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

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18-4 Industrial Communication Systems

split.into.a.port-specic.part.and.general.information.about.the.clock..Finally,.the.protocol.engine.has.

access.to.the.timekeeping.part—the.clock.itself..A.clock.in.IEEE.1588.can.be.adjusted.and.read.out.by.

the.protocol.engine.

18.6 Boundary Clocks

It.is.easy.to.broaden.the.concept.shown.for.the.ordinary.clocks.by.simply.duplicating.the.PTP.ports,.

each.having.access.to.a.common.clock.data.set.and.sharing.the.clock..In.order.to.implement.this.

communication.in.terms.of.intra-node.time.exchange.between.the.PTP.ports,.ordinary.clock.data.

sets.are.needed.. e.IEEE.1588.standard.calls.these.generalized.clocks.boundary clocks..A.model.

for.a.boundary.clock.with.a.general.number.of.n.ports.is.shown.in.Figure.18.4..It.has.to.be.noted.

that.only.one.port.may.act.as.master.node,.which.implies.that.the.local.clock.will.only.be.set.by.one.

instance.

Node clock

Protocol engine

Event port

Port

configuration

data and foreign

master data

PTP communication path

Clock

correction

Message

assembling

Configuration

storage

Protocol state

engine

General

port

Timestamped

message receipt

and

transmission

Message receipt

and

transmission w/o

timestamping

Ordinary clock

data sets

FIGURE 18.3 Model.of.an.ordinary.PTP.clock.(ModelOrdinaryPTPClock.pdf).

Clock Synchronization in Distributed Systems 18-5

18.7 Precision time Protocol, IEEE 1588–2008 (PtPv2)

Version.1.of.IEEE.1588.was.kept.as.simple.as.possible,.but.growing.industrial.interest.in.precision.clock.

synchronization.soon.made.clear.that.an.improved.version.of.the.standard.was.needed..By.the.time.

of.writing,.the.second.ballot.on.IEEE.1588v2.has.started.and.the.nal.publication.of.the.standard.is.

expected.to.be.in.the.second.half.of.2008.. e.new.version.of.the.standard.will.have.several.improve-

ments

.[IEEE.1588v2]:

•

. Mappings..As.the.main.clauses.of.the.standard.specify.a.communication.protocol,.rather.then.

actual.payload.positions.in.packets,.mappings.to.communication.protocols.such.as.IP.have.to.be.

made..As.an.improvement.to.version.1,.additional.mappings.such.as.to.UDP.in.IPv4.and.IPv6,.

DeviceNet,.raw.Ethernet,.PROFINET,.and.ControlNet.will.be.made.

•

. Type length value elds..Version.1.suered.from.the.problem.that.formally,.messages.could.

not.be.extended.with.arbitrary.information..Such.extensions.have.been.added.via.partly.user-

denable

.type.length.value.(TLV).elds..Nevertheless,.not.only.proprietary.elds.are.dened.

via.TLVs..Also,.standardized.functionality,.like.security.updates,.is.transported.via.these.elds.

•

. Transparent clocks..Under.the.name.“on-the-y.timestamping”.[Gaderer2004],.version.2.supports.

transparent.clocks,.which.are.basic.network.elements.that.timestamp.messages.when.entering.

Node clock

Protocol engine

Event port

Port

configuration

data and

foreign

master data

PTP communication path

(Technology 1)

Clock

correction

Message

assembling

Configuration

storage

Protocol state

engine

General

port

Timestamped

message receipt and

transmission

Message receipt and

transmission w/o

timestamping

Ordinary

clock

data sets

Port 1

…

Port 3Port 2

Port n

(Technology 2)

(Technology n)

(Technology 3)

FIGURE 18.4 Model.of.a.PTP.boundary.clock.(PTPBoundaryClock.pdf).

18-6 Industrial Communication Systems

and.exiting,.thus,.enabling.a.system.to.calculate.the.processing.time.of.a.packet.in.devices.. is.is.

requ

ired

.for.swit

ches,

.whic

.do.not.have.a.pred

ictable

.forw

arding

.for.pack

ets

.or.in.ring.and.line.

stru

ctures

.as.impl

emented

.in.PROF

INET.

. is.addi

tion

.was.intr

oduced

.when.it.turn

.out.that.

the.conc

ept

.of.boun

dary

.cloc

.of.versi

.1.sue

.from.dic

ulties

.caus

.by.casc

aded

.cont

rol

loop

.whi

.may.exh

ibit

.sta

bility

.pro

blems

.ae

.sev

eral

.hop

•

. In.prin

ciple,

.the.sync

hronization

.accur

acy

.with.IEEE.1588

.can.get.bett

.than.1.ns..Comp

ared

.to.

vers

ion

.1,.this.was.made.poss

ible

.by.the.incr

eased

.leng

.of.sever

.eld

.in.the.prot

ocol

.allo

wing

.for.

a.high

.resol

ution

.of.para

meters

.and.time

stamps.

. is.exte

nsion

.reect

.the.incr

eased

.dema

.for.

accur

acy

.in.cloc

.sync

hronization.

.Rela

ted

.to.it.and,.amon

.othe

rs,

.moti

vated

.by.telec

.appl

ica-

tions

.is.the.incr

eased

.sync

hronization

.rate.of.the.new.vers

ion.

.Fina

lly,

.eld

.to.comp

ensate

.asym

metry

.have.been.incl

uded,

.whic

.can.be.used.to.comp

ensate

.inhe

rently,

.die

rent

.dela

.for.send.

and.recei

.chan

nels.

.Howe

ver,

.also.meas

ures

.outs

ide

.of.the.scope.of.the.prot

ocol

.can.be.made.(e.g.

usag

.of.cali

brated

.cabl

.or.know

ledge

.of.the.chan

nel

.as.it.is.the.case.in.powe

rline).

•

. Opti

onal support of unicast messages.

.Gene

rally

.spea

king,

.IEEE.1588.assu

mes

.that.one.mast

.is.

elec

ted

.and.via.that.mast

.seve

ral

.slav

.are.sync

hronized.

.Of.cour

se,

.one.woul

.take

.if.avai

able,

.mult

icast

.mess

ages

.for.sync

hronization,

.as.the.time.to.be.dist

ributed

.is.the.same.for.all.

node

.atta

ched

.to.this.mast

er.

.Howe

ver,

.in.some.case

.for.exam

ple,

.if.the.comm

unication

.tech

nology

.does.not.supp

ort

.mult

icast

.or.die

rent

.time

scales

.need.to.be.used

.unic

ast

.is.need

ed.

. is.

will.be.pos

sible

.in.ver

sion

.2.

•

. Proles..Within.the.scope.of.standardization,.proles.are.in.general.needed.to.apply.a.special,.

reduc

.set.of.value

.and.rang

.to.a.more.gene

ral

.stan

dard

.in.orde

.to.appl

.it.for.a.speci

.use.case..

In.the.case.of.IEEE.1588

.pro

les

.are.needed.in.orde

.to.reect.the.grow

ing

.numb

.of.appl

ication

area

.For.exam

ple,

.in.the.new.vers

ion,

.a.speci

.pro

.for.telec

.appl

ications,

.PROF

INET,

.etc.

can.be.den

ed.

.For.all.value

.not.den

.in.a.speci

c

.pro

le,

.the.defa

ult

.value

.of.the.stan

dard

.appl

•

. Alte

rnative best master clock algorithms.

.Espe

cially,

.the.tele

com

.indu

stry

.dema

nds

.long.hold

over

time

.aer.a.mast

.fail

ure.

. is.is.ree

cted

.by.the.possi

bility

.for.alte

rnate

.best.mast

.cloc

(BMC

)

.alg

orithms.

•

. Expe

rimental specications.

.As.it.can.alre

ady

.be.pred

icted

.that.new.feat

ures

.such.as.secu

rity

(whi

.is.curr

ently

.not.supp

orted

.at.all).or.cumu

lative

.freq

uency

.ose

.to.the.mast

.are.need

ed,

some.expe

rimental,

.non-

normative

.clau

ses

.have.been.adde

.For.some.of.thos

.clau

ses,

.furt

her

rese

arch

.and.case.stud

ies

.are.need

ed;

.howe

ver,

.the.expe

rimental

.anne

.ease

.the.star

.for.such.

inve

stigations

.[Tr

eytl2007,Gaderer2006a].

18.8 Network time Protocol

With.the.introduction.of.distributed.applications.in.the.Internet,.a.new.requirement.arose:.the.synchro-

nization

.betw

een

.the.part

icipating

.comp

uters.

.For.exam

ple,

.simpl

.appl

ications

.like.the.sync

hroniza-

tion

.of.les.on.two.serv

ers,

.like.the.well

-known

.rsyn

.appl

ication,

.use.time

stamps

.of.les.to.dist

inguish

betw

een

.olde

.and.newe

. is.prob

lem

.was.solv

.with.the.prob

ably

.olde

.appl

ication

.laye

.proto

col

of.the.Inte

rnet

.stil

.in.use:. e.netw

ork

.time.proto

col

.(NTP

. e.open.proto

col

.stan

dard

.NTP.is.a.

spec

ication

.for.a.TCP/

IP-based

.cloc

.sync

hronization

.sche

.for.netw

ork

.devi

ces.

. e.main.work.on.

NTP.was.cont

ributed

.by.Davi

.L..Mill

. e.stan

dardization

.was.done.via.requ

est

.for.comm

ent

.(RFC

)

docu

ments—the

.key.cont

ribution

.can.be.foun

.in.[Mil

ls1991a].

.NTP.is.a.very.care

fully

.desig

ned

.proto

col

.almo

.to.ever

.exte

.inve

stigated.

.It.cove

rs,

.in.oppo

site

.to.IEEE.1588

.all.aspe

cts

.like.the.cont

rol

.of.

the.cloc

.and.resp

ective

.filt

.algo

rithms

.[Gur

ewitz2003,Mills1998b,Mills1998a]

.with.a.wide.rang

of.app

lication

.stu

dies

.[Ri

chards2006].

.late

.versi

.of.NTP.is.4;.neve

rtheless,

.in.the.mean

while,

.a.simpl

ied

.versi

.call

.SNTP.has.

been.spe

cied

.and.is.in.use

Clock Synchronization in Distributed Systems 18-7

18.9 Network time Protocol Strata

A.basic.principle.in.NTP.is.to.assign.to.every.clock.a.so-called.stratum.. is.hierarchically.organized.

system.of.clock.strata.denes.the.accuracy.of.a.computer’s.clock.or.more.precisely.the.distance.to.a.ref-

erence

.clock..Within.NTP,.device.strata.are.identied.with.an.integer.value.starting.at.stratum-0..Like.

illustrated.in.Figure.18.5,.these.strata.are.dened.as.follows:

•

. Stratum-0.devices.have.highly.accurate.clocks.such.as.cesium.or.rubidium.time.normals..It.

has.to.be.noted.that.devices.with.this.stratum.are.never.connected.directly.to.a.network—

time

.and.frequency.transfer.happens.via.serial.interfaces.or.transmission.of.defined.electrical.

pulses.

•

. Stratum-1.nodes.are.always.directly.connected.to.devices.with.stratum-0.. e.usual.way.is.to.have.

these.nodes.act.as.time.servers.for.stratum-2.clients.

•

. Stratum-2.nodes.communicate.via.NTP.with.stratum-1.servers..It.is.possible.for.these.devices.to.

peer.with.other.stratum-2.nodes.in.order.to.obtain.a.more.robust.time.source.for.all.other.devices.

in.the.so-called.peer group..Again,.in.the.usual.case,.these.devices.will.act.as.servers.for.stratum-3.

clocks.

•

. Stratum-3.is.the.class.for.the.next.hierarchical.layer.in.NTP.. ese.nodes.perform.basically.the.

same.tasks.as.NTP.devices.in.the.layer.below,.with.the.dierence.that.time.is.obtained.from.

stratum-2.devices..In.general,.NTP,.depending.on.the.version,.allows.strata.higher.than.stratum-3.

(up.to.256.depending.on.the.version)..Nevertheless,.future.developments.will.most.likely.restrict.

the.NTP.strata.to.8–16.

.study.by.Minar.in.1999.[Minar1999].revealed.that.at.the.time.of.the.study,.at.least.175,000.hosts.use.

NTP.in.the.Internet..Among.those.are.more.then.300.stratum-1.servers.and.20,000.and.80,000.stratum-2.

and.stratum-3.servers,.respectively.

Stratum-0

Stratum-2

Stratum-1

Stratum-2 peer group

for increased accuracy

Direct link

e.g., RS232

NTP link

Stratum-3

FIGURE 18.5 Example.for.an.NTP.network.with.node.strata.(NTPStratums.pdf).

18-8 Industrial Communication Systems

18.10 architecture, Protocol, and algorithms

 e.idea.behind.the.NTP.is.to.distribute.a.common.notion.of.time.among.all.nodes.of.a.network.in.

terms.of.a.reference.clock.. is.goal.can.be.achieved.in.quite.dierent.strategies:

•

. Minimization.of.clock oset,.which.tries.to.minimize.the.sampled.dierence.between.the.local.

and.the.reference.time.

•

. Alignment.of.the.frequency.. is.is.equal.to.the.rst.strategy,.with.the.exception.that.the.absolute.o-

set

.of.periodically.generated.clock.signals.is.not.considered,.but.the.stability.of.the.oset.optimized.

•

. e.alignment.of.the.absolute.clock.value.in.terms.of.minimizing.clock.oset.and.optimizing.the.

dierence.between.all.nodes.. is.criterion.is.equal.to.internal.clock.synchronization.(alignment.

of.all.nodes.of.a.network.with.no.respect.to.the.absolute.time.scale),.with.the.exception.that.NTP.

inherently.assumes.an.alignment.to.an.absolute.correct.time.source.of.the.overall.master.

18.11 NtP Clock Synchronization Hardware requirements

Like.any.other.clock.synchronization.system,.NTP.has.to.make.certain.assumptions.regarding.the.clocks..

As.NTP.aims.to.synchronize.hosts.within.the.Internet,.one.assumption.is.that.the.architecture.of.every.

node.is.similar.to.a.clock.in.a.PC..One.further.assumption.of.NTP.is.that.a.network.node.does.not.neces-

sarily

.need.to.be.powered.up.all.time..However,.this.requires.keeping.up.with.the.unstoppable.progress.of.

time.even.during.power.down..In.practice,.this.is.usually.done.with.a.so-called.real-time.clock.(RTC).. is.

clock.is.battery-powered.and.thus,.running.also.during.power-down.times..For.cost.and.energy.eciency.

reasons,.these.clocks.are.usually.equipped.with.32,768.kHz.crystals.. is.frequency.is.very.convenient,.

since.32,768.transitions.can.be.represented.as.2

,.which.means.that.a.15.bit.counter.can.be.used.without.

further.fractional.parts.to.represent.1.s..In.common.PC.architectures,.the.content.of.this.clock.is.loaded.

during.boot-up.into.a.tick.register..Usual.clock.registers.for.NTP.are.48.bit.wide,.and.updated.by.adding.

the.32.bit.wide.clock-adjust.register,.with.an.interrupt.typically.scheduled.every.1–20.ms.. e.content.is.

also.called.processor.cycle.counter.(PCC)..As.these.registers.are.updated.by.means.of.soware.routines.

and.without.the.possibility.to.schedule.future.updates.(such.as.provided.in.high.precision.clocks.like.in.

SynUTC),.writing.to.these.registers.has.to.be.done.with.high.priority.[Mills1998a]..Additionally,.the.16.bit.

wide.dri-compensation.register.is.used.to.compensate.the.frequency.oset.of.the.processor.clock.

18.12 Synchronization algorithms of NtP

Figure.18.6.shows.the.ve.components.of.the.NTP:

•

. e.data lter.is.instantiated.on.the.client.side.for.every.server.. e.main.task.of.this.unit.is.to.

calculate.the.delay.through.round-trip.measurements.and.adjust.the.contents.of.each.message.

accordingly.

Network

Data filter

Peer selection

Clock combining Loop filter

Adjustable

clock

Data filter

FIGURE 18.6 System.concept.of.a.typical.NTP.node.(NTPSystemConcept.pdf).

Clock Synchronization in Distributed Systems 18-9

•. e.second.component.in.the.data.ow.is.the.so-called.peer selection.. is.unit.is.included.due.to.

the.fac

.tha

.not.onl

.del

ayed

.mes

sages

.can.arr

ive

.fro

.a.nod

.but.als

.mut

ually

.wro

.clo

cks

can.pre

sent

.the

mselves

.as.mas

ters.

.Rea

sons

.for.wro

.hos

.in.NTP.net

works

.can.be.bro

ken

ser

ver

.clo

cks

.or.att

acks

.wit

.the.goa

.to.cha

nge

.the.tim

.on.a.net

work

.hos

. e.lat

ter

.can.be.

tac

kled

.by.aut

hentication

.of.clo

.ser

vers,

.whi

.is.nev

ertheless

.not.su

cient

.for.the.rs

.iss

of.acc

identally

.wro

.clo

cks.

.In.ord

.to.avo

.pro

blems

.res

ulting

.fro

.suc

.bro

ken

.clo

cks,

.a.

par

ameter-driven

.de

cision

.of.so

rting

.ou

.th

ose

.cl

ocks

.is.do

.in.pe

.se

lection.

•

. Ae

.the.pee

.sel

ection

.is.don

e—from

.a.mes

sage

.poi

.of.vie

w—oen

.con

tradicting

.inf

ormation

reg

arding

.the.os

.and.dri



.of.the.loc

.clo

.exi

st.

. es

.dat

.hav

.to.be.mer

ged

.in.the.clo

combining

.co

mponent.

•

. e.las

.two.com

ponents,

.nam

ely

.the.loo

p lter

.and.vol

tage-controlled oscillator

.(VC

O),

.hav

.the.

tas

.of.con

ditioning

.the.con

trol

.sign

.wit

.res

pect

.to.con

trol

.cir

cuit

.sta

bility

.and.adj

usting

.the.

loc

.cl

ock,

.re

spectively.

.rs

.ele

ment

.of.the.NTP.sign

.ow.is.the.dat

.lt

er.

. is.ele

ment

.is.pro

vided

.to.eli

minate

.sta

tistical

.err

ors

.as.wel

.as.byz

antine

.nod

es.

. e.lt

.col

lects

.dat

.on.a.per

-peer

.basi

.and.cal

culates

.two.

mea

sures:

.the.lt

er dispersion

.ε

,.and.the.select dispersion.ε

.. e.dispersion.is.calculated.with.ϑ

(0.≤.i.≤.n).

as.the.os

.of.the.ith.sam

ple

.and.the.sam

ple

.di

erence

.ε

.=.|ϑ

.−.ϑ

|.. e.dispersion,.ε

.is.then.dened.

rel

ative

.to.th

.jth.en

try

.as.we

ighted

.su

ε ε

j ij

−

∑

Both.ε

.and.ε

.are.dened.by.protocol.parameters,.namely.NTP.FILTER.and.NTP.SELECT,.which.

are.usu

ally

.set.to.val

ues

.les

.tha

.0.5.and.eva

luated

.rel

ative

.to.the.rs

.ent

.ε

..In.that.context,.w.

is.the.wei

ghting

.fac

tor,

.whi

.is.use

.to.bui

.an.exp

onential

.ave

rage,

.wit

.a.val

.of.w.<.0.5

. e.

sec

ond,

.fun

ctional

.par

.the.pee

.sel

ection

.is.mos

.imp

ortant

.for.the.sta

bility

.of.NTP

.As.soo

.as.new.

os

ets

.to.a.ref

erence

.clo

.are.est

imated,

.thi

.alg

orithm

.dec

ides

.whi

.pee

.is.use

.as.syn

chroniza-

tion

.sou

rce.

.Of.cou

rse,

.als

.the.una

vailability

.of.os

.mea

surements

.(e.

g.,

.due.to.tim

eouts)

.are.als

use

.to.(de

-)select

.pee

rs.

. e.act

ual

.pee

.sel

ection

.is.don

.by.ass

embling

.of.the.can

didates

.in.a.sor

ted

lis

. is.lis

.rep

resents

.the.ava

ilable

.pee

.sor

ted

.by.str

atum

.and.syn

chronization

.dis

persion.

.Als

sev

eral

.san

ity

.che

cks

.to.det

ect

.def

ective

.nod

.are.a.kno

ck-out

.cri

terion

.to.rem

ove

.def

ective

.nod

es.

.In.

cas

.tha

.the.res

ult

.of.thi

.san

ity

.che

.is.an.emp

.lis

.the.loc

.nod

.con

tinue

.run

ning

.wit

.the

loc

.una

djusted

.clo

.fre

quencies.

.In.the.oth

.cas

.if.the.lis

.hol

.mor

.tha

.one.nod

.the

.sta

tis-

tical

.equ

ivalent

.os

ets

.hav

.to.be.com

bined.

. e.clo

.ass

embly

.is.don

.by.con

struction

.of.so-

called

na

.clo

.cor

rection,

.η

,.for.all.nodes..Moreover,.the.synchronization.distance,.λ

,.is.also.internally.

req

uired

.by.the.pre

viously

.des

cribed

.alg

orithms.

. e.val

.of.1/λ

.is.then.used.as.weight.of.each.clock.

os

.in.the.lis

.vir

tual

.con

trol

.loo

.is.na

lly

.clo

sed

.by.the.loo

.lt

.and.the.VCO

.res

pectively.

. e.tas

.of.the

two.par

.is.to.ada

.the.cal

culated

.set

tings

.fro

.NTP.to.the.act

ual

.clo

.of.the.dev

ice.

. is.is.don

.by.

adj

usting

.os

.and.fre

quency—both

.alg

orithms

.are.imp

lemented

.in.a.fee

dback

.loo

. e.gen

eral

.str

uc-

ture

.of.the.dis

ciplining

.is.sho

.in.Fig

ure

.18.

.In.gen

eral,

.thi

.dis

ciplining

.str

ucture

.is.a.sub

ordinate

con

trol

.loo

. e.set.val

.ϑ

.is.taken.from.the.above-described.clock.selection.part.of.NTP.. e.value.is.

com

pared

.to.the.con

trol

.pha

.of.the.var

iable

.fre

quency

.osc

illator,

.whi

.is.the.act

ual

.dri

ver

.of.the.nod

clo

ck.

. e.di

erence

.bet

ween

.ϑ

.and.ϑ

.is.aerward.used.in.the.clock.lter,.which.is.actually.a.tapped.

del

.li

ne,

.wh

ereas

.th

.ta

.is.se

lected

.by.an.al

gorithm

.de

scribed

.in.[M

ills2000].

Aerward,

.the.lt

ered

.sign

.is.use

.in.the.loo

.lt

.(Fi

gure

.18.

7).

. is.lt

.adj

usts

.the.clo

.via.the.

cal

culation

.of.thr

.vari

ables.

.Fir

st,

.the.pha

.ose

.is.cal

culated

.and.dir

ectly

.fed.bac

.to.the.clo

.adj

ust-

ment

.uni

.Seco

nd,

.the.fre

quency

.ose

.is.cal

culated

.via.two.way

.a.pha

.pred

iction

.and.a.fre

quency

18-10 Industrial Communication Systems

prediction.(Figure.18.8).. e.phase-locked.loop.(PLL).predicts.a.frequency.oset.as.integral.of.V

τ,.whereas.

the.frequency-locked.loop.(FLL).use.an.exponential.average.of.V

/τ.. ese.two.values.are.combined.via.a.

simple.summarization..It.has.to.be.specically.noted.that.the.averaging.interval.of.these.two.components.is.

chosen.with.respect.to.the.Allan-intercept,.in.order.to.gain.an.optimal.behavior.of.the.structure.

references

[Anceaume1997].Anceaume,.E..and.Puaut,.I.A.,.Taxonomy.of.clock.synchronization.algorithms,.Campus

Universitaire de Beaulieu,

.Rennes.Cedex,.France,.1997,.25.pp.

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. Part. II. (Industrial. Information. Technology),. Section. 7. (Integration.

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[Eidson2005]

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.Catania,.Italy,.2004.

Phase

detector

Clock adjust

Predictor

Clock filter

VFO

NTP

Loop filter

FIGURE 18.7 NTP. clock. discipline. (NTPClockDiscipline.pdf).. (Redrawn. from. Mills,. D.,. IEEE/ACM Trans.

Netw.,

.6(5),.505,.1998.)

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Frequency

PLL predict

Phase

correct

adj

PLL

FLL

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Mills,.D.,.IEEE/ACM Trans. Netw.,.6(5),.505,.1998.)

Clock Synchronization in Distributed Systems 18-11

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ustrial

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.J.,.and.Schmid

.R.,.IEEE.Sta

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for.a.Pre

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.for.Netw

orked

.Mea

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.and.Con

trol

.Syst

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[IEEE1588v2]

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.J..(ed

.),

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Inc.,.Ne

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Com

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1993,.6,.211–219.

[Mi

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.D.L.,.Int

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.time.pro

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Communications,

.1991,.39,.1482–1493.

[Mi

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.(Versio

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.of.Del

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.Newa

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DE,.1998.

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.D.,.Ada

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.hyb

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.IEEE/

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.Mil

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the

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.1992,.43,.676–686.

[Pa

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.and.pit

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.Vie

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.200

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24.

19-1

19.1 Introduction

Nowadays,.almost.any.industrial.network.uses.applications.based.on.IP.networks.[1].. is.means.that.

peop

.cont

rolling

.thes

.netw

orks

.need.to.know.at.leas

.the.basic

.of.qual

ity

.of.serv

ice

.(QoS

)

.for.IP.net-

orks.

.Ever

.devi

.used.in.an.indu

strial

.netw

ork

.is.goin

.to.send.and.rece

ive

.IP.pack

ets

.but,.depe

nding

on.the.natu

.of.the.indu

strial

.appl

ication

.we.use.in.the.devi

ce,

.the.way.to.proc

ess

.the.IP.pack

ets

.can.be.

deci

sive

.to.the.objec

tives

.we.want.to.get.thro

ugh

.the.devi

ce.

. e.IP.netw

ork

.is.the.fund

amental

.infr

structure

.for.ever

.indu

strial

.netw

ork

.so,.alth

ough

.we.cann

.chan

.the.deta

ils

.of.the.IP.impl

ementa-

tion

.in.the.oper

ating

.syst

ems

.of.the.devi

ces,

.we.need.to.know.how.to.get.the.maxi

mum

.perf

ormance

and.reli

ability

.of.our.netw

ork

.for.ever

.indu

strial

.appl

ication

.we.deci

.to.use..It.is.also.impo

rtant

.to.

note.that.the.devi

ces

.we.can.use.to.mana

.and.impl

ement

.thes

.QoS.char

acteristics

.are.not.the.same.

devi

ces

.we.use.to.impl

ement

.our.indu

strial

.appl

ications

.but.the.usua

.comm

unication

.devi

ces

.in.ever

IP.netw

ork—switches

.and.rout

ers.

.It.is.so.possi

ble

.to.say.that.the.QoS.requ

irements

.in.indu

strial

.net-

orks

.depe

.on.the.indu

strial

.appl

ications

.we.want.to.use.and,.as.a.cons

equence,

.of.the.comp

lexity

of.the.netw

ork.

.If.the.netw

ork

.uses.rout

ers,

.we.can.appl

.spec

ial

.feat

ures

.in.the.rout

ers

.to.impr

ove

.our.

QoS;.but.if.the.net

work

.onl

.nee

.swi

tches,

.we.can.onl

.use.a.res

tricted

.set.of.QoS.fea

tures.

Any

.qual

ied

.netw

ork

.prof

essional

.must.know.the.limi

.for.the.netw

orks

.in.his.or.her.orga

niza-

tion.

.It.is.nece

ssary

.to.know.how.to.get.the.maxi

mum

.of.the.feat

ures

.of.the.netw

ork

.devi

ces

.used

.bein

swit

ches,

.rout

ers,

.or.any.netw

ork

.secu

rity

.elem

ent

.that.can.modi

.the.netw

ork

.perf

ormance,

.as.re-

alls

.or.int

rusion

.det

ection

.sys

tems.

Until

rece

ntly,

ther

was

not

much

say

abou

how

impr

ove

the

serv

ices

oer

netw

orks.

was.enou

.that.the.netw

ork

.does.its.job.prop

erly

.24.h/da

.But.today

.and.in.almo

.ever

.orga

nization,

it.is.nece

ssary

.a.netw

ork

.be.able.to.adapt.the.chan

ging

.need

.for.the.die

rent

.kind.of.appl

ications

.using.

it.as.a.mean

.of.tran

smission

.and.comm

unication.

. ere.will.be.appl

ications

.that.need.more.band

width

Quality of Service

19.1. Introduction.....................................................................................19-1

19.2

. Rela

tionship

.wit

.Inf

ormation

.Sec

urity

.Top

ics..........................19-3

19.3

. Qual

ity

.of.Ser

vice

.for.IP.Net

works...............................................19-4

Integrated.Services.(IntServ).Model. •. Dierentiated.Services.

(DiServ).Model. •. Classication.and.Marking. •. Queuing.

andCongestion.Management

19.4. Special.Considerations.for.Managing.the.Quality.

ofS

ervice.......................................................................................19-11

Congestion.Avoidance. •. High.Availability.Solutions.for.the.Routers

References................................................................................................... 19-14

Gabriel Diaz Orueta

Spanish University of

Dist

ance

Edu

cation,

UNE

Elio San

Cristobal Ruiz

Spanish University of

Dist

ance

Edu

cation,

UNE

Nuria Oliva Alonso

Spanish University of

Dist

ance

Edu

cation,

UNE

Manuel Castro Gil

Spanish University of

Dist

ance

Edu

cation,

UNE

19-2 Industrial Communication Systems

at.specic.moments.and.other.applications.that.impose.no.more.than.a.certain.delay.for.its.signal..

Ift

hese

.con

ditions

.are.not.rea

ched

.simp

ly,

.the.cli

ent

.side.of.the

.app

lications

.wil

.not.wor

.pro

perly

and.we.wi

.be.lo

sing

.qu

ality.

Technically

.spe

aking,

.net

work

.QoS.can.be.de

ned

.as.the.app

lication

.of.fun

ctionality

.and.fea

tures

to.act

ively

.man

age

.and.sat

isfy

.net

working

.req

uirements

.of.app

lications

.sen

sitive

.to.del

ay,

.los

.or.del

var

iation

.(j

itter),

.gu

arantying

.al

.th

.av

ailability

.of.ba

ndwidth

.fo

.cr

itical

.ap

plication

.o

ws.

Using

.Qo

.me

thods

.an

.te

chnologies

.[2

,3]

.se

veral

.be

nets

.ar

.re

ached:

•

. Con

trol over resources:

.It.can.be.det

ermined

.whi

.net

work

.res

ources

.(ba

ndwidth,

.equ

ipment,

etc

.are.bei

.use

.For.exa

mple,

.cri

tical

.tra

c

.as.voi

.or.vid

.ove

.IP.may.con

sume

.a.lin

.and.

QoS.helps.control.the.use.of.the.link.by.dropping.low-priority.packets.

•

. Mor

e ecient use of network resources:

. Usin

. net

work

. ana

lysis

.man

agement

. and. acc

ounting

tool

.it.can.be.det

ermined

.how.tra

c

.is.han

dled,

.and.whi

.tra

c

.sho

.lat

ency,

.jit

ter,

.or.pac

ket

los

.Qo

.too

.[1

]

.ca

.be.us

.to.tu

.th

.ha

ndling

.of.th

.tr

ac.

•

. Coe

xistence of critical applications:

.By.usi

.QoS.tec

hnologies,

.it.is.pos

sible

.tha

.mos

.cri

tical

app

lications

.rec

eive

.the.mos

.eci

ent

.use.of.the.net

work.

.It.is.pos

sible,

.for.exa

mple,

.tha

.tim

sen

sitive

.mul

timedia,

.whi

.req

uires

.ban

dwidth

.and.min

imized

.del

ay,

.get.the

.whi

.oth

.app

li-

cations

.in.the.sam

.lin

.rec

eive

.fai

.ser

vice

.wit

hout

.int

erfering

.wit

.cri

tical

.app

lications

.tra

c.

 ese

.nee

.are.dir

ectly

.lin

ked

.wit

.the.ide

.of.QoS

.see

.as.a.sum.of.val

.and.war

ranty,

.bei

.onl

a.part.of.the.framework.of.services.management.for.best.practices.IT.recommendations,.as.ITIL.or.

Cob

IT[4].

. e.val

.of.the.net

work

.is.wha

.we.sea

rch

.usin

.a.par

ticular

.net

work,

.the.par

ticular

.com

munication

.bet

ween

.di

erent

.kin

.of.dev

ices.

.For.the.cas

.of.an.ind

ustrial

.net

work,

.the.val

.is.goi

.to.

be.wha

.we.wan

.to.get.wit

.com

munication

.bet

ween

.ser

vers

.and.cli

ents

.in.our.ind

ustrial

.net

work.

. e.

war

ranty

.is.muc

.mor

.lin

ked

.wit

.the.qua

lity,

.mea

ning

.a.de

ned

.set.of.fea

tures

.for.a.con

crete

.net

work

and.ca

.be.ex

pressed

.as.th

.le

vel

.of.se

rvice

.of.th

.ne

twork

.fo

.a.nu

mber

.of.fe

atures:

•

. Ava

ilability

.or.ho

.mu

.ti

.is.gu

aranteed

.th

.ne

twork

.is.up.co

ntinuously

•

. Cap

acity

.or.how.man

.res

ources

.(de

dicated

.ban

dwidth,

.del

ay,

.etc

.are.ava

ilable

.for.eac

.of.the.

app

lications

.an

.ho

.is.th

.ca

pacity

.to.re

act

.to.a.ch

ange

.in.th

.ne

.of.re

sources

•

. Network.security.or.the.ability.to.protect.any.piece.of.information,.following.a.security.policy.

tha

.inc

lude

.fac

.lik

.the.con

dentiality,

.int

egrity,

.or.aut

hentication

.for.any.kin

.of.acc

ess

.eve

nts

in.th

.ne

twork

•

. Con

tinuity

.as.a.sum.of.the.oth

.fea

tures,

.but.spe

cially

.tho

ught

.for.dis

aster

.inc

idents

.suc

.as.a.

re.or.a.o

Once

.the.net

work

.man

agement

.sta



.has.bui

.a.Net

work

.Pol

icy

.for.all.the

.fea

tures,

.it.is.nec

essary

.to.

kno

.the.tool

.for.imp

lementing

.eac

.of.the.dir

ectives

.in.the.pol

icy

.and

.for

tunately,

.we.hav

.now.rea

tec

hnical

.so

lutions

.in.pl

ace

.fo

.th

ese

.ki

nds

.of.sit

uations,

.an

.we.ha

.en

ough

.re

liable

.st

andards.

.res

.of.thi

.cha

pter

.pre

sents

.an.int

roduction

.on.how.to.man

age

.all.the

.fea

tures

.to.get.the.bes

QoS.for.IP.industrial.networks,.although.practically.every.single.piece.of.information.in.the.chapter.

can.be.app

lied

.to.any.IP.net

work.

. e.rea

.di

erence

.bet

ween

.ind

ustrial

.IP.net

works

.and.non

industrial

IP.net

works

.are.the.app

lications

.we.run.on.the

.and

.for.IP.QoS

.we.onl

.nee

.to.kno

.som

.gen

eral

cha

racteristic

.of.the

.app

lications.

. e.nex

.sec

tion

.sho

.the.imp

ortant

.con

nections

.for.QoS

.pre

vi-

ously

.de

ned,

.wit

.the.basi

.of.inf

ormation

.sec

urity

.topi

.and.the.eo

rts

.dev

eloped

.in.the.las

.yea

to.bui

.som

.sta

ndards

.for.man

aging

.inf

ormation

.sec

urity

.topi

.rel

ated

.wit

.ind

ustrial

.net

works.

.we.go.in.dept

.of.the.cla

ssical

.ter

minology,

.tec

hniques,

.and.arc

hitectures

.use

.toda

.for.get

ting

a.goo

.QoS.in.any.IP.net

work,

.sho

wing

.the.two.mos

.imp

ortant

.mod

els:

.Int

Serv

.and.Di

Serv.

.Als

.we.

ana

lyze

.how.the.com

munication

.dev

ices

.in.the.net

work

.can.cla

ssify

.and.mar

.any.IP.pac

ket

.and.how.to.

man

age

.the.pos

sible

.tra

c

.con

gestion

.we.can.see.in.the.sam

.dev

ices.

.To.end

.we.ana

lyze

.two.pos

sible

situ

ations

.we.mus

.man

age

.onl

.if.our.IP.ind

ustrial

.net

work

.use

.rou

ters:

.the.con

gestion

.avo

idance

.and.

hig

.av

ailability

.so

lutions

.fo

.ro

uters.