Data and Computer Data and Computer CommunicationsCommunications

Eighth EditionEighth Editionby William Stallingsby William Stallings

Lecture slides by Lawrie BrownLecture slides by Lawrie Brown

Chapter 15 – Local Area NetworkChapter 15 – Local Area Network OverviewOverview

Local Area NetworkLocal Area Network Overview Overview

The whole of this operation is described in The whole of this operation is described in minute detail in the official British Naval minute detail in the official British Naval History, and should be studied with its excellent History, and should be studied with its excellent charts by those who are interested in its charts by those who are interested in its technical aspect. So complicated is the full story technical aspect. So complicated is the full story that the lay reader cannot see the wood for the that the lay reader cannot see the wood for the trees. I have endeavored to render intelligible trees. I have endeavored to render intelligible the broad effects.the broad effects.—The World Crisis, Winston Churchill

LAN Applications (1)LAN Applications (1) personal computer LANspersonal computer LANs

low costlow cost limited data ratelimited data rate

back end networksback end networks interconnecting large systems (mainframes and large interconnecting large systems (mainframes and large

storage devices)storage devices)• high data ratehigh data rate• high speed interfacehigh speed interface• distributed accessdistributed access• limited distancelimited distance• limited number of deviceslimited number of devices

LAN Applications (2)LAN Applications (2) storage area networks (SANs)storage area networks (SANs)

separate network handling storage needsseparate network handling storage needs detaches storage tasks from specific serversdetaches storage tasks from specific servers shared storage facilityshared storage facility

• eg. hard disks, tape libraries, CD arrayseg. hard disks, tape libraries, CD arrays accessed using a high-speed networkaccessed using a high-speed network

• eg. Fibre Channeleg. Fibre Channel improved client-server storage accessimproved client-server storage access direct storage to storage communication for backupdirect storage to storage communication for backup

Storage Area NetworksStorage Area Networks

LAN Applications (3)LAN Applications (3) high speed office networkshigh speed office networks

desktop image processingdesktop image processing high capacity local storagehigh capacity local storage

backbone LANsbackbone LANs interconnect low speed local LANsinterconnect low speed local LANs reliabilityreliability capacitycapacity costcost

LAN ArchitectureLAN Architecture topologiestopologies transmission mediumtransmission medium layoutlayout medium access controlmedium access control

LAN TopologiesLAN Topologies

Bus and TreeBus and Tree used with multipoint mediumused with multipoint medium transmission propagates throughout medium transmission propagates throughout medium heard by all stationsheard by all stations full duplex connection between station and tapfull duplex connection between station and tap

allows for transmission and receptionallows for transmission and reception need to regulate transmissionneed to regulate transmission

to avoid collisions and hoggingto avoid collisions and hogging terminator absorbs frames at end of mediumterminator absorbs frames at end of medium tree a generalization of bustree a generalization of bus headend connected to branching cablesheadend connected to branching cables

Frame Frame TransmissionTransmissiononon Bus LAN Bus LAN

Ring TopologyRing Topology a closed loop of repeaters joined by point to point a closed loop of repeaters joined by point to point

linkslinks receive data on one link & retransmit on anotherreceive data on one link & retransmit on another

links unidirectionallinks unidirectional stations attach to repeatersstations attach to repeaters

data in framesdata in frames circulate past all stationscirculate past all stations destination recognizes address and copies framedestination recognizes address and copies frame frame circulates back to source where it is removedframe circulates back to source where it is removed

media access control determines when a station media access control determines when a station can insert framecan insert frame

Frame Frame TransmissionTransmission

Ring LANRing LAN

Star TopologyStar Topology each station connects to central nodeeach station connects to central node

usually via two point to point linksusually via two point to point links either central node can broadcasteither central node can broadcast

physical star, logical busphysical star, logical bus only one station can transmit at a timeonly one station can transmit at a time

or central node can act as frame switchor central node can act as frame switch

Choice of TopologyChoice of Topology reliabilityreliability expandabilityexpandability performanceperformance needs considering in context of:needs considering in context of:

mediummedium wiring layoutwiring layout access controlaccess control

Bus LAN Bus LAN Transmission Media (1)Transmission Media (1)

twisted pairtwisted pair early LANs used voice grade cableearly LANs used voice grade cable didn’t scale for fast LANsdidn’t scale for fast LANs not used in bus LANs nownot used in bus LANs now

baseband coaxial cablebaseband coaxial cable uses digital signallinguses digital signalling original Ethernetoriginal Ethernet

Bus LAN Bus LAN Transmission Media (2)Transmission Media (2)

broadband coaxial cablebroadband coaxial cable as in cable TV systemsas in cable TV systems analog signals at radio frequenciesanalog signals at radio frequencies expensive, hard to install and maintainexpensive, hard to install and maintain no longer used in LANsno longer used in LANs

optical fiberoptical fiber expensive tapsexpensive taps better alternatives availablebetter alternatives available not used in bus LANsnot used in bus LANs

less convenient compared to star topology twisted pairless convenient compared to star topology twisted pair coaxial baseband still used but not often in new coaxial baseband still used but not often in new

installationsinstallations

Ring and Star UsageRing and Star Usage ringring

very high speed links over long distancesvery high speed links over long distances single link or repeater failure disables networksingle link or repeater failure disables network

starstar uses natural layout of wiring in buildinguses natural layout of wiring in building best for short distancesbest for short distances high data rates for small number of deviceshigh data rates for small number of devices

Choice of MediumChoice of Medium constrained by LAN topologyconstrained by LAN topology capacitycapacity reliabilityreliability types of data supportedtypes of data supported environmental scopeenvironmental scope

Media AvailableMedia Available Voice grade unshielded twisted pair (UTP)Voice grade unshielded twisted pair (UTP)

Cat 3 phone, cheap, low data ratesCat 3 phone, cheap, low data rates Shielded twisted pair / baseband coaxialShielded twisted pair / baseband coaxial

more expensive, higher data ratesmore expensive, higher data rates Broadband cableBroadband cable

even more expensive, higher data rateeven more expensive, higher data rate High performance UTPHigh performance UTP

Cat 5+, very high data rates, witched star topology Cat 5+, very high data rates, witched star topology Optical fibreOptical fibre

security, high capacity, small size, high costsecurity, high capacity, small size, high cost

LAN Protocol ArchitectureLAN Protocol Architecture

IEEE 802 Layers (1)IEEE 802 Layers (1) PhysicalPhysical

encoding/decoding of signalsencoding/decoding of signals preamble generation/removalpreamble generation/removal bit transmission/receptionbit transmission/reception transmission medium and topologytransmission medium and topology

IEEE 802 Layers (2)IEEE 802 Layers (2) Logical Link Control Logical Link Control

interface to higher levelsinterface to higher levels flow and error controlflow and error control

MediaMedia Access Control Access Control on transmit assemble data into frame on transmit assemble data into frame on receive disassemble frameon receive disassemble frame govern access to transmission mediumgovern access to transmission medium for same LLC, may have several MAC optionsfor same LLC, may have several MAC options

LAN Protocols in ContextLAN Protocols in Context

Logical Link Control Logical Link Control transmission of link level PDUs between transmission of link level PDUs between

stationsstations must support multiaccess, shared mediummust support multiaccess, shared medium but MAC layer handles link access detailsbut MAC layer handles link access details addressing involves specifying source and addressing involves specifying source and

destination LLC usersdestination LLC users referred to as service access points (SAP)referred to as service access points (SAP) typically higher level protocoltypically higher level protocol

LLC ServicesLLC Services based on HDLCbased on HDLC unacknowledged connectionless serviceunacknowledged connectionless service connection mode serviceconnection mode service acknowledged connectionless serviceacknowledged connectionless service

LLC ProtocolLLC Protocol modeled after HDLCmodeled after HDLC asynchronous balanced mode asynchronous balanced mode

connection mode (type 2) LLC service connection mode (type 2) LLC service unacknowledged connectionless serviceunacknowledged connectionless service

using unnumbered information PDUs (type 1) using unnumbered information PDUs (type 1) acknowledged connectionless serviceacknowledged connectionless service

using 2 new unnumbered PDUs (type 3)using 2 new unnumbered PDUs (type 3) permits multiplexing using LSAPspermits multiplexing using LSAPs

MACMAC Frame Format Frame Format

Media Access ControlMedia Access Control wherewhere

centralcentral• greater control, single point of failuregreater control, single point of failure

distributeddistributed• more complex, but more redundantmore complex, but more redundant

howhow synchronoussynchronous

• capacity dedicated to connection, not optimalcapacity dedicated to connection, not optimal asynchronousasynchronous

• in response to demandin response to demand

Asynchronous SystemsAsynchronous Systems round robinround robin

each station given turn to transmit dataeach station given turn to transmit data reservationreservation

divide medium into slotsdivide medium into slots good for stream trafficgood for stream traffic

contentioncontention all stations contend for timeall stations contend for time good for bursty trafficgood for bursty traffic simple to implementsimple to implement tends to collapse under heavy loadtends to collapse under heavy load

MAC Frame HandlingMAC Frame Handling MAC layer receives data from LLC layerMAC layer receives data from LLC layer fieldsfields

MAC controlMAC control destination MAC addressdestination MAC address source MAC addresssource MAC address LLCLLC CRCCRC

MAC layer detects errors and discards framesMAC layer detects errors and discards frames LLC optionally retransmits unsuccessful framesLLC optionally retransmits unsuccessful frames

BridgesBridges connects similar LANsconnects similar LANs identical physical / link layer protocolsidentical physical / link layer protocols minimal processingminimal processing can map between MAC formatscan map between MAC formats reasons for usereasons for use

reliabilityreliability performanceperformance securitysecurity geographygeography

Bridge FunctionBridge Function

Bridge Design AspectsBridge Design Aspects no modification to frame content or formatno modification to frame content or format no encapsulationno encapsulation exact bitwise copy of frameexact bitwise copy of frame minimal buffering to meet peak demandminimal buffering to meet peak demand contains routing and address intelligencecontains routing and address intelligence may connect more than two LANsmay connect more than two LANs bridging is transparent to stationsbridging is transparent to stations

Bridge Protocol ArchitectureBridge Protocol Architecture IEEE 802.1DIEEE 802.1D MAC level MAC level bridge does not need LLC layerbridge does not need LLC layer can pass frame over external comms systemcan pass frame over external comms system

capture framecapture frame encapsulate itencapsulate it forward it across linkforward it across link remove encapsulation and forward over LAN linkremove encapsulation and forward over LAN link e.g. WAN linke.g. WAN link

Connection of Two LANsConnection of Two LANs

Bridges andBridges and

LANsLANs with withAlternativeAlternative

RoutesRoutes

Fixed RoutingFixed Routing complex large LANs need alternative routescomplex large LANs need alternative routes

for load balancing and fault tolerancefor load balancing and fault tolerance bridge must decide whether to forward framebridge must decide whether to forward frame bridge must decide LAN to forward frame tobridge must decide LAN to forward frame to can use fixed routing for each source-destination can use fixed routing for each source-destination

pair of LANspair of LANs done in configurationdone in configuration usually least hop routeusually least hop route only changed when topology changesonly changed when topology changes widely used but limited flexibilitywidely used but limited flexibility

Spanning TreeSpanning Tree bridge automatically develops routing tablebridge automatically develops routing table automatically updates routing table in automatically updates routing table in

response to changesresponse to changes three mechanisms:three mechanisms:

frame forwardingframe forwarding address learningaddress learning loop resolutionloop resolution

Frame ForwardingFrame Forwarding maintain forwarding database for each portmaintain forwarding database for each port

lists station addresses reached through each portlists station addresses reached through each port for a frame arriving on port X:for a frame arriving on port X:

search forwarding database to see if MAC address is search forwarding database to see if MAC address is listed for any port except Xlisted for any port except X

if address not found, forward to all ports except Xif address not found, forward to all ports except X if address listed for port Y, check port Y for blocking if address listed for port Y, check port Y for blocking

or forwarding stateor forwarding state if not blocked, transmit frame through port Yif not blocked, transmit frame through port Y

Address LearningAddress Learning can preload forwarding databasecan preload forwarding database when frame arrives at port X, it has come form the when frame arrives at port X, it has come form the

LAN attached to port XLAN attached to port X use source address to update forwarding use source address to update forwarding

database for port X to include that addressdatabase for port X to include that address have a timer on each entry in databasehave a timer on each entry in database if timer expires, entry is removedif timer expires, entry is removed each time frame arrives, source address checked each time frame arrives, source address checked

against forwarding databaseagainst forwarding database if present timer is reset and direction recordedif present timer is reset and direction recorded if not present entry is created and timer setif not present entry is created and timer set

Spanning Tree AlgorithmSpanning Tree Algorithm address learning works for tree layoutaddress learning works for tree layout in general graph have loopsin general graph have loops for any connected graph there is a spanning tree for any connected graph there is a spanning tree

maintaining connectivity with no closed loopsmaintaining connectivity with no closed loops IEEE 802.1 Spanning Tree Algorithm finds thisIEEE 802.1 Spanning Tree Algorithm finds this

each bridge assigned unique identifiereach bridge assigned unique identifier exchange info between bridges to find spanning treeexchange info between bridges to find spanning tree automatically updated whenever topology changesautomatically updated whenever topology changes

Loop of BridgesLoop of Bridges

Interconnecting LANs - HubsInterconnecting LANs - Hubs activeactive central element of star layout central element of star layout each station connected to hub by two UTP lineseach station connected to hub by two UTP lines hubhub acts as a repeater acts as a repeater limited limited to about 100 m by UTP propertiesto about 100 m by UTP properties opticaloptical fiber may be used out to 500m fiber may be used out to 500m physicallyphysically star, logically bus star, logically bus transmissiontransmission from a station seen by all others from a station seen by all others ifif two stations transmit at the same time two stations transmit at the same time have a have a

collisioncollision

Two Level Hub TopologyTwo Level Hub Topology

Buses, Hubs and SwitchesBuses, Hubs and Switches bus bus configurationconfiguration

allall stations share capacity of bus stations share capacity of bus (e.g. 10Mbps) (e.g. 10Mbps) only one station transmitting at a timeonly one station transmitting at a time

hubhub uses star wiring to attach stations uses star wiring to attach stations transmissiontransmission from any station received by hub and from any station received by hub and

retransmitted on all outgoing linesretransmitted on all outgoing lines onlyonly one station can transmit at a time one station can transmit at a time total total capacity of LAN is 10 Mbpscapacity of LAN is 10 Mbps

can improvecan improve performance using a layer 2 switch performance using a layer 2 switch can switch multiple frames between separate portscan switch multiple frames between separate ports multiplying capacity of LANmultiplying capacity of LAN

Shared Shared Medium Medium Bus and Bus and

HubHub

Layer 2 Switch BenefitsLayer 2 Switch Benefits no changeno change to attached devices to convert bus to attached devices to convert bus

LAN or hub LAN to switched LANLAN or hub LAN to switched LAN e.g. e.g. Ethernet LANs use Ethernet Ethernet LANs use Ethernet MAC MAC protocol protocol

have dedicated capacity equal to original LAhave dedicated capacity equal to original LANN assumingassuming switch has sufficient capacity to keep up switch has sufficient capacity to keep up

with all deviceswith all devices scales easilyscales easily

additional devices attached to switch by increasing additional devices attached to switch by increasing capacity of layer 2 capacity of layer 2

Types of Layer 2 SwitchTypes of Layer 2 Switch store-and-forward switchstore-and-forward switch

accepts accepts frame on input lineframe on input line, buffers, buffers briefly, routes to briefly, routes to destination portdestination port

see delaysee delay between sender and receiver between sender and receiver better integritybetter integrity

cut-through switchcut-through switch useuse destination address at beginning of frame destination address at beginning of frame switchswitch begins repeating frame onto output line as begins repeating frame onto output line as

soon assoon as destination address recognizeddestination address recognized highesthighest possible throughput possible throughput risk risk of propagating bad framesof propagating bad frames

Layer 2 Switch vs BridgeLayer 2 Switch vs Bridge LayerLayer 2 switch can be viewed as full-duplex hub 2 switch can be viewed as full-duplex hub incorporates logic to function as multiport bridgeincorporates logic to function as multiport bridge differences between switches & bridges:differences between switches & bridges:

bridge frame handling done in softwarebridge frame handling done in software switchswitch performs frame forwarding in hardware performs frame forwarding in hardware bridge bridge analyzeanalyzess and forward and forwardss one frame at a time one frame at a time switchswitch cancan handle multiple frames at a time handle multiple frames at a time bridgebridge uses store-and-forward operation uses store-and-forward operation switch canswitch can have cut-through have cut-through operationoperation

hence bridgehence bridge have suffered commercially have suffered commercially

Layer 2 Switch ProblemsLayer 2 Switch Problems broadcast broadcast overloadoverload

users share common MAC broadcast addressusers share common MAC broadcast address broadcast broadcast frameframes are delivered to all devices s are delivered to all devices

connected by layer 2 switches and/or bridgesconnected by layer 2 switches and/or bridges broadcast frames can create broadcast frames can create big big overheadoverhead broadcast stormbroadcast storm from malfunctioning from malfunctioning devicesdevices

lack lack of multiple linksof multiple links limits performance & reliabilitylimits performance & reliability

Router ProblemsRouter Problems typically use subnetworks connected by routerstypically use subnetworks connected by routers

limits broadcasts to single subnetlimits broadcasts to single subnet supports multiple paths between subnetsupports multiple paths between subnet

routersrouters do all IP-level processing in software do all IP-level processing in software high-speed LANs and high-performance layer 2 high-speed LANs and high-performance layer 2

switches pump millions of packets per secondswitches pump millions of packets per second softwaresoftware-based router only able to handle well under -based router only able to handle well under

a million packets per seconda million packets per second

Layer 3 SwitchesLayer 3 Switches Solution:Solution: layer 3 switches layer 3 switches

implement implement packet-forwarding logic of router in packet-forwarding logic of router in hardwarehardware

twotwo categories categories packet packet by packet by packet flow flow basedbased

Packet by Packet or Packet by Packet or Flow BasedFlow Based

packet by packetpacket by packet operates like aoperates like a traditional router traditional router order order of magnitude increase in performance of magnitude increase in performance

compared to software-based routercompared to software-based router flowflow-based switch -based switch

enhances performance by identifying flows of enhances performance by identifying flows of IP packetsIP packets with same with same source and destination source and destination

by observing ongoing traffic or using a special by observing ongoing traffic or using a special flow label in packet header flow label in packet header ((IPv6IPv6))

a predefined route is used for identified flowsa predefined route is used for identified flows

Typical Typical Large Large LAN LAN

OrganizationOrganizationDiagramDiagram

SummarySummary LAN topologies and mediaLAN topologies and media LAN protocol architectureLAN protocol architecture bridges, hubs, layer 2 & 3 switchesbridges, hubs, layer 2 & 3 switches