Simulation of Local Area Networks pdf pdf
Simulation of
Local
Area
Networks
Simulation of
Local
Area
Networks
Matthew N. 0.
Sadiku, Ph.D.
I
AssociateProfessor
Departmentof ElectricalEngineeing
TempleUniversity
Philadelphia,Pennsylvania
Mohammad Ilyas, Ph.D.
Professor
Departmentof ComputerScienceandEngineering
FloridaAtlanticUniversity
BocaRaton,
Boca Raton London New York
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Library of Congress Cataloging-in-Publication Data
Sadiku, Matthew N. O.
Simulation of local area networks / Matthew N. O. Sadiku, Mohammad Ilyas
p. cm.
Includes bibliographical references and index.
ISBN 0-8493-2473-4
1. Local area networks (Computer networks)--Mathematical models
I. Ilyas, Mohammad, 1953- II. Title.
TK5105.7.S22 1994
004.6’8’01135133—dc20
DNLM/DLC
for Library of Congress
94-23413
A Library of Congress record exists under LC control number: 94023413
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ISBN 13: 978-1-315-89753-0 (hbk)
ISBN 13: 978-1-351-07663-0 (ebk)
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Dedicated t o our families:
Chris, Ann,andJoyce
Parveen,Safia,Omar,andZakia
PREFACE
One of the growing areas in the communication industry is the inter-
networkingof theincreased proliferation of computers,particularlyvia
local area networks (LANs). A LAN is a datacommunication system,
usually owned by a singleorganization, that enablessimilaror dissim-
ilar digital devices t o talk t o each other over a common transmission
medium.
Establishing theperformance characteristics of a LAN before put-
ting it into use is of paramount importance. It gives the designer the
freedom and flexibilityt o adjust various parameters of the network at
theplanningstage. Thiswaythedesigner eliminatestherisk of unfore-
seen bottlenecks, underuse or overuse of resources, andfailuret o meet
targeted systemrequirements.
Thecommon approachest o performance evaluation applyanalyt-
ical models, simulation models, or hybrid models. Simulation models
allow systems analyststoevaluate the performanceof existing or pro-
posed systemsunder different conditionsthat lie beyond what analytic
models canhandle. Unlikeanalyticmodels,simulationmodels can pro-
videestimatesof virtuallyanynetwork performancemeasure.
Sofarthere aretwo categories of texts onsimulation.Textsinthe
first categorycover thegeneral principlesof simulationandapplythem
t o aspecificsystemforillustrationpurposes;theyarenotspecificenough
t o help a beginner apply those principles in developing a simulation
modelfor a LAN.Textsinthesecond categoryinstruct readers onhow
t o applyspecial-purpose languages(such asGPSS,GASP,SIMSCRIPT,
SIMULA, SLAM, and RESQ) in constructing a simulation model for
computersystemsincludingLANs. Besidesthefactthattheselanguages
arestillevolvingandarelimited,thereader mustfirstlearnthespecific
simulation languagebefore asimulationmodelcan be developed.
Thistext hastwomajoradvantagesovertheseexistingtexts. First,
it uses C, a well developed general-purpose language that is familiar
t o most analysts. This avoids the need for learning a new simulation
languageorpackage. Second,thetextspecificallyappliesthesimulation
principlest o localareanetworks. Inaddition,thetextisstudentoriented
and issuitablefor classroom useorself-learning.
Thetext is intended for LAN designers who want to analyze the
performance of their designs using simulation. It may be used for a
one-semester course on simulation of LANs. The main requirements
for students taking such a course are introductory LAN course and a
knowledge of a high-level language,preferably C. Although familiarity
with probability theory andstatisticsisuseful,itisnot required.
vii
Thebook consists ofeight chapters. Each chapter has alist of ref-
erences to the literature, and there is bibliography at the end of the
book. Chapter 1provides a brief review of local area networks, and
Chapter 2 givestheanalytical models of popular LANs-token-passing
busandring networks,CSMAICD,andstarnetwork. Chapter 3 covers
thegeneralprinciplesofsimulation,andChapter4dealswithfundamen-
talconceptsinprobabilityandstatisticrelatingt o simulationmodeling.
MaterialsinChapters3 and4arespecificallyappliedindevelopingsimu-
lationmodelsontoken-passing LANs,CSMAICDLANs,andStarLANs
in Chapters5,6,and 7 respectively. Thecomputer codes in Chapters5
t o 7 aredivided intosegments and a detailed explanationof each seg-
ment is presented t o give a thorough understanding of the simulation
models. Theentirecodesareputtogetherintheappendices. Itishoped
thattheideasgainedin learninghow t o simulatethese common LANs
canbe applied t o other communicationsystems.
Theauthorsare indebted to various students and colleagues who
have contributed t o thisbook. We areparticularly indebted toGeorge
Paramanisand Sharuhk Murad for working on some of thesimulation
models as special graduateprojects. Special thanks are due to Robert
Sternof CRCPressforprovidingexperteditorialguidanceontheman-
uscript. Finally, we owe much to our families for their patience and
support while preparing thematerial. Tothemthisbookisdedicated.
Table of Contents
1 LOCAL AREA NETWORKS
1.1 Definition ofa LAN
1.2 Evolutionof LANs
1.3 LANTechnology
1.3.1 Network Topology and Access-Control
1.3.2 Transmission Media
1.3.3 Transmission Techniques
1.4 Standardization of LANs
1.5 LANArchitectures
1.5.1 The OSI Model
1.5.2 The Seven OSI Layers
1.5.3 The IEEE Modelfor LANs
1.6 Performance Evaluation
1.6.1 Channel Utilization
1.6.2 Delay, Power, andEffective Transmission Ratio
1.7 Summary
References
Problems
1
2
2
3
4
7
7
7
8
9
9
10
11
11
. . . . 13
14
15
16
2 ANALYTICAL MODELS OF LAN
17
2.1 Introduction
2.2 Token-Passing Ring
2.2.1 BasicOperation
2.2.2 Delay Analysis
2.3 Token-Passing Bus
2.3.1 Basic Operation
2.3.2 DelayAnalysis
2.4 CSMA/CD Bus
2.4.1 Basic Operation
2.4.2 DelayAnalysis
2.5 Star
2.5.1 Basic Operation
2.5.2 Delay Analysis
2.6 Performance Comparisons
2.7 Summary
References
Problems
17
19
19
20
27
27
28
31
32
33
35
36
37
38
40
40
41
3 SIMULATION MODELS
43
3.1 Introduction
3.2 Why Simulation?
43
44
IX
3.3 Characteristics of SimulationModels
3.3.1 Continuous/Discrete Models . .
3.3.2 Deterministic/Stochastic Models
3.3.3 Time/Event Models . . . . .
3.3.4 Hardware/Software Models . .
3.4 Stagesof Model Development . . .
3.5 Common Mistakes in Simulation . .
3.6 Summary . . . . . . . . . . . .
References . . . . . . . . . . . .
Problems . . . . . . . . . . . .
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4 PROBABILITY AND STATISTICS
4.1 Introduction . . . . . . . . . . . .
4.2 Characterizationof RandomVariables
4.3 Common ProbabilityDistributions . .
4.3.1 Uniform Distribution . . . . . .
4.3.2 Triangular Distribution . . . . .
4.3.3 Exponential Distribution . . . .
4.3.4 Poisson Distribution . . . . . .
4.3.5 Normal Distribution . . . . . .
4.3.6 Lognormal Distribution . . . . .
4.3.7 Gamma(Erland) Distribution . .
4.4 Generation of RandomNumbers . . .
4.5 Generation of RandomVariates . . .
4.5.1 Inverse Transformation Method .
4.5.2 Rejection Method . . . . . . .
4.6 Estimationof Error . . . . . . . . .
4.7 Summary . . . . . . . . . . . . .
References . . . . . . . . . . . . .
Problems . . . . . . . . . . . . .
45
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45
46
46
50
51
52
53
55
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5 SIMULATION OF TOKEN-PASSING LANS
83
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 83
5.2 Operation of Token-Passing RingLANs . . . . . . . . . . 83
5.3Operationof Token-Passing BusLANs . . . . . . . . . . 86
5.4 SimulationModel . . . . . . . . . . . . . . . . . . . 87
5.4.1 Assumptions . . . . . . . . . . . . . . . . . . . . 89
5.4.2 Inputandoutputvariables . . . . . . . . . . . . . 89
5.4.3 Descriptionof thesimulationmodel . . . . . . . . . 90
5.4.4 Typicalsimulationsessions . . . . . . . . . . . . 103
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . 104
References . . . . . . . . . . . . . . . . . . . . . . 104
Problems . . . . . . . . . . . . . . . . . . . . . . 105
6 SIMULATION OF CSMA/CD LANS
107
6.1 Introduction . . . . . . . . . . . . . . .
6.2 Operation of CSMA/CD LANs . . . . . .
6.2.1 Nonpersistent andppersistent CSMA/CD
6.3 SimulationModel . . . . . . . . . . . .
6.3.1 Assumptions . . . . . . . . . . . . .
6.3.2 Input and outputvariables . . . . . .
6.3.3 Description of simulationmodel . . . .
6.3.4 Typicalsimulationsessions . . . . . .
6.4 Summary . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . .
Problems . . . . . . . . . . . . . . . .
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109
110
112
112
114
127
132
132
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7 SIMULATION OF STAR LANs
7.1 Introduction . . . . . . . . . . . .
7.2 Operation of StarLANs . . . . . . .
7.3 SimulationModel . . . . . . . . .
7.3.1 Assumptions . . . . . . . . . .
7.3.2 Inputand outputvariables . . .
7.3.2 Description of thesimulationmodel
7.3.3 Typicalsimulationsessions . . .
7.5 Summary . . . . . . . . . . . . .
References . . . . . . . . . . . . .
Problems . . . . . . . . . . . . .
135
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135
135
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8 SIMULATION LANGUAGES
8.1Introduction . . . . . . . .
8.2 GPSS . . . . . . . . . . .
8.3 SIMSCRIPT . . . . . . . .
8.4 GASP . . . . . . . . . .
8.5 SIMULA . . . . . . . . .
8.6 SLAM . . . . . . . . . .
8.7 RESQ . . . . . . . . . .
8.8 Criteriafor LanguageSelection
8.9 Summary . . . . . . . . .
References . . . . . . . . .
Problems . . . . . . . . .
Selected Bibliography
Appendix A-Simulation
Appendix B-Simulation
Appendix C-Simulation
Index
151
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151
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154
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161
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. . . 167
169
program for token-passing LANs 175
program for CSMA/CD LANs 183
program for star LANs
191
195
Chapter 1
Local Area Networks
Successful people m a k e decisions quickly as soon a s all t h e facts are available and change t h e m v e r y slowly if ever. Unsuccessful people m a k e
decisions v e r y sloruly, and change t h e m o f t e n and quickly.
-Napoleon Hill
Inordert o fullyparticipate intheinformationage,onemust be ablet o
communicate with othersin a multitude of ways. Everyone is familiar
with telephone communications and theuse of television as a medium
for transmitting information. However, the greatest interest today is
centered on computer generated data,and itstransmission has become
themost rapidly developing facet of thecommunication industry. The
overall communicationproblemmaybe viewed asinvolvingthree types
of networks:
Local areanetworks (LANs) providing communicationsover a rel-
ativelysmallarea
Metropolitan areanetworks (MANS)operatingover afew hundred
kilometers
Wideareanetworks(WANs)providingcommunicationsoverseveral
kilometers, acrossthenation,or aroundtheglobe
WANs, such as theARPANET inthe US, have existed for several
years, but the need for LANs has been identified much more recently.
Although the two types of communication networks employ identical
principles,theircharacteristicsarequitedifferent. InaWAN,whichmay
spancontinents,thetransmissionmediaarerelativelyexpensivebecause
of thelargeextent of thenetworks. Transmission rates in aWAN may
range from 2,400t o about 50,000bits per second, whereas in a LAN
they are much higher, typically from 1t o 10 million bits per second.
2
Simulation of Local Area Networks
In a WAN,thedataarrivalrate is low enough t o permit processors t o
ensure error-free transmission and message integrity. This is not the
casewith aLAN becauseof itsmuch higher datarate. TypicallyWANs
usetheexistingtelephonenetworkforcommunications(ormorerecently
thenationalpacket datanetwork),whereas LANsuseprivatelyinstalled
coaxialcableortwisted-pair wires.
In this and the next chapter, we briefly review thefundamentals
of local areanetworks necessary for therest of thebook. Thematerial
in the chapter is also discussed in manyjournal papers and textbooks
which aregiven in thereference list attheend of the chapter.
1.1 Definition of a LAN
A LAN is a data communication system, usually owned by a single
organization,that allowssimilaror dissimilar digital devices t o talkt o
eachotheroveracommontransmissionmedium.Accordingt o theIEEE,
A local area network is distinguished from other types of data networks i n that communication is usually confined t o a moderate geographic area such as a single ofice building, a warehouse, or a
campus, and can depend on a physical communications channel of
moderate-to-high data rate which has a consistently low error rate.
Thuswe mayregard a LAN asa resource-sharing datacommunication
network withthefollowingcharacteristics [l,21:
Short distances (0.1t o 10km)
Highspeed (1t o 16Mbps)
Low cost (intheregion of $3,000)
Lowerror rate ( 1 0 to10-l')
~ ~
Ease of access
Highreliabilitylintegrity.
Thenetwork may connect datadevices such as computers,termi-
nals,massstoragedevices,andprinters/plotters. Through thenetwork
thesedevicescaninterchangedatainformationsuchasfiletransfer,elec-
tronicmail,andword processing.
1.2 Evolution of LANs
LANs, asdatacommunication networks, resulted fromthemarriage of
two different technologies: telecommunications and computers. Data
communicationtakes advantageof CATVtechnologyt o produce better
performanceatlowercosts. Recent developmentsof largescaleandvery
largescale (LSIand VLSI) integrated circuits haverapidly reduced the
cost of computation andmemoryhardware. Thishasresulted inwidely
available low-cost personal computers, intelligent terminals, worksta-
tions, andminicomputers. However, other expensive resources such as
high-qualityprinters,graphicplotters,anddisk storagearebest shared
in a geographically limitedareausing a LAN.
Local Area Networks
3
Research in LANs began intheearly 1970s,spurred by increasing
requirements for resource sharing in multiple processor environments.
In manycases theserequirements first appeared inuniversity campuses
or research laboratories [3]. Ethernet,thefirst bus contention technol-
ogy, originated a t Xerox Corporation's Palo Alto Research Center, in
themid-1970s [4]. Called Ethernet aftertheconcept inclassical physics
ofwavetransmissionthroughanether,thedesignborrowed manyofthe
techniques andcharacteristicsoftheAlohanetwork,a packet radionet-
work developed a t the University of Hawaii. Since the introduction of
Ethernet,networksusinganumberoftopologiesandprotocolshavebeen
developed andreported. Typicalexamples arethetoken-ring topology
developed intheUS, mainlya t MIT,butnowthesubject of IBM devel-
opment work in Europe, and the Cambridgering,which was produced
a t Cambridge University Computer Laboratory in the UK. T h e 1980s
have been a decadeof rapidmaturationfor LANs.
LANs represent a comparatively newfield of activityandcontinue
t o hold the public interest. Thisis mirrored by the numerous courses
beingoffered inthesubject,bythemanyconferencesessionsdevotedt o
LANs,by the research and developmentwork onLANs being pursued
botha t theuniversities andin industries, andby theincreasingamount
of literaturedevotedt o LANs.
All this interest is generated by the LAN's promise as a means
of interconnecting various computers or computer-related devices into
systems t h a t are more useful than their individualparts. T h e goal of
LANsist o providealargenumber of devices withinexpensive yet high-
speed local communications.
Communicationbetweencomputersisbecomingincreasinglyimpor-
tant as d a t a processing becomes a commodity. Local area networking
is a very rapidly growing field. Continued efforts are being made for
furthertechnological developments andinnovationsintheorganization
of these networksfor maximaloperationalefficiency.
Today's LANs areontheedgeof broadband speeds,andnew LAN
proposals call for higher speeds-e.g., a proposed 16 Mbps token ring
LAN and a 100Mbps fiber distributed d a t a interface (FDDI). Higher
speedsareneeded t o matchtheincreasing speed of thePCsandt o sup-
port diskless workstations and computer-aided design/manufacturing
(CAD/CAM) terminals t h a t rely on LANs for interconnection of file
servers. Also,thesuccessof LANshasled t o severalattemptst o extend
high-speed d a t a networkingbeyondthelocalpremises, acrossmetropoli-
t a n andwide areaenvironments.
1.3 LAN Technology
T h e typesoftechnologiesused t o implementLANs are as diverse as the
200orsoLAN vendors. Bothvendorsand users arecompelled t o make
a choice. Thischoiceisusuallybased onseveral criteria such as [5-101:
Simulation of Local Area Networks
Network topologyand architecture
Access control
Transmissionmedium
Transmissiontechnique
Adherence tostandards
Performance in terms of channel utilization,delay,power, and ef-
fectivetransmission ratio
Thefirstfourcriteriaarethemajortechnologicalissuesandareof great
concernwhendiscussingLANs. Theseissuesaresometimesinterrelated.
Forexample,someaccessmethodsareonlysuitableforsometopologies
or with certain transmission techniques.
1.3.1 Network Topology and Access Control
Thetopology of a network is theway in which thenodes (orstations)
areinterconnected. In spite of the proliferation of LAN products, the
vast majorityof LANs conform t o one of three topologies and one of
ahandful of medium-access control protocols summarized in Table 1.1.
Thebasicformsof thetopologies areshowninFigure 1.l.
Ring
Bus
Star
Figure 1.1 Local network topologies.
In thering topology,all nodes are connected together in a closed
loop. Information passes from node t o node on the loop and is regen-
erated (byrepeaters) at each node (called an activeinterface). A bus
topology uses a single, open-ended transmission medium. Each node
tapsintothe medium in away that does not disturb thesignalon the
Local Area Networks
5
bus(thusitiscalled apassiveinterface). Startopologyconsistsofacen-
tral controllingnode with star-like connections tovarious other nodes.
Although thering topologyis popular in Europe,the bus is themost
commontopologyinthe US.
Table 1.1 LAN Topology and Medium-Access Protocol.
1. Ringtopology
ControlledAccess
Token
Slotted
BufferlRegister Insertion
2. Bus topology
ControlledAccess
Token
Multilevel MultipleAccess(MLMA)
RandomAccess
Carrier Sense MultipleAccess (CSMA)
(l-persistent, p-persistent, nonpersistent)
Carrier Sense MultipleAccesswith
Collision Detection (CSMA/CD)
CSMA/CD with DynamicPriorities (CSMA/CD-CP)
CSMA/CD with Deterministic Contention
Resolution (CSMA/CD-DCR)
3. Startopology
4. Hybridtopology
Ring-star
Ring-bus
Bus-star
CSMA/CD-token ring
CSMA/CD-token bus
Both ring and bus topologies,lacking any central node,must use
somedistributedmechanismtodeterminewhichnodemayusethetrans-
mission medium atanygiven moment. Various flow control and access
strategieshave been proposed or developed for inserting and removing
messages from ring and bus LANs. The most popular ones are the
CarrierSenseMultipleAccesswithCollision Detection(CSMA/CD) for
broadcast buses andtokenpassing for rings andbuses.
InCSMA/CD,eachbusinterfaceunit (BIU),before attemptingto
transmit dataontothe channel,first listens or senses if the channel is
idle. An active BIU transmits its data only if the channel is sensed
idle. If thechannel issensedbusy,theBIU defers itstransmission until
6
Simulation of Local Area Networks
the bus becomes clear. In this contention-type access scheme, collision
occurswhen twoor more nodes attemptt o transmit a t thesametime.
Duringthecollision,thetwoormoremessagesbecomegarbled andlost.
Inthetoken-passing protocol, anemptyoridletoken (someunique
bit pattern or signal) is passed around theringor bus. Any node may
removethetoken,insert amessage,andappendthetoken. Whenanode
has d a t a t o transmit,it grabs the token, changes the token t o a busy
state (another bit pattern) and appends itspacket t o the busy token.
At the end of the transmission,the node issues another idletoken. A
node has channel access right only when itgets the idletoken. Figure
1.2showsthepacket formatfortoken bus andtoken ringtopologies.
>l
1
Preamble
Start
1
Control
2 or6
D~~~~~~
2or6
,"S:
>O
Data
4
FCS
1
Bytes
End
(a) Token Bus
(c) Token Format
Figure 1.2 Packetformatfortoken busandtoken ring.
Anumberofattemptshavebeenproposed t o develophybridtopolo-
gies and access techniques t h a t combine random and time-assignment
access.
Local Area Networks
1.3.2 T r a n s m i s s i o n M e d i a
Thetransmission mediumisthephysical pathconnecting thetransmit-
ter t o the receiver. Any physical medium that is capable of carrying
information in anelectromagneticformispotentialy suitableforuse on
aLAN.Inpractice,themediaused aretwisted-pair cable,coaxialcable,
andopticalfiber.
Twisted-pair cableisgenerallyusedforanalogsignalsbuthasbeen
successfullyusedfordigitaltransmissions. Itislimitedinspeedt o afew
megabitsandisoftensusceptiblet o noise. Ring,bus, andstarnetworks
can allusetwisted-pair cableasamedium.
Coaxial cable consists of a central conductor and a conducting
shield. Itprovides asubstantialperformanceimprovement overtwisted-
pair: ithashigher capacity,cansupportalargernumber of devices, and
canspangreater distances.
Optical fiber transmits light or infrared rays instead of electrical
signals. It demonstrateshigher capacity than coaxial cable and is not
susceptible t o noise or electrical fluctuations. Although there arestill
sometechnicaldifficultieswithopticalfiber,itmaywellbe themedium
choiceof futurenetworks.
1.3.3 T r a n s m i s s i o n Techniques
Therearetwotypesof transmissiontechniques: basebandsignalingand
broadband signaling. In spiteof the hot debateand controversy about
the merits of one technique over the other, it appears that the two
techniques will coexist for sometime,fillingdifferent needs.
Baseband signalingliterallymeansthatthesignalisnotmodulated
atall. Itistotallydigital. Theentirefrequencyspectrumisusedtoform
the signal, which is transmitted bidirectionally on broadcast systems
such asbuses. Baseband networks arelimitedin distance duet o signal
attenuation.
Broadband signaling is a technique by which information is fre-
quency modulated ontoanalogcarrier waves. Thisallowsvoice, video,
and datat o be carried simultaneously. Although it is more expensive
thanbasebandbecauseoftheneedformodemsateachnode,itprovides
larger capacity.
1.4 Standardization of LANs
Theincompatibilityof LAN productshasleft themarketsmallandun-
decided. One way t o increase the market size is t o develop standards
that can be used by the numerous LAN product manufacturers. The
most obvious advantage of standards is that they facilitate the inter-
change of databetween diverse devicesconnected t o LAN.Thedriving
force behind thestandardization efforts isthedesire by LAN users and
vendors t o have "open systems'' in which any standard computer de-
8
Simulation of Local Area Networks
vicewould be abletointeroperatewithothers. Attemptstostandardize
LAN topologies,protocols,andmodulation techniques have been made
by organizationssuch asthoseshown inTable 1.2.
Table 1.2 LAN StandardizationActivities
Organization
Standards
IS0
T C 97/SC6
CSMA/CD, token bus,
token ring,slottedring
SG 18
Connectionbetween ISDN and LAN
802
Logicallink control,CSMA/CD,
token bus,token ring,
metropolitanareanetwork
T C 24
CSMA/CD, token bus,token ring
CCITT
IEEE
ECMA
The International Standards Organization (ISO) is perhaps the
most prominent of these,anditisresponsiblefortheseven-layer model
of network architecture,initiallydeveloped for WANs, called therefer-
ence model for open systems interconnection (OSI).TheInternational
Telegraph and Telephone ConsultativeCommittee (CCITT),based in
Geneva,isapart oftheInternationalTelecommunications Union (ITU)
and is heavily involved in all aspects of datatransmission. It has pro-
duced standardsin Europe butnot intheUS. In the US theAmerican
National StandardInstitute (ANSI),theNational Bureau of Standards
(NBS),andtheIEEEareperhapsthemostimportantbodies. TheIEEE
802committeehasdeveloped LAN protocol standardsforthelowertwo
layers of the ISO's OS1 reference model, namely the physical and link
controllayers. Thephysical standard defineswhat manufacturers must
provideintermsofaccesstotheirhardwareforauserorsystemsintegra-
tor. More recently,theEuropean ComputerManufacturers Association
(ECMA)hasfollowedalongthesamelines. Althoughnetworkstandards
arebeing developed bythevariousorganizations,standardizationisstill
uptothemanufacturers. TheI S 0 protocolshavetheadvantageofinter-
national backing,and most manufacturers have madethe commitment
toimplementthemeventually.
1.5 LAN Architectures
Anetwork architectureis aspecificationofthesetoffunctionsrequired
for a user at a location tointeract with another user at another loca-
tion. These interconnect functions include determination of the start
and end of a message, recognition of a message address, management
of acommunication link,detection andrecovery of transmission errors,
Local Area Networks
9
andreliableandregulated delivery of data. Ageneralnetwork architec-
ture thus describes the interfaces, algorithms, and protocols by which
processes atdifferent locationsand/or a t heterogeneous machinescould
communicate. Althoughthearchitecturesdevelopedbydifferentvendors
arefunctionallyequivalent,theydonot provideforeasyinterconnection
of systemsof different make.
1.5.1 The OS1 Model
As mentioned intheprevious section,theneed for standardization and
compatibility at all levels has compelled the International Standards
Organization(ISO)toestablishageneralseven-layer hierarchical model
for universal intercomputer communication. This arhitecture, known
as the Open Systems Interconnection model (see Figure 1.3), defines
seven layersof communicationprotocols,withspecificfunctionsisolated
a t each level. TheOS1model is a reference model for theexchange of
informationamongsystemsthatareopent o oneanotherforthispurpose
by virtue of their mutual use of the applicable standards. Theseven
hierarchical layersarehardware andsoftwarefunctionalgroupingswith
specific well-defined tasks. The OS1model states thepurpose of each
layer anddescribestheservicesprovided byeach withinitslayerandto
theadjacent higher and lowerlayers. Detailsof theimplementation of
each layer of OS1model depend on thespecificsof theapplication and
thecharacteristics of thecommunicationchannel employed.
1.5.2 The Seven OS1 Layers
The application layer,level 7,istheonetheuser sees. It provides ser-
vices directly comprehensible toapplication programs: login,password
checks,network transparencyfor distribution of resources, fileanddoc-
ument transfer,andindustry-specific protocols.
Thepresentation layer,level 6, is concerned with interpretingthe
data.Itrestructuresdatatoorfromthestandardizedformatusedwithin
anetwork,textcompression,codeconversion,fileformatconversion,and
encryption.
Thesession layer,level 5,manages addresstranslation and access
security. It negotiates t o establish a connection with another node on
thenetwork and then t o managethedialogue. Thismeans controlling
thestarting,stopping,andsynchronizationoftheconversion.
Thetransportlayer,level4,performserror control,sequencecheck-
ing,handling of duplicatepackets,flowcontrol,andmultiplexing. Here
it is determined whether the channel is t o be point-to-point (virtual)
with ordered messages, isolated messages with no order, or broadcast
messages. It is the last of the layers concerned with communications
between peer entities in the systems. The transport layer and those
above are referred t o as the upper layers of the model, and they are
Simulation of Local Area Networks
10
independent of theunderlyingnetwork. Thelowerlayersareconcerned
with datatransfer acrossthenetwork.
Thenetwork layer,level 3, providesaconnectionpathbetween sys-
tems,includingthecasewhere intermediatenodesareinvolved. Itdeals
with message packetization, message routing for datatransfer between
nonadjacent nodes orstations,congestion control,and accounting.
7
Application
1
Presentation
5
Session
Transport
3
Network
Data Link
Physical
Figure 1.3 Relationshipbetween theOS1modelandIEEELAN layers.
The data link layer, level 2, establishes the transmission protocol,
howinformationwillbetransmitted,acknowledgmentofmessages,token
possession, error detection, and sequencing. It prepares the packets
passed downfromthenetwork layerfortransmissiononthenetwork. It
takes a raw transmission and transforms it into a linefree from error.
Hereheaders andframinginformationareaddedorremoved. Withthese
gothetimingsignals,check-sum, andstation addresses,aswell as the
controlsystemfor access.
Thephysical layer, level 1,is that part that actuallytouches the
transmissionmediumor cable;thelineisthepoint within thenode or
device where thedataisreceived andtransmitted. Itensures thatones
arrive as ones and zeros as zeros. It encodes and physically transfers
messages (rawbits in a stream)between adjacent stations. It handles
voltages, frequencies, direction, pin numbers, modulation techniques,
signaling schemes, ground loop prevention, and collision detection in
theCSMA/CD accessmethod.
1.5.3 The IEEE Model for LANs
TheIEEE has formulated standards for the physical and logical link
layers for three types of LANs, namely, token buses, token rings, and
Local Area Networks
11
CSMA/CD protocols. Figure1.3illustratesthecorrespondence between
the three layers of the OS1 and the IEEE 802 reference models. The
physical layer specifiesmeansfortransmittingandreceivingbits across
various types of media. The media-access-control layer performs the
functionsneeded t o control accesst o thephysicalmedium. Thelogical-
link-control layer isthecommon interface t o thehigher softwarelayers.
1.6 Performance Evaluation
Therehasbeen muchresearch intotheperformanceof thevariousmedi-
umaccessprotocols [5,6,11,121.LANsbeingcomplexsystems,model-
ingof themmust be done atvarious levelsof detail. Inthissection,we
present simpleperformancemodels of LANs. Theperformance is mea-
sured in termsof channel utilization, delay,power, and effectivetrans-
mission ratio. More rigorous performancemodels of LANs are covered
in thenext chapter.
1.6.1 Channel Utilization
A performance yardstick is the maximumthroughput achievablefor a
given channel capacity. For example, how many megabits per second
(Mbps)of datacan actuallybetransmittedfor achannel capacityof 10
Mbps? Itiscertain thatafractionof thechannel capacityisused up in
formof overhead-acknowledgments, retransmission,token delay,etc.
Channel capacity is the maximum possible data rate, that is,the
signaling rate on the physical channel. It is also known as the data
rate or transmission rate and will be denoted by R in bits per second.
Throughput S istheamountof "user data" thatiscarried by theLAN.
Channelutilization U istheratioofthroughputt o channelcapacity-i.e.
Itisindependent of themediumaccesscontrol. Itisobviousthat U = 1
in anidealsituation.
In analyzing LAN performance, the two most useful parameters
arethechannel capacityor datarateR of themediumandtheaverage
maximumsignalpropagation delayP . Theirproduct ( R P , inbits)isthe
numberofbitsthatcanexistinthechannelbetweentwonodesseparated
by the maximum distance determined by the propagation delay. We
definetheratio
Lengthof datapath - R P
Length of packet
PL
where PLis thepacket length in bits. Thequantity ais a normalized
nondimensional measure used in determining the upper bound on uti-
lization;itsreciprocaliscalledtheeffectivetransmissionratio. Realizing
that P L / R isthetimeneeded t o transmita packet,
a=
Simulation of Local Area Networks
P - Propagationtime
P L / R Transmissiontime
a=-
(1.3)
If thethroughput Sisdefined astheactualnumber of bitstransmitted
per second,then
S=
Lengthof packet
Transmission time Propagationtime
+
Substituting(1.4)into(1.1)gives
Thusutilizationisinverselyrelatedtoa . Typicalvaluesof a rangefrom
0.01to0.1accordingtoStallings[13].Theidealcaseoccurs when there
isnooverheadand a = 0. Theratio a cannowbeusedtodefinechannel
utilization bounds for amediumaccessprotocol. Fortoken ring or bus,
whereTl isthepackettransmissionperiodandT2isatokentransmission
period. It can be shown that [6,131
where N isthenumber of active nodes orstations. In atoken bus,the
optimalcaseoccurswhen thelogicalorderingof nodes (thesequenceof
nodes through which thetoken ispassed, asshown inFigure 2.4) isthe
sameasthephysicalorder. In thiscase, Eq. (1.7)applies. Intheworst
case,thelogicalorderingof nodesforcesthepropagation delaybetween
nodes toapproach theend-to-end delay. Forthiscase,Tz = a and
Local Area Networks
1.6.2 Delay, P o w e r , a n d EffectiveT r a n s m i s s i o n R a t i o
Packet delayistheperiod of timebetween themomentatwhich anode
becomes active (i.e.,when it has data t o transmit) and the moment
atwhich the packet issuccessfully transmitted. Throughput delay de-
scribes
Local
Area
Networks
Simulation of
Local
Area
Networks
Matthew N. 0.
Sadiku, Ph.D.
I
AssociateProfessor
Departmentof ElectricalEngineeing
TempleUniversity
Philadelphia,Pennsylvania
Mohammad Ilyas, Ph.D.
Professor
Departmentof ComputerScienceandEngineering
FloridaAtlanticUniversity
BocaRaton,
Boca Raton London New York
CRC Press is an imprint of the
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First published 1995 by CRC Press
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Reissued 2018 by CRC Press
© 1995 by CRC Press, Inc.
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Library of Congress Cataloging-in-Publication Data
Sadiku, Matthew N. O.
Simulation of local area networks / Matthew N. O. Sadiku, Mohammad Ilyas
p. cm.
Includes bibliographical references and index.
ISBN 0-8493-2473-4
1. Local area networks (Computer networks)--Mathematical models
I. Ilyas, Mohammad, 1953- II. Title.
TK5105.7.S22 1994
004.6’8’01135133—dc20
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ISBN 13: 978-1-315-89753-0 (hbk)
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CRC Press Web site at http://www.crcpress.com
Dedicated t o our families:
Chris, Ann,andJoyce
Parveen,Safia,Omar,andZakia
PREFACE
One of the growing areas in the communication industry is the inter-
networkingof theincreased proliferation of computers,particularlyvia
local area networks (LANs). A LAN is a datacommunication system,
usually owned by a singleorganization, that enablessimilaror dissim-
ilar digital devices t o talk t o each other over a common transmission
medium.
Establishing theperformance characteristics of a LAN before put-
ting it into use is of paramount importance. It gives the designer the
freedom and flexibilityt o adjust various parameters of the network at
theplanningstage. Thiswaythedesigner eliminatestherisk of unfore-
seen bottlenecks, underuse or overuse of resources, andfailuret o meet
targeted systemrequirements.
Thecommon approachest o performance evaluation applyanalyt-
ical models, simulation models, or hybrid models. Simulation models
allow systems analyststoevaluate the performanceof existing or pro-
posed systemsunder different conditionsthat lie beyond what analytic
models canhandle. Unlikeanalyticmodels,simulationmodels can pro-
videestimatesof virtuallyanynetwork performancemeasure.
Sofarthere aretwo categories of texts onsimulation.Textsinthe
first categorycover thegeneral principlesof simulationandapplythem
t o aspecificsystemforillustrationpurposes;theyarenotspecificenough
t o help a beginner apply those principles in developing a simulation
modelfor a LAN.Textsinthesecond categoryinstruct readers onhow
t o applyspecial-purpose languages(such asGPSS,GASP,SIMSCRIPT,
SIMULA, SLAM, and RESQ) in constructing a simulation model for
computersystemsincludingLANs. Besidesthefactthattheselanguages
arestillevolvingandarelimited,thereader mustfirstlearnthespecific
simulation languagebefore asimulationmodelcan be developed.
Thistext hastwomajoradvantagesovertheseexistingtexts. First,
it uses C, a well developed general-purpose language that is familiar
t o most analysts. This avoids the need for learning a new simulation
languageorpackage. Second,thetextspecificallyappliesthesimulation
principlest o localareanetworks. Inaddition,thetextisstudentoriented
and issuitablefor classroom useorself-learning.
Thetext is intended for LAN designers who want to analyze the
performance of their designs using simulation. It may be used for a
one-semester course on simulation of LANs. The main requirements
for students taking such a course are introductory LAN course and a
knowledge of a high-level language,preferably C. Although familiarity
with probability theory andstatisticsisuseful,itisnot required.
vii
Thebook consists ofeight chapters. Each chapter has alist of ref-
erences to the literature, and there is bibliography at the end of the
book. Chapter 1provides a brief review of local area networks, and
Chapter 2 givestheanalytical models of popular LANs-token-passing
busandring networks,CSMAICD,andstarnetwork. Chapter 3 covers
thegeneralprinciplesofsimulation,andChapter4dealswithfundamen-
talconceptsinprobabilityandstatisticrelatingt o simulationmodeling.
MaterialsinChapters3 and4arespecificallyappliedindevelopingsimu-
lationmodelsontoken-passing LANs,CSMAICDLANs,andStarLANs
in Chapters5,6,and 7 respectively. Thecomputer codes in Chapters5
t o 7 aredivided intosegments and a detailed explanationof each seg-
ment is presented t o give a thorough understanding of the simulation
models. Theentirecodesareputtogetherintheappendices. Itishoped
thattheideasgainedin learninghow t o simulatethese common LANs
canbe applied t o other communicationsystems.
Theauthorsare indebted to various students and colleagues who
have contributed t o thisbook. We areparticularly indebted toGeorge
Paramanisand Sharuhk Murad for working on some of thesimulation
models as special graduateprojects. Special thanks are due to Robert
Sternof CRCPressforprovidingexperteditorialguidanceontheman-
uscript. Finally, we owe much to our families for their patience and
support while preparing thematerial. Tothemthisbookisdedicated.
Table of Contents
1 LOCAL AREA NETWORKS
1.1 Definition ofa LAN
1.2 Evolutionof LANs
1.3 LANTechnology
1.3.1 Network Topology and Access-Control
1.3.2 Transmission Media
1.3.3 Transmission Techniques
1.4 Standardization of LANs
1.5 LANArchitectures
1.5.1 The OSI Model
1.5.2 The Seven OSI Layers
1.5.3 The IEEE Modelfor LANs
1.6 Performance Evaluation
1.6.1 Channel Utilization
1.6.2 Delay, Power, andEffective Transmission Ratio
1.7 Summary
References
Problems
1
2
2
3
4
7
7
7
8
9
9
10
11
11
. . . . 13
14
15
16
2 ANALYTICAL MODELS OF LAN
17
2.1 Introduction
2.2 Token-Passing Ring
2.2.1 BasicOperation
2.2.2 Delay Analysis
2.3 Token-Passing Bus
2.3.1 Basic Operation
2.3.2 DelayAnalysis
2.4 CSMA/CD Bus
2.4.1 Basic Operation
2.4.2 DelayAnalysis
2.5 Star
2.5.1 Basic Operation
2.5.2 Delay Analysis
2.6 Performance Comparisons
2.7 Summary
References
Problems
17
19
19
20
27
27
28
31
32
33
35
36
37
38
40
40
41
3 SIMULATION MODELS
43
3.1 Introduction
3.2 Why Simulation?
43
44
IX
3.3 Characteristics of SimulationModels
3.3.1 Continuous/Discrete Models . .
3.3.2 Deterministic/Stochastic Models
3.3.3 Time/Event Models . . . . .
3.3.4 Hardware/Software Models . .
3.4 Stagesof Model Development . . .
3.5 Common Mistakes in Simulation . .
3.6 Summary . . . . . . . . . . . .
References . . . . . . . . . . . .
Problems . . . . . . . . . . . .
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4 PROBABILITY AND STATISTICS
4.1 Introduction . . . . . . . . . . . .
4.2 Characterizationof RandomVariables
4.3 Common ProbabilityDistributions . .
4.3.1 Uniform Distribution . . . . . .
4.3.2 Triangular Distribution . . . . .
4.3.3 Exponential Distribution . . . .
4.3.4 Poisson Distribution . . . . . .
4.3.5 Normal Distribution . . . . . .
4.3.6 Lognormal Distribution . . . . .
4.3.7 Gamma(Erland) Distribution . .
4.4 Generation of RandomNumbers . . .
4.5 Generation of RandomVariates . . .
4.5.1 Inverse Transformation Method .
4.5.2 Rejection Method . . . . . . .
4.6 Estimationof Error . . . . . . . . .
4.7 Summary . . . . . . . . . . . . .
References . . . . . . . . . . . . .
Problems . . . . . . . . . . . . .
45
45
45
45
46
46
50
51
52
53
55
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5 SIMULATION OF TOKEN-PASSING LANS
83
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . 83
5.2 Operation of Token-Passing RingLANs . . . . . . . . . . 83
5.3Operationof Token-Passing BusLANs . . . . . . . . . . 86
5.4 SimulationModel . . . . . . . . . . . . . . . . . . . 87
5.4.1 Assumptions . . . . . . . . . . . . . . . . . . . . 89
5.4.2 Inputandoutputvariables . . . . . . . . . . . . . 89
5.4.3 Descriptionof thesimulationmodel . . . . . . . . . 90
5.4.4 Typicalsimulationsessions . . . . . . . . . . . . 103
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . 104
References . . . . . . . . . . . . . . . . . . . . . . 104
Problems . . . . . . . . . . . . . . . . . . . . . . 105
6 SIMULATION OF CSMA/CD LANS
107
6.1 Introduction . . . . . . . . . . . . . . .
6.2 Operation of CSMA/CD LANs . . . . . .
6.2.1 Nonpersistent andppersistent CSMA/CD
6.3 SimulationModel . . . . . . . . . . . .
6.3.1 Assumptions . . . . . . . . . . . . .
6.3.2 Input and outputvariables . . . . . .
6.3.3 Description of simulationmodel . . . .
6.3.4 Typicalsimulationsessions . . . . . .
6.4 Summary . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . .
Problems . . . . . . . . . . . . . . . .
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109
110
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7 SIMULATION OF STAR LANs
7.1 Introduction . . . . . . . . . . . .
7.2 Operation of StarLANs . . . . . . .
7.3 SimulationModel . . . . . . . . .
7.3.1 Assumptions . . . . . . . . . .
7.3.2 Inputand outputvariables . . .
7.3.2 Description of thesimulationmodel
7.3.3 Typicalsimulationsessions . . .
7.5 Summary . . . . . . . . . . . . .
References . . . . . . . . . . . . .
Problems . . . . . . . . . . . . .
135
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135
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8 SIMULATION LANGUAGES
8.1Introduction . . . . . . . .
8.2 GPSS . . . . . . . . . . .
8.3 SIMSCRIPT . . . . . . . .
8.4 GASP . . . . . . . . . .
8.5 SIMULA . . . . . . . . .
8.6 SLAM . . . . . . . . . .
8.7 RESQ . . . . . . . . . .
8.8 Criteriafor LanguageSelection
8.9 Summary . . . . . . . . .
References . . . . . . . . .
Problems . . . . . . . . .
Selected Bibliography
Appendix A-Simulation
Appendix B-Simulation
Appendix C-Simulation
Index
151
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154
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161
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169
program for token-passing LANs 175
program for CSMA/CD LANs 183
program for star LANs
191
195
Chapter 1
Local Area Networks
Successful people m a k e decisions quickly as soon a s all t h e facts are available and change t h e m v e r y slowly if ever. Unsuccessful people m a k e
decisions v e r y sloruly, and change t h e m o f t e n and quickly.
-Napoleon Hill
Inordert o fullyparticipate intheinformationage,onemust be ablet o
communicate with othersin a multitude of ways. Everyone is familiar
with telephone communications and theuse of television as a medium
for transmitting information. However, the greatest interest today is
centered on computer generated data,and itstransmission has become
themost rapidly developing facet of thecommunication industry. The
overall communicationproblemmaybe viewed asinvolvingthree types
of networks:
Local areanetworks (LANs) providing communicationsover a rel-
ativelysmallarea
Metropolitan areanetworks (MANS)operatingover afew hundred
kilometers
Wideareanetworks(WANs)providingcommunicationsoverseveral
kilometers, acrossthenation,or aroundtheglobe
WANs, such as theARPANET inthe US, have existed for several
years, but the need for LANs has been identified much more recently.
Although the two types of communication networks employ identical
principles,theircharacteristicsarequitedifferent. InaWAN,whichmay
spancontinents,thetransmissionmediaarerelativelyexpensivebecause
of thelargeextent of thenetworks. Transmission rates in aWAN may
range from 2,400t o about 50,000bits per second, whereas in a LAN
they are much higher, typically from 1t o 10 million bits per second.
2
Simulation of Local Area Networks
In a WAN,thedataarrivalrate is low enough t o permit processors t o
ensure error-free transmission and message integrity. This is not the
casewith aLAN becauseof itsmuch higher datarate. TypicallyWANs
usetheexistingtelephonenetworkforcommunications(ormorerecently
thenationalpacket datanetwork),whereas LANsuseprivatelyinstalled
coaxialcableortwisted-pair wires.
In this and the next chapter, we briefly review thefundamentals
of local areanetworks necessary for therest of thebook. Thematerial
in the chapter is also discussed in manyjournal papers and textbooks
which aregiven in thereference list attheend of the chapter.
1.1 Definition of a LAN
A LAN is a data communication system, usually owned by a single
organization,that allowssimilaror dissimilar digital devices t o talkt o
eachotheroveracommontransmissionmedium.Accordingt o theIEEE,
A local area network is distinguished from other types of data networks i n that communication is usually confined t o a moderate geographic area such as a single ofice building, a warehouse, or a
campus, and can depend on a physical communications channel of
moderate-to-high data rate which has a consistently low error rate.
Thuswe mayregard a LAN asa resource-sharing datacommunication
network withthefollowingcharacteristics [l,21:
Short distances (0.1t o 10km)
Highspeed (1t o 16Mbps)
Low cost (intheregion of $3,000)
Lowerror rate ( 1 0 to10-l')
~ ~
Ease of access
Highreliabilitylintegrity.
Thenetwork may connect datadevices such as computers,termi-
nals,massstoragedevices,andprinters/plotters. Through thenetwork
thesedevicescaninterchangedatainformationsuchasfiletransfer,elec-
tronicmail,andword processing.
1.2 Evolution of LANs
LANs, asdatacommunication networks, resulted fromthemarriage of
two different technologies: telecommunications and computers. Data
communicationtakes advantageof CATVtechnologyt o produce better
performanceatlowercosts. Recent developmentsof largescaleandvery
largescale (LSIand VLSI) integrated circuits haverapidly reduced the
cost of computation andmemoryhardware. Thishasresulted inwidely
available low-cost personal computers, intelligent terminals, worksta-
tions, andminicomputers. However, other expensive resources such as
high-qualityprinters,graphicplotters,anddisk storagearebest shared
in a geographically limitedareausing a LAN.
Local Area Networks
3
Research in LANs began intheearly 1970s,spurred by increasing
requirements for resource sharing in multiple processor environments.
In manycases theserequirements first appeared inuniversity campuses
or research laboratories [3]. Ethernet,thefirst bus contention technol-
ogy, originated a t Xerox Corporation's Palo Alto Research Center, in
themid-1970s [4]. Called Ethernet aftertheconcept inclassical physics
ofwavetransmissionthroughanether,thedesignborrowed manyofthe
techniques andcharacteristicsoftheAlohanetwork,a packet radionet-
work developed a t the University of Hawaii. Since the introduction of
Ethernet,networksusinganumberoftopologiesandprotocolshavebeen
developed andreported. Typicalexamples arethetoken-ring topology
developed intheUS, mainlya t MIT,butnowthesubject of IBM devel-
opment work in Europe, and the Cambridgering,which was produced
a t Cambridge University Computer Laboratory in the UK. T h e 1980s
have been a decadeof rapidmaturationfor LANs.
LANs represent a comparatively newfield of activityandcontinue
t o hold the public interest. Thisis mirrored by the numerous courses
beingoffered inthesubject,bythemanyconferencesessionsdevotedt o
LANs,by the research and developmentwork onLANs being pursued
botha t theuniversities andin industries, andby theincreasingamount
of literaturedevotedt o LANs.
All this interest is generated by the LAN's promise as a means
of interconnecting various computers or computer-related devices into
systems t h a t are more useful than their individualparts. T h e goal of
LANsist o providealargenumber of devices withinexpensive yet high-
speed local communications.
Communicationbetweencomputersisbecomingincreasinglyimpor-
tant as d a t a processing becomes a commodity. Local area networking
is a very rapidly growing field. Continued efforts are being made for
furthertechnological developments andinnovationsintheorganization
of these networksfor maximaloperationalefficiency.
Today's LANs areontheedgeof broadband speeds,andnew LAN
proposals call for higher speeds-e.g., a proposed 16 Mbps token ring
LAN and a 100Mbps fiber distributed d a t a interface (FDDI). Higher
speedsareneeded t o matchtheincreasing speed of thePCsandt o sup-
port diskless workstations and computer-aided design/manufacturing
(CAD/CAM) terminals t h a t rely on LANs for interconnection of file
servers. Also,thesuccessof LANshasled t o severalattemptst o extend
high-speed d a t a networkingbeyondthelocalpremises, acrossmetropoli-
t a n andwide areaenvironments.
1.3 LAN Technology
T h e typesoftechnologiesused t o implementLANs are as diverse as the
200orsoLAN vendors. Bothvendorsand users arecompelled t o make
a choice. Thischoiceisusuallybased onseveral criteria such as [5-101:
Simulation of Local Area Networks
Network topologyand architecture
Access control
Transmissionmedium
Transmissiontechnique
Adherence tostandards
Performance in terms of channel utilization,delay,power, and ef-
fectivetransmission ratio
Thefirstfourcriteriaarethemajortechnologicalissuesandareof great
concernwhendiscussingLANs. Theseissuesaresometimesinterrelated.
Forexample,someaccessmethodsareonlysuitableforsometopologies
or with certain transmission techniques.
1.3.1 Network Topology and Access Control
Thetopology of a network is theway in which thenodes (orstations)
areinterconnected. In spite of the proliferation of LAN products, the
vast majorityof LANs conform t o one of three topologies and one of
ahandful of medium-access control protocols summarized in Table 1.1.
Thebasicformsof thetopologies areshowninFigure 1.l.
Ring
Bus
Star
Figure 1.1 Local network topologies.
In thering topology,all nodes are connected together in a closed
loop. Information passes from node t o node on the loop and is regen-
erated (byrepeaters) at each node (called an activeinterface). A bus
topology uses a single, open-ended transmission medium. Each node
tapsintothe medium in away that does not disturb thesignalon the
Local Area Networks
5
bus(thusitiscalled apassiveinterface). Startopologyconsistsofacen-
tral controllingnode with star-like connections tovarious other nodes.
Although thering topologyis popular in Europe,the bus is themost
commontopologyinthe US.
Table 1.1 LAN Topology and Medium-Access Protocol.
1. Ringtopology
ControlledAccess
Token
Slotted
BufferlRegister Insertion
2. Bus topology
ControlledAccess
Token
Multilevel MultipleAccess(MLMA)
RandomAccess
Carrier Sense MultipleAccess (CSMA)
(l-persistent, p-persistent, nonpersistent)
Carrier Sense MultipleAccesswith
Collision Detection (CSMA/CD)
CSMA/CD with DynamicPriorities (CSMA/CD-CP)
CSMA/CD with Deterministic Contention
Resolution (CSMA/CD-DCR)
3. Startopology
4. Hybridtopology
Ring-star
Ring-bus
Bus-star
CSMA/CD-token ring
CSMA/CD-token bus
Both ring and bus topologies,lacking any central node,must use
somedistributedmechanismtodeterminewhichnodemayusethetrans-
mission medium atanygiven moment. Various flow control and access
strategieshave been proposed or developed for inserting and removing
messages from ring and bus LANs. The most popular ones are the
CarrierSenseMultipleAccesswithCollision Detection(CSMA/CD) for
broadcast buses andtokenpassing for rings andbuses.
InCSMA/CD,eachbusinterfaceunit (BIU),before attemptingto
transmit dataontothe channel,first listens or senses if the channel is
idle. An active BIU transmits its data only if the channel is sensed
idle. If thechannel issensedbusy,theBIU defers itstransmission until
6
Simulation of Local Area Networks
the bus becomes clear. In this contention-type access scheme, collision
occurswhen twoor more nodes attemptt o transmit a t thesametime.
Duringthecollision,thetwoormoremessagesbecomegarbled andlost.
Inthetoken-passing protocol, anemptyoridletoken (someunique
bit pattern or signal) is passed around theringor bus. Any node may
removethetoken,insert amessage,andappendthetoken. Whenanode
has d a t a t o transmit,it grabs the token, changes the token t o a busy
state (another bit pattern) and appends itspacket t o the busy token.
At the end of the transmission,the node issues another idletoken. A
node has channel access right only when itgets the idletoken. Figure
1.2showsthepacket formatfortoken bus andtoken ringtopologies.
>l
1
Preamble
Start
1
Control
2 or6
D~~~~~~
2or6
,"S:
>O
Data
4
FCS
1
Bytes
End
(a) Token Bus
(c) Token Format
Figure 1.2 Packetformatfortoken busandtoken ring.
Anumberofattemptshavebeenproposed t o develophybridtopolo-
gies and access techniques t h a t combine random and time-assignment
access.
Local Area Networks
1.3.2 T r a n s m i s s i o n M e d i a
Thetransmission mediumisthephysical pathconnecting thetransmit-
ter t o the receiver. Any physical medium that is capable of carrying
information in anelectromagneticformispotentialy suitableforuse on
aLAN.Inpractice,themediaused aretwisted-pair cable,coaxialcable,
andopticalfiber.
Twisted-pair cableisgenerallyusedforanalogsignalsbuthasbeen
successfullyusedfordigitaltransmissions. Itislimitedinspeedt o afew
megabitsandisoftensusceptiblet o noise. Ring,bus, andstarnetworks
can allusetwisted-pair cableasamedium.
Coaxial cable consists of a central conductor and a conducting
shield. Itprovides asubstantialperformanceimprovement overtwisted-
pair: ithashigher capacity,cansupportalargernumber of devices, and
canspangreater distances.
Optical fiber transmits light or infrared rays instead of electrical
signals. It demonstrateshigher capacity than coaxial cable and is not
susceptible t o noise or electrical fluctuations. Although there arestill
sometechnicaldifficultieswithopticalfiber,itmaywellbe themedium
choiceof futurenetworks.
1.3.3 T r a n s m i s s i o n Techniques
Therearetwotypesof transmissiontechniques: basebandsignalingand
broadband signaling. In spiteof the hot debateand controversy about
the merits of one technique over the other, it appears that the two
techniques will coexist for sometime,fillingdifferent needs.
Baseband signalingliterallymeansthatthesignalisnotmodulated
atall. Itistotallydigital. Theentirefrequencyspectrumisusedtoform
the signal, which is transmitted bidirectionally on broadcast systems
such asbuses. Baseband networks arelimitedin distance duet o signal
attenuation.
Broadband signaling is a technique by which information is fre-
quency modulated ontoanalogcarrier waves. Thisallowsvoice, video,
and datat o be carried simultaneously. Although it is more expensive
thanbasebandbecauseoftheneedformodemsateachnode,itprovides
larger capacity.
1.4 Standardization of LANs
Theincompatibilityof LAN productshasleft themarketsmallandun-
decided. One way t o increase the market size is t o develop standards
that can be used by the numerous LAN product manufacturers. The
most obvious advantage of standards is that they facilitate the inter-
change of databetween diverse devicesconnected t o LAN.Thedriving
force behind thestandardization efforts isthedesire by LAN users and
vendors t o have "open systems'' in which any standard computer de-
8
Simulation of Local Area Networks
vicewould be abletointeroperatewithothers. Attemptstostandardize
LAN topologies,protocols,andmodulation techniques have been made
by organizationssuch asthoseshown inTable 1.2.
Table 1.2 LAN StandardizationActivities
Organization
Standards
IS0
T C 97/SC6
CSMA/CD, token bus,
token ring,slottedring
SG 18
Connectionbetween ISDN and LAN
802
Logicallink control,CSMA/CD,
token bus,token ring,
metropolitanareanetwork
T C 24
CSMA/CD, token bus,token ring
CCITT
IEEE
ECMA
The International Standards Organization (ISO) is perhaps the
most prominent of these,anditisresponsiblefortheseven-layer model
of network architecture,initiallydeveloped for WANs, called therefer-
ence model for open systems interconnection (OSI).TheInternational
Telegraph and Telephone ConsultativeCommittee (CCITT),based in
Geneva,isapart oftheInternationalTelecommunications Union (ITU)
and is heavily involved in all aspects of datatransmission. It has pro-
duced standardsin Europe butnot intheUS. In the US theAmerican
National StandardInstitute (ANSI),theNational Bureau of Standards
(NBS),andtheIEEEareperhapsthemostimportantbodies. TheIEEE
802committeehasdeveloped LAN protocol standardsforthelowertwo
layers of the ISO's OS1 reference model, namely the physical and link
controllayers. Thephysical standard defineswhat manufacturers must
provideintermsofaccesstotheirhardwareforauserorsystemsintegra-
tor. More recently,theEuropean ComputerManufacturers Association
(ECMA)hasfollowedalongthesamelines. Althoughnetworkstandards
arebeing developed bythevariousorganizations,standardizationisstill
uptothemanufacturers. TheI S 0 protocolshavetheadvantageofinter-
national backing,and most manufacturers have madethe commitment
toimplementthemeventually.
1.5 LAN Architectures
Anetwork architectureis aspecificationofthesetoffunctionsrequired
for a user at a location tointeract with another user at another loca-
tion. These interconnect functions include determination of the start
and end of a message, recognition of a message address, management
of acommunication link,detection andrecovery of transmission errors,
Local Area Networks
9
andreliableandregulated delivery of data. Ageneralnetwork architec-
ture thus describes the interfaces, algorithms, and protocols by which
processes atdifferent locationsand/or a t heterogeneous machinescould
communicate. Althoughthearchitecturesdevelopedbydifferentvendors
arefunctionallyequivalent,theydonot provideforeasyinterconnection
of systemsof different make.
1.5.1 The OS1 Model
As mentioned intheprevious section,theneed for standardization and
compatibility at all levels has compelled the International Standards
Organization(ISO)toestablishageneralseven-layer hierarchical model
for universal intercomputer communication. This arhitecture, known
as the Open Systems Interconnection model (see Figure 1.3), defines
seven layersof communicationprotocols,withspecificfunctionsisolated
a t each level. TheOS1model is a reference model for theexchange of
informationamongsystemsthatareopent o oneanotherforthispurpose
by virtue of their mutual use of the applicable standards. Theseven
hierarchical layersarehardware andsoftwarefunctionalgroupingswith
specific well-defined tasks. The OS1model states thepurpose of each
layer anddescribestheservicesprovided byeach withinitslayerandto
theadjacent higher and lowerlayers. Detailsof theimplementation of
each layer of OS1model depend on thespecificsof theapplication and
thecharacteristics of thecommunicationchannel employed.
1.5.2 The Seven OS1 Layers
The application layer,level 7,istheonetheuser sees. It provides ser-
vices directly comprehensible toapplication programs: login,password
checks,network transparencyfor distribution of resources, fileanddoc-
ument transfer,andindustry-specific protocols.
Thepresentation layer,level 6, is concerned with interpretingthe
data.Itrestructuresdatatoorfromthestandardizedformatusedwithin
anetwork,textcompression,codeconversion,fileformatconversion,and
encryption.
Thesession layer,level 5,manages addresstranslation and access
security. It negotiates t o establish a connection with another node on
thenetwork and then t o managethedialogue. Thismeans controlling
thestarting,stopping,andsynchronizationoftheconversion.
Thetransportlayer,level4,performserror control,sequencecheck-
ing,handling of duplicatepackets,flowcontrol,andmultiplexing. Here
it is determined whether the channel is t o be point-to-point (virtual)
with ordered messages, isolated messages with no order, or broadcast
messages. It is the last of the layers concerned with communications
between peer entities in the systems. The transport layer and those
above are referred t o as the upper layers of the model, and they are
Simulation of Local Area Networks
10
independent of theunderlyingnetwork. Thelowerlayersareconcerned
with datatransfer acrossthenetwork.
Thenetwork layer,level 3, providesaconnectionpathbetween sys-
tems,includingthecasewhere intermediatenodesareinvolved. Itdeals
with message packetization, message routing for datatransfer between
nonadjacent nodes orstations,congestion control,and accounting.
7
Application
1
Presentation
5
Session
Transport
3
Network
Data Link
Physical
Figure 1.3 Relationshipbetween theOS1modelandIEEELAN layers.
The data link layer, level 2, establishes the transmission protocol,
howinformationwillbetransmitted,acknowledgmentofmessages,token
possession, error detection, and sequencing. It prepares the packets
passed downfromthenetwork layerfortransmissiononthenetwork. It
takes a raw transmission and transforms it into a linefree from error.
Hereheaders andframinginformationareaddedorremoved. Withthese
gothetimingsignals,check-sum, andstation addresses,aswell as the
controlsystemfor access.
Thephysical layer, level 1,is that part that actuallytouches the
transmissionmediumor cable;thelineisthepoint within thenode or
device where thedataisreceived andtransmitted. Itensures thatones
arrive as ones and zeros as zeros. It encodes and physically transfers
messages (rawbits in a stream)between adjacent stations. It handles
voltages, frequencies, direction, pin numbers, modulation techniques,
signaling schemes, ground loop prevention, and collision detection in
theCSMA/CD accessmethod.
1.5.3 The IEEE Model for LANs
TheIEEE has formulated standards for the physical and logical link
layers for three types of LANs, namely, token buses, token rings, and
Local Area Networks
11
CSMA/CD protocols. Figure1.3illustratesthecorrespondence between
the three layers of the OS1 and the IEEE 802 reference models. The
physical layer specifiesmeansfortransmittingandreceivingbits across
various types of media. The media-access-control layer performs the
functionsneeded t o control accesst o thephysicalmedium. Thelogical-
link-control layer isthecommon interface t o thehigher softwarelayers.
1.6 Performance Evaluation
Therehasbeen muchresearch intotheperformanceof thevariousmedi-
umaccessprotocols [5,6,11,121.LANsbeingcomplexsystems,model-
ingof themmust be done atvarious levelsof detail. Inthissection,we
present simpleperformancemodels of LANs. Theperformance is mea-
sured in termsof channel utilization, delay,power, and effectivetrans-
mission ratio. More rigorous performancemodels of LANs are covered
in thenext chapter.
1.6.1 Channel Utilization
A performance yardstick is the maximumthroughput achievablefor a
given channel capacity. For example, how many megabits per second
(Mbps)of datacan actuallybetransmittedfor achannel capacityof 10
Mbps? Itiscertain thatafractionof thechannel capacityisused up in
formof overhead-acknowledgments, retransmission,token delay,etc.
Channel capacity is the maximum possible data rate, that is,the
signaling rate on the physical channel. It is also known as the data
rate or transmission rate and will be denoted by R in bits per second.
Throughput S istheamountof "user data" thatiscarried by theLAN.
Channelutilization U istheratioofthroughputt o channelcapacity-i.e.
Itisindependent of themediumaccesscontrol. Itisobviousthat U = 1
in anidealsituation.
In analyzing LAN performance, the two most useful parameters
arethechannel capacityor datarateR of themediumandtheaverage
maximumsignalpropagation delayP . Theirproduct ( R P , inbits)isthe
numberofbitsthatcanexistinthechannelbetweentwonodesseparated
by the maximum distance determined by the propagation delay. We
definetheratio
Lengthof datapath - R P
Length of packet
PL
where PLis thepacket length in bits. Thequantity ais a normalized
nondimensional measure used in determining the upper bound on uti-
lization;itsreciprocaliscalledtheeffectivetransmissionratio. Realizing
that P L / R isthetimeneeded t o transmita packet,
a=
Simulation of Local Area Networks
P - Propagationtime
P L / R Transmissiontime
a=-
(1.3)
If thethroughput Sisdefined astheactualnumber of bitstransmitted
per second,then
S=
Lengthof packet
Transmission time Propagationtime
+
Substituting(1.4)into(1.1)gives
Thusutilizationisinverselyrelatedtoa . Typicalvaluesof a rangefrom
0.01to0.1accordingtoStallings[13].Theidealcaseoccurs when there
isnooverheadand a = 0. Theratio a cannowbeusedtodefinechannel
utilization bounds for amediumaccessprotocol. Fortoken ring or bus,
whereTl isthepackettransmissionperiodandT2isatokentransmission
period. It can be shown that [6,131
where N isthenumber of active nodes orstations. In atoken bus,the
optimalcaseoccurswhen thelogicalorderingof nodes (thesequenceof
nodes through which thetoken ispassed, asshown inFigure 2.4) isthe
sameasthephysicalorder. In thiscase, Eq. (1.7)applies. Intheworst
case,thelogicalorderingof nodesforcesthepropagation delaybetween
nodes toapproach theend-to-end delay. Forthiscase,Tz = a and
Local Area Networks
1.6.2 Delay, P o w e r , a n d EffectiveT r a n s m i s s i o n R a t i o
Packet delayistheperiod of timebetween themomentatwhich anode
becomes active (i.e.,when it has data t o transmit) and the moment
atwhich the packet issuccessfully transmitted. Throughput delay de-
scribes