L&T Project Report

7/31/2019 L&T Project Report

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INDUSTRIAL TRAINING REPORT

(JUNE JULY 2007)

Submitted in the partial fulfillment of the requirements

For the 8th Semester Curriculum Degree of

Bachelor of Technology

in

Electronics and Communication Engineering

of

Bharati vidhyapeeths college of engineering, Delhi

Submitted to:

Mrs. Anuradha basu, H.O.D.,

Electronics and Communication Engineering Department


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ACKNOWLEDGEMENT

It is not possible to prepare a project report without the assistance

encouragement of other people. This one is certainly no exception. On thevery outset of this report, I would like to extend my sincere heartfelt obligation

towards all the personages. Without their active guidance, help, cooperation

encouragement, I would not have made headway in the project.

First, I take this opportunity to acknowledge my institution Bharati

vidyapeeths college of engineering Where I am pursuing my B.Tech.

I would like to express my sincere thanks to Mr. Hemant Kumar (Sales

Engineer L&T, New Delhi) who gave me the opportunity to work with such an

esteemed organization.

I owe profound sense of regards gratitude towards Mr. Vivek kumar who has

continuously guided me supported in all the tasks by giving me valuable

suggestions.

I owe debt of gratitude to all the employees who has given me enough

support, cooperation and guidance in clearing the doubts advising me in the

right time to make this project a real learning experience.

Thanking You

Bharat Bhushan Verma


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INTRODUCTION

The points of study about optical fiber communication and network and other technologies used

for communication are:-

1. List of figures.(1)

2. List of tables...(2)

3. Introduction to Larsen & Toubro

4. Introduction of optical fiber.

i. Types of optical fiber

ii. Principle of operation

iii. Application of optical fiber

iv. Optical fiber communication

5. PDH (Plesiochronous hierarchy).

i. Introduction of PDH

ii. Weaknesses of PDH

iii. Implementation of PDH

6. SONET/SDH (Synchronous Optical Network/Synchronous digital hierarchy.

i. Introduction of SDH

ii. Sonnet/SDH rates

iii. Structure of sonnet/SDH structure

iv. Difference from PDH

v. Basic unit of transmission

vi. Advantages of SDH


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vii. Layered model of SDH

viii. SDH elements

ix. Topologies of SDH

x. Usage of SDH element in SDH technology

xi. Next generation SONET/SDH

7. Wireless Communication.

i. Introduction of wireless communication

ii. Application of wireless technology

8. Internet

i. Introduction

ii. Internet vs. web

iii. Internet technology

iv. Data transfer over internet

v. Social impact of internet

9. GPRS

i. Introduction

ii. Security perspective of GPRS

10.Intranet

i. Introduction

ii. Characteristics of intranet

iii. Benefits of intranet

11.GSM (Global System for Mobile communication).

i. Introduction to GSM

ii. Advantages of GSM

iii. Disadvantages of GSM


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List of figures

Figure no. Figure name Page no.1 single-mode fiber

2 Multi-mode optical fiber

3 Step index multi-mode optical fiber
http://en.wikipedia.org/wiki/Single-mode_fiberhttp://en.wikipedia.org/wiki/Single-mode_fiber


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4 Graded index multi-mode optical fiber

5 Propagation analysis

6 Propagation of light in glass with minimum loss

7 Optical fiber communication

8 Chain of Add/drop multiplexer

9 SDH frame

10 STM-1 frame structure

11 Layers model of SDH

12 Terminal multiplexer

13 Add/drop multiplexer

14 Linear bus

15 Dual unidirectional ring

16 Breakdown of one ring

17 Bi-directional Ring

18 Switching of data in breakdown ring

19 Mesh topology

20 Star topology

21 chain topologies of ADM

22 Potentially open Firewall to unauthorized users

23 Using a web based service provider

24 Data transaction through firewall


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List of Tables

Table no. Table name Page no.

1 SDA/SONET rates

2 Comparison of cellular, analog modem


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Larsen & Toubro

Larsen & Toubro Limited (L&T) is a USD 8.5 Billion technology, engineering, construction and

manufacturing company. It is one of the largest and most respected companies in India's private

sector and is a company that infuses engineering with imagination.

Seven decades of a strong, customer-focused approach and the continuous quest for world-class

quality have enabled it to attain and sustain leadership in all its major lines of business .

L&T has an international presence, with a global spread of offices. A thrust on international

business has seen overseas earnings grow significantly. It continues to grow its overseas

manufacturing footprint, with facilities in China and the Gulf region.

The company's businesses are supported by a wide marketing and distribution network, and have

established a reputation for strong customer support.

L&T believes that progress must be achieved in harmony with the environment. A commitment to

community welfare and environmental protection are an integral part of the corporate vision .

L&T was founded in Bombay (Mumbai) in 1938 by two Danish engineers, Henning Holck-Larsen

and Soren Kristian Toubro.

Beginning with the import of machinery from Europe, L&T rapidly took on engineering and

construction assignments of increasing sophistication. Today, the company sets global engineering

benchmarks in terms of scale and complexity with Operating Divisions covering:

Engineering & Construction Projects (E&C)

Heavy Engineering (HED)

Engineering Construction & Contracts (ECC)

Electrical & Automation (EBG)

Machinery & Industrial Products (MIPD)

Information Technology & Engineering Services (ITES)


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INTRODUCTION OF OPTICAL FIBER

An optical fiber is made up of the core, (carries the light pulses), the cladding

(reflects the light pulses back into the core) and the buffer coating (protects the core

and cladding from moisture, damage, etc.). Together, all of this creates a fiber optic

which can carry up to 10 million messages at any time using light pulses.

Fiber optics is the overlap ofapplied science and engineering concerned with the

design and application of optical fibers. Optical fibers are widely used in fiber-optic

communications, which permits transmission over longer distances and at higher

bandwidths (data rates) than other forms of communications. Fibers are used insteadof metal wires because signals travel along them with less loss and are also immune

to electromagnetic interference. Fibers are also used for illumination, and are

wrapped in bundles so they can be used to carry images, thus allowing viewing in

tight spaces. Specially designed fibers are used for a variety of other applications,

includingsensors and fiber lasers. In optical fiber communication, Digital signals are

transmitted in the form intensity modulated light signal which is trapped in the glass

core. In optical fiber communication the light is launched into the fiber by using the

light such as LASER and LEDs. Optical fibers are widely used as the backbone of

network. In now days, the Current optical fiber provides transmission rate of 4.5 Mbps

to 9.5 Mbps. Light is kept in the core of the optical fiber by total internal reflection.This causes the fiber to act as a waveguide.

TYPES OF OPTICAL FIBER

There are basically two types of optical fiber.

Single mode optical fiber

Multi-mode optical fiber

Single mode optical fiber: - The optical fiber which supports only one mode is

known as single mode optical fiber. Single-mode fibers are used for most

communication links longer than 550 meters (1,800 ft).

Fiber with a core diameter less than about ten times the wavelength of the

propagating light cannot be modeled using geometric optics. Instead, it must be

analyzed as an electromagnetic structure, by solution of Maxwell's equations as
http://en.wikipedia.org/wiki/Applied_sciencehttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Bandwidth_(computing)http://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Fiber_laserhttp://en.wikipedia.org/wiki/Core_(optical_fiber)http://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Waveguide_(optics)http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Electromagnetismhttp://en.wikipedia.org/wiki/Maxwell's_equationshttp://en.wikipedia.org/wiki/Applied_sciencehttp://en.wikipedia.org/wiki/Engineeringhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Fiber-optic_communicationhttp://en.wikipedia.org/wiki/Bandwidth_(computing)http://en.wikipedia.org/wiki/Attenuationhttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Sensorhttp://en.wikipedia.org/wiki/Fiber_laserhttp://en.wikipedia.org/wiki/Core_(optical_fiber)http://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Waveguide_(optics)http://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Electromagnetismhttp://en.wikipedia.org/wiki/Maxwell's_equations


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reduced to the electromagnetic wave equation. The electromagnetic analysis may

also be required to understand behaviors such as speckle that occur when coherent

light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one

or more confined transverse modes by which light can propagate along the fiber. Fiber

supporting only one mode is called single-mode or mono-mode fiber(as shown in fig

(1)).The most common type of single-mode fiber has a core diameter of 810micrometers and is designed for use in the near infrared. The mode structure

depends on the wavelength of the light used, so that this fiber actually supports a

small number of additional modes at visible wavelengths.

Fig (1)

The structure of a typicalsingle-mode fiber.

1. Core: 8 m diameter

2. Cladding: 125 m dia.

3. Buffer: 250 m dia.

4. Jacket: 400 m dia.

Multi-mode optical fiber: - Fibers which support many propagation paths ortransverse modesare called multi-mode fibers (MMF)(as shown in fig (2)). Multi-mode fibers generally have a

larger core diameter, and are used for short-distance communication links and for applications

where high power must be transmitted.

Fig (2)

There are two types of Multi-mode optical fiber on the basis of the boundary between the core and

cladding: -

Step index multi-mode optical fiber

Graded index multi-mode optical fiber.
http://en.wikipedia.org/wiki/Electromagnetic_wave_equationhttp://en.wikipedia.org/wiki/Specklehttp://en.wikipedia.org/wiki/Coherence_(physics)http://en.wikipedia.org/wiki/Transverse_modehttp://en.wikipedia.org/wiki/Near_infraredhttp://en.wikipedia.org/wiki/Single-mode_fiberhttp://en.wikipedia.org/wiki/Single-mode_fiberhttp://en.wikipedia.org/wiki/Transverse_modehttp://en.wikipedia.org/wiki/Multi-mode_fiberhttp://en.wikipedia.org/wiki/Electromagnetic_wave_equationhttp://en.wikipedia.org/wiki/Specklehttp://en.wikipedia.org/wiki/Coherence_(physics)http://en.wikipedia.org/wiki/Transverse_modehttp://en.wikipedia.org/wiki/Near_infraredhttp://en.wikipedia.org/wiki/Single-mode_fiberhttp://en.wikipedia.org/wiki/Transverse_modehttp://en.wikipedia.org/wiki/Multi-mode_fiber


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Step index multi-mode optical fiber: - In a step-index multi-mode fiber, rays of

light are guided along the fiber core by total internal reflection. Rays that meet the

core-cladding boundary at a high angle (measured relative to a line normal to the

boundary), greater than the critical angle for this boundary, are completely reflected.

The critical angle (minimum angle for total internal reflection) determined by the

difference in index of refraction between the core and cladding materials. Rays thatmeet the boundary at a low angle are refracted from the core into the cladding, and

do not convey light and hence information along the fiber. The critical angle

determines the acceptance angle of the fiber, often reported as a numerical aperture.

A high numerical aperture allows light to propagate down the fiber in rays both close

to the axis and at various angles, allowing efficient coupling of light into the fiber.

However, this high numerical aperture increases the amount of dispersion as rays at

different angles have different path lengths and therefore take different times to

traverse the fiber(as shown in fig(3)).

Fig (3)

Graded index multi -mode optical fiber: - In graded-index fiber, the index of

refraction in the core decreases continuously between the axis and the cladding. This

causes light rays to bend smoothly as they approach the cladding, rather than

reflecting abruptly from the core-cladding boundary. The resulting curved paths

reduce multi-path dispersion because high angle rays pass more through the lower-

index periphery of the core, rather than the high-index center. The index profile is

chosen to minimize the difference in axial propagation speeds of the various rays in

the fiber. This ideal index profile is very close to a parabolic relationship between the

index and the distance from the axis (as shown in fig (4)).

Fig (4)
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Principle of operation

An optical fiber is a cylindricaldielectric waveguide(non conducting waveguide) that transmits

light along its axis, by the process oftotal internal reflection. The fiber consists of a core

surrounded by a claddinglayer, both of which are made ofdielectric materials. To confine the

optical signal in the core, the refractive index of the core must be greater than that of the cladding.The boundary between the core and cladding may either be abrupt, instep-index fiber, or gradual,

ingraded-index fiber

Index of refraction

The index of refraction is a way of measuring the speed of light in a material. Light

travels fastest in a vacuum, such as outer space. The actual speed of light in avacuum is about 300,000 kilometers (186 thousand miles) per second. Index of

refraction is calculated by dividing the speed of light in a vacuum by the speed of

light in some other medium. The index of refraction of a vacuum is therefore 1, by

definition. The typical value for the cladding of an optical fiber is 1.46. The core value

is typically 1.48. The larger the index of refraction, the slower light travels in that

medium. From this information, a good rule of thumb is that signal using optical fiber

for communication will travel at around 200 million meters per second. Or to put it

another way, to travel 1000 kilometers in fiber, the signal will take 5 milliseconds to

propagate. Thus a phone call carried by fiber between Sydney and New York, a 12000

kilometer distance, means that there is an absolute minimum delay of 60 milliseconds

(or around 1/16th of a second) between when one caller speaks to when the other

hears. (Of course the fiber in this case will probably travel a longer route, and there

will be additional delays due to communication equipment switching and the process

of encoding and decoding the voice onto the fiber).

Total internal reflection

When light traveling in a dense medium hits a boundary at a steep angle (larger than

the "critical angle" for the boundary), the light will be completely reflected. This effect

is used in optical fibers to confine light in the core. Light travels along the fiber

bouncing back and forth off of the boundary. Because the light must strike the

boundary with an angle greater than the critical angle, only light that enters the fiber

within a certain range of angles can travel down the fiber without leaking out. This

range of angles is called the acceptance cone of the fiber. The size of this acceptance

cone is a function of the refractive index difference between the fiber's core and

cladding.In simpler terms, there is a maximum angle from the fiber axis at which light

may enter the fiber so that it will propagate, or travel, in the core of the fiber. The

sine of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a
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larger NA requires less precision to splice and work with than fiber with a smaller NA.

Single-mode fiber has a small NA.The behavior of waves in the step index optical

fiber, graded index optical fiber and single mode optical fiber are shown in fig (5).

Fig (5)

Application of optical fiber

Optical fiber communication

Optical fiber can be used as a medium for telecommunication and networking

because it is flexible and can be bundled as cables. It is especially advantageous for

long-distance communications, because light propagates through the fiber with little

attenuation compared to electrical cables. This allows long distances to be spanned

with few repeaters. Additionally, the per-channel light signals propagating in the fiber

have been modulated at rates as high as 111 gigabits per second by NTT, although

10 or 40 Gb/s is typical in deployed systems. Each fiber can carry many independent

channels, each using a different wavelength of light (wavelength-division multiplexing

(WDM)). The net data rate (data rate without overhead bytes) per fiber is the per-

channel data rate reduced by the FEC overhead, multiplied by the number of

channels (usually up to eighty in commercial dense WDM systems as of 2008). The

current laboratory fiber optic data rate record, held by Bell Labs in Villarceaux, France,

is multiplexing 155 channels, each carrying 100 Gb/s over a 7000 km fiber. Nippon

Telegraph and Telephone Corporation have also managed 69.1 Tb/s over a single

240 km fibre (multiplexing 432 channels, equating to 171 Gb/s per channel). Bell Labs

also broke a 100 Petabit per second kilometer barrier (15.5 Tb/s over a single

7000 km fiber).For short distance applications, such as creating a network within anoffice building, fiber-optic cabling can be used to save space in cable ducts. This is

because a single fiber can often carry much more data than many electrical cables,

such as 4 pair Cat-5 Ethernet cabling. Fiber is also immune to electrical interference;

there is no cross-talk between signals in different cables and no pickup of

environmental noise. Non-armored fiber cables do not conduct electricity, which

makes fiber a good solution for protecting communications equipment located in high
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voltage environments such as power generation facilities, or metal communication

structures prone to lightning strikes. They can also be used in environments where

explosive fumes are present, without danger of ignition. Wiretapping is more difficult

compared to electrical connections, and there are concentric dual core fibers that are

said to be tap-proof.

Fiber optic sensors

Fibers have many uses in remote sensing. In some applications, the sensor is itself

an optical fiber. In other cases, fiber is used to connect a non-fiber optic sensor to a

measurement system. Depending on the application, fiber may be used because of its

small size, or the fact that no electrical power is needed at the remote location, or

because many sensors can be multiplexed along the length of a fiber by using

different wavelengths of light for each sensor, or by sensing the time delay as light

passes along the fiber through each sensor. Time delay can be determined using a

device such as an optical time-domain reflectometer.

Optical fibers can be used as sensors to measure strain, temperature, pressure

and other quantities by modifying a fiber so that the quantity to be measured

modulates the intensity, phase, polarization, wavelength or transit time of light in the

fiber. Sensors that vary the intensity of light are the simplest, since only a simple

source and detector are required. A particularly useful feature of such fiber optic

sensors is that they can, if required, provide distributed sensing over distances of up

to one meter.

Extrinsic fiber optic sensors use an optical fiber cable, normally a multi-mode one,

to transmit modulated light from either a non-fiber optical sensor, or an electronic

sensor connected to an optical transmitter. A major benefit of extrinsic sensors is theirability to reach places which are otherwise inaccessible. An example is the

measurement of temperature inside aircraftjet engines by using a fiber to transmit

radiation into a radiation pyrometer located outside the engine. Extrinsic sensors can

also be used in the same way to measure the internal temperature of electrical

transformers, where the extreme electromagnetic fields present make other

measurement techniques impossible. Extrinsic sensors are used to measure vibration,

rotation, displacement, velocity, acceleration, torque, and twisting.

Optical fiber communication

In recent years it has become apparent that fiber-optics are steadily replacing copper

wire as an appropriate means of communication signal transmission. They span the

long distances between local phone systems as well as providing the backbone for

many network systems. Other system users include cable television services,

university campuses, office buildings, industrial plants, and electric utility companies.

A fiber-optic system is similar to the copper wire system that fiber-optics is replacing.
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The difference is that fiber-optics use light pulses to transmit information down fiber

lines instead of using electronic pulses to transmit information down copper lines.

Looking at the components in a fiber-optic chain will give a better understanding of

how the system works in conjunction with wire based systems.

At one end of the system is a transmitter. This is the place of origin for informationcoming on to fiber-optic lines. The transmitter accepts coded electronic pulse

information coming from copper wire. It then processes and translates that

information into equivalently coded light pulses. A light-emitting diode (LED) or an

injection-laser diode (ILD) can be used for generating the light pulses. Using a lens,

the light pulses are funneled into the fiber-optic medium where they travel down the

cable. The light (near infrared) is most often 850nm for shorter distances and

1,300nm for longer distances on Multi-mode fiber and 1300nm for single-mode fiber

and 1,500nm is used for longer distances. Think of a fiber cable in terms of very long

cardboard roll (from the inside roll of paper towel) that is coated with a mirror on the

inside.

If you shine a flashlight in one end you can see light come out at the far end - even if

it's been bent around a corner. Light pulses move easily down the fiber-optic line

because of a principle known as total internal reflection. "This principle of total

internal reflection states that when the angle of incidence exceeds a critical value,

light cannot get out of the glass; instead, the light bounces back in. When this

principle is applied to the construction of the fiber-optic strand, it is possible to

transmit information down fiber lines in the form of light pulses. The core must a very

clear and pure material for the light or in most cases near infrared light (850nm,

1300nm and 1500nm). The core can be Plastic (used for very short distances) but

most are made from glass. Glass optical fibers are almost always made from pure

silica, but some other materials, such as fluorozirconate, fluoroaluminate, andchalcogenide glasses, are used for longer-wavelength infrared applications. There are

three types of fiber optic cable commonly used: single mode, multimode and plastic

optical fiber (POF).Transparent glass or plastic fibers which allow light to be guided

from one end to the other with minimal loss(as shown in fig (8)).

Fig (6)

Fiber optic cable functions as a "light guide," guiding the light introduced at one end

of the cable through to the other end. The light source can either be a light-emitting

diode (LED)) or a lasers. The light source is pulsed on and off, and a light-sensitive
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receiver on the other end of the cable converts the pulses back into the digital ones

and zeros of the original signal(as shown in fig (9)).

Fig (7)

Even laser light shining through a fiber optic cable is subject to loss of strength,

primarily through dispersion and scattering of the light, within the cable itself. The

faster the laser fluctuates, the greater the risk of dispersion. Light strengtheners,

called repeaters, may be necessary to refresh the signal in certain applications.

While fiber optic cable itself has become cheaper over time - a equivalent length ofcopper cable cost less per foot but not in capacity. Fiber optic cable connectors and

the equipment needed to install them are still more expensive than their copper

counterparts.

Plesiochronous digital hierarchy (PDH)

The Plesiochronous Digital Hierarchy (PDH) is a technology used in

telecommunications networks to transport large quantities of data over digital

transport equipment such as fiber optic and microwave radio systems. The term

plesiochronous is derived from Greek plesio, meaning near, and chronos, time, and

refers to the fact that PDH networks run in a state where different parts of the
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network are nearly, but not quite perfectly, synchronized.PDH is typically being

replaced by Synchronous Digital Hierarchy (SDH) or Synchronous optical networking

(SONET) equipment in most telecommunications networks.PDH allows transmission of

data streams that are nominally running at the same rate, but allowing some

variation on the speed around a nominal rate. By analogy, any two watches are

nominally running at the same rate, clocking up 60 seconds every minute. However,there is no link between watches to guarantee they run at exactly the same rate, and

it is highly likely that one is running slightly faster than the other. The development of

digital transmission systems started in the early 70s, and was based on the Pulse

Code Modulation (PCM) method. In the early 80's digital systems became more

and more complex, yet there was huge demand for some features that were not

supported by the existing systems.

The demand was mainly to high order multiplexing through a hierarchy of increasing

bit rates up to 140 Mbps or 565 Mbps in Europe. The problem was the high cost of

bandwidth and digital devices. The solution that was created then, was a

multiplexing technique, allowed for the combining of slightly non synchronous rates,

referred to as plesiochronous, which lead to the term plesiochronous digital

hierarchy (PDH).

Weaknesses of PDH were: -

No world standard on digital format (three incompatible regional standards - European,North American and Japanese).

No world standard for optical interfaces. Networking is impossible at the optical level.

Rigid asynchronous multiplexing structure.

Limited management capability.

Implementation of PDH

The basic data transfer rate is a data stream of 2048 Kbit/s. For speech transmission,

this is broken down into thirty 64 Kbit/s channels plus two 64 Kbit/s channels used for

signaling and synchronization. Alternatively, the entire bandwidth may be used for

non-speech purposes, for example, data transmission.

The data rate is controlled by a clock in the equipment generating the data. The rateis allowed to vary by 50 ppm of 2.048 Mbit/s. This means that different data

streams can be (probably are) running at slightly different rates to one another.

In order to move multiple data streams from one place to another, they are

multiplexed in groups of four. This is done by taking 1 bit from stream #1, followed by

1 bit from stream #2, then #3, then #4. The transmitting multiplexer also adds

additional bits in order to allow the far end receiving multiplexer to decode which bits
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belong to which data stream, and so correctly reconstitute the original data streams.

These additional bits are called "justification" or "stuffing" bits.

Because each of the four data streams is not necessarily running at the same rate,

some compensation has to be introduced. The transmitting multiplexer combines the

four data streams assuming that they are running at their maximum allowed rate.This means that occasionally, (unless the 2 Mbit/s really is running at the maximum

rate) the multiplexer will look for the next bit but it will not have arrived. In this case,

the multiplexer signals to the receiving multiplexer that a bit is "missing". This allows

the receiving multiplexer to correctly reconstruct the original data for each of the four

2 Mbit/s data streams, and at the correct, different, plesiochronous rates. The

resulting data stream from the above process runs at 8,448 Kbit/s (about 8 Mbit/s).

Similar techniques are used to combine four 8 Mbit/s together, plus bit stuffing,

giving 34 Mbit/s. Four 34 Mbit/s gives 140. Four 140 gives 565. 565 Mbit/s is the

rate typically used to transmit data over a fiber optic system for long distance

transport. Recently, telecommunications companies have been replacing their PDH

equipment with SDH equipment capable of much higher transmission rates. 2.048

Mbit/s 8.448 Mbit/s 34.368 Mbit/s 139.264 Mbit/s Multiplex levels

Synchronous optical network/synchronous digital

hierarchy
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SDH (Synchronous Digital Hierarchy) is an international standard for high speed

telecommunication over optical/electrical networks which can transport digital signals

in variable capacities. It is a synchronous system which intends to provide a more

flexible, yet simple network infrastructure.

SONET was developed in the United States through ANSI T1X1.5 committee.ANSI work commenced in 1985 with the CCITT (now ITU) initiating astandardization effort in 1986. The US wanted a data rate close to 50Mbps. Butthe Europeans wanted the data rate to be around 150 Mbps. A compromisewas reached and the US data rates were made subset of ITU specification,known formally as Synchronous Digital Hierarchy (SDH).

SONET/SDH networks are configured as linear networks, whereSONET/SDH nodes knows as Add Drop Multiplexers (ADMs) are hookedtogether in a line as shown in figure-1. There may be two or four fibers

between the two consecutive ADMs with one set serving as protection orback up.

Add/drop multiplexers (ADMs) are places where traffic enters andleaves(as shown In fig (10)). The traffic can be at various levels in the SONET/SDH hierarchy (see Table-1). We will learn more about ADMs later.

Fig (8)

Also SONET network elements can receive signals from a variety of facilities such as DS1, DS3,ATM, Internet, and LAN/MAN/WAN. They can also receive signals from a variety of networktopologies. We will study how all this is done in subsequent sections. In addition SDH signals mayalso be connected with a SONET and vice versa. In this case, circuitry translates specific SDHinformation into its SONET equivalent, and vice versa.

SONET/SDH Rates:

The SONET frame in its electrical nature is called Synchronous Transport Signal-level N(STS-N). The SDH equivalent is called Synchronous Transport Module level N (STM-N). Afterconversion into optical pulses it is known as Optical Carrier level N. The line rates for differentlevels of SONET and SDH signals are shown in Table-1 below.


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signal Designation Line Rate

(Mbp

s)SONET SDH Optical

STS-1

STS-3

STS-12

STS-48

STS-192

STM-0

STM-1

STM-4

STM-16

STM-64

OC-1

OC-3

OC-12

OC-18

OC-192

51.85

155.52

622.08

2488.32

9953.28

Table-1:


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You need not worry about the different levels of SONET /SDH at this stage. I had given detailedexplanation of these levels later. I feel, to understand SDH easily, it is better to have knowledge ofSONET initially. This is the reason I devoted major portion of this article to SONET. Except interms of terminology there are no major differences between the two.

Synchronous Optical Networking (SONET) or Synchronous Digital Hierarchy (SDH) arestandardized multiplexing protocols that transfer multiple digital bit streams overoptical fiberusing lasers orlight-emitting diodes (LEDs). Lower rates can also be transferred via an electricalinterface. The method was developed to replace the Plesiochronous Digital Hierarchy (PDH)system for transporting larger amounts oftelephone calls and data traffic over the same fiber wirewithout synchronization problems. SONET generic criteria are detailed in Telcordia TechnologiesGeneric Requirements document GR-253-CORE. Generic criteria applicable to SONET and othertransmission systems (e.g., asynchronous fiber optic systems or digital radio systems) are found inTelcordia GR-499-CORE.SONET and SDH, which is basically the same, were originally designed to transport

circuit mode communications (e.g., T1, T3) from a variety of different sources. Theprimary difficulty in doing this prior to SONET/SDH was that the synchronization

sources of these different circuits were different. This meant each circuit was actually

operating at a slightly different rate and with different phase. SONET/SDH allowed for

the simultaneous transport of many different circuits of differing origin within one

single framing protocol. In a sense, then, SONET/SDH is not itself a communications

protocolper se, but a transport protocol.

Due to SONET/SDH's essential protocol neutrality and transport-oriented features,

SONET/SDH was the obvious choice for transporting Asynchronous Transfer Mode

(ATM) frames. It quickly evolved mapping structures and concatenated payload

containers to transport ATM connections. In other words, for ATM (and eventuallyother protocols such as TCP/IP and Ethernet), the internal complex structure

previously used to transport circuit-oriented connections is removed and replaced

with a large and concatenated frame (such as STS-3c) into which ATM frames, IP

packets, or Ethernet are placed.

Both SDH and SONET are widely used today. SONET in the U.S. and Canada and SDH

in the rest of the world. Although the SONET standards were developed before SDH,

their relative penetrations in the worldwide market dictate that SONET is considered

the variation.

The two protocols are standardized according to the following:

Synchronous Digital Hierarchy (SDH) standard was originally defined by the ETSI orEuropean Telecommunications Standards Institute

Synchronous Optical Networking (SONET) standard as defined by GR-253-CORE

fromTelcordia andT1.105 from American National Standards Institute
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We can use SDH when: -

When networks need to increase capacity, SDH simply acts as a means ofincreasing transmission capacity.

When networks need to improve flexibility, to provide services quickly or torespond to new change more rapidly.

when networks need to improve survivability for important user services.

When networks need to reduce operation costs, which are becoming a heavyburden.

Structure of SONET/SDH signals

SONET and SDH often use different terms to describe identical features or functions.

This can cause confusion and exaggerate their differences. With a few exceptions,

SDH can be thought of as a superset of SONET.

The protocol is an extremely heavily multiplexed structure, with the header

interleaved between the data in a complex way. This is intended to permit the

encapsulated data to have its own frame rate and to be able to float around relative

to the SDH/SONET frame structure and rate. This interleaving permits a very low

latency for the encapsulated data. Data passing through equipment can be delayed

by at most 32 microseconds, compared to a frame rate of 125 microseconds and

many competing protocols buffer the data for at least one frame or packet before

sending it on. Extra padding is allowed for the multiplexed data to move within the

overall framing because it being on a different clock than the frame rate. The decision

to allow this at most of the levels of the multiplexing structure makes the protocolcomplex, but gives high all-around performance.

Difference from PDH

Synchronous networking differs from Plesiochronous Digital Hierarchy (PDH) in that

the exact rates that are used to transport the data are tightly synchronized across the

entire network, using atomic clocks. This synchronization system allows entire inter-

country networks to operate synchronously, greatly reducing the amount of buffering

required between elements in the network.

Both SONET and SDH can be used to encapsulate earlier digital transmission

standards, such as the PDH standard, or used directly to support either Asynchronous

Transfer Mode (ATM) or so-called packet over SONET/SDH (POS) networking. As such,

it is inaccurate to think of SDH or SONET as communications protocols in and of

themselves, but rather as generic and all-purpose transport containers for moving
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both voice and data. The basic format of an SDH signal allows it to carry many

different services in its virtual container (VC) because it is bandwidth-flexible.

The basic unit of transmission

The basic unit of framing in SDH is a STM-1 (Synchronous Transport Module level 1),

which operates at 155.52 Mbps. SONET refers to this basic unit as an STS-3c

(synchronous transport signal - 3, concatenated), but its high-level functionality,

frame size, and bit-rate are the same as STM-1.

SONET offers an additional basic unit of transmission, the STS-1 (synchronous

transport signal - 1), operating at 51.84 Mbps - exactly one third of an STM-1/STS-3c.

That is, in SONET the associated OC-3 signal will be composed of three STS-1s (or,

more recently in packet transport, the OC-3 signal will carry a single concatenated

STS-3c.) Some manufacturers also support the SDH equivalent: STM-0.

FramingIn packet-oriented data transmission such as Ethernet, a packet frame usually

consists of a header and a payload. The header is transmitted first, followed by the

payload (and possibly a trailer, such as a CRC). In synchronous optical networking,

this is modified slightly. The header is termed the overhead and instead of being

transmitted before the payload, is interleaved with it during transmission. Part of the

overhead is transmitted, then part of the payload, then the next part of the overhead,

then the next part of the payload, until the entire frame has been transmitted. In the

case of an STS-1, the frame is 810 octets in size while the STM-1/STS-3c frame is

2430 octets in size. For STS-1, the frame is transmitted as 3 octets of overhead,

followed by 87 octets of payload. This is repeated nine times over until 810 octets

have been transmitted, taking 125 microseconds. In the case of an STS-3c/STM-1

which operates three times faster than STS-1, 9 octets of overhead are transmitted,

followed by 261 octets of payload. This is also repeated nine times over until 2,430

octets have been transmitted, also taking 125 microseconds. For both SONET and

SDH, this is normally represented by the frame being displayed graphically as a block:

of 90 columns and 9 rows for STS-1; and 270 columns and 9 rows for STM1/STS-3c.

This representation aligns all the overhead columns, so the overhead appears as a

contiguous block, as does the payload.

The internal structure of the overhead and payload within the frame differs slightly

between SONET and SDH, and different terms are used in the standards to describe

these structures. Their standards are extremely similar in implementation making it

easy to interoperate between SDH and SONET at particular bandwidths.

In practice, the terms STS-1 and OC-1 are sometimes used interchangeably, though

the OC-N format refers to the signal in its optical form. It is therefore incorrect to say

that an OC-3 contains 3 OC-1s: an OC-3 can be said to contain 3 STS-1s.
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SDH frame

Fig (9)

A STM-1 Frame. The first 9 columns contain the overhead and the pointers. For the sake of

simplicity, the frame is shown as a rectangular structure of 270 columns and 9 rows but, in

practice, the protocol does not transmit the bytes in this order.

Fig (10)

For the sake of simplicity, the frame is shown as a rectangular structure of 270 columns and 9

rows. The first 3 rows and 9 columns contain regenerator section overhead (RSOH) and the last 5

rows and 9 columns contain multiplex section overhead (MSOH). The 4th row from the top

contains pointers

The STM-1 (synchronous transport module level - 1) frame is the basic transmission

format for SDH or the fundamental frame or the first level of the synchronous digital

hierarchy. The STM-1 frame is transmitted in exactly 125 microseconds, therefore

there are 8000 frames per second on a fiber-optic circuit designated OC-3 (OpticalCarrier-3). The STM-1 frame consists of overhead and pointers plus information

payload. The first 9 columns of each frame make up the Section Overhead and

Administrative Unit Pointers, and the last 261 columns make up the Information

Payload. The pointers (H1, H2, H3 bytes) identify administrative units (AU) within the

information payload.
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Carried within the information payload, which has its own frame structure of 9 rows

and 261 columns, are administrative units identified within the information payload

by pointers. Within the administrative unit is one or more virtual containers (VC). VCs

contain path overhead and VC payload. The first column is for path overhead; its

followed by the payload container, which can itself carry other containers.

Administrative units can have any phase alignment within the STM frame, and thisalignment is indicated by the pointer in row four,

The section overhead of a STM-1 signal (SOH) is divided into two parts: the

regenerator section overhead (RSOH) and the multiplex section overhead (MSOH).

The overheads contain information from the system itself, which is used for a wide

range of management functions, such as monitoring transmission quality, detecting

failures, managing alarms, data communication channels, service channels, etc.

The STM frame is continuous and is transmitted in a serial fashion, byte-by-byte, row-

by-row.

STM1 frame contains

1 octet = 8 bit

Total content : 9 x 270 octets = 2430 octets

overhead : 8 rows x 9 octets

pointers : 1 row x 9 octets

payload : 9 rows x 261 octets

Period : 125 sec

Bitrate : 155.520 Mbps (2430 octets x 8 bits x 8000 frame/s )or 270*9*64Kbps :

155.52Mbps

Actual payload capacity : 150.336 Mbps (2349 x 8 bits x 8000 frame/s)

The transmission of the frame is done row by row, from the left to right and top to

bottom.

Transport overhead

The transport overhead is used for signaling and measuring transmission error rates,

and is composed as follows:

Section overhead- called RSOH (regenerator section overhead) in SDH terminology: 27

octets containing information about the frame structure required by the terminal equipment.

Line overhead - called MSOH (multiplex section overhead) in SDH: 45 octets containing

information about alarms, maintenance and error correction as may be required within the network.

Pointer It points to the location of the J1 byte in the payload.
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Path virtual overhead

Data transmitted from end to end is referred to as path data. It is composed of two

components:

Payload overhead (POH): 9 octets used for end to end signaling and error measurement.

Payload: user data (774 bytes for STM-0/STS-1, or 2340 octets for STM-1/STS-3c)

For STS-1, the payload is referred to as the synchronous payload envelope (SPE),

which in turn has 18 stuffing bytes, leading to the STS-1 payload capacity of 756

bytes.[1]

The STS-1 payload is designed to carry a full PDH DS3 frame. When the DS3 enters a

SONET network, path overhead is added, and that SONET network element (NE) is

said to be a path generator and terminator. The SONET NE is said to be line

terminating if it processes the line overhead. Note that wherever the line or path isterminated, the section is terminated also. SONET regenerators terminate the

section but not the paths or line.

An STS-1 payload can also be subdivided into 7 VTGs (virtual tributary groups). Each

VTG can then be subdivided into 4 VT1.5 signals, each of which can carry a PDH DS1

signal. A VTG may instead be subdivided into 3 VT2 signals, each of which can carry a

PDH E1 signal. The SDH equivalent of a VTG is aTUG2; VT1.5 is equivalent to VC11,

and VT2 is equivalent toVC12.

Three STS-1 signals may be multiplexed by time-division multiplexing to form the next

level of the SONET hierarchy, the OC-3 (STS-3), running at 155.52 Mbps. Themultiplexing is performed by interleaving the bytes of the three STS-1 frames to form

the STS-3 frame, containing 2,430 bytes and transmitted in 125 microseconds.Higher

speed circuits are formed by successively aggregating multiples of slower circuits,

their speed always being immediately apparent from their designation. For example,

four STS-3 or AU4 signals can be aggregated to form a 622.08 Mbps signal designated

as OC-12 or STM-4.The highest rate that is commonly deployed is the OC-192 or STM-

64 circuit, which operates at rate of just under 10 Gbps. Speeds beyond 10 Gbps are

technically viable and are under evaluation. [Few vendors are offering STM-256 rates

now, with speeds of nearly 40Gbps]. Where fiber exhaustion is a concern, multiple

SONET signals can be transported over multiple wavelengths over a single fiber pair

by means of wavelength-division multiplexing, including dense wavelength division

multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM). DWDM

circuits are the basis for all modern transatlantic cable systems and other long-haul

circuits.

Advantages of SDH
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First world standard in digital format.

First optical Interfaces.

Transversal compatibility reduces networking cost. Multivendor environment drives pricedown

Flexible synchronous multiplexing structure. Easy and cost-efficient traffic add-and-drop and cross connect capability.

Reduced number of back-to-back interfaces improves network reliability and serviceability.

Powerful management capability.

New network architecture. Highly flexible and survivable self healing rings available.

Backward and forward compatibility: Backward compatibility to existing PDH

Forward compatibility to future B-ISDN, etc

Layered model of SDH

The layers of SDH according to the OSI model:

Fig (11)

SDH elements

The most common SDH elements are :


28/48

The terminal multiplexer is used to multiplex local tributaries (low rate) to the stm-N

(high rate) aggregate(as shown in fig(11)). The terminal is used in the chain topology

as an end element.

Fig (12)

The regenerator is used to regenerate the (high rate) stm-N in case that the distance

between two sites is longer than the transmitter can carry.

The Add and Drop Multiplexer (ADM) passes the (high rate) stm-N through from his

one side to the other and has the ability to drop or add any (low rate) tributary (as

shown in fig (12)). The ADM used in all topologies.

Fig (13)

The synchronous digital cross connect receives several (high rate) stm-N and switches

any of their (low rate) tributaries between them. It is used to connect between several

topologies.

Topologies of SDH

The linear bus (chain) topology used when there is no need for protection and the

demography of the sites is linear(as shown in fig (13)).


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Fig (14)

The ring topology is the most common and known of the sdh topologies

it allows great network flexibility and protection.

The protected ring topologies are:-In fig (14) we can see Dual Unidirectional

Ring . The normal data flow is according to ring A (red). Ring B (blue) carries

unprotected data which is lost in case of breakdown or it carries no data at all.

Fig (15)

In case of breakdown rings A & B become one ring without the broken segment, as

shown in fig(15).

Fig (16)


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The Bi-directional Ring allows data flow in both directions. For example if data from

one of the sites has to reach a site which is next to the left of the origin site it will flow

to the left instead of doing a whole cycle to the right as shown in fig(16).

Fig(17)

In case of breakdown some of the data is lost and the important data is

switched. For example if data from a site should flow to its destination through the

broken segment, it will be switched to the other side instead the broken segment, as

shown in fig (17).

Fig (18)

The mesh topology allows even the most paranoid network manager to sleep well at

nights Because of the flexibility and redundancy that it gives


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Fig (19)

The Star topology is used for connecting far and less important sites to the network

Fig (20)

Usage of SDH elements in SDH topologies

The Terminal multiplexer can be used to connect two sites in a high rate

connection.

The Add and Drop Multiplexer (ADM) is used to build the chain topologies in the fig

(12). At the ends of the chain usually a Terminal Multiplexer is connected.

The Add and Drop Multiplexer (ADM) is used to build the ring topology. At each site we

have the ability to add & drop certain tributaries, as shown in fig(18).


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Fig (21)

Next-generation SONET/SDH

SONET/SDH development was originally driven by the need to transport multiple PDH

signals like DS1, E1, DS3 and E3 along with other groups of multiplexed 64 kbps

pulse-code modulated voice traffic. The ability to transport ATM traffic was another

early application. In order to support large ATM bandwidths, the technique of

concatenation was developed, whereby smaller multiplexing containers (eg, STS-1)

are inversely multiplexed to build up a larger container (eg, STS-3c) to support large

data-oriented pipes.

One problem with traditional concatenation, however, is inflexibility. Depending on

the data and voice traffic mix that must be carried, there can be a large amount of

unused bandwidth left over, due to the fixed sizes of concatenated containers. Forexample, fitting a 100 Mbps Fast Ethernet connection inside a 155 Mbps STS-3c

container leads to considerable waste. More important is the need for all intermediate

NEs to support the newly introduced concatenation sizes. This problem was later

overcome with the introduction of Virtual Concatenation.

Virtual concatenation (VCAT) allows for a more arbitrary assembly of lower order

multiplexing containers, building larger containers of fairly arbitrary size (e.g., 100

Mbit/s) without the need for intermediate NEs to support this particular form of

concatenation. Virtual Concatenation increasingly leverages X.86 or Generic Framing

Procedure (GFP) protocols in order to map payloads of arbitrary bandwidth into the

virtually concatenated container.

Link Capacity Adjustment Scheme (LCAS) allows for dynamically changing the

bandwidth via dynamic virtual concatenation, multiplexing containers based on the

short-term bandwidth needs in the network.The set of next generation SONET/SDH

protocols to enable Ethernet transport is referred to as Ethernet over SONET/SDH

(EoS).
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Wireless communication

Wireless communication is the transfer of information over a distance without the use ofenhanced electrical conductors or "wires. The distances involved may be short (a few meters as intelevision remote control) or long (thousands or millions of kilometers for radio communications).When the context is clear, the term is often shortened to "wireless". Wireless communication isgenerally considered to be a branch oftelecommunications.

It encompasses various types of fixed, mobile, and portable two-way radios, cellular telephones,personal digital assistants (PDAs), and wireless networking. Other examples of wirelesstechnology include GPS units, garage door openers and or garage doors, wireless computer mice,keyboards and headsets, satellite televisionand cordlesstelephones.

Wireless operations permits services, such as long range communications, that are impossible or

impractical to implement with the use of wires. The term is commonly used in thetelecommunications industry to refer to telecommunications systems (e.g. radio transmitters andreceivers, remote controls, computer networks, network terminals, etc.) which use some form ofenergy (e.g. radio frequency (RF), infrared light, laserlight, visible light, acoustic energy, etc.) totransfer information without the use of wires. Information is transferred in this manner over bothshort and long distances.

The term "wireless" has become a generic and all-encompassing word used to describecommunications in which electromagnetic waves or RF (rather than some form of wire) carry a
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signal over part or the entire communication path. Common examples of wireless equipment in usetoday include:

Professional LMR (Land Mobile Radio) and SMR (Specialized Mobile Radio) typicallyused by business, industrial and Public Safety entities.

ConsumerTwo Way Radio including FRS (Family Radio Service), GMRS (General MobileRadio Service) and Citizens band ("CB") radios.

The Amateur Radio Service (Ham radio).

Consumer and professional Marine VHF radios.

Cellular telephones and pagers: provide connectivity for portable and mobile applications,both personal and business.

Global Positioning System (GPS): allows drivers of cars and trucks, captains of boats andships, and pilots of aircraft to ascertain their location anywhere on earth.

Cordless computer peripherals: the cordless mouse is a common example; keyboards andprinters can also be linked to a computer via wireless.

Cordless telephone sets: these are limited-range devices, not to be confused with cellphones.

Satellite television: allows viewers in almost any location to select from hundreds ofchannels.

Wireless gaming: new gaming consoles allow players to interact and play in the same gameregardless of whether they are playing on different consoles. Players can chat, send text messagesas well as record sound and send it to their friends. Controllers also use wireless technology. Theydo not have any cords but they can send the information from what is being pressed on thecontroller to the main console which then processes this information and makes it happen in thegame. All of these steps are completed in milliseconds.

Wireless networking (i.e. the various types of unlicensed 2.4 GHz Wi-Fi devices) is used tomeet many needs. Perhaps the most common use is to connect laptop users who travel fromlocation to location. Another common use is for mobile networks that connect via satellite. Awireless transmission method is a logical choice to network a LAN segment that must frequentlychange locations. The following situations justify the use of wireless technology:

To span a distance beyond the capabilities of typical cabling,

To provide a backup communications link in case of normal network failure,

To link portable or temporary workstations,

To overcome situations where normal cabling is difficult or financially impractical, or

To remotely connect mobile users or networks.

Applications of wireless technology

Security systems: - Wireless technology may supplement or replace hard wired implementationsin security systems for homes or office buildings.
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Television remote control: - Modern televisions use wireless (generally infrared) remote controlunits. Now radio waves are also used.

Cellular telephone (phones and modems): - These instruments use radio waves to enable theoperator to make phone calls from many locations worldwide. They can be used anywhere thatthere is a cellular telephone site to house the equipment that is required to transmit and receive the

signal that is used to transfer both voice and data to and from these instruments.

Wi-Fi: - Wi-Fi is a wireless local area networkthat enables portable computing devices to connecteasily to the Internet. Standardized as IEEE 802.11 a, b, g, n, Wi-Fi approaches speeds of sometypes of wired Ethernet. Wi-Fi hot spots have been popular over the past few years. Somebusinesses charge customers a monthly fee for service, while others have begun offering it for freein an effort to increase the sales of their goods.

Wireless energy transfer: - Wireless energy transfer is a process whereby electrical energy istransmitted from a power source to an electrical load that does not have a built-in power source,without the use of interconnecting wires.

Computer Interface Devices: - Answering the call of customers frustrated with cord clutter, many

manufactures of computer peripherals turned to wireless technology to satisfy their consumer base.Originally these units used bulky, highly limited transceivers to mediate between a computer and akeyboard and mouse, however more recent generations have used small, high quality devices,some even incorporating Bluetooth. These systems have become so ubiquitous that some usershave begun complaining about a lack of wired peripherals. Wireless devices tend to have a slightlyslower response time than their wired counterparts, however the gap is decreasing. Initial concernsabout the security of wireless keyboards have also been addressed with the maturation of thetechnology.

Internet

The Internet is a global system of interconnected computer networks that use the standard InternetProtocol Suite (TCP/IP) to serve billions of users worldwide. It is a network of networks thatconsists of millions of private, public, academic, business, and government networks of local toglobal scope that are linked by a broad array of electronic and optical networking technologies.The Internet carries a vast array of information resources and services, most notably the inter-linked hypertext documents of the World Wide Web (WWW) and the infrastructure to supportmail. Most traditional communications media, such as telephone and television services, arereshaped or redefined using the technologies of the Internet, giving rise to services such as Voiceover Internet Protocol (VoIP) and IPTV. Newspaper publishing has been reshaped into Web sites,blogging, and web feeds. The Internet has enabled or accelerated the creation of new forms ofhuman interactions throughinstant messaging, Internet forums, and social networking sites. Theorigins of the Internet reach back to the 1960s when the United States funded research projects ofits military agencies to build robust, fault-tolerant and distributed computer networks. Thisresearch and a period of civilian funding of a new U.S. backbone by the National ScienceFoundation spawned worldwide participation in the development of new networking technologiesand led to the commercialization of an international network in the mid 1990s, and resulted in thefollowing popularization of countless applications in virtually every aspect of modern human life.As of 2009, an estimated quarter of Earth's population uses the services of the Internet. TheInternet has no centralized governance in either technological implementation or policies foraccess and usage; each constituent network sets its own standards. Only the overreaching
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definitions of the two principal name spaces in the Internet, the Internet Protocol address space andthe Domain Name System, are directed by a maintainer organization, the Internet Corporation forAssigned Names and Numbers (ICANN). The technical underpinning and standardization of thecore protocols (IPv4 and IPv6) is an activity of the Internet Engineering Task Force (IETF), a non-profit organization of loosely affiliated international participants that anyone may associate with by

contributing technical expertise.

Internet vs. Web

The terms Internet and World Wide Web are often used in everyday speech without muchdistinction. However, the Internet and the World Wide Web are not one and the same. The Internetis a global data communications system. It is a hardware and software infrastructure that providesconnectivity between computers. In contrast, the Web is one of the services communicated via theInternet. It is a collection of interconnected documents and otherresources, linked byhyperlinksand URLs.

Internet technology

The complex communications infrastructure of the Internet consists of its hardware componentsand a system of software layers that control various aspects of the architecture. While the hardwarecan often be used to support other software systems, it is the design and the rigorousstandardization process of the software architecture that characterizes the Internet and provides thefoundation for its scalability and success. The responsibility for the architectural design of theInternet software systems has been delegated to the Internet Engineering Task Force (IETF).[9] TheIETF conducts standard-setting work groups, open to any individual, about the various aspects ofInternet architecture. Resulting discussions and final standards are published in a series ofpublications, each called a Request for Comments (RFC), freely available on the IETF web site.The principal methods of networking that enable the Internet are contained in specially designatedRFCs that constitute the Internet Standards. Other less rigorous documents are simply informative,experimental, or historical, or document the best current practices (BCP) when implementingInternet technologies.

The Internet Standards describe a framework known as the Internet Protocol Suite. This is a modelarchitecture that divides methods into a layered system of protocols (RFC 1122, RFC 1123). Thelayers correspond to the environment or scope in which their services operate. At the top is theApplication Layer, the space for the application-specific networking methods used in softwareapplications, e.g., a web browser program. Below this top layer, the Transport Layer connectsapplications on different hosts via the network (e.g., clientserver model) with appropriate dataexchange methods. Underlying these layers are the core networking technologies, consisting oftwo layers. The Internet Layer enables computers to identify and locate each other via Internet

Protocol (IP) addresses, and allows them to connect to one-another via intermediate (transit)networks. Lastly, at the bottom of the architecture, is a software layer, the Link Layer, thatprovides connectivity between hosts on the same local network link, such as a local area network(LAN) or a dial-up connection. The model, also known asTCP/IP, is designed to be independentof the underlying hardware which the model therefore does not concern itself with in any detail.Other models have been developed, such as the Open Systems Interconnection (OSI) model, butthey are not compatible in the details of description, nor implementation, but many similaritiesexist and the TCP/IP protocols are usually included in the discussion of OSI networking.
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