Unit 3 - Transmission Media And Network Topology
Transmission media is a pathway that carries the information from sender to
receiver. We use different types of cables or waves to transmit data. Data is
transmitted normally through electrical or electromagnetic signals.
MAGNETIC MEDIA
One of the most convenient way to transfer data from one computer to another,
even before the birth of networking, was to save it on some storage media and
transfer physical from one station to another. Though it may seem old-fashion
way in today‟s world of high speed internet, but when the size of data is huge, the
magnetic media comes into play.
For example, a bank has to handle and transfer huge data of its customer, which
stores a backup of it at some geographically far-away place for security reasons
and to keep it from uncertain calamities. If the bank needs to store its huge backup
data then its, transfer through internet is not feasible.The WAN links may not
support such high speed. Even if they do; the cost too high to afford.
In these cases, data backup is stored onto magnetic tapes or magnetic discs, and
then shifted physically at remote places.
TWISTED PAIR CABLE
A twisted pair cable is made of two plastic insulated copper wires twisted together
to form a single media. It consists of 2 separately insulated conductor wires wound
about each other. Generally, several such pairs are bundled together in a protective
sheath. They are the most widely used Transmission Media.
Twisted Pair is of two types:
1. Unshielded Twisted Pair (UTP):
This type of cable has the ability to block interference and does not depend on
a physical shield for this purpose. It is used for telephonic applications.
Advantages:
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Least expensive
Easy to install
High speed capacity
Disadvantages:
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Easily affected to external interference
Lower capacity and performance in comparison to STP
Short distance transmission
2. Shielded Twisted Pair (STP):
This type of cable consists of a special jacket to block external interference. It
is used in fast-data-rate Ethernet and in voice and data channels of telephone
lines.
Advantages:
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Better performance at a higher data rate in comparison to UTP
 Eliminates crosstalk
 Comparitively faster
Disadvantages:
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Comparitively difficult to install and manufacture
More expensive
Bulky
COAXIAL CABLE
Coaxial cable has two wires of copper. The core wire lies in the center and it is
made of solid conductor. The core is enclosed in an insulating sheath. The second
wire is wrapped around over the sheath and that too in turn encased by insulator
sheath. This all is covered by plastic cover.
Coaxial cables are copper cables with better shielding than twisted pair cables, so
that transmitted signals may travel longer distances at higher speeds.
Coaxial cables are widely used for cable TV connections and LANs.
Because of its structure, the coax cable is capable of carrying high frequency
signals than that of twisted pair cable. The wrapped structure provides it a good
shield against noise and cross talk.
Advantages of Coaxial Cables
 Excellent noise immunity
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Signals can travel longer distances at higher speeds,
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Can be used for both analog and digital signals
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Inexpensive as compared to fiber optic cables
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Easy to install and maintain
Disadvantages of Coaxial Cables
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Expensive as compared to twisted pair cables
Not compatible with twisted pair cables
OPTICAL FIBER OR FIBER OPTIC CABLE
Fiber optic cable is ideal cable for fast data transmission. Fiber optic cable
transmits light signals rather than electrical signals. These cables have high
bandwidth.
It uses the concept of reflection of light through a core made up of glass or plastic.
The core is surrounded by a less dense glass or plastic covering called the cladding.
It is used for transmission of large volumes of data.
Optical fiber is rapidly replacing copper wires in telephone lines, internet
communication and even cable TV connections because transmitted data can
travel very long distances without weakening.
Advantages of Optical Fiber
 High bandwidth
 Immune to electromagnetic interference
 Suitable for industrial and noisy areas
 Signals carrying data can travel long distances without weakening
Disadvantages of Optical Fiber
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Optical fiber cables are expensive
Sophisticated technology required for manufacturing, installing and
maintaining optical fiber cables
Light waves are unidirectional, so two frequencies are required for full
duplex transmission.
3.2 Network Topology And Advantages and Disadvantages
The Hardware configuration also called as network Topology. “The network
topology defines the way in which computers, printers, and other devices are
connected. A network topology describes the layout of the wire and devices as well
as the paths used by data transmissions.”
Topology is a map of network, its structure of physical existence.
Bus Topology :
· The Bus topology is often used when a network installation is small, simple or
temporary.
· The Bus is a Passive Topology because it does not amplify or regenerate
thesignal. When one computer sends a signal up (& down) the wire, all the
computers on the network receive the information, but only one (whichmatches the
address) accepts the information. The rest discard the message.
· Only one computer at a time can send a message; therefore the number of
computers attached to a bus network can significantly affect the speed of the
network. A computer must wait until the bus is free before it can transmit.
· Another important issue in bus networks is „Termination‟.The electrical signal
from a transmitting computer is free to travel the entire length of the cable.
Without termination, when the signal reaches the end of the wire, it bounces back
& travels back up the wire.
· When the signal echoes back & forth along an without terminated bus, it is called
„ringing‟. To stop the signals from ringing, you attach „terminator‟ at either end of
the segment. Terminator absorbs the electrical energy & stops the reflections.
Cables can‟t be left without terminated in a bus network.
Advantages of the Bus –
♦ The bus is simple, reliable in very small networks, easy to use, & easy to
understand.
♦ The bus requires the least amount of cable to connect the computers together & is
therefore less expensive than other cabling arrangement.
♦ It is easy to extend a bus. Two cables can be joined into one longer cable with a
BNC (British Naval Connector) barrel connector, making a longer cable &
allowing computers to join the network.
♦ A repeater can also be used to extend a bus; a repeater boosts the signal & allows
it to travel a longer distance.
Disadvantages of the Bus –
♦ Heavy network traffic can slow a bus considerably. Because any computer can
transmit at any time, & computers on most bus networks do not coordinate with
each other to reserve times to transmit, a bus network with a lot of computers can
spend a lot of its bandwidth (capacity for transmitting information) with computers
interrupting each other instead of communicating.
♦ Each barrel connector weakens the electrical signal, & too many may prevent the
signal from being correctly received all along the bus.
♦ A cable break or loose connector will also cause reflections & bring down the
whole network, causing all network activity to stop.
Ring Topology:
In a ring topology each computer is connected to the next computer, with the last
one connected to the first.
Rings are used in high performance networks, networks requiring that bandwidth
be reserved for time – sensitive features such as video & audio or when
performance is needed when a large no. of clients access the network.
Every computer is connected to the next computer in the ring, & each retransmits
what it receives from the previous computer.
The message flow around the ring in one direction. Since each computer transmits
what it receives, a ring is an „active‟ network & is not subject to the signal loss
problems. There is no termination because there is no end of the ring. Some ring
networks do „token passing‟. A short message called a „token‟ is passed around the
ring until a computer wishes to send information to another computer. Each
computer in sequence receives the token & the information & passes them to the
next computer until either the electronic
address matches the address of a computer or the token returns to its origin.
The receiving computer returns a message to the originator indicating that the
message has been received. The sending computer then creates another token &
places it on the network, allowing another station to capture the token & begin
transmitting. The token circulates until a station is ready to send &capture the
token.
Advantages of the
Ring –
1) Because every computer is given equal access to the token, no one
computer can get control of whole the network.
2)The fair sharing of the network allows the network to degrade gracefully
(continue to function in a useful, if slower, manner rather than fail once capacity is
exceeded) as more users are added.
Disadvantages of the Ring –
1.Failure of one computer on the ring can affect the whole network.
2.It is difficult to troubleshoot a ring network.
3.Adding or removing computers break up the network.
Star topology: In a star topology all the cables run from the computer to a central
location, where they are all connected by a device called „Concentrator‟.
Concentrator can be a HUB or Switch.
Stars are used in concentrated networks, where the endpoints are directly reachable
from a central location; when network expansion is expected; & when the greater
reliability of a star topology is needed.
Each computer on a star network communicates with a central hub that resends the
message either to all the computers (in a broadcast star network) or only to the
destination computer (in a switched star network).
Advantages of the Star –
It is easy to modify & add new computers to a star network without disturbing
the rest of the network.
The center of a star network is a good place to
diagnose network faults.
Some hubs that have microprocessor are called intelligent hubs. They provide
centralized monitoring & management of the network.
Single computer failures do not necessarily bring down the whole star network.
The hub can detect a network fault & isolate the offending computer or network
cable & allow the rest of the network to continue operating.
You can use several cable types in the same network with a hub that can
accommodate (find a room for) multiple cable types.
Disadvantages of the Star –
If a central hub fails, the whole network fails to operate.
It costs more to cable a star network because all network cables must be pulled
to one central point, requiring more cable than other networking topologies.
Tree topology:
Tree topology also called as hierarchical topology.
In this topology there is a root which is a main server who is responsible to handle
all activities related with that network. Normally this topology use severclient
model. All services and security provided by main server. Main server can have
multiple sub servers and their clients.
The hierarchical topology is one of the more common topologies found today. The
software to control the network is relatively simple and the topology provides a
concentration point for control and error resolution.
The Main Server at the highest point in the hierarchy usually controls the network.
Another implementation of this topology is a combination of different topologies
in hierarchical fashion. Tree topologies integrate multiple star topologies together
onto a bus.
Here BUS is a made from Hub networking devices, In its simplest form, only hub
devices connect directly to the tree bus, and each hub functions as the “root” of a
tree of devices. This bus/star hybrid approach supports future expandability of the
network much better than a bus (limited in the number of devices due to the
broadcast traffic it generates) or a star (limited by the number of hub connection
points) alone.
Mesh Topology
The mesh topology connects all devices (nodes) to each other for redundancy and
fault tolerance. It is used in WANs to interconnect LANs and for mission critical
networks like those used by governments.
In mesh, every device is connected to every other device, this Increase fault
tolerance but involves extra work. If media break, data transfer can take alternative
routes. Caballing is complicated here. Implementing the mesh topology is
expensive and difficult.
Advantages of the Mesh
The major advantage of the mesh topology is fault tolerance. Other advantages
include guaranteed communication channel capacity & the fact that mesh networks
are relatively easy to troubleshoot.
Disadvantages of the Mesh
Disadvantages include the difficulty of installation and reconfiguration, as well as
the cost of maintaining redundant link. Considerations When Choosing a Topology
Money: A linear bus network may be the least expensive way to install a network;
you do not have to purchase concentrators.
Length of cable needed: The linear bus network uses shorter lengths of cable.
Future growth: With a star topology, expanding a network is easily done by
adding another concentrator.
Cable type: The most common cable in schools is unshielded twisted pair, which
is most often used with star topologies.
3.3 Wireless Transmission
Radio waves also are omnidirectional, meaning that they travel in all directions
from the source, so the transmitter and receiver do not have to be carefully aligned
physically. Sometimes omnidirectional radio is good, but sometimes it is bad. At
all frequencies, radio waves are subject to interference from motors and other
electrical equipment.
a)Microwave Transmission
Above 100 MHz, the waves travel in nearly straight lines and can therefore be
narrowly focused. Concentrating all the energy into a small beam by means of a
parabolic antenna (like the familiar satellite TV dish) gives a much higher
signal-to-noise ratio, but the transmitting and receiving antennas must be
accurately aligned with each other. In addition, this directionality allows
multiple transmitters lined up in a row to communicate with multiple receivers
in a row without interference, provided some minimum spacing rules are
observed.
Microwaves travel in a straight line, so if the towers are too far apart, the earth
will get in the way. Consequently repeaters are needed periodically. The higher the
towers are, the farther apart they can be. microwave communication is so widely
used for long-distance telephone communication, mobile phones, television
distribution, and other purposes that a severe shortage of spectrum has developed.
It has several key advantages over fiber.
b) Infrared Transmission
Unguided infrared waves are widely used for short-range communication.The
remote controls used for televisions, VCRs, and stereos all use infrared
communication. They are relatively directional, cheap, and easy to build but have a
major drawback: they do not pass through solid objects.
the fact that infrared waves do not pass through solid walls well is also a plus. It
means that an infrared system in one room of a building will not interfere with a
similar system in adjacent rooms or buildings: you cannot control your neighbor‟s
television with your remote control.
Therefore, no government license is needed to operate an infrared system, in
contrast to radio systems, which must be licensed outside the ISM bands. Infrared
communication has a limited use on the desktop, for example, to connect notebook
computers and printers with the IrDA (Infrared Data Association) standard, but
it is not a major player in the communication game.
-------------------------------------------Best of Luck----------------------------------------