which along with data is sent by
the source computer. This token then passes to next node, which checks if the signal is intended to
it. If yes, it receives it and passes the empty to into the network, otherwise passes token along with
the data to next node. This process continues until the signal reaches its intended destination.
The nodes with token are the ones only allowed to send data. Other nodes have to wait for an empty
token to reach them. This network is usually found in offices, schools and small buildings.
Figure 10: The ring topology
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Advantages of Ring Topology
Transmitting network is not affected by high traffic or by adding more nodes, as only the
nodes having tokens can transmit data.
Transmission of data is fairly simple as it only travels in one direction
No data collisions
Cheap to install and expand
Extra computers can be added easily with little effect on performance - although you have to
shut down the network to be able to do this.
There is no need for network server to control the connectivity between workstations.
Each computer has equal access to resources.
Disadvantages of Ring Topology
Indentifying and troubleshooting network problem is difficult.
Because of the closed loop, adding or deleting a new nodes can be difficult and may disturbs
the network activity.
Failure of one computer disturbs the whole network.
If a single machine is switched off, the network doesn't work
If a cable breaks, the network doesn't work
Data must pass through every computer until it reaches its destination. This can make it
slower than other network layouts.
Network is highly dependent on the wire which connects different components
2.8.4) Mesh Topology
A mesh topology network offers superior redundancy and reliability. In a mesh network topology,
each of the network node, computer and other devices, are interconnected with one another by
separate cabling (See Figure 11). Every node not only sends its own signals but also relays data
from other nodes. In fact a true mesh topology is the one where every node is connected to every
other node in the network. This type of topology is very expensive as there are many redundant
connections, thus it is not mostly used in computer networks. It is commonly used in wireless
networks. Flooding or routing technique is used to transmit data over mesh topology.
Routing:- In routing, the nodes have a routing logic, as per the network requirements. Like routing
logic to direct the data to reach the destination using the shortest distance. Or, routing logic which
has information about the broken links, and it avoids those node etc. We can even have routing
logic, to re-configure the failed nodes.
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Flooding:- In flooding, the same data is transmitted to all the network nodes, hence no routing logic
is required. The network is robust, and the its very unlikely to lose the data. But it leads to unwanted
load over the network.
Figure 11: The mesh topology
Mesh topology is a point-to-point connection to other nodes or devices. All the network nodes are
connected to each other. Mesh has n(n-2)/2 physical channels to link n devices.
Types of Mesh Network topologies
1.
Partial Mesh Topology: This is far more practical as compared to full mesh topology. Here,
some of the systems are connected in similar fashion as in mesh topology while rests of the
systems are only connected to 1 or 2 devices. It can be said that in partial mesh, the
workstations are ‘indirectly’ connected to other devices. This one is less costly and also
reduces redundancy.
2.
Full Mesh Topology: Each and every nodes or devices are connected to each other. Even after
considering the redundancy factor and cost of this network, its main advantage is that the
network traffic can be redirected to other nodes if one of the nodes goes down. Full mesh
topology is used only for backbone networks.
Advantages of Mesh Topology
Each connection can carry its own data load. Data can be transmitted from different devices
simultaneously. This topology can withstand high traffic.
Backup capabilities. Even if one of the components fails there is always an alternative
present. So data transfer doesn’t get affected.
Expansion and modification in topology can be done without disrupting other nodes
Fault is diagnosed easily.
Provides security and privacy.
It is robust.
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Disadvantages of Mesh Topology
Set-up (Installation and configuration) and maintenance of this topology is very difficult. Even
administration of the network is tough.
Because it uses more cables the overall cost of this network is way too high as compared to other
network topologies.
Bulk wiring is required.
There are high chances of redundancy in many of the network connections.
2.8.5) Hybrid Topology
Hybrid, as the name suggests, is mixture of two different things. Similarly in this type of topology
we integrate/combined two or more different standard topologies to form a resultant topology which
has good points(as well as weaknesses) of all the constituent basic topologies rather than having
characteristics of one specific topology. This combination of topologies is done according to the
requirements of the organization. For example, if there exists a ring topology in one office
department while a bus topology in another department, connecting these two will result in Hybrid
topology. Remember connecting two similar topologies cannot be termed as Hybrid topology. StarRing and Star-Bus networks are most common examples of hybrid network.
Star-Bus
The star bus is a combination of the bus and star topologies. In a star-bus topology, several star
topology networks are linked together with linear bus trunks. Figure 12 shows a typical star-bus
topology.
In this topology, one computer goes down/fails, it will not affect the rest of the network. The other
computers can continue to communicate. However, if the central component, or hub, that attaches
all computers in a star, fails, then you have big problems since no computer will be able to
communicate. If a hub is linked to other hubs, those connections will be broken as well.
Figure 12: The star-bus topology
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Star-Ring
The star-ring (sometimes called a star-wired ring) appears similar to the star-bus. Both the star-ring
and the star bus are centered in a hub that contains the actual ring or bus. In the Star-Ring topology,
the computers are connected to a central component as in a star network. These components,
however, are wired to form a ring network. Figure 13 shows a star-ring network. Linear-bus trunks
connect the hubs in a star bus, while the hubs in a star-ring are connected in a star pattern by the
main hub. Like the star-bus topology, if a single computer fails, it will not affect the rest of the
network. By using token passing, each computer in a star-ring topology has an equal chance of
communicating. This allows for greater network traffic between segments than in a star-bus
topology.
Figure 13: The star-ring topology
Advantages of Hybrid Topology
Reliable: Unlike other networks, fault detection and troubleshooting is easy in this type of
topology. The part in which fault is detected can be isolated from the rest of network and
required corrective measures can be taken, WITHOUT affecting the functioning of rest of
the network.
Scalable: It's easy to increase the size of network by adding new components, without
disturbing existing architecture.
Flexible: Hybrid Network can be designed according to the requirements of the organization
and by optimizing the available resources. Special care can be given to nodes where traffic
is high as well as where chances of fault are high.
Effective: Hybrid topology is the combination of two or more topologies, so we can design
it in such a way that strengths of constituent topologies are maximized while there
weaknesses are neutralized. For example we saw Ring Topology has good data reliability
(achieved by use of tokens) and Star topology has high tolerance capability (as each node is
not directly connected to other but through central device), so these two can be used
effectively in hybrid star-ring topology.
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Disadvantages of Hybrid Topology
Complexity of Design: One of the biggest drawback of hybrid topology is its design. Its not
easy to design this type of architecture and its a tough job for designers. Configuration and
installation process needs to be very efficient.
Costly Hub: The hubs used to connect two distinct networks, are very expensive. These
hubs are different from usual hubs as they need to be intelligent enough to work with
different architectures and should be function even if a part of network is down.
Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a lot
of cables, cooling systems, sophisticate network devices, etc.
2.8.6) Tree Topology
Tree Topology integrates the characteristics of Star and Bus Topology. Earlier we saw how in
Physical Star network Topology, computers (nodes) are connected by each other through central
hub. And we also saw in Bus Topology, work station devices are connected by the common cable
called Bus. After understanding these two network configurations, we can understand tree topology
better. In Tree Topology, the number of Star networks are connected using Bus. This main cable
seems like a main stem of a tree, and other star networks as the branches. It is also called Expanded
Star Topology. Ethernet protocol is commonly used in this type of topology. The diagram below
will make it clear.
Figure 14: The tree topology
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(a) Clustered star topology
(b) Hierarchical Star Topology
Advantages of Tree Topology
It is an extension of Star and bus Topologies, so in networks where these topologies can't be
implemented individually for reasons related to scalability, tree topology is the best
alternative.
Expansion of Network is possible and easy.
Here, we divide the whole network into segments (star networks), which can be easily
managed and maintained.
Error detection and correction is easily done.
Each segment is provided with dedicated point-to-point wiring to the central hub.
If one segment is damaged, other segments are not affected.
Disadvantages of Tree Topology
Heavily cabled and Costly.
Central hub fails, network fails.
Because of its basic structure, tree topology, relies heavily on the main bus cable, if it breaks
whole network is crippled.
As more and more nodes and segments are added, the maintenance becomes difficult.
Scalability of the network depends on the type of cable used.
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Selecting a Topology
There are many factors to consider when deciding which topology best suits the needs of an
organization. Some of those factors to be taken into consideration while choosing a Network
topology are
1) Scale of your project (in terms of number of components to be connected).
2) Amount of traffic expected on the network.
3) Budget allotted for the network i.e. amount of money you are willing to invest.
4) Required response time
The following table provides some guidelines for selecting a topology
Topology Advantages and Disadvantages
Topology
Bus
Advantages
Disadvantages
Use of cable is economical.
Media is inexpensive and easy to
work with.
System is simple and reliable.
Bus is easy to extend.
Easy to connect a node to a
bus/trunk
•
•
•
•
Ring
Star
•
•
•
•
Mesh
•
Network can slow down in heavy
traffic.
Problems are difficult to isolate. If the
cable is damaged anywhere along its
length or either end of the cable loses its
termination, the entire network fail.
Terminators are required at both ends of
the backbone cable
Not advisable to install in a large
building
Lacks central control
Failure of one computer can impact the
rest of the network, because each
computer acts as a repeater to boost the
signal and send it to the next computer.
Problems are hard to isolate.
Network reconfiguration disrupts
operation.
•
System provides equal access for all •
computers.
Performance is even/uniform
despite many users.
•
There is no reliance on a central
•
hub. all messages pass through all
devices.
Modifying system and adding new • If the centralized point fails, the
computers is easy.
network fails. Hence the network
vulnerable to breakdown as it is
Centralized monitoring and
essentially controlled by one device
management are possible.
Failure of one computer does not
affect the rest of the network.
Faults are easier to locate and
isolate
System is expensive to install because it
System provides increased
redundancy and reliability as well as uses a lot of cabling.
ease of troubleshooting.
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2.9 Logical Topologies example: Token ring LANs
One method of transmitting data around a ring is called token passing. (A token is a special series of
bits that travels around a token-ring network. Each network has only one token.) The token is
passed from computer to computer until it gets to a computer that has data to send. Figure 11 shows
a token ring topology with the token. The sending computer modifies the token, puts an electronic
address on the data, and sends it around the ring.
The data passes by each computer until it finds the one with an address that matches the address on
the data. The receiving computer returns a message to the sending computer indicating that the data
has been received. After verification, the sending computer creates a new token and releases it on
the network. The token circulates within the ring until a workstation needs it to send data.
Therefore the token ring network uses a logical ring topology – the token travels around in a circle
from computer to computer. However, the physical topology of a token ring network is a star – the
wires connecting the computers to each other are connected via a central hub. This is sometimes
referred to as a “star-shaped ring” network.
Figure 15: The token ring topology
The token ring avoids a common problem with bus topologies. If there are many computers on the
network a bus will often be busy, seriously affecting network performance. However, with a token
ring the network is never busy – each computer must simply wait for the token to arrive and add its
message.
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2.10 Network Models/Architecture
Computer networks can be discriminated into various types based on how the computers in a
network are configured and how they access information, or simply based on the roles of the
computer found in the network. Accordingly it classified as follow:
1. Peer-to-peer network
2. Client-server network
3. Hybrid network
Peer-To-Peer Network
In such network configuration, all the connected systems (computers and printers) can
communicate directly with each other without relying on other intermediary server. In a peer-topeer network, there are no dedicated servers, and there is no hierarchy among the computers. All the
computers are equal and therefore are known as peers. Each computer functions as both a client and
a server, and there is no administrator responsible for the entire network. The user at each computer
determines what data on that computer is shared on the network. Peer-to-peer networks are also
sometimes called workgroups.
Implementation of Peer-to-Peer network is relatively simple, because each computer functions as a
client and a server, there is no need for a powerful central server or for the other components
required for a high-capacity network.
Peer-to-peer networks can be less expensive than server-based networks. Although appropriate for
small networks (can connect up to 10 computers). Also, the networking software does not require
the same standard of performance and level of security as the networking software designed for
dedicated servers. However, it lacks the ability to implement centralized management of users, files,
and security features and tends to slow down performance as the number of connected computers
increase.
Figure 16: Peer-to-peer network architecture
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Peer-to-peer networks are good choices for environments where:
•
There are 10 users or fewer
•
Users share resources, such as printers, but no specialized servers exist
•
Security is not an issue
•
The organization and the network will experience only limited growth within the
foreseeable future
Where these factors apply, a peer-to-peer network will probably be a better choice than a serverbased network.
Advantages of Peer-to-peer networking
It is easy to install and so is the configuration of computers on this network,
All the resources and contents are shared by all the peers, unlike server-client architecture
where Server shares all the contents and resources.
P2P is more reliable as central dependency is eliminated. Failure of one peer doesn’t affect
the functioning of other peers. In case of Client –Server network, if server goes down whole
network gets affected.
There is no need for full-time System Administrator. Every user is the administrator of his
machine. User can control their shared resources.
The over-all cost of building and maintaining this type of network is comparatively very
less.
Disadvantages(drawbacks) of Peer to peer architecture
In this network, the whole system is decentralized thus it is difficult to administer. That is
one person cannot determine the whole accessibility setting of whole network.
Security in this system is very less viruses, spywares, Trojans, etc malwares can easily
transmitted over this P-2-P architecture.
Data recovery or backup is very difficult. Each computer should have its own back-up
system
Lot of movies, music and other copyrighted files are transferred using this type of file
transfer. P2P is the technology used in torrents.
Peer to peer networks are good to connect small number (around 10) of computer and places
where high level of security is not required. In case of business network where sensitive data
can be present this type of architecture is not advisable or preferred.
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Client-Server Network
In an environment with more than 10 users, a peer-to-peer network with computers acting as both
servers and clients will probably not be adequate. Therefore, most networks have dedicated servers.
A dedicated server is one that functions only as a server and is not used as a client or workstation.
Servers are described as "dedicated" because they are not themselves clients, and because they are
optimized to service requests from network clients quickly and to ensure the security of files and
directories. Server-based networks have become the standard models for networking.
Client/Server architecture consists of client computers (workstations) that require a network service
and servers that provide the network service. Servers are usually very powerful computers both in
processing power and in their storage capacity. Different servers can be configured depending on
the need of the organizations, such as:
File servers – provide centralized data storage which will be shared among the LAN users.
Print server – control one or more printers and provide print service to LAN users.
Database server – centrally store database and provide data to client application in the LAN.
Web server – store web pages that will be viewed using browsers in the network.
Email server – used to manage and control email communications across the network.
As networks increase in size (as the number of connected computers, and the physical distance and
traffic between them, grows), more than one server is usually needed. Spreading the networking
tasks among several servers ensures that each task will be performed as efficiently as possible.
Servers must perform varied and complex tasks. Servers for large networks have become
specialized to accommodate the expanding needs of users. For example, a network may have
separate servers for file storage, printing, email and for storing and running application software.
Figure 17: Client-Server network architecture
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Advantages of Client Server Networks
Centralization: Unlike P2P, where there is no central administration, here in this
architecture there is a centralized control. Servers help in administering the whole set-up.
Access rights and resource allocation is done by Servers.
Proper Management: All the files are stored at the same place. In this way, management of
files becomes easy. Also it becomes easier to find files.
Back-up and Recovery possible: As all the data is stored on server it's easy to make a backup of it. Also, in case of some break-down if data is lost, it can be recovered easily and
efficiently. While in peer computing we have to take back-up at every workstation
Upgrading and Scalability in Client-server set-up: Changes can be made easily by just
upgrading the server. Also new resources and systems can be added by making necessary
changes in server.
Accessibility: From various platforms in the network, server can be accessed remotely.
Security: Rules defining security and access rights can be defined at the time of set-up of
server.
Easily file update: As new information is uploaded in database, each workstation need not
have its own storage capacities increased (as may be the case in peer-to-peer systems). All
the changes are made only in central computer on which server database exists.
Disadvantages of Client Server Architecture
Congestion in Network: Too many requests from the clients may lead to congestion, which
rarely takes place in P2P network. Overload can lead to breaking-down of servers. In peerto-peer, the total bandwidth of the network increases as the number of peers increase.
Not as robust: As a P2P and if the server fails, the whole network goes down. Also, if you
are downloading a file from server and it gets abandoned due to some error, download stops
altogether. However, if there would have been peers, they would have provided the broken
parts of file.
Cost: It is very expensive to install and manage this type of computing.
Need professional IT people to maintain the servers and other technical details of network.
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Selecting Network Configuration
The distinction between peer-to-peer and server-based networks is important because each type has
different capabilities. The type of network you choose to implement will depend on various factors
such as the:
Size of the organization.
Level of security required.
Type of business.
Level of administrative support available.
Amount of network traffic.
Needs of the network users.
Network budget.
Size:- Peer-to-peer networks are also called workgroups. The term "workgroup" implies a small
group of people. There are typically 10 or fewer computers in a peer-to-peer network.
Cost:- Peer-to-peer networks are relatively simple. Because each computer functions as a client and
a server, there is no need for a powerful central server or for the other components required for a
high-capacity network. Peer-to-peer networks can be less expensive than server-based networks.
Operating Systems:- In a peer-to-peer network, the networking software does not require the same
standard of performance and level of security as the networking software designed for dedicated
servers. Dedicated servers function only as servers and not as clients or workstations. Peer-to-peer
networking is built into many operating systems. In those cases, no additional software is required
to set up a peer-to-peer network.
Administration:- In a typical peer-to-peer network, no system manager oversees administration
for the entire network. Instead, individual users administer their own computers.
Security:- On a computer network, security (making computers and data stored on them safe from
harm or unauthorized access) consists of setting a password on a resource, such as a directory, that
is shared on the network. All peer-to-peer network users set their own security, and shared resources
can exist on any computer rather than on a centralized server only; consequently, centralized control
is very difficult to maintain. This lack of control has a big impact on network security because some
users may not implement any security measures at all. If security is an issue, a server-based network
might be a better choice. In a server-based environment, one administrator who sets the policy and
applies it to every user on the network can manage security.
Hardware Considerations:- Client computer hardware can be limited to the needs of the user
because clients do not need the additional random access memory (RAM) and disk storage needed
to provide server services. A typical client computer often has no more than a Pentium processor
and 32 megabytes (MB) of RAM.
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Number of Users:- A server-based network can support thousands of users. This type of network
would be impossible to manage as a peer-to-peer network, but current monitoring and networkmanagement utilities make it possible to operate a server-based network for large numbers of users.
Comparison of Network Types
Consideration
Peer-to-Peer Network
Server-Based Network
Size
Good for 10 or fewer computers Limited only by server and
network hardware
Security
Security established by the user
of each computer
Extensive and consistent
resource and user security
Administration
Individual users responsible for
their own administration; no
full-time administrator
necessary
Centrally located for network
control; requires at least one
knowledgeable administrator
2.11 Survey of Network Operating System (NOS)
The core of a network is the network operating system. Just as a computer cannot operate without
an operating system, a network of computers cannot operate without a network operating system.
A network operating system must support mechanisms that enable applications to communicate
with one another: for example, applications that enable multiple computers to work jointly on a
single task, such as a mathematical calculation. A network operating system must also support
multiple processors, clusters of disk drives, and data security features. Finally, a network operating
system must be reliable and be able to recover quickly from errors.
Depending upon the network operating system's manufacturer, a desktop computer's networking
software can be added either to the computer's own operating system or be integrated with it.
Network operating system software is integrated into a number of popular operating systems,
including Microsoft Windows. In short the NOS loaded on server has the following functions:
NOS manages network resources
Controls the flow of data
Maintain security
Tracks user accounts
NOS enables clients to access remote drives as if they were on the clients own machine
Allow servers to process requests from clients and decide whether that client can use a
particular resource
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Examples of NOS:- Novel Netware,
- Windows NT server
- Windows 2000/2k server
- Windows 2000/2k advanced server
- UNIX
Note: Part of the NOS must run from the client, and part of it must run from the server
All NOSs can exist within a network and they can operate with one another. This feature is called
Interoperability which makes it easier for corporations/business/company with different client and
server to create a network even though the client and the servers all use different operating systems.
Desktop operating system listed below include the software necessary to establish communication
with a network server. The network software that is built into your computer operating system
handles the communication between your workstation and the network server
- Windows 95
- Windows 98/ first edition / second edition
- Windows NT workstation
- Windows Millennium (me)
- Windows 2000 professional
- Windows XP professional/Home
NOS has two components known as Network server software and Network client software. In most
cases, software must be installed on the server and client for interoperability.
Network server software
Installed on a file server
Controls file access from the server hard disk
Manages the print queue
Tracks user id and password
Network client software
Installed on the local hard disk of each workstation
Gathers your login information
Handles drive mapping
And directs printout to the network printer
The server and client software components do not necessarily have to match the network client
software that is provided as part of windows 95, 98, NT workstation, and 2000 professional that
allows you to access servers running a variety of software including Linux, UNIX and NOvel
Netware, windows NT server and windows 2000 server.
Note: Many desktop operating systems have some server capabilities. You could for example set
up a small network by installing windows 98 on server and windows 95, 98, NT workstation, or
2000 professional on the workstations.
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Summary of Key Points
What Is a Network?
•
The primary reasons for networking are to share information, to share hardware and
software (reducing cost), and to centralize administration and support
•
Potential disadvantages of computer networks are lack of security when dealing with
sensitive information, and the danger of computer viruses infecting the system
•
A local area network (LAN) is the smallest form of network and is the building block for
larger networks
•
A wide area network (WAN) is a collection of LANs and has no geographical limitation
•
The locality principle in computer networking states that a computer is more likely to
communicate with a computer that is nearby, than with one that is distant
•
A campus area network (CAN/MAN) is a collection of LANs linked together on a university
or college campus
•
A metropolitan area network (MAN) is a collection of LANs linked together within a town
or city
Network Topologies
•
The physical layout of computers on a network is called a topology
•
There are four primary topologies: star, bus, ring, and mesh
•
Topologies can be physical (actual wiring) or logical (the way they work)
•
In a bus topology, the computers are connected in a linear fashion on a single cable
•
Bus topologies require a terminator on each end of the cable
•
In a star topology, all computers are connected to a centralized hub
•
Mesh topologies connect all computers in a network to one another with separate cables
•
Hybrid topologies combine one or more of the primary topology types. Common examples
of hybrid topologies are the star bus and star-ring topologies
•
In token-ring topology, the computers are connected physically in a star shape, but logically
in a ring or circle. The data is passed from one computer to another around the circle
Network Models
•
Networks are classified into two principal groups based on how they share information:
peer-to-peer networks and server-based networks
•
In a peer-to-peer network, all computers are equal. They can either share their resources or
use resources on other computers
•
In server-based network, one or more computers act as servers and provide the resources to
the network. Other computers are the clients and use the resources provided by the server
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