Wilamowski B.M., Irwin J.D. The Industrial Electronics Handbook. Second Edition: Industrial Communication Systems

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2-6 Industrial Communication Systems

Principally.at.the.receiver,.the.series.of.bits.has.to.be.recovered.by.means.of.a.clock.signal.. is.timing.

signal.is.needed.to.identify.the.boundaries.between.the.bits;.respectively,.the.centre.of.the.bit.has.to.be.

detected.indicating.the.bit.value.with.maximum.signal.power..Two.methods.are.used.providing.timing:

•

. Asynchronous.communications.with.independent.transmit.and.receive.clock:.A.simple.method.

where.transmit.and.receive.clock.are.independently.set.to.the.same.clock.rate..Bit.synchroniza-

tion

.within.a.single.byte.is.attained.by.a.start.bit.providing.the.timing.for.detecting.the.data.

bits.(Figure.2.8)..Synchronization.ends.with.a.stop.bit.. e.data.rate.is.limited.mainly.caused.by.

increasing.synchronization.loss,.e.g.,.because.of.noise.disturbing.the.start.bit..A.parity.bit.is.oen.

employed.to.validate.correct.reception.. is.type.of.communication.is.primarily.used.for.con-

necting

.printer,.terminal,.and.modem.

•

. Synchronous.communication.using.synchronized.transmit.and.receive.clocks:. is.more.com-

plex

.method.supports.also.high.data.rates.(~Gbps)..Timing.is.derived.from.edges.in.the.bit.stream.

that.are.used.to.synchronize.the.receive.clock..Mostly,.cyclic.redundancy.check.is.used.to.validate.

correct.reception.

•

. A.lot.of.additional.coding.and.modulation.techniques.are.used,.which.are.only.mentioned.shortly.

•

. Digital. frequency. modulation. (FM). and. modied. frequency. modulation. (MFM). related. to.

Manchester.coding.and.primarily.used.for.magnetic.recording.

•

. Dierential.Manchester.encoding,.where.an.additional.transition.at.the.beginning.of.the.bit.time.

is.only.included.when.coding.a.zero.bit.

•

. Non–return.to.zero.invert.(NRZ-I),.where.the.coding.signal.level.depends.on.the.previous.one..

With.that,.frequent.level.changes.allow.better.synchronizing.

•

. Block.codes.4B/5B,.5B/6B,.and.8B/10B.use.a.greater.number.of.bits.(5,.6,.or.10.instead.of.4,.5,.or.8).

for.coding.the.bit.group.with.the.goal.of.using.code.combination.for.reliable.clock.synchroniza-

tion.

.5B/6B.and.8B/10B.additionally.allow.DC.balance.

•

. Block.code.8B/6T.uses.ternary.coding.(three.voltage.levels.−V,.0,.+V).

•

. Pulse.amplitude.modulation.5.(PAM-5).uses.ve.voltage.levels.(e.g.,.−2,.−1,.0,.1,.2.V).where.one.

level.(0.V).is.used.for.trellis.error.correction..PAM-5.is.utilized.by.Ethernet.100BASE-T2.over.

two.wire.pairs.and.1000BASE-T.over.four.wire.pairs..One.type.of.10.Gbps.Ethernet.over.wire.

(Standard. IEEE. 802.3an).makes. use.of. a.pulse-amplitude. modulation. with.16. discrete. levels.

(PAM-16).over.UTP.or.STP.cable.

–V

‘1’ bit ‘0’ bit

–V

1 1 1 1 1 1 000 0

–V

FIGURE 2.7 Manchester.encoding.

0 1 2

Stop

Start Data bits

3 4 5 6 7 P

Idle

FIGURE 2.8 Asynchronous.communication:.example.with.8.data.bits.(0–7),.a.parity.bit.P.and.a.single.stop.bit.

with.length.of.1.data.bit.

Media 2-7

2.2.6 Standards

Numerous.standards.in.dierent.versions.were.developed.over.the.years.which.are.used.in.the.area.of.

indu

strial

.com

munication.

.Tab

.2.1.giv

.an.ove

rview

.of.som

.oe

.use

.sys

tems.

2.2.7 Data transmission Utilizing Existing Cable Infrastructure

In.special.cases,.e.g.,.within.buildings,.existing.cable.infrastructure.primarily.not.intended.for.data.

tran

smission

.can.be.empl

oyed

.for..Espe

cially

.used.for.home.appl

ications,

.powe

r line communication

or.powe

r line networking

.allo

.data.tran

sfer

.via.AC.powe

.supp

.line

.Appl

ications

.rang

.from.nar-

owband

.home.cont

rol

.up.to.broa

dband

.home.netw

orking

.and.inte

rnet

.acce

.with.data.rate

.≥1.Mbps

Powe

.line.data.tran

smission

.is.base

.on.medi

.to.high.carr

ier

.freq

uencies

.(nar

rowband

.9–50

.kHz,.

broa

dband

.1.6–

.MHz).modu

lated

.by.die

rent

.modu

lation

.form

ats,

.e.g.,.OFDM.(see.Sect

ion

.2.4.

3).

Indu

ced

.by.devi

.on.and.o.swit

ching

.and.by.nonl

inearities

.causi

.harm

onics,

.powe

.line

.are.inhe

ently

.a.nois

.envi

ronment.

.Powe

.line

.are.unsh

ielded

.and,.ther

efore,

.radi

ate

.signa

.they.carr

.whic

may.inte

rfere

.with.near

.equi

pment.

.Powe

.line.tech

nology

.can.also.be.used.for.in-v

ehicle

.netw

orks.

Othe

.cabl

.whic

.can.be.empl

oyed

.for.data.tran

smissions

.are.tele

phone

.wire

.using.Digi

tal

.Subs

criber

Line.(DS

.tec

hnology.

2.3 Optical Links

2.3.1 Physical Properties

 e.core.and.cladding.are.the.two.key.elements.of.an.optical.ber.. e.core.is.the.inner.part.of.the.ber,.

thro

ugh

.whic

.ligh

.is.guid

ed.

. e.clad

ding

.surr

ounds

.it.comp

letely.

. e.refr

active

.inde

.of.the.core.is.

high

.than.that.of.the.clad

ding,

.so.ligh

.in.the.core.stri

kes

.the.boun

dary

.with.the.clad

ding

.at.a.glan

cing

angl

.con

ned

.in.the.cor

.by.the.pri

nciple

.of.tot

al internal reection.

Optical

.comm

unication

.is.the.fast

est

.comm

unication.

. e.tend

ency

.of.the.ligh

.to.trav

.in.pack

ets

thro

ugh

.the.narr

.core.is.not.beca

use

.of.photo

.size.but.of.spee

.photo

.are.many.orde

.of.magn

tude

.smal

ler

.than.the.core

.Tran

smission

.of.ligh

.by.optic

.ber

.is.not.100%.eci

ent.

.Some.ligh

.is.lost

causi

.atte

nuation

.of.the.signa

.Atte

nuation

.meas

ures

.the.redu

ction

.in.signa

.stre

ngth

.by.comp

aring

outp

.powe

.with.inpu

.powe

.Meas

urements

.are.made.in.deci

bels

.(dB)

.Optic

.ber

.are.uniq

in.allo

wing

.high

-speed

.signa

.tran

smission

.at.low.atte

nuation.

. e.degr

.of.atte

nuation

.depe

nds

.on.

the.wave

length

.of.ligh

.tran

smitted.

. is.make

.oper

ating

.wave

length

.an.impo

rtant

.feat

ure

.of.a.ber.

syst

em.

.Ligh

.inte

nsity

.atte

nuation

.has.no.dire

.eec

.on.the.band

width

.of.the.elec

trical

.signa

.bein

tran

sported.

. ere.is.a.dire

.corr

elation,

.betw

een

.the.S/N.of.the.ber.rece

iver

.elec

tronic

.circ

uits

.and.

the.usab

.reco

vered

.optic

.signa

.For.comm

ercially

.avai

lable

.ber

.atte

nuation

.rang

.from.appr

oxi-

mately

.0.15

–0.5

.dB/k

.for.singl

e-mode

.ber

. e.grad

ual

.redu

ction

.in.atte

nuation

.loss

.in.all.the.four.

gene

rations

.of.opti

cal

.com

munication

.sys

tems

.(OC

Ss)

.hav

.bee

.des

cribed

.in.Tab

.2.2

Dispersion

.is.the.spre

ading

.or.broa

dening

.of.ligh

.pulse

.as.they.prop

agate

.thro

ugh

.ber

.Disp

ersion

limi

.ber.tran

smission

.capa

city.

.Disp

ersion

.incr

eases

.with.dist

ance,

.so.the.maxi

mum

.tran

smission

.rate.

decre

ases

.with.the.dist

ance.

. ere.are.die

rent

.forms.of.disp

ersion,

.incl

uding

.mate

rial

.and.wave

guide.

. e.

materials.used.to.create.the.ber.cable.use.dierent.refractive.indices;.therefore,.each.wavelength.moves.

at.a.die

rent

.speed.insi

.the.ber.cabl

. is.mean

.that.some.wave

lengths

.arri

.prio

.to.othe

.and.a.

sign

.puls

.disp

erses

.over.a.broa

der

.rang

. is.is.also.call

.smea

ring.

. e.cent

.core.crea

tes

.the.

wave

guide.

. e.shap

.and.the.refr

active

.inde

.insid

.the.core.can.crea

.the.disp

ersion

.or.spre

ading

of.the.pulse..Info

rmation-carrying

.capa

city

.(ban

.widt

h/bit

.rate

)

.is.very.impo

rtant

.in.all.type

.of.com-

unications.

. e.more.bits.that.can.pass.thro

ugh

.a.syst

.in.a.give

.time

.the.more.info

rmation

.it.can.

carr

. e.band.widt

.enha

ncement

.has.been.the.majo

.area.of.resea

rch

.sinc

.1974

. e.deve

lopments

.in.

band

width

.are.show

.in.chro

nological

.orde

.in.Tabl

.2.2.

2-8 Industrial Communication Systems

2.3.2 types and Media access

Total.internal.reection.of.rays.is.a.rst.approximation.of.light.guiding.in.bers..Core-cladding.struc-

ture

.and.mate

rial

.comp

osition

.are.the.key.facto

.in.dete

rmining

.ber.prop

erties.

.Optic

.ber

.are.

clas

sied

.into.singl

.and.mult

imode

.ber

.Furt

hermore,

.the.ber

.are.cate

gorized

.in.term

.of.thei

refr

active

.inde

.pro

le.

. e.most.impo

rtant

.type

.of.ber

.are.singl

e-mode

.(ste

.inde

x),

.mult

imode

.step.

inde

.and.mul

timode

.gra

ded

.ind

ex.

.thic

kness

.of.a.sing

le-mode

.ber.toda

.is.appr

oximately

.8.3–

.μm.at.the.cent

er.

. e.oute

.clad

ding

.is.appr

oximately

.125.μm.thic

. e.sing

le-mode

.ber.is.the.focu

.of.most.of.the.acti

vities

.toda

Mult

imode

.ber.has.give

.way.to.sing

le-mode

.ber.thro

ughout

.the.publ

.carr

ier

.netw

orks.

. us,.

sing

le-mode

.ber.has.an.adva

ntage

.over.all.othe

.type

.sinc

.it.can.hand

.mult

iple

.sign

als

.with

out

any.atte

nuation

.for.long.dist

ances.

.A.sing

le-mode

.step.inde

.ber.betw

een

.the.two.ends.enab

les

.the.

deve

lopers

.to.spee

.up.the.inpu

.beca

use

.ther

.is.no.conc

ern

.abou

.vary

ing

.path.leng

ths.

.If.the.glas

coul

.be.made.very.thin.and.very.pure.in.the.cent

er,

.the.ligh

.woul

.have.no.choi

.but.to.foll

.the.

same.path.ever

.time

Multimode

.ber

.as.the.name.impl

ies,

.has.mult

iple

.path

.wher

.the.ligh

.can.reac

.the.end.of.the.ber

In.mult

imode

.step.inde

.ber

.the.core.has.a.unif

orm

.inde

.with.a.shar

.chan

.at.the.boun

dary

.of.the.

clad

ding.

. e.thic

kness

.of.the.glas

.is.cruc

ial

.to.the.pass

age

.of.the.ligh

.and.the.path.used.to.prop

agate

from.one.end.to.the.othe

. is.ber.is.the.thic

kest

.form.of.ber

.havi

.a.core.of.120–

400

.μm.thic

. e.

ligh

.is.both.refr

acted

.and.ree

cted

.insid

.the.enca

sed

.ber

.Ligh

.beam

.in.the.ber.prop

agate

.by.using.

path

.of.die

rent

.leng

ths

.causi

.die

rent

.dela

ys.

. ese.ber

.are.less.in.prac

tice

.beca

use

.of.low.car-

ying.capacity.. ey.have.a.relatively.wide.core,.but.since.they.are.not.graded,.the.light.bounces.widely.

thro

ugh

.the.be

.exh

ibiting

.hig

.lev

els

.of.mod

.dis

persion.

.a.mult

imode

.grad

ed-index

.ber

.the.core.inde

.is.not.unif

orm;

.it.is.high

est

.at.the.cent

.and.

decr

eases

.unti

.it.matc

hes

.the.clad

ding.

.By.usin

.a.grad

ing

.of.the.dens

ity

.of.the.glas

.from.the.cent

core.out,.the.capa

city

.of.the.ber.can.be.incr

eased.

. e.path.in.the.oute

.part.of.the.glas

.is.long

.than.

the.path.in.the.cent

.of.the.glas

. is.mean

.that.ligh

.arri

ves

.at.die

rent

.time

.beca

use

.the.path.

leng

ths

.are.die

rent.

.Grad

ing

.the.cent

.core.to.have.a.high

.leve

.of.refr

action

.and.the.oute

.part

of.the.glas

.to.be.thin

ner

.(and.thus.less.refr

active)

.can.expl

oit

.the.char

acteristics

.of.the.glas

.to.get.

appr

oximately

.the.same.leng

.of.a.wave.on.the.cabl

.and.ther

efore

.incr

ease

.the.spee

.of.thro

ughput.

 e.bett

.the.grad

ing

.of.the.inde

.the.more.thro

ughput

.one.can.expe

ct.

.Curr

ently,

.the.two.form

of.grad

ed-index

.ber.eith

.a.62.5.or.a.50.μm.cent

.core.with.a.125.μm.oute

.clad

ding

.of.glas

.are.

bein

.used

TABLE 2.2 Comparison.of.the.Generations.of.Optical.Communication

Serial.# Parameter

First.

Generation

Second.

Generation

 ird.

Generation

Fourth.

Generation

1 Deploying

.yea

r 1974 1978 1982 1994

2 Operating

Wave

length

820

.nm 1,330.nm 1,550.nm 1,550.nm

3 Atten

uation 20

.dB/km.

reduce

.to.

5.dB/km

0.5.dB/km 0.25.dB/km 0.15.dB/km

4 Max.

.Ban

dwidth

.for.

Communic

ation

.Mbps 140.Mbps 1.Gbps 10.Gbps.

expand

able

.to.

50.Tbps

5 Rep

eater

.Dist

ance 8–10

.km 10.km 50.km 100.km.EDFA.

(erbium.dope

ber.amp

lier)

Media 2-9

2.3.3 transmitters and receivers

Optical.transmitters.generate.the.signals.carried.by.ber-optic.communication.systems..Light.sources.

compatible.to.the.properties.of.optical.bers.are.used.in.optical.communication..Visible.red.light–

emitting

.diodes.(LEDs).are.used,.that.transmit.wavelengths.better.than.the.near-infrared..Near-infrared.

LEDs.and.semiconductor.laser.made.on.gallium.arsenide.and.gallium.aluminum.arsenide.emitting.at.

750–900.nm.are.used.with.glass.optical.bers.for.relatively.short.links.and.moderate.speed.systems..

 ese.light.sources.have.been.used.in.the.rst.generation.of.optical.communication.. e.second.and.

third.generations.of.OCSs.have.been.using.the.semiconductor.laser..Its.operating.wavelength.is.1300.nm.

through.glass.bers.and.has.a.loss.of.0.35–0.5.dB/km.at.this.wavelength.. e.transmission.window.

for.the.fourth.generation.of.optical.communication.is.1550.nm.and.the.attenuation.is.0.15–0.5.dB/km..

Optical.bers.doped.with.the.rare.earth.erbium.also.generate.light.near.1550.nm,.but.they.are.used.

more.oen.to.amplify.an.optical.signal.that.has.traveled.a.long.distance..Erbium-doped.ber.ampli-

er

.(EDFA).has.revolutionized.optical.communication..It.has.wide.bandwidth.(20–70.nm),.high.gain.

(20–40.dB),.high.output.power.(>200.mW),.modulation.format,.wavelength.insensitive,.low.distortion,.

and.low.noise.

Photomultipliers

.respond.to. incident.light.by.delivering. charge.to.the. anode.. Photoelectrons.are.

accelerated.toward.a.series.of.electrodes.(dynodes).. ese.are.maintained.at.successively.higher.poten-

tial

.with.respect.to.the.cathode..On.striking.a.dynode.surface,.each.electron.causes.the.emission.of.

several.secondary.electrons,.which.in.turn.are.accelerated.toward.next.dynode,.and.multiplication.con-

tinues.

.Silicon.photodiodes.are.one.of.the.most.popular.radiation.detectors.. ey.have.small.size,.high.

quantum.eciency,.wavelength.range.from.0.4.to.1.μm,.good.linearity.response,.large.bandwidth,.sim-

ple

.biasing.requirements,.and.relatively.low.cost..Most.fast.photodiodes.require.internal.amplication..

 is.useful.amplication.of.photocurrent.is.achieved.in.the.avalanche.photodiode..A.basic.p–n.junction.

is.operated.under.very.high.reverse.bias..Carriers.traversing.the.depletion.region.gain.sucient.energy.

and.enable.further.carriers.to.be.excited.across.energy.gap.by.impact.excitation.

Optimum

.performance.of.the.OCS.is.achieved.by.the.proper.alignment.. e.generic.representation.of.

an.OCS.is.shown.in.Figure.2.9..It.is.composed.of.three.major.blocks:.optical.transmitter,.ber.link,.and.

optical.receiver..An.optical.source,.which.is.generally.a.LASER,.signal.is.fed.to.a.modulator’s.input.

and.the.second.input.in.the.form.of.electrical.signal.is.fed.to.the.modulator’s.other.input.. is.input.in.

case.of.optical.modulator.can.be.used.to.directly.modulate.the.optical.signal.up.to.5.Gbps..Several.kinds.

of.modulators.are.being.used.for.communication.purposes,.the.Mach–Zender.modulator.is.the.most.

common.one.. e.modulated.signal.is.transmitted.through.the.ber.link,.which.can.span.over.hundreds.

to.thousands.of.kilometers.. e.ber.link,.in.addition.to.regenerative.repeaters.and.optical.ampliers,.

also.employs.splitters.to.distribute.or.drop.optical.signal.on.any.particular.link.. e.splitters.can.be.

thought.of.as.toll.points.at.the.main.highway.where.the.routing.of.the.trac.is.to.be.decided.

Other users

Fiber

transmission

Optical

source

Modulator

Detector

Demodulator

Output data

signal

Electronic

intelligence

(destination)

Splitter

Input data

signal

Electronic

intelligence

(origin)

FIGURE 2.9 Generic.representation.of.an.OCS.

2-10 Industrial Communication Systems

 e.optical.detector.detects.the.modulated.optical.signal.and.feeds.that.detected.signal.to.a.demodulator..

 e.dem

odulator

.usu

ally

.com

pares

.the.inc

oming

.sig

nal

.wit

.thr

eshold

.val

ues

.and.gen

erates

.the.ele

ctrical

sign

al.

. is.ele

ctrical

.sign

.is.fur

ther

.mod

ied

.to.pro

per

.dat

.seq

uence

.if.nec

essary,

.and.na

lly

.it.is.

rep

roduced

.at.the.des

tination.

2.3.4 Multiplexing

Wavelength. division.multiplexing.(WDM).technology.uses.multiple.wavelengths. to. transmit.infor-

mation

.ove

.a.sing

.be

.Coa

rse

.WDM.(CW

DM)

.has.wid

.cha

nnel

.spa

cing

.(20.nm).and.is.of.low.

cos

.Den

.WDM.(DW

DM)

.has.hig

.cap

acity

.and.den

.cha

nnel

.spa

cing

.(0.

.nm).tha

.all

ows

.simu

taneous

.tra

nsmission

.of.16+.wav

elengths.

.Fir

.WDM.net

works

.use

.jus

.two.wav

elengths,

.131

.and.

155

.nm..Tod

ay’s

.DWD

.sys

tems

.uti

lize

.16,.32,.64,.128

.or.mor

.wav

elengths

.in.the.155

.nm.win

dow.

.Eac

.of.the

.wav

elengths

.pro

vides

.an.ind

ependent

.cha

nnel.

. e.ran

.of.sta

ndardized

.cha

nnel

gri

.inc

ludes

.50,.100

.200

.and.100

.GHz.spa

cing.

.Wav

elength

.spa

cing

.pra

ctically

.dep

ends

.on.las

.lin

wid

.and.opti

cal

.lt

.ban

dwidth.

.Eac

.opti

cal

.cha

nnel

.can.car

.any.tra

nsmission

.for

mat

.(di

erent

asy

nchronous

.bit.rat

es,

.ana

log

.or.dig

ital).

.Wav

elength

.is.use

.as.ano

ther

.dim

ension

.to.tim

.and.spa

ce.

Mul

tiplexing

.is.use

.thr

oughout

.com

munications

.bec

ause

.it.gre

atly

.inc

reases

.the.tra

nsmission

.cap

acity

and.re

duces

.sy

stem

.co

sts.

2.3.5 Implementations and Standards

 e.fundamental.requirements.for.making.optical.bers.sound.deceptively.simple.. e.material.that.is.

tra

nsparent

.can.be.dra

.into.thi

.be

.wit

.a.dis

tinct

.cor

e-cladding

.str

ucture.

.It.sho

uld

.be.uni

form

alo

.the.len

gth

.of.the.be

.and.can.sur

vive

.in.the.desi

red

.wor

king

.env

ironment.

.Gla

.is.the.mos

com

mon

.mat

erial

.use

.in.opti

cal

.be

rs.

.Ord

inary

.gla

.is.a.non

crystalline

.com

pound

.of.sili

ca.

.Man

di

erent

.var

iations

.of.gla

.be

.hav

.bee

.dev

eloped.

.Sim

ple

.gla

ss-clad

.be

.are.mad

.by.col

lapsing

a.low

-index

.tub

.onto.a.hig

her-index

.rod

.Ref

ractive

.ind

ices

.of.mos

.opti

cal

.gla

sses

.are.bet

ween

.1.4

and.1.8

.Imp

urities

.lim

.tra

nsmission

.of.sta

ndard

.gla

sses.

.Fib

ers

.are.dra

.fro

.the.bot

tom

.of.a.hot.

pre

form.

. e.pre

form

.has.a.phy

sical

.mak

.up.ide

ntical

.to.the.na

.be

.exc

ept

.tha

.it.is.muc

.wid

and.sho

rter.

. e.pre

form

.is.hea

ted

.ver

.pre

cisely,

.and.a.thi

.str

and

.of.gla

.is.pul

led

.o.on.one.end

. e.

dia

meter

.of.thi

.str

and

.is.con

trolled

.ver

.car

efully

.thr

ough

.var

iances

.in.hea

ting

.and.pul

ling

.ten

sion.

 is.str

and

.is.the.opti

cal

.be

.con

taining

.bot

.the.cor

.and.the.cla

dding.

.Onc

.the.be

.is.pul

led

.o.

the.pre

form,

.it.mus

.be.car

efully

.and.slo

wly

.coo

led,

.cov

ered

.wit

.the.na

.coa

ting,

.and.wou

.on.to.

ree

ls.

.All.sili

.be

.are.use

.for.com

munications.

.Har

d-clad

.sili

.be

.are.use

.for.ill

umination

.and.

bea

.del

ivery.

.Fib

ers

.mad

.of.uo

ride

.and.cha

lcogenide

.gla

sses

.tra

nsmit

.inf

rared

.wav

elength,

.whi

sili

.b

.ab

sorbs.

Splices

.are.use

.to.join.end

.of.be

.to.eac

.oth

.per

manently.

. is.is.don

.by.fusi

.and.mec

hani-

cal

.mea

ns.

. e.cri

tical

.fac

tor

.in.spl

icing

.is.the.be

.join

.spe

cial

.mea

sures

.are.ado

pted

.to.gua

rantee

tha

.the.lig

.pas

ses

.thr

ough

.the.join

.wit

hout

.los

. e.join

.is.mec

hanically

.sec

ure

.and.har

.to.bre

easi

ly.

. e.pur

pose

.of.a.be

r-optic

.con

nector

.is.to.ec

iently

.con

vey

.the.opti

cal

.sign

.fro

.one.lin

.to.

the.next..Typically,.connectors.are.plugs.and.are.mated.to.precision.couplers.or.sleeves.(female)..Some.

con

nectors

.are.desi

gned

.to.join.be

.end

.in.ord

.to.red

uce

.re

ections

.res

ulting

.fro

.the.gla

ss-to-air-

to-glass

.tra

nsition.

. e.opti

cal

.los

.in.a.be

r-optic

.con

nector

.is.the.pri

mary

.mea

sure

.of.dev

ice

.qua

lity.

Ret

urn

.los

.is.the.opti

cal

.pow

.tha

.is.re

ected

.towa

.the.sou

rce

.by.a.con

nector.

.Con

nector

.ret

urn

los

.in.a.sing

le-mode

.lin

.can.di

use

.bac

.into.the.las

.cav

ity,

.deg

rading

.its.sta

bility.

.In.a.mul

timode

lin

.ret

urn

.los

.can.cau

.ext

raneous

.sign

als,

.red

ucing

.ove

rall

.per

formance.

.To.mak

.sur

.tha

.lig

is.pas

sing

.thr

ough

.the.sys

tem

.pro

perly,

.tes

ting

.is.gua

ranteed.

. is.is.don

.wit

.a.mod

ied

.typ

.of.

as

hlight

.dev

ice

.and.tak

.onl

.a.few.min

utes

.to.per

form.

.A.cal

ibrated

.lig

.sou

rce

.put

.inf

rared

.lig

into.one.end.of.the.be

.and.a.cal

ibrated

.met

.mea

sures

.the.lig

.arr

iving

.at.the.oth

.end.of.the.be

 e.equ

ipment

.use

.for.tes

ting

.is.cal

led

.an.opti

cal

.tim

.dom

ain

.re

ectometer

.(OT

DR).

. is.dev

ice

Media 2-11

uses.light.backscattering.to.analyze.bers..OTDR.takes.a.snapshot.of.the.ber’s.optical.character-

istics.

.It.send

.a.high

-powered

.puls

.into.the.ber.and.meas

ures

.the.ligh

.scat

tered

.back.towa

.the.

inst

rument.

.OTDR.can.be.used.to.loca

.ber.brea

ks,

.spli

ces,

.and.conn

ectors,

.as.well.as.to.meas

ure

loss

.In.orde

.to.comp

letely

.spec

ify

.a.ber

-optic

.cabl

.four.prim

ary

.perf

ormance

.cate

gories

.must.be.

quan

tied:

.inst

allation

.spec

ications,

.envi

ronmental

.spec

ications,

.ber.spec

ications,

.and.opti

cal

spec

ications.

2.4 Wireless Links

Although.a.wireless.radio.link.has.many.advantages.with.respect.to.mobility,.the.list.of.technical.

hurd

les

.to.be.tack

led

.is.impr

essive.

. e.wire

less

.link.depe

nds

.on.many.para

meters

.like.dist

ance,

ree

ctors,

.inte

rferers,

.weat

her

.cond

ition,

.and.much.more

.In.orde

.to.achi

eve

.a.desi

red

.link.qual

ity,

larg

.safe

.marg

ins

.are.need.to.be.used.and.suit

able

.modu

lation

.form

ats

.have.to.be.chos

en.

.A.link.

will.neve

.guar

antee

.100%.perf

ect

.tran

smission

.of.the.info

rmation;

.high

.BERs.than.for.wire

.link

have.to.be.expe

cted.

2.4.1 Physical Properties

2.4.1.1 Wavelength

Solving

.the.wave.equa

tion

.(bas

.on.Maxw

ell’s

.form

ulas)

.turn

.out.that.elec

tromagnetic

.wave

.prop

gate

.with.the.spee

c =1 µε

.with.μ.denoting.the.permeability.and.ε.denoting.the.permittivity.of.the.

medi

.the.wave.is.prop

agating

.in..For.air,.whic

.is.the.typi

cal

.tran

sport

.medi

um,

.the.free

-space

.per-

eability

.and.perm

ittivity

.μ

.and.ε

,.respectively,.can.be.used.as.a.rst.approximation,.resulting.in.a.

prop

agation

.spee

.of.abou

.the.spee

.of.ligh

.c.≈.c

.(the.error.due.to.approximation.is.<10

−3

)..Hence,.the.

wave

length

.in.air.can.be.wri

tten

.as

λ ≈

(2.2

)

with

.f.den

oting

.the.rad

.wav

e’s

.fre

quency.

Because

.of.havi

.no.boun

dary

.cond

itions

.like.in.an.elec

tromagnetic

.wave

guide,

.the.prop

agation

spee

.c.re

ects

.the.pha

.vel

ocity.

.As.thi

.is.a.con

stant

.val

ue,

.no.dis

persion

.occ

urs.

2.4.1.2

therm

al Noise

.rang

.of.wire

less

.comm

unication

.syst

ems

.is.limi

ted

.by.seve

ral

.reas

ons;

.one.of.them.bein

.a.too.

smal

.signa

.and,.ther

efore,

.a.too.low.signa

l-to-noise

.rati

.(SNR

. e.powe

.spec

tral

.densi

.of.ther

mal

nois

.of.a.resis

tor

.can.be.calc

ulated

.by.

⏤

.=.4K

TR with.k

.denoting.Boltzmann’s.constant,.T.denoting.

the.temp

erature,

.and.R.is.the.resis

tor

.valu

.As.ante

nnas

.of.wire

less

.comm

unication

.syst

ems

.on.eart

are.typi

cally

.oper

ated

.at.or.arou

.room.temp

erature,

.the.nois

.powe

.for.a.give

.band

width

.B.can.be.

expr

essed

.as.P

.=.k

TB,.or.with.the.power.in.dBm.by

. P B

n( )

log( ).

dBm

= − +174 10 . (2.3)

.very.comm

only

.used.dBm.den

.deci

bels

.rela

tive

.to.1.mW,.mean

ing

.0.dBm.=.1.mW..For.exam

ple,

a.wire

less

.syst

.with.band

width

.B.=.2.MHz.rece

ives

.=.7.9.fW.(−111.dBm).noise.power.

2.4.1.3

Chan

nel Capacity

.orde

.to.dete

rmine

.the.maxi

mum

.data.rate.that.can.be.achi

eved

.over.a.wire

less

.link

.the.Shan

non–

Hartley

.theo

rem

.can.be.used

.It.give

.an.uppe

.boun

.for.the.chan

nel

.capa

city

.for.an.erro

r-free

.tran

mission

.rat

.of.ran

dom

.dat

. e.cha

nnel

.cap

acity

.C.(bi

ts/s)

.is.giv

.by

2-12 Industrial Communication Systems

C B= +log ( )

1 SNR

(2.4)

where

.B.is.the.channel.bandwidth.and.the.signal.to.noise.ratio.(SNR).=.P

.(signal.power.P

.and.noise.

power.P

2.4.1.4

Free-Space Path Loss, Fresnel Zone

For

.determination.of.the.required.transmit.power.levels.and/or.receiver.sensitivities,.it.is.important.to.

know.the.attenuation.of.the.air-link.between.transmit.and.receive.antenna..In.the.most.simple.case.of.

two.antennas.that.are.in.line.of.sight.(e.g.,.point-to-point.radio.systems),.the.so-called.free-space.path.

loss.L

.equals













(2.5)

with

.d.as.the.distance.between.the.transmitter’s.and.receiver’s.antenna..Note.that.the.loss.depends.on.

the.frequency:.Doubling.the.transmission.frequency.increases.the.path.loss.by.four.

Having

.a.direct.line.of.sight.path.between.both.antennas.is.not.the.only.prerequisite.for.applying.

formula.2.5..Radio.waves.do.not.propagate.only.at.a.direct.line.from.the.transmitter.to.the.receiver..

 ey.occupy.a.certain.volume.dened.by.the.concentric.Fresnel.ellipsoids..By.ensuring.no.or.almost.

no.obstacles.(20%.obstruction.as.a.rule.of.thumb).to.be.within.the.rst.Fresnel.ellipsoid/rst.Fresnel.

zone,.the.wireless.link.can.be.assumed.not.to.be.inuenced.(no.additional.attenuation).. is.is.shown.

in.Figure.2.10.

.rst.Fresnel.zone’s.radius,.F

,.at.a.given.point.can.be.calculated.by

d d

1 2

(2.6)

For

.example,.if.someone.plans.to.run.a.point-to-point.radio.link.at.2.45.GHz.over.a.distance.d.=.d

.+.d

.=.1.km,.

the.maximum.cross-section.radius.r.(see.Figure.2.10).becomes.r.=.5.5.m.. is.means.that.the.antennas.need.to.

be.mounted.5.5.m.higher.than.an.obstacle.in.the.middle.of.the.radio.link.

2.4.1.5

antennas

Each

.wireless.transmission.needs.antennas.for.reception.and.transmission..An.antenna.is.characterized.

by.lots.of.parameters.like.size,.bandwidth,.gain,.return.loss.(matching.to.system.impedance),.and.many.

FIGURE 2.10 First.Fresnel.zone.of.a.line.of.sight.radio.link.

Media 2-13

others..For.planning.a.radio.link,.the.most.important.parameter.is.the.antenna’s.gain:. e.gain.is.the.

ratio.of.the.radiation.intensity.P

rad

(ϕ,.ϑ).in.the.angular.direction.ϕ,.ϑ.to.the.antenna.input.power.P

( , )

φ ϑ π

φ ϑ

= 4

rad

(2.7)

.the.atenna.gain.is.specied.without.an.angular.dependency,.the.maximum.value.G.=.max.(G(ϕ,.ϑ)).is.

meant.. e.gain.G.represents.the.maximum.intensity.compared.to.the.average.intensity.(Figure.2.11).

.theoretical.antenna.that.emits.the.entire.input.power.provided.uniformly.into.all.angular.direc-

tions,

.a.so.called.isotropic.antenna,.has.a.gain.of.1.. e.physically.feasible.Hertzian.dipole.or.short.

dipole.has.a.gain.of.1.76.dBi.(dB.relative.to.isotropic.antenna)..It.consists.of.two.wires.with.a.total.

length.<<λ.and.has.the.feed.point.in.the.center.of.the.antenna..Sometimes,.also.gain.measures.in.dBd.

units.are.found:.these.refer.to.the.short.dipole..For.example,.4.2.dBd.=.(4.2.+.1.76).dBi.=.5.96.dBi.

2.4.1.6

Non–Line of Sight Channels and Moving antennas/Objects

 ere

.are.much.more.problems.with.generic.wireless.links.. e.most.important.impairments.are.as.follows:

•

. Interference:. e.assumption.that.the.range.of.a.communication.link.is.limited.by.thermal.noise.

is.only.true.when.operating.at.an.exclusive.frequency.that.is.not.reused.by.someone.else.within.

reception.range.of.a.wireless.receiver..It.is.much.more.typical.that.the.link.is.limited.by.interfer-

ence

.from.other.users..For.example,.a.cellular.phone.system.where.the.same.frequency.is.reused.

some.radio.cells.away.can.be.mentioned..Another.example.is.a.license-free.industrial,.scientic,.

and.medical.(ISM).radio.frequency.band..Because.everyone.can.operate.radio.equipment.within.

those.bands,.the.range.of.a.wireless.local.area.network.(LAN).is.oen.limited.due.to.other.access.

points.or.other.devices.operating.at.the.same.frequency:.For.802.11b/g.wireless.LANs.this.could.

be.a.Bluetooth.radio.as.well.as.a.microwave.oven.

•

. Multipath,.Delay Spread,.Fading:.For.the.majority.of.wireless.links,.there.is.not.just.a.single.propa-

gation

.path.from.the.transmitter.to.the.receiver..For.a.cellular.system,.e.g.,.the.received.signal.

might.be.the.line.of.sight.signal.from.the.base.station,.plus.reections/diractions.from.buildings,.

plus.multiple.reections.from.parked.cars,.and.so.on..Hence,.the.received.eld.strength.is.deter-

mined

.by.the.superposition.of.all.paths.at.the.point.of.the.antenna..Due.to.the.dierent.phase.shis.

of.the.waves,.this.results.in.constructive.and.destructive.interference.as.well,.making.the.instanta-

neous

.received.power.level.dependent.on.the.receiver’s.position.and.the.positions.of.all.reectors.

.variation.of.the.channel’s.attenuation.over.time.is.called.fading..Depending.on.the.trans-

mission

.bandwidth.B,.dierent.behavior.occurs:.For.narrowband.channels,.fading.can.be.repre-

sented

.by.a.time-varying.gain.and.phase.(at.fading)..But.in.case.of.a.broadband,.transmission.

fading.needs.to.be.represented.as.a.frequency-dependent.function.(frequency.selective.fading)..

Hence,.the.estimation.of.the.channel.and.equalization.might.be.required.on.the.receiver.side,.

making.the.wireless.receiver.much.more.complicated..

Maximum intensity

Average intensity

FIGURE 2.11 Antenna.directional.characteristics.(2D.equivalent).

2-14 Industrial Communication Systems

Fading.is.also.distinguished.by.the.rate.at.which.the.channel.changes:.When.moving.a.radio.

rec

eiver,

.the.rad

.cha

nnel

.cha

nges

.due.to.sha

dowing

.of.lar

.bui

ldings,

.whi

.res

ults

.in.a.hig

her

cha

nnel

.att

enuation

.for.som

.tim

.(sl

.fad

ing).

.Mov

ing

.a.rec

eiver

.wit

hin

.an.ind

ustrial

.env

ronment

.wit

.man

.re

ectors,

.the.cha

nnel

.und

ergoes

.fas

.cha

nges

.due.to.the.hig

.numb

.of.

mul

tipaths

.(f

ast

.fa

ding).

•

. Dop

pler Spread:

.If.the.env

ironment

.and

/or

.the.rad

.ter

minals

.are.mov

ing,

.Dop

pler

.fre

quency

shi



.occ

urs.

.In.mul

tipath

.env

ironments,

.thi

.wil

.lea

.simu

ltaneously

.to.sev

eral

.posi

tive

.and.

neg

ative

.fr

equency

.sh

is.

2.4.1.7

Lin

k Budget

For

.pla

nning

.a.rad

.sys

tem,

.a.lin

.bud

get

.cal

culation

.nee

.to.be.don

. is.can.be.eit

her

.use

.for.

det

ermining

.a.wir

eless

.lin

k’s

.ran

.und

.a.giv

.tra

nsmit

.pow

.or,.in.the.oth

.way.rou

nd,

.it.can.be.

use

.for.cal

culating

.the.req

uired

.tra

nsmit

.pow

.for.a.cer

tain

.lin

.ran

ge.

.For.a.lin

.of.sigh

.rad

.lin

thi

.ca

.be.do

.by.(a

.va

lues

.in.de

cibels)

P P G L L L G L

rx tx tx tx m rx rx

EIRP,tx

= + − − − + −

  

(2.

with

the

.re

ceived

.an

.tr

ansmitted

.po

wer

,.P

.in.dBm

the

.tr

ansmit

.an

.re

ceive

.an

tenna

.ga

,.G

.in.dBi

the

.ca

ble/connector

.lo

sses

.on.tr

ansmitter

.an

.re

ceiver

.sid

,.L

.in.dB

the

.fr

ee-space

.an

.mi

scellaneous

.lo

sses

,.L

.in.dB

.ee

ctively

.rad

iated

.pow

EIRP,tx

.denotes.the.feed.power.of.an.isotropic.antenna.generating.the.

sam

.max

imum

.el

.str

ength

.as.the.ant

enna

.use

.for.lin

.bud

get

.cal

culation.

. er

efore,

.alm

ost

.all.

rad

.reg

ulations

.lim

EIRP,tx

.instead.of.the.transmitter.power.P

.as.it.provides.a.fair.comparison.of.

max

imum

.po

wer

.de

nsities.

.mis

cellaneous

.los

ses,

,.are.very.important.for.the.reliability.of.the.link:. ey.include.additional.

los

ses

.due.to.cha

nging

.wea

ther

.con

ditions,

.com

ponent

.var

iations,

.imp

erfect

.ali

gnment

.of.the.ant

ennas

to.ea

.ot

her,

.an

.so.on

.In.ca

.of.mu

ltipath

.en

vironments,

.th

.al

.ac

count

.fo

.lo

sses

.du

.to.fa

ding.

2.4.2 types and Media access

Because.of.the.electromagnetic.spectrum.being.a.limited.resource,.access.to.frequencies.is.regulated.

by.nat

ional

.aut

horities.

.Dep

loying

.a.rad

.sys

tem,

.the

refore,

.req

uires

.get

ting

.a.loc

.lic

ense.

.Typ

ically,

lic

ensing

.a.sys

tem

.wit

.a.wid

.ban

dwidth

.and.hig

.tra

nsmit

.pow

.is.not.as.eas

.as.for.a.low

-power

nar

rowband

.sys

tem.

. e.mor

.ban

dwidth

.and.tra

nsmit

.pow

.is.req

uired,

.the.mor

.di

cult

.it.is.to.get.

a.li

cense

.an

d/or

.th

.mo

.ex

pensive

.th

.li

cense

.wi

.be

Only

. a. sma

. fra

ction

. of. the. ban

dwidth

. use

. now

adays

. is. ava

ilable

. for. unl

icensed

. use

Fur

thermore,

.fre

quencies

.are.not.equ

ivalent:

.Dep

ending

.on.the.app

lication,

.a.hig

her

.or.low

.fre

quency

.is.sui

table.

2.4.2.1

Frequencies for Wireless Links

Covering

.all.fre

quency

.ban

.in.det

ail

.wou

.go.bey

ond

.the.sco

.of.thi

.boo

.Gen

erally,

.it.can.be.sai

tha

.low

.fre

quencies

.all

.lon

ger

.ran

ges

.due.to.les

.fre

e-space

.att

enuation

.sho

.bet

ter

.pen

etration

into.bui

ldings,

.but.nee

.lar

ger

.ant

ennas,

.and.all

.onl

.low.dat

.rat

es.

.In.con

trary,

.hig

her

.fre

quencies

hav

.hig

her

.att

enuation,

.sho

.les

.pen

etration-performance,

.but.all

.hig

.dat

a-rate

.app

lications

.and.

sma

ll/integrated

.an

tennas.

Media 2-15

As.an.example,.the.three.most.popular.license-free.ISM.frequency.bands.shall.be.listed:

•

. 433.

05 … 434.79

MHz (I

TU-Region 1 only: Europe, Africa, Middle East):

.Use

.for.spe

ech

.app

lica-

tions

.(ba

by-phones,

.two

-way

.rad

ios)

.and.low

-speed

.dat

.com

munications

.(re

motes);

.dis

tances

.of.

1.km.an

.mo

.ca

.be.ac

hieved;

.go

.pe

netration

.of.wa

lls;

.hi

.nu

mber

.of.us

ers.

•

. 2.4

… 2.5

GHz

(worldwide):

.Mai

nly

.used.for.wir

eless

.LAN

.(IE

.802

.11b/g/n),

.Blu

etooth

.rad

ios,

vid

.tra

nsmitters,

.and.also.by.mic

rowave

.ove

ns;

.hig

.cha

nnel

.ban

dwidths

.all

.for.hig

h-speed

dat

.com

munications;

.typ

ical

.ran

ges

.are.bel

.200.m;.poo

.pen

etration

.of.wal

ls;

.ver

.hig

.num

ber

of.user

•

. 5.72

5 … 5.875

GHz (w

orldwide):

.Mai

nly

.use

.for.wir

eless

.LAN

.(IE

.802

.11a),

.hig

.tra

nsmit

powers.allow.compensation.of.higher.free-space.attenuation;.high.channel.bandwidths.allow.for.

hig

h-speed

.da

.co

mmunications;

.ve

.po

.wa

.pe

netration;

.st

ill

.lo

.nu

mber

.of.us

ers.

2.4.2.2

Media access

While

.cel

lular

.sys

tems

.acc

ess

.the.wir

eless

.cha

nnel

.coo

rdinated

.by.a.mas

ter

.(e.

g.,

.a.bas

.sta

tion),

.the.

opp

osite

.is.tru

.for.ISM.ban

ds.

.As.sev

eral

.sys

tems

.coe

xist

.(ty

pically

.not.kno

wing

.fro

.eac

.oth

er),

.a.

coo

rdinated

.acc

ess

.is.not.pos

sible.

.Als

.for.man

.ISM.ban

ds,

.a.sys

tem

.is.all

owed

.to.tra

nsmit

.a.rad

sign

al,

.eve

.if.the.cha

nnel

.is.in.use.by.som

eone

.els

.To.pre

vent

.ISM.ban

.fro

.bec

oming

.use

less

.due.

to.too.mu

.in

terference,

.se

veral

.re

quirements

.ha

.to.be.fu

llled:

•

. Max

imum transmit power:

.Lim

iting

.the.tra

nsmit

.pow

.res

ults

.in.a.max

imum

.ran

.for.the.

com

munication

.sy

stem

.an

.th

us,

.li

mits

.th

.di

stance

.wh

ere

.in

terference

.ca

.oc

cur.

•

. Max

imum bandwidth:

.By.lim

iting

.a.sys

tem’s

.rad

.ban

dwidth,

.a.sing

.sys

tem

.is.not.abl

.to.blo

the.en

tire

.IS

.ba

.an

.th

erefore,

.co

llisions

.ar

.re

duced.

•

. Fix

ed channel list:

.De

ning

.a.se

.of.ce

nter

.fr

equencies

.re

duces

.co

llisions.

•

. Max

imum duty cycle and maximum transmit time:

.Bot

.req

uirements

.red

uce

.the.tim

.a.sys

tem

.is.

all

owed

.to.us

.th

.ra

dio

.ch

annel.

.As.a.co

nsequence,

.th

.pr

obability

.fo

.a.co

llision

.re

duces.

•

. Lis

ten before talk:

.All

owing

.a.sys

tem

.to.use.a.cha

nnel

.onl

.in.cas

.it.is.not.in.use.giv

.a.hig

red

uction

.of.co

llisions

.bu

.in

creases

.ac

cess

.ti

mes

.an

.la

tency.

2.4.3 Modulation

Typical.modulation.formats.of.present.communication.systems.are.as.follows.(detailed.information.is.

giv

.by.[J

P95]

.an

.[S

H88]):

•

. Gau

ssian minimum shi keying (GMSK):

. is.non

linear

.mod

ulation

.for

mat

.has.ver

.poo

.spe

tral

.ec

iency

.(bi

.rat

.per.ban

dwidth)

.and.can

.the

refore,

.be.use

.for.low.dat

.rat

.onl

.Typ

ical

app

lications

.are.low.pow

er,

.low.dat

a-rate

.sen

sor

.tra

nsmitters.

.GMS

.is.als

.use

.by.the.cel

lular

glo

bal

.sy

stem

.fo

.mo

bile

.co

mmunication

.(G

SM)

.sy

stem.

•

. Sin

gle carrier pulse-amplitude modulation (PAM) with high bit rates:

.Her

.the.car

rier

.fre

quency

is.mul

tiplied

.by.a.com

plex

.val

.rep

resenting

.the.tra

nsmission

.symb

ol.

. is.fac

tor

.rem

ains

.con

stant

.unt

.the.nex

.symb

.is. tra

nsmitted.

. e.com

plex

.val

ues

.rep

resenting

.the.symb

ols

.are.

spread.across.the.complex.plane.but.centered.around.the.origin.. erefore,.the.resulting.transmit.

sign

.is.alt

ered

.in.mag

nitude

.and.pha

.dep

ending

.aro

und

.the.tra

nsmit

.symb

ol.

.Als

.sev

eral

bit

.are.pac

ked

.into.a.sing

.symb

ol.

. is.can.be.one.bit

/symbol

.for.bin

ary

.pha

se-shi

.key

ing

(BP

SK),

.two.bit

s/symbol

.for.qua

drature

.pha

se-shi

.key

ing

.(QP

SK),

.or.eve

.mor

.bit

s/symbol

.for.

qua

drature

.amp

litude

.mod

ulation

.(QA

.for

mats.

.A.hig

.dat

.rat

.is.ach

ieved

.by.usin

.a.lar

symb

.al

phabet

.(m

any

.bi

.pe

.tr

ansmit

.sy

mbol)

.an

.hi

.sy

mbol

.ra

tes.

•

. Ort

hogonal frequency division multiplex (OFDM):

.Bec

ause

.cla

ssic

.PAM.use

.a.lar

.symb

.alp

ha-

bet

.(QA

M-16,

.QAM

-64)

.and.hig

.symb

.rat

es,

.eac

.bit.is.“sp

read”

.acr

oss

.the.ent

ire

.sys

tem’s

ban

dwidth.

.In.cas

.of.a.fre

quency

.sel

ective

.cha

nnel

.(e.

g.,

.due.to.mul

tipath),

.the.cha

nnel

.res

ponse