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

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52-8 Industrial Communication Systems

52.5.2 WiMaX-3GPP Integration

 e.third.Generation.Partnership.Project.(3GPP).unites.telecommunications.standards.bodies.as.orga-

nizational

.part

ners,

.orig

inally

.esta

blished

.to.prod

uce

.tech

nical

.spec

ications

.and.tech

nical

.repo

rts

.for.a.

3G.mobi

.syst

.base

.on.evol

ved

.GSM.core.netw

orks

.and.the.radi

.acce

.tech

nologies

.that.they.sup-

ort

.(i.e

.univ

ersal

.terr

estrial

.radi

.acce

.[UTR

A],

.both.freq

uency-division

.dupl

.[FDD

]

.and.time

division

.dupl

.[TDD

]

.mode

.[16]

. e.3GPP

-WiMAX

.inte

gration

.pres

ents

.inte

rworking

.appr

oaches

by.whic

.a.3GPP.oper

ator

.empl

oying

.3GPP.tech

nologies

.(GSM

/GPRS,

.EDGE,.UMTS

.and/

.HSDP

prov

ides

.subs

criber

.acce

.to.3GPP.PS.and.to.the.IEEE.802.

.pack

.serv

ice

.plan

.Tigh

tly

.and.loos

ely

coup

led

.inte

rworking

.of.WiMAX.with.3GPP.enta

ils

.tigh

tly

.coup

led

.and.loos

ely

.coup

led

.inte

rworking

appr

oaches

.betw

een

.the.WiMAX.AN.and.the.3GPP.core.netw

ork

.take

.by.3GPP.in.stan

dard

.TS.23.2

34.

Whil

.orig

inally

.deve

loped

.for.WLAN

.the.idea

.in.TS.23.2

.are.now.the.basis.for.ongo

ing

.3GPP.

disc

ussions

.on.ext

ending

.the.sco

.to.inc

lude

.oth

.IP-b

ased

.acc

ess

.tec

hnologies,

.inc

luding

.WiMA

52.6 Conclusion

While.WiMAX.may.seem.as.technology.competing.with.existing.broadband.alternatives.such.as.DSL.

and.cabl

.it.rath

.has.a.dist

inctive,

.comp

lementary

.plac

.in.the.mode

.broad

band

.Inte

rnet

.acce

.oer

ings.

. e.WiMA

.alre

ady

.has.esta

blished

.itse

.as.a.broad

band

.alte

rnative

.of.choi

.in.rura

.and.remo

area

.with.the.poss

ibility

.of.back

haul

.and.over

lay

.for.othe

.tech

nologies

.as.well

.Rega

rdless

.of.the.pre-

ictions

.made.rega

rding

.this.tech

nology,

.it.is.alre

ady

.obvi

ous

.that.the.time

.of.WiMA

.are.yet.to.come

references

. 1.. ITU..Telecommunications.indicators.update—2004..URL.from.January.2009:.http://www.itu.int/

ITU-D/ict/statistics/

. 2.. Ahso

.S..and.Ilyas,.M..(Eds.),.WiMAX: A

pplications,

.CRC.Press,.Taylo

.&.Franci

.Grou

.Boca.

Rato

.FL,.2008.

. 3.. IEEE.802.LAN/MAN.Sta

ndards

.Commi

ttee,

.URL.fro

.Jan

uary

.2009:.htt

p://www.ieee802.org/

. 4.. IEEE.802.16.Work

ing

.Grou

.on.Broad

band

.Wireles

.Access.Stan

dards,

.URL.from.Janu

ary

.2009:.

http://ie

ee802.org/16/index.html

. 5.. IEEE.802.16.Tas

.Gro

.d,.URL.fro

.Jan

uary

.2009:.htt

p://www.ieee802.org/16/tgd/

. 6.. IEEE. Std. 802.16e:. Publica

tion

. Hist

ory,

. URL. from. Janu

ary

. 2009:. http://w

ww.ieee802.org/16/

pubs/80216e.html

. 7.. IEEE.802.16.Tas

.Gro

.e,.URL.fro

.Jan

uary

.2009:.htt

p://www.ieee802.org/16/tge/

. 8.. Andre

ws,

.J.G.,.Ghosh,.A.,.and.Muha

med,

.R.,.Fund

amentals of WiMAX: Understanding Broadband

Wireless Networking,

.Pren

tice

.Hal

.Upp

.Saddle.Riv

er,

.NJ,.2007.

. 9.. A. service. of. digita

. bridge. communica

tions,

. URL. from. Janu

ary

. 2009:. http://w

ww.

digitalbridgecommunications.com/

. 10.. A.next.genera

tion

.mob

ile

.inter

net

.fro

.Clea

rwire,

.URL.fro

.Jan

uary

.2009:.htt

p://www.clear.com

. 11.. WiMax.com.Broad

band

.Solu

tions,

.Inc,.URL.from.Janu

ary

.2009:.http://w

ww.wimax.com/education

. 12.. CNET.News,.Decl

.McCul

lagh:

.Inte

l’s

.rst.WiMax.boar

.URL.from.Janu

ary

.2009:.http://n

ews.

cnet.com/Photos-Welcoming-the-WiMax-chip/2009–1006_3–5675441.html

. 13.. Katz,.M.D..and.Fitzek,.F.H.P.,.WiMAX Evolution, Emerging Technologies and Applications,.John.

Wiley.&.Son

.Chich

ester,

.U.K.,.2009.

. 14.. Kumar

.A.,.Mobi

le Broadcasting with WiMAX: Principles, Technology, and Applications,

.Else

vier,

Amst

erdam,

.the.Neth

erlands,

.2008.

. 15.. WiMAX. Forum,. URL. from. Janu

ary

. 2009:. http://w

ww.wimaxforum.org/;

. http://w

ww.ieee802.

org/16/pubs/80216e.html

. 16.. e.mob

ile

.bro

adband

.sta

ndards,

.URL.fro

.Jan

uary

.2009:.htt

p://www.3gpp.org/

53-1

53.1 Introduction

 e.world.of.industrial.communications.shows.increasing.interest.toward.wireless.eldbuses,.i.e.,.the.

use.of.wire

less

.comm

unications

.to.inte

rconnect

.devi

ces

.at.eld.leve

.sens

ors;

.actu

ators;

.inst

ruments;

cont

rollers;

.and.so.on.[W08

,FFM07].

.In.part

icular,

.the.adve

.of.hybr

.wire

d/wireless

.netw

orks

.prom

ises

.to.grea

tly

.impr

ove

.eci

ency

.and.scal

ability.

.Howe

ver,

.the.succ

ess

.of.this.kind.of.solu

tions

.will.

depe

.on.the.sta

ndardization

.pro

cess

.tha

.ens

ures

.mul

tivendor

.com

patibility

.and.int

eroperability.

.big.step.towar

.the.deve

lopment

.of.a.trul

.open.and.stan

dard

.solu

tion

.has.been.done.than

.to.

the.intr

oduction

.of.the.IEEE.802.

15.4

.spec

ications

.[I06

.In.part

icular,

.this.stan

dard

.body.crea

ted

.a.

physi

cal

.laye

.(PHY

)

.desc

ription

.that.main

tains

.good.perf

ormances

.in.pres

ence

.of.nois

.and.inte

rfer-

ences,

.but.it.is.simpl

.enou

.to.allo

.syst

.desig

ner

.to.prod

uce

.low.powe

.cons

umption

.and.low-

ost

.devi

ces.

.In.fact

.a.wire

less

.sens

.for.indu

strial

.appl

ications

.must.work.in.a.nois

.envi

ronment

on.one.hand.and.must.ensu

.an.over

all

.devi

.life

span

.and.cost.comp

arable

.with.trad

itional

.wire

syst

ems,

.on.the.othe

.one.. e.adopt

ion

.of.a.dire

.sequ

ence

.spre

.spec

trum

.(DSS

.stra

tegy

.miti

gates

inte

rferences

.and.ose

.quad

rature

.phas

.shi.keyi

.with.half

-sine

.shap

ing

.(OQP

SK-HSS)

.modu

lation

ensu

res

.good.spec

tral

.occu

pancy.

.Howe

ver,

.the.adopt

ion

.of.carr

ier

.sens

.mult

iple

.acce

.with.coll

ision

avoi

dance

.(CSM

A/CA)

.at.the.medi

.acce

.cont

rol

.(MAC

)

.leve

.pose

.seve

.limi

.on.the.resp

ect

.of.

real

-time

.dea

dlines,

.a.typ

ical

.req

uirement

.of.ind

ustrial

.app

lications.

.orde

.to.over

come

.such. limi

tations,

.a.lot.of.solu

tions

.empl

oying

.radi

.comp

liant

.with. IEEE.

802.

15.4-PHY

.but.using.time.divi

sion

.mult

iple

.acce

.(TDM

.proto

col

.at.the.MAC.laye

.have.appe

ared

WirelessHART,

ISA100.11a, and OCARI

53.1. Introduction.....................................................................................53-1

53.2

. Wirel

essHART.................................................................................53-2

 e.WirelessHART.Physical.Layer. •. e.WirelessHART.Data.

LinkLayer. •. Time.Keeping. •. e.WirelessHART.Network.Layer.

and.Topologies. •. e.WirelessHART.Upper.Layers

53.3. ISA100.11a.........................................................................................53-8

 e.ISA100.11a.Physical.Layer. •. e.ISA100.11a.Data.Link.

Layer. •. Time.Keeping. •. e.ISA100.11a.Network.Layer.

andTopologies. •. e.ISA100.11a.Upper.Layers

53.4. OCARI.............................................................................................53-10

Physical.Layer. •. MAC.Layer. •. Network.Layer.

andTopologies. •. Application.Layer

53.5. Coexistence.of.the. ree.Protocols.............................................53-13

53.6

. Exam

ple

.of.Pla

tform

.Pro

viders...................................................53-15

53.7

. Conc

lusions....................................................................................53-16

References...................................................................................................53-16

Tuan Dang

EDF Research and

Deve

lopment

Emiliano Sisinni

University of Brescia

53-2 Industrial Communication Systems

recently..In.particular,.in.this.chapter.are.described.the.proposal.of.the.Hart.Communication.Foundation.

(HCF),.also.known.as.WirelessHART.[SHM07,KHP08],.the.proposal.of.the.International.Society.of.

Automation.(ISA),.also.known.as.ISA100.[ISA09],.and.the.Optimization.of.Ad.hoc.Communications.

in.Industrial.networks.(OCARI.[D08]).project,.funded.by.the.French.national.research.council—ANR.

53.2 WirelessHart

 e.HART•.(Highway.Addressable.Remote.Transducer).Communication.Protocol.is.one.of.the.earliest.

eldbus.to.appear.on.the.market..It.was.originally.designed.to.preserve.compatibility.and.improve.commu-

nication

.over.legacy.4–20.mA.analog.instrumentation.wiring,.sharing.the.pair.of.wires.used.by.the.older.

system..According.to.some,.considering.the.huge.number.of.4–20.mA.devices.installed.throughout.the.

world,.the.HART.protocol.is.probably.the.most.diused.protocols.for.industrial.communications.today.

.protocol.was.proposed.by.Rosemount.Inc..in.the.mid-1980s.for.their.smart.eld.instruments..

It.was.based.on.the.Bell.202.communications.standard.that.AT&T.developed.for.a.modem.operating.

in.the.audio.frequency.range..Starting.from.1986,.it.was.made.an.open.protocol,.thus.speeding.up.its.

enhancement,.as.proved.by.several.successive.revisions.to.the.specication.

.regards.performances,.it.has.been.developed.having.in.mind.process.automation.as.the.target.

scenario,.and.when.used.in.multichannel.I/O.system,.it.allows.for.about.one.transaction.per.second.

(if.frequency.shi.keying.(FSK).modulation.is.adopted),.with.latencies.on.the.same.order.of.magnitude.

(depending.on.the.number.of.nodes)..In.addition,.it.is.able.to.work.on.distances.up.to.1500.m.

WirelessHART

.[H09].is.an.extension.of.wired.HART;.it.has.been.developed.with.the.aim.of.preserv-

ing

.the.same.performance.of.the.wired.counterpart.(or.even.improve.them),.allowing.the.implementa-

tion

.of.a.truly.autonomous.sensor..In.fact,.as.suggested.by.its.name,.it.uses.radio.frequencies.(RF).as.the.

communication.medium.with.a.nominal.coverage.area.of.about.100.m.(line.of.sight).and.devices.may.

be.powered.either.by.wire,.battery,.solar,.or.a.combination.of.these.sources..Communicating.devices.

are.described.in.terms.according.to.the.well-known.Open.Systems.Interconnection.(OSI).Seven-layer.

communication.model.[H88];.a.comparison.with.the.traditional.OSI.model.communications.layers.is.

given.in.Figure.53.1.

OSI layer WirelessHART features

Presentation

Secured session between network devices

Reliable stream transport, negotiated segment

Power-optimized redundant path, mesh to the edge network

Secure and reliable, Time synced, TDMA/CSMA,

frequency agile with ARQ

2.4 GHz wireless, 802.15.4 based radios, 10 dBm Tx power

Command oriented. Predefined data types and application

procedures. Auto-segmented transfer of large data sets.

Application

Presentation

Session

Transport

Network

Data link

Physical

FIGURE 53.1 OSI.Seven-layer.model.and.WirelessHART.comparison.

WirelessHART, ISA100.11a, and OCARI 53-3

53.2.1 the WirelessHart Physical Layer

 e.lowest.layer.on.the.OSI.model.is.the.PHY;.it.is.responsible.for.sending.the.bits.across.the.network.

media..It.enables.the.transmission.and.reception.of.the.protocol.data.across.the.physical.radio.channel..

In.particular,.the.WirelessHART.PHY.is.based.on.a.tailored.adoption.of.the.IEEE.STD.802.15.4-2006.

[KHP08],.in.order.to.reduce.the.overall.device.cost.. is.standard.is.intended.to.conform.to.regulations.

in.Europe,.Canada,.Japan,.China,.and.the.United.States..Only.a.simplied.set.of.services.are.dened.

and.only.the.2.4.GHz.band,.which.uses.OQPSK-HSS.with.DSSS.modulation,.is.supported..In.order.to.

ensure.worldwide.availability,.channel.26.(the.one.center.around.2480.MHz).is.not.supported.since.it.

is.not.legal.in.many.locales..In.this.way,.transceivers.readily.available.“o.the.shelf”.or.custom.radio.

transceivers.that.are.compliant.to.IEEE.802.15.4.can.be.adopted..Transceivers.must.only.support.some.

additional.restrictions.on.channel.switching.time.and.power.up.sequence.to.satisfy.the.medium.access.

strategy.requirements.and.lower.the.power.consumption.. e.radio.must.be.capable.to.set.the.transmit-

ting

.power.(from.−10.up.to.10.dBm).and.must.provide.some.mechanisms.to.perform.the.so-called.clear.

channel.assessment.(in.particular,.only.the.CCA.mode.2,.carrier.sense,.of.IEEE.802.15.4.is.supported)..

Finally,.the.radio.can.operate.in.two.dierent.states:.awake,.during.normal.operation;.and.sleep,.when.

it.is.inactive.

53.2.2 the WirelessHart Data Link Layer

According.to.the.OSI.model,.the.data.link.layer.(DLL).is.the.lowest.information-centric.layer,.which.exploits.

services.oered.by.the.PHY.entities.and.provides.basic.low-level.messaging.among.dierent.peer.enti-

ties.

.It.provides.only.local.addressing;.it.can.forward.messages.to.neighbors,.i.e.,.devices.within.the.local.

addressing.domain,.but.cannot.modify.message.addresses..Addresses.at.the.DLL.have.only.local.scope.

and.can.be.duplicated.in.other.local.links.. e.MAC.layer.is.part.of.the.DLL.and.sits.directly.on.top.of.the.

PHY;.since.it.controls.the.radio,.it.has.a.large.impact.on.the.overall.energy.consumption,.and.hence,.the.

lifetime.of.a.node..MAC.protocols.decide.when.and.how.nodes.(devices).may.access.the.shared.medium,.

i.e.,.the.radio.channel,.and.try.to.ensure.that.no.two.nodes.are.interfering.with.each.other’s.transmissions..

WirelessHART.uses.TDMA.and.channel.hopping.to.control.accesses.to.the.network..It.is.well.known.

that.TDMA.is.a.technique.that.provides.collision-free,.deterministic.communications.and.it.is.the.prefer-

able

.solution.when.time.deadlines.must.be.satised..When.the.TDMA.approach.is.used,.communications.

between.devices.can.occur.in.time.slots..Time.slots.are.grouped.together.in.structures.called.superframes,.

which.are.repeated.continuously..Each.device.must.be.capable.to.manage.several.superframes..Both.the.

slot.size.and.the.superframe.length.(in.number.of.slots).are.xed.and.form.a.network.cycle.with.a.xed.

repetition.rate..A.schematization.of.this.mechanism.is.proposed.in.Figure.53.2.

Destination listens for incoming message

Source now listening

Data

Cycle N

Superframe

Slot

Cycle N + 1

ACK

FIGURE 53.2 Basics.of.WirelessHART.TDMA.mechanism.

53-4 Industrial Communication Systems

For.successful.and.ecient.TDMA.communications,.all.nodes.within.the.same.network.must.share.a.

common.sense.of.time;.in.other.words,.synchronization.of.clocks.between.devices.is.critical..Common.

sources.of.clock.dri.include.temperature.and.aging..Consequently,.tolerances.on.timekeeping.and.time.syn-

chronization

.mechanisms.are.specied.to.ensure.that.devices.“exactly”.know.when.the.start.of.a.slot.occurs.

.format.of.a.timeslot.is.shown.in.Figure.53.3.

.this.gure,.it.is.shown.how.a.unicast.message.exchange.occurs.between.a.transmitting.source.

device.(call.it.“A”).that.is.transmitting.to.a.destination.device.(call.it.“B”),.which.is.listening..Assuming.

that.both.nodes.are.already.synchronized,.then.“B”.knows.when.to.expect.the.rst.bit.of.the.preamble.

of.the.message.coming.from.“A”.(TsMaxPacket.is.the.duration.of.the.longest.packet.allowed.by.the.IEEE.

802.15.4.standard,.i.e.,.4.256.ms).

 ere

.is.an.initial.CCA.(which.starts.with.a.TsCCAOset.=.1.8.±.0.1.ms.delay.and.lasts.TsCCA.=.

0.128.ms).that.is.not.strictly.needed.but.has.been.taken.into.account.in.order.to.improve.coexistence.

with.other.wireless.networks..Within.the.slot,.the.actual.transmission.of.the.source.message.has.a.

delay.with.respect.to.the.beginning.of.a.slot.(TsTxOset.=.2.12.±.0.1.ms.in.the.Figure.53.3).. is.short.

time.delay.allows.the.source.and.destination.to.set.their.frequency.channel.and.allows.the.receiver.to.

begin.listening.on.the.correct.channel..Also,.the.receiver.has.a.delay.with.respect.to.time.slot.beginning.

(TsRxOset.=.1.12.±.0.1.ms.in.Figure.53.3),.needed.to.set.up.the.radio..In.addition,.since.there.is.a.toler-

ance

.on.clocks,.the.receiver.has.a.guard.interval.(TsRxWait.=.2.2.±.0.1.ms.in.Figure.53.3),.i.e.,.it.must.

start.to.listen.before.the.ideal.transmission.start.time.and.continue.listening.aer.that.ideal.time.

Once

.the.transmission.is.complete,.the.communication.direction.is.reversed.and.the.destination.

device.sends.an.acknowledgment.packet.(ACK).whether.it.received.the.source.device.message.success-

fully

.or.with.a.specic.class.of.detected.errors.. e.switching.among.the.transmitting.and.the.receiving.

state.of.the.radio.must.be.no.longer.than.TsRxTx.=.0.192.ms,.while.the.source.node.waits.TsRxAckDelay.=.

0.8.±.0.1.ms.from.the.end.of.the.message.to.the.listening.of.the.ACK.(the.guard.time.for.the.ACK.arrival.

can.be.small,.since.the.two.nodes.are.tightly.synchronized.by.the.arrival.of.the.message.from.the.source.

to.the.destination);.on.the.contrary,.the.destination.node.must.send.the.ACK.after.TsTxAckDelay.=.

1.0.±.0.1.ms.from.the.end.of.the.message.

Communicating

.devices.are.assigned.not.only.to.a.superframe.and.time.slot.but.have.also.a.specic.

channel.oset,.by.means.of.which.channel.hopping.is.implemented.. is.3-tuple.forms.the.so-called.

communications.link,.i.e.,.the.opportunity.to.establish.a.connection.between.communicating.devices..

All.devices.must.support.multiple.links.. e.number.of.possible.links.is,.typically,.equal.to.the.number.

of.channels.utilized.by.a.network.times.the.number.of.slots.in.the.superframe..For.example,.the.use.of.

15.channels.and.9,000.slots.per.superframe.results.in.135,000.possible.links.

Time slot starts here

TsRxTx

TsCCAOffset

TsTXOffset

TsCCA

TsRxOffset TsRxWait TsTxACKDelay

TsACK

Data

ACK

≤TsMaxPacket TsRxACKDelay

TsACKWait

FIGURE 53.3 Packet.timing.and.time.slot.format..Brighter.box.is.for.reception.state.and.darker.box.is.for.

transmitting.state.of.devices.

WirelessHART, ISA100.11a, and OCARI 53-5

Usually,.only.two.devices.are.assigned.to.a.given.slot.(source.and.sink.of.the.transaction)..However,.

shared.time.slots.are.also.considered;.since.collisions.may.occur,.the.traditional.CSMA/CA.MAC.strat-

egy

.is.adopted.

Channel

.hopping.(see.Figure.53.4).is.combined.with.TDMA.in.order.to.enhance.reliability..It.pro-

vides

.frequency.diversity,.which.can.avoid.interferers.and.reduce.multipath.fading.eects.

Channel

.hopping.means.also.channel.diversity,.which.is.each.slot.may.be.used.on.multiple.channels.

at.the.same.time.by.dierent.nodes.. is.can.be.achieved.by.creating.links.on.the.same.slot,.but.with.

dierent.channel.osets.. e.assignment.of.a.link.occurs.in.a.centralized.way,.by.the.network.manager.

described.in.the.following.subsection..In.addition,.the.channel.blacklisting.mechanism,.which.allows.

the.network.administrator.to.restrict.the.channel.hopping.of.network.devices.network-wide.to.selected.

channels.in.the.RF.band,.has.also.been.implemented.. is.feature.could.be.useful.in.a.crowded.RF.envi-

ronment,

.increasing.coexistence..However,.it.must.be.noticed.that.WirelessHART.communication.(like.

WiFi,.Bluetooth,.and.other.wireless.communication).is.very.random.and.bursty.in.nature;.consequently,.

blacklisting.seldom.provides.tangible.benets.

53.2.2.1

the Data Link Layer Datagram

.subsection.species.the.format.of.the.data.link.datagram.(DLPDU,.see.Figure.53.5).

Each

.DLPDU.consists.of.the.following.elds:

•

. A.single.byte.set.to.0x41

•

. A.1.byte.address.specier

•

. e.1.byte.sequence.number

5 MHz 3 MHz

2.405

F (GHz)

Cycle N – 1Cycle N – 2 Cycle N + 1 Cycle N + 2

Time slot

Link

Time

Channel

Superframe

FIGURE 53.4 e.frequency.diversity.mechanism.of.WirelessHART.

Data link layer

Data link payload

Physical layer

Preamble Delimiter Length

X41

Address

specier

Sequence

number

Network

Destination

address

Source

address

DLPDU

specier

MIC CRC

FIGURE 53.5 e.datagram.at.the.data.link.layer.

53-6 Industrial Communication Systems

•. e.2.byte.network.ID

•

. Destination.and.source.addresses.either.of.which.can.be.2.or.8.byte.long

•

. A.1.byte.DLPDU.specier

•

. e.DLL.payload

•

. A.4.byte.keyed.message.integrity.code.(MIC)

•

. A.2.byte.ITU-T.CRC16.(imposed.by.the.IEEE.802.15.4)

.particular,.while.the.DLL.packet.is.unencrypted,.the.MIC.is.enciphered.for.link.layer.authentication.

of.the.received.datagram;.it.is.generated.and.conrmed.using.CCM*.mode.(combined.counter.with.

CBC-MAC.[corrected]).in.conjunction.with.the.AES-128.block.cipher.to.provide.authentication.of.the.

originator.

53.2.3 time Keeping

It.is.possible.to.maintain.the.nodes.clock.skew.under.the.time.slot.guard.time.using.regular.message.

exchange.and.low-level.timestamping..A.keep-alive.exchange.must.occur.at.least.every.30.s..Devices.

transmit.the.rst.bit.of.their.packet.as.close.to.the.ideal.start.time.as.possible.in.their.time.slot.. e.

receiving.node.measures.the.time.of.arrival.of.this.rst.bit.in.its.own.time.frame..In.this.way,.the.desti-

nation

.knows.the.dierence.(oset).between.its.time.slot.and.the.transmitter’s.time.slot.. is.informa-

tion

.is.then.sent.back.to.the.source.in.the.ACK.packet.in.order.to.compensate.for.it.

53.2.4 the WirelessHart Network Layer and topologies

 e.WirelessHART.protocol.allows.to.implement.a.sensor.and.actuator.mesh.communication.system.

(see.Figure.53.6).

Every

.WirelessHART.network.contains.dierent.kind.of.devices.(logical.and/or.physical):

•

. One.and.only.one.security.manager.that.distributes.encryption.keys.to.the.network.manager.of.

each.network

•

. One.and.only.one.active.network.manager,.whose.aim.is.to.form.the.network,.schedule.resources,.

congure.routing.paths,.etc.

•

. At.least.one.gateway,.whose.aim.is.to.interconnect.eld.devices.with.the.plant.automation.system.

by.means.of.access.points

•

. Several.eld.devices.that.are.connected.with.the.process,.i.e.,.devices.with.sensors.and.actuators

All

.communications.occur,.e.g.,.by.moving.data.from.the.gateway,.through.the.intermediate.devices,.to.

the.packet’s.destination..Each.movement.of.a.packet.from.one.device.to.another.along.the.route.to.the.

Host

application

Gateway

Access point

Network

manager

Handheld device

Field

devices

Security

manager

FIGURE 53.6 WirelessHART.network.topology.

WirelessHART, ISA100.11a, and OCARI 53-7

packet’s.nal.destination.is.called.a.hop..WirelessHART.networks.can.be.implemented.in.a.wide.variety.

of.topologies..In.a.high-performance.scenario,.a.star.topology.is.probably.preferable.(i.e.,.all.devices.are.

one.hop.to.the.gateway),.while.a.multi-hop.mesh.topology.is.useful.for.a.less.demanding.(e.g.,.monitoring).

application..Obviously,.any.topology.in.between.can.also.be.realized.

 ere

.can.be.also.devices.that.lack.the.connection.with.the.process.but.have.only.communication.

facilities;.they.are.routers,.handheld.devices.(used.for.commissioning.and/or.maintenance.purposes),.

and.adapters.(used.to.connect.legacy.hardware.with.the.wireless.network).

Routing

.of.data.packets.is.based.on.graph.routing.rather.than.on.the.(optional).source.address.rout-

ing.

.Each.pair.of.nodes.is.interconnected.by.several.graphs,.i.e.,.a.directed.list.of.paths.that.connect.

them..Both.upstream.(toward.the.gateway).and.downstream.graphs.are.used..Only.the.network.man-

ager,

.who.is.responsible.for.correctly.conguring.each.graph,.knows.the.entire.route;.the.graph.infor-

mation

.within.a.node.only.indicates.the.next.hop.destinations..Consequently,.devices.in.a.graph.route.

must.be.congured.prior.to.its.use;.i.e.,.it.must.contain.a.list.of.all.the.links.that.can.be.used.to.forward.

a.packet.along.the.graph..Graph.routing.is.redundant,.highly.reliable,.and.should.be.used.for.normal.

communications.

.the.contrary,.a.source.route.is.a.single,.directed.route.between.a.source.and.a.destination.device..

 e.source.route.is.statically.specied.in.the.packet.itself;.for.this.reason,.it.is.vulnerable.and.it.is.used.

only.during.the.commissioning.phase.

53.2.4.1

the Network Layer Datagram

.network.layer.datagram.consists.of.the.following.elds.(see.Figure.53.7):

•

. A.1.byte.control.eld

•

. e.1.byte.time-to-live.(TTL).hop.counter

•

. e.least-signicant.2.byte.of.the.absolute.slot.number.(latency.count)

•

. A.2.byte.graph.ID

•

. e.(nal).destination.and.(original).source.addresses

•

. Optional.routing.elds

•

. A.security.layer.is.encapsulated.within.this.datagram.together.with.the.enciphered.payload

•

. Enciphered.network.protocol.data.unit.(NPDU).payload

Communication

.between.a.pair.of.network.addresses.is.organized.in.sessions.for.security.purposes..

Four.sessions.are.generally.set.up.as.soon.as.any.device.joins.the.network,.even.if.additional.sessions.

may.be.added:

•

. Unicast.session.between.the.device.and.the.network.manager—it.is.used.by.the.network.manager.

to.manage.the.device.

•

. Broadcast.session.between.the.device.and.the.network.manager—it.is.used.to.globally.manage.

devices..For.example,.this.can.be.used.to.roll.a.new.network.key.out.to.all.network.devices..All.

devices.in.the.network.have.the.same.key.for.this.session.

Security

control

Counter MIC

ASN

snippet

Control TTL

Graph

Destination

address

Source

address

(Expanded routing

information)

Network layer

Security layer

Enciphered payload

FIGURE 53.7 e.network.layer.datagram.

53-8 Industrial Communication Systems

•. Unicast.between.the.device.and.the.gateway—it.carries.normal.communications.(e.g.,.process.

data

)

.bet

ween

.the.gat

eway

.and.the.dev

ice.

•

. Broad

cast

.sess

ion

.betw

een

.the.devic

.and.the.gatew

ay—it

.is.used.by.the.gatew

.to.send.the.

iden

tical

.app

lication

.dat

.to.all.dev

ices.

.addi

tion,

.each.devi

.alwa

.has.a.join.sessi

.that.cann

.be.dele

ted.

. e.join.key.is.uniq

ue;

.it.is.the.

only.key.that.can.be.writ

ten

.by.dire

ctly

.conn

ecting

.to.main

tenance

.port.physi

cally

.impl

emented

.on.the.

devi

.itse

lf.

.It.can.also.be.writ

ten

.by.netw

ork

.mana

ger.

.In.fact

.only.the.netw

ork

.mana

ger

.may.crea

or.mod

ify

.ses

sions

.and.the

.cor

responding

.key

53.2.5 the WirelessHart Upper Layers

A.simple.transport.layer.is.provided.in.WirelessHART,.which.ensures.data.exchange.reliability..It.imple-

ments

.a.mast

er–slave

.tran

saction,

.wher

.a.“mas

ter”

.issu

.a.requ

est

.pack

.and.one.or.more.“sla

ves”

repl

.with.a.resp

onse

.pack

et;

.or.a.slav

.cycl

ically

.publ

ishes

.a.resp

onse

.pack

et.

. e.appl

ication

.laye

(APL

.inst

ead,

.is.the.same.of.the.wire

.coun

terpart.

.HART.appl

ication

.laye

.is.comm

and

.base

.with.

stan

dard

.data.type

.and.proc

edures.

.Univ

ersal,

.Comm

.Prac

tice,

.Devi

.Fami

ly,

.and.Wirel

ess

.com-

ands

. are. spec

ied.

. Exte

nsive

. stan

dard

. and. devi

ce-specic

. stat

. are. avai

lable,

. incl

uding

. qual

ity

asse

ssment

.and.stat

.for.all.proc

ess

.vari

ables.

.A.deta

iled

.disc

ussion

.of.thes

.comm

ands

.is.outsi

.the.

scop

.of.thi

.cha

pter.

53.3 ISa100.11a

 e.ISA100.[ISA09].committee.was.formed.by.International.Society.of.Automation.(ISA).to.satisfy.the.

need.of.a.wire

less

.manu

facturing

.and.cont

rol

.syst

ems.

.In.part

icular,

.the.ISA1

0.11a

.is.devo

ted

.to.the.

deve

lopment

.of.a.reli

able

.and.secu

.comm

unication

.syst

.for.both.nonc

ritical

.moni

toring

.and.pro-

ess

.cont

rol

.appl

ications,

.whic

.can.toler

ate

.late

ncies

.on.the.orde

.of.100.ms..ISA1

00.11a

.netw

orks

.can.

have.a.very.rich.stru

cture.

.Seve

ral

.mult

i-hop

.netw

orks

.havi

.a.mesh.topol

ogy,

.each.one.buil

.amon

devi

ces

.with.rout

ing

.capa

bilities

.to.whic

.simpl

.non-

routing

.devi

ces

.are.asso

ciated,

.can.be.inte

rcon-

nected

.via.a.back

bone.

.Rout

ing

.supp

ort

.for.inte

r-mesh

.comm

unications

.is.prov

ided

.and.each.mesh.

can.have.one.or.more.gate

ways

.to.othe

.type

.of.plan

.netw

orks.

.Howe

ver,

.it.must.be.unde

rlined

.that.

the.ISA1

00.11a

.is.a.very.rece

.stan

dard

.(it.has.been.oci

ally

.appr

oved

.by.the.ISA.asso

ciation

.duri

Septe

mber

.200

9),

.and.onl

.pre

liminary

.imp

lementations

.are.ava

ilable.

53.3.1 the ISa100.11a Physical Layer

Similarly.to.WirelessHART,.ISA100.11a.also.utilizes.radios.operating.in.the.2.4.GHz.ISM.band.and.

base

.on.the.IEEE.802.

15.4-2006

.stan

dard.

.Also.in.this.case

.the.last.chan

nel

.is.not.used.to.be.comp

li-

ant

.with.regu

lations

.in.some.coun

tries

.and.some.requ

irements

.on.swit

ching

.time.of.tran

sceivers

.are.

prov

ided

.in.ord

.to.imp

lement

.a.fre

quency

.agi

lity

.mec

hanism.

53.3.2 the ISa100.11a Data Link Layer

Also,.the.DLL.has.many.similarities.with.the.one.implemented.in.WirelessHART..ISA100.11a.denes.

devi

ces

.accor

ding

.to.thei

.role.in.the.netw

ork;

.basi

cally,

.ther

.is.a.syst

.mana

ger

.that.mana

ges

.the.net-

ork,

.a.securi

.mana

ger

.that.mana

ges

.keys

.rout

ers,

.and.eld.devi

ces

.(oth

.role

.are.den

ed,

.for.inst

ance,

for.cloc

.sourc

ing

.or.mult

itier

.netw

orking

.but.are.not.repo

rted

.for.sake.of.simp

licity).

.Time.slot

.supe

frames,

.and.link

.are.the.buil

ding

.bloc

.that.perm

.comm

unications

.to.occur

.A.time

slot

.is.a.sing

le,

.non-

epeating

.peri

.of.time

;

.a.supe

rframe

.is.a.coll

ection

.of.time

slots

.repe

ating

.on.a.cycl

.schedu

le;

.link

.are.

conn

ections

.betw

een

.devi

ces

.(eac

.link.refe

.to.one.time

slot

.or.a.grou

.of.time

slots

.with

.a.supe

rframe).

Howe

ver,

.in.the.ISA.specs

.time.slot

.do.not.have.a.xed.leng

.but.must.be.alig

ned

.to.a.250.ms.boun

dary.

WirelessHART, ISA100.11a, and OCARI 53-9

In.addition,.the.compatibility.with.the.IEEE.802.15-4-2006.has.been.preserved;.in.other.words,.the.packet.

at.the.DLL.preserves.the.header.and.footer.specied.in.the.IEEE.standard;.see.Figure.53.8..Nevertheless,.it.

must.be.highlighted.that.protocol.itself.is.dierent;.there.are.no.IEEE.MAC.full.function.devices.(FFDs),.

the.superframe.is.dened.in.a.dierent.way;.even.if.CSMA/CA.is.used.in.both.solutions,.details.are.not.the.

same.and.the.IEEE.MAC.backo.and.retry.mechanism.is.not.used..Instead,.the.ISA.standard.implements.

its.own.retries,.involving.spatial.diversity.(retries.to.multiple.devices).and.frequency.diversity.(retries.on.

multiple.radio.channels)..Neighbor.discovery.and.joining.are.also.implemented.dierently.

 ree

.general.modes.of.operation.are.supported:.the.slotted.channel.hopping;.the.slow.channel.hop-

ping;

.and.hybrid.combinations.of.slotted.and.slow.hopping..In.slotted.hopping,.each.timeslot.uses.a.

dierent.radio.channel.in.a.hopping.pattern.and.it.is.intended.to.accommodate.one.message.and.its.

acknowledgment.(see.Figure.53.9)..On.the.contrary,.in.slow.hopping,.a.collection.of.contiguous.times-

lots

.is.grouped.on.a.single.radio.channel..Timeslot.(bandwidth).allocation.to.devices.may.be.centralized,.

i.e.,.handled.by.the.system.manager,.or.delegated..For.instance,.a.simple.form.of.delegation.is.based.on.

channel.hopping.osets;.a.frequency.oset.is.assigned.to.each.infrastructure.device.and.it.is.free.to.cre-

ate

.superframes.using.that.oset.

According

.to.the.specs,.packets.forwarding.within.the.wireless.network.is.performed.at.the.DLL,.

using.the.same.graph.routing.adopted.also.by.the.WirelessHART.solution..Routes.are.congured.by.

the.system.manager,.based.on.reports.from.devices.that.indicate.instantaneous.and.historical.quality.

of.wireless.connectivity.to.their.immediate.neighbors.. e.system.manager.accumulates.these.reports.

of.link.quality.to.make.routing.decisions.. ese.reports.are.standardized,.but.not.the.routing.decision.

process.within.the.system.manager.

Data link layer

Physical layer

Preamble Delimiter Length

IEEE 802.15.4

MAC header

ISA100.11a

DL header

Data link payload

(possibly encrypted)

Authenticated by MMIC

MMIC FCS

FIGURE 53.8 ISA100.11a.datagram.structure.(DL.Header:.additional.ISA100.Data.Link.layer.header).

Time

Chan.

Time slot

C D

Link

FIGURE 53.9 ISA100.11a.link.and.channel.hopping.concept.