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

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24-2 Industrial Communication Systems

real-time.Ethernet.[KG04,EI03,F05]),.this.chapter.focuses.solely.on.protocols.that.have.already.been.

stan

dardized

.by.the.ARI

.com

mittee.

.Sect

ion

.24.2.of.this.chapt

er,

.the.main.evol

ution

.of.avio

nics

.syst

ems

.and.the.stan

dardization

.unde

taken

.by.ARIN

.are.brie

y

.revi

ewed.

. e.clas

sic

.avio

nics

.cont

ext

.is.pres

ented

.in.Sect

ion

.24.3.and.inte

grated

.modu

lar

.avio

nics

.conc

epts

.are.outl

ined

.in.Sect

ion

.24.4

.ARIN

.629.proto

cols

.for.cont

rolling

acce

.to.a.mult

iplexed

.data.bus.are.desc

ribed

.in.Sect

ion

.24.5

.Sect

ion

.24.6.is.devo

ted

.to.the.pres

entation

of.ARIN

.664.avio

nics

.full

-duplex

.Ether

net

.mech

anisms.

.End-

to-end

.dela

.anal

ysis

.meth

ods

.are.sum-

arized

.in.Sec

tion

.24.

24.2 avionics Systems Evolution and arINC Context

Avionics.is.the.generic.name.given.to.the.electronic.systems.installed.in.an.aircra..It.includes,.for.

exam

ple,

.the.calc

ulators

.and.thei

.sow

are,

.the.sens

ors

.and.actu

ators,

.and.all.the.comm

unication

.link

betw

een

.thes

.elem

ents.

.Each.func

tion

.of.an.airc

ra

.is.impl

emented

.thro

ugh

.a.give

.avio

nics

.syst

em:

comm

and

.cont

rols,

.autop

ilot,

.navi

gation,

.info

rmation

.disp

lay,

.and.cabi

.mana

gement.

.In.the.1950

avio

nics

.were.very.simpl

.stan

dalone

.syst

ems

.in.whic

.each.func

tion

.was.exec

uted

.using.a.singl

.calc

lator.

.Mode

.avio

nics

.bega

.in.the.1960

.with.the.repl

acement

.of.anal

.devi

ces

.by.thei

.digi

tal

.equi

alents.

.Sinc

.then

.the.comp

lexity

.of.avio

nics

.has.cont

inually

.incr

eased:

.More.and.more.anal

.devi

ces

are.beco

ming

.outd

ated

.and.requ

ire

.new.digi

tal

.repl

acements,

.and.new.func

tions

.are.cont

inuously

.bein

adde

. ese.func

tions

.bene

t

.from.impr

ovements

.in.elec

tronics

.and.hard

ware

.exec

ution

.and.comm

nication

.arch

itectures.

. ey.can.be.answ

ers

.to.the.new.need

.inhe

rent

.in.the.evol

ution

.of.civi

.avia

tion

or.simpl

.bett

.answ

ers

.to.exis

ting

.prob

lems,

.like.the.y-by

-wire

.comm

and

.syst

em.

.A.few.fact

.clea

rly

illu

strate

.the.grow

.of.the.role.of.avio

nics

.in.civi

lian

.airc

ra:

.From.1983.(A31

.to.1993.(A34

0),

.the.

numbe

.of.onbo

ard

.avio

nics

.syst

ems

.incr

eased

.by.almo

.50%.(fro

.77.to.115)

.whil

.total.proc

essing

powe

.was.mult

iplied

.four

fold,

.from.60.to.250.Mips

.Desig

ning

.and.manu

facturing

.new.civi

lian

.airc

ra

has.led.to.an.incr

ease

.in.the.numbe

.of.embed

ded

.syst

ems

.and.func

tions

.and.cons

equently,

.an.incr

ease

in.com

munication

.nee

.[CC

93].

Avionics

.syst

ems

.are.subje

.to.volu

.and.weig

.limi

tations

.and.must.also.oper

ate

.corr

ectly

.even.

in.seve

.cond

itions:

.heat

.vibr

ations,

.elec

tromagnetic

.inte

rferences,

.etc..Howe

ver,

.the.main.char

acter-

istics

.lie.in.the.safe

.leve

.they.requ

ire.

.Foll

owing

.ARP.4754.[SAE9

6],

.syst

ems

.are.clas

sied

.acco

rding

to.the.eects.of.their.failure:.catastrophic,.hazardous,.major,.minor,.or.no.eect..For.example,.a.system.

is.clas

sied

.as.“cat

astrophic”

.if.its.fail

ure

.can.lead.to.the.total.loss.of.an.airc

ra.

. e.segr

egation

.of.crit

cal

.func

tions

.lead

.to.a.dist

ributed

.arch

itecture

.with

out

.any.cent

ralized

.cont

rol:

.the.syst

ems

.coop

erate

thro

ugh

.data.exch

ange

.than

.to.data.buse

.Typi

cally,

.clas

sic

.avio

nics

.uses.piec

.of.equi

pment

.that.can.

be.seen.as.hard

ware

.and.sow

are

.blac

.boxe

.whic

.are.each.resp

onsible

.for.one.give

.func

tion.

. e.

physi

cal

.devi

ces

.adopt.a.stan

dardized

.modu

.form

.call

.line.repl

aceable

.unit

.(LRU

. ese.modu

les

inco

rporate

.memo

.and.proc

essing

.reso

urces

.and.prov

ide

.a.stan

dardized

.appl

ication

.inte

rface

.(API

)

to.thes

.reso

urces.

. is.stan

dardization

.ease

.main

tenance

.acti

vity,

.since.any.piec

.of.equi

pment

.can.be.

easil

.repl

aced

.by.anot

her

.LRU,.prov

ided

.the.appr

opriate

.code.has.been.load

ed.

.New.conc

epts

.base

.on.

bett

.shar

ing

.of.exec

ution

.and.comm

unication

.reso

urces

.were.prop

osed

.in.the.1990

.IMA.intr

oduced

the.den

ition

.of.a.gene

ric

.plat

form

.call

.“cab

inet”

.comp

osed

.of.stan

dard

.exec

ution

.and.tran

smission

modu

les,

.call

.line.repl

aceable

.modu

les

.(LRM

);

.this.will.be.pres

ented

.in.Sect

ion

.24.3

. e.glob

.IMA.

arch

itecture

.is.thus.comp

osed

.of.modu

les

.hous

.in.cabi

nets

.dist

ributed

.thro

ughout

.the.airc

ra,

.lead

ing

.to.new.int

racabinet

.and.int

ercabinet

.com

munication

.nee

ds.

Aeronautical

. Radi

. Inco

rporated

. (ARI

NC)

. prov

ided

. lead

ership

. in. deve

loping

. spec

ications

. and.

stan

dards

.for.avio

nics

.equi

pment.

.Most.of.the.prop

osed

.spec

ications

.and.stan

dards

.were.deve

loped

and.main

tained

.by.the.Airl

ines

.Elec

tronic

.Engi

neering

.Comm

ittee

.(AEE

.comp

rising

.membe

.that.

repr

esent

.airl

ines,

.gove

rnments,

.and.ARIN

.ARIN

.stan

dardized

.the.trad

itional

.ARIN

.429.mono

emitter

.data.bus.[ARI

01],

.whic

.has.been.wide

.used.in.clas

sic

.avio

nics

.arch

itectures

.cont

ext.

. e.ARIN

629.mult

iplexed

.data.bus.[ARI

99]

.was.desig

ned

.with.IMA.in.mind

.IMA.itse

.is.comp

osed

.of.two.

Embedded Networks in Civilian Aircraft Avionics Systems 24-3

ARINC.standards:.ARINC.651.for.the.description.of.the.modular.hardware.architecture.[ARI91].and.

ARINC.653.for.the.modular.soware.architecture.[ARI97]..More.recently,.the.ARINC.664.has.provided.

an.answer.to.the.problem.of.increased.communication.by.multiplexing.huge.amounts.of.communica-

tion

.data.over.a.full-duplex.switched.Ethernet.network.[ARI02,ARI03]..It.has.become.the.reference.

communication.technology.in.the.context.of.civilian.aircra.avionics.since.it.provides.a.backbone.

network.for.the.avionics.platform.

24.3 Classic avionics and arINC 429

ARINC.429.is.the.earliest.and.most.commonly.used.standard.for.civilian.aircra.avionics.and.has.

been.installed.on.the.majority.of.Airbus.and.Boeing.aircra..It.employs.a.unidirectional.data.bus.

standard.known.as.the.Mark.33.digital.information.transfer.system.(DITS)..Messages.are.transmit-

ted

.over.two.twisted.pairs.by.the.owner.of.the.bus.to.other.system.units.at.a.bit.rate.of.either.12.5.

or.100.kbps.. e.specication.denes.electrical.transmission.characteristics.and.protocol.data.unit.

features.[ARI01].

.ARINC.429.protocol.implements.a.mono-emitter.broadcast.bus.with.up.to.20.receivers..Messages.

sent.by.the.sole.transmitter.consist.of.a.single.32.bits.data.word.. e.label.eld.of.the.word.denes.the.

type.of.data.that.is.contained.in.the.rest.of.the.word..ARINC.429.data.words.use.ve.primary.elds:.

parity,.SSM,.data,.SDI,.and.label..As.represented.in.Figure.24.1,.the.ARINC.429.data.word.is.composed.

as.follows:.bit.32.is.the.parity.bit;.bits.31.and.30.contain.the.sign/status.matrix.(SSM).eld.that.contains.

equipment.conditions,.operational.modes,.or.validity.of.data.content;.bits.29.through.11.contain.the.

data;.bits.10.and.9.provide.a.source/destination.identier.(SDI);.and.bits.8.through.1.contain.a.label.

identifying.the.data.and.associated.parameters.

.label.is.an.important.part.of.the.message..It.is.used.to.identify.the.transmitted.data.and.to.deter-

mine

.the.data.type.. e.emitter.broadcasts.the.data.(with.a.given.periodicity).on.the.bus.and.does.not.

know.which.equipment.will.receive.a.given.instance.of.data..Each.receiver.lters.the.data.according.to.

the.label.of.each.data.. e.SDI.is.used.to.identify.the.receiver.to.which.the.data.is.destined..If.a.given.

label.can.be.sent.on.dierent.data.buses,.it.can.also.be.used.to.identify.the.source.of.the.transmission.(as.

units.are.assigned.digital.identication.numbers.called.equipment.id).

ARINC

.429.is.confronted.with.an.increasing.number.of.intercommunicating.avionics.functions.. e.

(point.to.multi-point.and.unidirectional).characteristics.of.the.ARINC.429.protocol.mean.that.the.avi-

onics

.system.must.include.an.ARINC.429.bus.for.each.communication.path.as.depicted.in.Figure.24.2..

32 31 30

P SSM

29 11

10 9

8 1

Data SDI Label

FIGURE 24.1 ARINC.429.data.word.

G1 G2

FIGURE 24.2 Classic.avionics.architecture.

24-4 Industrial Communication Systems

Moreover,.an.ARINC.429.transmitter.can.transmit.to.a.maximum.of.20.receivers..While.the.ARINC.

429.data.bus.provides.high.determinism,.point-to-point.wiring.has.become.a.major.problem.in.systems.

composed.of.multiple.emitting.units.

24.4 Integrated Modular avionics

 e.IMA.concept.has.introduced.the.sharing.of.execution.and.communication.resources..IMA.hard-

ware

.architecture,. as.described. in. the. standard.ARINC.651.[ARI91],.is.represented. in.Figure.24.3..

Execution.and.communication.resources.are.described.as.standard.LRMs.that.are.installed.in.common.

cabinets.. ree.types.of.modules.have.been.designed:.core.modules.for.application.execution,.input–

output

.modules.for.communications.with.non-IMA.equipment.(existing.LRUs),.and.gateway.modules.

for.communicating.among.cabinets.. e.ARINC.659.back.plane.data.bus.standard.has.been.used.for.

communication.between.modules.hosted.in.the.same.cabinet.[ARI93]..Communication.among.cabinets.

needs.new.aircra.data.buses.such.as.ARINC.629.[ARI99].

ARINC

.653.[ARI97].describes.how.many.avionics.subsystems.can.share.the.execution.resources.of.

an.IMA.architecture..In.an.ARINC.653.environment,.an.avionics.subsystem.is.no.longer.implemented.

on.a.dedicated.CPU.or.LRU.but.is.viewed.as.a.partition,.to.which.is.assigned.a.time.window.to.execute.

its.code.on.a.shared.core.processing.module.(CPM)..Each.partition.becomes.a.virtual.CPU.or.LRU..

A.robust.partitioning.concept.is.introduced.to.guarantee.isolation.between.subsystems.running.on.the.

same.execution.computer..Spatial.isolation.is.guaranteed.by.protecting.the.address.space.of.each.parti-

tion

.(for.example,.by.a.memory.management.unit)..Temporal.isolation.is.based.on.the.static.allocation.

of.CPU.time.to.each.partition..Time.partitioning.guarantees.that.each.user.partition.obtains.a.slice.of.

time.for.execution.as.determined.by.the.integrator.as.a.worst-case.execution.time.(easier.to.evaluate.for.

systems.that.have.mostly.periodic.processes).. e.main.objective.is.to.guarantee.that.an.avionic.subsys-

tem

.running.in.a.given.partition.will.have.no.eect.on.other.subsystems.running.in.other.partitions..

Partitions.communicate.with.each.other.by.exchanging.messages.through.communication.ports..Two.

types.of.communication.ports.can.be.used:.sampling.ports.(periodic.data—only.the.last.value.of.data.

is.stored).and.queuing.ports.(all.the.values.of.data.are.stored)..A.logical.channel.concept.is.used.to.link.

communicating.ports.(multicast.scheme—1.emitter.and.N.receivers).independently.of.the.underlying.

data.bus.or.network.stack..Mechanisms.are.needed.in.order.to.schedule.inputs–outputs.according.to.the.

needs.of.each.partition.(for.example,.port.sizes.and.depths.must.be.determined.on.a.worst.case.commu-

nication

.basis)..In.conclusion,.the.ARINC.653.interface.between.application.soware.and.the.run-time.

Backplane Backplane

F1 F3

Eq F3

ARINC 429

API

Aircraft data bus

API

Eq F2 Eq F1

ARINC 429

APEX/C APEX/C APEX/C APEX/C APEX/C APEX/C APEX/C APEX/C

FIGURE 24.3 IMA.avionics.architecture.

Embedded Networks in Civilian Aircraft Avionics Systems 24-5

executive.called.application.executive.(APEX).is.an.important.step.in.developing.avionics.applications.

with

out

.kno

wing

.the.cha

racteristics

.of.the.tar

get

.pro

cessing

.and.com

municating

.arc

hitecture.

.rst.exam

ples

.of.IMA.impl

ementation,

.such.as.the.Boei

.777.airp

lane

.info

rmation

.mana

gement

syst

.(AIM

.show.many.pote

ntial

.bene

ts:

.adapt

ability

.(tha

nks

.to.the.modu

larity

.of.the.arch

itecture,

it.is.easy.to.con

gure

.the.airc

ra

.for.a.give

.missi

on),

.main

tainability

.(the.stan

dardization

.of.the.mod-

les

.simpl

ies

.the.main

tenance),

.cost.redu

ctions

.(easi

.main

tenance

.and.hard

ware

.evol

ution

.cut.down.

cost

s),

.and.a.decr

ease

.in.weig

.and.volu

.(few

.devi

ces

.are.need

.than.with.prev

ious

.arch

itectures).

Consi

dering

.thes

.bene

ts,

.Airb

.chos

.the.IMA.arch

itecture

.for.the.A380

.and.Boei

.reus

.the.

IMA.conc

epts

.for.the.787.airc

ra.

.Sinc

.the.arch

itecture

.is.inde

pendent

.of.the.chos

.comm

unication

meth

.used

.we.will.pres

ent

.in.foll

owing

.sect

ions

.the.ARIN

.629.mult

iplexed

.data.bus,.whic

.was.

chos

.for.the.Boei

.777.AIMS.and.the.ARIN

.664.or.AFDX

.whic

.was.chos

.as.the.basis.of.the.

IMA.arc

hitecture

.of.the.A38

24.5 arINC 629 Multiplexed Data Bus

Development.of.the.ARINC.629.[ARI99].multiplexed.data.bus.began.when.avionics.systems.designers.real-

ized

.that.the.ARIN

.429.sing

.tran

smitter/multiple

.recei

ver

.conc

ept

.coul

.not.cope.with.the.incr

easing

amou

.of.inte

rsystem

.data.tran

sfer

.requ

ired

.for.evol

ving

.comm

ercial

.airc

ra.

. e.prim

ary

.advan

tages

.of.

a.mult

i-transmitter

.data.bus.such.as.the.ARIN

.629.incl

ude

.the.abil

ity

.to.move.more.data.betw

een

.LRUs.

at.high

.rate

.usin

.fewe

.wire

.Also.they.are.gene

rally

.more.reli

able

.and.prov

ide

.an.arch

itecture

.allo

ing

.the.inte

gration

.of.comp

lex

.syst

ems.

.Howe

ver,

.mult

itransmitter

.buses.have.been.main

.used.in.the.

cont

ext

.of.mili

tary

.airc

ra.

. e.MIL-

STD-1553

.data.bus.stan

dard

.[MIL

78]

.has.been.deve

loped

.for.mili

tary

and.spac

.syst

ems.

.It.impl

ements

.a.bus.arch

itecture

.in.whic

.all.the.devi

ces

.atta

ched

.to.the.bus.are.capa

ble

of.recei

ving

.and.tran

smitting

.data

.Howe

ver,

.it.uses.a.comm

and/response

.(cen

tralized)

.mecha

nism

.that.

does.not.sati

sfy

.the.term

inal

.inde

pendence

.needed.for.cert

ication

.of.comm

ercial

.airc

ra.

.More

over,

.clas

sic

.Ethe

rnet

.arch

itecture

.and.its.rand

.CSMA

-CD

.mediu

.acces

.cont

rol

.mecha

nism

.cann

.guar

antee

peri

odic

.tra

. e.digi

tal

.auto

nomous

.term

inal

.acces

.comm

unication

.(DAT

AC)

.proj

ect

.enge

ndered

the.ARIN

.629.[ARI

99],

.an.avio

nics

.indu

stry

.digi

tal

.comm

unications

.stan

dard,

.whic

.was.chose

.as.the.

prim

ary

.digi

tal

.comm

unication

.syst

.on.the.Boei

.777.

.stan

dard

.allo

.the.tran

smission

.of.mess

ages

.at.a.2.Mbps.seri

.data.rate.on.twis

ted

.pair.cond

uctors.

A.mess

age

.has.vari

able

.leng

.and.is.comp

rised

.of.up.to.31.word.stri

ngs.

.Each.word.stri

.has.vari

able

.leng

and.cont

ains

.a.16.bit.labe

.word.and.up.to.256.data.word

.(eac

.16.bit.word.is.actu

ally

.tran

smitted

.with.

3.addi

tional

.bits.of.sync

hronization

.and.a.pari

.bit)

.Each.LRU.comm

unicating

.on.the.ARIN

.629.data.bus.

cont

ains

.a.term

inal

.cont

roller,

.whic

.impl

ements

.the.bus.prot

ocol

.acce

.logi

.that.dete

rmines

.when.the.ter-

inal.will.transmit.. e.terminal.listens.to.the.bus.and.waits.for.series.of.quiet.periods,.one.of.unique.dura-

tion

.befo

.tran

smitting

.(CSM

.like.mech

anism

.+.TDMA.like.allo

cation

.by.the.den

ition

.of.preas

signed

wait

ing

.time

s).

.Only.one.term

inal

.is.allo

wed

.to.tran

smit

.at.a.time

.and.once.a.term

inal

.has.tran

smitted,

.it.

must.wait.befo

.it.tran

smits

.agai

.to.give.time.to.all.othe

.term

inals

.whic

.want.to.tran

smit

.on.the.bus..

 e.ARI

.629.sta

ndard

.sup

ports

.two.alt

ernative

.data.lin

.lev

.pro

tocols,

.the.basi

.pro

tocol

.(BP

)

.and.the.

comb

ined

.prot

ocol

.(CP)

.whic

.canno

.coex

ist

.on.the.same.bus.due.to.funda

mental

.die

rences.

24.5.1 Basic Protocol

 e.BP.provides.an.equal.priority.access.to.the.bus.for.each.terminal,.giving.the.opportunity.to.transmit.

peri

odic

.data

.Howe

ver,

.as.show

.in.Figu

.24.4

.if.the.cycl

.is.too.shor

.to.tran

smit

.the.data.from.all.

term

inals,

.the.bus.auto

matically

.lose

.this.peri

odic

.beha

vior

.in.favo

.of.an.aper

iodic

.mode.wher

all.mess

ages

.are.tra

nsmitted,

.wit

hout

.dat

.los

For

.a.give

.term

inal

.i.(fro

.a.set.of.n.conn

ected

.to.the.bus)

.TGi.repr

esents

.the.uniq

.time.(ter

mi-

nal

.gap)

.duri

.whic

.a.term

inal

.must.wait.aer.bus.acti

vity

.befo

.star

ting

.its.own.tran

smission.

.For.

all.n.term

inals,

.TI.repr

esents

.a.comm

.time.inte

rval

.(tra

nsmit

.inte

rval),

.sepa

rating

.two.succ

essive

24-6 Industrial Communication Systems

transmissions.of.each.terminal..Finally,.the.synchronization.gap.(SG).is.a.common.quiet.time,.longer.

than.any.individual.TG,.which.is.used.to.synchronize.all.the.terminals.at.the.beginning.of.a.new.cycle.

 ese

.three.timers.are.used.at.each.terminal.in.the.following.manner:

•

. e.TI.timer.starts.immediately.every.time.the.terminal.begins.transmitting.

•

. e.SG.timer.starts.immediately.every.time.the.bus.is.sensed.quiet.. is.timer.may.be.reset.before.

it.has.elapsed.if.any.activity.is.detected.on.the.bus.. is.timer.does.not.aect.periodic.mode.but.in.

aperiodic.mode,.aer.it.has.elapsed.we.know.that.all.the.terminals.have.had.transmit.access.to.the.

bus.(as.SG.timer.is.greater.than.each.TGi.timer).and.so.a.new.cycle.can.begin.

•

. A.TGi.timer.will.start.aer.SG.has.elapsed..It.also.starts.immediately.every.time.the.bus.is.sensed.

quiet.. is.timer.may.also.be.reset.before.it.has.elapsed.if.any.bus.activity.is.detected.

.all.three.timers.have.elapsed,.the.local.terminal.begins.transmitting..As.all.TGi.are.dierent,.only.

one.terminal.will.be.allowed.to.transmit.aer.any.quiet.time..In.aperiodic.mode,.the.terminals.are.

scheduled.in.ascending.order.of.their.TGs.

.assignment.of.timer.values.is.done.on.a.per-system.basis.and.must.address.a.trade-o.between.

bus.cycle.time.(all.terminals.transmit.exactly.once.within.a.cycle).and.terminal.transmission.frequency.

(all.terminals.must.be.allowed.to.transmit.at.a.frequency.suitable.for.data.update.rate.requirements).

24.5.2 Combined Protocol

 e.CP.has.been.proposed.for.combining.periodic.and.aperiodic.data.transmission..As.shown.in.Figure.24.5,.

three.levels.of.transmission.are.handled:.level.one.for.periodic.transmissions,.level.two.for.shorter.(and.higher.

priority).aperiodic.transmissions,.and.level.three.for.longer.(and.lower.priority).aperiodic.transmissions.

.with.BP,.each.terminal.has.a.unique.preassigned.TG.. e.transmit.interval.(TI).is.only.applicable.dur-

ing

.the.rst.periodic.transmission.in.each.cycle;.for.all.other.terminals,.a.concatenation.event.(CE).forces.

all.unelapsed.TI.timers.to.be.canceled.. is.has.the.eect.of.compressing.periodic.transmissions.at.the.start.

of.each.cycle.(separated.only.by.TG.delays)..Two.types.of.SGs.are.dened:.the.periodic.synchronization.gap.

(PSG),.which.is.used.for.cycle.level.synchronization;.and.the.aperiodic.synchronization.gap.(ASG),.which.is.

needed.to.handle.transition.in.a.cycle.between.levels.1.and.(and.between.levels.2.and.3)..In.order.to.guarantee.

Basic protocol : periodic mode (when TI is sufficiently large)

Basic protocol : aperiodic mode (when TI is too small)

M2 M3

TI (terminal 1)

TI (terminal 2)

Free

space

TI (terminal 3)

TI (terminal 1)

TI (terminal 2)

TI (terminal 3)

FIGURE 24.4 ARINC.629.basic.protocol.

Embedded Networks in Civilian Aircraft Avionics Systems 24-7

a.xed.cycle.duration.(needed.for.level.1.periodic.transmissions),.an.aperiodic.time-out.(AT).is.used.to.avoid.

transmission.of.aperiodic.data.that.could.make.the.cycle.overow..Level.3.aperiodic.messages.are.allowed.to.

span.multiple.cycles.(backlog.messages.have.a.higher.priority.than.current.cycle.level.three.messages)..Level.

2.aperiodic.messages.must.be.transmitted.in.the.current.cycle.otherwise.they.are.lost;.the.transmit.interval.

must.be.calculated.so.as.to.give.enough.space.for.both.levels.1.and.2.messages.

.assignment.of.timer.values.is.similar.to.BP.mode:.ASG.is.equivalent.to.SG.(greater.than.each.TGi).

and.PSG.must.be.greater.than.ASG.(the.standard.denes.PSG.as.ve.times.the.selected.ASG).

.conclusion,.implementing.an.ARINC.629.data.bus.requires.the.development.of.a.global.avionics.

system.communication.scheme.in.order.to.guarantee.deterministic.transmission.ordering.for.critical.

data..BP.delivers.true.real-time.behavior.for.systems.requiring.periodic.and.xed.length.messages.(peri-

odic

.mode)..Variable.length.periodic.messages.or.aperiodic.messages.result.in.variable.length.cycles.

(aperiodic.mode)..CP.protocol.oers.stable.periodic.response.time.for.systems,.which.require.some.

combination.of.periodic.and.aperiodic.transmissions.(with.no.interference.of.aperiodic.messages.on.

periodic.messages)..However,.taking.into.account.sporadic.or.aperiodic.messages.can.lead.to.an.under-

utilization

.of.the.data.bus..In.fact,.the.main.drawback.of.the.ARINC.629.has.been.the.cost.of.interfacing.

an.ecient.but.complex.avionics-specic.communication.protocol.

24.6 arINC 664: avionics Full-Duplex Ethernet

Avionics.full-duplex.Ethernet.(AFDX).is.part.of.the.ARINC.664.standard.and.is.based.on.classic.switched.

Ethernet.technology.but.introduces.a.virtual.link.(VL).paradigm,.which.is.an.important.concept.for.

characterizing.the.incoming.trac.of.the.network.

24.6.1 Full-Duplex Switched Ethernet

Reusing.Ethernet.technology.for.avionics.presents.many.advantages:.high.throughput.oered.to.the.

connected.units.(100.Mbps.compared.to.a.few.Mbps.with.the.ARINC.629.standard),.high.connectivity.

given.by.the.network.structure,.a.mature.industrial.standard,.and.a.signicantly.lower.connection.cost.

than.with.a.proprietary.or.aeronautical.specic.protocol..Despite.these.advantages,.Ethernet.was.not.

used.for.critical.systems.in.previous.aircra.because.of.its.random.physical.medium.access.protocol:.

CSMA/CD.[IEEE98]..An.embedded.network.must.have.determinism.properties.such.as.the.bounded.

transmission.delay.of.any.data.. e.CSMA/CD.protocol.fails.to.oer.such.a.guarantee.because.of.poten-

tial

.collisions.on.the.physical.medium.during.the.transmission.and.because.of.random.(binary.

exponential.back-o.[BEB]).retransmission.algorithms.

.solve.this.problem,.the.rst.assumption.was.to.adopt.a.switched.Ethernet.technology;.all.units.are.

directly.connected.by.a.point-to-point.link.to.an.Ethernet.switch.since.cascading.switches.oer.the.desired.

M2 M3

ASG

Level 1 Level 2 Level 3 Level 3

ASG ASG PSG

FIGURE 24.5 ARINC.629.combined.protocol.

24-8 Industrial Communication Systems

connectivity.. is.reduces.the.possible.collision.domain.from.the.entire.network.to.the.single.link.between.

two.elements.. e.second.assumption.consisted.in.using.full-duplex.links:.each.avionics.subsystem.is.

connected.to.a.switch.via.a.full-duplex.link.comprised.of.two.twisted.pairs.(one.pair.for.transmission.and.

one.pair.for.reception)..In.fact,.full-duplex.switched.Ethernet.eliminates.the.possibility.of.transmission.

collisions.on.links:.the.CSMA-CD.medium.access.control.protocol.is.no.longer.necessary.. is.eliminates.

the.inherent.indeterminism.of.vintage.Ethernet.and.the.collision.frame.loss,.and.shis.the.problem.to.the.

switch.level.where.various.ows.enter.into.competition.for.the.use.of.switch.resources.. e.switch.imple-

ments

.the.classic.IEEE.802.1d.bridging.algorithm.[IEEE98]:.Reception.and.transmission.buers.are.used.

in.the.switch.for.storing.multiple.incoming.and.outgoing.Ethernet.frames.. e.role.of.the.switch.is.to.

lter.and.to.retransmit.frames.from.the.incoming.buers.to.the.outgoing.buers.. e.store.and.forward.

bridging.mechanism.reads.the.destination.addresses.of.each.received.frame.and.retransmits.them.accord-

ing

.to.the.port.ID.stored.in.the.forwarding.table.of.the.switch..If.a.temporary.congestion.appears.on.the.

output.port.of.a.switch,.it.can.signicantly.increase.end-to-end.delays.of.frames.and.even.lead.to.frame.

loss.through.buer.overow.. is.is.why.dedicated.mechanisms.have.been.added.to.the.classic.full-duplex.

Ethernet.in.order.to.guarantee.the.determinism.of.an.AFDX.network.

24.6.2 the arINC 664 Standard

 e.AFDX.has.been.initiated.by.Airbus.for.the.evolution.of.the.A380.aircra.toward.IMA.as.represented.

in.Figure.24.6..AFDX.concepts.have.been.standardized.as.in.ARINC.664.with.the.help.of.many.avionics.

manufacturers:.Airbus,.Boeing,.Rockwell.Collins,.Honeywell,.etc..[ARI02,ARI03].. is.standard.adapts.

existing.Ethernet.standards,.describes.the.global.communication.system.of.the.aircra,.and.focuses.on.

the.interconnection.of.domains.with.dierent.safety.levels..In.particular,.it.explains.how.the.critical.

avionics.domain,.responsible.for.aircra.control,.can.be.connected.to.the.open-world.domain.

.precisely,.the.standard.denes.two.kinds.of.embedded.networks:.compliant.networks.and.pro-

led

.networks..Compliant.networks.conform.exactly.to.existing.standards:.IEEE.802.3,.802.1d.[IEEE98].

and.can.be.used.in.the.lowest.safety-level.domains,.such.as.the.open-world.domain..Proled.networks.

deviate. from. existing. standards,. when. specic. deviations. are. needed. to. achieve. required. levels. of.

GWMIOMCPM

Switch

Open world environment

Ethernet

429

Ethernet

AFDX network

Avionics world

LRU environment

429

GWM CPM

IOM

LRU environment

FIGURE 24.6 AFDX.IMA.avionics.architecture.

Embedded Networks in Civilian Aircraft Avionics Systems 24-9

performance.and.safety..Among.these.proled.networks.is.dened.the.subset.of.deterministic.networks.

whose.main.characteristics.can.be.summarized.as.follows:

•

. Static conguration:. e.entire.network.conguration.must.be.static.and.precisely.dened.before.any.

take-o.. is.includes.the.network.topology,.the.number.of.connected.units,.the.switch.parameters,.

and.switching.tables..Algorithms.such.as.ARP,.GMRP,.and.Spanning-Tree.must.not.be.implemented.

•

. Flow segregation and error connement:. e.main.safety.principle.is.that.errors.must.be.locally.

contained,.which.means.that.a.local.error.must.not.be.propagated.in.the.network.or.cause.the.

malfunction.of.another.system.. is.led.to.the.recommendation.to.implement.a.ltering.function.

in.the.switches,.whose.role.is.to.check.all.incoming.frames.and.discard.invalid.ones.including.

incorrect.length,.unknown.sender.or.destination.address,.and.corrupted.frames.(FCS.check)..

Filtering.operations.prior.to.relaying.the.frames.and.dierentiating.the.service.oered.to.each.

ow.are.mandatory.for.robust.segregation.of.ows.

•

. Controlled trac:. e.main.characteristic.of.deterministic.networks.is.that.they.guarantee.a.given.

quality.of.service.that.is.provable.(bounded.delay.or.jitter,.frame.loss.probability.by.congestion,.

and.availability.rate)..Each.trac.source.must.assume.a.given.trac.pattern.for.each.ow.(needed.

regulation.is.managed.by.trac.shaping.mechanisms.at.end.system.level).. e.network.has.to.

provide.mechanisms.in.order.to.check.that.no.source.will.transmit.more.data.than.allowed.(trac.

policing.mechanisms.are.needed.at.the.switch.level.to.discard.frames.if.the.source.has.transmitted.

too.many.frames.for.a.given.ow.over.a.short.period).

.conclusion,.an.ARINC.664.deterministic.network.is.based.upon.the.Ethernet.frame.denition.and.

IEEE.802.1d.switching.protocol.but.includes.specic.mechanisms.in.order.to.guarantee.the.determin-

ism

.of.avionic.communications.. e.main.AFDX.specic.assumptions.deal.with.the.static.denition.of.

avionic.ows.that.have.to.respect.a.bandwidth.envelope.(burst.and.rate).at.each.network.ingress.point.

24.6.3 Virtual Link Paradigm

 e.end-to-end.trac.ow.characterization.is.standardized.by.ARINC.664.in.part.7.where.the.VL.

paradigm.is.presented.[ARI03].. e.idea.behind.this.concept.is.to.segregate.the.ows.for.safety.reasons..

Yet,.each.VL.can.also.be.seen.as.a.virtual.ARINC.429.bus.dedicated.to.one.emitting.end.system.. is.

makes.it.possible.to.reuse.existing.avionics.components.or.concepts,.which.decreases.the.transition.

time.between.ARINC.429.and.ARINC.664.technologies.. e.denition.of.VLs.is.based.on.the.concept.

of.virtual.communication.channels.. us,.it.is.possible.to.statically.dene.all.ows.(VL),.which.enter.a.

network,.as.shown.in.Figure.24.7,.which.corresponds.to.an.AFDX.network.test.conguration.provided.

by.Airbus.for.industrial.research.

113

821

820

143

358

1207

113

S1 S8

S4 S7

132

457

708

160

857

1156

142

FIGURE 24.7 Example.of.AFDX.architecture.conguration.

24-10 Industrial Communication Systems

 e.AFDX.network.architecture.is.composed.of.several.interconnected.switches.. e.inputs.and.out-

puts

.of.the.network.are.the.end.systems.(the.little.circles.in.Figure.24.7)..Each.end.system.is.connected.to.

exactly.one.switch.port.and.each.switch.port.is.connected.to.either.an.end.system.or.another.switch.. e.

links.are.all.full.duplex..In.Figure.24.7,.the.values.on.the.end.systems.indicate.the.number.of.VL.that.are.

dispatched.between.the.end.systems.and.a.given.switch.. us,.the.VL.concept.of.virtual.communication.

channels.has.the.advantage.of.statically.dening.the.ows,.which.enter.the.network,.and.associating.

some.performance.properties.to.each.ow..Each.VL.can.be.statically.mapped.on.the.network.of.inter-

connected

.AFDX.switches..Transmitting.an.Ethernet.frame.from.one.end.system.to.another.is.based.

on.a.VL.identier,.which.is.used.for.the.deterministic.routing.of.each.VL.(the.switch.forwarding.tables.

are.statically.dened.aer.allocation.of.all.VL.on.the.AFDX.network.architecture)..Each.VL.denes.a.

logical.unidirectional.connection.from.one.source.end.system.to.one.or.more.destination.end.systems..

For.example,.Figure.24.8.illustrates.dierent.kinds.of.VL:.vx.is.a.unicast.VL.with.path.{e3−S3−S4−e8},.

while.v6.is.a.multicast.VL.with.paths.{e1−S1−S2−e7}.and.{e1−S1−S4−e8}.

.VL.denition.includes.the.bandwidth.allocation.gap.(BAG).value,.the.minimum.frame.size.(S

min

),.

and.the.maximum.frame.size.(S

max

).. e.BAG.is.the.minimum.delay.between.two.consecutive.frames.

of.the.associated.VL.(which.actually.denes.a.VL.as.a.sporadic.ow)..BAG.and.S

max

.values.guarantee.an.

allocated.bandwidth.for.each.VL..Moreover,.a.jitter.value.is.associated.to.each.VL.to.establish.an.upper.

bound.on.the.maximum.admissible.jitter.aer.multiplexing.dierent.regulated.VL.ows.

24.6.4 Virtual Link Properties

From.the.avionics.systems.designer’s.point.of.view,.classic.ARINC.429.buses.have.many.interesting.

features..For.example,.the.single-emitter.assumption.implies.a.dedicated.link.oered.to.the.emitter,.and.

thus.guaranteed.access.to.the.bus,.guaranteed.bandwidth,.and.high.determinism..Moreover,.the.com-

munication

.paradigm.used.by.many.avionic.applications.is.derived.from.the.ARINC.429’s.properties.

behavior,.which.has.been.generalized.on.IMA.through.APEX.port.paradigm.[ARI97].. is.explains.why.

the.VL.concept.is.of.importance.in.the.denition.of.AFDX;.it.allows.direct.replacement.of.ARINC.429.

buses,.on.a.deterministic.ARINC.664.network.

24.6.4.1

VL Bandwidth Guarantee

Each

.avionic.function.denes.its.VL.bandwidth.requirements.in.the.form.of.two.parameters:.the.BAG.

and.the.maximum.frame.size.(S

max

)..When.the.VL.has.no.jitter,.the.BAG.represents.the.minimum.

interval.between.the.rst.bits.of.two.consecutive.frames.. us,.the.bandwidth.oered.to.a.VL.is.the.one.

obtained.when.emitting.a.maximum-sized.frame.every.BAG,.the.latter.being.specied.by.an.integer.(k),.to.

give.(create).a.2

.interval.(in.milliseconds).. e.minimal.BAG.is.thus.1.ms.for.k.=.0,.and.when.combined.

with.standard.maximum.Ethernet.frames,.the.maximum.bandwidth.oered.attributed.to.a.single.VL.

can.be.up.to.12.Mbps.

.network.integrator.collates.all.the.VL.bandwidth.requirements.and.veries.that.the.sum.of.VL.

bandwidths.on.any.physical.link.of.the.full-duplex.switched.Ethernet.network.does.not.exceed.the.

v6, v8

v7, v9

vx, v1

v2, v3

v1, v3

v6, v7

v6, v8, v 9

vx, v2

v6, v8, v9

vx, v6, v7

v2, v5

v1, v3, v4

e10

FIGURE 24.8 Example.of.AFDX.VLs.conguration.

Embedded Networks in Civilian Aircraft Avionics Systems 24-11

100.Mbps.capacity..Moreover,.the.network.integrator.is.responsible.for.VL.placement:.A.multipath.

VL.(a.tree.originating.from.the.emitter).has.to.be.statically.allocated.on.the.network’s.physical.topology.

and.the.path.can.be.chosen.from.among.several.possibilities.

One

.must.note.that.guaranteeing.bandwidth.on.the.network.requires.that.all.VLs.remain.compliant.

with.their.specied.BAG.and.S

max

.parameters;.otherwise,.the.sum.of.some.VL.bandwidths.could.possi-

bly

.exceed.the.100.Mbps.capacity..For.this.reason,.every.VL.is.shaped.inside.the.emitter.end.system,.and.

all.incoming.VLs.are.also.controlled.by.policers.at.the.switch.level;.frames.exceeding.the.VL’s.contract.

are.deleted.by.the.switches.(leaky.bucket.algorithm).as.represented.in.Figure.24.9.

24.6.4.2

VL Latency Guarantee

.communication.latency.of.an.ARINC.429.data.bus.is.very.easy.to.measure..It.is.simply.the.time.

required.to.emit.data.plus.a.negligible.propagation.time;.it.is.absolutely.deterministic.

.picture.is.completely.dierent.for.a.VL.on.an.AFDX.network..Obviously,.the.latency.of.a.frame.

cannot.be.predicted.as.it.depends.on.whether.or.not.the.frame.will.be.delayed.by.other.frames.sharing.a.

section.of.its.path.. us,.a.strong.form.of.determinism.cannot.be.obtained;.a.weaker.form.of.determin-

ism

.is.proposed,.it.is.based.on.VL.end-to-end.latency.upper.bounding.methods..VLs.competing.for.a.

given.switch.output.port.generate.contentions.in.the.output.port.queue.and.can.be.delayed.depending.

on.the.number.of.conuent.VLs.and.their.characteristics.(maximum.frame.size.for.example).

.course,.end-to-end.latencies.have.to.be.compared.to.the.real-time.constraints.of.avionics.commu-

nications,

.but.in.reality,.these.constraints.are.very.dierent.from.one.avionics.function.to.another.. us,.

in.the.early.prototype.specications,.a.maximum.latency.of.1.ms.per.switch.crossing.was.integrated.into.

the.switch.specication..When.four.or.ve.switches.are.crossed,.the.maximum.obtained.latency.is.com-

parable

.with.the.highest.“freshness”.required.for.some.critical.data..At.100.Mbps,.1.ms.of.continuous.

emission.of.minimum-sized.frames.represents.nearly.150.frames.exiting.from.an.output.port’s.queue..

Of.course,.the.physical.size.of.the.switches’.queues.is.the.actual.constraint:.overowing.this.queue.

capacity.necessarily.implies.lost.frames.

24.6.4.3

VL Jitter Issues

Jitter

.is.the.third.parameter.of.a.VL,.after.the.BAG.and.S

max

..As.the.AFDX.definition.in.the.ARINC.

664.standard.states,.jitter.is.considered.null.at.the.output.of.the.end.system.shapers.applied.to.each.

VL..Since.the.VLs.are.multiplexed.in.the.end.system.just.after.having.been.shaped,.as.represented.

in.Figure.24.10,.jitter.is.non-null.at.the.output.of.the.multiplexer.and.is.limited.by.two.bounds:.the.

first.one.comes.from.the.calculation.of.the.multiplexer’s.latency.and.the.second.is.an.absolute.value.

of.500.μs.

.value.has.been.chosen.as.half.the.minimal.BAG.in.order.to.avoid.burst.occurrences.where.a.

frame.pertaining.to.some.VLs.would.be.overtaken.by.a.following.frame.of.the.same.VL..Of.course,.

avoiding.this.problem.in.end.systems.does.not.prevent.it.from.occurring.later.down.the.network.

1 + Jit/BAG

Credit

BAGBAGBAGBAGBAGBAGBAG

FIGURE 24.9 AFDX.VLs.shaping.and.policing.