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

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2-16 Industrial Communication Systems

needs.to.be. equalized.by. the.receiver.that. requires. estimation.of.the.channel..If. the.channel.

res

ponse

.has.mul

tiple

.zer

.in.the.fre

quency

.dom

ain,

.it.mig

.hap

pen

.tha

.equ

alization

.is.not.

pos

sible

.an

.lo

nger,

.re

sulting

.in.a.hi

.BE

.OFD

.app

roach

.is.di

erent

.to.tha

.the.sing

le,

.hig

.bit.rat

.car

rier

.is.spl

.into.mul

tiple

low

.bit.rat

.car

riers

.(mu

lticarrier

.mod

ulation).

. is.has.som

.ben

ets:

.the.sub

-channels

.hav

low.ban

dwidth

.and.at.fad

ing

.can.be.ass

umed

.for.eac

.sub

-channel

.(ev

.if.the.ent

ire

.ban

width

.acr

oss

.all.sub

-channels

.sho

.fre

quency

.sel

ective

.fad

ing).

.Hen

ce,

.now.cha

nnel

.est

imation/

equalization

.is.nec

essary.

.Fur

thermore,

.if.the.cha

nnel’s

.fre

quency

.res

ponse

.has.nul

.on.som

sub

-carriers,

.on

.th

ose

.sh

.bi

.er

rors.

.By.ap

propriate

.co

ding,

.th

.ov

erall

.BE

.ca

.be.ke

.lo

•

. Ult

ra wideband (UWB):

. is.ver

.new.tec

hnology

.spr

eads

.the.dat

.bit

.ove

.ban

dwidths

.in.exc

ess

of.500.MHz

. is.can.be.don

.by.the.gen

eration

.of.ext

remely

.sho

.pul

ses

.in.the.ran

.of.som

100.ps.for.exa

mple.

. is.res

ults

.in.hig

.imm

unity

.to.mul

tipath

.fad

ing

.whi

.hav

ing

.the.pot

ential

to.pro

vide

.ver

.hig

.dat

.rat

.(>5

.MBi

t/s).

.UWB.reu

ses

.fre

quency

.ban

.alr

eady

.occ

upied

.by.

oth

.com

munication

.sys

tems.

.Due.to.UWB

’s

.bro

adband

.nat

ure,

.the.spe

ctral

.pow

.den

sity

.is.

ver

.lo

.ex

isting

.co

mmunication

.sy

stems

.ar

.ha

rdly

.a

ected

.by.UW

2.4.4 Bit Coding, Multiplexing

2.4.4.1 Bit Coding

Because

.of.the.rel

atively

.hig

.BER

.of.a.wir

eless

.cha

nnel

.(du

.to.fad

ing),

.muc

.eo

.has.to.be.spe

.on.

for

ward

.er

ror

.co

rrection.

.Ty

pically,

.tw

.di

erent

.ty

pes

.of.bi

.er

rors

.ne

.to.be.co

nsidered:

•

. Sin

gle bit errors:

.Due.to.ver

.sma

.rec

eived

.sign

.str

ength

.the.SNR.dec

reases,

.lea

ding

.to.hig

her

BER

.By.usin

.err

.cor

recting

.cod

es,

.it.is.pos

sible

.to.dra

matically

.red

uce

.the.BER.by.sac

ricing

the.dat

.rat

. e.IEE

.802

.11x

.sta

ndard

.uses.cod

ing

.rat

.dow

.to.0.5

.mea

ning

.eac

.tra

nsmitted

bit.is.ac

companied

.by.an.er

ror-correction

.bi

.an

.th

us,

.re

ducing

.th

.us

able

.bi

.ra

.by.50

•

. Bur

st errors:

.Bec

ause

.of.the.tim

e-varying

.fad

ing

.pro

perties

.of.a.wir

eless

.lin

.a.com

plete

.los

.of.

the.rec

eption

.oe

.occ

urs

.for.sho

.tim

.int

ervals.

.Bec

ause

.err

or-correction

.cod

.can

not

.dea

wit

.bur

.err

ors,

.int

erleaving

.wil

.be.app

lied.

. is.mea

.tha

.the.dat

.bits.are.spr

ead

.ove

tim

.If.a.sho

.bur

.err

.occ

urs,

.the.bur

.err

.is.spr

ead

.ove

.the.bit.str

eam

.by.the.rec

eiver’s

de-

interleaver.

.As.a.con

sequence,

.the.blo

.of.err

oneous

.bit

.is.spr

ead

.into.mul

tiple

.sing

.bit.

err

ors.

.No

.th

.er

ror-correction

.co

.ca

.be.us

.fo

.ca

lculating

.th

.co

rrect

.bi

ts.

2.4.4.2

Mul

tiplexing

Sharing

.a.wi

reless

.fr

equency

.ba

.ca

.be.do

.by.on

.or.a.co

mbination

.of.th

.fo

llowing

.th

ree

.wa

ys:

•

. Fre

quency division multiple access (FDMA):

.Eac

.com

munication

.lin

.use

.its.own.fre

quency.

Hen

ce,

.no.co

ordination

.be

tween

.th

.di

erent

.li

nks

.is.re

quired.

•

. Tim

e division multiple access (TDMA):

.Mul

tiple

.use

.sha

.one.fre

quency

.on.the.basi

.of.tim

slo

ts.

. i

.ac

cess

.sc

heme

.re

quires

.co

ordination

.of.al

.tr

ansmitters.

•

. Cod

e domain multiple access (CDMA):

.Her

.all.use

.use.the.sam

.fre

quency

.at.the.sam

.tim

(co

ntinuously).

.To.dis

tinguish

.bet

ween

.mul

tiple

.use

rs,

.eac

.tra

nsmitter

.enc

odes

.the.dat

.bit

by.a.spr

eading

.seq

uence

.(di

rect

.seq

uence

.spr

ead

.spe

ctrum,

.DSS

S).

.By.usin

.di

erent

.spr

eading

seq

uences

.wit

.low.cro

ss-correlation,

.it.is.pos

sible

.to.sep

arately

.ext

ract

.a.sing

.tra

nsmitter’s

.bit

at.th

.re

ceiver

.sid

.fo

.ea

.us

er.

•

. Fre

quency hopping spread spectrum (FHSS):

.By.rap

idly

.cha

nging

.the.cen

ter

.fre

quency

.in.a.pse

udo-

random

.seq

uence

.(kn

own

.by.the.tra

nsmitter

.and.rec

eiver),

.the.tra

nsmission

.is.spr

ead

.in.ban

width.

. is.is.not.a.rea

.mul

tiplexing

.tec

hnique

.as.tra

nsmissions

.on.the.sam

.fre

quency

.can.occ

but.he

lps

.to.re

duce

.BE

.in.mu

ltiuser

.ap

plications.

Media 2-17

 e.cellular.GSM.system.uses.a.combination.of.FDMA.and.TDMA.(with.eight.timeslots)..On.the.

cont

rary,

.UMT

.is.bas

.on.a.comb

ination

.of.FDM

.and.CDM

2.4.5 realizations and Standards

Table.2.3.presents.a.summary.of.several.radio.communication.standards.

references

[JP95].J..G..Proakis,.Digital Communications,.McGraw-Hill,.New.York.(1995).

[NS08]

.Natio

nal

.Semicond

uctor,

.LVDS Ow

ner’s Manual,

.Natio

nal

.Semicond

uctor,

.Santa.Clara,.Califo

rnia

(2008).

[SH88]

.S..Hay

kin,

.Dig

ital Communications,

.John.Wile

.&.Son

.New.Yor

.(1988).

[TI04]

.Texas.Ins

truments,

.App

lication

.Repo

.SLLA067A,.Texas.Ins

truments,

.Dal

las,

.Texas.(2004).

TABLE 2.3 Overview.of.Dierent.Communication.Standards

Standard

EIRP,tx

(dBm)

Required.P

(dBm)

Net.Bit.Rate.

(Mbit/s) Modulation

Frequency.

(GHz)

Bandwidth.

(MHz)

.radio +80 −116 n.a.

FM ~0.1 0.2

GSM

.mob

ile +33 −110

0.384 GMSK …/0.9/1.8/… 0.2

UMTS

.mob

ile +33 −124

/−113

14.4 W-CDMA 1.9…2.2 4.7

GPS +46 −127.5

0.00005 DSSS 1.575 2.0

Bluetooth

.1.2 +20 −70 2.1

FHSS ISM

.2.45 80.0

IEEE

.802.11a +30 −82…−65

54 OFDM 5.2…5.8 16.6

IEEE

.802.11g +20 −82…−65

54 OFDM ISM.2.45 16.6

IEEE

.802.11p

+44.8

−85…−68

27 OFDM 5.85…5.92 8.3

For.speech.transmission.(values.for.base.stations,.mobiles.do.not.perform.that.good).

For.384.kbit/s.data.transmission.(values.for.base.stations,.mobiles.do.not.perform.that.good).

SNR.=.−16.dB.(received.power.is.16.dB.below.thermal.noise.power).

Depending.on.coding.rate.and.sub-carrier.modulation.

Wireless.access.in.vehicular.environments.(WAVE),.e.g.,.car-to-car.communication.

+44.8.dBm.EIRP.is.for.the.United.States,.additional.limitations.apply.per.FCC.47CFR.[B8],.90.375.

3-1

3.1 Introduction

 e.technical.details.of.media.access.are.generally.implemented.in.OSI.layer.1.and.2..Usually.the.nature.

of.OSI.laye

.1.dete

rmines

.the.oper

ation

.cond

itions

.and.feasi

ble

.acce

.mech

anisms

.that.can.be.impl

mented

.in.lay

.2.

dia

.acce

.on.full

-duplex

.medi

.is.oen.a.simpl

.task.beca

use

.each.stat

ion

.can.send.data.inde

pen-

dently

.of.each.othe

.On.the.othe

.hand

.medi

.acce

.on.half

-duplex

.medi

.is.typi

cally

.nont

rivial

.and.

requ

ires

.sol

utions

.for.the.fol

lowing

.iss

ues:

•

. Acce

.arb

itration

•

. Coll

ision

.tre

atment

•

. Acce

.fai

rness

•

. Access.priorities.for.selected.stations.or.messages

•

. Qual

ity

.of.ser

vice

.(Qo

.req

uirements

.(qu

euing)

•

. Real

-time

.req

uirements

Furthermore,

. most. inte

resting

. for. each. acce

. mech

anism

. is. the. ques

tion

. abou

. medi

. util

ization.

Typi

cally,

.ther

.is.a.natu

ral

.trad

e-o

.betw

een

.real

-time

.supp

ort

.and.maxi

mum

.medi

.util

ization,

.whic

migh

.be.solv

.only.by.incr

easing

.the.comp

lexity

.of.the.syst

.(for.exam

ple,

.by.intr

oducing

.a.base.

stat

ion

.whi

.is.typ

ically

.use

.in.wir

eless

.dat

a/voice

.com

munications).

.subs

equent

.sect

ions

.prov

ide

.a.deta

iled

.prob

lem

.desc

ription,

.espe

cially

.consi

dering

.half

-duplex

medi

. acce

. in. more. deta

il,

. and. expl

ain

. possi

ble

. solu

tions.

. Histo

rical

. and. curr

ent

. exam

ples

. are.

alsog

iven.

Media Access Methods

3.1. Introduction.......................................................................................3-1

3.2

. Full

-Duplex

.Med

.Acc

ess................................................................3-2

3.3

. Sync

hronous

.Acc

ess

.Arb

itration

.Con

cepts...................................3-2

Static.Timeslot.Mechanisms. •. Dynamic.Timeslot.Mechanisms

3.4. Statistic.Access.Arbitration.Concepts............................................3-3

Aloha.Mechanisms. •. Pure.Aloha. •. Slotted.Aloha

3.5. Carrier.Sense.Mechanisms.with.Exponential.Backo................3-4

Using.Collision.Detection. •. Using.Collision.Avoidance. •. .

TokenMechanisms. •. Polling.Mechanisms

3.6. Other.Media.Access.Issues...............................................................3-6

Access.Fairness. •. Access.Priorities. •. Quality.of.Service. •. .

HiddenStations

References.......................................................................................................3-7

Herbert Haas

Vienna University

of Tec

hnology

Manfred Lindner

Vienna University

of Tec

hnology

3-2 Industrial Communication Systems

3.2 Full-Duplex Media access

If. the. communication. medium. supports. full-duplex. access,. most. communication. problems. are.

inhe

rently

.sol

ved.

.Ful

.dup

lex

.can.be.ach

ieved

.usin

.the.fol

lowing

.met

hods:

•

. Sepa

rated

.wire.pair

.with

.cabl

es,

.one.for.tran

smitter

.(TX)

.the.othe

.for.rece

iver

.(RX)

. is.is.

the.mos

.imp

ortant

.sol

ution

.in.pra

ctical

.imp

lementations

.(e.

g.,

.use

.in.100

BaseT).

•

. Sepa

rated

.TX.and.RX.be

.in.opti

cal

.com

munication

.env

ironments.

•

. Sepa

rated

.fre

quencies,

.one.fre

quency

.for.TX,.the.oth

.for.RX.

•

. Sepa

rated

.wave

lengths,

.one.for.TX,.one.for.RX,.with

.ber

. is.meth

.is.typi

cally

.more.

expe

nsive

.tha

.usin

.sep

arate

.be

rs.

•

. Echo.canc

ellation—both

.stat

ions

.send.and.recei

.at.the.same.time.over. the.same. wire

. is.

scheme.has.been.implemented,.for.example,.on.Gigabit.Ethernet.on.twisted.pair.media.(1000BaseT).

•

. Alte

rnating

.TX.and.RX.time

slots—this

.meth

.is,.for.exam

ple,

.supp

orted

.on.ISDN.(Basi

.Rate.

Inte

rfaces,

.BRI

)

.lin

es.

When

.full

-duplex

.comm

unication

.is.supp

orted,

.a.stat

ion

.will

ing

.to.tran

smit

.a.fram

.does.not.need.to.

consi

der

.the.tran

smission

.stat

.of.the.othe

.stat

ion.

.Inst

ead,

.the.curr

ent

.stat

ion

.can.imme

diately

.init

ate

.the.tran

smission.

.Any.issu

.rega

rding

.QoS,.bloc

king,

.or.real

-time

.requ

irements

.must.be.solv

.by.

the.actu

.netw

ork

.node.(e.g

.an.Ether

net

.swit

ch)

.using.queu

ing

.and.sche

duling

.to.deci

.what.shou

be.for

warded

.rs

.to.the.nex

.lin

Consequently,

.half

-duplex

.medi

.dese

rve

.clos

.atte

ntion,

.espe

cially

.beca

use

.seve

ral

.mode

.syst

ems

such.as.wireless.networks.are.typically.half.duplex..Furthermore,.the.half.duplex.mode.is.preferred.

in.many.real

-time

.comm

unication

.syst

ems

.(e.g

.Ether

net

.Powe

rlink)

.in.orde

.to.avoi

.brid

ges

.whic

woul

.int

roduce

.som

.add

itional

.lat

ency.

3.3 Synchronous access arbitration Concepts

Access.on.half-duplex.media.requires.algorithms.that.select.exactly.one.station.at.each.time.for.trans-

mission.

.For.a.part

icular

.stat

ion,

.the.prob

lem

.aris

.when.to.send.data

. e.algo

rithms

.can.be.divi

ded

in.det

erministic

.and.sta

tistical

.app

roaches.

Obviously,

.acce

.arbi

tration

.is.the.cent

ral

.mech

anism

.that.must.be.tune

.and.cont

rolled

.to.achi

eve

or.imp

rove

.fai

rness,

.pre

cedence,

.QoS

.and.rea

l-time.

3.3.1 Static timeslot Mechanisms

A.technically.simple.approach.to.handle.half-duplex.media.access.is.to.assign.a.dedicated.timeslot.of.

xed.size.to.each.stat

ion.

.If.ther

.are.N.time

slots

.with

.a.peri

odic frame,

.then.the.medi

.can.supp

ort

up.to.N.sta

tions

.(ho

wever,

.usu

ally

.N.−.1.bec

ause

.one.tim

eslot

.is.oe

.use

.for.man

agement

.pur

poses).

.mode.is.also.know

.as.time.divi

sion

.mult

iple

.acce

.(TDM

.and.is.gene

rally

.best.suite

.for.isoc

hro-

nous

.serv

ices

.such.as.tele

phony,

.wher

.the.comm

unication

.patt

erns

.can.be.desc

ribed

.by.call

.or.circ

uits

.(ste

ady

data.rate

.bloc

king,

.non-

bursty).

.TDMA.is.imple

mented,

.for.exam

ple,

.in.GSM,.UMTS

.DECT

.or.TETR

.some.case

.(e.g

.TETR

A),

.mult

iple

.stat

ions

.that.wish.to.comm

unicate

.with.each.othe

.can.stat

isti-

cally

.shar

.a.part

icular

.time

slot.

. e.major.disa

dvantage

.is.that.one.stat

ion

.cann

.util

ize

.unus

.time

lots

.easil

.(wit

hout

.signi

cant

.incr

ease

.of.the.over

all

.mana

gement

.comp

lexity;

.an.exam

ple

.is.GSM)

 ere

fore,

.TDM

.is.not.a.goo

.cho

ice

.to.tra

nsmit

.bur

sty

.dat

.tra

c.

3.3.2 Dynamic timeslot Mechanisms

In.order.to.achieve.better.media.utilization,.TDMA.can.be.implemented.with.dynamically.negotiated.

time

slot

.sizes

.Ever

.addi

tional

.stat

ion

.regi

stering

.at.the.cent

ral

.base.stat

ion

.will.be.assig

ned

.a.dedi

cated

Media Access Methods 3-3

timeslot.at.the.expense.of.the.timeslot.sizes.of.other.existing.stations.. at.is,.assuming.a.fair.environ-

ment,

.the.base.stat

ion

.woul

.crea

.new.time

slots

.by.divi

ding

.the.whol

.time.peri

.(typ

ically

.call

.a.

supe

rframe)

.by.the.tota

.numb

.of.sta

tions.

Alternatively,

.time

slot

.sizes.may.rema

.cons

tant

.but.the.base.stat

ion

.may.assig

.a.set.of.D.time

slots

for.part

icular

.stat

ions.

. at.is,.a.singl

.stat

ion

.may.be.assig

ned

.a.singl

.time

slot

.or.D.time

slots,

.assu

ing

.the.sup

erframe

.con

sists

.of.N.>.D.tim

eslots.

.solu

tion

.achi

eves

.a.mix.of.both.real

-time

.beha

vior

.and.good.trun

.util

ization

.and.is.ther

efore

comm

.in.mod

ern

.wir

eless

.cel

lular

.net

work

.sol

utions,

.inc

luding

.WiMA

.GSM

.and.UMT

3.4 Statistic access arbitration Concepts

3.4.1 aloha Mechanisms

 e.DARPA-founded.Aloha.media.access.protocol.has.been.invented.in.1971.to.interconnect.several.

mini

computers

.dist

ributed

.over.the.isla

nds

.of.Hawa

.via.a.singl

.wire

less

.chan

nel

.[Abr

70].

.Alth

ough

today.Aloh

.is.seld

.used.prac

tically,

.it.is.fund

amental

.to.unde

rstand

.the.Aloh

.mech

anisms

.(esp

cially

.the.dra

wbacks)

.bef

ore

.stu

dying

.mod

ern

.con

cepts

.suc

.as.car

rier

.sen

.mul

tiple

.acc

ess

.(CS

MA).

All

.clas

sic

.Aloh

.meth

ods

.desc

ribed

.belo

.used.xed.fram

.sizes.(cel

ls)

.and.did.not.supp

ort

.carr

ier

sens

.(CS

3.4.2 Pure aloha

 e.basic.idea.of.Aloha.is.implemented.in.Pure Aloha:

. 1..Ever

.sta

tion

.may.tra

nsmit

.whe

never

.it.wan

. 2.. Upon. coll

ision

. with. anot

her

. stat

ion

. (no. ackn

owledgment

. rece

ived

. on. a. dedi

cated

. broa

dcast

chan

nel),

.the.inv

olved

.sta

tion

.may.ret

ransmit

.the.mes

sage

.ae

.a.ran

dom

.tim

Due

.to.this.rath

.anar

chistic

.acce

.meth

od,

.the.coll

ision

.prob

ability

.is.rela

tively

.high

.Let.T.deno

the.xed.mess

age

.leng

.in.seco

nds

.and.R.the.rate.of.both.mess

ages

.and.retr

ansmissions.

.Assu

ming

that.the.star

.of.mess

ages

.and.retr

ansmissions

.can.be.desc

ribed

.via.a.Pois

son

.proc

ess,

.the.prob

ability

of.zero.mess

ages

.with

.time.T.is.exp(

−RT).

.Howe

ver,

.a.coll

ision

.occu

.if.a.part

icular

.mess

age

.is.sent.at.

time.t.and.anot

her

.mess

age

.appe

ars

.with

.[t.−.T,.t.+.T].. at.is,.a.coll

ision

.occu

.if.anot

her

.(orm

ore)

mess

age(s)

.appe

.with

.a.time.inte

rval

.of.2T.s.. ere

fore,

.the.coll

ision

.prob

ability

.is.1.−.exp(

−2TR).

Henc

.the.aver

age

.numbe

.of.retr

ansmission

.per.seco

.is.R[1.−.exp(

−2TR)]

.and.addi

.the.mess

age

rate.r.lea

.to.R.=.r.+.R(1.−.exp

(−2TR)),

.or.equ

ivalently,

.r.=.R.exp

(−2TR).

Multiplying

.both.sides.by.T.give

.a.rela

tion

.betw

een

.the.chan

nel

.util

ization

.rT.and.total.chan

nel

tra

. e.chan

nel

.util

ization

.reac

hes

.a.maxi

mum

.valu

.of.1/2e.=.0.18

.when.RT.reac

hes

.0.5.. at.

is,.the.chan

nel

.is.satu

rated

.as.soon.as.two.mess

ages

.per.mess

age

.dura

tion

.time.T.are.sent.on.aver

age.

At.maxi

mum,

.onl

.18.

.of.the.cha

nnel

.ban

dwidth

.can.be.uti

lized.

Since

.the.mess

age

.rate.r.is.the.prod

uct

.of.the.numbe

.of.stat

ions

.k.time

.the.aver

age

.singl

e-station

mess

age

.rate.λ,.the.maxi

mum

.numbe

.of.stat

ions

.is.dete

rmined

.sole

.by.λ.and.T,.that.is,.k

max

.=.1/2eλT.

3.4.3 Slotted aloha

 e.throughput.of.Aloha.can.be.doubled.if.all.transmissions.must.take.place.within.xed.timeslots.

of.size.T,.whic

.is.usua

lly

.equa

.to.the.fram

.size.. at.is,.all.stat

ions

.main

tain

.a.sync

hronized

.cloc

and.tran

smissions

.must.star

.at.the.begi

nning

.of.a.time

slot.

.In.this.situa

tion,

.coll

isions

.can.only.occu

with

.a.singl

.time

slot

.T.and.not.with

.2T.as.in.the.case.of.Pure.Aloh

. ere

fore,

.using.the.same.

consi

derations

.as.abo

ve,

.the.max

imum

.cha

nnel

.uti

lization

.is.36.

8%.

Today,

.Slot

ted

.Aloh

.is.stil

.used

.for.exam

ple,

.in.RFID.netw

orks

.beca

use

.the.algo

rithm

.can.be.impl

mented

.on.simp

.ASI

.str

uctures

.and.the.thr

oughput

.req

uirements

.are.rel

atively

.low

3-4 Industrial Communication Systems

3.5 Carrier Sense Mechanisms with Exponential Backoff

3.5.1 Using Collision Detection

 e.most.important.achievement.was.made.by.Robert.Metcalfe.in.1972.when.he.created.Ethernet-based.

on.Carr

ier

.Sens

.Mult

iple

.Acce

. with. Coll

ision

. Dete

ction

.(CSM

A/CD).

.Actu

ally,

.thre

. impo

rtant

impr

ovements

.[Ch

a00]

.wer

.mad

.com

pared

.to.the.Alo

.pri

nciple:

. 1..Befo

.star

ting

.a.tran

smission,

.a.stat

ion

.must.sens

.for.an.exis

ting

.carr

ier

.to.avoi

.imme

diate

coll

isions.

. is.is.the.CS.par

.of.the.pro

tocol.

. 2.. Even.duri

.a.tran

smission,

.the.tran

smitting

.stat

ion

.must.sens

.for.a.coll

ision.

. is.is.the.CD.part.

of.the.proto

col.

.Ten.Mega

bit

.IEEE.802.

.and.Ether

net

.use.Manc

hester

.codi

.with.a.nega

tive

.DC.

leve

.duri

.coll

isions,

.each.stat

ion

.meas

ures

.an.incr

ease

.of.the.aver

age

.DC.leve

.When.the.col-

ision

.is.dete

cted,

.the.send

.must.cont

inue

.the.tran

smission

.for.the.dura

tion

.of.32.bits

. is.jam.

signal.ensures.that.all.involved.senders.back.o.for.a.random.time.

. 3..If.the.netw

ork

.diam

eter

.is.rela

tively

.larg

.so.that.the.signa

.prop

agation

.time.t

.becomes.signi-

cant

.comp

ared

.to.the.tran

smission

.dela

,.frames.“detach”.from.the.senders.and.collisions.may.

appe

.som

ewhere

.on.the.cab

le.

. 4.. To.guar

antee

.that.all.coll

isions

.are.dete

ctable,

.it.is.requ

ired

.that.fram

.have.a.mini

mum

.size.

soth

.any.coll

ision

.signa

.can.prop

agate

.back.to.the.send

er.

.In.gene

ral,

.the.dime

nsions

.of.the.

netw

ork

.size.and.the.min

imum

.fra

.size.mus

.sat

isfy

.2t

.≤.t

. 5.. e.rand

.backo



.time.is.actu

ally

.the.most.impo

rtant

.inno

vation.

.Aer.dete

ction

.of.a.coll

ision

each.send

.calc

ulates

.a.backo



.time

.whic

.is.the.prod

uct

.of.the.slot

time

.(a.fund

amental

.base.

time.unit

)

.time

.a.rand

omly

.chos

.numbe

.from.the.inte

rval

.[0.….2

],.with.k.=.min{number.of.

retr

ansmission

.atte

mpts,

.10}.. e.slot.time.is.den

.as.the.tran

smission

.dela

.of.a.64.byte.fram

.trun

cated binary exponential backo

.algo

rithm

.ensu

res

.that.the.prob

ability

.of.repe

ated

.coll

isions

is.expo

nentially

.redu

ced.

. e.backo



.is.trun

cated

.by.k.=.10,.so.up.to.1024.pote

ntial

.time

slots

.can.be.

occu

pied

.by.sta

tions

.(th

eoretically).

 ese

.impr

ovements

.enab

.Ether

net

.to.reac

.thro

ughputs

.clos

.to.the.physi

cal

.band

width,

.in.par-

icular

.95%.with.smal

.fram

.and.clos

.to.100%.with.larg

.fram

.[Bog

88].

.It.is.impo

rtant

.to.unde

stand

.that.coll

isions

.in.CSMA

/CD-based

.netw

ork

.tech

nologies

.are.part.of.the.arbi

tration

.proc

ess;

.the.

occu

rrence

.of.col

lisions

.doe

.not.ind

icate

.a.bad.desi

gn.

3.5.2 Using Collision avoidance

In.certain.environments,.most.importantly.wireless.media.within.the.RF.range,.the.concept.of.CD.is.

not.imp

lementable

.bec

ause

. 1.. e.rati

.betw

een

.TX.and.RX.powe

.is.typi

cally

.“dra

matic”

.(e.g

.110.dBm)

.so.that.“lis

ten

.whil

talk

”

.wou

.ful

.sat

urate

.(if.not.dam

age)

.the.RX.cir

cuits.

. 2.. Due.to.the.long.dist

ances,

.the.tran

smission

.time.is.oen.smal

ler

.than.twic

.the.signa

.prop

aga-

tion

.time

;

.henc

.coll

ision

.signa

.(i.e

.the.sum.of.two.or.more.fram

es)

.migh

.not.reac

.each.

send

er.

 erefore,

.wire

less

.tech

nologies,

.such.as.IEEE.802.

.WLAN

.util

ize

.a.coll

ision

.avoi

dance

.(CA).meth

with.the.fol

lowing

.pro

perties:

•

. Coll

isions

.shou

.be.avoi

ded

.if.possi

ble.

. ere

fore,

.a.send

.will

ing

.to.tran

smit

.a.fram

.must.rst.

list

.whet

her

.the.medi

.is.non-

occupied,

.and.addi

tionally

.star

.a.rand

.time

.Only.aer.

expi

ration

.of.that.time

.the.fram

.can.be.sent

. is.init

ial backo

.meth

.redu

ces

.the.coll

ision

prob

ability

.if.mul

tiple

.sta

tions

.wai

.for.the.med

uim

.to.bec

ome

.idl

Media Access Methods 3-5

•. Each.frame.must.be.acknowledged.by.the.sender..CSMA/CA.interprets.the.absence.of.acknowl-

edgment

.fra

mes

.as.col

lision.

.Obv

iously,

.ack

nowledgment

.fra

mes

.hav

.a.hig

her

.pri

ority

.tha

.dat

fra

mes

.an

.sh

ould

.no

.su

er

.fr

.th

.in

itial

.ba

cko.

•

. In.ord

.to.giv

.cer

tain

.(ma

nagement)

.fra

mes

.pre

cedence

.ove

.dat

.fra

mes,

.dat

.fra

mes

.mus

.not.

onl

.awa

.the.ini

tial

.bac

ko

.del

.but.als

.a.con

stant

.int

er-frame

.spa

.(th

.DIF

.in.IEE

.802

.11).

Man

agement

.fra

mes,

.for.exa

mple,

.ack

nowledgments,

.are.sen

.imm

ediately

.ae

.the.med

ium

.is.

idl

.an

.a

.a.sh

ort

.in

terframe

.sp

ace

.(S

IFS

.in.IE

.80

2.11,

.wh

ere

.SI

.<.DI

FS)

.is.ov

er.

.cas

.of.col

lisions,

.aga

.a.tru

ncated binary exponential backo

.alg

orithm

.ens

ures

.tha

.the.pro

bability

of.re

peated

.co

llisions

.is.ex

ponentially

.re

duced.

Generally,

.the.throughput.of.CSMA/CA.transmissions.is.approximately.50%.lower.than.the.nominal.

bit.ra

.be

cause

.ea

.pa

cket

.mu

.aw

ait

.an.ac

knowledgment.

3.5.3 token Mechanisms

Statistic.but.collision-free.medium.access.methods.can.be.implemented.via.a.token.mechanism.. e.

pri

nciple

.is.simp

le:

.A.sing

.“tok

en”

.(e.

g.,

.a.bit.ag

)

.is.han

ded

.ove

.fro

.one.sta

tion

.to.the.oth

er,

.all

ow-

ing

.eac

.sta

tion

.tha

.pos

sesses

.the.toke

.to.ini

tiate

.a.dat

.tra

nsmission.

.Ae

.a.tra

nsmission,

.the.sta

tion

mus

.fo

rward

.th

.tok

.to.th

.ne

.st

ation.

Logically,

.the.net

work

.mus

.fol

low

.a.rin

.topo

logy

.bec

ause

.the.sing

.toke

.mus

.“ro

tate”

.thr

ough

.all.

sta

tions.

. is.met

hod

.has.bee

.imp

lemented

.in.IEE

.802

.(tok

.bus

.the.phy

sical

.med

ium

.is.a.bus

)

.or.

IEE

.80

2.5

.(to

ken

.ri

ng,

.al

.th

.ph

ysical

.me

dium

.is.a.ri

ng).

Obviously

.toke

n-based

.acc

ess

.arb

itration

.pro

vides

.sta

tistic

.acc

ess

.to.the.med

ium

.bec

ause

.idl

.sta

tions

.simp

.for

ward

.the.toke

.wit

hout

.any.fra

.bei

.sen

.How

ever,

.not

.tha

.the.arb

itration

.its

elf

.is.

det

erministic,

.i.

e.,

.th

.or

der

.is.fo

reseeable.

.gen

eral,

.toke

.mec

hanisms

.pre

vent

.cha

nnel occupation,

.i.e

.oth

.tha

.wit

.CSM

A/CD-based

net

works,

.it.is.not.pos

sible

.tha

.par

ticular

.sta

tions

.are.pre

ferred

.by.a.ran

dom

.bac

ko

.alg

orithm

.dur

ing

spe

cic

.ne

twork

.co

nditions.

Moreover,

.the.rot

ating

.toke

.pro

vides

.eac

.sta

tion

.per

iodic

.tra

nsmission

.opp

ortunities

.wit

.gua

ran-

teed

.max

imum

.int

erval

.dur

ation.

. is.max

imum

.toke

.rot

ation

.tim

.(MT

RT)

.dep

ends

.on.the.numb

of.st

ations

.an

.th

.ma

ximum

.tr

ansmission

.de

lay

.pe

.st

ation.

Since

.the.toke

.is.alw

ays

.for

warded

.det

erministically

.and.the.MTR

.is.a.kno

.con

stant,

.toke

based

.acc

ess

.arb

itration

.is.wel

.sui

ted

.in.rea

l-time

.env

ironments.

.Tod

ay,

.toke

.mec

hanisms

.are.mai

nly

use

.in.e

.bu

.sy

stems,

.su

.as.Pr

obus

.(I

.61

158/IEC

.61

784).

3.5.4 Polling Mechanisms

Polling.mechanisms.logically.resemble.token.passing;.the.only.dierence.is.that.a.dedicated.central.

pol

ling

.sta

tion

.is.req

uired

.(wh

ile

.toke

.mec

hanisms

.do.not.nee

.ded

icated

.sta

tions,

.exc

ept

.for.toke

mon

itoring

.an

.re

establishment

.ta

sks).

Instead

.of.a. toke

.tha

.rot

ates

.thr

ough

.all. sta

tions,

.a. cen

tral

.pol

ling

.ins

tance

.pol

.eac

.sta

tion

whe

ther

.it.wan

.to.tra

nsmit

.dat

.If.the.sta

tion

.doe

.not.wan

.to.tra

nsmit,

.the.cen

tral

.sta

tion

.rec

eives

som

.so

.of.ne

gative

.ac

knowledgment

.an

.im

mediately

.po

lls

.th

.ne

.st

ation.

.po

lling

.ar

bitration

.me

chanism

.ha

.th

.fo

llowing

.di

stinguishing

.pr

operties:

•

. er

.is.no.tok

.tha

.can.be.los

.or.tha

.need

.to.be.mon

itored

.and.mai

ntained.

. er

efore,

.pol

ling

met

hods

.are.usu

ally

.sim

pler

.to.imp

lement

.tha

.tok

.sche

mes.

•

. is. simplicity. allows. for. scalability:. e. central. polling. instance. may. not. only. poll. normal.

sta

tions

.but.als

.oth

.pol

ling

.ins

tances

.tha

.are.res

ponsible

.to.pol

.a.sub

set

.of.sta

tions.

. is.way

a.hi

erarchical

.tr

.of.po

lling

.re

alms

.ca

.be.im

plemented.

•

. e.ce

ntral

.po

lling

.in

stance

.is.a.sin

gle

.po

int

.of.fa

ilure.

3-6 Industrial Communication Systems

Polling-based.access.arbitration.has.been.used.with.IEEE.802.12.and.is.still.implemented.in.several.

real

-time

.syst

ems.

. e.stan

dard

.IEEE.802.

.den

.an.optio

nal

.poll

ing

.mode.for.real

-time

.serv

ices

thou

.it.is.only.seld

.impl

emented

.beca

use

.queu

e-based

.CSMA

/CA

.is.prac

tically

.suc

ient.

.Anot

her

more.rec

ent

.exa

mple

.is.Ethe

rnet

.POW

ERLINK,

.an.Ethe

rnet-based

.rea

l-time

.pro

tocol.

3.6 Other Media access Issues

Although.the.problem.of.access.arbitration.is.the.most.important.issue,.this.short.section.shall.highlight.

othe

.iss

ues

.tha

.mus

.be.con

sidered

.wit

.eac

.imp

lementation.

3.6.1 access Fairness

Regarding.the.issue.of.fairness,.the.two.main.questions.are

•

. Can.the.arb

itration

.mec

hanism

.pre

vent

.sing

.sta

tions

.fro

.dom

inating

.the.med

ium?

•

. Are.the

.sce

narios

.whe

.sing

.sta

tions

.are.not.sch

eduled

.for.a.lon

.dur

ation

.(“s

tarvation”)?

Obviously,

.only.stat

istic

.arbi

tration

.meth

ods

.may.resu

.in.unfai

.situ

ations.

.One.exam

ple

.for.a.fairn

ess-

enforcing

.mech

anism

.is.to.intr

oduce

.a.post

-backo

.in.CSMA

/CA-based

.syst

ems.

. is.conc

ept

.requ

ires

.that.

ever

.stat

ion

.that.succ

essfully

.occu

pied

.the.chann

.must.star

.a.post

-backo

.time

.aer.tran

smission,

.so.

that.unsu

ccessful

.stat

ions

.are.luck

ier

.in.the.next.arbi

tration

.cycl

3.6.2 access Priorities

In.many.applications,.several.stations.or.frames.(especially.control.frames).must.be.given.a. higher.

acce

.prio

rity.

.If.stat

istical

.acce

.meth

ods

.are.used.(e.g

.CSMA

.prio

rities

.can.be.impl

emented

.by.

tuni

.backo



.time

.In.dete

rministic

.acce

.sche

mes

.(e.g

.Toke

.prio

rity

.info

rmation

.(ag

.migh

.be.

signa

led

.wit

hin

.the.toke

.Obv

iously,

.TDM.met

hods

.pro

vide

.the.sam

.acc

ess

.pri

ority

.to.eac

.sta

tion.

3.6.3 Quality of Service

Again,.the.problem.of.QoS.is.basically.an.issue.with.statistical.access.schemes.because.token.or.polling.

prin

ciples

.inhe

rently

.prov

ide

.cons

traints

.on.dela

.Obvi

ously,

.QoS.is.not.an.issu

.with.(syn

chronous)

TDM.tec

hniques.

With

.stat

istical

.acces

.schem

es,

.prio

ritized

.queu

ing

.and.conse

quently

.tuned.back

o

.is.the.only.meth

to.impl

ement

.QoS..For.exam

ple,

.IEEE.802.

11e

.den

.up.to.eigh

.queu

.per.stat

ion

.(pra

ctically,

.WiFi.

Mult

iMedia

.only.requ

ires

.four.queu

es)

.to.die

rentiate

.high

.or.lowe

.prio

rity

.pack

ets.

.Each.queu

.can.be.

cons

idered

.as.a.virt

ual

.stat

ion

.with.its.own.back

o

.cons

traints.

. at.is,.the.trun

cated

.bina

.expo

nential

back

o

.algo

rithm

.is.impl

emented

.for.each.queu

.separ

ately

.whil

.the.rang

.of.poss

ible

.rand

.numb

ers

(the.cont

ention window,

.CW).has.a.die

rent

.size.in.each.queu

.depe

nding

.on.the.prio

rity.

.QoS.mech

anism

.does.not.guar

antee

.that.high

-priority

.fram

.are.alwa

.pref

erred

.when.low-

riority

.queu

.are.also.lle

.rath

er,

.this.mech

anism

.prov

ides

.stat

istical QoS—which

.turn

.out.to.be.

suc

ient

.for.pra

ctical

.int

eractive

.voi

.and.vid

.ser

vices.

3.6.4 Hidden Stations

In.wireless.environments,.some.stations.may.interfere.with.sessions.between.other.pairs.of.stations.. is.

is.espec

ially

.an.impo

rtant

.issu

.when.the.dist

urbing

.stat

ions

.cann

.detec

.the.peer

.of.exis

ting

.sessi

ons.

For

.exam

ple,

.assu

.that.stat

ion

.A.comm

unicates

.with.stat

ion

.B..Anot

her

.stat

ion

.C.can.dete

.A.

but.not.B.. ere

fore,

.CSMA

/CA

.on.C.will.not.take.care.abou

.B.and.stat

ion

.A.will.sue

.from.coll

isions

caus

.by.the.simul

taneous

.tran

smissions

.of.B.and.C..In.this.exam

ple,

.stat

ions

.B.and.C.are.“hid

den”

from.eac

.oth

er.

Media Access Methods 3-7

One.(optional).solution.dened.in.IEEE.802.11.(WLAN).is.to.announce.a.transmission.via.a.dedicated.

req

uest-to-send

.(RT

.fra

me,

.whi

.are.con

rmed

.by.the.rec

eiver

.by.a.cle

ar-to-send

.(CT

.fra

me.

.Bot

fra

mes,

.RTS.and.CTS

.car

.an.est

imation

.of.the.tota

.han

dshake

.dur

ation.

.Hid

den

.sta

tions

.wil

.rec

eive

eit

her

.th

.RT

.or.th

.CT

.fr

ame

.an

.sh

all

.be.sil

ent

.du

ring

.th

.an

nounced

.ha

ndshake

.du

ration.

.is,.the.tota

.tra

nsaction

.con

sists

.of.one.RTS

.one.CTS

.one.dat

.and.one.ack

nowledgment

.fra

me.

Cle

arly,

.thi

.fou

r-way

.han

dshake

.dec

reases

.the.thr

oughput

.of.pla

.CSM

A/CA

.by.app

roximately

.50%

;

onl

.25

.of.th

.ba

ndwidth

.re

mains

.fo

.da

.th

roughput.

.rea

lity,

.the.RTS

/CTS

.mec

hanism

.is.typ

ically

.onl

.use

.in.poi

nt-to-multipoint

.wir

eless

.bri

dging

sce

narios,

.whe

.the.con

current

.non

-headend

.bri

dges

.use.dir

ectional

.ant

ennas

.(po

inting

.to.the.sing

hea

d-end

.br

idge)

.an

.th

erefore

.ca

nnot

.se

.ea

.ot

her.

references

[Abr70].N..Abramson,. e.Aloha.system—Another.alternative.for.computer.communications,.SRMA.

Aeros

pace

.Res

earch,

.Te

chnical

.Rep

ort,

.Ap

ril

.1970.

[Bog88]

.D..R..Boggs,.J..C..Mogul

.and.C..A..Kent,.Mea

sured

.cap

acity

.of.an.Eth

ernet:

.Myth

.and.rea

ity,

.in.Pro

ceedings of the SIGCOMM‘88 Symposium on Communications Architectures and Protocols,

Sta

nford,

.CA,.pp

.222–234,.ACM.SI

GCOMM,

.Augu

.1988.

[Ch

a00]

.C..E..Sp

urgeon,

.Et

hernet,

.in.e D

enitive Guide,

.O’

Reilly,

.Se

bastopol,

.CA,.2000.

4-1

4.1 Introduction

Wireless.networks.allow.communication.between.devices.over.a.wireless.medium.using.electromagnetic.

wave

. ey.oer.exi

bility

.and.spee

.when.sett

ing

.up.the.netw

ork,

.need.less.main

tenance

.than.cabl

.net-

orks,

.and.allo

.mobi

lity.

. ere.are.die

rent

.ways.of.clas

sifying

.thes

.netw

orks,

.the.most.comm

.of.which.

is.base

.on.thei

.purp

ose

.and.radi

.rang

.Depe

nding

.on.thes

.aspe

cts,

.wire

less

.netw

orks

.can.be.sepa

rated

into.four.grou

ps:

.wire

less

.pers

onal

.area.netw

orks

.(WPA

N),

.wire

less

.loca

.area.netw

orks

.(WLA

N),

.wire

less

metr

opolitan

.area.netw

orks

.(WMA

N),

.and.wire

less

.wide.area.netw

orks

.(WWA

N),

.see.Figu

.4.1.

•

. Wire

less personal area networks

.inte

rconnect

.devi

ces

.in.a.smal

.area

. e.stan

dard

.used.is.IEEE.

802.

15.1

.base

.on.the.Blue

tooth

.spec

ications.

. e.IEEE.802.

15.4

.stan

dard

.[WMA

C03],

.appr

oved

in.2004.and.prom

oted

.by.the.ZigB

.Alli

ance,

.has.been.deve

loped

.to.enab

.appl

ications

.with.

rela

xed

.band

width

.and.dela

.requ

irements,

.wher

.the.emph

asis

.is.devi

.batt

ery

.life

time

.maxi

mization.

. es

.app

lications

.wil

.be.run.on.pla

tforms

.suc

.as.sen

sors.

•

. Wire

less local area networks

.are.tech

nologies

.base

.on.High.Perf

ormance

.Radi

.LAN.(Hip

erLAN),

a.grou

.of.the.stan

dard

.grou

.Euro

pean

.Tele

communications

.Stand

ards

.Inst

itute

.(ETS

I),

.and.

Wi-Fi.sta

ndardized

.und

.IEE

.802

.11

.ser

ies.

Routing in Wireless

Networks

4.1. Introduction.......................................................................................4-1

4.2

. Rout

ing

.Pro

tocols

.and.Cla

ssication.............................................4-4

4.3

. Rout

ing

.Pro

tocol

.Fam

ilies

.for.Ad.Hoc.Net

works........................4-5

Proactive.Routing.Protocols. •. Reactive.Routing.Protocols

4.4. Routing.Protocol.Families.for.Wireless.Sensor.Networks..........4-6

Flat.Routing.Protocols. •. Hierarchical.Routing.Protocols. •. .

Location-Based.Routing.Protocols

4.5. Summary.of.the.Main.Routing.Protocols.in.Wireless.

Netw

orks.............................................................................................4-7

Optimized.Link-State.Routing.Protocol. •. TopologyDissemination.

Basedon.Reverse.Path.Forwarding. •. Dynamic.SourceRouting.

Protocol. •. Ad.Hoc.On-Demand.Distance.Vector.

RoutingProtocol. •. Dynamic.MANET.On-Demand.Routing.

Protocol. •. Sensor.Protocols.for.Information.via.Negotiation. •. Low.

Energy.Adaptive.Clustering.Hierarchy. •. Geographic.Adaptive.

Fidelity

4.6. Conclusions......................................................................................4-13

Acknowledgment......................................................................................... 4-14

Abbreviations...............................................................................................4-14

References..................................................................................................... 4-14

Teresa

Albero-Albero

Universidad Politécnica

de Val

encia

Víctor-M.

Sempere-Payá

Universidad Politécnica

de Val

encia