ECM Notes Chapter 5

7/24/2019 ECM Notes Chapter 5

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By yuNKaer

ECM 241

CHAPTER 5Prcatical Comm System

TERM DEFINITION/FUNCTION/

PROCESS/APPLICATION/CHARACTERISTIC/

ADVANTAGE/DISADVANTAGE/EXTRA

NOTES/TYPES/ELEMENTS/PARTS/LOSSES/REASON

Fiber Optic System(FOS)

-Comm. system with info is put on a light beam &

transmitted thru a guided cable

Light freq used in FOS

Basic element of single-ch fos

Main element in optical fiber comm. link

1)

Driving circuitry

a)

Serveas electrical interference bween i/p

circuitry & light source

b) Convert V source to I source to drive light

source

2)

Light source(2 types)

a)

Light Emitting Diode(LED)

b)

Light Amplification by Stimulated Emission of

Radiation(LASER)

c)

Convert electrical energy to optical energy

(amount of light emitted proportional to

amount of drive current)

3)

Light source-to-fiber coupler

a)

Couple light emitted by source into optical

fiber cable

4) Fiberto-detector coupler

a) Couple as much light as possible from fiber

cable into light detector

5)

Light detector

a)

PIN(p-type-intrinsic-n-type) diode

b)

APD(avalanche photodiode)

c)

Convert light energy to electrical

energy/Current

6) Fiber optic cable

a) Transfer signal in form of light from a point to

otherpoint

b)

It size almost same as human hair

Part in FO cable(typical layer in FO)

1) Core

- Where light passing thru

2)

Cladding

-

Ensure light waves remain within core

- Protect fiber core from scratches

- Strengthen fiber core

3) Protective coating

- Provides 1st

level shock

-

Resistance to damage & moisture to fiber

4)

Kevlar Sleeve(protecting jacket)-

Strong & act as insulator

5) Plastic outer jacket

Typical layer in FO cable

FO cable in flat ribbon form

- Easier to use

- More space efficient (for multiple fiber)

Typical construction of flat ribbon FO cable


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Type of FO

a) Step index

-centre core about diameter-large light-to-fiber aperture

-allows more light to enter cable

-light rays propagated in zig zag pattern

-many path

-all light rays dont follow same path w. diff

propagation time

-modal dispersion exist

b)

Graded index

-centre core about diameter

-non-uniform refractive index( max at centre & toward side of cable)

- light rays propagated by refraction

-continuous bending of light rays at diff angles

-light rays travel at outermost area have great

distance but velocity than the innermost

-all rays arrive at end point almost at same time

-less modal dispersion

c) Single mode

-very small core diameter(

diameter)-only single path of light can propagated

-all light rays follow same path, take same

amount of time to travel

-zero modal dispersion

Modal dispersion

-The pulse at end of cable is lower in amplitude due

to attenuation of light in cable & in duration due to

diff time arrival of light rays.

Advantages of FO cable over Metallic cable

a. Wider bandwidth

b.

Lower attenuation

over long distance

c.

Light weight glass/plastic is lighter than copper

d. Small in size

FO cable hv smaller diameter than electrical

cable

e. strength

-manufactured w. very high tensile strengths

-can be bent/twisted w.out damage

f. security

-can be tapped as easily as electrical cable

-dont radiate signals

g.

Interference immunity-free from EMI,RFI or EMP

-no optical interference & crosstalk

h. Greater safety

-dont carry electricity

-no shock hazard, arching, sparks & short cct

-its insulator - not susceptible to lightning strikes

i.

System reliability & easy maintenance

j. No problem of corrosions

Lossesin FO

1.

Scattering losses-refer to radiation of propagation light

-due to microscopic imperfection of fiber

-refer to light loss bcoz of light waves entering

wrong incident angle & lost in cladding(called as

Rayleigh scattering loss)

-due to refraction

2.

Absorption losses

-refer to how light wave soaked-up in fiber core

-due to impurity of glass/plastic

-due to any imperfection

3.

Bending losses-due to light rays outside sharp bend cant travel

fast enough to keep up w other rays & lost

4.

Splicing losses

-permanent joining of 2 fibers

-due to fiber misalignments, fiber end cleanliness

or different size

5. Coupling losses

-represent a large source of loss in commercial

fiber optic system

-due to misalignment


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Mobile & Cellular Comm

Each zone

-sub-divided into cell (hexadiagonal-shaped) that fit

together to form honey-comb pattern

-allocate a fixed no. of cellular voice channels

1 cluster, 7 cells

Cellular telephone system

-allows large no. of users to share limited no. ofcommon usage of radio CH

Figure 1

Concept of cellular

1) Cellular system allows freq reused in same area by

splitting entire region into many smaller cells

2) Each cell hv radio base station(RBS)

3)

Due to low power signal from RBS, same freq can

be used for other cell(not adjacent, adjacent cell

hv diff freq to avoid interference)

4) Each mobile user assigned a freq & communicates

w. closest RBS

5) When user crossed to other cell, its linked to new

RBS

6)

RBS are linked together(so conversation can pass

from one cell to another)

Physical Size of Cells

a. Macrocells

-

Radius - 1.8km to 28km

-

Base station transmit power - 1 W to 6 W

b. Microcells

- Radius -


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Mobile Unit

-It communicates directly with RBS over free-

space/air

-RBS communicates directly with MTSO over

dedicated data link (microwave/coaxial cable/focable)

RBS

-RBS serves as central control for all users w.in cell

-It consists of low-power radio transceiver, power

amplifier & cell-site controller

-provides interface bween mobile phone & MTSO

Mobile Telephone Switching Office(MTSO)

-it control CH assignment, call processing, setup &

termination, includes signalling, switching,supervision & allocating RF CH

-provides centralized administration & maintenance

pt. for entire network interfaces w. public phone

network over wire line voice trunks for conventional

wired telephone & interface w. data links

-most MTSO connected to SS7 signalling network,

which allow cell phone to operate outside their

service are

Satellite Comm. System

A satellite system consist of:1)

Transponder(radio repeater on the sky)

2)

Ground-based station(control operation)

3) User network of earth station(provides facilities

for transmission & reception of comm. traffics

thru satellite system)

Comm. Satellite

-a radio relay station in orbit above the earth that

receives, amplifies & redirects analog & digital signals

carried on specific RF

Satellite process

-Receivea signal from ground station

-Do amplification& translatefrequency

-Broadcastit to earth stations that are able to receive

transmission

-A satellite begins at single earth station

-Passes thru satellite

-End at one/more earth stations

-It utilize microwave terminals both on satellite & in

earth station for high reliable & high capacity system

Satellite Transponder

-act like repeater

-consist of receiver & transmitter

Main function of Satellite Transponder

-pick up transmitted signalfrom transmitter on earth

-amplify signal

-convert carrier freq to another freq

-retransmit amplified signalto receiver on earth

-BPF limits total noise

-LNA amplifies received signal & feeds it to freq

converter which converts high-band uplink freq to

low-band downlink freq

-each RF satellite CH requires a satellite transponder

Satellite System Links

Uplink

-path of satellite signal from earth transmitter to

receiver of satellite

Downlink

-path of satellite signal from satellite transmitter to

receiver on earth

p/s: uplink & downlink use diff carrier freq to avoid

interference & freq conversion is done in transponder


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Earth Stations

a)

Modulator

-

Convert i/p basebands signals to FM, PSK or

QAM modulated intermediate freq(IF)

b) Up-converter

-

Consists of a mixer & BPF converts IF to anappropriate RF carrier freq

c)

HPA

- Provides adequate i/p sensitivity & o/p

power to propagated the signal to satellite

transponder

a) LNA

- Highly sensitive & low-noise device amplifies

received signal

b) RF-to-IF down-converter

-

Mixer & BPF combination which convertsreceived RF signal to an IF

c)

Demodulator

-

Detects info signals & feed to appropriate

baseband o/p

Frequency Allocations

Most common bands used for satellite comm. are:

i.

C-band(4-8GHz) for voice & data tel.comm.

ii. Ku-band(12-18GHz) for TV-broadcasting

iii. At these bands signal attenuation is at

minimum

Satellite Orbit

3 basic typesof orbits

1) Polar

-north-south orbit

-used for navigation, weather satellites,

meteorological & land resource satellite systems

2)

Elliptical inclined

-used for Russian domestic systemsw. inclination

of 63 & a 12-hour orbit period but visible for 8

hours only

-3 satellites needed for continuous coverage a

day

3)

Circular equatorial

-called geosynchronous orbit

-at height of about 35.8km

-24 hour orbit period

-its angular speed is equal to rotational speed of

earth

-so, it appears stationary/motionless over fixed pt

on earth

-its visible from 1/3 of earths surface

-3 satellites needed for full coverage of

earth(except on polar regions)

Advantages of Circular equatorial

a)

Satellite remains almost stationary relative to

earth station

- So, computer-controlled tracking of satellite

minimized

b) Permanently in view

- So, no breaks in transmission

-

No need to switch from one satellite to

another


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c) Due to high altitude, it can cover large area on

earth & large no. of earth stations may

intercommunicate

d)

Effects of Doppler shift are negligible

-

Doppler shift is change in apparent freq ofradiation to & from satellite caused by

motion of satellite to & from earth station

Disadvantages of Circular equatorial

a)

Latitudes greater than 81.25 North & South are

not covered

b) Due to high latitudes, received signal power which

is inversely proportional to square of distance is

very weak & signal propagation delay is about

300msec

Applications of Satellite Comm

a. Weather satellite

- Provide meteorologist w. scientific data to

predict weather conditions

b.

Earth observation satellite

-

Allow scientist to gather valuable data about

earths ecosystem

c. Navigation satellite

- GPS technology

- Provide building exact location on earth to

w.in a few metersd.

Digital audio broadcasting

e.

Satellite TV broadcasting, Direct broadcast

satellite, Direct To Home(DTH) satellite

f. Vehicle tracking

g. Mobile comm.

h. Video teleconferencing

i.

Satellite internets

Comparison bween Satellite System & Terrestrial

Microwave System

ADVANTAGES DISADVANTAGES-

Access wide zone

-

Wide bandwidth

-

High reliability

-Distance insensitive cost

-Terrestrial infrastructure

independent

-

Rapid installation

-Low cost per added site

-Uniform servicecharacteristics

-

Single provider

-

High initial cost

-

Very far away from

earth & propagation

delay bween time msg

sent & delivered about

300msec for up &

downlink

- Inside building-satellite

phone cant work

-

Regulatory licensing

requirement

-

Limited orbital parking

slots

Public Switching Telephone Networks (PSTN)

-Definition: It is global collection of interconnects

originally designed to support cct-switched voice

comm.

-

Provide traditional telephone service

Types of Subscribers

a) Private-line cct/dedicated cct

-customer lease equipments, trans media &

services from tel. companies/service providers

on permanent basis

b) Public subscribers

-customer share equipments thtre available to

all public subscribers to network

PSTN Primary Characteristica)

Analog access(300Hz-3.4kHz)

b) Cct-switched duplex connection

c) Basic PSTN network link supports 64kbps

d) PSTN utilizes SS7 signalling protocol

e)

Immobility

f)

Many functions in common w. another bearer

network

PSTN Teleservices

- PSTN interface is analog

-

Fax & data traffic must be converted to analogsignals in freq band 0.3 to 3kHz w. help of built-in

modem/stand-alone modem

-

Modem adapts signal to PSTN

- By converting info from digital to analog & vv

Terminals can be connected to PSTN

-

Fixed telephone

-

Fax service

-

Data communication

-

Video, multimedia & teleworking

-

Cordless phone


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Integrated Services Digital Network (ISDN)

- 1st

tel.comm. service designed specially for

sending voice, video & data over digital phone

lines/normal phone wires system

-

Support data transfer rates of 64kbps

-

Requires adapters at both ends of tans- ISDN is no longer popular

ISDN Channels

-

2 bearer CH/B-CH & 1 data CH/D-CH

-

B-CH

- Carry end-users actual data

- In US it always 64kbps

- In elsewhere it always 56kbps

- D-CH

-

Contains data like signalling error, framing dll

-

For BRI it is 16kbps-

For PRI it is 64kbps

2 typesof ISDN Services

a. Primary Rate Interface(PRI)

-

In US

- 23 B-cH (64kbps), 1 D-cH (64kbps)

-total=1.544Mbps

- In EU & elsewhere

- 30 B-cH (56kbps), 1 D-cH (64kbps)

-total=1.544Mbps

-

Reason- US use 8-bit word w. echo cancellation

-Elsewhere use 7-bit word & no echo

cancellation

- Used in larger envi. Bcoz it provides er

bandwidth

b. Basic Rate Interface(BRI)

-

2 B-CH & 1 D-CH

-

B-CH (US=64kbps, elsewhere=56kbps)

-

D-CH(16kbps)

-

Data transmitted=144kbps

-

Usable user data=128kbps- Typically use in small office/home connection

BRI

Broadband ISDN(B-ISDN)

-ori version of ISDN employs baseband trans

-B-ISDN use broadband trans & able to support trans

rates of 1.5Mbps

-B-ISDN requires fiber optic cables-intended to extend integration of both services

thru.out rest of end-to-end path using FO/radio

media

-support trans from 2Mbps up to much higher rate

ISDN Pro & Conts

ADVANTAGES DISADVANTAGES

-Governed by world-wide

set of standards

-provides symmetrical

transfer rates

-consistent transfer rates

-Possibility of multiple

simultaneous calls

-more reliable than POTS

-external power supply is

required

-special digital

phone/terminal adapter

required to talk to existing

POTS service

-very expensive to

upgrade from central

office switch to ISDN

Applications

ISDN application

a)

Alternative to analog connections

b)

Commonly used in radio broadcasting

c) Videoconferences(provides simultaneous

voice,video & texting trans)


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By yuNKaer

ECM Notes Chapter 5

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