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

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47-10 Industrial Communication Systems

the.earliest-deadline-rst.scheduling.mechanism.[13]..Second,.static.scheduling.eases.synchronization.

between.safety.chips.mandatory.for.the.close.cooperation.between.the.safety.chips..Both.chips.start.at.

the.same.time.with.the.execution.of.function.in.the.same.order.

.state.machine.controls.the.behavior.of.the.node..According.to.inputs.received,.it.decides.to.

which.state.to.switch.to..In.a.SafetyLon.node.four.states.are.specied.as.illustrated.in.Figure.47.8..

Aer.a.reset.the.node.is.in.POWER.UP.state.and.runs.through.the.start-up.procedure..For.example,.

the.hardware.is.tested,.the.hardware.interfaces.are.initialized.and.conguration.parameters.are.

copied.from.the.ash.memory.to.the.RAM.due.to.performance.reasons..In.case.of.no.error,.the.

node.enters.the.RUN.state.where.the.node.is.operating..If.safety-critical.failures.are.detected,.the.node.

switches.to.FAIL.SAFE.state..In.this.state,.the.functionality.of.the.node.is.limited.to.a.minimum.

that.does.not.jeopardize.safety.. e.fail.safe.state.is.only.le.when.the.critical.fault.was.eliminated.

by.an.operator..MODIFY.state.is.used.to.congure.the.node.(e.g.,.to.make.a.safe.binding)..In.this.

state,.the.node.provides.only.such.functionality.necessary.to.execute.conguration.requests.and.

send.the.responses.

47.6 the SafetyLon tools

SafetyLon.provides.two.types.of.tools:.a.development.tool.called.application.builder.that.gives.the.user.

the.possibility.to.congure.the.user-application.(e.g.,.specifying.the.safe.and.non-safe.network.variable.

types)..Moreover,.a.SafetyLon.management.tool.makes.the.safety-related.commissioning,.conguration.

of.the.rmware.and.user-application,.maintenance.of.the.safety-related.application,.and.decommission-

ing

.of.nodes.or.removal.of.bindings.possible.

.development.tool.is.a.non-safe.tool.that.gets.a.script.le.as.input.(see.Figure.47.9).which.among.

LonWorks.specic.parameters.includes

•

. e.type.of.EN.14908.controller

•

. e.name.and.type.of.non-safe.network.variables.and.the.name.and.type.of.safe.network.variables

Additionally,

.the.application.builder.interfaces.with.the.LonMark.device.resource.API.where.the.

format.and.size.of.the.network.variables.is.taken.from.

The

.output.of.the.application.builder.are.plain.text.c-.and.header-files.containing.all.informa-

tion

.required.to.run.the.user-application.as.intended.(e.g.,.a.file.with.the.network.variable.table.

that.holds.the.SafetyLon.message.as.well.as.the.safety-related.payload.of.the.message)..The.files.are.

Run

No failure

Safety critical failure

Safety critical

failure

Safety critical

failure

Modify

Start safety-related

conguration

Stop safe-related

conguration

Safety-related conguration

Fail safe

Power up

FIGURE 47.8 Node.state.machine.

SafetyLon 47-11

linked.to.the.user-application.and.the.safety-related.firmware..Since.a.non-safe.tool.is.used,.it.is.

not.allowed.to.upload.all.the.user-application.remotely.as.safety.integrity.cannot.be.ensured.with.

a.high.probability.

.SafetyLon.management.tool.is.divided.into.a.SafetyLon.application.(SLA).interfacing.with.the.

local.network.service.(LNS).platform.[14]..Additionally,.a.SafetyLon.library.(SLL).is.used.to.store.all.

SafetyLon.specic.data.and.provide.SafetyLon.functionality.as.shown.in.Figure.47.10.

SLA

.provides.services.to.congure.one.or.several. safe.nodes,. depending.on.the.network.system.

design.. e.application.includes.an.interface.to.the.SLL.and.it.is.instantiating.the.SLL.within.the.ini-

tialization

.process..With.functions.used.from.this.library,.the.application.congures.the.safe.nodes..

 e.user-interface.shows.the.operator.the.resources.of.every.safe.node.that.are.changeable..To.obtain.

information.from.devices,.the.application.uses.the.interface.to.the.LNS.Object.Server.API..It.is.required.

because.the.SLA.is.a.standard.LNS.plug-in.

.SLA.instantiates.the.LNS.Object.Server.and.then.it.is.passed.through.to.the.SLL.together.with.

at.least.LNS.network.and.system.object.during.initialization..As.a.consequence,.both.soware.parts.

interact.with.the.same.instance.of.LNS.Object.Server.. e.event-sink.for.handling.LNS.events.is.imple-

mented

.in.the.SLA.. us,.the.changes.in.the.LNS.Object.Server.event.handling.should.not.inuence.the.

functionality.of.the.SLL.

Input source file(s)

SafetyLon application builder

Output source files for

SafetyLon user-application

Safety-related

firmware

Safety-related binary

for SafetyLon node

LonMark device

resource API

FIGURE 47.9 Application.builder.

SafetyLon

application

SafetyLon library

SafetyLon

application

SafetyLon

application

LON

FIGURE 47.10 Structure.of.SafetyLon.management.tool.

47-12 Industrial Communication Systems

 e.SLL.soware.architecture.consists.of.two.interfaces.and.one.application..An.API.handles.the.con-

nection

.to.LNS.Object.Server,.another.API.is.the.interface.for.the.SLA.to.the.SLL.. e.handling.of.LNS.

Object.Server.is.described.in.the.Echelon.guidelines.of.the.LNS.application.developer’s.kit.[15].

From

.the.implementation.point.of.view,.the.SLL.is.divided.into.three.parts.. e.SLL.is.used.by.the.client..

It.provides.functions.to.congure.a.safe.node..It.uses.the.SafetyLon.message.part.to.build.and.check.the.

management.messages.. e.message.is.sent.by.the.SLL.to.the.safe.node.. e.current.conguration.of.a.

safe.node.including.the.safe.address.is.stored.in.the.SafetyLon.database..It.manages.and.stores.the.safe.

addresses..Additionally,.it.saves.all.conguration.information.

.SafetyLon.management.tool.supports.the.three-step.process.explained.in.Section.47.3.applied.to.

congure.and.commission.a.node..It.uses.explicit.messages.and.a.dened.message.structure.(cf..Figure.

47.11).dierent.from.the.one.used.to.exchange.sensor.or.actuator.data.among.nodes.. erefore,.it.can.

be.explicitly.distinguished.between.a.message.sent.from.a.management.unit.and.messages.sent.from.a.

SafetyLon.node,.respectively.

. management.message. is. embedded. into. the. payload. of.an. explicit. message.. It. starts. with. a.

Command.ID.specifying.the.command.to.be.carried.out.at.node.side.(e.g.,.assign.a.safe.address.to.a.

network.variable).. e.destination.safe.address.(SADR).is.a.unique.safe.address.of.the.node.that.shall.be.

addressed.. e.transaction.ID.is.a.consecutive.number.that.makes.a.correlation.of.request.and.response.

possible..Data.are.a.conguration.parameter.such.as.a.safe.address.of.a.network.variable..CRC16,.nally,.

is.a.2.byte.CRC.appended.to.the.message.to.ensure.data.integrity..In.case.of.a.response.message,.an.addi-

tional

.byte.is.included:.an.error.byte.that.is.set.by.the.node.(e.g.,.the.network.variable.is.not.available).

Tool related data

1 1

11 Code

Command ID

Source safe address

Transaction ID

Error code

Safety-related parameter

CRC

1 2Max. 343

Byte

(a)

Tool related data

1 1

11 Code

Command ID

Destination safe address

Transaction ID

Safety-related parameter

CRC

2Max. 35

(b)

Byte

FIGURE 47.11 (a). Request. message. of. SafetyLon. management. tool.. (b). Response. message. of. SafetyLon.

management.tool.

SafetyLon 47-13

acronyms

1oo2. One.out.of.two

COTS

. Com

mercial

.o.th

.sh

elf

CRC

. Cyc

lic

.re

dundancy

.ch

eck

LNS

. Loc

.ne

twork

.se

rvice

LON

. Loc

.op

erating

.ne

twork

LSWord

. Lea

.sig

nicant

.wo

MSWord

. Mos

.sig

nicant

.wo

SADR

. Saf

.ad

dress

SFF

. Saf

.fa

ilure

.fr

action

SIL

. Saf

ety

.in

tegrity

.le

vel

SLA

. Saf

etyLon

.ap

plication

SLL

. Saf

etyLon

.li

brary

. Dan

gerous

.un

detected

. Dan

gerous

.de

tected

. Saf

.de

tected

. Saf

.un

detected

references

. 1.. International.Electrotechnical.Commission..IEC.61508—Functional.safety.of.electric/electronic/

programmable

.ele

ctronic

.sa

fety-related

.syst

ems.

.IEC,.Gen

eva,

.Sw

itzerland,

.1998.

. 2.. Euro

pean

. Nor

. EN. 14908—Open. dat

. communic

ation

. in. buildin

. aut

omation,

. cont

rols

. and.

buildin

.ma

nagement—control

.net

work

.pr

otocol.

.CEN,.2005.

. 3.. T..Nova

.P..Fisc

her,

.M..Holz,.M..Kiev

iet,

.and.T..Tam

andl.

.Safe.commis

sioning

.and.main

tenance

pro

cess

.for.a.saf

.system..In.Pro

ceedings of the Seventh IEEE International Workshop on Factory

Communication Systems,

.Dres

den,

.Ger

many,

.pp

.225–232,.2008.

. 4.. Ins

titute

.of.Elec

trical

.and.Elec

tronics

.Engin

eers

.Com

puter

.Societ

.IEEE.Sta

ndard

.for.So

ware

Safet

.Pl

ans.

.IEEE.St

.1228,.1994.

. 5.. C..Hat

ton.

.Saf

er C—Developing Soware for High-Integrity and Safety-Critical Systems.

.McGra

w-Hill

Boo

.Co

mpany

.Eur

ope,

.Ber

kshire,

.U.K.,.1995.

. 6.. T.. Nov

. and. T.. Tam

andl.

. Arc

hitecture

. of. a. saf

. nod

. for. a. el

dbus

. sys

tem.

. In. Pro

ceedings

of the Fih IEEE International Conference on Industrial Informatics,

. Vol

. 1,. Vie

nna,

. Aus

tria,

pp.

101–106,

.200

. 7.. Int

ernational

.Elec

trotechnical

.Commi

ssion.

.IEC.61508—Func

tional

.saf

ety

.of.elec

tric/electronic/

programmable

.ele

ctronic

.saf

ety-related

.sys

tems—Part

.2:.Req

uirements

.for.ele

ctrical/electronic/pro-

grammable

.ele

ctronic

.saf

ety-related

.sys

tems.

.IEC

.Gen

eva,

.Swi

tzerland,

.200

. 8.. T..Tam

andl

.and.P..Preinin

ger.

.Online.self.tests.for.microco

ntrollers

.in.saf

ety

.rel

ated

.system

.In.

Pro

ceedings of the Fih IEEE International Conference on Industrial Informatics,

.Vol

.1,.Vienna,.

Austria,.pp..137–142,.2007.

. 9.. P..Wrat

.and.M..Kiev

iet.

.Sic

herheitstechnik für Komponenten und Systeme.

.Hüt

hig

.Verl

ag,

.Heidel

berg,

Germ

any,

.p..203,.2007.

. 10.. H..Höl

scher

.and.J..Rader..Micr

ocomputers in Safety Technique, an Aid to Orientation for Developer

and Manufacturer.

.TÜV.Rh

einland,

.Co

logne,

.Ger

many,

.p..50,.1986,.Ch

apter

.7.

. 11.. H..Höl

scher,

.J..Rader..Micr

ocomputers in Safety Technique, an Aid to Orientation for Developer and

Manufacturer.

.TÜV.Rh

einland,

.Co

logne,

.Ger

many,

.p..86,.1986,.Ch

apter

.7.

. 12.. T..Nova

.and.B..Sev

cik.

.Netw

ork

.time.sync

hronization

.in.a.saf

.aut

omation

.netw

ork.

.In.Pro

ceedings

of the Seventh IEEE International Workshop on Factory Communication Systems,

.Dresden,.Germany,.

pp..305–313,.2008.

47-14 Industrial Communication Systems

. 13.. W..Wolf..Computers as Components, Principles of Embedded Computing System Design..Morgan.

Kauf

man

.Pub

lishers,

.San.Fra

ncisco,

.CA,.pp

.377,.2001.

. 14.. P..Fisc

her,

.M..Holz,.and.M..Mentze

.Netw

ork

.man

agement

.for.a.saf

.communic

ation

.in.an.unsa

envir

onment.

.In.Pro

ceedings of the Fih IEEE International Conference on Industrial Informatics,

Vol

.1,.Vienn

.Au

stria,

.pp

.131–136,.2007.

. 15.. Ech

elon

.Co

rporation,

.LNS.pr

ogrammer’s

.guide,.Turb

.edi

tion,

.Ec

helon,

.San.Jos

.CA,.2004.

48-1

48.1 Introduction

Many.industrial.automation.applications.are.designed.to.be.very.exible.in.order.to.manufacture.prod-

ucts

.with.incr

eased

.eci

ency.

. us,.a.exi

ble

.comm

unication

.infr

astructure

.is.requ

ired,

.lead

ing

.to.a.

grow

ing

.dema

.for.wire

less

.net

works.

.Wirel

ess

.netw

orks

.ful

ll

.thes

.requ

irements

.beca

use

.the

.oer.

more.exi

bility,

.cost.redu

ctions,

.and.high

.mobi

lity

.to.the.autom

ation

.syst

.as.comp

ared

.to.thei

wire

.cou

nterparts.

Since

.comm

unication

.in.futu

.indu

strial

.auto

mation

.syste

.will.most.like

.be.real

ized

.by.real

-time

Ether

net

.proto

cols

.like.PROF

INET

.[IEC

06a,IEC06b],

.Ether

net/IP

.[Eth0

9],

.etc..(cf..Chapt

.37.of.this.

book

.the.corr

esponding

.wire

less

.tech

nology

.at.the.eld.leve

.has.to.be.suit

able

.and.care

fully

.cho-

.with.respe

.to.the.appl

ication

.requ

irements,

.in.term

.of.both.depe

ndable

.and.temp

oral

.beha

vior.

 ere

fore,

.IEEE.802.

.[IEE

07]

.wire

less

.loca

.area.netw

orks

.(WLA

Ns)

.are.an.inte

resting

.opti

.beca

use

they.were.init

ially

.desig

ned

.to.be.the.wire

less

.exte

nsion

.of.Ethe

rnet

.[IEE

05].

.Some.inte

resting

.appl

ication

scen

arios

.are.over

head

.mono

rail

.syste

.or.auto

mated

.guid

.vehi

cles

.(AGV

.that.can.be.foun

.in.the.

area.of.logi

stics.

.For.inst

ance,

.AGVs.are.mobi

.trans

port

.syste

.that.can.opera

.auto

nomously.

. ey.

carr

.load

.and.use.eith

.fork.lis.or.a.conv

eyor

.syste

.to.move.the.obje

cts

.to.be.trans

ported.

.Basi

cally,

they.are.used.to.deli

ver

.work.piec

.to.spec

ic

.manu

facturing

.proc

esses

.and.have.to.be.quit

.exi

ble

.by.

den

ition.

.In.a.typi

cal

.scen

ario,

.the.mobi

.syste

.cons

ists

.of.a.loca

.prog

rammable

.logi

.cont

roller

.(PLC

)

to.cont

rol

.the.vehi

cle.

. e.loca

.PLC.also.comm

unicates

.with.the.cent

ral

.cont

roller

.that.is.respo

nsible

.for.

overa

.coor

dination

.and.task.assig

nment.

. e.task

.are.then.inde

pendently

.exec

uted

.by.the.auto

nomous

syste

ms.

. e.requ

irements

.are.dete

rmined

.by.the.velo

city

.and.the.numb

.of.vehi

cles

.in.the.enti

.syste

Wireless Local

Area Networks

48.1. Introduction.....................................................................................48-1

48.2

. e.802

.11

.Fam

ily............................................................................48-2

48.3

. Physi

cal

.Lay

er...................................................................................48-2

Frequency.Bands. •. Modulation.Techniques

48.4. Medium.Access.Control.................................................................48-3

Distributed.Coordination.Function. •. Point.Coordination.Function

Enhanced.Distributed.Channel.Access. •. HCF.Controlled.

ChannelAccess. •. Direct.Link.Protocol.and.Block.ACK

48.5. Limitations.of.DCF.and.HCF.for.QoS.Support.

inInd

ustrial

.Env

ironments...........................................................48-8

48.6

. Secu

rity

.Mec

hanisms......................................................................48-9

48.7

. Fast.Han

dover..................................................................................48-9

Mechanisms.on.the.AP.Side. •. Mechanisms.on.the.Client.Side

48.8. Future.Enhancements...................................................................48-11

References...................................................................................................48-11

Henning Trsek

Ostwestfalen-

Lippe

Uni

versity

Appl

ied

Sci

ences

Juergen Jasperneite

Ostwestfalen-

Lippe

Uni

versity

Appl

ied

Sci

ences

Lucia Lo Bello

University of Catania

Milos Manic

University of Idaho

Idah

Fal

48-2 Industrial Communication Systems

48.2 the 802.11 Family

 e.rst.wireless.standard.of.the.IEEE.802.family.was.the.802.11.one,.approved.by.the.IEEE.in.1997,.and.

thus.refe

rred

.to.as.the.802.

11-1997

.[IEE9

7].

. is.stan

dard,

.appr

oved

.in.1999

.is.now.obso

lete.

. e.most.

popu

lar

.stan

dards

.in.the.802.

.fami

.are.thos

.den

.by.the.802.

11a,

.802.

11b,

.and.802.

11g

.proto

cols

as.ame

ndments

.to.the.ori

ginal

.802

.11-1997

.sta

ndard.

. ey.are.sum

marized

.in.Tab

.48.

Several

.impo

rtant

.amen

dments

.to.the.orig

inal

.stan

dard

.foll

owed,

.such.as.the.802.

11e

.[IEE

05e]

.den

ing

.a.set.of.qual

ity

.of.serv

ice

.(QoS

)

.enha

ncements

.to.supp

ort

.dela

y-sensitive

.appl

ications,

.the.802.

11i

[IEE

04i]

.intr

oducing

.secu

rity

.enha

ncements,

.such.as.key.mana

gement

.and.dist

ribution,

.encr

yption,

and.auth

entication,

.and.802.

11n

.addi

.mult

iple-input

.mult

iple-output

.(MIM

.and.othe

.newe

.fea-

ures.

.Othe

.stan

dards

.in.the.fami

.(c–f

.h,.j).are.serv

ice

.amen

dments

.and.exte

nsions

.or.corr

ections

.to.

ious

.spe

cications.

.curr

ent

.IEEE.802.

11-2007

.stan

dard

.[IEE

07]

.is.a.singl

.docu

ment

.that.merg

.eigh

.amen

dments

(802

.11a,

.b,.d,.e.g.,.h,.i,.j).with.the.1999.versi

.of.the.802.

.stan

dard

.[IEE9

7].

.Howe

ver,

.two.new.amen

ments

.to.this.versi

on,

.whic

.are.802.

11k

.for.radi

.reso

urce

.mana

gement

.[IEE

08k]

.and.802.

11r

.for.a.fast.

basic.serv

ice

.set.(BSS

)

.tran

sition

.[IEE

08r],

.have.been.rati

ed

.in.the.mean

time,

.and.othe

.are.alre

ady

avai

lable

.as.rs

.dra

s.

48.3 Physical Layer

 e.physical.layer.of.the.original.802.11.[IEE97].standardized.three.wireless.data-exchange.techniques..

 ey.were.the.infr

ared

.(IR)

.the.freq

uency-hopping

.spre

.spec

trum

.(FHS

S),

.and.the.dire

.sequ

ence

spre

.spec

trum

.(DSS

S).

. e.phys

ical

.laye

.in.802.

.is.spli

.into.the.Phys

ical

.Laye

.Conv

ergence

.Prot

ocol

(PLC

.and.the.Phys

ical

.Medi

.Depe

ndent

.(PMD

)

.sub-

layers.

. e.PLCP.prep

ares/parses

.data.unit

tran

smitted/received

.usin

.vari

ous

.802.

.medi

.acce

.tech

niques.

. e.PMD.perf

orms

.data.tran

smis-

sion/reception

.and.modu

lation/demodulation

.dire

ctly

.acce

ssing

.air.unde

.the.guid

ance

.of.the.PLCP

48.3.1 Frequency Bands

 e.original.intention.of.the.internationally.designated.Industrial,.Scientic,.and.Medical.(ISM).radio.

band

.was.to.gene

rate

.and.to.use.loca

.unli

censed

.Radi

.Freq

uency

.(RF).elec

tromagnetic

.eld

.for.

indu

strial,

.sci

entic,

.med

ical,

.and.dom

estic

.pur

poses

.exc

luding

.com

munications.

.Inte

rnational

.Tele

communication

.Unio

.(ITU

)

.exte

nds

.the.orig

inal

.inte

ntion

.to.radi

.com-

unication

.serv

ices

.oper

ating

.with

.thes

.band

.that.must.acce

.harm

ful

.inte

rferences

.that.may.be.

caus

.by.ISM.appl

ications

.[ITU

09b].

. e.den

ition,

.as.well.as.the.prec

ise

.freq

uency

.rang

es,

.cent

.fre-

uencies,

.and.the.avai

lability

.are.den

.by.the.ITU-

Recommendations

.(ITU

-R)

.on.radi

.regu

lations:

ITU-

.5.1

38,

.5.1

50,

.5.2

80,

.and.15.

.[IT

U09a].

Although

.typi

cally

.asso

ciated

.with.the.micr

owave

.oven.oper

ating

.freq

uency

.of.2.45.GHz,.ISM.band

have.also.been.shar

.with.lice

nse-free,

.erro

r-tolerant

.comm

unications

.appl

ications,

.such.as.wire

less

LANs.(IEE

.802.

11b/g

.at.2.45.GHz).[IEE

07],

.Blue

tooth

.(IEE

.802.

15.1

.at.2.45.GHz).[IEE

05a],

.ZigB

(IEE

. 802.

15.4

. at. 2.45.GHz). [IEE

06],

. and. cord

less

. phon

. comm

unications

. oper

ating

. at. 900.MHz,.

2.45.GHz,.5.8.GHz,.and.other.frequencies.

TABLE 48.1 WLAN.Technology.Overview

Technology Release.Date

Spectrum.

(GHz)

Max.Data.Rate.

(Mbps)

Outdoor.Range.

(m)

IEEE

.802.11a Oc

.1999 5.0 54 35

IEEE

.802.11b Oct..1999 2.4 11 42

IEEE

.802.11g Jun

.2003 2.4 54 42

Wireless Local Area Networks 48-3

 e.a,.b,.and.g.encoding.protocols.of.working.group.11.of.the.IEEE.802.wireless.LAN.standards.

comm

ittee

.use.the.ISM.band

.(802

.11a

.oper

ates

.at.5.GHz,.whil

.802.

11b/g

.oper

ates

.in.the.unli

censed

2.45.GHz.band

.and.802.

11n

.at.2.45.and.5.GHz)

. is.impl

ies

.that.802.

11b/g/n,

.oper

ating

.in.an.unre

gu-

lated

.freq

uency

.band.can.incu

.inte

rference

.from.micr

owave

.oven

.cord

less

.phon

es,

.and.othe

.appl

ances

.using.the.same.2.4.GHz.rang

. ere

fore,

.coex

istence

.has.to.be.consi

dered

.in.any.case

.and.is.of.

major.imp

ortance

.for.the.suc

cessful

.dep

loyment

.of.WLA

Ns.

48.3.2 Modulation techniques

Modulation.techniques.are.techniques.typically.used.in.telecommunications.to.transmit.a.message..

A.high.freq

uency

.peri

odic

.(sinu

soid)

.wave

form

.is.used.as.a.carr

ier

.signa

.with.ampl

itude,

.phas

.and.

freq

uency

.modu

lation.

. e.rece

iving

.unit.perf

orming

.the.inve

rse

.oper

ation

.of.modu

lation

.is.know

as.the.dem

odulator.

. e.mod

.(Mo

dulator-Demodulator)

.is.cap

able

.of.per

forming

.bot

.ope

rations.

Wireless

.comm

unication

.proto

cols

.use.vari

ous

.modu

lation

.tech

niques.

.Whil

.Blue

tooth

.and.the.

earl

.802.

.LANs.use.the.Freq

uency-Hopping

.Sprea

.Spect

rum

.signa

ling

.meth

.(FHS

S),

.802.

11b

and.ZigB

. 802.

15.4

.use. Dire

.Sequ

ence

. Sprea

.Spect

rum

. (DSS

. signa

ling.

. e. 802.

11a

. uses. the.

Orth

ogonal

.Fre

quency-Division

.Mul

tiplexing

.(OF

DM)

.sys

tem.

Frequency

.hopp

ing

.was.the.rst.step.in.the.evol

ution

.to.DSSS.and.othe

.data.tran

smission

.tech

niques.

 e.idea.was.to.tran

smit

.via.a.pred

ened

.freq

uency

.hopp

ing

.patt

ern

.know

.to.both.the.tran

smitter

.and.

the.rece

iver.

. e.802.

.freq

uency

.hopp

ing

.sepa

rates

.the.whol

.2.4.GHz.ISM.band.into.1.MHz-s

paced

chan

nels.

. e.tran

smitter

.has.to.chan

.chan

nels

.at.leas

.2.5.time

.per.seco

.(eve

.400.ms.or.less

 is.allo

.deal

ing

.with.high.ener

.inte

rference

.in.a.narr

.band

.as.well.as.the.mutu

.inte

rference

of.two.FHS

.tra

nsmitters

.posi

tioned

.clo

.to.eac

.oth

er.

With

.Dire

.Sequ

ence

.Sprea

.Spect

rum

.(DSS

.signa

ling,

.the.carr

ier

.signa

.occu

.over.the.full.

band

width

.(spe

ctrum)

.of.a.devi

ce’s

.tran

smitting

.freq

uency.

. e.data.signa

.at.the.send

ing

.stat

ion

.is.

combi

ned

.with.a.high

.data.rate.bit.sequ

ence,

.or.a.chip

ping

.code.that.divi

des

.the.user.data.acco

rding

to.a.spre

ading

.rati

. e.chip

ping

.code.is.a.redu

ndant

.bit.patt

ern

.for.each.bit.that.is.tran

smitted,

.whic

incr

eases

.the.signa

l’s

.resis

tance

.to.inte

rference.

.If.one.or.more.bits.in.the.patt

ern

.are.dama

ged

.duri

transmission,.the.original.data.can.be.recovered.due.to.the.redundancy.of.the.transmission.

Complementary

.Code.Keyi

.(CCK

)

.is.used.in.conj

unction

.with.DSSS.tech

nology.

.CCK.is.a.set.of.

64.8-bit.code.word

.used.to.enco

.data.for.5.5.and.11.Mbps.data.rate

.in.the.2.4.GHz.band.of.802.

11b

wire

less

.netw

orking.

. e.code.word

.have.uniq

.math

ematical

.prop

erties

.that.allo

.them.to.be.

corr

ectly

.dist

inguished

.from.one.anot

her

.by.a.rece

iver

.even.in.the.pres

ence

.of.subs

tantial

.nois

.and.

mult

ipath

.inte

rference.

.CCK.appl

ies

.soph

isticated

.math

ematical

.form

ulas

.to.DSSS.code

.perm

itting

the.code

.to.repr

esent

.a.grea

ter

.volu

.of.info

rmation

.per.cloc

.cycl

.(11.Mbps.of.data.rath

.than.the.

2.Mbps.in.the.ori

ginal

.sta

ndard).

.CCK.doe

.not.wor

.wit

.FHS

Orthogonal

.freq

uency-division

.mult

iplexing

.(OFD

.is.a.Freq

uency-Division

.Mult

iplexing

.(FDM

)

.sche

util

ized

.as.a.digi

tal

.mult

icarrier

.modu

lation

.meth

od.

.OFDM.spli

.the.radi

.sign

.into.mult

iple

.smal

ler

.sub-

ignals

.that.are.then.tran

smitted

.simu

ltaneously

.at.die

rent

.freq

uencies

.to.the.rece

iver.

.OFDM

.ther

efore,

redu

ces

.the.amou

.of.cros

.talk.in.sign

.tran

smissions.

.OFDM.is.used.in.the.802.

11a/g,

.the.Euro

pean

.alte

native

.for.the.IEEE.8021

.High.Perf

ormance

.Radio.LAN.(HIP

ERLAN/2),

.and.in.802.

.(WiM

AX).

48.4 Medium access Control

According.to.the.original.IEEE.802.11.MAC.protocol.[IEE97],.the.architecture.of.the.MAC.sub-layer.

incl

udes

.a.mand

atory

.Dist

ributed

.Coor

dination

.Func

tion

.(DCF

)

.and.an.opti

onal

.Poin

.Coor

dination

Func

tion

.(PCF

.More

over,

.due.to.the.limi

tations

.of.this.arch

itecture

.for.tran

smitting

.time

-critical

.tra

ows

.real

-time

.enha

ncements

.were.intr

oduced.

. ey.are.addre

ssed

.by.the.stan

dard

.amen

dment

.IEEE.

802.

11e

.[IEE

05e].

.It.prov

ides

.advan

ced

.QoS.capab

ilities

.by.addin

.the.hybr

.coor

dination

.func

tion

(HCF

.whic

.dene

.two.new.acce

.mech

anisms.

. e.over

all

.MAC.arch

itecture

.is.show

.in.Figu

.48.1

48-4 Industrial Communication Systems

 e.DCF.is.totally.distributed.and.can.be.used.within.both.ad.hoc.and.infrastructure.network.con-

gurations.

.Conversely,.the.PCF.is.centralized.and.can.be.used.only.in.infrastructure.network.congu-

rations.

.In.the.DCF,.each.station.senses.the.shared.channel.and.transmits.when.it.nds.a.free.channel.

according.to.a.carrier-sense.mechanism..In.the.PCF,.a.coordinator.station.polls.the.other.nodes,.thus.

enabling.them.to.transmit.in.a.collision-free.way.. e.Enhanced.Distributed.Channel.Access.(EDCA).

mechanism.is.based.on.DCF.medium.access,.except.for.the.ability.to.assign.dierent.priorities.to.vari-

ous

.kinds.of.trac.ows..HCF-Controlled.Channel.Access.(HCCA).denes.a.parameterized.QoS.sup-

port.

.It.also.relies.on.a.polling.procedure.with.an.underlying.time-division.multiple.access.(TDMA).

principle.but.is.more.exible.when.compared.to.the.legacy.of.the.PCF.

Currently,

.the.most.widely.used.channel.access.mechanisms.are.the.DCF.and.the.EDCA,.while.the.

PCF.and.the.HCCA.have.received.little.attention.so.far,.especially.where.commercial.products.are.con-

cerned,

.because.the.available.chipsets.still.lack.support.for.these.mechanisms..In.the.following,.the.four.

dierent.access.methods.are.outlined.

48.4.1 Distributed Coordination Function

 e.IEEE. 802.11. DCF. operating. mode. is. based. on.Carrier-Sense. Multiple.Access.with. a. Collision.

Avoidance.(CSMA/CA).protocol.and.on.a.random.backo.time.following.a.busy.medium.condition.. e.

random.backo.time.is.intended.to.reduce.the.collision.probability.between.multiple.stations.access-

ing

.a.shared.medium.at.the.point.where.collisions.would.most.likely.occur.. e.highest.probability.of.a.

collision.exists.just.aer.the.medium.becomes.idle.following.a.busy.medium.because.multiple.stations.

could.have.been.waiting.for.the.medium.to.become.available.again.. is.is.the.situation.in.which.the.

random.backo.procedure.comes.into.action.to.resolve.medium.contention.conicts.

Before

.starting.transmission,.a.node.listens.to.the.channel.for.a.time.called,.a.Distributed.Interframe.

Space.(DIFS),.to.assess.whether.the.channel.is.idle.or.not..If.the.channel.is.idle,.each.node.generates.

a.random.backo.interval.in.order.to.reduce.the.probability.of.collisions.with.other.nodes.trying.to.

access.the.sensed.idle.channel.at.the.same.time..Each.node.decreases.its.backo.counter.as.long.as.the.

wireless.channel.is.sensed.to.be.idle..If.the.counter.has.not.reached.zero,.and.the.channel.becomes.

busy.again,.the.backo.counter.is.frozen.and.reloaded.as.soon.as.the.channel.is.sensed.to.be.idle.again.

for.aDIFS.

When

.the.backo.interval.is.over,.and.if.the.channel.is.still.idle,.the.transmission.starts.. e.random.

backo.interval,.expressed.as.a.number.of.time.slots,.is.generated.in.the.set.{0,CW.–.1},.where.CW.denotes.

the.contention.window.size.. e.initial.value.of.the.contention.window.is.CWmin..In.the.case.of.an.unsuc-

cessful

.transmission.(due.to.collisions.or.losses),.the.CW.is.doubled.up.to.a.maximum.value.CWmax..

MAC

PCF

HCF

EDCA HCCA

DCF

Legacy support for

contention-free

services, optional

Support for

prioritized QoS

Support for

parameterized QoS

Support for

contention services

FIGURE 48.1 MAC.architecture.with.HCF.

Wireless Local Area Networks 48-5

Aer.experiencing.a.maximum.number.of.collisions,.a.packet.is.dropped..In.the.case.of.a.successful.

tra

nsmission,

.the.CW.val

.is.res

.to.CWm

.bef

ore

.the.ran

dom

.bac

ko

.int

erval

.is.sel

ected.

According

.to.the.IEE

.802

.11

.MAC.pro

tocol,

.an.exp

licit

.ack

nowledgment

.(AC

.fra

.has.to.be.sen

by.the.rec

eiver

.to.not

ify

.the.tra

nsmitter

.of.the.suc

cessful

.rec

eption

.of.a.dat

.fra

me.

. e.tim

.int

erval

bet

ween

.the.rec

eption

.of.a.dat

.fra

.and.the.tra

nsmission

.of.the.rel

evant

.ACK.is.de

ned

.as.the.Sho

Int

erframe

.Spac

.(SI

FS).

. is.sma

.gap.bet

ween

.tra

nsmissions

.giv

.pri

ority

.to.the.ong

oing

.fra

me-

exchange

.seq

uence,

.by.pre

venting

.oth

.nod

.tha

.hav

.to.wai

.for.the.med

ium

.to.be.idl

.for.a.lon

ger

tim

.in

terval

.(e

.g.,

.at.le

ast

.fo

.th

.DI

.ti

me),

.fr

.ac

cessing

.th

.ch

annel.

Physical

.and.vir

tual

.car

rier-sense

.fun

ctions

.are.use

.to.det

ermine

.the.sta

.of.the.med

ium.

.Whe

eit

her

.fun

ction

.ind

icates

.a.bus

.med

ium,

.the.med

ium

.is.con

sidered

.bus

.oth

erwise

.it.is.con

sidered

.idl

 e.phy

sical

.car

rier-sense

.mec

hanism

.is.pro

vided

.by.the.802

.11

.PHY

.whi

.the.vir

tual

.car

rier-sense

mec

hanism

.is.pro

vided

.by.the.MAC.and.is.ref

erred

.to.as.the.Net

work

.All

ocation

.Vec

tor

.(NA

V).

. e.

NAV.mai

ntains

.a.pre

diction

.of.fut

ure

.tra

c

.on.the.med

ium

.bas

.on.the.dur

ation

.inf

ormation

.tha

.is.

eit

her

.ann

ounced

.in.Req

uest

.to.Sen

d/Clear

.to.Sen

.(RT

S/CTS)

.fra

mes

.pri

.to.the.act

ual

.dat

.exc

hange

or.ca

.be.us

.by.th

.PC

.an

.th

.HC

.RTS

/CTS

.mec

hanism

.is.ver

.ee

ctive

.in.red

ucing

.the.len

gth

.of.the.fra

mes

.inv

olved

.in.the.con

tention

.pro

cess.

.In.fac

.ass

uming

.per

fect

.cha

nnel-sensing

.by.eve

.sta

tion,

.col

lisions

.may.onl

.occ

whe

.two.or.mor

.sta

tions

.sta

.RTS.tra

nsmission

.wit

hin

.the.sam

.tim

.slo

.If.bot

.sou

rces

.emp

loy

.the.

RTS

/CTS

.mec

hanism

.(th

.dec

ision

.to.use.RTS

/CTS

.dep

ends

.on.the.pac

ket

.len

gth,

.as.wil

.be.exp

lained

subsequently),.collisions.would.only.occur.while.transmitting.the.RTS.frames.and.would.promptly.be.

det

ected

.by.the.sou

rce

.lac

king

.the.CTS.res

ponses.

. e.RTS

/CTS

.the

refore,

.sign

icantly

.low

ers

.the.tem

poral

.ove

rhead

.of.a.col

lision

.(i.

e.,

.col

lisions

.are.muc

.sho

rter

.in.tim

e).

. is.fea

ture

.is.ben

ecial

.for.tim

constrained

.tr

ac.

However,

.the.RTS

/CTS

.mec

hanism

.has.som

.dra

wbacks

.as.the.add

itional

.RTS.and.CTS.fra

mes

.add.a.

pro

tocol

.ove

rhead

.and.thu

.red

uce

.pro

tocol

.ec

iency

.esp

ecially

.for.sho

.dat

.fra

mes.

.For.thi

.rea

son,

eac

.sta

tion

.can.be.con

gured

.to.use.the.RTS

/CTS

.mec

hanism

.eit

her

.alw

ays,

.or.nev

er,

.or.onl

.on.fra

mes

lon

ger

.th

.a.sp

ecied

.le

ngth.

.the.typ

ical

.con

trol

.tra

c

.exc

hanged

.on.the.fac

tory

.oo

.con

sists

.of.sho

.fra

mes,

.the.use.of.the.

RTS

/CTS

.mec

hanism

.to.tra

nsmit

.thi

.kin

.of.tra

c

.is.not.adv

isable.

.In.add

ition,

.the.RTS

/CTS

.mec

ha-

nism

.can

not

.be.use

.for.bro

adcast

.and.mul

ticast

.tra

nsmissions,

.as.the

.are.mul

tiple

.rec

ipients

.for.the.

RTS

.an

.th

.po

tentially,

.mu

ltiple

.co

ncurrent

.se

nders

.of.th

.CT

.in.re

sponse.

.RTS

/CTS

.mec

hanism

.can.be.suc

cessfully

.exp

loited

.to.cop

.wit

.the.hid

den node problem,

.whi

ari

ses

.whe

.the

.are.nod

.tha

.are.out.of.ran

.of.som

.oth

.nod

.bel

onging

.to.the.sam

.wir

eless

net

work,

.thu

.cre

ating

.a.pot

ential

.for.col

lisions

.wit

.tra

nsmissions

.fro

.tho

.nod

es.

.Bef

ore

.sen

ding

.a.

pac

ket,

.th

.tr

ansmitter

.se

nds

.an.RT

.fr

ame

.an

.wa

its

.fo

.a.CT

.fr

ame

.fr

.th

.NA

.rec

eption

.of.a.CTS.fra

.not

ies

.the.tra

nsmitter

.tha

.the.cha

nnel

.is.cle

.in.the.rec

eiver

.are

.so.

the.rec

eiver

.is.abl

.to.rec

eive

.the.pac

ket.

.Any.oth

.nod

.in.the.rec

eiver

.are

.wil

.hea

.the.CTS.eve

.if.it.

can

not

.hea

.the.RTS.and.wil

.avo

.acc

essing

.the.cha

nnel

.ae

.hea

ring

.the.CTS

.eve

.if.its.car

rier

.sen

mec

hanism

.ind

icates

.tha

.the.cha

nnel

.is.idl

. e.sou

rce

.is.onl

.all

owed

.to.tra

nsmit

.the.dat

.pac

ket

.if.

the.CTS.fra

.is.cor

rectly

.rec

eived

.wit

hin

.a.dur

ation

.cal

led

.the.CTS.Tim

eout.

.Mor

.det

ails

.on.the.DCF.

fun

ction

.ca

.be.fo

und

.in.[I

EE97].

48.4.2 Point Coordination Function

 e.PCF.access.method.uses.a.point.coordinator.(PC),.acting.as.the.polling.master,.which.determines.

whi

.sta

tion

.cur

rently

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. e.ope

ration

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uire

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itional

coo

rdination

.tha

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cied

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ndard

.[IE

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.to.per

mit

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ient

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ration

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.whe

mul

tiple

.poi

nt-coordinated

.net

works

.are.ope

rating

.on.the.sam

.cha

nnel,

.and.to.sha

.an.ove

rlapping

phy

sical

.sp

ace.