Business Data Communications and Networking - Chapter 6: Local Area Networks

Business Data Communications and Networking 8th Edition Jerry Fitzgerald and Alan DennisJohn Wiley & Sons, Inc Prof. M. UlemaManhattan CollegeComputer Information Systems1Copyright 2005 John Wiley & Sons, IncChapter 6Local Area Networks2Copyright 2005 John Wiley & Sons, IncOutlineLAN ComponentsTraditional Ethernet (IEEE 802.3)Switched EthernetBest Practice LAN designImproving LAN Performance3Copyright 2005 John Wiley & Sons, IncWhy Use a LANInformation sharingHaving users access the same files, exchange information via email, or use InternetEx: single purchase order database accessed by all users on the LANResults in improved decision makingResource sharing Having hardware devices shared by all usersPrinters, Internet connectionsHaving software packages shared by all users on a LANResults in reduced cost4Copyright 2005 John Wiley & Sons, IncSharing Software on a LANPurchase software on a per seat basis Install software on a server for all to useNo need to have a copy on every computer on the LANReduces costSimplifies maintenance and upgradesExampleLAN: a 30 client network Purchase only a 10-seat license for a software program (instead of purchasing 20 copies of the same program)Assumes that only 10 users would simultaneously use the software5Copyright 2005 John Wiley & Sons, IncLAN MeteringUsed to control the number of copies of a software used on a LANTypically comes with many software packages used on LANsKeeps track of the usersProhibits using more copies of the package than the licensed numberHelps to minimize Copyright violations40% of SW used in the world is illegal, $13B Loss6Copyright 2005 John Wiley & Sons, IncNetwork TypesDedicated server network A server (computer) permanently assigned a specific taskMost popular network type90% of all LANsPeer-to-peer networkNo dedicated servers used7Copyright 2005 John Wiley & Sons, IncDedicated Server NetworksRequires one or more dedicated computers (servers)Permanently assigned a specific task (Web server, e-mail server, file server or print server)Enable users to share files, printers, etc.,May form a powerful enterprise network replacing mainframesMay form a server farm (many servers part of a network)Runs a server network operating system (NOS)Windows NT, LINUXAlso requires a special communication software to enable communications with client computers8Copyright 2005 John Wiley & Sons, IncTypes of Dedicated ServersCommon Types: Web servers, e-mail servers, database serversOthersFile serversAllows many users to share the same files on a common disk driveTypically with restricted access Print servesHandle print requestsCould be a separate computer or a “black box”Remote Access ServersEnable users to dial in and out of the LAN by phone (via modems)9Copyright 2005 John Wiley & Sons, IncPeer-to-Peer NetworksRequires no dedicated serverAny computer can act as both a client or a serverMore appropriate for small networksAdvantage: Lower cost No dedicated server, generally the most expensive network componentDisadvantage: Generally slower than dedicated server networksEach computer may be in use as a client and a server at the same timeDifficult to manage10Copyright 2005 John Wiley & Sons, IncBasic LAN ComponentsClient computerNetwork serverNetwork Interface Cards (NICs)Network CablesHubs and SwitchesNetwork Operating System (NOS)NICNOS11Copyright 2005 John Wiley & Sons, IncNetwork Interface Cards (NICs)Contains physical and data link layer protocolsIncludes a unique data link layer address (called a MAC address), placed in them by their manufacturerIncludes a socket allowing computers to be connected to the network Organizes data into frames and then sends them out on the networkCan be plugged into the PCMCIA port (for laptops and notebooks)Also called network cards and network adapters12Copyright 2005 John Wiley & Sons, IncNetwork CablesUsed to connect a computer physically to the networkTypes of cablesUntwisted wire pairs (UTP) – leading LAN cable typeShielded twisted pair (STP)Coaxial cable – heavy, not flexibleOptical fiber – high capacity, just beginning in LANs May include multiple different types cables Requires a special connector. Example: BALUN (Balanced-Unbalanced) connects UTP and Coaxial Cable13Copyright 2005 John Wiley & Sons, IncNetwork CablesTable 6.1 from page 192 goes here14Copyright 2005 John Wiley & Sons, IncHubsAct as junction boxes, linking cables from several computers on a networkUsually sold with 4, 8, 16 or 24 portsMay allow connection of more than one kind of cabling, such as UTP and coax.Repeat (reconstruct and strengthen) incoming signalsImportant since all signals become weaker with distanceExtends the maximum LAN segment distance15Copyright 2005 John Wiley & Sons, IncPlanning for LAN InstallationsCritically important with today’s LAN explosionsCheapest point to install the cable: During the construction of the buildingVery expensive to add cable to existing building (Labor, construction material, etc.)Built-in LAN cable planSimilar to power and phone linesWiring closet on each floor with LAN hubsCables from each room connected to hubs in the closetInstall 20-50 % more cable than need (future planning)If needed, simply add more hubs16Copyright 2005 John Wiley & Sons, IncNetwork Operating SystemsSoftware that controls the LANParts of NOSServer version of NOS Runs on the network serversClient version of NOS Runs on the client computersDirectory ServiceProvide information about resources on the LANNetwork ProfilesIndicate the resources available in the network and authorized users17Copyright 2005 John Wiley & Sons, IncNOS Server SoftwareEnables servers to operateHandles all network functionsPerforms data link, network, and application layer functions Acts as the application software by executing and responding to the requests sent to them by clientsReplaces the normal OS on the serverOptimized to provide better performance and faster response time (for its limited number of operations)ExamplesMS Windows NTNovell NetWare18Copyright 2005 John Wiley & Sons, IncNOS Client SoftwareProvides data link and network layer functionsInteracts with application software and computer’s own operating systemIncluded in most OS packages such as Windows 98 and Windows 200019Copyright 2005 John Wiley & Sons, IncDirectory ServiceProvide information about resources on the LANActive Directory Service (ADS) by MicrosoftWorks like a DNS serviceADS servers (aka, domain servers) act as DNS servers as wellResources organized into a treeEach branch contains a domain (a group of resources)A domain has a server (domain controller) Responsible for resolving address information (textual name of resource  network address)Responsible for managing authorizationA tree can be linked to other trees creating “forest”Uses Lightweight Directory Service Protocol (LDAP) to interact with client computers20Copyright 2005 John Wiley & Sons, IncProfilesNetwork ProfilesKept by serversSpecify resources available for use by other computersInclude data files, printers, etc.Configured when the LAN is established, and updatedUser profilesOne profile for each user, used for security reasonsDescribe what each user on a LAN has access toIncludes access codes assigned to devices and usersOnly the user with a correct code can use a specific device21Copyright 2005 John Wiley & Sons, IncEthernet (IEEE 802.3)Used by almost all LANs todayOriginally developed by a consortium of Digital Equipment Corp., Intel and XeroxStandardized as IEEE 802.3Types of EthernetShared EthernetUses hubs Switched EthernetUses switches22Copyright 2005 John Wiley & Sons, IncTopologyBasic geometric layout of the networkThe way computers on the network interconnectedLogical TopologyHow the network works conceptuallyLike a logical data flow diagram (DFD) or Like a logical entity relation diagram (ERD)Physical TopologyHow the network is physically installedLike physical DFD or physical ERD23Copyright 2005 John Wiley & Sons, IncShared Ethernet’s Logical TopologyViewed logically as a bus topologyAll messages from any computer flow onto the central cable (bus)A computer receive messages from all other computers, whether the message is intended for it or notWhen a frame is received by a computer, the first task is to read the frame’s destination address to see if the message is meant for it or notHalf-duplex circuit(Bus cable)24Copyright 2005 John Wiley & Sons, IncShared Ethernet’s Physical TopologyAppears to be a physical star topologyComputers linked into the central hub 25Copyright 2005 John Wiley & Sons, IncMultiple Hub Ethernet DesignAlso common to link is to use multiple hubs to form more complex physical topologies.26Copyright 2005 John Wiley & Sons, IncMedia Access Control (MAC)Uses a contention-based protocol called CSMA/CD (Carrier Sense Multiple Access / Collision Detect)Frames can be sent by two computers on the same network at the same timeThey will collide and become garbledCan be termed as “ordered chaos” Tolerates, rather than avoids, collisions27Copyright 2005 John Wiley & Sons, IncCSMA/CDCarrier Sense (CS): Listen to the bus to see if another computer is transmittingBefore sending anythingTransmit when no one is transmittingMultiple Access (MA): All computers have access to the network mediumCollision Detect (CD): Declared when any signal other than its own detectedIf a collision is detectedWait a random amount of time and then resend itMust be random to avoid another collision28Copyright 2005 John Wiley & Sons, IncEthernet Physical Media Format[Value1]Base/Broad[-Value2]Data Rate for Medium (e.g., 10 = 10Mbps)Baseband Mode (only one (digital) channel)Broadband (analog) cable transmissions (more than one channel (e.g., cable TV))maximum distance possible (in 100 of meters) or cable type T= twisted pair, F =fiber)29Copyright 2005 John Wiley & Sons, IncCoaxial Cable Ethernets10Base-5Thick Ethernet, uses thick coaxOriginal Ethernet specificationCapable of running 500 meters between hubsNow uncommon10Base-2Thin Ethernet, uses thin coaxCapable of running 200 meters between hubsBecame popular in the early 1990s as a cheaper alternative to 10Base-5Now uncommon30Copyright 2005 John Wiley & Sons, IncTwisted Pair Ethernets10Base-TUses Cat 3 and Cat 5 UTP, very inexpensiveRuns up to 100 metersCommon but rapidly losing ground to 100Base-T100Base-TUses Cat 5 UTPAlso called Fast Ethernet, replaced 10Base-T in sales volumeCombined 10/100 Ethernet Some segments run 10Base-T and some run 100Base-T31Copyright 2005 John Wiley & Sons, IncFiber Optic based Ethernets1000Base-T (1 GbE)Gigabit Ethernet. Maximum cable length is only 100 m for UTP cat5Fiber Optic based (1000Base-LX) runs up to 440 meters10 GbE10 Gbps Ethernet. Uses fiber and is typically full duplex40 GbE 40 Gbps Ethernet. Uses fiber and is typically full duplex.32Copyright 2005 John Wiley & Sons, Inc1.       NameMaximum Data Rate Cables10Base-510 MbpsCoaxial10Base-210 MbpsCoaxial10Base-T10 MbpsUTP cat 3, UTP cat 5100Base-T100 MbpsUTP cat 5, fiber1000Base-T1 GbpsUTP cat 5, UTP cat 5e, UTP cat 6, fiber10 GbE10 GbpsUTP cat 5e, UTP cat 6, UTP cat 7, fiber40 GbE40 GbpsfiberSummary - Ethernet Media Types33Copyright 2005 John Wiley & Sons, IncSwitched Ethernet TopologyUses switches (instead of hubs) Designed to support a small set of computers (16 to 24) in one LANLooks similar to a hub, but very different inside Designed to support a group of point-to-point circuitsNo sharing of circuitsBoth Logical and physical topology of the network becomes a star topologySwitch reads destination address of the frame and only sends it to the corresponding portWhile a hub broadcasts frames to all ports 34Copyright 2005 John Wiley & Sons, IncBasic Switch OperationPort 1234Forwarding TablePort MAC AdrUsed to make forwarding decisionsLayer 22 A1 S3 B4 CWhen a frame is received, the switch reads its [data link layer] destination address and sends the frame out the corresponding port in its forwarding table.35Copyright 2005 John Wiley & Sons, IncLearning Switch OperationSwitch starts by working like a hub With an empty forwarding tableIt gradually fills its forwarding table by learning about the nodesReads the source MAC address of the incoming frame and records it to the corresponding port numberReads the destination MAC address. If not in the Table then it broadcasts the frame to all portsWaits for the destination computers to respond, and repeats the first step Forwarding TablePort MAC Adr36Copyright 2005 John Wiley & Sons, IncModes of Switch OperationsCut through switchingRead destination address and start transmittingWithout waiting for the entire message is receivedLow latency; but may waste capacity (errorred messages) Only on the same speed incoming and outgoing circuitsStore and forward switchingWait until the whole message is received, perform error control, and then transmit itLess wasted capacity; slower networkCircuit speeds may be differentFragment free switchingRead the first 64 byte segment (contains the header)Perform error check, if it is okay then start transmittingCompromise between previous two modes37Copyright 2005 John Wiley & Sons, IncMAC in Switched Ethernet Each circuit shared by a computer and the switchStill CSMA/CD media access control usedEach device (computer or switch) listens before transmittingMultiple messages can be sent at the same time.Computer A can send a message to computer B at the same time that computer C sends one to computer DTwo computers send frames to the same destination at the same timeSwitch stores the second frame in memory until it finishes sending the first, then forwards the second38Copyright 2005 John Wiley & Sons, IncPerformance ComparisonRuns at up to 90% capacity on 100Base-TCapable of using about only 50% of capacity (10BaseT) before collisions become a problem39Copyright 2005 John Wiley & Sons, IncFactors in LAN DesignEffective Data RatesData Link Protocol EfficiencyMAC Protocol EfficiencyCosts40Copyright 2005 John Wiley & Sons, IncEffective Data RatesMaximum speed in bits the hardware layers can provideDepends onNominal data rate (provided by Physical layer)10Base-T  10 MbpsError rate (determines retransmissions)Efficiency of data link layer protocolPercentage of transmission that contains user dataDepends on the number of overhead bitsEfficiency of MAC protocol How well the MAC protocol can use the nominal data rate41Copyright 2005 John Wiley & Sons, IncData Link Protocol EfficiencyDepends on a typical packet size33-byte overhead in a 1500-byte packet 97.7% efficiency (assuming no retransmission)33-byre overhead in a 9000 byte (jumbo) packet 99.6% efficiency 33-byre overhead in a 150 byte (small) packet 82% efficiencyAverage efficiency on a LANDepends on the traffic patterns; TypicallyA number of small HTTP or SMT packetsFollowed by a large number of larger packets97% presents a reasonable estimate for LAN traffic42Copyright 2005 John Wiley & Sons, IncMAC Protocol EfficiencyCSMA/CD works well in low traffic LANs>>>>>> Figure 6.7 goes hereResponse time vs. utilization: a good indicatorWorks well when it is under 50% capacityExamples:10Base-T: 50% capacity x 97% efficiency x 10 Mbps rate 4.85 Mbps (shared by all computers on the LAN)100Base-T: 80% capacity x 97% efficiency x 100 Mbps 78 Mbps (total effective rate)43Copyright 2005 John Wiley & Sons, IncEffective Rate for a Computer Depends on number of computers using the LAN simultaneouslyA typical LAN has 20 users; but not all of them use the LAN at the same timeExamples: 2 simultaneous users on a 10Base-T4.85 Mbps / 2  2.425 Mbps / per computer10 simultaneous users on a 10Base-T4.85 Mbps / 10  485 Kbps / per computer10 simultaneous users on a 100Base-T78 Mbps / 10  7.8 Mbps / per computer44Copyright 2005 John Wiley & Sons, IncEffective Rates for Switched EthernetsDramatic improvements (No sharing)95% capacity efficiencyExamples:10Base-T: 95% capacity x 97% efficiency x 10 Mbps rate 9.2 Mbps 100Base-T: 95% capacity x 97% efficiency x 100 Mbps 92 Mbps1 GbE: implemented in full duplex (1 Gbps each direction) 1.8 GbpsPer computer efficiencySame as aboveNot affected by the traffic (since each has own circuit)45Copyright 2005 John Wiley & Sons, IncEffective Rate Estimates >>>>>Figure 6.6 Goes here46Copyright 2005 John Wiley & Sons, IncCostsVery cheapShared 10Base-T (old technology)Relatively inexpensiveShared 100Base-T (old technology)Switched EthernetVery expensive1 GbE10 GbE47Copyright 2005 John Wiley & Sons, IncBest Practice RecommendationsSwitched 10Base-TLess susceptible to response time delaysMore robust as traffic increasesProvides the best cost-performance tradeoffCosts almost the same as Shared 10Base-TCategory 5 or 5e cablesCosts almost the same as cat3Provides room for upgrades to 100Base-T or 100Base-TFiberLAN with very high traffic needsUsed with switched 100Base-T or 1 GbECurrently expensive48Copyright 2005 John Wiley & Sons, IncBest Practice Recommendations>>>>>>>>Figure 6.9 goes here49Copyright 2005 John Wiley & Sons, IncImproving LAN PerformanceThroughput: Used often as a measure of LAN performanceTotal amount of user data transmitted in a given period of timeTo improve throughput (thus LAN performance) Identify and eliminate bottlenecksBottlenecksPoints in the network where congestion is occurringCongestionNetwork or device can’t handle all of the demand it is experiencing50Copyright 2005 John Wiley & Sons, IncIdentifying Network BottlenecksPotential PlacesNetwork serverNetwork circuit (especially LAN-BN connection)Client’s computer (highly unlikely, unless too old)How to find itCheck the server utilization (during poor performance)If high (>60%), Then the server is the bottleneckIf low (<40%), Then the network circuit is the bottleneckIf between (40% - 60%)Both the server and circuits are the bottlenecks 51Copyright 2005 John Wiley & Sons, IncImproving Server PerformanceSoftware improvements Choose a faster NOSFine tune network and NOS parameters such asAmount of memory used for disk cacheNumber of simultaneously open filesAmount of buffer spaceHardware improvements Add a second serverUpgrade the server’s CPUIncrease its memory spaceAdd more hard disksAdd a second NIC to the server52Copyright 2005 John Wiley & Sons, IncImproving Disk Drive PerformanceEspecially important, since disk reads are the slowest task the server needs to doRedundant Array of Inexpensive Disks (RAID)Replacing one large drive with many small ones Can be used to both improve performance and increase reliability Building redundancy into the hard drivesA hard drive failure does not result in any loss of data53Copyright 2005 John Wiley & Sons, IncImproving Circuit CapacityUpgrade to a faster protocol Means upgrading the NICs and possible cablesExamples:Upgrading the network from 10Base-T to 100Base-TUpgrading the segment to the server from 10Base-T to 100Base-TIncrease number of segments to serverAdding additional NIC cards to the serversIncreasing the number of ways to access to serverIdeal number of NICs/server = 3More NICs may affect server’s processing capacity54Copyright 2005 John Wiley & Sons, IncNetwork Segmentationbeforeafter55Copyright 2005 John Wiley & Sons, IncReducing Network DemandMove files to client computersSuch as heavily used software packagesInstall disk caching software on client machinesReduces the need to access files stored on the serverMove user demands to off peak timesTell network users about peak usage timesTypically: Early morning and after lunchEncourage users to not use the network as heavily during these timesDelay some network intensive jobs to off-peak timesRun heavy printing jobs at night56Copyright 2005 John Wiley & Sons, IncImproving LAN Performance - SummaryIncrease Server PerformanceSoftware: Fine-tune the NOS settingsHardware: Add more servers and spread the network applications across the servers to balance the loadUpgrade to a faster computerIncrease the server's memoryIncrease the number and speed of the server's hard disk(s)Upgrade to a faster NICIncrease Circuit CapacityUpgrade to a faster circuitSegment the networkReduce Network DemandMove files from the server to the client computersIncrease the use of disk caching on client computers Change user behavior57Copyright 2005 John Wiley & Sons, IncImplications for ManagementCost of LAN equipment dropping quicklyCommodity marketFlood of vendors into the marketVarying quality of productsDifficult to justify the purchase of high quality LAN equipmentBecame more common everywhereLook for applications to take advantage of thisMore network enabled devices to deal withNetworked printers, scanners, vending machines, etc., 58Copyright 2005 John Wiley & Sons, IncCopyright 2005 John Wiley & Sons, Inc. All rights reserved. Reproduction or translation of this work beyond that permitted in section 117 of the 1976 United States Copyright Act without express permission of the copyright owner is unlawful. Request for further information should be addressed to the Permissions Department, John Wiley & Sons, Inc. The purchaser may make back-up copies for his/her own use only and not for distribution or resale. The Publisher assumes no responsibility for errors, omissions, or damages caused by the use of these programs or from the use of the information herein. 59Copyright 2005 John Wiley & Sons, Inc