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

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ISO/OSI Model 1-5

1.2.1.7 Layer 7: application Layer

.application.layer.provides.an.interface.that.can.be.used.by.the.application..It.contains.services.for.

the.application,.which.can,.for.example,.provide.access.to.distributed.databases.or.other.high-level.

services.. e.application.layer.strongly.depends.on.what.the.applications.(or.the.operating.system).

above.it.need,.and.is.therefore.usually.designed.to.meet.the.requirements.of.these.applications.

.stated.earlier,.the.seven.layers.cannot.provide.interoperability.by.themselves..Proles,.which.create.

alayer.on.top.of.the.ISO/OSI.model,.can.help.to.reach.the.level.of.interworkability.

1.3 Vertical and Horizontal Communication

Each.communication.node.(i.e.,.a.device.that.participates.in.communication).contains.at.least.one.

communication.stack,.which.is.dened.by.the.function.layers.1–7.. erefore,.we.distinguish.between.

horizontal.and.vertical.communication.as.shown.in.Figure.1.3.

.layers.communicate.using.a.logical.connection.(horizontal.communication).or.the.dierent.layer.

protocols.(e.g.,.application.protocol.or.session.protocol),.respectively.. e.term.“protocol”.is.ambigu-

ously

.used.for.both.the.complete.communication.stack.and.the.separate.(seven).protocols.of.the.stack..

 e.protocol.contains.not.only.data.denitions.that.dene.the.kind.of.information.that.is.exchanged,.

but.it.also.denes.the.set.of.rules.that.communicating.entities.have.to.use,.in.terms.of.appropriate.reac-

tions

.to.incoming.information,.handling.of.error.situations,.or.timing.constraints..Information.ows.

vertically.(vertical.communication).through.the.layers,.which.oer.services.to.higher.layers.. e.follow-

ing

.section.will.go.into.more.detail.on.this.

.we.look.at.the.protocols.of.the.separate.layers—on.which.we.want.to.focus.in.the.following—we.see.

a.set.of.rules.that.are.assigned.to.the.same.layer.. ey.are.dened.by.the.type.of.control.information,.the.

procedure,.and.its.behavior;.this.is.done.in.three.denitions:.(a).denition.of.the.states,.(b).denition.of.

the.transitions,.and.(c).the.denition.of.the.timing.. ey.should.be.realized.in.a.way.that.they.can.each.

be.replaced.separately.in.the.stack.

.seven.layers.of.the.ISO/OSI.model.can.be.assigned.to.dierent.block.functions.. e.le.side.of.

Figure.1.4.shows.the.separation.between.point-to-point.connections,.i.e.,.the.connection.between.two.units.

without.another.unit.(e.g.,.a.router).is.connected.in.between,.and.end-to-end.connections.. is.means.that.

on.a.connecting.line.between.two.end.devices,.the.three.lower.layers.have.to.be.processed.by.each.device.on.

the.way.from.end.device.to.end.device,.while.the.upper.four.layers.are.only.processed.by.the.end.devices.

Figure

.1.4.on.the. right. side.separates.between. transport-oriented. protocols. and. the. upper.three.

application-oriented.protocols.. e.transport-oriented.protocols.do.not.process.payload.data,.while.the.

application-oriented.protocols.depend.on.the.according.application.

.indicates.the.importance.of.the.transport.layer:.layer.four.is.the.rst.that.shows.the.end-to-end.

property.of.the.communications.. is.layer.shall.guarantee.that.all.data,.which.are.sent,.do.receive.the.

other.side.completely.and.in.correct.order..On.the.other.side,.it.is.the.lowest.layer.that.can.still.be.dened.

independently.from.the.application

Vertical communication

(services)

Horizontal (logical) communication

(protocols)

FIGURE 1.3 Layer.architecture.

1-6 Industrial Communication Systems

1.4 Dynamic Behavior of Services and Protocols

Figure.1.5.shows.a.common.example.of.communication.that.runs.over.an.intermediate.station,.e.g.,.a.router..

One.side,.e.g.,.the.le.side,.starts.transmitting.a.message,.which.is.done.by.the.application.using.the.applica-

tion

.layer..Communication.runs.vertically.down.to.layer.1,.horizontally.through.the.router.to.the.receiving.

device,.and.vertically.up.to.the.application.in.the.receiving.device.. is.communication.can.also.be.seen.as.

parallel.horizontal.communications.that.run.between.each.layer.as.peer-to-peer.communications.

Due

.to.the.modularization.that.the.OSI.model.requires,.all.layers.operate.simultaneously..Consequently,.

the.layers.can.operate.independently,.the.interfaces.can.be.dened.easily.and.systematically,.and.the.dier-

ent

.tasks.can.be.clearly.assigned.to.the.dierent.layers.

Each

.layer.in.the.OSI.model.contains.a.denition.of.the.protocol.and.the.services.. e.protocol.speci-

cation

.contains.the.exact.number.of.functions.and.messages.(protocol.data.units,.PDUs)—so.to.say.

the.“language”.of.the.layer.. e.layer.itself.is.characterized.by.the.procedures.that.operate.on.the.PDUs.

.service.denitions.can.be.explained.as.follows:.two.opposing.entities.(the.service.users,.SU).

exchange.information.using.protocols.(peer-to-peer.protocols).by.using.the.service.providers.(SP).. us,.

a.service.provider.oers.services.to.a.service.user,.so.that.the.service.user.can.fulll.its.task..Service.

communication.runs.vertically,.where.lower.layers.oer.services.to.higher.layers.(with.the.exception.of.

the.lowest.layer).

Seen

.from.a.logical.perspective,.the.layers.of.entities.communicate.with.each.other.(i.e.,.layer.n.of.

one.entity.communicates.with.layer.n′.of.the.other.entity)..In.the.vertical.direction,.service.primitives.

are.passed.on.using.the.service.access.points.(SAP).as.shown.in.Figure.1.6.. ere.is.no.communica-

tion

.dened.between.layer.n.−.1.and.layer.n.+.1.. e.communication.between.two.adjacent.layers.can.

be.compared.to.master–slave.communication,.because.a.master.does.not.communicate.with.a.slave,.

End-to-end

protocols

Application-

oriented

protocols

Transport-

oriented

protocols

Point-to-point

FIGURE 1.4 Layer.build-up.as.blocks.

Application layer protocol AL

Subnet a Subnet a

Converter

Information ew

Presentation layer

Application layer

Session layer

Transport layer

Network layer

Link layer

Physical layer

Internal subnet-protocol

Network protocol NL

Physical layer protocol LL

Link layer protocol LL

Application layer protocol PL

FIGURE 1.5 Communication.over.subnets.

ISO/OSI Model 1-7

it.rather.requests.a.service.from.the.slave;.the.slave.itself.can.again.be.the.master.of.underlying.slaves.

(Figure.1.7).. is.is.also.called.a.cascaded.system.. e.service.user.can.also.be.seen.as.an.instance,.which.

runs.a.process.with.the.opposing.instance.(i.e.,.the.peer.user).

.communication.stack.according.to.ISO/OSI.is.therefore.a.parallel.working.system,.which.does.not.

have.priority.control.(at.least.in.the.rst.denition.of.the.model).. is.has.been.soened.later.by.adding.

a.management,.which.will.be.described.later.

.SAP.can.be.implemented.in.soware.(e.g.,.as.a.subroutine).or.in.hardware.(e.g.,.as.memory).. e.

decision.is.up.to.the.developer.

. vertical. communication. uses. predened. primitives. to. unify. the. sequence. of. communication.

in.all.systems.(Figure.1.8).. e.sequence.of.primitives.is.as.follows:.Request,.Indication,.Response,.and.

Conrmation..Based.on.these.primitives,.we.can.derive.dierent.communication.relations.. ree.examples.

Logical connection

Peer protocol

Denition of protocols

Denition of services

Remote station

SAP: Service access point

Layer n – 1

Layer n

Layer n + 1

FIGURE 1.6 Services,.protocols,.and.service.access.points.

SAP

Primitive

Peer- to-peer

protocol

FIGURE 1.7 e.SAP.is.a.logical.interface.between.the.service.user.(SU).and.the.service.provider.

SAP

(n – 1)-Entity

SAP

Conrm

Request

Indication

-Entity SUn-Entity SU

Response

FIGURE 1.8 Predened.service.primitives.

1-8 Industrial Communication Systems

are.unconrmed.service.(using.only.Request.and.Indication),.conrmed.service.(using.Request,.Indication,.

Response,.and.Conrmation),.and.conrmed.service.with.local.signicance.(using.all.four.primitives,.but.

without.being.able.to.link.Response.and.Conrmation,.because.the.communication.channel.is.only.unidi-

rectional).

.In.the.latter.case,.a.Conrmation.is.initiated.by.a.Request,.which.has.the.big.disadvantage.that.

the.user.is.led.to.believe.that.a.conrmation.has.been.sent—which.is.not.the.case.*

.process.that.happens.between.the.layers.is.described.in.Figure.1.9.[Leo.00]..Layer.n.of.a.commu-

nication

.stack.shall.send.a.piece.of.information.to.an.opposing.layer.. is.information.is.called.service.

data.unit.(SDU).. e.interface.process.in.layer.n.attaches.an.interface.control.unit.information.(ICI).

to.the.head.of.the.SDU,.which.together.builds.the.interface.data.unit.(IDU).that.is.transferred.over.the.

SAP.. e.layer.below.the.ICI.is.decoded.and.a.new.process.is.triggered.. e.following.sequence.(i.e.,.

the.execution.of.a.protocol).is.to.be.seen.as.a.logical.process,.with.the.exception.of.the.lowest.layer,.the.

physical.layer..For.all.other.layers,.we.can.say.that.layer.n.−.1.packs.the.SDU.into.a.PDU.by.following.a.

procedure.dened.by.the.ICI.by.prexing.a.protocol.control.information.(PCI).to.the.SDU.. e.PDU.is.

now.transmitted,.which.means.that.the.layer.n.−.1.uses.its.layers.underneath.and.acts.as.a.service.user..

Finally,.layer.1.physically.transmits.information.as.shown.in.Figure.1.10.. e.SDUs.can.be.physically.

measured.as.well.as.all.header,.i.e.,.the.PCI.of.all.layers.

For.some.implementations.this.is.sucient,.assuming.that.errors.occur.rarely.and.can.be.caught.on.higher.layers.

SAP

ICI

SDU

Interface

(Schnittstelle)

SDU

PCI

PDU

PCI

ICI SDU

IDU

n – 1

FIGURE 1.9 Communication. between. two.consecutive. layers. (ICI,. interface. control. unit. information;. IDU,.

interface.data.unit;.PCI,.protocol.control.information;.PDU,.protocol.data.unit;.and.SAP,.service.access.point).

PCI (n – 4) PCI (n – 3) PCI (n – 2) PCI (n – 1) PCI (n) SDU (n)

PDU (n)

PDU (n – 1)

PDU (n – 2)

PDU (n – 3)

PDU (n – 4)

FIGURE 1.10 Data.frame.that.can.be.physically.measured.on.the.connection.between.two.end.devices.

ISO/OSI Model 1-9

1.5 Extensions, Benets, and Discussion

 e.ISO/OSI.model.is.the.rst.model.of.its.kind..It.provides.a.template.for.the.design.of.protocols.and.

has.proven.useful.over.decades..Since.its.denition,.many.dierent.protocols.have.been.dened.using.

the.ISO/OSI.model..Telecommunication.relies.on.it,.just.like.eldbus.systems.in.automation.use.it..It.

has.been.rened.over.the.years,.for.example,.by.introducing.a.management.for.automatic.handling.of.

dierent.protocol.layers.as.shown.in.Figure.1.11.

.developer.has.to.decide.how.to.add.management.systems..For.smaller.stacks,.one.management.

for.the.whole.stack.will.be.sucient,.for.bigger.stacks.each.layer.will.get.its.own.management,.and.addi-

tionally,

.the.whole.stack.will.have.an.integral.management..In.such.a.system,.it.is.necessary.to.introduce.

priorities,.since.error.handling.has.to.be.considered.

.main.advantages.as.we.see.them.today.are.modularity,.a.clear.top-down.design,.independence.

of.the.layers,.the.possibility.to.handle.errors.in.each.layer,.the.functional.description,.and.the.result-

ing

.possibility.to.implement.in.either.hardware.or.soware..Disadvantages.of.a.design.according.to.

the.ISO/OSI.model.are.the.overhead,.which.can.become.considerable,.and.the.fulllment.of.real-time.

requirements.

references

[Hay. 88]. Hayes,. J.. P.,. Computer Architecture and Organization, McGraw-Hill Series in Computer

Organization and Architecture,

.2nd.edn,.McGraw-Hill,.New.York,.1988,.p..500.

[Leo

.00].Leon-Garcia,.A..and.Widjaja,.I.,.Communication Networks,.McGraw-Hill,.New.York,.2000,.p..51.

[Loy

.05].Loy,.D.,.Fundamentals.of.LonWorks/EIA-709,.ANSI/EIA-709.protocol.standard.(LonTalk),.in:.

e Industrial Communication Technology Handbook,.CRC.Press,.Boca.Raton,.FL,.Zurawski,.R..

(Ed.),.2005,.p..35-1.

[Ols

.92].Olsen,.A.,.Færgrgemand,.O.,.Møller-Pedersen,.B.,.Reed,.R.,.and.Smith,.J..R..W.,.Systems Engineering

Using SDL 92,

.Elsevier.Science.B..V.,.Amsterdam,.the.Netherlands,.1994,.ISBN.0-444-89872-7.

[Zur

.05].Zurawski,.R..(Ed.),.e Industrial Information Technology Handbook,.CRC.Press,.Boca.Raton,.

FL,.2005,.p..37-1.

Layer 7 Layer 7

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Layer 6

Layer 5

Layer 4

Layer 3

Layer 2

Layer 1

Management

FIGURE 1.11 Adding.management.units..Mn:.management.n.{n:.1.to.n}.

2-1

2.1 Introduction

 is.chapter.discusses.physical.mechanisms.for.transferring.digital.data.. e.three.dominant.media.

type

.and.thei

.appl

ications

.are.pres

ented:

.data.tran

smission

.over.cabl

.over.optic

.link

.and.over.

wire

less

.radi

.freq

uency

.(RF).link

.Prop

erties

.of.each.medi

.are.conc

isely

.intr

oduced

.with.resp

ect

to.physi

cal

.char

acteristics,

.alte

rnatives

.of.real

ization,

.topol

ogies,

.bit.codi

ng,

.and.stan

dards.

.Medi

.for.

spec

ial

.loc

.dat

.exc

hange,

.e.g

.inf

rared

.tra

nsmission,

.are.not.con

tained

.in.thi

.cha

pter.

2.2 Wired Links

2.2.1 Physical Properties

Data.transmission.of.digital.signals.over.cables.is.based.on.electromagnetic.wave.propagation,.which.is.used.

for.movi

.data.from.one.locat

ion

.to.anot

her.

.Data.bits.are.code

.as.shor

.puls

.or.fast.chan

ging

.modu

la-

tions

.of.wave.prop

erties

.(amp

litude,

.freq

uency,

.phase

. e.funda

mental

.rela

tion

.of.wave.prop

agation

.is

v f= ⋅ λ

(2.1)

where

.is.the.pha

.vel

ocity

.of.a.sinu

soidal

.ele

ctromagnetic

.wav

.is.the.fre

quency

.is.the.wav

elength

Each

.cabl

.has.a.char

acteristic impedance Z

.and.has.to.be.matched.at.both.ends.with.equal.terminating.

resis

tors

..Mismatch.at.an.end.leads.to.a.reection.where.the.wave.front,.e.g.,.a.pulse,.is.traveling.back.

Media

2.1. Introduction.......................................................................................2-1

2.2

. Wired.Lin

ks........................................................................................ 2-1

Physical.Properties. •. Cable.Types.andOperational.

Characteristics. •. Single-Ended.and.Dierential.

Transmission. •. Simplex.and.DuplexCommunication. •.

BitEncoding. •. Standards. •. Data.Transmission.Utilizing.Existing.

Cable.Infrastructure

2.3. Optical.Links......................................................................................2-7

Physical.Properties. •. Types.and.Media.Access. •. Transmitters.and.

Receivers. •. Multiplexing. •. Implementations.and.Standards

2.4. Wireless.Links.................................................................................. 2-11

Physical.Properties. •. Types.and.MediaAccess. •. Modulation. •.

BitCoding,.Multiplexing. •. Realizations.and.Standards

References..................................................................................................... 2-17

Herbert Schweinzer

Vienna University

of Tec

hnology

Saleem Farooq

Shaukat

COMSATS Institute of

Info

rmation

Tec

hnology

Holger Arthaber

Vienna University

of Tec

hnology

2-2 Industrial Communication Systems

to.the.sender.with.an.amplitude.and.sign.depending.on.the.mismatch’s.impedance..Cable.mismatch.

mostly.reduces.the.achievable.bit.rate.and,.therefore,.should.be.avoided.

.form.of.a.rectangular.pulse.remains.equal.as.long.as.the.pulse.frequency.components.are.trans-

mitted

.with.the.same.attenuation.and.velocity..Frequency.dependence.of.phase.velocity.is.called.disper-

sion.

.Mostly,.cable.attenuation.(in.dB/m).increases.with.the.frequency.. is.and.increasing.cable.length.

are.limiting.the.maximum.rate.of.transmitted.bits.considering.a.maximal.bit.error.rate.(BER)..Dening.

the.distance.as.length.of.the.signal.transmission.path.between.the.sending.and.the.receiving.system.and.

the.signaling.rate.as.the.bit.rate.at.which.data.have.to.be.passed.to.the.receiving.device,.a.characteristic.

relation.can.be.stated.as.dened.by.Figure.2.1.

Dierent

.transmission.standards,.such.as.TIA/EIA–485,.low-voltage.dierential.signaling.(LVDS),.

multiplex.LVDS.(M-LVDS),.and.current-mode.logic.(CML).provide.solutions.for.various.needs.in.terms.

of.bit.rate.and.line.length.(see.Table.2.1.for.details.of.the.standards)..As.shown.in.Figure.2.1,.as.the.cable.

length.increases,.the.speed.at.which.the.information.is.transmitted.must.be.lowered.in.order.to.keep.

the.BER.down..More.details.with.regard.to.physical.properties,.transmission.types.and.standards,.and.

devices.can.be.found.in.[TI04,NS08].

2.2.2 Cable types and Operational Characteristics

Besides.of.cable.data.transmission.characteristics,.the.main.important.aspect.is.cable.protection.against.

electromagnetic.inuences.of.the.environment,.which.gets.growing.importance.with.increasing.cable.

length.and.strong.and.high-frequent.electromagnetic.elds.near.the.cable..Two.parameters.inuence.

protecting.the.cable.against.inuences:.cable.type.and.construction.

Basically,

.a.cable.has.two.lines.. is.unshielded.type.can.be.additionally.protected.by.shielding.where.

the.two.lines.are.placed.within.a.conductive.hose..Mostly.connected.with.ground.potential.at.one.end,.

this.hose.reduces.the.impact.of.electromagnetic.noise.

Several

.cable.constructions.are.well.suited.for.supporting.high.bit.rates:.coaxial.cable.and.one.or.

multiple.twisted.pairs.. e.coaxial.cable.is.set.up.with.a.line.in.the.center.of.a.conductive.hose.that

delivers.a.shielding.of.the.inner.signal.line.by.the.hose.at.ground.potential..Primarily.used.for.unbal-

anced

.signal.transmission.as.described.in.the.following,.a.coaxial.cable.can.be.additionally.shielded.

by.an.outer.hose.

10,000

1,000

CML/ECL

LVDS

M-LVDS

485/422

100

0.1

0.1 1 10 100 1,000 10,000

Distance (m)

Signaling rate (Mbps)

M-LVDS

CML/ECL

FIGURE 2.1 Signaling.rate.versus.cable.length.

Media 2-3

Twisted.pair.cables.are.based.on.two.twisted.lines.in.an.unshielded.(UTP).or.shielded.cable.(STP)..

Due.to.twisting,.the.two.lines.are.similarly.inuenced.by.electromagnetic.noise..Especially,.if.operated.

with.dierential.signal.transmission.as.described.in.the.following,.this.inuence.aects.only.the.com-

mon

.mode.(CM).

2.2.3 Single-Ended and Differential transmission

Two.forms.of.operation.of.electrical.interface.circuits.are.used.for.data.transmission.via.cables:.single-

ended

.(or.unbalanced,.SE).and.dierential.(or.balanced).transmission.

Figure

.2.2.shows.the.principle.of.SE.transmission..A.single.signal.line.carries.the.data.coding.signal.

referring.at.both.ends.to.a.common.ground.line..Advantages.of.SE.transmission.are.simplicity.and.low.

cost.of.implementation,.e.g.,.used.with.the.frequently.applied.standard.TIA/EIA-232.serial.communica-

tion

.with.many.handshaking.lines..Cabling.costs.can.be.kept.to.a.minimum.with.short.distance.com-

munication,

.depending.on.data.throughput..For.longer.distances.and/or.noisy.environments,.shielding.

and.additional.ground.lines.are.essential..Twisted.pair.cables.are.recommended.for.line.lengths. of.

Driver

Receiver

Data

out

Signal line

Common ground

out

GND

Data

FIGURE 2.2 SE.transmission.with.terminating.resistor.at.the.end.

TABLE 2.1 Overview.over.Standards.for.Digital.Data.Transmission.and. eir.Characteristics

Standard

Product.

Family

Max..data.

Rate.per.Line

Max..Data.Rate.

per.Device Max..Distance Topology

TIA/EIA–232

(ITU–T.V.28)

TIA/EIA–232 512.kbps 20.m Point-to-point.

(simplex)

CAN

.(ISO.11898) CAN 1.Mbps 40.m Multipoint.

(Multiplex)

TIA/EIA

.422.

(ITU–T.V.11)

TIA/EIA–422 10.Mbps 10.m.(1,200.m) Multidrop.

(distributed.

simplex)

TIA/EIA

.485.

(ISO8482)

TIA/EIA.485 35.Mbps 10.m.(1,200.m) Multipoint.

(Multiplex)

USB

.1.1

USB.2.0

USB 12.Mbps

480.Mbps

5.m Multipoint.

(Multiplex)

IEEE1394–1995

IEEE1394a–2000

IEEE1394b–2002

Firewire 100–400/

800

.Mbps

800.Mbps

4.5.m.>.100.m Multipoint.

(Multiplex)

TIA/EIA–899

(Multiplex.LVDS)

M–LVDS 500.Mbps

0.5

.m.(∼30.m)

Multipoint

(Multiplex)

TIA/EIA–644

(LVDS)

LVDS 2.Gbps 4.ch:.

1,600/800.Mbps

.m.(∼30.m)

Point-to-point

(simplex)

IEEE

.P802.3z Gigabit.

Ethernet

1.25.Gbps 1.25.Gpbs.full.

duplex

<10.m Point-to-point.

(simplex)

IEEE

.P802.3ae 10.Gigabit.

Ethernet

2.5.Gbps 4.ch:.10.0.Gbps.

full.duplex

<10.m Point-to-point.

(simplex)

2-4 Industrial Communication Systems

more.than.1.m.. ese.additional.eorts.signicantly.reduce.the.cost.advantages..Disadvantage.of.SE.

transmission.is.the.poor.noise.immunity.originated.by.the.way.of.ground.wiring.

For

.balanced.or.dierential.transmission,.a.pair.of.signal.lines.is.necessary.together.with.an.addi-

tional

.ground.connection.(see.Figure.2.3).. e.signal.is.transmitted.by.a.sender.with.dierential.outputs.

in.a.way.that.the.signal.is.placed.on.one.line.and.the.inverted.signal.on.the.second.line.. e.receiver.

detects.the.voltage.dierence.between.its.inputs..Good.noise.performance.comes.from.the.use.of.a.

twisted.pair.whereby.noise.is.coupled.into.both.signal.lines.in.the.same.way..Due.to.the.CM.rejection.

capability.of.a.dierential.amplier,.this.noise.is.without.eect..Twisted-pair.cables.with.correct.ter-

mination

.and.dierential.signaling.allow.very.high.data.rates.up.to.10.Gbps..Only.cost.is.the.primary.

disadvantage.of.dierential.transmission..Furthermore,.high.data.rates.require.a.very.well-dened.cable.

impedance.and.correct.termination.to.avoid.reections.

2.2.4 Simplex and Duplex Communication

Media.access.for.data.transmission.can.be.performed.in.dierent.topologies.in.respect.of.directly.realiz-

able

.communication.links.. ese.topologies.are.point-to-point,.multidrop,.and.multipoint.

Point-to-point

.or.simplex.communication.is.characterized.by.one.sender.and.one.receiver.per.sig-

nal

.line.or.line.pair.of.a.cable.(Figure.2.4)..Data.transmission.is.possible.only.in.one.direction,.i.e.,.

unidirectional..Being.the.most.elementary.topology,.it.has.the.advantage.of.enabling.well-controlled.

cable.impedances.necessary.for.very.high.signaling.rates..All.signaling.technologies.may.be.used.for.

point-to-point.links..However,.very.fast.differential.signaling.technologies.are.primarily.designed.

for.point-to-point.signal.transmission.

.topology.with.one.sender.and.multiple.receivers.is.called.multidrop.or.distributed simplex.(Figure.

2.5)..Only.unidirectional.transfer.is.possible..Terminating.the.cable.on.the.far.receiver.side.is.advis-

able

.only.when.the.signal.driver.is.on.the.opposite.end.of.the.cable.from.the.terminated.receiver..In.

all.other.cases.(e.g.,.driver.connected.to.the.middle.of.the.bus),.the.bus.needs.to.be.terminated.at.both.

ends.of.the.bus.

.multipoint.or.multiplex.topology.is.implemented.with.many.transmitters.and.many.receivers.

per.line.(Figure.2.6)..In.practice,.this.solution.is.oen.realized.with.combined.transmitter–receiver.

pairs.called.transceivers..Any.combination.of.receivers,.transmitters,.and.transceivers.is.possible.for.

this.topology..Transmission.is.possible.in.both.direction,.i.e.,.bidirectional,.and.all.signal.drivers.and.

receivers.are.sharing.the.same.single.interconnect..A.frequently.implemented.variant.of.the.multipoint.

Di. driver

Di. receiver

Data

out

Signal line

Data

out

Di.

–

GND

out

–

FIGURE 2.3 Dierential.transmission.

Single ended Dierential

R D

FIGURE 2.4 Point-to-point.communication.link.

Media 2-5

topology.is.the.half-duplex.topology,.which.consists.of.two.driver/receiver.pairs.that.transmit.and.receive.

signals.between.two.points.over.a.single.interconnect..Since.sharing.of.a.single.interconnect.requires.

that.only.one.transmitter.is.active.at.any.time,.a.full-duplex.topology.implying.concurrent.transmission.

in.both.directions.has.to.be.realized.with.two.lines.in.parallel.

Physical

.connection.of.multiple.drivers.and.receivers.to.a.common.signal.line.raises.a.design.chal-

lenge

.mainly.encountered.by.impedance.discontinuities.that.device.loading.and.device.connections.

(stubs).introduce.on.the.common.bus.

.topologies.presented.above.can.be.used.with.single.line.or.multiple.lines.in.parallel,.which.corre-

spondingly

.increases.the.rate.of.transferred.bits..On.the.other.hand,.parallel.lines.increase.the.cost.because.

of.the.need.of.parallel.transmitters.and.receivers.and.of.cable.cost.according.to.the.cable.length..Parallel.

data.transmission.moreover.requires.provisions.for.synchronous.data.clocking,.which.is.basically.imple-

mented

.by.a.separate.clock.line.. ese.aspects.are.responsible.for.mostly.using.single.or.very.few.parallel.

lines.in.communication.networks.with.larger.line.lengths..In.this.case,.a.separate.clock.line.can.be.saved.

and.replaced.by.coding.techniques.allowing.for.synchronizing.the.receiver.clock.as.described.as.follows.

2.2.5 Bit Encoding

For.bit.transmission,.a.frame.is.serialized.and.sent.across.a.communications.link.to.the.destination..As.

dened.by.the.open.systems.interconnection.(OSI).model,.the.physical.layer.(Layer.1).denes.the.rep-

resentation

.of.each.bit.as.a.voltage,.current,.phase,.or.frequency.. e.following.basic.schemes.are.used:

•

. Return.to.zero,.RZ.(pulse.signaling)

•

. Non–return.to.zero.transmission,.NRZ.(level.signaling)

•

. Manchester.encoding.or.Manchester.phase.encoding.(edge/phase.signaling)

.earlier.transmission.techniques,.pulses.were.used.to.represent.bits,.e.g.,.RZ.in.which.a.logic.1.is.

represented.by.a.pulse.and.a.logic.0.by.the.absence.of.a.pulse..In.NRZ.transmission,.each.data.bit.is.rep-

resented

.by.a.level..A.high.level.may.represent.a.logic.1.and.a.low.level.a.logic.0,.or.vice.versa.

Manchester

.encoding.(Figure.2.7).uses.a.still.dierent.scheme.where.a.logic.1.is.represented.by.a.

transition.in.a.particular.direction.(usually.a.rising.edge).in.the.centre.of.each.bit..A.transition.in.the.

opposite.direction.(downward.in.this.case).is.used.to.represent.a.logic.0..Compared.with.NRZ,.advan-

tage

.of.this.coding.is.that.the.bit.timing.is.intrinsically.indicated.by.the.edges.characterizing.the.bits.

Single ended Dierential

R R

FIGURE 2.5 Multidrop.topology.

Single ended

D D

R R

Dierential

FIGURE 2.6 Multipoint.topology.