COM 302: DATA COMMUNICATIONS AND NETWORKING
Module 2
DATA COMMUNICATIONS EQUIPMENT
Data communications equipment (DCE) refers to computer hardware devices used to establish, maintain and
terminate communication network sessions between a data source and the receiving device.
One example of data communications equipment would be a modem. The modem is used primarily to preform
signal exchange. This means for example the modem that is connected to your computer can receive a transmission
from your computer and send it over a LAN to another computer or workstation. A simpler example of this data
transformation or signal exchange would be your cable box. Your cable provider sends data through the cables into
your cable box your cable box in turn collects the data that was sent and converts it or exchanges it and makes it
understandable to you the viewer in the form of a video that you can watch on your television.
DCE may also be responsible for providing timing over a serial link. In a complex network which uses directly
connected routers to provide serial links, one serial interface of each connection must be configured with a clock
rate to provide synchronisation. So, DCEs consist of devices and connections within the network which connect the
communications circuit from the data source and the destination.
The data terminal equipment (DTE) is the end instrument that is used to convert user input into digital signals and
vice versa. This device communicates with the DCE.
The data terminal equipment serves as the functional part of the data station which can be referred to as the data
sink. This is where the data will originate, or it’s source. It also allows for the data communication control function
to be performed.
There can be many different configurations of data terminal equipment: it can be a simple laptop, or it can be a
number of different devices connected together. An example of this would be a computer, printer, and an external
hard drive all connected and having the ability to each send information to the data communications equipment. For
an example, you turn on your computer and have a memo to write for your company. You open up a word
document, type out the memo and proceed to send the memo in an email as an attachment. When you click send,
your computer becomes the data terminal equipment. So, that message is sent from your computer (the DTE) to
your boss’s computer. Your boss’s computer then becomes another data terminal equipment device. When your
boss opens up the email and downloads the attachment to view the memo, his computer, now the DTE digitally
converts the email attachment and opens it in a word document so your boss can read the memo.
COMMUNICATION COMPONENTS
1. Modems
A MODEM is a MOdulation-DEModulation device that converts the discrete stream of digital "on-off" electric pulses
used by computers into the analog wave patterns used for transmission of the human voice. Demodulator (recovers)
the digital data from the transmitted signal. A special type of MODEM called an acoustic coupler is often used in
libraries and information units with portable machines, but internal and external direct-connect MODEMs are
generally used at permanent stations.
2. Multiplexers
A multiplexer is a device used for transmission of several messages over a single channel using predetermined
frequencies within the full bandwidth. The multiplexer operates on the principle that individual channel may require
only a small account of actual transmission time; thus, the multiplexer acts almost as a timeshare computer
allocating use of the single communication lines on a priority basis. A multiplexer is capable of accepting as many as
45 separate channels for transmitting data on a single communication line.
3. Concentrators (Data Switch)
A concentrator is a switch that allocates a particular input to a particular output. While a multiplexer combines the
inputs on a high-speed line; the concentrator allocates a particular input to the line for the duration of its
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information transfer. The allocation of the output line to a specific input depends on when the request is made, the
speed of the input line, and the class or importance of the request.
4. Front-end Processors
Front-end processors are installed to handle communication related functions for a mainframe computer so that the
later can be fully used for processing applications, be it an inquiry to a database, a printing job or updating a master
file, etc... The aim of the front-end processor is to provide an inter-face between the mainframe computer and the
network so that the data is passed to and from the mainframe efficiently. The front-end processor is responsible for
supervision of the input/output controllers or channels attached to the network, for providing buffering and partial
processing of in-coming and outgoing data, for the assembling and dissembling of messages and for error handling.
The Need for Modulation
A message signal cannot travel a long distance because of its low signal strength. In addition to this, physical
surroundings, the addition of external noise and travel distance will further reduce the signal strength of a message
signal. So, in order to send the message signal to a long distance, we need to increase the signal strength of a
message signal. This can be achieved by using a high frequency or high energy signal called carrier signal. A high
energy signal can travel to a larger distance without getting affected by external disturbances. We take the help of
such high energy signal to transmit the message signal. This high energy or high frequency signal is known as carrier
signal.
The low energy message signal is mixed with the high energy or high frequency carrier signal to produce a new high
energy signal which carries information to a larger distance.
The question arises how the message signal should be added to the carrier signal. The solution lies in changing some
characteristics (amplitude, frequency or phase) of a carrier signal in accordance with the amplitude of the message
signal. This process is called modulation. Modulation means to “change”.
The Message signal contains information whereas the carrier signal contains no information. Carrier signal is used
just to transmit the information to a long distance. At the destination, the message signal is consumed whereas the
carrier signal is wasted.
In modulation process, the characteristics of the carrier signal is changed but the message signal characteristics will
not be changed. The carrier signal does not contain any information so even if we change the characteristics of the
carrier signal, the information contained in it will not be changed. However, the message signal contains information
so if we change the characteristics of the message signal, the information contained in it will also change. Therefore,
we always change the characteristics of the carrier signal but not the message signal.
Modulation allows the transmission to occur at high frequency while it simultaneously allows the carrying of the
message signal.
Modulation Definition
Modulation is the process of mixing a low energy message signal with the high energy carrier signal to produce a
new high energy signal which carries information to a long distance.
or
Modulation is the process of changing the characteristics (amplitude, frequency or phase) of the carrier signal, in
accordance with the amplitude of the message signal.
A device that performs modulation is called modulator.
Example:
The modulation process can be understood with a simple example. The below figure shows the amplitude
modulation.
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The first figure shows the modulating signal or message signal which contains information, the second figure shows
the high frequency carrier signal which contains no information and the last figure shows the resultant amplitude
modulated signal.
From the above three figures, it can be observed that the amplitude of the carrier signal is varied in accordance with
the instant amplitude of the message signal.
Types of Signals in Modulation
In modulation process, three types of signals are used to transmit information from source to destination. They are
the message signal, carrier signal, and modulated signal.
1) Message signal
The signal which contains a message to be transmitted to the destination is called a message signal. The message
signal is also known as a modulating signal or baseband signal.
The original frequency range of a transmission signal is called baseband signal. The message signal or baseband
signal undergoes a process called modulation before it gets transmitted over the communication channel. Hence,
the message signal is also known as the modulating signal.
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2) Carrier signal
The high energy or high frequency signal which has characteristics like amplitude, frequency, and phase but contains
no information is called a carrier signal. It is also simply referred to as a carrier. Carrier signal is used to carry the
message signal from transmitter to receiver. The carrier signal is also sometimes referred to as an empty signal.
3) Modulated signal
When the message signal is mixed with the carrier signal, a new signal is produced. This new signal is known as a
modulated signal. The modulated signal is the combination of the carrier signal and modulating signal.
Need for (Benefits of) Modulation
Modulation is extremely necessary in communication system because of the following reasons:
1) Avoids mixing of signals
One of the basic challenges facing by the communication engineering is transmitting individual messages
simultaneously over a single communication channel. A method by which many signals or multiple signals can be
combined into one signal and transmitted over a single communication channel is called multiplexing.
We know that the sound frequency range is 20 Hz to 20 KHz. If the multiple baseband sound signals of same
frequency range (I.e. 20 Hz to 20 KHz) are combined into one signal and transmitted over a single communication
channel without doing modulation, then all the signals get mixed together and the receiver cannot separate them
from each other. We can easily overcome this problem by using the modulation technique.
By using modulation, the baseband sound signals of same frequency range (I.e. 20 Hz to 20 KHz) are shifted to
different frequency ranges. Therefore, now each signal has its own frequency range within the total bandwidth.
After modulation, the multiple signals having different frequency ranges can be easily transmitted over a single
communication channel without any mixing and at the receiver side, they can be easily separated.
2) Increase the range of communication
The energy of a wave depends upon its frequency. The greater the frequency of the wave, the greater the energy
possessed by it. The baseband audio signals frequency is very low so they cannot be transmitted over large
distances. On the other hand, the carrier signal has a high frequency or high energy. Therefore, the carrier signal can
travel large distances if radiated directly into space.
The only practical solution to transmit the baseband signal to a large distance is by mixing the low energy baseband
signal with the high energy carrier signal. When the low frequency or low energy baseband signal is mixed with the
high frequency or high energy carrier signal, the resultant signal frequency will be shifted from low frequency to high
frequency. Hence, it becomes possible to transmit information over large distances. Therefore, the range of
communication is increased.
3) Wireless communication
In radio communication, the signal is radiated directly into space. The baseband signals have very low frequency
range (I.e. 20 Hz to 20 KHz). So it is not possible to radiate baseband signals directly into space because of its poor
signal strength. However, by using the modulation technique, the frequency of the baseband signal is shifted from
low frequency to high frequency. Therefore, after modulation, the signal can be directly radiated into space.
4) Reduces the effect of noise
Noise is an unwanted signal that enters the communication system via the communication channel and interferes
with the transmitted signal.
A message signal cannot travel for a long distance because of its low signal strength. Addition of external noise will
further reduce the signal strength of a message signal. So in order to send the message signal to a long distance, we
need to increase the signal strength of the message signal. This can be achieved by using a technique called
modulation.
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In modulation technique, a low energy or low frequency message signal is mixed with the high energy or high
frequency carrier signal to produce a new high energy signal which carries information to a long distance without
getting affected by the external noise.
5) Reduces height of antenna
When the transmission of a signal occurs over free space, the transmitting antenna radiates the signal out and
receiving antenna receives it. In order to effectively transmit and receive the signal, the antenna height should be
approximately equal to the wavelength of the signal to be transmitted.
Now,
The audio signal has a very low frequency (I.e. 20 Hz to 20 kHz) and longer wavelength, so if the signal is transmitted
directly into space, the length of the transmitting antenna required would be extremely large.
For instance, to radiate an audio signal frequency of 20 kHz directly into space, we would need an antenna height of
15,000 meters.
The antenna of this height is practically impossible to construct.
On the other hand, if the audio signal (20 Hz) has been modulated by a carrier wave of 200 MHz. Then, we would
need an antenna height of 1.5 meters.
The antenna of this height is easy to construct.
Types of Modulation
Basically, the modulation is classified into two types: analog modulation and digital modulation.
Analog modulation
In analog modulation, the analog signal (sinusoidal signal) is used as a carrier signal that modulates the analog
message signal. In analog modulation, the characteristics (amplitude, frequency or phase) of the carrier signal is
varied in accordance with the amplitude of the message signal.
The four basic types of analog modulation are as follows:
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1) Amplitude modulation
Amplitude modulation is a type of modulation where the amplitude of the carrier signal is varied (changed) in
accordance with the amplitude of the message signal while the frequency and phase of carrier signal remain
constant.
The above figure shows the amplitude modulation.
The first figure shows the modulating signal or message signal which contains information, the second figure shows
the high frequency carrier signal which contains no information and the last figure shows the resultant amplitude
modulated signal.
From the above three figures, it can be observed that the amplitude of the carrier signal is varied in accordance with
the instant amplitude of the message signal.
2) Frequency modulation
Frequency modulation is a type of modulation where the frequency of the carrier signal is varied (changed) in
accordance with the amplitude of the message signal while the amplitude and phase of carrier signal remain
constant.
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The above figure shows the frequency modulation.
The first figure shows the modulating signal or message signal, the second figure shows the high frequency carrier
signal which contains no information and the last figure shows the resultant frequency modulated signal.
From the above three figures, it can be observed that the frequency of the carrier signal is varied in accordance with
the instant amplitude of the message signal.
3) Phase modulation
Phase modulation is a type of modulation where the phase of the carrier signal is varied (changed) in accordance
with the amplitude of the message signal while the amplitude of carrier signal remains constant.
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The above figure shows the phase modulation.
The first figure shows the modulating signal or message signal, the second figure shows the high frequency carrier
signal which contains no information and the last figure shows the resultant phase modulated signal.
From the above three figures, it can be observed that the phase of the carrier signal is varied in accordance with the
instant amplitude of the message signal.
In this type of modulation, when the phase is changed it also affects the frequency so this modulation also comes
under frequency modulation.
The frequency and phase modulation come under angle modulation. When the frequency or phase of the carrier
signal is varied (changed) in accordance with the amplitude of the message signal, then it is called angle modulation.
4) Analog pulse modulation
In amplitude, frequency and phase modulation techniques, the carrier and message signals are continuous signals
(sinusoidal signals). However, in analog pulse modulation, the carrier signal is a discontinuous signal (series of pulses)
and message signal is a continuous signal (sinusoidal signal).
Analog pulse modulation is the process of changing the characteristics (pulse amplitude, pulse width or pulse
position) of the carrier pulse, in accordance with the amplitude of the message signal.
The analog pulse modulation is again classified as,
1. Pulse amplitude modulation
2. Pulse width modulation
3. Pulse position modulation
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Example: The analog pulse modulation can be understood with a simple example. The below figure shows the pulse
amplitude modulation.
The first figure shows the message signal (continuous signal), the second figure shows the carrier pulse train (series
of pulses) which contains no information and the last figure shows the resultant pulse amplitude modulated (PAM)
signal.
From the above three figures, it can be observed that the amplitude of the series of carrier pulses are varied
(changed) in accordance with the instant amplitude of the message signal while the width and positions of the
carrier pulses remain constant.
Analog modulation is more sensitive to noise. Noise is an unwanted signal that enters the communication system via
the communication channel and interferes with the transmitted signal. The noise signal degrades the transmitted
signal (signal containing information). Therefore, this drawback can be overcome by the digital modulation
technique.
Digital modulation
The digital modulation technique is employed for efficient communication. The main advantage of the digital
modulation over analog modulation include high noise immunity, available bandwidth, and permissible power. In
digital modulation, the modulating signal or message signal is converted from analog to digital.
Pulse Code Modulation (PCM)
In digital modulation, the modulation technique used is Pulse Code Modulation (PCM). The pulse code modulation is
the method of converting an analog signal into a digital signal I.e. 1s and 0s. As the resultant signal is a coded pulse
train, this is called as pulse code modulation.
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COM 302: DATA COMMUNICATIONS AND NETWORKING
MODULE 3
MULTIPLEXING
Multiplexing is a technique by which different analog and digital streams of transmission can be
simultaneously processed over a shared link. Multiplexing divides the high capacity medium into low
capacity logical medium which is then shared by different streams.
Communication is possible over the air (radio frequency), using a physical media (cable), and light (optical
fibre). All mediums are capable of multiplexing.
When multiple senders try to send over a single medium, a device called Multiplexer divides the physical
channel and allocates one to each. On the other end of communication, a De-multiplexer receives data
from a single medium, identifies each, and sends to different receivers.
1. Frequency Division Multiplexing
When the carrier is frequency, FDM is used. FDM is an analog technology. FDM divides the spectrum or
carrier bandwidth in logical channels and allocates one user to each channel. Each user can use the channel
frequency independently and has exclusive access of it. All channels are divided in such a way that they do
not overlap with each other. Channels are separated by guard bands. Guard band is a frequency which is
not used by either channel.
2. Time Division Multiplexing
TDM is applied primarily on digital signals but can be applied on analog signals as well. In TDM the shared
channel is divided among its user by means of time slot. Each user can transmit data within the provided
time slot only. Digital signals are divided in frames, equivalent to time slot i.e. frame of an optimal size
which can be transmitted in given time slot.
TDM works in synchronized mode. Both ends, i.e. Multiplexer and De-multiplexer are timely synchronized
and both switch to next channel simultaneously.
When channel A transmits its frame at one end,the De-multiplexer provides media to channel A on the
other end.As soon as the channel A’s time slot expires, this side switches to channel B. On the other end,
the De-multiplexer works in a synchronized manner and provides media to channel B. Signals from
different channels travel the path in interleaved manner.
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3. Wavelength Division Multiplexing
Light has different wavelength (colors). In fiber optic mode, multiple optical carrier signals are multiplexed
into an optical fiber by using different wavelengths. This is an analog multiplexing technique and is done
conceptually in the same manner as FDM but uses light as signals.
Further, on each wavelength time division multiplexing can be incorporated to accommodate more data
signals.
4. Code Division Multiplexing
Multiple data signals can be transmitted over a single frequency by using Code Division Multiplexing. FDM
divides the frequency in smaller channels but CDM allows its users to full bandwidth and transmit signals
all the time using a unique code. CDM uses orthogonal codes to spread signals.
MULTIPLEXERS VS CONCENTRATORS
Multiplexing can promote more economical use of a communication channel; it acts as a communication
interface, combining the input streams from several devices into a single stream that can be sent over a
single channel to the computer system. This allows a single communication channel (typically voice-grade)
to substitute for many slower sub voice channels that might otherwise have been operating at less than
full capacity. Once the computer system has completed processing, the data is sent to the multiplexer,
which then routes the data to the appropriate device.
A concentrator differs from a multiplexer in that it allows data to be transmitted from only one terminal
at a time over a communication channel. The concentrator polls the terminals one at a time to see if they
have any messages to send. When a communication channel is free, the first terminal ready to send or
receive data will get control of the channel and continue to control it for the length of the transaction. The
use of a concentrator relies on the assumption that not all terminals will be ready to send or receive data
at a given time.
DATA TRANSMISSION MODES
The way in which data is transmitted from one place to another is called data transmission mode. It is also
called the data communication mode. It indicates the direction of flow of information. Sometimes, data
transmission modes are also referred to as directional modes.
When a person is giving a lecture, information is conveyed in one direction. Similarly, during a conversation
between two persons, spoken messages are exchanged in both directions. These messages may be
exchanged alternatively or simultaneously.
There are three data transmission modes.
(i) Simplex mode:
In simplex mode, data is transmitted in only one direction. A terminal can only send data and cannot
receive it or it can only receive data but cannot send data.
Today, this mode of data communication is not popular, because most of the modem communications
require two-way exchange of data. However, this mode of communication is used in business field at
certain point-of-sale terminals in which sales data is entered without a corresponding reply. The other
examples of simplex communication modes are Radio and T.V transmissions.
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In computer system, the keyboard, monitor and printer are examples of simplex devices. The keyboard can
only be used to enter data into computer, while monitor and printer can only accept (display/print) output.
(ii) Half-Duplex Mode:
In half-duplex mode, data can be transmitted in both directions but only in one direction at a time. In this
mode, data is sent and received alternatively. It is like a one-lane bridge where two-way traffic must give
way in order to cross the other.
In half-duplex mode, at a time only one end transmits data while other end receives. In addition, it is
possible to perform error detection and request the sender to re-transmit information.
The Internet browsing is an example of half duplex. When we issue a request to download web document
or webpage, it is downloaded and displayed before we issue another request.
(iii) Full-Duplex Mode:
In full-duplex mode, data can be transmitted in both directions at the same time on the same channel. It is
the fastest directional mode of communication. The telephone communication system is an example of
full-duplex communication mode.
Types of Data Transmission Modes:
There are two types of data transmission modes. These are:
(i) Parallel Transmission.
(ii) Serial Transmission.
(i) Parallel Transmission:
In parallel transmission, a group of bits of data flow at the same time (in parallel) through separate
communication lines. It is very fast data transmission. The automobile traffic on a multi-lane highway is an
example of parallel transmission.
Inside the computer, usually a group of bits of data flow from one component to another at the same time.
If a computer uses 32-bit internal structure, then 32-bits of data can be transferred from one component
of computer to another at the same time. Parallel transmission is commonly used to transfer data from
computer to printer.
(ii) Serial Transmission:
In serial data transmission, a group of bits of data flow in sequential order through single communication
line. The flow of traffic on one-lane residential street is an example of serial data transmission mode. Serial
transmission is typically slower than parallel transmission, because data is sent sequentially in a bit-by-bit
on a single communication line.
The telephone line system uses the serial transmission to transmit data from one location to another. In
computer system, mouse also uses serial transmission to send the command signals inside the computer.
Synchronous and Asynchronous Transmissions:
Synchronous Transmission:
In synchronous transmission, data is transmitted block-by-block or word-byword at the same time. Each
block may contain several bytes of data. In this mode, data is saved before sending. A large volume of data
can be transmitted at a time. The data transmission is very fast. It is most commonly used by remote
communication systems.
In synchronous transmission, a special communication device known as 'synchronized clock' is required to
schedule the transmission of information. This special communication device or equipment is expensive.
Asynchronous Transmission:
In asynchronous transmission, data is transmitted one byte at a 'time'. For example, the data is
transmitted character-by-character as the user types it on a keyboard. In this mode, data is not saved
before sending.
An asynchronous line that is idle (not being used) is identified with a value 1, also known as mark state.
This value is used by the communication devices to find whether the line is idle or free.
In asynchronous transmission, a special signal is sent by sender to the receiver before sending the
message. It is known as start bit. A start bit has a value of 0, also called a space state. Thus, when the line
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switches from a value of 1 to a value of 0, the receiver is alerted for receiving the message. The
asynchronous transmission is most commonly used by microcomputers.
Transmission Mediums in Computer Networks
Data is represented by computers and other telecommunication devices using signals. Signals are
transmitted in the form of electromagnetic energy from one device to another. Electromagnetic signals
travel through vacuum, air or other transmission mediums to move from one point to another (from
sender to receiver).
Electromagnetic energy (includes electrical and magnetic fields) consists of power, voice, visible light, radio
waves, ultraviolet light, gamma rays etc.
Transmission medium is the means through which we send our data from one place to another. The first
layer (physical layer) of Communication Networks OSI Seven-layer model is dedicated to the transmission
media.
Factors to be considered while selecting a Transmission Medium
1. Transmission Rate
2. Cost and Ease of Installation
3. Resistance to Environmental Conditions
4. Distances
Bounded or Guided Transmission Media
Guided media, which are those that provide a conduit from one device to another, include Twisted-Pair
Cable, Coaxial Cable, and Fibre-Optic Cable.
A signal travelling along any of these media is directed and contained by the physical limits of the medium.
Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the
form of electric current. Optical fibre is a cable that accepts and transports signals in the form of light.
Twisted Pair Cable
This cable is the most commonly used and is cheaper than others. It is lightweight, cheap, can be installed
easily, and they support many different types of network. Some important points:
 Its frequency range is 0 to 3.5 kHz.
 Typical attenuation is 0.2 dB/Km @ 1kHz.
 Typical delay is 50 µs/km.
 Repeater spacing is 2km.
A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted
together. One of these wires is used to carry signals to the receiver, and the other is used only as ground
reference. The receiver uses the difference between the two. In addition to the signal sent by the sender
on one of the wires, interference(noise) and crosstalk may affect both wires and create unwanted signals.
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If the two wires are parallel, the effect of these unwanted signals is not the same in both wires because
they are at different locations relative to the noise or crosstalk sources. This results in a difference at the
receiver.
Twisted Pair is of two types:
 Unshielded Twisted Pair (UTP)
 Shielded Twisted Pair (STP)
Unshielded Twisted Pair Cable
It is the most common type of telecommunication when compared with Shielded Twisted Pair Cable which
consists of two conductors usually copper, each with its own colour plastic insulator. Identification is the
reason behind coloured plastic insulation.
UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4 pair cable
use RJ-45 connector.
Advantages of Unshielded Twisted Pair Cable
 Installation is easy
 Flexible
 Cheap
 It has high speed capacity,
 100-meter limit
 Higher grades of UTP are used in LAN technologies like Ethernet.
It consists of two insulating copper wires (1mm thick). The wires are twisted together in a helical form to
reduce electrical interference from similar pair.
Disadvantages of Unshielded Twisted Pair Cable
 Bandwidth is low when compared with Coaxial Cable
 Provides less protection from interference.
Shielded Twisted Pair Cable
This cable has a metal foil or braided-mesh covering which encases each pair of insulated conductors.
Electromagnetic noise penetration is prevented by metal casing. Shielding also eliminates crosstalk.
It has the same attenuation as unshielded twisted pair. It is faster the unshielded and coaxial cable. It is,
however, also more expensive than coaxial and unshielded twisted pair.
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Advantages of Shielded Twisted Pair Cable
 Easy to install
 Performance is adequate
 Can be used for Analog or Digital transmission
 Increases the signalling rate
 Higher capacity than unshielded twisted pair
 Eliminates crosstalk
Disadvantages of Shielded Twisted Pair Cable
 Difficult to manufacture
 Heavy
Applications of Shielded Twisted Pair Cable
 In telephone lines to provide voice and data channels. The DSL lines that are used by the telephone
companies to provide high-data-rate connections also use the high-bandwidth capability of
unshielded twisted-pair cables.
 Local Area Network, such as 10Base-T and 100Base-T, also use twisted-pair cables.
Coaxial Cable
Coaxial is called by this name because it contains two conductors that are parallel to each other. Copper is
used in this cable as centre conductor which can be a solid wire or a standard one. It is surrounded by PVC
installation, a sheath which is encased in an outer conductor of metal foil, braid or both.
Outer metallic wrapping is used as a shield against noise and as the second conductor which completes the
circuit. The outer conductor is also encased in an insulating sheath. The outermost part is the plastic cover
which protects the whole cable.
Here the most common coaxial standards.
 50-Ohm RG-7 or RG-11 : used with thick Ethernet.
 50-Ohm RG-58 : used with thin Ethernet
 75-Ohm RG-59 : used with cable television
 93-Ohm RG-62 : used with ARCNET.
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Coaxial Cable Standards
Coaxial cables are categorized by their Radio Government (RG) ratings. Each RG number denotes a unique
set of physical specifications, including the wire gauge of the inner conductor, the thickness and the type of
the inner insulator, the construction of the shield, and the size and type of the outer casing. Each cable
defined by an RG rating is adapted for a specialized function, as shown in the table below:
Coaxial Cable Connectors
To connect coaxial cable to devices, we need coaxial connectors. The most common type of connector
used today is the Bayonet Neill-Concelman (BNC) connector. The below figure shows 3 popular types of
these connectors: the BNC Connector, the BNC T connector and the BNC terminator.
The BNC connector is used to connect the end of the cable to the device, such as a TV set. The BNC T
connector is used in Ethernet networks to branch out to a connection to a computer or other device. The
BNC terminator is used at the end of the cable to prevent the reflection of the signal.
There are two types of Coaxial cables:
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1. BaseBand
This is a 50 ohm (Ω) coaxial cable which is used for digital transmission. It is mostly used for LAN's.
Baseband transmits a single signal at a time with very high speed. The major drawback is that it needs
amplification after every 1000 feet.
2. BroadBand
This uses analog transmission on standard cable television cabling. It transmits several simultaneous signal
using different frequencies. It covers large area when compared with Baseband Coaxial Cable.
Advantages of Coaxial Cable
 Bandwidth is high
 Used in long distance telephone lines.
 Transmits digital signals at a very high rate of 10Mbps.
 Much higher noise immunity
 Data transmission without distortion.
 The can span to longer distance at higher speeds as they have better shielding when compared to
twisted pair cable
Disadvantages of Coaxial Cable
 Single cable failure can fail the entire network.
 Difficult to install and expensive when compared with twisted pair.
 If the shield is imperfect, it can lead to grounded loop.
Aplications of Coaxial Cable
 Coaxial cable was widely used in analog telephone networks, where a single coaxial network could
carry 10,000 voice signals.
 Cable TV networks also use coaxial cables. In the traditional cable TV network, the entire network
used coaxial cable. Cable TV uses RG-59 coaxial cable.
 In traditional Ethernet LANs. Because of its high bandwidth, and consequence high data rate,
coaxial cable was chosen for digital transmission in early Ethernet LANs. The 10Base-2, or Thin
Ethernet, uses RG-58 coaxial cable with BNC connectors to transmit data at 10Mbps with a range of
185 m.
Fibre Optic Cable
A fibre-optic cable is made of glass or plastic and transmits signals in the form of light.
For better understanding we first need to explore several aspects of the nature of light.
Light travels in a straight line as long as it is mobbing through a single uniform substance. If ray of light
travelling through one substance suddenly enters another substance (of a different density), the ray
changes direction.
The below figure shows how a ray of light changes direction when going from a more dense to a less dense
substance.
Bending of a light ray
As the figure shows:
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
If the angle of incidence I(the angle the ray makes with the line perpendicular to the interface
between the two substances) is less than the critical angle, the ray refracts and moves closer to the
surface.
 If the angle of incidence is greater than the critical angle, the ray reflects (makes a turn) and travels
again in the denser substance.
 If the angle of incidence is equal to the critical angle, the ray refracts and moves parallel to the
surface as shown.
Note: The critical angle is a property of the substance, and its value differs from one substance to another.
Optical fibres use reflection to guide light through a channel. A glass or plastic core is surrounded by a
cladding of less dense glass or plastic. The difference in density of the two materials must be such that a
beam of light moving through the core is reflected off the cladding instead of being refracted into it.
Internal view of an Optical fibre
Propagation Modes of Fibre Optic Cable
Current technology supports two modes (Multimode and Single mode) for propagating light along optical
channels, each requiring fibre with different physical characteristics. Multimode can be implemented in
two forms: Step-index and Graded-index.
Multimode Propagation Mode
Multimode is so named because multiple beams from a light source move through the core in different
paths. How these beams move within the cable depends on the structure of the core as shown in the
below figure.
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

In multimode step-index fibre, the density of the core remains constant from the centre to the
edges. A beam of light moves through this constant density in a straight line until it reaches the
interface of the core and the cladding.
The term step-index refers to the suddenness of this change, which contributes to the distortion of
the signal as it passes through the fibre.
In multimode graded-index fibre, this distortion gets decreases through the cable. The word index
here refers to the index of refraction. This index of refraction is related to the density. A gradedindex fibre, therefore, is one with varying densities. Density is highest at the centre of the core and
decreases gradually to its lowest at the edge.
Single Mode
Single mode uses step-index fibre and a highly focused source of light that limits beams to a small range of
angles, all close to the horizontal. The single-mode fibre itself is manufactured with a much smaller
diameter than that of multimode fibre, and with substantially lower density.
The decrease in density results in a critical angle that is close enough to 90 degree to make the propagation
of beams almost horizontal.
Fibre Optic Cable Connectors
There are three types of connectors for fibre-optic cables, as shown in the figure below.
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The Subscriber Channel (SC) connector is used for cable TV. It uses push/pull locking system. The StraightTip (ST) connector is used for connecting cable to the networking devices. MT-RJ is a connector that is the
same size as RJ45.
Advantages of Fibre Optic Cable
Fibre optic has several advantages over metallic cable:
 Higher bandwidth
 Less signal attenuation
 Immunity to electromagnetic interference
 Resistance to corrosive materials
 Light weight
 Greater immunity to tapping
Disadvantages of Fibre Optic Cable
There are some disadvantages in the use of optical fibre:
 Installation and maintenance
 Unidirectional light propagation
 High Cost
Applications of Fibre Optic Cable
 Often found in backbone networks because its wide bandwidth is cost-effective.
 Some cable TV companies use a combination of optical fibre and coaxial cable thus creating a
hybrid network.
 Local-area Networks such as 100Base-FX network and 1000Base-X also use fibre-optic cable.
UnBounded or UnGuided Transmission Media
Unguided medium transport electromagnetic waves without using a physical conductor. This type of
communication is often referred to as wireless communication. Signals are normally broadcast through
free space and thus are available to anyone who has a device capable of receiving them.
The below figure shows the part of the electromagnetic spectrum, ranging from 3 kHz to 900 THz, used for
wireless communication.
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Unguided signals can travel from the source to the destination in several ways: Gound propagation, Sky
propagation and Line-of-sight propagation as shown in below figure.
Propagation Modes
 Ground Propagation: In this, radio waves travel through the lowest portion of the atmosphere,
hugging the Earth. These low-frequency signals emanate in all directions from the transmitting
antenna and follow the curvature of the planet.
 Sky Propagation: In this, higher-frequency radio waves radiate upward into the ionosphere where
they are reflected back to Earth. This type of transmission allows for greater distances with lower
output power.
 Line-of-sight Propagation: in this type, very high-frequency signals are transmitted in straight lines
directly from antenna to antenna.
We can divide wireless transmission into three broad groups:
1. Radio waves
2. Micro waves
3. Infrared waves
Radio Waves
Electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are normally called radio waves.
Radio waves are omnidirectional. When an antenna transmits radio waves, they are propagated in all
directions. This means that the sending and receiving antennas do not have to be aligned. A sending
antenna send waves that can be received by any receiving antenna. The omnidirectional property has
disadvantage, too. The radio waves transmitted by one antenna are susceptible to interference by another
antenna that may send signal suing the same frequency or band.
Radio waves, particularly with those of low and medium frequencies, can penetrate walls. This
characteristic can be both an advantage and a disadvantage. It is an advantage because, an AM radio can
receive signals inside a building. It is a disadvantage because we cannot isolate a communication to just
inside or outside a building.
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Omnidirectional Antenna for Radio Waves
Radio waves use omnidirectional antennas that send out signals in all directions.
Applications of Radio Waves
 The omnidirectional characteristics of radio waves make them useful for multicasting in which there
is one sender but many receivers.
 AM and FM radio, television, maritime radio, cordless phones, and paging are examples of
multicasting.
Micro Waves
Electromagnetic waves having frequencies between 1 and 300 GHz are called micro waves. Micro waves
are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This means that
the sending and receiving antennas need to be aligned. The unidirectional property has an obvious
advantage. A pair of antennas can be aligned without interfering with another pair of aligned antennas.
The following describes some characteristics of microwaves propagation:
 Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in
direct sight of each other, towers that are far apart need to be very tall.
 Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage
if receivers are inside the buildings.
 The microwave band is relatively wide, almost 299 GHz. Therefore, wider sub-bands can be
assigned and a high date rate is possible.
 Use of certain portions of the band requires permission from authorities.
Unidirectional Antenna for Micro Waves
Microwaves need unidirectional antennas that send out signals in one direction. Two types of antennas are
used for microwave communications: Parabolic Dish and Horn.
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A parabolic antenna works as a funnel, catching a wide range of waves and directing them to a common
point. In this way, more of the signal is recovered than would be possible with a single-point receiver.
A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcast up a stem and deflected
outward in a series of narrow parallel beams by the curved head. Received transmissions are collected by
the scooped shape of the horn, in a manner similar to the parabolic dish, and are deflected down into the
stem.
Applications of Micro Waves
Microwaves, due to their unidirectional properties, are very useful when unicast(one-to-one)
communication is needed between the sender and the receiver. They are used in cellular phones, satellite
networks and wireless LANs.
There are 2 types of Microwave Transmission :
1. Terrestrial Microwave
2. Satellite Microwave
Advantages of Microwave Transmission
 Used for long distance telephone communication
 Carries 1000's of voice channels at the same time
Disadvantages of Microwave Transmission
 It is very costly
Terrestrial Microwave
For increasing the distance served by terrestrial microwave, repeaters can be installed with each antenna.
The signal received by an antenna can be converted into transmittable form and relayed to next antenna
as shown in below figure. It is an example of telephone systems all over the world
There are two types of antennas used for terrestrial microwave communication :
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1. Parabolic Dish Antenna
In this every line parallel to the line of symmetry reflects off the curve at angles in a way that they intersect
at a common point called focus. This antenna is based on geometry of parabola.
2. Horn Antenna
It is a like gigantic scoop. The outgoing transmissions are broadcast up a stem and deflected outward in a
series of narrow parallel beams by curved head.
Satellite Microwave
This is a microwave relay station which is placed in outer space. The satellites are launched either by
rockets or space shuttles carry them.
These are positioned 36000 Km above the equator with an orbit speed that exactly matches the rotation
speed of the earth. As the satellite is positioned in a geo-synchronous orbit, it is stationery relative to earth
and always stays over the same point on the ground. This is usually done to allow ground stations to aim
antenna at a fixed point in the sky.
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Features of Satellite Microwave
 Bandwidth capacity depends on the frequency used.
 Satellite microwave deployment for orbiting satellite is difficult.
Advantages of Satellite Microwave
 Transmitting station can receive back its own transmission and check whether the satellite has
transmitted information correctly.
 A single microwave relay station which is visible from any point.
Disadvantages of Satellite Microwave
 Satellite manufacturing cost is very high
 Cost of launching satellite is very expensive
 Transmission highly depends on whether conditions, it can go down in bad weather
Infrared Waves
Infrared waves, with frequencies from 300 GHz to 400 THz, can be used for short-range communication.
Infrared waves, having high frequencies, cannot penetrate walls. This advantageous characteristic prevents
interference between one system and another, a short-range communication system in on room cannot be
affected by another system in the next room.
When we use infrared remote control, we do not interfere with the use of the remote by our neighbours.
However, this same characteristic makes infrared signals useless for long-range communication. In
addition, we cannot use infrared waves outside a building because the sun's rays contain infrared waves
that can interfere with the communication.
Applications of Infrared Waves
 The infrared band, almost 400 THz, has an excellent potential for data transmission. Such a wide
bandwidth can be used to transmit digital data with a very high data rate.
 The Infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has
established standards for using these signals for communication between devices such as
keyboards, mouse, PCs and printers.
 Infrared signals can be used for short-range communication in a closed area using line-of-sight
propagation.
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COM 302: DATA COMMUNICATIONS AND NETWORKING
Module 4
COMMUNICATION PROTOCOLS
Communications devices have to agree on many physical aspects of the data to be exchanged before
successful transmission can take place. Rules defining transmissions are called protocols.
A protocol is a set of rules which governs how data is sent from one point to another. In data
communications, there are widely accepted protocols for sending data. Both the sender and receiver must
use the same protocol when communicating. They are required to exchange messages in or between
computing systems and are required in communications.
Communications protocols cover authentication, error detection and correction, and signalling. They can
also describe the syntax, semantics, and synchronization of analog and digital communications.
Communications protocols are implemented in hardware and software. There are thousands of
communications protocols that are used everywhere in analog and digital communications. Computer
networks cannot exist without them.
There are many properties of a transmission that a protocol can define. Common ones include: packet size,
transmission speed, error correction types, handshaking and synchronization techniques, address
mapping, acknowledgement processes, flow control, packet sequence controls, routing, address
formatting.
Popular protocols include: File Transfer Protocol (FTP), TCP/IP, User Datagram Protocol (UDP), Hypertext
Transfer Protocol (HTTP), Post Office Protocol (POP3), Internet Message Access Protocol (IMAP), Simple
Mail Transfer Protocol (SMTP).
SYNCHRONOUS AND ASYNCHRONOUS TRANSMISSION
A protocol establishes a means of communicating between two systems. As long as the sender and
receiver each use the same protocol, information can be reliably exchanged between them. We will look at
two common protocols used in serial data communications, the first is known as Asynchronous, the
second as Synchronous.
ASYNCHRONOUS TRANSMISSION
The asynchronous protocol evolved early in the history of telecommunications. It became popular with the
invention of the early tele-typewriters that were used to send telegrams around the world.
Asynchronous systems send data bytes between the sender and receiver by packaging the data in an
envelope. This envelope helps transport the character across the transmission link that separates the
sender and receiver. The transmitter creates the envelope, and the receiver uses the envelope to extract
the data. Each character (data byte) the sender transmits is preceded with a start bit, and suffixed with a
stop bit. These extra bits serve to synchronize the receiver with the sender.
In asynchronous serial transmission, each character is packaged in an envelope, and sent across a single
wire, bit by bit, to a receiver. Because no signal lines are used to convey clock (timing) information, this
method groups data together into a sequence of bits (five - eight), then prefixes them with a start bit and
appends the data with a stop bit.
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Nowadays, electronic clocks that provide the timing sequences
necessary to decode the incoming signal have replaced the
electromechanical motors.
This method of transmission is suitable for slow speeds less than about 32000 bits per second. In addition,
notice that the signal that is sent does not contain any information that can be used to validate if it was
received without modification. This means that this method does not contain error detection information,
and is susceptible to errors.
In addition, for every character that is sent, an additional two bits is also sent. Consider the sending of a
text document which contains 1000 characters. Each character is eight bits, thus the total number of bits
sent are 10000 (8 bits per character plus a start and stop bit for each character). These 10000 bits is
actually 1250 characters, meaning that an additional 250 equivalent characters are sent due to the start
and stop bits. This represents a large overhead in sending data, clearly making this method an inefficient
means of sending large amounts of data.
Summary
Transmission of these extra bits (2 per byte) reduce data throughput. Synchronization is achieved for each
character only. When the sender has no data to transmit, the line is idle and the sender and receiver are
NOT in synchronization. Asynchronous protocols are suited for low speed data communications, and there
is no method of error checking inherent in this protocol.
SYNCHRONOUS TRANSMISSION
One of the problems associated with asynchronous transmission is the high overhead associated with
transmitting data. Another problem is the complete lack of any form of error detection. This means the
sender has no method of knowing whether the receiver is correctly recognizing the data being transmitted.
In synchronous transmission, greater efficiency is achieved by grouping characters together, and doing
away with the start and stop bits for each character. We still envelop the information in a similar way as
before, but this time we send more characters between the start and end sequences. In addition, the start
and stop bits are replaced with a new format that permits greater flexibility. An extra ending sequence is
added to perform error checking.
A start type sequence, called a header, prefixes each block of characters, and a stop type sequence, called
a tail, suffixes each block of characters. The tail is expanded to include a check code, inserted by the
transmitter, and used by the receiver to determine if the data block of characters was received without
errors. In this way, synchronous transmission overcomes the two main deficiencies of the asynchronous
method, that of inefficiency and lack of error detection.
There are variations of synchronous transmission, which are split into two groups, namely character
orientated and bit orientated. In character orientated, information is encoded as characters. In bit
orientated, information is encoded using bits or combinations of bits, and is thus more complex than the
character orientated version. Binary synchronous is an example of character orientated, and High-Level
Data Link Control (HDLC) is an example of bit orientated.
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In asynchronous transmission, if there was no data to transmit, nothing was sent. We relied on the start bit
to start the motor and thus begin the preparation to decode the incoming character. However, in
synchronous transmission, because the start bit has been dropped, the receiver must be kept in a state of
readiness. This is achieved by sending a special code by the transmitter whenever it has no data to send.
In bit orientated protocols, the line idle state is changed to 7E,
which synchronizes the receiver to the sender. The start and
stop bits are removed, and each character is combined with
others into a data packet.
User data is prefixed with a header field, and suffixed with a
trailer field which includes a checksum value (used by the
receiver to check for errors in sending).
The header field is used to convey address information (sender and receiver), packet type and control
data. The data field contains the user’s data (if it can't fit in a single packet, then multiple packets are used
and numbered). Generally, it has a fixed size. The tail field contains checksum information which the
receiver uses to check whether the packet was corrupted during transmission.
Summary
Asynchronous transmission is suited for low speed serial transmission and does not include error checking
as part of the protocol. Each character is contained in an envelope of a start and stop bit. It is inefficient.
Synchronous transmission can achieve higher speeds than asynchronous. In addition, it is an error checking
protocol, and much more efficient because it groups characters together into blocks.
Some common networking protocols for routing, addressing schemes, interoperability and naming
conventions:
1. TCP/IP
Transmission Control Protocol, TCP A connection-based Internet protocol responsible for breaking data
into packets, which the IP protocol sends over the network. IP is located at the TCP/IP Internet layer which
corresponds to the network layer of the OSI Model. IP is responsible for routing packets by their IP address.
Internet Protocol, IP is a connectionless protocol. This means, IP does not establish a connection between
source and destination before transmitting data, thus packet delivery is not guaranteed by IP. Instead, this
must be provided by TCP. TCP is a connection-based protocol and, is designed to guarantee delivery by
monitoring the connection between source and destination before data is transmitted. TCP places packets
in sequential order and requires acknowledgment from the receiving node that they arrived properly
before any new data is sent.
2. IPX/SPX
IPX/SPX is the primary protocol of Novell NetWare (in particular, versions 4.0 and earlier, though it can be
used on all versions). Internetwork Packet Exchange/Sequenced Packet Exchange developed by Novell and
is used primarily on networks that use the Novell NetWare network operating system. The IPX and SPX
protocols provide services similar to those offered by IP and TCP. Like IP, IPX is a connectionless network
layer protocol. SPX runs on top of IPX at the transport layer and, like TCP, provides connection oriented,
guaranteed delivery. IPX/SPX provides many of the same features as TCP/IP, and is a routable transport
protocol that allows networks to be segmented. However, network segmentation with IPX/SPX is done
with network numbers and not with subnet masks. IPX/SPX is also similar to TCP/IP because IPX/SPX relies
on internal protocols for network communication.
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IPX
IPX is similar to the operation of UDP of TCP/IP. IPX is a connectionless datagram transfer service. Because
it is connectionless, like UDP, it does not require any preliminary connection setup to transmit the data
packets. A disadvantage to connectionless communication is that flow control and error correction are not
provided during network communication. In addition, packet delivery is not guaranteed. IPX also provides
addressing and routing of packets within and between network segments.
SPX
SPX is similar to the operation of TCP of TCP/IP. SPX is connection-oriented data transfer over IPX. Because
SPX is connection oriented, flow control and error correction are provided along with packet delivery
acknowledgments. SPX allows a single packet to remain unacknowledged at one time. If a packet is
unacknowledged, the packet is retransmitted a total of 8 times. If there’s no acknowledgment, SPX
considers the connection failed.
3. FTP (File Transfer Protocol)
One of the earliest uses of the Internet, long before Web browsing came along, was transferring files
between computers. The File Transfer Protocol (FTP) is used to connect to remote computers, list shared
files, and either upload or download files between local and remote computers.
FTP runs over TCP, which provides a connection-oriented, guaranteed data-delivery service. FTP is a
character-based command interface, although many FTP applications have graphical interfaces. FTP is still
used for file transfer purposes, most commonly as a central FTP server with files available for download.
Web browsers can make FTP requests to download programs from links selected on a Web page.
4. SMTP (Simple Mail Transfer Protocol)
SMTP is a standard electronic-mail protocol that handles the sending of mail from one SMTP to another
SMTP server. To accomplish the transport, the SMTP server has its own MX (mail exchanger) record in the
DNS database that corresponds to the domain for which it is configured to receive mail.
When equipped for two-way communication, mail clients are configured with the address of a POP3 server
to receive mail and the address of an SMTP server to send mail. The clients can configure server
parameters in the properties sheets of the mail client, basing the choices on an FQDN or an IP address.
SMTP uses TCP for communication and operates on port 25. Simple Mail Transfer Protocol (SMTP) is the
application-layer protocol used for transmitting e-mail messages. SMTP is capable of receiving e-mail
messages, but it's limited in its capabilities. The most common implementations of SMTP are in
conjunction with either POP3 or IMAP4. For example, users download an e-mail message from a POP3
server, and then transmit messages via an SMTP server
5. HTTP (Hypertext Transfer Protocol)
HTTP is often called the protocol of the Internet. HTTP received this designation because most Internet
traffic is based on HTTP. When a user requests a Web resource, it is requested using HTTP. The following is
a Web request:
http://www.example.com
When a client enters this address into a Web browser, DNS is called to resolve the Fully Qualified Domain
Name (FQDN) to an IP address. When the address is resolved, an HTTP get request is sent to the Web
server. The Web server responds with an HTTP send response. Such communication is done several times
throughout a single session to a Web site. HTTP uses TCP for communication between clients and servers.
HTTP operates on port 80.
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6. HTTPS (Hypertext Transfer Protocol Secure)
HTTP is for Web sites using additional security features such as certificates. HTTPS is used when Web
transactions are required to be secure. HTTPS uses a certificatebased technology such as VeriSign.
Certificate-based transactions offer a mutual authentication between the client and the server. Mutual
authentication ensures the server of the client identity, and ensures the client of the server identity.
HTTPS, in addition to using certificate-based authentication, encrypts all data packets sent during a session.
Because of the encryption, confidential user information cannot be compromised. To use HTTPS, a Web
site must purchase a certificate from a third-party vendor such as VeriSign, CertCo, United States Postal
Service, or other certificate providers. When the certificate is issued to a Web site from a third-party
vendor, the Web site is using trusted communication with the client. The communication is trusted
because the third party is not biased toward either the Web site or the client. To view a certificate during a
HTTPS session, simply double-click the lock icon in the lower-right area of the Web browser. HTTPS
operates on port 443 and uses TCP for communication.
7. ARP / RARP (Address Resolution Protocol / Reverse Address Resolution Protocol)
The Address Resolution Protocol (ARP) is an Internet layer protocol that helps TCP/IP network components
find other devices in the same broadcast domain. ARP uses a local broadcast (255.255.255.255) at layer 3
and FF:FF:FF:FF:FF:FF at layer 2 to discover neighboring devices. Basically stated, you have the IP address
you want to reach, but you need a physical (MAC) address to send the frame to the destination at layer 2.
ARP resolves an IP address of a destination to the MAC address of the destination on the same data link
layer medium, such as Ethernet. Remember that for two devices to talk to each other in Ethernet (as with
most layer 2 technologies), the data link layer uses a physical address (MAC) to differentiate the machines
on the segment. When Ethernet devices talk to each other at the data link layer, they need to know each
other’s MAC addresses.
RARP is sort of the reverse of an ARP. In an ARP, the device knows the layer 3 address, but not the data link
layer address. With a RARP, the device doesn’t have an IP address and wants to acquire one. The only
address that this device has is a MAC address. Common protocols that use RARP are BOOTP and DHCP
8. NTP (Network Time Protocol)
The Network Time Protocol is used to synchronize the time of a computer client or server to another
server or reference time source, such as a radio or satellite receiver or modem. It provides accuracy's
typically within a millisecond on LANs and up to a few tens of milliseconds on WANs.
A protocol performs the following functions:
1. Data sequencing. It refers to breaking a long message into smaller packets of fixed size. Data sequencing
rules define the method of numbering packets to detect loss or duplication of packets, and to correctly
identify packets, which belong to same message.
2. Data routing. Data routing defines the most efficient path between the source and destination.
3. Data formatting. Data formatting rules define which group of bits or characters within packet constitute
data, control, addressing, or other information.
4. Flow control. A communication protocol also prevents a fast sender from overwhelming a slow receiver.
It ensures resource sharing and protection against traffic congestion by regulating the flow of data on
communication lines.
5. Error control. These rules are designed to detect errors in messages and to ensure transmission of
correct messages. The most common method is to retransmit erroneous message block. In such a case, a
block having error is discarded by the receiver and is retransmitted by the sender.
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6. Precedence and order of transmission. These rules ensure that all the nodes get a chance to use the
communication lines and other resources of the network based on the priorities assigned to them.
7. Connection establishment and termination. These rules define how connections are established,
maintained and terminated when two nodes of a network want to communicate with each other.
8. Data security. Providing data security and privacy is also built into most communication software
packages. It prevents access of data by unauthorized users.
9. Log information. Several communication software are designed to develop log information, which
consists of all jobs and data communications tasks that have taken place. Such information may be used
for charging the users of the network based on their usage of the network resources.
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COM 302: DATA COMMUNICATIONS AND NETWORKING
Module 5
COMPUTER NETWORKS
A Computer Network is the interconnection of multiple devices, generally termed as hosts connected
using multiple paths for the purpose of sending/receiving data or media.
There are also multiple devices or mediums which helps in the communication between two different
devices which are known as Network devices or components. There are multiple components that go into
computer networks, but not all of these are needed in every type of network. Some of the more common
computer network components include:
1. Network Interfaces
Every device on the network has to have a network interface of some design. The network interface is
sometimes referred to as a NIC (network interface card) and may be integrated into the computer's
motherboard or may be a separate card. The NIC is the component that takes information from the
computer and sends it out onto the network wire, or into the air in the case of a wireless network.
2. Repeater
A repeater operates at the physical layer. Its job is to regenerate the signal over the same network before
the signal becomes too weak or corrupted so as to extend the length to which the signal can be
transmitted over the same network. An important point to be noted about repeaters is that they do not
amplify the signal. When the signal becomes weak, they copy the signal bit by bit and regenerate it at the
original strength. It is a 2 port device.
3. Hubs
When you connect multiple computers on a network, they all plug into a central device called a hub. The
hub has the job of moving the network signal from one wire to another. In the case of a basic hub, the
signal from one computer is sent to all other computers on the hub, and each NIC decides whether to pass
the information to the computer or just drop it if it is not the intended recipient.
4. Switches
Switches are really smart hubs in that they are able to build tables that keep up with which computer is on
which switch port. With this intelligence, a switch does not transmit all information to all other computers
on the switch, just to the destination computer. Switching technology helps to reduce congestion on a
network and should be used for networks of 10 or more computers.
5. Routers
Routers are really smart switches in that they are aware of other networks, while hubs and switches are
only aware of the network they facilitate. Routers are used to connect one local area network (LAN) to
another, many times across long distances via commercial data carriers. Another way routers are smart is
they can dynamically update their routing information, detecting when one route to a network is down,
and checking to see if another route is available.
6. Gateway
A gateway, as the name suggests, is a passage to connect two networks together that may work upon
different networking models. They basically work as the messenger agents that take data from one system,
interpret it, and transfer it to another system. Gateways are also called protocol converters and can
operate at any network layer. Gateways are generally more complex than switch or router.
7. Media (Cabling)
Of course, none of these networking devices work well unless they are connected to each other, and that is
done with various media. The most ubiquitous media is commonly called Ethernet cabling, which is actually
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one of several categories of unshielded twisted pair (UTP) wiring. In addition, there is fibre optic cable,
which is more expensive and uses laser or LED light rather than electrical pulses. Wireless networks have
also become popular due to the ease and low expense of setting up a network. The "media" for a wireless
network is the air, through which the wireless NICs transmit radio signals that carry information.
8. Software
Software is the intelligence that causes all of the components to function together. The most popular
network software today uses what is known as the TCP/IP protocol suite, or stack. The suite is constructed
from actual layers of software, where each has its own function. These layers play by the same set of rules
so heterogeneous computer systems can communicate with each other, regardless of differences in
hardware or operating systems. Software in a network can either be local, or network.
Local Operating System - A local operating system allows personal computers to access files, print to a local
printer, and have and use one or more disk and CD drives that are located on the computer. Examples are
Ubuntu, Linux, Windows 7, Windows 8, Windows 10 etc.
Network Operating System - The network operating system is a program that runs on computers and
servers, and allows the computers to communicate over the network, sharing files and hardware devices
with one another. Examples include Windows Server 2012, Oracle Solaris, etc.
THE OSI NETWORK REFERENCE MODEL
OSI stands for Open Systems Interconnection. It was developed by ISO – ‘International Organization of
Standardization ‘ in the year 1974. It is a 7-layer architecture with each layer having specific functionality
to performed. All these 7 layers work collaboratively to transmit the data from one device to another across
the globe.
1. Physical Layer (Layer 1):
The lowest layer of the OSI reference model is the physical layer. It is responsible for the actual physical
connection between the devices. The physical layer contains information in the form of bits. When
receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the
Data Link layer, which will put the frame back together.
The functions of the physical layer are :
1. Bit synchronization: The physical layer provides the synchronization of the bits by providing a clock.
This clock controls both sender and receiver thus providing synchronization at bit level.
2. Bit rate control: The physical layer also defines the transmission rate i.e. the number of bits sent
per second.
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3. Physical topologies: Physical layer specifies the way in which the different, devices/nodes are
arranged in a network i.e. bus, star or mesh topology.
4. Transmission mode: Physical layer also defines the way in which the data flows between the two
connected devices. The various transmission modes possible are: Simplex, half-duplex and fullduplex.
*Hub, Repeater, Modem, Cables are examples of physical layer devices.
2. Data Link Layer (DLL) (Layer 2):
The data link layer is responsible for the node to node delivery of the message. The main function of this
layer is to make sure data transfer is error free from one node to another, over the physical layer. When a
packet arrives in a network, it is the responsibility of DLL to transmit it to the Host using its MAC address.
Data Link Layer is divided into two sub layers :
1. Logical Link Control (LLC)
2. Media Access Control (MAC)
Packet received from Network layer is further divided into frames depending on the frame size of NIC
(Network Interface Card). DLL also encapsulates Sender and Receiver’s MAC address in the header.
The Receiver’s MAC address is obtained by placing an ARP(Address Resolution Protocol) request onto the
wire asking “Who has that IP address?” and the destination host will reply with its MAC address.
The functions of the data Link layer are :
1. Framing: Framing is a function of the data link layer. It provides a way for a sender to transmit a set
of bits that are meaningful to the receiver. This can be accomplished by attaching special bit
patterns to the beginning and end of the frame.
2. Physical addressing: After creating frames, Data link layer adds physical addresses (MAC address)
of sender and/or receiver in the header of each frame.
3. Error control: Data link layer provides the mechanism of error control in which it detects and
retransmits damaged or lost frames.
4. Flow Control: The data rate must be constant on both sides else the data may get corrupted thus,
flow control coordinates that amount of data that can be sent before receiving acknowledgement.
5. Access control: When a single communication channel is shared by multiple devices, MAC sub-layer
of data link layer helps to determine which device has control over the channel at a given time.
* Packet in Data Link layer is referred as Frame.
** Data Link layer is handled by the NIC (Network Interface Card) and device drivers of host machines.
*** Switches & Bridges are examples of Data Link Layer devices.
3. Network Layer (Layer 3):
Network layer works for the transmission of data from one host to the other located in different networks.
It also takes care of packet routing i.e. selection of shortest path to transmit the packet, from the number
of routes available. The sender and receiver’s IP address are placed in the header by network layer.
The functions of the Network layer are :
1. Routing: The network layer protocols determine which route is suitable from source to destination.
This function of network layer is known as routing.
2. Logical Addressing: In order to identify each device on internetwork uniquely, network layer
defines an addressing scheme. The sender & receiver’s IP address are placed in the header by
network layer. Such an address distinguishes each device uniquely and universally.
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* A segment in the Network layer is referred to as PACKET .
** Network layer is implemented by networking devices such as routers.
*** Network Layer, Data Link Layer and Physical Layer are also known as Lower Layers or Hardware
Layers.
4. Transport Layer (Layer 4):
Transport layer provides services to application layer and takes services from network layer. The data in
the transport layer is referred to as Segments. It is responsible for the End to End delivery of the complete
message. Transport layer also provides the acknowledgement of the successful data transmission and retransmits the data if error is found.
• At sender’s side:
Transport layer receives the formatted data from the upper layers, performs Segmentation and also
implements Flow & Error control to ensure proper data transmission. It also adds Source and Destination
port number in its header and forwards the segmented data to the Network Layer.
Note: The sender needs to know the port number associated with the receiver’s application.
Generally this destination port number is configured, either by default or manually. For example, when a
web application makes a request to a web server, it typically uses port number 80, because this is the
default port assigned to web applications. Many applications have default port assigned.
• At receiver’s side:
Transport Layer reads the port number from its header and forwards the Data which it has received to the
respective application. It also performs sequencing and reassembling of the segmented data.
The functions of the transport layer are:
1. Segmentation and Reassembly: This layer accepts the message from the (session) layer, and breaks
the message into smaller units. Each of the segment produced has a header associated with it. The
transport layer at the destination station reassembles the message.
2. Service Point Addressing: In order to deliver the message to correct process, transport layer header
includes a type of address called service point address or port address. Thus by specifying this
address, transport layer makes sure that the message is delivered to the correct process.
The services provided by transport layer:
1. Connection Oriented Service: It is a three phase process which include
– Connection Establishment
– Data Transfer
– Termination / disconnection
In this type of transmission the receiving device sends an acknowledgment back to the source after
a packet or group of packet is received. This type of transmission is reliable and secure.
2. Connection less service: It is a one phase process and includes Data Transfer. In this type of
transmission, the receiver does not acknowledge receipt of a packet. This approach allows for much
faster communication between devices. Connection oriented Service is more reliable than
connection less Service.
* Data in the Transport Layer is called as Segments.
** Transport layer is operated by the Operating System. It is a part of the OS and communicates with the
Application Layer by making system calls.
5. Session Layer (Layer 5):
This layer is responsible for the establishment of connection, maintenance of sessions, authentication and
also ensures security.
The functions of the session layer are :
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1. Session establishment, maintenance and termination: The layer allows the two processes to
establish, use and terminate a connection.
2. Synchronization: This layer allows a process to add checkpoints which are considered as
synchronization points into the data. These synchronization points help to identify the error so that
the data is re-synchronized properly, and that ends of the messages are not cut prematurely and
data loss is avoided.
3. Dialog Controller: The session layer determines which device will communicate first and the
amount of data that will be sent.
6. Presentation Layer (Layer 6):
Presentation layer is also called the Translation layer. The data from the application layer is extracted here
and manipulated as per the required format to transmit over the network.
The functions of the presentation layer are:
1. Translation: For example, ASCII to EBCDIC.
2. Encryption/ Decryption: Data encryption translates the data into another form or code. The
encrypted data is known as the cipher text and the decrypted data is known as plain text. A key
value is used for encrypting as well as decrypting data.
3. Compression: Reduces the number of bits that need to be transmitted on the network.
7. Application Layer (Layer 7):
At the very top of the OSI Reference Model stack of layers, we find Application layer which is implemented
by the network applications. These applications produce the data, which has to be transferred over the
network. This layer also serves as window for the application services to access the network and for
displaying the received information to the user.
Ex: Application – Browsers, Skype Messenger etc.
**Application Layer is also called as Desktop Layer.
The functions of the Application layer are :
1.
2.
3.
4.
Network Virtual Terminal
FTAM-File transfer access and management
Mail Services
Directory Services
SCENARIO:
Let’s consider a scenario where a user wants to send a message through some Messenger application
running in his browser. The “Messenger” here acts as the application layer which provides the user with an
interface to create the data. This message or so called Data is compressed and encrypted (if it is secure
data). A notification for communication is sent to the receiving device, and the message is converted into
bits (0’s and 1’s) so that it can be transmitted.
OSI model acts as a reference model and is not implemented in Internet because of its late invention. The
most popular network software today uses what is known as the TCP/IP protocol suite, or stack. The suite
is constructed from actual layers of software, where each has its own function. While the seven-layer OSI
model -- Physical, Data Link, Network, Transport, Session, Presentation and Application -- is the starting
point for network stacks, the Internet (TCP/IP) model has four layers that combine several of the seven OSI
layers into the other layers.
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OSI MODEL TO TCP/IP MODEL
NETWORK TOPOLOGY
Computers in a network have to be connected in some logical manner. The layout pattern of the
interconnections between computers in a network is called network topology. You can think of topology as
the virtual shape or structure of the network. Network topology is also referred to as 'network
architecture.'
Devices on the network are referred to as 'nodes.' The most common nodes are computers and peripheral
devices. Network topology is illustrated by showing these nodes and their connections using cables. There
are a number of different types of network topologies, including point-to-point, bus, star, ring, mesh, tree
and hybrid. Let's review these main types.
1. Point-to-Point
Point-to-point topology is the simplest of all the network topologies. The network consists of a direct link
between two computers. This is faster and more reliable than other types of connections since there is a
direct connection. The disadvantage is that it can only be used for small areas where computers are in
close proximity.
2. Bus
Bus topology uses one main cable to which all nodes are directly connected. The main cable acts as a
backbone for the network. One of the computers in the network typically acts as the computer server. The
first advantage of bus topology is that it is easy to connect a computer or peripheral device. The second
advantage is that the cable requirements are relatively small, resulting in lower cost.
One of the disadvantages is that if the main cable breaks, the entire network goes down. This type of
network is also difficult to troubleshoot. For these reasons, this type of topology is not used for large
networks, such as those covering an entire building.
3. Star
In star topology, each computer is connected to a central hub using a point-to-point connection. The
central hub can be a computer server that manages the network, or it can be a much simpler device that
only makes the connections between computers over the network possible.
Star topology is very popular because the start-up costs are low. It is also easy to add new nodes to the
network. The network is robust in the sense that if one connection between a computer and the hub fails,
the other connections remain intact. If the central hub fails, however, the entire network goes down. It
also requires more cable than bus topology and is, therefore, more expensive.
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4. Ring
In ring topology, the computers in the network are connected in a circular fashion, and the data travels in
one direction. Each computer is directly connected to the next computer, forming a single pathway for
signals through the network. This type of network is easy to install and manage.
If there's a problem in the network, it is easy to pinpoint which connection is defective. It is also good for
handling high-volume traffic over long distances since every computer can act as a booster of the signal.
On the downside, adding computers to this type of network is more cumbersome, and if one single
computer fails, the entire network goes down.
5. Mesh
In mesh topology, every node has a direct point-to-point connection to every other node. Because all
connections are direct, the network can handle very high-volume traffic. It is also robust because if one
connection fails, the others remain intact. Security is also high since data travels along a dedicated
connection.
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COM 302: DATA COMMUNICATIONS AND NETWORKING
Module 6
NETWORKS (CONT’D)
Generally, networks are distinguished based on their geographical span. A network can be as small as
distance between your mobile phone and its Bluetooth headphone and as large as the internet itself,
covering the whole geographical world,
Personal Area Network
A Personal Area Network (PAN) is the smallest network which is very personal to a user. This may include
Bluetooth enabled devices or infra-red enabled devices. PAN has connectivity range up to 10 meters. PAN
may include wireless computer keyboard and mouse, Bluetooth enabled headphones, wireless printers
and TV remotes.
For example, a Piconet is a Bluetooth-enabled Personal Area Network which may contain up to 8 devices
connected together in a master-slave fashion.
Local Area Network
A computer network spanned inside a building and operated under single administrative system is
generally termed as Local Area Network (LAN). Usually, LAN covers an organization’ offices, schools,
colleges or universities. Number of systems connected in LAN may vary from as least as two to as much as
16 million.
LAN provides a useful way of sharing the resources between end users. The resources such as printers, file
servers, scanners, and internet are easily sharable among computers.
FIGURE: A SIMPLE LAN CONFIGURATION ARCHITECTURE
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LANs are composed of inexpensive networking and routing equipment. It may contain local servers serving
file storage and other locally shared applications. It mostly operates on private IP addresses and does not
involve heavy routing. LAN works under its own local domain and controlled centrally.
LAN uses either Ethernet or Token-ring technology. Ethernet is most widely employed LAN technology and
uses Star topology, while Token-ring is rarely seen.
LAN can be wired, wireless, or in both forms at once.
Metropolitan Area Network
The Metropolitan Area Network (MAN) generally expands throughout a city such as cable TV network. It
can be in the form of Ethernet, Token-ring, ATM, or Fibre Distributed Data Interface (FDDI).
Metro Ethernet is a service which is provided by Internet Service Providers (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.
FIGURE: A SIMPLE MAN CONFIGURATION ARCHITECTURE
The typical backbone of MAN is high-capacity and high-speed fibre optics. MAN works in between Local
Area Network and Wide Area Network. MAN provides uplink for LANs to WANs or internet.
Wide Area Network
As the name suggests, the Wide Area Network (WAN) covers a wide area which may span across provinces
and even a whole country. Generally, telecommunication networks are Wide Area Network. These
networks provide connectivity to MANs and LANs. Since they are equipped with very high speed backbone,
WANs use very expensive network equipment. A WAN may be managed by multiple administration.
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FIGURE 6.3: A TYPICAL WAN CONFIGURATION
Internetwork
A network of networks is called an internetwork, or simply the internet. It is the largest network in
existence. The internet hugely connects all WANs and it can have connection to LANs and Home networks.
Internet uses TCP/IP protocol suite and uses IP as its addressing protocol.
The internet enables its users to share and access enormous amount of information worldwide. It uses
WWW, FTP, email services, audio and video streaming etc. At huge level, internet works on Client-Server
model.
Internet uses very high speed backbone of fibre optics. To inter-connect various continents, fibres are laid
under sea as submarine communication cable.
The Evolution of the Internet
The concept of Internet was originated in 1969 and has undergone several technological & Infrastructural
changes as discussed below:
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





The origin of Internet devised from the concept of Advanced Research Project Agency Network
(ARPANET).
ARPANET was developed by United States Department of Defence.
The basic purpose of ARPANET was to provide communication among the various bodies of
government.
Initially, there were only four nodes, formally called Hosts.
In 1972, the ARPANET spread over the globe with 23 nodes located at different countries and thus
became known as Internet.
At the end of the cold war, the Internet was released to the general public, and especially
universities for telecommunication purposes.
In time, with invention of new technologies such as TCP/IP protocols, DNS, www, browsers,
scripting languages etc., the Internet continues to grow as the primary medium to publish and
access information over the web.
Advantages
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Internet covers almost every aspect of life one can think of. Here, we will discuss some of the advantages
of Internet:
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


Internet allows us to communicate with the people at remote locations. There are various
applications available on the web that uses the Internet as a medium for communication. One can
find various social networking sites such as Facebook, twitter, Yahoo, Flickr, etc.
Internet helps us to access any kind of information over the internet. Information regarding various
topics such as Technology, Health & Science, Social Studies, Geographical Information, Information
Technology, Products etc. can be surfed with help of a search engine.
Internet also serves a medium for entertainment. Some of the various modes for entertainment
over internet include Online Television, Online Games, Songs, Videos, and Social Networking Apps.
Internet provides concept of electronic commerce, that allows the business deals to be conducted
on electronic systems
Disadvantages
There exists also, a few disadvantages of the Internet discussed below:
 There is always the chance that personal information such as name, address, credit card number
used over the Internet will fall into the wrong hands. Therefore, one should be very careful while
sharing such information. One should use credit cards only through authenticated sites.
 Another disadvantage is Spamming. Spams are the unwanted or junk e-mails received from
different places. These e-mails serve no purpose and lead to denial or obstruction of normal
service.
 Virus can easily be spread to the computers connected to internet. Such virus attacks may cause
the system to crash or important data may get deleted.
 Also a major concern of internet is pornography. There are many pornographic sites that can be
found on the Internet with negative moral effects, especially on children.
 There are various websites (especially blogs) that do not provide the authenticated information.
This leads to misinformation among many people.
The Intranet
Intranet is defined as private network of computers within an organization with its own server and firewall.
Alternatively, we can define Intranet as a system in which multiple PCs are networked and connected to
each other but are not available to the world outside of the intranet.
Usually each company or organization has their own Intranet network and members or employees of that
company can access the computers in their intranet. As every computer on internet is identified by a
unique IP address, likewise each computer in an Intranet is also identified by an IP Address, which is unique
among the computers in that Intranet.
Benefits
The Intranet is a very efficient and reliable network system for any organization. It is beneficial as almost
every advantage of the internet also applies to the Intranet. However, some additional and specific
benefits of intranets are:
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
Communication: Intranet offers easy and cheap communication within an organization. Employees
can communicate using chat, e-mail or blogs.

Collaboration: Information is distributed among the employees as according to requirement and it
can be accessed by the authorized users, resulting in enhanced teamwork.

Business Management: It is also possible to deploy applications that support business operations.

Security: Since information shared on intranet can only be accessed within an organization,
therefore there is reduced opportunities for information theft.

Specific Users: Intranet targets only specific users within an organization therefore, once can know
exactly whom he is interacting with.

Immediate Updates: Any changes made to information are reflected immediately to all the users.
Internet vs. Intranet
Despite the similarities between Intranets and Internet, there are some differences between the two.
Following are the differences between Internet and Intranet:
Intranet
Localized Network.
Doesn't have access to Intranet
More Expensive
More Security
More Reliability
Internet
Worldwide Network
Have access to Internet.
Less Expensive
Less Security
Less Reliability
LOCAL AREA NETWORK TOPOLOGIES
When choosing a networking system in communication, it is important to analyse the effectiveness and
usefulness of the networking type, to the particular context. In doing so, it is appropriate to understand
that each topology system has its merits and demerits.
Star Topology
Devices and computers on a star network connect to a central hub, (which may be a hub, a router or a
switch). The hub acts as a junction to connect different nodes present in the Star Network, and at the same
time it manages and controls the whole of the network.
Advantages
 Signals don’t necessarily get transmitted to all the workstations. A sent signal reaches the intended
destination after passing through no more than 3-4 devices and 2-3 links.
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


Easy to connect new nodes or devices. In star topology new nodes can be added easily without
affecting rest of the network. Similarly, components can also be removed easily.
Centralised management. This helps in monitoring the network.
Failure of one node or link doesn’t affect the rest of network. At the same time, it is easy to detect
the failure and troubleshoot it.
Disadvantages
 A major disadvantage of this network topology is that if the central hub fails, all computers
connected to that hub would be disconnected.
 Dependent on a central hub rather than an individual and reliable network source.
 The use of hub, a router or a switch as central device increases the overall cost of the network.
 Performance and as well number of nodes which can be added in such topology is depended on
capacity of central device.
 More software is required to run the network efficiently
Bus Topology
in the bus setup, all the nodes are connected to a single cable, by the help of interface connectors. This
central cable is the backbone of the network and is known as Bus. Every workstation communicates with
the other device through this Bus.
Advantages
 Easy to implement and extend
 Well suited for temporary networks (quick setup)
 Initially less expensive than other topologies
 The linear bus network is well-suited for LAN
Disadvantages
 Difficult to administer/troubleshoot.
 Limited cable length and number of stations.
 If there is a problem with the cable, the entire network goes down.
 Maintenance costs may be higher in the long run.
 Performance degrades as additional computers are added or on heavy traffic.
 Low security (all computers on the bus can see all data transmissions).
 One virus in the network will affect all of them
 Proper termination is required. (loop must be in closed path).
 If one node fails, the whole network will shut down.
 If many computers are attached, the amount of data flowing causes the network to slow down.
Ring Topology
A computer network configuration where each network computer and device are connected to each other
forming a large circle. Each packet is sent around the ring until it reaches its final destination. Any
computer that wants to transmit a packet uses a token, without which a message cannot be transmitted.
Today, the ring topology is seldom used. Below is a visual example of a simple computer setup on a
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network using a ring topology.
Advantages
 Data is quickly transferred
 The use of tokens to transmit data ensures reliability of data.
 The transmission of data is relatively simple as packets travel in one direction only.
 Adding additional nodes has very little impact on bandwidth
 It prevents network collisions because of the media access method or architecture required
Disadvantages
 Data packets must pass through every computer between the sender and recipient therefore this
makes it slower.
 If any of the nodes fail, then the ring is broken and data cannot be transmitted successfully.
 It is difficult to troubleshoot the ring.
 Because all stations are wired together, to add a station you must shut down the network
temporarily.
 In order for all computers to communicate with each other, all computers must be turned on.
 Total dependence upon the one cable
Hybrid Topology
Tailor-made Integration of two or more different topologies to form a resultant topology. The network
adopts the characteristics and functioning of the combined topologies, rather than just the one.


Advantages
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 the required corrective
measures can be taken, WITHOUT affecting the functioning of rest of the network.
It is easy to increase the size of network by adding new components, without disturbing existing
architecture.
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

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.
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 maximised while their weaknesses are neutralised. 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.
Disadvantages
 One of the biggest drawbacks of the hybrid topology is its design. It is not easy to design this type of
architecture and it is a tough job for designers. Configuration and installation process needs to be
very efficient.
 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.
 As hybrid architectures are usually larger in scale, they require a lot of cables, cooling systems,
sophisticate network devices, etc.
MESH
In a mesh network topology, each of the network node, computer and other devices, are interconnected
with one another. 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 in
mesh topology.
Advantages
 It can handle heavy traffic, as there are dedicated paths between any two network nodes.
 The arrangement of the network nodes is such that it is possible to transmit data from one node to
many other nodes at the same time.
 The failure of a single node does not cause the entire network to fail as there are alternate paths
for data transmission.
 Point-to-point contact between every pair of nodes, makes it easy to identify faults.
Disadvantages
 A lot of cabling is required. Thus, the costs incurred in setup and maintenance are high.
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

The arrangement wherein every network node is connected to every other node of the network,
many connections serve no major purpose. This leads to redundancy of many network connections.
Owing to its complexity, the administration of a mesh network is difficult.
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