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Part IV

Local Area Networks

(LANs)

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Classification TerminologyClassification Terminology

Network technologies classified into three broad categories

Local Area Network (LAN)Metropolitan Area Network (MAN)Wide Area Network (WAN)

LAN and WAN most widely deployed

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The Local Area Network (LAN)

The Local Area Network (LAN)

Engineering classificationExtremely popular (most networks are LANs)Many LAN technologies exist

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Key Features of a LANKey Features of a LAN

High throughputRelatively low costLimited to short distanceOften rely on shared media

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Scientific Justification ForLocal Area Networks

Scientific Justification ForLocal Area Networks

A computer is more likely to communicate with computers that are nearby than with computers that are distant

Known as the locality principle

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TopologyTopology

Mathematical termRoughly interpreted as “geometry for curved

surfaces”

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Network TopologyNetwork Topology

Specifies general “shape” of a networkHandful of broad categoriesOften applied to LANPrimarily refers to interconnectionsHides details of actual devices

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Star TopologyStar Topology

Central component of network known as hubEach computer has separate connection to

hub

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Ring TopologyRing Topology

No central facilityConnections go directly from one computer

to another

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Bus TopologyBus Topology

Shared medium forms main interconnectEach computer has a connection to the

medium

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Example Bus Network: Ethernet

Example Bus Network: Ethernet

Most popular LANWidely usedIEEE standard 802.3Several generations

Same frame formatDifferent data ratesDifferent wiring schemes

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Shared Medium in a LANShared Medium in a LAN

Shared medium used for all transmissionsOnly one station transmits at any timeStations “take turns” using mediumMedia Access Control (MAC) policy ensures

fairness

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Illustration of Ethernet Transmission

Illustration of Ethernet Transmission

Only one station transmits at any timeSignal propagates across entire cableAll stations receive transmissionCSMA/CD media access scheme

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CSMA/CD ParadigmCSMA/CD Paradigm

Multiple Access (MA)Multiple computers attach to shared mediaEach uses same access algorithm

Carrier Sense (CS)Wait until medium idleBegin to transmit frame

Simultaneous transmission possible

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CSMA/CD Paradigm(continued)

CSMA/CD Paradigm(continued)

Two simultaneous transmissionsInterfere with one anotherCalled collision

CSMA plus Collision Detection (CD)Listen to medium during transmissionDetect whether another station’s signal interferesBack off from interference and try again

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Backoff After CollisionBackoff After Collision

When collision occursWait random time t1, 0 < t1 < dUse CSMA and try again

If second collision occursWait random time t2, 0 < t2 < 2*d

Double range for each successive collisionCalled exponential backoff

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Media Access on a Wireless Net

Media Access on a Wireless Net

Limited rangeNot all stations receive all transmissionsCannot use CSMA/CD

Example in diagramMaximum transmission distance is dStations 1 and 3 do not receive each other’s

transmissions

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CSMA/CACSMA/CA

Used on wireless networksBoth sides send small message followed by data

transmission“X is about to send to Y”“Y is about to receive from X”Data from sent from X to Y

Purpose: inform all stations in range of X or Y before transmission

Known as Collision Avoidance (CA)

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Identifying a DestinationIdentifying a Destination

All stations on shared-media LAN receive all transmissions

To allow sender to specify destinationEach station assigned unique numberKnown as station’s addressEach frame contains address of intended

recipient

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Assignment of physical address

Assignment of physical address

The stations may get their address in different ways:

StaticConfigurableDynamic

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Ethernet AddressingEthernet Addressing

Standardized by IEEEEach station assigned by unique 48-bit

addresse.g. 00:30:65:52:2E:96 in hexadecimal form

Address assigned when network interface card (NIC) manufactured (In most cases)

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Ethernet Address Recognition

Ethernet Address Recognition

Each frame contains destination addressAll stations receive a transmissionStation discards any frame addresses to

another stationImportant: interface hardware, not software,

checks address

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Possible DestinationsPossible Destinations

Packet can be sent to:Single destination (unicast)All stations on network (broadcast)Subset of stations (multicast)

Address used to distinguish

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Advantages of Address Alternatives

Advantages of Address Alternatives

UnicastEfficient for interaction between two computers

BroadcastEfficient for transmitting to all computers

MulticastEfficient for transmitting to a subset of

computers

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Broadcast on EthernetBroadcast on Ethernet

All 1s address specifies broadcast(FF:FF:FF:FF:FF:FF in hexcode)

SenderPlaces broadcast address in frameTransmits one copy on shared networkAll stations receive copy

Receive always accepts frame that contains this address

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MulticastMulticast

Half of addresses reserved for multicastNetwork interface card

Always accepts unicast and broadcastCan accept zero or more multicast addresses

SoftwareDetermines multicast address to acceptInforms network interface card

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Promiscuous ModePromiscuous Mode

Designed for testing / debuggingAllows interface to accept all packetsAvailable on most interface hardware

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Identifying Frame Contents

Identifying Frame Contents

Integer type field tells recipient the type of data being carried

Two possibilitiesSelf-identifying or explicit type (hardware record

type)Implicit type (application sending data must

handle type)

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Conceptual Frame FormatConceptual Frame Format

HeaderContains address and type informationLayout fixed

PayloadContains data being sent

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Illustration Of Ethernet Frame

Illustration Of Ethernet Frame

Sender placesSender’s address in sourceRecipient’s address in destinationType of data in frame typeCyclic redundancy check in CRC

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Example Ethernet TypesExample Ethernet Types

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When Network HardwareDoes Not Include Types

When Network HardwareDoes Not Include Types

Sending and receiving computers must agreeTo only send one type of dataTo put type information in first few octets of

payloadMost systems need type information

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Illustration of TypeInformation Added to Data

Illustration of TypeInformation Added to Data

In practiceType information small compared to data carriedFormat of type information standardized

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A Standard For Type Information

A Standard For Type Information

Defined by IEEEUsed when hardware does not include type fieldCalled LLC / SNAP header

Logical Link ControlSubNetwork Attachment Point

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Demultiplexing On TypeDemultiplexing On Type

Network interface hardwareReceives copy of each transmitted frameExamines address and either discards or acceptsPasses accepted frame to system software

Network device softwareExamines frame typePasses frame to correct software module

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Network AnalyzerNetwork Analyzer

Device used for testing and maintenanceListens in promiscuous modeProduces

Summaries (e.g., % of broadcast frames)Specific items (e.g., frames from a given address)

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Ethernet WiringEthernet Wiring

Three schemes Correspond to three generationsAll use same frame format

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Original Ethernet WiringOriginal Ethernet Wiring

Used heavy coaxial cableFormal name 10Base5Called thicknet

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Second Generation Ethernet Wiring

Second Generation Ethernet Wiring

Used thinner coaxial cableFormal name 10Base2Called thinnet

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Modern Ethernet WiringModern Ethernet Wiring

Uses a hubFormal name 10Base-TCalled twisted pair Ethernet

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Ethernet Wiring In An Office

Ethernet Wiring In An Office

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A Note About Ethernet Topology

A Note About Ethernet Topology

ApparentlyOriginal Ethernet used bus topologyModern Ethernet uses star topology

In fact, modern Ethernet isPhysical starLogical busCalled star-shaped bus

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Higher Speed EthernetsHigher Speed Ethernets

Fast EthernetOperates at 100 MbpsFormally 100Base-TTwo wiring standards10/100 Ethernet devices available

Gigabit EthernetOperates at 1000 Mbps (1 Gbps)Slightly more expensive

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Another LAN Using Bus Topology

Another LAN Using Bus Topology

LocalTalkDeveloped by Apple Corp.Simple to useSlow by current standards

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Illustration Of LocalTalkIllustration Of LocalTalk

Transceiver required per stationTransceiver terminates cable

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Ring TopologyRing Topology

Second most popular LAN topologyBits flow in single directionSeveral technologies exist

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Token PassingToken Passing

Used with ring topologyGuarantees fair accessToken

Special (reserved) messageSmall (a few bits)

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Token Passing ParadigmToken Passing Paradigm

StationWaits for the token to arriveTransmits one packet around ringTransmits token around ring

When no station has data to sendToken circulates continuously

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Token Passing Ring Transmission

Token Passing Ring Transmission

Station waits for token before sendingSignal travels around entire ringSender receives its own transmission

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Strengths of Token Ring Approach

Strengths of Token Ring Approach

Easy detection ofBroken ringHardware failuresInterference

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Weaknesses of Token Ring Approach

Weaknesses of Token Ring Approach

Broken wire disables entire ringPoint-to-point wiring

Awkward in office environmentDifficult to add / move stations

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Token Passing Ring Technologies

Token Passing Ring Technologies

ProNet-10Operated at 10 Mbps

IBM Token RingOriginally operated at 4 MbpsLater version operated at 16 Mbps

Fiber Distributed Data Interconnect (FDDI)Operated at 100 Mbps

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FDDI TerminologyFDDI TerminologyFDDI

Uses optical fibersHigh reliabilityImmune to interference

CDDIFDDI over copperSame frame formatSame data rateLess noise immunity

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FDDI Hub TechnologyFDDI Hub Technology

Part of FDDI standardStations attach to hubSame frame format and data rate as FDDICalled star-shaped ring

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FDDI Failure RecoveryFDDI Failure Recovery

Uses two ringsAutomatic failure recoveryTerminology

Dual-attachedCounter rotatingSelf healing

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Illustration of FDDIFailure Recovery

Illustration of FDDIFailure Recovery

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Another Example of aPhysical Star TopologyAnother Example of aPhysical Star Topology

Asynchronous Transfer Mode (ATM)Designed by telephone companiesIntended to accommodate

VoiceVideoData

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ATMATM

Building block known as ATM switchEach station connects to switchSwitches can be interconnected

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Details of ATM ConnectionDetails of ATM Connection

Full-duplex connectionsTwo fibers required

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ATM CharacteristicsATM Characteristics

High data rates (e.g. 155 Mbps)Fixed size packets

Called cellsImportant for voice

Cell size is 53 octets48 octets of data5 octets of header

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SummarySummary

Local Area NetworksDesigned for short distanceUse shared mediaMany technologies exist

Topology refers to general shapeBusRingStar

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Summary (continued)Summary (continued)

AddressUnique number assigned to stationPut in frame headerRecognized by hardware

Address formsUnicastBroadcastMulticast

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Summary (continued)Summary (continued)

Type informationDescribes data in frameSet by senderExamined by receiver

Frame formatHeader contains address and type informationPayload contains data being sent

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Summary (continued)Summary (continued)

LAN technologiesEthernet (bus)IBM Token RingFDDI (ring)ATM (star)

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Summary (continued)Summary (continued)

Wiring and topologyCan distinguish

Logical topologyPhysical topology (wiring)

Hub allows Star-shaped busStar-shaped ring