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

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Real-Time Systems 17-7

17.3 Design Paradigms for real-time Systems

17.3.1 Centralized vs. Distributed architectures

A.centralized.architecture.for.a.real-time.system.comes.with.the.advantage.that.no.communication.

betw

een

.syst

.comp

onents

.has.to.be.consi

dered,

.whic

.save

.cost.and.desig

.eor

.On.the.othe

hand

.the

.are.sev

eral

.rea

sons

.for.bui

lding

.dis

tributed

.rea

l-time

.sys

tems:

•

. Perf

ormance:

.A.cent

ralized

.syst

.migh

.not.be.able.to.prov

ide

.all.the.nece

ssary

.comp

utations

with

. the. requ

ired

. time. fram

es.

. us,. task

. are. to. be. exec

uted

. conc

urrently

. on. netw

orked

hard

ware.

•

. Comp

lexity:

.Even.if.it.is.possi

ble

.to.impl

ement

.a.real

-time

.syst

.with.a.cent

ralized

.arch

itecture,

a.dist

ributed

.solu

tion

.migh

.come.in.with.a.lowe

.comp

lexity

.rega

rding

.sche

duling

.deci

sions,

sepa

ration

.of.conc

erns,

.etc..In.many.case

.a.comp

lex

.mono

lithic

.syst

.woul

.be.too.comp

lex

.in.

order.to.be.veried.and.accepted.as.a.dependable.system.

•

. Faul

t tolerance:

.Ultr

a-dependable

.syst

ems

.that.are.to.be.depl

oyed

.in.safe

ty-critical

.appl

ications

are.expe

cted

.to.have.a.mean.time.to.fail

ure

.of.bett

.than.10–9.h.[Sur

95].

.Singl

.comp

onents

cann

.prov

ide

.this.leve

.of.depen

dability;

.there

fore,

.a.syst

.must.be.desi

gned

.as.a.dist

ributed

syst

.with.redu

ndant

.comp

onents

.in.order.to.prov

ide

.its.servi

.desp

ite

.of.part

icular

.faul

ts.

•

. Inst

rumentation:

.Using.a.dist

ributed

.sens

or/actuator

.netw

ork

.allo

.to.perf

orm

.the.inst

rumenta-

tion

.of.sens

ors

.and.actu

ators

.dire

ctly

.at.the.devi

ce.

. e.inte

rface

.to.the.part

icular

.devi

ces

.is.then.

impl

emented

.as.a.stan

dardized

.digi

tal

.netw

ork

.inte

rface,

.avoi

ding

.possi

ble

.nois

.pick

.over.long.

anal

.tra

nsmission

.lin

.and.red

ucing

.the.com

plexity

.in.ins

trumentation

.[El

m03].

.distributed.approach.requires.a.well-suited.communication.system.for.connecting.its.dierent.

comp

onents.

.Such.a.comm

unication

.syst

.must.prov

ide

.hard.real

-time

.guar

antees,

.a.high.leve

.of.

depe

ndability,

.and.mea

.for.fau

.iso

lation

.and.dia

gnostics.

17.3.2 Composability and Scalability

In.a.distributed.real-time.system,.the.nodes.interact.via.the.communication.system.to.execute.an.overall.

appl

ication.

.An.arch

itecture

.supp

orts

.comp

osability

.if.it.foll

ows

.from.corr

ect

.veri

cation

.and.test

ing

.of.

the.comp

onents

.of.the.syst

.(i.e

.its.node

.and.the.comm

unication

.syst

em)

.that.the.over

all

.syst

.is.

corr

ect.

. ink.of.a.comp

lex

.real

-time

.syst

.inte

grating

.comp

onents

.from.many.manu

facturers.

.Each.

manu

facturer

.does.test

.on.the.prov

ided

.comp

onents.

.Howe

ver,

.with

out

.supp

orting

.comp

osability,

.the.

over

all

.syst

.cann

.be.prov

.to.work.corr

ectly.

. us,.the.inte

grated

.syst

.has.to.be.eval

uated

agai

.whic

.come

.with.high.eor

.and.cost

.for.each.smal

.chan

.in.its.comp

onents.

.For.real

-time

syst

ems,

.comp

osability

.in.the.valu

.doma

.and.comp

osability

.in.the.time.doma

.need.to.be.ensu

red.

An.arch

itecture

.supp

orting

.comp

osability

.supp

orts

.then.a.two-

level

.desig

.appr

oach:

. e.desig

.of.the.

comp

onents

.(pos

sibly

.done.by.part

icular

.manu

facturers)

.and.the.over

all

.syst

.desig

.do.not.inu

ence

each.oth

.as.soo

.as.the.com

posability

.pri

nciple

.hol

.[Ko

p02].

Scalability

.of.a.real

-time

.syst

.also.heav

ily

.depe

nds

.on.comp

osability,

.since.it.requ

ires

.new.comp

nents

.not.to.inte

ract

.with.the.stab

ility

.of.alre

ady

.esta

blished

.serv

ices.

.If.netw

ork

.reso

urces

.are.mana

ged

dyna

mically,

.it.must.be.asce

rtained

.that.aer.the.inte

gration

.of.the.new.comp

onents

.even.at.the.crit

ical

inst

ant,

.i.e.,.when.all.node

.requ

est

.the.netw

ork

.reso

urces

.at.the.same.inst

ant,

.the.time

liness

.of.all.com-

unication

.req

uests

.can.be.sat

ised.

17.3.3 time-triggered vs. Event-triggered Systems

 ere.are.two.major.design.paradigms.for.constructing.real-time.systems,.the.event-triggered.and.the.

time

-triggered

.appr

oach.

.Even

t-triggered

.syst

ems

.foll

.the.prin

ciple

.of.reac

tion

.on.dema

nd.

.In.such.

17-8 Industrial Communication Systems

systems,.the.environment.enforces.temporal.control.onto.the.system.in.an.unpredictable.manner.(interrupts)..

Conv

ersely,

.time

-triggered

.syst

ems

.deri

.cont

rol

.from.the.glob

.prog

ression

.of.time

;

.thus

.the.conc

ept

of.tim

.tha

.app

ears

.in.the.pro

blem

.sta

tement

.is.als

.use

.as.a.basi

.mec

hanism

.for.the.sol

ution.

.even

t-triggered

.appr

oach

.is.well

-suited

.for.spor

adic

.acti

ons

.and.data

.low-

power

.slee

.mode

and.best

-eort

.so.real

-time

.syst

ems

.with.high.util

ization

.of.reso

urces.

.Even

t-triggered

.syst

ems

.can.

easil

.adapt.to.onli

.chan

ges

.and.are.exi

ble.

.Howe

ver,

.they.do.not.idea

lly

.cope.with.the.dema

nds

.for.

pred

ictability,

.dete

rminism,

.and.guar

anteed

.late

ncies

.as.a.high.anal

ytical

.eor

.woul

.be.requ

ired

.to.

prov

.suc

.pro

perties.

Conversely,

.the.time

-triggered

.appr

oach

.[Elm

07]

.provi

des

.a.low.jitt

.for.mess

age

.tran

smission

.and.

task.exec

ution.

. e.pred

ictable

.comm

unication

.sche

.simp

lies

.diag

nosis

.of.timi

.fail

ures

.and.the.

peri

odically

.tran

smitted

.mess

ages

.enab

.a.shor

.and.bound

.erro

.dete

ction

.late

ncy

.for.timi

.and.

omis

sion

.erro

rs.

. e.princ

iple

.of.resou

rce

.adeq

uacy

.guar

antees

.the.nomi

native

.mess

age

.throu

ghput

inde

pendently

.of.the.netw

ork

.load

.More

over,

.the.time

-triggered

.para

digm

.avoid

.bus.coni

cts

.usin

a.time.divi

sion

.mult

iple

.acce

.(TDM

.sche

me,

.thus.elim

inating

.the.need.for.an.expl

icit

.bus.arbi

tra-

tion.

. e.down

side

.of.time

-triggered

.syst

ems

.is.limi

ted

.exi

bility,

.wors

.avera

.perfo

rmance,

.the.

perm

anent

.full.resou

rce

.util

ization,

.and.a.high

.eor

.(clo

.synch

ronization)

.in.esta

blishing

.the.

syst

em.

.Supp

ort

.to.onlin

.chan

ges

.can.be.provi

ded,

.but.at.the.expe

nse

.of.eci

ency

.(see.Sect

ion

.17.4

.2).

Most.stat

e-of-the-art

.safet

y-critical

.syst

ems

.are.base

.on.a.time

-triggered

.appr

oach.

.Furt

hermore,

hybr

.syst

ems

.like.the.Flex

Ray

.bus.[Fle

05]

.or.TTE.[Kop0

.have.gain.atte

ntion

.in.the.last.year

17.3.4 Comparison of the real-time SupportProvided

by Notable

appro

aches

In.order.to.preserve.the.real-time.capabilities,.real-time.support.in.industrial.networks.has.to.be.pro-

vided

.from.the.medi

.acce

.cont

rol

.(MAC

)

.laye

.up.to.the.appl

ication

.laye

. ere.are.seve

ral

.pos-

ble

.tech

niques

.to.hand

.a.shar

.medi

um.

.Tabl

.17.1.list

.some.nota

ble

.exam

ples.

.A.rst.appr

oach

.is.

thro

ugh

.cont

rolled-access

.proto

cols,

.whic

.can.be.furt

her

.clas

sied

.as.eith

.cent

ralized

.or.dist

ributed

ones

. e.seco

.appr

oach

.is.repr

esented

.by.unco

ntrolled-access

.proto

cols

.(e.g

.thos

.base

.on.the.

carr

ier-sense

.mult

iple

.acce

.proto

col),

.whic

.can.be.exte

nded

.with.addi

tional

.feat

ures

.to.impr

ove

.thei

real

-time

.beh

avior.

.cont

rolled-access

.cent

ralized

.meth

ods,

.ther

.is.a.cent

ral

.node.acti

.as.poll

ing

.mast

.that.gran

the.stat

ions

.the.righ

.to.acce

.to.the.chan

nel.

.In.this.case

.the.time

liness

.of.the.tran

smissions

.on.the.

netw

ork

.is.up.to.such.a.cent

ral

.node

.so.it.turn

.into.a.loca

.sche

duling

.prob

lem

.at.that.node

.Nota

ble

exam

ples

.of.thes

.acce

.meth

ods

.in.indu

strial

.netw

orks

.are.foun

.in.the.Ether

net

.Powe

rlink

.[Eth0

7],

Pro

bus

.DP.[PR

O07],

.and.Blu

etooth

.[IE

E05]

.pro

tocols.

.In.the.lit

erature,

.it.has.bee

.sho

.that.the.use.

of.a.real

-time

.sche

duling

.algo

rithm

.(ins

tead

.of.a.non-

real-time

.one.such.as.roun

d-robin

.or.Firs

.In,.

Firs

.Out.(FIF

.algo

rithms)

.combi

ned

.with.a.prop

.timi

.anal

ysis

.make

.it.possi

ble

.to.comp

ute

.the.

resp

onse

.time.of.real

-time

.mess

ages

.and.avoi

.dead

line

.miss.[Tov

99a,Col07].

.In.cont

rolled-access

.dis-

ributed

.proto

cols,

.tran

smissions

.are.rule

.acco

rding

.to.a.dist

ributed

.mech

anism,

.invo

lving

.eith

.the.

circ

ulation

.of.a.physi

cal

.token

.or.a.virt

ual

.token.passi

ng,

.or.a.TDMA.poli

.that.allo

cates

.tran

smission

inst

ants

.to.each.node

.In.thes

.case

.to.enfo

rce

.a.time

.beha

vior

.on.the.netw

ork

.and.thus.to.achi

eve

tran

smission

. time

liness,

. suit

able

. time

. and. poli

cies

. [Mal

95,Tov99b]

. or. a. glob

. sync

hronization

TABLE 17.1 Classications.of.Notable.MAC.Layer.Protocols

Controlled.Access Uncontrolled.Access

Centralized Distributed CSMA/CD

.(CSMA/col

lision

.detec

tion)

Master/slave Token

.pas

sing

.(virt

ual

.or.physic

al) CSMA/BA

.(CSMA/col

lision

.bit

wise

.arbi

tration)

TDMA CSMA/DCR

.(CSMA/deter

ministic

.colli

sion

.reso

lution)

Timed

.tok

en CSMA/CA

.(CSMA/col

lision

.avo

idance)

Real-Time Systems 17-9

framework.[Kop97].are.required,.respectively..Notable.examples.of.networks.provided.with.these.access.

meth

ods

.are.Pro

bus

.[ES9

6],

.P-NE

.[ES9

6],

.TTP/

.[Kop9

7],

.TTE.[Kop

05].

.Conv

ersely,

.in.unco

ntrolled

proto

cols,

.such.as.Carr

ier

.Sens

.Mult

iple

.Acce

.(CSM

.base

.ones

.each.stat

ion

.deci

des

.auton

mously

.when.to.tran

smit,

.obey

ing

.only.to.the.proto

col-specic

.rule

.When.coll

isions

.may.occu

.like.

in.the.CSMA

/CD

.proto

col,

.the.one.used.by.the.Ether

net,

.achi

eving

.pred

ictability

.beco

mes

.an.issu

and.suit

able

.tech

niques,

.such.as.tra

.smoo

thing,

.have.to.be.adopt

.to.boun

.the.medi

.acce

time.[Kwe

04,LoB05].

.A.die

rent

.situa

tion

.aris

.with.the.CSMA

/BA,

.wher

.the.coll

isions

.are.non-

estructive

.for.the.high

est

.prio

rity

.mess

age,

.whic

.then.goes.thro

ugh

.with

out

.dela

.CSMA

/BA

.is.used.

in.the.CAN.bus.and.seve

ral

.stud

ies

.exis

.whic

.show.that.it.is.possi

ble

.to.calc

ulate

.the.resp

onse

.time.

of.real

-time

.mess

ages

.over.the.CAN.bus.[Tin

94,Bri06].

.In.CSMA

/CA

.coll

isions

.may.occu

.thus.the.

medi

.acce

.time.is.non-

predictable.

. e.high

est

.prob

ability

.of.a.coll

ision

.exis

.just.aer.the.mediu

becom

.idle.foll

owing

.a.busy.mediu

.becau

.mult

iple

.stat

ions

.coul

.have.been.wait

ing

.for.the.medi

to.bec

ome

.ava

ilable

.aga

in.

17.4 Design Challenges in real-time Industrial

Communication Systems

17.4.1 real-time and Security

Real-time.industrial.communication.systems.can.be.subject.to.dierent.security.threats..Such.attacks.

can.be.mess

age

.inte

grity,

.fake.mess

ages,

.intr

usion,

.impe

rsonation,

.deni

.of.serv

ice

.(DoS

.etc..DoS.

atta

cks

.can.be.dic

ult

.to.hand

.due.to.the.cons

trained

.reso

urces

.avai

lable

.in.the.embed

ded

.netw

orks

used. to. supp

ort

.auto

mation

.appl

ications.

.DoS. atta

cks

.can.be. hand

led

.eith

.by.prev

ention

.or. by.

dete

ction.

.A.comm

.requ

irement

.for.prev

enting

.DoS.atta

cks

.is.the.possi

bility

.to.limi

.physi

cal

.acce

to.the.netw

ork.

.Dete

ction

.is.typi

cally

.done.by.obse

rving

.the.netw

ork

.beha

vior

.to.dete

.unco

mmon

situa

tions

.and,.in.case.of.dete

ction,

.by.acti

.with.comba

.meas

ures.

.In.[Gra

08],

.a.solu

tion

.combi

ning

dete

ction

.and.prev

ention

.is.prop

osed

.for.autom

ation

.netw

orks.

.As.indu

strial

.comm

unications

.oen.

deal.with.safe

ty-critical

.real

-time

.syst

ems,

.secu

rity

.serv

ices

.must.also.be.take

.into.consi

deration

.from.

the.poin

.of.view.of.eci

ent

.reso

urce

.util

ization.

.A.bala

nced

.harm

onization

.amon

.depe

ndability,

.secu

rity,

.and.real

-time

.aspe

cts

.must.then.be.perf

ormed.

.In.a.fail

ure

.situa

tion,

.when.reso

urces

.beco

.scar

ce,

one.must.prio

ritize

.cert

ain

.appl

ications/users/services

.over.othe

.to.be.able.to.reta

.the.most.crit

ical

appl

ications

.[Fal

08].

.Die

rent

.depe

ndability,

.secu

rity,

.and/

.real

-time

.requ

irements

.will.lead.to.dif-

erent

.solu

tions

.and,.for.the.most.dema

nding

.serv

ices,

.the.supp

orting

.mech

anisms

.coul

.be.the.most.

expe

nsive

.one

.at.lea

.in.ter

.of.res

ource

.usa

ge.

17.4.2 real-time and Flexibility

 e.specication.of.the.requirements.of.real-time.systems.is.a.task.that.can.be.hard.due.to.incomplete.

know

ledge

.of.the.appl

ication

.and.of.the.dyna

mic

.scen

arios

.wher

.the.syst

ems

.will.oper

ate.

.If.the.requ

ire-

ments

.are.not.full

.know

.befo

.runt

ime

.or.if.the.oper

ating

.cond

itions

.chan

.duri

.oper

ation

.or.if.it.

is.requ

ired

.to.add.or.remo

.syst

.comp

onents

.duri

.oper

ation,

.then.exi

bility

.is.an.impo

rtant

.issu

For.indu

strial

.cont

rol

.syst

ems,

.Zoit

.[Zoi

09]

.disc

usses

.an.appr

oach

.for.reco

ngurable

.real

-time

.appl

ica-

tions

.base

.on.the.real

-time

.exec

ution

.mode

.of.the.new.Inte

rnational

.Elec

trotechnical

.Comm

ission

(IEC

)’s

.fam

ily

.of.sta

ndards

.(IE

.614

99).

.the.leve

.of.the.comm

unication

.proto

col,

.die

rent

.appr

oaches

.for.exi

bility

.can.be.iden

tied.

As.disc

ussed

.prev

iously,

.even

t-triggered

.comm

unication

.syst

ems

.such.as.the.ones.base

.on.CAN.can.

reac

.prom

ptly

.to.comm

unication

.requ

ests

.that.can.be.issu

.at.any.inst

ant

.in.time

.lett

ing

.the.bus.

avai

lable

.in.the.abse

nce

.of.even

.to.comm

unicate.

.On.the.othe

.side,.time

-triggered

.syst

ems

.such.as.

Time

-Triggered

.Proto

col

.(TTP

)

.[TTA

03]

.are.less.exi

ble

.beca

use

.comm

unication

.must.take.plac

.at.

pred

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17-10 Industrial Communication Systems

predened.static.modes.or.allow.reservation.of.empty.slots.to.include.messages.whose.transmission.is.

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ided

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line;

.bu

.th

.wa

stes

.ba

ndwidth,

.as.th

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ots

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.re

served

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.if.th

.ar

.no

.us

ed.

.spi

.of.thi

.div

ision,

.a.num

ber

.of.hyb

rid

.pro

tocols

.has. bee

.def

ined

.in. the. las

.20.yea

rs.

The.mos

.pro

minent

.hyb

rid

.pro

tocols

.are.ARI

NC-629

.[AE

E94]

.fro

.the.avi

onics

.dom

ain,

.Fle

xRay

[Fl

e05]

. fro

. the. aut

omotive

. fie

ld,

. as. wel

. as. TTE. [Ko

p05].

. Bot

. the

. pro

tocols

. ena

ble

. onl

ine

sch

eduling

.usi

.FTD

.tha

.is.a.fle

xible

.TDM

.Ano

ther

.exa

mple

.is.FTT

-CAN

.[Al

m02],

.whi

use

.a.tim

e-triggered

.app

roach

.whe

.per

iodic

.tra

ffic

.is.con

veyed

.wit

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trol

.of.a.cen

tralized

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patcher

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xisting

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.“le

gacy”

.eve

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pecific

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While

.TDM

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.sch

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.ext

ensions

.to.mak

.it.mor

.ex

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.hav

.bee

.pro

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mple,

.P-N

.el

dbus

.[ES

96]

.use

.a.vir

tual

.toke

n-passing

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roach

.tha

.ena

bles

.a.nod

.to.tra

ns-

mit

.ear

lier

.if.the.pre

vious

.slo

.was.not.use

.to.tra

nsmit

.a.str

eam.

. is.tec

hnique

.was.por

ted

.to.Ethe

rnet

and.ca

lled

.VT

PE,

.vi

rtual

.tok

en-passing

.Eth

ernet

.[C

ar03].

17.4.3 Offering real-time Support to Wireless Communication

In.wireless.industrial.networks,.due.to.the.error-prone.nature.of.the.wireless.channel,.the.deterministic.

vie

.on.sch

edulability

.of.a.set.of.rea

l-time

.str

eams

.is.no.lon

ger

.app

licable

.and.it.sho

uld

.be.rep

laced

.by.

a.pro

babilistic

.vie

.bas

.on.the.pro

bability

.of.ful

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.ind

ustrial-related

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.may.tak

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ket

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ses

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ount

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.Und

.thi

.per

spective,

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l-time

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eams

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sidered

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edulable

.(fo

.xe

.ass

umptions

.on.the.wir

eless

.cha

nnel

.err

.pro

cess)

.whe

.all.of.

its.str

eams

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ieve

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g-term

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mple,

.99%.[Wil

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.22,.1206–1216,.1999.

TA03]

.TTA

Group,

.Spe

cication

.of.the.TTP

.Pro

tocol

.V1.

.200

.Ava

ilable

.at.htt

p://www.ttagroup.org

[Wil08]

.A..Wil

lig,

.Recen

.and.emerg

ing

.top

ics

.in.wire

less

.indu

strial

.communic

ations:

.A.sele

ction,

.IEEE

Tra

nsactions on Industrial Informatics,

.4(2),.102–124,.Ma

.2008.

[Zo

i09]

.A..Zoi

tl,

.Rea

l-time

.exec

ution

.for.IEC.61499,.Int

ernational

.Societ

.of.Aut

omation

.(ISA),.Noid

Indi

.2009.

18-1

18.1 Introduction

It.is.known.that.technologies.to.synchronize.clocks.are.not.an.exclusive.research.topic.of.the.computer.

age..As.a.matt

.of.fact

.the.cons

truction

.of.high

.accu

rate

.cloc

.and.the.resu

lting

.requ

irements

.for.

sync

hronization

.date.back.to.the.seve

nteenth

.cent

ury.

.Howe

ver,

.the.actu

.rese

arch

.for.the.usag

.in.

dist

ributed

.syst

ems

.bega

.with.the.intr

oduction

.of.netw

orked

.comp

uting.

. e.issu

.was.rst.disc

ussed

in.depth.with.the.topic.of.even

.orde

ring

.by.Lamp

ort

.[Lam

port1978].

.A.good

.gene

ral

.over

view

.on.

avai

lable

.algo

rithms

.can.be.foun

.in.[Anc

eaume1997].

.Late

.espe

cially

.with.the.upco

ming

.real

-time

netw

orks

.[Kop

etz1987,Palumbo1992,Liskov,Patt1994],

.the.prob

lem

.came.agai

.into.scie

ntic

.disc

us-

sion.

. is.chapt

.give

.an.over

view

.of.the.stat

.of.the.art.of.cloc

.sync

hronization

.proto

cols

.as.well.as.

thei

.app

lication

.in.rea

l-time

.net

works.

18.2 Precision time Protocol

 e.precision.time.protocol.(PTP.or.IEEE.1588).is.a.protocol.designed.for.highly.accurate.clock.syn-

chronization

.focu

sed

.espe

cially

.on.test.and.meas

urement,

.as.well.as.powe

.engi

neering

.(e.g

.subs

ta-

tion

.auto

mation),

.and.fact

ory

.auto

mation.

. e.stand

ard

.is.IEEE.appr

oved

.sinc

.2002.[IEE

E1588v1].

In. 2008

. a. posi

tive

. ballot. on. vers

ion

. 2. of. the. proto

col

. was. subm

itted

. to. the. IEEE. [IEE

E1588v2].

While. the. test. and. meas

urement

. indus

try

. needs. sync

hronized

. clock

. for. data. colle

ction

[Eid

son2005,Owen2005,

.Pleas

ant2005],

.other.clas

ses

.of.appl

ications,

.like.the.fact

ory

.auto

mation

.com-

unity,

.need.sync

hronized

.clock

.in.orde

.to.coor

dinate

.the.acce

.to.the.netwo

.and.thus.guar

antee

Clock Synchronization

in Distributed Systems

18.1. Introduction.....................................................................................18-1

18.2

. Prec

ision

.Tim

.Pro

tocol.................................................................18-1

18.3

. IEEE.158

.Sys

tem

.Mod

el................................................................18-2

18.4

. Serv

ice

.Acc

ess

.Poi

nts......................................................................18-3

18.5

. Ordi

nary

.Clo

cks..............................................................................18-3

18.6

. Boun

dary

.Clo

cks.............................................................................18-4

18.7

. Prec

ision

.Tim

.Pro

tocol,

.IEE

.158

8–2008

.(PT

Pv2)..................18-5

18.8

. Netw

ork

.Tim

.Pro

tocol..................................................................18-6

18.9

. Netw

ork

.Tim

.Pro

tocol

.Stra

ta.......................................................18-7

18.10

.Arch

itecture,

.Pro

tocol,

.and.Alg

orithms......................................18-8

18.11

.NTP.Clo

.Syn

chronization

.Har

dware

.Req

uirements.............18-8

18.12

.Sync

hronization

.Alg

orithms

.of.NTP.

..........................................18-8

References...................................................................................................18-10

Georg Gaderer

Austrian Academy

of Sci

ences

Patrick Loschmidt

Austrian Academy

of Sci

ences

18-2 Industrial Communication Systems

real. time. for. communication. [Sauter2005,. Brennan2005,Hansson2005].. IEEE. 1588. synchronizes.

clocks.in.a.master/slave.fashion,.whereas.the.master.is.the.node.with.the.highest.accuracy.. e.accu-

racy

.class.(also.called.stratum.in.version.1).can.of.course.be.the.same.at.every.node.in.the.network.(e.g.,.

same.oscillator.type.used)..However,.in.the.case.where.the.stratum.is.the.same,.a.unique.node.identier.

(layer.2.MAC.address.in.the.Ethernet.case).is.used.to.favor.one.node.. e.master.election.follows.a.cer-

tain

.message-based.election.algorithm.and.ends.up.with.the.decision.which.node.should.be.the.master.

for.the.next.synchronization.process..Aer.a.proper.master.is.elected,.synchronization.can.take.place.

.synchronization.process.in.IEEE.1588.networks.is.organized.in.two.stages..A.so-called.delay.

request.delay.response.process.measures.the.round-trip.delay,.to.determine.the.packet.delay.between.

master.and.slave.. is.is.done.upon.initiation.by.the.slave.with.a.telegram,.which.is.returned.by.the.

master.with.the.local.timestamp.of.reception..In.addition,.the.master.regularly.sends.out.synchroniza-

tion

.packets..Again,.these.packets.are.sent.in.a.two-stage.process.. e.rst.set.of.packets.is.timestamped.

with.an.estimation.of.the.sending.time.. e.actual.sending.time.(which.obviously.can.only.be.known.

aer.completing.the.transmission).is.sent.with.a.so-called.follow-up.packet..Figure.18.1.shows.this.syn-

chronization

.principle..A.detailed.description.of.this.process.together.with.implementation.issues.can.

be.found.in.[Eidson2006].

18.3 IEEE 1588 System Model

 e.starting.point.of.the.protocol.specied.in.IEEE.1588.is.a.distributed.system.of.nodes.. e.medium.as.

such.is.not.relevant.for.the.protocol..As.shown.in.Figure.18.2.in.general,.the.position.of.the.ingress.mes-

sage

.timestamp.point.and.the.egress.timestamp.point.are.not.required.to.be.identical,.since.two.dierent.

Master

Slave

Master time

Slave time

Delay request

Delay response

Sync packets

= Δ

FIGURE 18.1 Synchronization. packets. and. round-trip. delay. measurement. (1588_Sync_DelayRequResp_

packets.pdf).

Clock Synchronization in Distributed Systems 18-3

data.ows.handle.the.data.transmission.for.the.two.directions.. e.exact.timestamp.point.for.both.the.

DELAY_REQ.and.SYNC.is.unimportant.for.the.further.processing.of.the.protocol.as.long.as.the.position.is.

the.same.for.all.communication.partners.

.soon.as.a.node.detects.an.incoming.DELAY_REQ.or.SYNC.message,.timestamps.for.further.pro-

cessing

.are.generated..If.the.actual.implementation.is.not.able.to.detect.the.exact.time.of.incoming.mes-

sages,

.the.timestamp.has.to.be.corrected.in.an.appropriate.way.. e.time.for.a.message.to.travel.between.

the.ideal.timestamp.point.and.the.actual.timestamp.point.for.inbound.or.outbound.message.point.is.

referred.to.as.inbound.latency.and.outbound latency.for.IEEE.1588.

18.4 Service access Points

Beyond.the.logical.division.of.subnets,.PTP.relies.on.the.possibility.of.distinguishing.messages.with.the.

identication.of.the.incoming.service.access.point.(SAP).mapped.to.the.ISO.model..For.the.example.of.

user.datagram.protocol.(UDP),.which.is.dened.in.the.annex.of.PTP.as.a.possible.transport.protocol,.

this.means.that.synchronization.messages.(i.e.,.SYNC.and.FOLLOW_UP).are.sent.to.another.UDP.port.

than.all.other.messages.

18.5 Ordinary Clocks

Figure.18.3.shows.a.model.for.an.ordinary.PTP.clock..Each.PTP.clock.uses.a.communication.path.for.

message.exchange.with.other.PTP.nodes.. is.path.is.accessed.in.PTP.via.two.dierent.SAPs,.namely.the.

event.and.the.general.port..Both.hand.over.messages.to.the.protocol.engine.of.the.PTP.clock..In.order.

to.model.the.fact.that.a.clock.may.not.be.limited.to.only.one.PTP.port,.the.conguration.information.is.

Clock synchronization protocol

code

Outbound

message

stack

Access to communication media

Inbound

message

stack

Timestamp point for

outbound messages

Timestamp point for

inbound messages

Synchronization message

Ideal message timestamp

point

FIGURE 18.2 IEEE.1588.node.model.(PTPNodeModel.pdf).