Presentation on theme: "William Stallings Data and Computer Communications 7th Edition"— Presentation transcript:
1William Stallings Data and Computer Communications 7th Edition Chapter 15Local Area Network Overview
2General application areas of LANs Storage Area NetworksSeparate network to handle storage needsDetaches storage tasks from specific serversShared storage facility across high-speed networkHard disks, tape libraries, CD arraysMost SANs use fibre channelHigh-speed office networksDesktop image processors have increased data flow (e.g. graphics programs)High capacity local storage (tremendous load)
3General application areas of LANs Backbone LANs: interconnect several local LANs within a single large building or a cluster of buildings.Drawbacks of a single LAN strategy:Reliability: a service interruption may result in a major disruption for users.Capacity: a single LAN could be saturated as the number of devices grows.Cost: very-low-cost attachments may not be suitable for all requirements.
5LAN Architecture The key elements of a LAN are: Topologies: the way in which stations attached to the network are connected;Tree, Bus, Ring and Star.Transmission medium: STP, UTP, Coaxial or Optical Fiber.Layout: Design, Plan and Operation.Medium access control (Protocol)
7Frame Transmission on Bus LAN Stations attach to the bus through a tap.A tap: is a hardware interfacing which allows full-duplex operation.Transmission from any station propagates the length of the bus in both directions.
8Bus and Tree Both use multipoint medium For the Bus all stations attach directly to a linear transmission medium, or bus.Tree topology is a generalization of the bus medium; tree layout begins at a point known as the headend.Full duplex connection between station and tap (hardware interfacing), allows for transmission and reception.Transmission propagates throughout medium.Terminator absorbs frames at end of medium.
9Bus and Tree (cont.)Two problems present in the arrangement of the bus and tree topologies:Transmission from any one station can be heard by all stationsNeed to identify target station (Each station has unique address)A mechanism is needed to regulate transmissionTo avoid collisions, (where two stations attempt to transmit at the same time), use CA technique.To avoid hogging, (transmit long data blocks as a unit), split data in small blocks, frames.
10Ring TopologyThe network consists of a set of repeaters joined by point to point links in closed loopReceive data on one link and retransmit on anotherLinks unidirectional; data are transmitted in one direction.Stations attach to repeatersData in framesCirculate past all the other stationsDestination recognizes address and copies frameFrame circulates back to source where it is removedMedia access control determines when station can insert frame
12Star Topology Each station connected directly to central node Usually via two point to point linksTwo alternatives for the operation of the central node:Central node can broadcastPhysical star, logical bus, where a transmission from any node is received by other nodes.Only one station can transmit successfully at a timeIn this case the central element is referred to as a hub.Central node can act as frame switchAn incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.
13Choice of TopologyThe choice of topology depends on a variety of factors:ReliabilityExpandabilityPerformanceTheses factors are needed to be considered in context of:MediumWiring layoutAccess control techniques
14Bus LAN Transmission Media Twisted pairEarly LANs used voice grade cable, didn’t scale for fast LANsNot used in bus LANs nowBaseband coaxial cableUses digital signalling, used in original EthernetBroadband coaxial cableCarry analog signals at radio frequencies, as in cable TV systemsExpensive, hard to install and maintainNo longer used in LANsOptical fiberExpensive taps, and there exist better alternatives.Not used in bus LANs
15Ring and Star Usage Ring Star Very high speed links over long distancesSingle link or repeater failure disables networkStarUses natural layout of wiring in buildingBest for short distancesHigh data rates for small number of devices
16Choice of MediumChoice of medium is determined by a number of factors:Capacity: to support the expected network traffic.Reliability: to meet requirements for availability.Types of data supported: tailored to the application (voice, video, text, …).Environmental scope: to provide service over the range of environments required.
17Media Available Voice grade unshielded twisted pair (UTP) Cat 3 cheap, but low data ratesShielded twisted pair and baseband coaxialMore expensive than UTP but higher data ratesBroadband coaxial cableStill more expensive and higher data rateHigh performance UTPCat 5 and above, high data rate for small # of devicesSupports switched star topology for large installationsOptical fibreHigh capacity, small size and high cost of components
18BridgesA bridge or a router can be used to interconnect a single LAN to other LANs/WANs.Bridge is simplerConnects similar LANs that use identical protocols for physical and link layersBridges perform minimal processingRouters more general purpose: interconnect various LANs and WANs
19Why using bridge?Reliability: can break a large LAN into several parts (self-contained units) to increase reliability.Performance:Performance declines with an increase number of devices or the length of wires.Smaller LANs will give improved performance.Security: different types of traffic have different security needs. (e.g. accounting, personnel)Geography: separate LANs may be needed to support devices clustered in separate locations.
20Functions of a BridgeRead all frames transmitted on one LAN and accept those addressed to any station on the other LANUsing MAC protocol for second LAN, retransmit each frameDo the same the other way round
22Bridge Design AspectsThe bridge makes no modification to content or format of frame, nor encapsulation.Exact bitwise copy of frameThe bridge should contain enough buffering to meet peak demandsContains routing and address intelligenceMust be able to tell which frames to pass to each LAN.May be more than one bridge to crossMay connect more than two LANsBridging is transparent to stationsAppears to all stations on two or more LANs as if they are on one single LAN
23Bridge Protocol Architecture IEEE 802.1D defines the protocol architecture for MAC bridges.Station address is at this level (physical address)Bridge does not need LLC layerIt is relaying MAC frames
25Fixed RoutingIt is important to provide alternative routes in complex large LANs for:Load balancingFault toleranceBridge must decide whether to forward frameBridge must decide which LAN to forward frame onRouting selected for each source-destination pair of LANsDone in configurationUsually least hop routeOnly changed when topology changes
27Spanning Tree Bridge automatically develops routing table Automatically update in response to changesThe algorithm consists of three mechanism:Frame forwardingAddress learningLoop resolution
28Frame forwardingThe bridge maintains forwarding database for each port attached to a LANList station addresses reached through each portFor a frame arriving on port X:Search forwarding database to see if MAC address is listed for any port except XIf address not found, forward to all ports except XIf address listed for port Y, check port Y for blocking or forwarding stateBlocking prevents port from receiving or transmittingIf not blocked, transmit frame through port Y
29Address Learning Routing information can be preloaded into the bridge. Also, routing information can be learned.When frame arrives at port X, it has come from the LAN attached to port XUse the source address to update forwarding database for port X to include that addressTimer on each entry in database, to allow for changes in topology.Each time frame arrives, source address checked against forwarding databaseIf the element is in database, then update entry and reset timer.If the element is not in database, create a new entry, with a timer.
30Spanning Tree Algorithm Closed loop problem, result in updating the addresses incorrectly and then neither bridge can forward frames to a specific LAN (see figure in next slide)Address learning works for tree layouti.e. no closed loopsFor any connected graph there is a spanning tree that maintains connectivity but contains no closed loopsEach bridge assigned unique identifierExchange between bridges to establish spanning tree
32Layer 2 and Layer 3 Switches Now many types of devices for interconnecting LANsBeyond bridges and routers:Layer 2 switchesLayer 3 switches
33Hubs Active central element of star layout Each station connected to hub by two lines; for transmit and receiveHub acts as a repeater; hub repeats signal on outgoing line to each stationLine may consists of:Two unshielded twisted pairs limited to about 100 mOptical fiber may be used (max about 500 m)Physically star, logically busTransmission from any station received by all other stationsCollision will occur if two stations transmit at the same time.
34Hub LayoutsMultiple levels of hubs can be cascaded in hierarchical configuration.Each hub may have a mixture of stations and other hubs attached to it from belowFits well with building wiring practicesWiring closet on each floorHub can be placed in each oneEach hub services stations on its floor
36Buses and Hubs Bus configuration All stations share capacity of bus (e.g. 10Mbps)Only one station transmitting at a timeHub uses star wiring to attach stations to hubTransmission from any station received by hub and retransmitted on all outgoing linesOnly one station can transmit at a timeTotal capacity of LAN is 10 MbpsPerformance can be improved with layer 2 switch
39Layer 2 Switches Central hub acts as switch Incoming frame from particular station switched to appropriate output lineUnused lines can switch other trafficMore than one station transmitting at a timeMultiplying capacity of LAN
40Layer 2 Switch BenefitsNo change is required to software or hardware to the attached devices to convert bus LAN or hub LAN to switched LANe.g. for Ethernet LAN, each device uses Ethernet MAC protocolDevice has dedicated capacity equal to original LANAssuming switch has sufficient capacity to keep up with all devicesFor example if switch can sustain throughput of 20 Mbps, each device appears to have dedicated capacity for either input or output of 10 MbpsLayer 2 switch scales easily additional devices attached to the switch.
41Types of Layer 2 Switch Store-and-forward switch: Cut-through switch: Accepts frame on input line and buffers it briefly,Then routes it to appropriate output lineDelay between sender and receiverCut-through switch:Takes advantage of destination address that appears at beginning of MAC frame.Switch begins repeating frame onto output line as soon as it recognizes destination addressHighest possible throughputRisk of propagating bad frames no CRC check prior to retransmission
42Differences between Layer 2 Switch and Bridge Bridge frame handling done in softwareSwitch performs address recognition and frame forwarding in hardwareBridge only analyzes and forwards one frame at a timeSwitch has multiple parallel data pathsCan handle multiple frames at a timeBridge uses store-and-forward operationSwitch can have cut-through operationNew installations typically include layer 2 switches with bridge functionality rather than bridges
43Problems with Layer 2 Switches (1) As number of devices in building grows, layer 2 switches reveal some inadequaciesBroadcast overloadLack of multiple linksSet of devices and LANs connected by layer 2 switches have flat address spaceAll users share common MAC broadcast addressIf any device issues broadcast frame, that frame is delivered to all devices attached to network connected by layer 2 switches and/or bridgesIn large network, broadcast frames can create big overheadMalfunctioning device can create broadcast stormNumerous broadcast frames clog network
44Problems with Layer 2 Switches (2) Current standards for bridge protocols dictate no closed loopsOnly one path between any two devicesImpossible in standards-based implementation to provide multiple paths through multiple switches between devicesLimits both performance and reliability.Solution: break up network into subnetworks connected by routersIP-based routers employ sophisticated routing algorithmsAllow use of multiple paths between subnetworks going through different routers
45Problems with Routers Routers do all IP-level processing in software High-speed LANs and high-performance layer 2 switches pump millions of packets per secondSoftware-based router only able to handle well under a million packets per secondSolution: layer 3 switchesImplement packet-forwarding logic of router in hardwareTwo categories of layer 3 switches:Packet by packetFlow based
46Packet by Packet or Flow Based Switches Operates in same way as traditional routerPacket-by-packet switch can achieve an order of magnitude increase in performance compared to software-based routerFlow-based switch tries to enhance performance by identifying flows of IP packets:Packets that have the same source and destinationThis done by observing ongoing traffic or using a special flow label in packet header (allowed in IPv6 but not in IPv4)Once flow is identified, predefined route can be established through the network to speed up the forwarding process.Huge performance over a pure software-based router.
47Typical Large LAN Organization Thousands to tens of thousands of devicesDesktop systems links 10 Mbps to 100 Mbps Into a LAN controlled by a layer 2 switchWireless LAN connectivity available for mobile usersLayer 3 switches at local network's coreForm local backboneInterconnected at 1 GbpsConnect to layer 2 switches at 100 Mbps to 1 GbpsServers connect directly to layer 2 or layer 3 switches at 1 GbpsLower-cost software-based router provides WAN connectionCircles in diagram identify separate LAN subnetworksMAC broadcast frame limited to own subnetwork