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Telecom Essentials




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Communication Basics




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what is communication?

Communication is the process of

exchanging information between two

different points.

The information may be sent from one

point to another point through a wire /

cable or it may be sent on a particular

frequency (wireless) on air




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• Any Communication System requires the

following:

Transmitter

Channel

Receiver




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Transmitter

• It is the one which transmits the

information after processing the

normal signal (modulating signal) to

modulated signal.

• It has some elements like modulator

to combine low frequency

information with RF carrier and anamplifier to amplify the signal before

giving it to the antenna.




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Receiver

• It is the one, which receives themodulated information and retrieves

the actual information from the

modulated signal.• It has an amplifier to amplify the

weak signal and a demodulator to

separate the actual information fromthe modulated information.




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Channel

• It is the medium through which theinformation is being carried out.

• If the information is being carried

over wires, it is called wire line

communication.

• If the information is carried over aparticular frequency on air, it is called

wire less communication.




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Types of communication Systems :

Type Description Example

Simplex One way only FM radio,

Television etc..

Half

Duplex

Two way, only one at a

time

Walkie Talkie

Full

Duplex

Two way, both at the

same time

Mobile

Systems




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Simplex System

It is one way communication. It can

either transmit or receive. But only onefunction it does at any time.

Ex: Radio, Television, GPS Rx etc.




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Half Duplex System

It can do both the functions. i.e., it can

transmit as well as receive. But only onefunction at a time. That means, if it is

transmitting, it works like a transmitter and if it

is receiving, it works like a receiver.

• Ex: Police Walkie-Talkie




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Full Duplex System

It can do both the (Tx and Rx) functions at a

time.

• Ex: Telephone and mobile systems




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Full Duplex System




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DIFFERENCES BETWEEN WIRED & WIRELESS

COMMUNICATIONS:

• The main difference between the wired andwireless communication infrastructure is

the existence of the physical cabling.

• Wired communication consists of aphysical cable between two points,

whereas wireless communication doesn‟t

have any physical cabling between thepoints. The information will be sent on a

particular frequency over air.




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• FREQUENCY: The number of cycles per unit of

time is called the frequency.

frequency is most often measured in cycles

per second (cps) or Hertz (Hz) (60 cps = 60 Hz)

1000 Hz is often referred to as 1 kHz (kilohertz).




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velocity: Distance travelled per unit time

wavelength:

wavelength is the distance between

repeating units of a propagating wave of a

given frequency. It is commonly denoted by thegreek letter (λ). Examples of wave-like

phenonomena are light waves and sound

waves.

• Note: The wavelength, (λ)., is related to the

propagation velocity, v , and the frequency (f),

by = v /f .




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BANDWIDTH

Bandwidth is expressed in terms of thedifference between the highest-frequencysignal component and the lowest-frequencysignal component. Since the frequency of a

signal is measured in Hertz (the number of cycles of change per second), a givenbandwidth is the difference in hertzbetween the highest frequency the signaluses and the lowest frequency it uses.

(or)




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BW of a signal can be defined as the

range of frequencies in which most of theenergy or Power lies.

• For a Band limited signal the BW is welldefined.

• When a signal is not band limiteddefining BW becomes difficult and there isno universal definition. Thus this givesrise to BW Dilemma.




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Frequency Spectrum and its Applications

Range Frequency Band Application

VLF 3 KHz – 30 KHz Submarine Application

LF 30KHz to 300 KHz Navigational Application

MF 300KHz to 3MHz Cordless Phones, AM radio

HF 3 MHz to 30 MHz Aeronautical, Amateur radio

VHF 30 MHz to 300 MHz FM radio, TV Applications

UHF 300 MHz to 3GHz TV, Mobile Communication

SHF 3 GHz to 30 GHz Point to Point, Satellite Comm.

EHF 30 GHz to 300 GHz Point to Point microwave




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MODULATION




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What is modulation?

A technique in Telecom transmission

system where by an electromagnetic

signal (the modulating signal) is encoded

into one or more of the characteristics of

the another signal (carrier signal) toproduce a third signal (modulated signal).

Whose properties are matched to the

characteristics of medium over which it istransmitted.




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NEED OF MODULATION

• To reduce the noise and interference.

• Multiplexing and de multiplexing.

• To decrease the Antenna size.• To transmit the audio signal to far

distance.




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TYPES OF MODULATIONS

• Analog modulation.

• Digital modulation

Analog modulation:• The aim of analog modulation is to

transfer the analog signal over a

channel. (or) the modulation whichdeals with the analog signals




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ANALOG SIGNAL

An analog signal is any variable signal

which is continuous in both time and amplitude.




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Different types of Analog modulations are

• Amplitude modulation.

• Frequency modulation.• Phase modulation




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Amplitude modulation

If the Amplitude of the carrier

signal is varied according to the

amplitude of the modulating signal,it is called Amplitude Modulation




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FREQUENCY MODULATION

If the Frequency of the carrier signal

is varied according to the Amplitude of

the modulating signal, it is called

Frequency modulation




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DIGITAL MODULATION




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Digital Modulation Techniques

• There are different types of digital

modulation techniques, which are given

as follows.

• Pulse code modulation.

• Delta pulse code modulation.

• Adaptive delta code modulation.




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PULSE CODE MODULATION

• PCM is a digital representation of ananalog signal where the magnitude of the

signal is sampled regularly at uniform

intervals of time.

• They are three steps to continue DIGITAL

MODULATION, they are

• Sampling.• Quantization.

• Coding.

SAMPLING




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SAMPLING

• : It is the process of measuring the analog

signal at different instants of time.

• According to nyquist criteria Ns>=2fm,

the number of samples should be taken at

least twice the maximum frequency or

higher than that.

• If not, we will not get accurate output. We

will get the distorted output and this effect

is called as Aliasing effect.




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Quantization and coding

• QUANTIZATION: Approximating the

sampled signal with an accurate value.

• CODING: Representing the quantizing

signal with binary form i.e „0‟ and „1‟s




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Based on the sampling theorem,• Each analog sample is assigned a binary

code

– Analog samples are referred to as pulseamplitude modulation (PAM) samples

• The digital signal consists of block of n

bits, where each n-bit number is theamplitude of a PCM pulse




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DIGITAL DATA TRANSMISSION

D t C i ti T




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Data Communication Terms

• Data - entities that convey meaning, or

information

• Signals - electric or electromagnetic

representations of data

• Transmission - communication of data

by the propagation and processing of

signals




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• Reasons for Choosing Data and Signal

Combinations

• Digital data, digital signal

–Equipment for encoding is less expensivethan digital-to-analog equipment

• Analog data, digital signal

–Conversion permits use of modern digital

transmission and switching equipment




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• Digital data, analog signal –Some transmission media will only

propagate analog signals

–Examples include optical fiber and

satellite

• Analog data, analog signal – Analog data easily converted to analog

signal




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Analog Transmission

• Transmit analog signals without regard to

content

• Attenuation limits length of transmission

link

• Cascaded amplifiers boost signal‟s

energy for longer distances but cause

distortion – Analog data can tolerate distortion

– Introduces errors in digital data

Digital Transmission




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Digital Transmission

• Concerned with the content of the signal

• Attenuation endangers integrity of data

• Digital Signal

– Repeaters achieve greater distance – Repeaters recover the signal and retransmit

• Analog signal carrying digital data

– Retransmission device recovers the digitaldata from analog signal

– Generates new, clean analog signal

Digital to Analog mod lation Techniq es




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Digital to Analog modulation Techniques

There are many different modulationtechniques

• Amplitude shift key modulation (ASK)

• Frequency shift key modulation (FSK)• Binary-phase shift key modulation (BPSK)

• Quadrature -phase shift key modulation

(QPSK)

• Quadrature amplitude modulation (QAM)

Amplitude Shift Key Modulation




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Amplitude Shift Key Modulation

In this method the amplitude of the carrier assumes one of the two

amplitudes dependent on the logic

states of the input bit stream.

t s t f A c 2cos 1 binary

0 binary0

A typical output waveform of an ASK




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A typical output waveform of an ASK

modulator is shown in the figure below

Frequency Shift Key Modulation




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Frequency Shift Key Modulation

• In this method the frequency of the

carrier is changed to two different

frequencies depending on the logic state of

the input bit stream.

• The typical output waveform of an FSK is

shown below.

• Notice that logic high causes the centre

frequency to increase to a maximum and a

logic low causes the centre frequency to

decrease to a minimum




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• Values represented by different

frequencies (near carrier)

• Less susceptible to error than ASK

• Up to 1200bps on voice grade lines• High frequency radio

• Even higher frequency on LANs using

co-axial cable.




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• Two binary digits represented by two different

frequencies near the carrier frequency.

t s t f A 12cos

t f A 22cos

1 binary0 binary

where f 1 and f 2 are offset from carrier

frequency fc by equal but opposite amounts

Binary Frequency-Shift Keying (BFSK)

FSK




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FSK

Phase Shift Key Modulation




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Phase Shift Key Modulation

• With this method the phase of the carrier

changes between different phases

determined by the logic states of the input

bit stream.

• Phase of carrier signal is shifted to

represent data

• Differential PSK

• Phase shifted relative to previous

transmission rather than some reference

signal

Types of PSK




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Types of PSK

• There are several types of phase shift key

(PSK) methods.

• Two-phase (2 PSK)

• Four-phase (4 PSK)

• Eight-phase (8 PSK)

• Sixteen-phase (16 PSK)• Sixteen- quadrature amplitude (16 QAM)




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The 16 QAM is a composite modulator consisting of amplitude modulation and

phase modulation. The 2 PSK, 4 PSK, 8

PSK and 16 PSK modulators are generally

referred to as binary phase shift key

(BPSK) modulators and the QAM

modulators are referred to as quadrature

phase shift key (QPSK) modulators.

Two-Phase Shift Key Modulation (BPSK)




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Two-Phase Shift Key Modulation (BPSK)

• In this modulator the carrier assumes one

of two phases to represent binary digits. Logic

1 produces no phase change and logic 0

produces a 180° phase change.

t s

t f A c 2cos

t f A c2cos

1 binary

0 binary

t f A c 2cos

t f A c 2cos

1 binary

0 binary

BPSK




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BPSK

Quadrature Phase Shift Keying




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Quadrature Phase Shift Keying

• Four-level PSK (QPSK) : Each element

represents more than one bit

t s

42cos

t f A c 11

4

32cos

t f A c

4

32cos

t f A c

4

2cos

t f A c

01

00

10

QPSK




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QPSK

8PSK




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8PSK

Quadrature amplitude modulation




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Quadrature amplitude modulation

Quadrature amplitude modulation,

or QAM, is a big name for a relatively

simple technique. It is simply a

combination of amplitude modulation andphase shift keying




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TRANSMISSION MEDIUM

TRANSMISSION MEDIUM




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TRANSMISSION MEDIUM

The media through, which transmission

takes place is called as Transmission Media.

There are two types of transmission

media. They are

– GUIDED MEDIA

– UNGUIDED MEDIA.

GUIDED MEDIA




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GUIDED MEDIA

• This uses a cabling system that guides the

data signals along a specific path. The datasignals are bound by the cabling system. This

is also known as “bound media”.

• Different types of Guided media are as follows.

1. TWISTED PAIR

2. CO-AXIAL CABLE

3. FIBER OPTICS

TWISTED PAIR




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TWISTED PAIR

Twisted pair is the ordinary copper wire

that connects home and many business

computers to the telephone company.

To reduce crosstalk or electromagnetic

induction between pairs of wires, two

insulated copper wires are twisted around

each other. Each connection on twisted pair

requires both wires

Types of twisted pair cables:




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Types of twisted pair cables:

• There are two types of twisted pair cables.

They are,

• UNSHILEDED TWISTED PAIR (UTP)• SHEILDED TWISTED PAIR (STP)

Unshielded Twisted Pair (UTP) Cable




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Unshielded twisted pair (UTP) is the

most popular and is generally the bestoption.





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Shielded Twisted Pair Cable




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If twisted pair is enclosed in ashield that functions as a ground.

This is known as shielded twisted

pair (STP).





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• A disadvantage of UTP is that it

may be susceptible to radio and

electrical frequency interference.

• Shielded twisted pair (STP) issuitable for environments with

electrical interference; however, the

extra shielding can make the cablesquite bulky.

FIBER OPTICS




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FIBER OPTICS

Fiber optics (optical fibers) are long, thinstrands of very pure glass about the diameter of ahuman hair.

When light ray passes from one mediumto another (from glass to air) ,the ray isRefracted or bent at the boundary, theamount of refraction depends on the

properties of two media

Light rays in different medium




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g y

What is critical Angle?




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at s c t ca g e

Critical Angle

Critical Angle?




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g

The least angle of incidence at which

total internal reflection takes place.

The angle of incidence in a denser

medium, at an interface between the

denser and less dense medium, at which

the light is refracted along the interface




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When the critical angle is exceeded,

the light is totally reflected back into thedenser medium.

The critical angle varies with the

indices of refraction of the two media with

the relationship:

sin Ic = n´/n where Ic is the critical

angle; n´ the refractive index of the less

dense medium; and n the refractive index of the denser medium.




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The formula for critical angle is shown

where c is the critical angle, n 1 is the

refractive index of the less dense medium, and

n 2 is the refractive index of the denser medium.




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Critical Angle: In the above diagram, light is lost

along the interface until reaching the Critical

angle of 60 degrees.

Elements of Fiber




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Elements of Fiber:




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• Core - Thin glass center of the fiber where the

light travels• Cladding - Outer optical material surrounding

the core that reflects the light back into the

core

• Buffer coating - Plastic coating that protects

the fiber from damage and moisture

• Hundreds or thousands of these optical fibers

are arranged in bundles in optical cables. Thebundles are protected by the cable's outer

covering, called a jacket.

Types of Fibers:




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Optical fibers come in two types:

• Single-mode fibers• Multi-mode fibers

Types of Fibers:




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• Single-mode fibers have small cores (about

3.5 x 10-4 inches or 9 microns in diameter) andtransmit infrared laser light (wavelength =

1,300 to 1,550 nanometers).

• Multi-mode fibers have larger cores (about

2.5 x 10-3 inches or 62.5 microns in diameter)

and transmit infrared light (wavelength = 850 to

1,300 nm) from light emitting diodes (LEDs).

• Some optical fibers can be made from plastic.These fibers have a large core (0.04 inches or

1 mm diameter) and transmit visible red light

(wavelength = 650 nm) from LEDs.

Multimode Step-Index




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p




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• Density of the core remains constant from

the center to the edges.• Multiple beams from a light source move

through the core in different paths hence

the term Multimode.• Due to different beams arriving at different

times at the receiver the signal is distorted

(propagation delays). Thus this constantdensity multimode fiber finds less

applications.

Multimode Graded-Index




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• Multiple beams from a light source move through




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Multiple beams from a light source move through

the core in different paths hence the term

Multimode.

• The distortion of signal of step index is reduced in

Graded Index Fiber by Having variable densities.

(Index refers to index of refraction). density is

highest at the center of the core and decreasesgradually to its lowest at the edge. Each density

difference causes each beam to refract into a

curve.

• Also different beams intersect at regular intervals.

Placing the receiver carefully at one of the

intersection can construct the signal with greater

Precision.

Single Mode




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Single mode fiber




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g

In single mode fiber the diameter is

smaller & the density of core is lower. Thedecrease in density and dia results in

critical angle almost equal to 90 degrees to

make the propagation almost horizontal. All the beams arrive at the receiver

together that can be combined with less

distortion. Core sizes vary from 10 to50microns, while the cladding varies from 100

to 150 microns.

Fiber Construction




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Fiber Construction

How Does an Optical Fiber Transmit Light?




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Suppose you want to shine a flashlight

beam down a long, straight hallway. Just pointthe beam straight down the hallway -- light

travels in straight lines, so it is no problem.

What if the hallway has a bend in it? You couldplace a mirror at the bend to reflect the light

beam around the corner. What if the hallway is

very winding with multiple bends? You might

line the walls with mirrors and angle the beamso that it bounces from side-to-side all along

the hallway. This is exactly what happens in an

optical fiber




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• The light in a fiber-optic cable travels

through the core (hallway) by constantly

bouncing from the cladding (mirror-lined

walls), a principle called total internal

reflection.

• Because the cladding does not absorb any

light from the core, the light wave can travel

great distances. However, some of the lightsignal degrades within the fiber, mostly due

to impurities in the glass.




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• The extent that the signal degrades

depends on the purity of the glass andthe wavelength of the transmitted light

• (for example, 850 nm = 60 to 75

percent/km; 1,300 nm = 50 to 60percent/km; 1,550 nm is greater than 50

percent/km).

• Some premium optical fibers show muchless signal degradation -- less than 10

percent/km at 1,550 nm.

Fiber optic Elements




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• Light source: LED, Laser Diode.

• Receiver : Photodiode, PIN diode.

Advantages of optical fiber (over Copper & Coaxial cables)




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(over Copper & Coaxial cables)

• Low Signal Loss in Fiber Optics. The loss isnearly >20 dB/km in conventional medium and

0.2 dB/km to a maximum of 2 dB/km in Optical

Fiber

• Long Distance Transmission. 60km to 300 km.

• Due to low attenuation, repeaters are needed

about every 60kms compared to every 5kms for

copper, reducing cost.• Maintenance is very high for conventional

medium.




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• High Band width. Conventional mediumsupports 500 MHz to 700 MHz. Fiber Opticssupports several GHz. Todays networkssupport Tera Hertz.

• Since it uses light, it is not affected by power surges, EM interference or power failures.

Also not affected by corrosive chemicals in

the air • Fiber is small in size (thin) and less in weight.




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• Fiber is a dielectric material and no need for electrical power, hence no short circuits and no

earthing is required.

• Fiber does not leak light and it is difficult to tap,thus security is highest

• Reliability is high. Only failures in OFC is cable

cuts.

Disadvantages




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• Fiber optic cable is more expensive.Needs skilled job for cable joints by

splicing.

• Sophisticated equipment is required.

UNGUIDED TRANSMISSION




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• This consists of means of datasignals to travel but nothing to

guide them along a specific path.

• The data signals are not bound toa cabling media and as such are

often called unbound media.

UNGUIDED TRANSMISSION




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They are two types of unguidedtransmission are there, they are,

• Line of sight communication• Satellite communication

LINE OF SIGHT COMMUNICATION




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• This type of communication usesparabolic dishes of different diameters

(0.6,0.8,1.2mts etc.).

• These are fixed at certain height andfocuses a narrow beam .

• It achieves line of sight transmission to

receiving antenna.




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• Ground wave propagation follows thecurvature of the Earth.

• Ground waves have carrier frequencies

up to 2 MHz.

• AM radio is an example of ground wave

propagation




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• Ionospheric propagation bouncesoff of the Earth's ionospheric

layer in the upper atmosphere.

• It is sometimes called double hop

propagation. It operates in the

frequency range of 30 - 85 MHz.




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• Because it depends on theEarth's ionosphere, it changes

with the weather and time of day.

• The signal bounces off of the

ionosphere and back to earth.

Ham radios operate in this range.




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• Line of sight propagation transmits

exactly in the line of sight.

• The receive station must be in the view

of the transmit station.• It is sometimes called space waves or

Tropospheric propagation.

• It is limited by the curvature of the

Earth for ground-based stations (100

km, from horizon to horizon).

Terrestrial Microwave Line of Sight communication




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Reflected waves can cause problems.

Examples of line of sight propagation are:FM radio, microwave and satellite.

Satellite Communication




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Description of communication satellite

–Microwave relay station

–Used to link two or more ground-based

microwave transmitter/receivers –Receives transmissions on one frequency

band (uplink), amplifies or repeats the

signal, and transmits it on another frequency (downlink)

Applications




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–Television distribution

–Long-distance telephone transmission

–Private business networks

Case study1:




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• Transmission medias used in GSM,CDMA, etc.. (mobile Communication)

between different elements of the

network.

• Comparison of Optical Fiber with Micro

wave (Line of sight) communication.




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DATA TRANSMISSION

Synchronous Transmission




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The transmission of data in which bothstations are synchronized.

Codes are sent from the transmitting

station to the receiving station to establish thesynchronization, and data is then transmitted in

continuous streams

Synchronous Transmission




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Asynchronous Transmission The transmission of data in which each




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The transmission of data in which each

character is a self-contained unit with its

own start and stop bits.

Intervals between characters may be

uneven.

It is the common method of transmission

between a computer and a modem,

although the modem may switch to

synchronous transmission to communicate

with the other modem. Also called

"start/stop transmission."

Asynchronous Transmission




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Error detection




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Error detection is the ability to detect errors

caused by noise or other impairments duringtransmission from the transmitter to the receiver

Error correction has an additional feature

that enables identification and correction of the

errors

Error correction

Parity Check




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Basic method of checking data for errors

during a transmission or on a data storage

mechanism.

Parity works by setting a parity bit to an

even or odd number.

If the binary stream contains eight 0's or 1's,for example, we know that this is an even

number; however, if the parity bit indicates that

this number should be an odd number we wouldbe able to easily determine that the data is

corrupt.

Even Parity & Odd Parity




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• Parity bit appended to a block of data

• Even parity

– Added bit ensures an even number of 1s

• Odd parity

– Added bit ensures an odd number of 1s

• Example, 7-bit character [1110001]

– Even parity [11100010]

– Odd parity [11100011

Cyclic Redundancy Check (CRC)




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• The CRC is a very powerful but easily

implemented technique to obtain data reliability.

The CRC technique is used to protect blocks of

data called Frames.

• Using this technique, the transmitter appendsan extra n- bit sequence to every frame called

Frame Check Sequence (FCS). The FCS

holds redundant information about the frame

that helps the transmitter detect errors in the

frame.




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• The CRC is one of the most usedtechniques for error detection in data

communications. The technique gained

its popularity because it combines threeadvantages:

• Extreme error detection capabilities.

• Little overhead.• Ease of implementation.

Asynchronous Data Error Detection




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• Parity has been used to detect errors in

Asynchronous data stream.• The parity bit is an added error-detection bit

included with each character of the

asynchronous data stream.

In one respect, this is a disadvantage of using

parity. It adds additional bit to be transmitted

with each character, reducing the efficiency of

the data transfer.In other words, it takes more time to transmit a

data character with a parity bit than to send

one without a parity bit.




126

1. What is the efficiency of asynchronously transmitting

an ASCII character with 1 start and 2 stop bits? Give

two answers, one including the parity bit and one

without parity for comparison purposes.

• An ASCII character contains 7 bits of character

information. Transmitting an ASCII character

asynchronously as described above includes 3 overhead

bits (1 start and 2 stop bits) as well as a parity bit. Total

bits transmitted is 11 with parity and 10 without parity.

The efficiency in transmitting an ASCII character

message without parity is 7/10, or 70%. And with parity is7/11 or 63.7%.




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2. What is the state of the parity bit for an ASCII

character R, using both even and odd parity

systems?

Sol: An ASCII character R is 101 0010 in binary.

The total number of 1s in the character is

three, an odd number. For an odd parity system,the parity bit is a 0 to keep the count of 1s odd.

While in the even parity systems, the parity bit

is set to make the total number of 1s even. (3+1=4).




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3. Which parity (odd or even) is used with the ASCII

asynchronous data stream shown below? Two stopand 1 start bits are being used with each character.

Which character has an error in it?

Data are shown starting with the first received bit

(usually the LSB) at the left side of the screen wherethe sweep begins. Succeeding bits follow in correct

sequence until the last bit is displayed. What is the

intended message?

0000 1001 0110 1010 0110 1100 0010 1101 1000

0011 1111 0100 0010 011.

0000 1001 0110 1010 0110 1100 0010 1101

1000 0011 1111 0100 0010 011




129

000 00 0 00 00 0 0

• It shows the bits as they are sent and displays

them left (LSB) to right. The message starts withthe first character sent on the left.

• Each character starts with 1 start bit and ends

with 2 stop bits. Adding the parity bit and 7 codebits, each transmitted character contains 11 bits.

The first step is to separate the message into

characters:

0000 1001 011 0101 0011 011 0000 1011 011

0000 0111 111 0100 0010 011

0000 1001 011 0101 0011 011 0000 1011 011

0000 0111 111 0100 0010 011




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• Next strip out the start and stop bits:

• 0001 0010 1010 0110 0001 0110 0000 1111

1000 0100

• Then determine the parity used and the character that

has a bad parity bit. Most of the characters in themessage use even parity with a parity error in the

middle character .

• Remove the parity bits and reverse the order of each

character to place the LSB of each on the right tofacilitate their interpretation:

• 100 1000 110 0101 110 1000 111 0000 010 0001

100 1000 110 0101 110 1000 111 0000

010 0001




131

• Convert each binary group to its ASCIIequivalent using the ASCII table.

• It is the combination of characters H e h p !

• The error shows up in the middle character.

What if 2 bits in a single characters are bad?




132

What if 2 bits in a single characters are bad?

• A 2 bit change in a single character causes the

parity condition to be the same as if an error

did not occur. The sum of 1s in both cases

would result in the same state of the parity bit.

On the surface, this may seem like a limitationof parity checking for error detection.

• Essentially, it is – parity detection is reliable

only if a single error occurs in a character.

Thus, parity checking is limited to

environments that experience infrequent errors

in data transmissions.




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• To correct errors detected using parity, the

receiving station can only request that themessage containing the error be retransmitted.

• A system that is set to request retransmission

automatically in response to detecting an error includes Automatic Request for retransmission,

also known as Automatic Repeat Request

(ARQ) processing within its communications

software.

LRC Matrix Example




134




135




136

Multiple Access Methods

Multiple Access Method




137

It determines how several users

can share a medium with minimum

or no interference.

Types of Access Methods




138

Access MethodsFDMA (Frequency Division Multiple Access)

TDMA (Time Division Multiple Access)

CDMA (Code Division Multiple Access)




139

Frequency Division Multiple Access(FDMA) :

– An approach to sharing a channel by

separating the simultaneous users in

frequency




140

f

c

t

FDMA




141

Time division multiple access

(TDMA)

– Approach for allotting single-channel

usage amongst many users, by dividing the channel into slots of time

during which each user has access to

the medium.

TDMA




142

f

t

c

2.17.1

TDMA / FDD




143

f

c

2.18.1

t




144

Code Division Multiple Access(CDMA) :

– Approach for allotting the channel

usage amongst many users, by

converting the speech signals of all

users in to different codes.

CDMA




145

f

t

c

User Data : 1011 (to be transmitted)

How CDMA uses codes to the actual data?




146

User Data : 1011 (to be transmitted)

PN Code : 110101

Transmitting Data : 001010 110101 001010 001010

Receiving Data : 001010 110101 001010 001010PN Code : 110101

Final Data : 111111 000000 111111 111111

Received Data : 1011

How CDMA uses codes to the actual data?




147

What we do, we can undo




148




149

Generations of Mobile Communication




150

EVOLUTION MOBILE COMMUNICATION




151

AMPS

TACS

NMT

D-AMPS

IS-95

GSM

ANALOG

1G

DIGITAL

2G

CDMA 2000

UMTS/

W-CDMA

IMT-2000

3G

GPRS

2.5G

ANALOG MOBILE COMMUNICATION




152

AMPS

TACS

NMT

ANALOGSYSTEM

1G

– Advanced Mobile Phone Service

US based, 800 MHz band

– Total Access Communication

System UK based, AMPS in 900MHz band

– Nordic Mobile Telephone System

Scandinavian, Both in 450 MHz

and 900 MHz band

DIGITAL MOBILE COMMUNICATION




153

– Dual mode AMPS

US, Analog signaling and Digital voicecoding

– IS-95

US, CDMA based

– GLOBAL SYSTEMS FOR MOBILE

COMMUNICATIONS

European standard, Both in 900 MHz& 1800 MHz band

D-AMPS

IS-95

GSM

DIGITALSYSTEM

2G

Technology 1G 2G

2.5G 3G 4G

Comparison o f 4G with Other Mob i le Techno log ies




154

Design began 1970

1980

1985 1990 2000

Implemen-

tation

1984 1991

1999 2002 2010?

Service Analog voice

and

data

Digital voice and

data (SMS)

Higher-

capacity data

Transfer (internet)

Higher-

capacity data

Transfer withmore data

rates

Higher

capacity

multimedia

Standards AMPS,TACS,

CDMA,GSM,

GPRS,EDGE,

Cdma

20001XRTT

UMTS/WCDM

A,

CDMA2000

1xEVDO etc

Single

standard

Data Rate 1.9kbps 14.4kbps 384kbps 2Mbps 200Mbps

Multiplexing FDMA TDMA,CDMA

TDMA,CDMA CDMA CDMA?


Case Study 2:




155

• Different Generations in mobile

Communication.

• Multiple Access Methods used in that

particular generation.

• Communication Methods used in that

particular Generation.

• Data rates in different generations.




156

WAVE PROPAGATION




157

• Antenna radiates electromagnetic waves , and

these are waves travel, outward in all directions

with a velocity of 3*10^8m/secs.

• These waves are transverse in nature and are

therefore called transverse electromagneticwaves (TEM).

• For this type of waves, the electric field (E),

the magnetic field (H), and the direction of

propagation along which the wave travels with

Phase velocity (Vp) are mutually at right angles




158

PROPAGATION OF WAVES

• There are number of mechanisms by which




159

y

radio waves may travel from transmitting

antenna to a receiving antenna. The more

important of these are

• Ground waves

• Sky wave

• Space or Troposphere waves.

• The ground wave some times also called as

surface waves can exist when the transmittingand receiving antenna are close to surface of

the earth and vertically polarized.




160

• The Sky wave represents energy that reaches

the receiving antenna as result of bending of

the wave path, introduces by the ionization of

upper atmosphere

• The space wave represents energy that travelsfrom the transmitting to the receiving antenna

in the earth troposphere, the portion of the

earth atmosphere in first 15 kms adjacent to

earth surface.

• The space wave commonly consists of two




161

p y

components; one of these rays that is that travels

directly from transmitter to receiver. While the

other that reaches the receiver as a result of

reflection from the surface of earth.

• Ground waves are generally useful at very lowfrequencies (10to30kHz) and low frequencies

(30to300khz).

• AM radio broad casting medium frequency band

of (300to3000kHz) are transferred primarily by the

ground waves.

ANTENNA




162

• An antenna is a specialized transducer thatconverts radio-frequency (RF) fields into

alternating current (AC) or vice-versa. There

are two basic types: the receiving antenna,

which intercepts RF energy and delivers AC

to electronic equipment, and the transmitting

antenna, which is fed with AC from

electronic equipment and generates an RFfield.

Isotropic Radiator:

An isotropic radiator is a theoretical point




163

• An isotropic radiator is a theoretical point

source of waves which exhibits the samemagnitude or properties when measured

in all directions. It has no preferred

direction of radiation. It radiates uniformlyin all directions over a sphere centered

on the source. It is a reference radiator

with which other sources are compared.

• Directional antenna




164

• Directional antenna

• Antenna having a preference for a particular direction and radiating (receiving) a signalmore efficiently in (from) this direction than inother directions.

• Isotropic antenna

• Antenna transmitting (receiving) equalradiation in (from) all directions. Isotropicantenna is a idealized device that does notexist in reality. It is usually taken as a

reference when measuring directivity of actualantennas.

• Omni directional antenna

• Antenna transmitting (receiving) equal radiation




165

in (from) all directions. A typical example is a

whip antenna. Whip antenna's radiation power

is distributed equally in all directions in a plane

perpendicular to the whip.

• Dipole antenna• The dipole antenna or dipole aerial is one of the

most important and also one of the most widely

used types of antenna. It can be used on its

own, or there are many other types of antenna

that use the dipole as the basic element within

the antenna.




166

• Basic dipole facts:• The name dipole means two poles and

the antenna does in fact consist of two

"poles" or sections. These are normallyequal in length, making the antenna what

is termed a centre fed antenna.




167

• The power is applied to the dipole

antenna itself through a feeder.

Conversely if the dipole antenna is used

for receiving, the received signals are

taken away to the receiver through afeeder. The feeder serves to transfer the

power to or from the antenna with as little

loss as possible




168

• The basic dipole antenna configuration




169

p g

• The most common form of dipole has anelectrical length of half a wavelength. As a

result this antenna is called a half wave

dipole. As before the lengths of the wiresare both the same. As the total length of the

dipole is a half wavelength, this makes

each section or leg of the dipole a quarter

wavelength long.




170

• The folded dipole antenna




171

The folded dipole antenna

• The folded dipole antenna is probably onlyever seen as a TV antenna. It exhibits animpedance of 300 ohms whereas a half wavedipole is 75 ohms.




172

• One powerful advantage of a foldeddipole antenna is that is has a wide

bandwidth.

• This is the reason it was often used as aTV antenna for multi channel use.

• Folded dipole antennas were mainly

used in conjunction with Yagi antennas

The Yagi antenna

Th Y i t tl th




173

• The Yagi antenna or more correctly, the

Yagi - Uda antenna was developed by

Japanese scientists in the 1930's.

• It consists of a half wave dipole, a rear

"reflector" and may or may not have one

or more forward "directors". These are

collectively referred to as the "elements

of Yagi Antenna”.




174

Parabolic antenna

• The parabolic antenna is a high gain reflector




175

• The parabolic antenna is a high-gain reflector

antenna used for radio, television and datacommunications, and also for radio location

(RADAR), on the UHF and SHF parts of the

electromagnetic spectrum.

• The relatively short wavelength of

electromagnetic (radio) energy at these

frequencies allows reasonably sized reflectors

to exhibit the very desirable highly directionalresponse for both receiving and transmitting.




176

NOISE




177

• Noise : In communications, interferencethat destroys the integrity of signals on a

line.

• Noise can come from a variety of sources, including radio waves, nearby

electrical wires, lightening, and bad

connections.

Types of Noise

• Thermal Noise : The noise generated by




178

g y

thermal agitation of electrons in a conductor. Itis also called as Johnson Noise. (Or)

• white noise generated by thermal agitation of

electrons in a conductor or electronic device.

• It is produced by the thermal agitation of the

charges in an electric conductor and is

proportional to the absolute temperature of the

conductor.




179

:

Formula for the rms noise voltage:

Boltzmann constant k B =1,3806505×10-23 J/K

(joule/kelvin);

Absolute temperature T in kelvin K =273.15 + in °C

Bandwidth being considered Δ f =f 2 - f 1

= f max - f min in Hz; 20 kHz - 20 Hz = 19980 HzR= the resistance of the circuit element.

Noise Power




180

• Noise Power: The noise power, P , in

watts, is given by P = kT f , where k is

Boltzmann‟s constant in joules per Kelvin,

T is the conductor temperature in kelvins,

and f is the bandwidth in Hertz.

• Noise Figure: Noise figure NF or noise factor F :

The Noise factor of a transducer at a




181

specified input frequency is the ratio of (a/b)where "a and b" are:

a) the available Signal to Noise Ratio (SNR) at

the signal generator terminals per unit bandwidth

when the temperature of the input termination(generator or source) is (usually 20°C = 293.15 K)

and the bandwidth is limited by the transducer, to

b) the available SNR per unit bandwidth at the

output terminals of the transducer.

• Noise figure NF = 10 log (noise factor F ) in dB

• Noise temperature Te = T0 (F 1)




182

• Noise temperature T e = T 0 (F - 1)

T 0 is standard temperature, usually20°C = 293.15 K

Noise Figure of Cascaded Amplifier

• Noise Figure of Cascaded Amplifier :




183

Noise Figure of Cascaded Amplifier :

• In a cascade amplifier the final stage sees aninput signal that consists of the original signal

and noise amplified by each successive stage.

Each stage in the cascade chain amplifies

signals and noise from previous stages and

contributes some noise of its own. The overall

noise factor for a cascade amplifier can be

calculated from the Fris noise equation:




184

• where

F is the overall noise factor of N stages in

cascade

F1 is the noise factor of stage 1

F2 is the noise factor of stage 2

Fn is the noise factor of the nth stage

G1 is the gain of stage 1G2 is the gain of stage 2

Gn-1 is the gain of stage (n-1).

Atmospheric Noise :

• Atmospheric noise is radio noise caused

by natural atmospheric processes




185

by natural atmospheric processes,

primarily lightning discharges in

thunderstorms.

• All atmospheric noise is created by

weather. More specifically, this noise

comes from lightning flashes, with most of

the noise caused by cloud-to-ground

flashes because the currents in thosestrokes are much stronger than those of

cloud-to-cloud flashes.

Shot Noise

• Shot noise in electronic devices consists




186

Shot noise in electronic devices consists

of random fluctuations of the electriccurrent in an electrical conductor, which

are caused by the fact that the current is

carried by discrete charges (electrons).• This occurs not only in p-n junctions but

also in any conductor.

I l i i t f ( ti )

Impulse noise




187

• Impulse noise is a category of (acoustic)

noise which includes unwanted, almost

instantaneous (impulse-like) sharp sounds.

• Noises of the kind are usually caused by

electromagnetic interference, scratches on

the recording disks, and ill synchronization

in digital recording and communication.

OSI reference model

Th OSI (O S t I t ti ) d l




188

• The OSI (Open Systems Interconnection) model

was developed by the International StandardsOrganization.

• We can remember the layers in sequence byremembering the sentence “All People Seem ToNeed Data Processing”.




189

OSI reference model layers




190

• Physical Layer: Its responsible for conversion of stream

of bits into signals ,that can be

transmitted on the sender ‟s side. At thereceiving side the signal is transferred

back into the bit stream.

Data Link Layer

The main task of this layer involves




191

The main task of this layer involves

accessing the medium error correctionand detection flow control, services to

network layer, framing and

synchronization .It deals with frames for error detection CRC method and error

correction Hamming code.

Network Layer




192

It deals with Packets. It is responsiblefor routing and addressing of the packets,

Connection control if traffic is high. Also

responsible for establishing, maintainingand terminating.

Transport Layer




193

It is the heart of the whole layers. Itaccepts data from session layer passes

data and split into smaller blocks to

network layer.It is responsible for the end to end

connectivity. Efficient and cost effective

delivery .It also provides error freedelivery without any duplication.

Session Layer

It provides a means for the application




194

It provides a means for the application

to establish and use a connection called

session. Data exchange, dialogue

conversation checking and recovery.

Where failure between checkpoints isalso provided by this layer.

Presentation Layer




195

It is concerned with syntax and

symantax of data exchange. It also

provides network security and privacy(Encryption and Decryption).

Application Layer




196

It has user programs to do actual workfor which the computers are purchased.

It also provides the services to

presentation layer.• Ex: File transfer between two different

systems, Email, Picture storage.

Interaction between layers




197

• Each layer provides services to upper layer.




198

y p pp y

• Each layer communicates some information.• Message (M) is produced by an application

process in layer-5 and given to layer-4.

• It puts header in front of msg, and passes tolayer-3.

• Header includes control information like

sequence number. In some layers headers

can be also have time and size and other

control information.

• Generally there is no limit to size of message




199

sequence in Layer-3 it is broken up intosmaller units called packets, there M is divided

into M1 & M2

• In layer -3 decision is made to pass packet

through the lines.

• Layer-2 adds, both its header and trailer, and

then finally gives to layer-1.

• At the receiving, machine as message movesupwards, header & trailer are stripped off.

SWITCHING




200

• A provision of connection between two or more networks to permit transmission

and reception.

• Types of switching: • Circuit switching

• Packet Switching

Circuit Switching

• A link establishes between two




201

users during the total call duration.The Circuit ones establishes

produces no more propagation delay.

Time between end of dialing &ringing is 10sec and in that time is

hunting for the line to be connected.




202

• A type of communication in which adedicated channel or circuit is

established for the duration of

transmission. The most similar circuitswitching network is the telephone

system, which links together wire

segments to create a single unbroken

line for each telephone call.




203

• Circuit switching systems are ideal for communication that requires data to be

transmitted in real time. These are

sometimes called connection orientedn/w dedicated bandwidth is allocated for

entire duration of the call.

• Advantages:




204

Advantages:

• Good interactive response.

• Efficient for transferring large block of

data.

• Simple protocol.

• Ideal for voice.

Packet Switching

• It refers to protocols in which the

messages are divided into packets before they




205

messages are divided into packets before they

are sent, each packet is then transmitted

individually and can be even follow different

routes to its destinations. Once all the packets

forming a message receive at the destination,they are recompiled into the original msg.

• Currently this is the most popular method

for data transmission. In this, each message is

split into smaller units called packets and eachpacket has an identity header destination.

• Advantages:




206

– Response time is shorter than Messageswitching

– Can be used by the digital network

– System cannot be congested due to a

standard packet length

HEADER USER DATA TRAILER

Packet format:

BASIC MODES OF OPERATION OFPACKET SWITCHING COMMUNICATION




207

DATA GRAM

• It is used when a Message to be sent is greater than the maximum size of the packet




208

than the maximum size of the packet.

• Message splitted into packets by Transmitter stations are send to its node and Eachdatagram must have the full destination address.

• Each packet treated independently• Packets can take any practical route

• Packets may arrive out of order

• Packets may go missing

• Up to receiver to re-order packets and recover from missing packets.




209




210

• A virtual circuit is a circuit or pathbetween points in a network that appears

to be a discrete, physical path but is

actually a managed pool of circuitresources from which specific circuits are

allocated as needed to meet traffic

requirements

• A permanent virtual circuit (PVC) is a virtual




211

p ( )

circuit that is permanently available to the user just as though it were a dedicated or leased

line continuously reserved for that user. A

switched virtual circuit (SVC) is a virtual circuit

in which a connection session is set up for a

user only for the duration of a connection.




212

Frame Relay

• Frame relay consists of an efficient data

transmission technique used to send digital




213

transmission technique used to send digital

information quickly and cheaply in a relay of frames to one or many destinations from one

or many end-points.

• Network providers commonly implement framerelay for voice and data as an encapsulation

technique, used between local area networks

(LANs) over a wide area network (WAN).

• The frame-relay network handles the

transmission over a frequently-changing path

transparent to all end-users.

Frame Relay Devices

• DTEs generally are considered to be terminatingequipment for a specific network and typically are




214

q p p yp y

located on the premises of a customer. In fact,they may be owned by the customer. Examples of DTE devices are terminals, personal computers,routers, and bridges.

• DCEs are carrier-owned internetworking devices.The purpose of DCE equipment is to provideclocking and switching services in a network,which are the devices that actually transmit datathrough the WAN. In most cases, these are packetswitches. Figure 10-1 shows the relationshipbetween the two categories of devices.




215

X.25

• It is an International Telecommunication




216

Union-Telecommunication StandardizationSector (ITU-T) protocol standard for WAN

communications that defines how connections

between user devices and network devices are

established and maintained.• X.25 is designed to operate effectively

regardless of the type of systems connected to

the network. It is typically used in the packet-

switched networks (PSNs), such as the

telephone companies.




217

• Subscribers are charged based on their use of the network. The development of the X.25

standard was initiated in the 1970s. At that

time, there was a need for WAN protocols

capable of providing connectivity across publicdata networks (PDNs). X.25 is now

administered as an international standard by

the ITU-T.

X.25 Devices and Protocol Operation




218

• X.25 network devices fall into three general

categories:

• Data terminal equipment (DTE),

• Data circuit-terminating equipment (DCE), and

• Packet-switching exchange (PSE).

• Data terminal equipment devices are end




219

systems that communicate across the X.25network. They are usually terminals, personal

computers, or network hosts, and are located

on the premises of individual subscribers.

• DCE devices are communications devices,

such as modems and packet switches, that

provide the interface between DTE devices

and a PSE, and are generally located in thecarrier's facilities.




220

• PSEs are switches that compose thebulk of the carrier's network. They

transfer data from one DTE device to

another through the X.25 PSN. Figure

below illustrates the relationships among

the three types of X.25 network devices

DTEs, DCEs, and PSEs Make Up an X.25 N/W




221

Packet Assembler/Disassembler

The packet assemb ler/disassemb ler (PAD) is a




222

device commonly found in X.25 networks. PADsare used when a DTE device, such as a character-

mode terminal, is too simple to implement the full

X.25 functionality.

The PAD is located between a DTE device anda DCE device, and it performs three primary

functions:

• buffering (storing data until a device is ready to

process it),• packet assembly and

• packet disassembly.

• The PAD buffers data sent to or from the DTE




223

device. It also assembles outgoing data intopackets and forwards them to the DCE device.

(This includes adding an X.25 header.)

• Finally, the PAD disassembles incoming

packets before forwarding the data to the DTE.

(This includes removing the X.25 header.)

• Figure 17-2 illustrates the basic operation of

the PAD when receiving packets from the X.25WAN.

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