Business Data Communications and Networking - Chapter 7: Wireless 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 7Wireless Local Area Networks2Copyright 2005 John Wiley & Sons, IncOutlineIntroductionIEEE 802.11bIEEE 802.11aIEEE 802.11gBluetoothBest practice WLAN DesignImproving WLAN Performance3Copyright 2005 John Wiley & Sons, IncWireless LANs (WLANs)Use radio or infrared frequencies to transmit signals through the air (instead of cables)Basic CategoriesUse of Radio frequencies (FOCUS of this chapter)802.1x family of standards (aka, Wi-Fi)Use of Infrared frequencies (Optical transmission)Wi-Fi grown in popularity Eliminates cabling Facilitates network access from a variety of locationsAirports, cafes, restaurants, etc., Facilitates for mobile workers (as in a hospital)4Copyright 2005 John Wiley & Sons, IncPrincipal WLANs TechnologiesIEEE 802.11bStandardization started after .11a, but finished beforeIEEE 802.11aFirst attempt to standardization of WLANs; more complicated than .11bIEEE 802.11gBluetoothAlso an IEEE standard 802.155Copyright 2005 John Wiley & Sons, IncIEEE 802.11bReuses many Ethernet componentsDesigned to connect easily to Etherneta.ka., wireless EthernetAlso called Wi-FiMarketing ploy; sounds like Hi-FiVersions of .11bDirect Sequence Spread Spectrum (DSSS)Focus of this chapter (more popular)Frequency Hopping Spread Spectrum (FHSS)6Copyright 2005 John Wiley & Sons, IncVersions of IEEE 802.11bDirect Sequence Spread Spectrum (DSSS)Uses the entire frequency band to transmit informationCapable of data rates of up to 11 Mbps Fallback rates: 5.5, 2 and 1 Mbps. (Used when interference or congestion occurs)Dominates market place, because fasterFrequency Hopping Spread Spectrum (FHSS) Divides the frequency band into a series of channels Then changes its frequency channel about every half a second, based on a pseudorandom sequenceMore secure, Only capable of data rates of 1 or 2 Mbps7Copyright 2005 John Wiley & Sons, IncWLAN TopologyA wireless Access Point (AP) connected into an Ethernet SwitchSame as EthernetPhysical starLogical busUse the same radio frequencies, so take turns using the networkUses a NIC that transmits radio signals to the AP10Base-T or 100Base-T8Copyright 2005 John Wiley & Sons, IncComponents of WLANsNetwork Interface CardsAvailable for laptops as PCMCIA cards Available for desktops as standard cardsMany laptops come with WLAN cards built inAbout 100-500 feet max transmission rangeAccess Points (APs)Used instead of hubs; act as a repeaterMust hear all computers in WLANMessage transmitted twiceSender to AP, then AP to receiver 9Copyright 2005 John Wiley & Sons, IncMore on the APs and NICs 3 separate channels available for 802.11bAll devices using an AP must use the same channelWLAN functions as a shared media LANReduces the interferenceUsers can roam from AP to APInitially NIC selects a channel (thus an AP)Based on “strength of signal” from an APDuring roaming, if NIC sees another AP with a stronger signal, attaches itself to this AP Usually a set of APs installed to provide geographical coverage and meet traffic needsNICs selects a less busy channel if its current channel becomes busy (too many users)10Copyright 2005 John Wiley & Sons, IncAntennas used in WLANs Omni directional antennasTransmit in all directions simultaneouslyUsed on most WLANsDipole antenna (rubber duck)Transmits in all direction (vertical, horizontal, up, down)Directional antennasProject signal only in one directionFocused area; stronger signal; farther rangesMost often used on inside of an exterior wallTo reduce the security issueA potential problem with WLANsFigures 7.2 and 7.3 will be used here11Copyright 2005 John Wiley & Sons, IncWLAN Media Access ControlUses CSMA/CACA  collision avoidanceA station waits until another station is finished transmitting plus an additional random period of time before sending anythingMay use two MAC techniques simultaneouslyDistributed Coordination Function (DCF)Also called “Physical Carrier Sense Method” Point Coordination Function (PCF)Also called “Virtual Carrier Sense Method”Optional: (can be set as “always”, “never”, or “just for certain frame sizes”12Copyright 2005 John Wiley & Sons, IncDistributed Coordination FunctionRelies on the ability of computers to physically listen before they transmitWhen a node wants to send a message:First listens to make sure that the transmitting node has finished, then Waits a period of time longerEach frame is sent using stop-and-wait ARQBy waiting, the listening node can detect that the sending node has finished and Can then begin sending its transmissionACK/NAK sent a short time after a frame is received, Message frames are sent a somewhat longer time after (ensuring that no collision will occur)13Copyright 2005 John Wiley & Sons, IncPoint Coordination Function (PCF)Solves Hidden Node problem Two computers can not detect each other’s signalsA computer is near the transmission limits of the AP at one end and another computer is near the transmission limits at the other end of the AP’s rangePhysical carrier sense method will not workSolutionFirst send a Request To Send (RTS) signal to the APRequest to reserve the circuit and durationAP responds with a Clear To Send (CTS) signal,Also indicates duration that the channel is reservedComputer wishing to send begins transmitting14Copyright 2005 John Wiley & Sons, IncMessage Delineation>>>>>> Figure 7.4 goes here15Copyright 2005 John Wiley & Sons, IncPreamble of 802.11b PacketsUsed to mark the start of the packetAlways transmitted at 1 MbpsSub fields of PreambleLong preamble version16 sync bytes of alternating 1’s and 0’s1 byte of start of frame delimiter (1010101)Short preamble version7 sync bytes1 byte of start of frame16Copyright 2005 John Wiley & Sons, IncPhysical Layer Convergence Protocol (PLCP)Used to indicate data rates and packet lengthTransmitted at 1 Mbps (long preamble) or at 2 Mbps (short preamble)Fields of PLCPSignal rate (1 byte)Which of the four speeds to be usedService field (1 byte)Reserved for future useLength field (2 bytes)Length of the payload in 8-bit bytesHeader error check field (2 bytes)CRC-16 (if any error found, packet is discarded)17Copyright 2005 John Wiley & Sons, IncFields of Payload HeaderFrame control (2 bytes)Indicates version of the 802.11b protocolContains any ACK/NAK and RTS/CTS signalsDestination address (6 bytes)AP-NIC: Address of NIC; NIC-AP-NIC: Address of APAddress 3 (6 bytes)NIC-AP-NIC: Address of the NICSource address (6 bytes)AP-NIC: Address of AP; NIC-AP-NIC: Address of NICSequence control (2 bytes)Contains packet number for error controlAddress 4 (6 bytes)Used only for NIC-NIC communications18Copyright 2005 John Wiley & Sons, IncOther FieldsLogical Link Control Protocol Data Unit (LLC PDU)Same as in 802.3 EthernetPhysical trailer4-byte CRC-32 used in Ethernet19Copyright 2005 John Wiley & Sons, IncData Transmission in PLVia radio wavesAnalog mediumDigital computer data transmitted using analog transmission (Translations done by NIC and AP)Frequency and bandwidth (range of frequencies)2.4000 – 2.4835 GHz  83.5 MHz bandwidth in USATransmission83.5 MHz divided into 3 channels  22 MHz each (with 3 MHz guard bands between channels)Data capacity of the circuit:Number of bits sent on each symbol x symbol rateMax symbol rate: depends on bandwidth and SNR22 MHz  22 million symbols/second (perfect conditions)20Copyright 2005 John Wiley & Sons, IncBit Transmission in DSSSEach bit converted into a special code8-bit or 11-bit code (designed to reduce interference)Called spreading a bit into many bits across spectrum1-Mbps DSSUses an 11-bit Barker sequence codeTransmitted using binary phase shift keying (BPSK) (1 bit per symbol)11 Mbps signaling rate  1 Mbps data rate2-Mbps DSSUses the same 11-bit codeTransmits using Quadrature phase shift keying (QPSK) (2 bits per symbol)11 Mbps signaling rate  2 Mbps data rate21Copyright 2005 John Wiley & Sons, Inc1Mbps DSSS with Barker code>>>>>> Figure 7.5 goes here22Copyright 2005 John Wiley & Sons, IncIEEE 802.11aOperates in a 5 GHz frequency rangeTotal bandwidth is 300 MHzFaster data rates possible: Up to 54 Mbps6, 9, 12, 18, 24, 36, 48, and 54 MbpsUses the same topology as .11bReduced range because of higher speed50 meters ( 150 feet)Highest speed achievable within 15 meter23Copyright 2005 John Wiley & Sons, IncIEEE 802.11a CoverageProvides 4-12 channels (depending on configuration)Important for coverage; takes more .11a AP to cover the same area (small range)Make it possible to locate many APs in the same area to increase capacity Figure 7.6 goes here24Copyright 2005 John Wiley & Sons, Inc802.11a Media Access ControlSame as .11bSimilar packet formatPreamble and PLCP Header: transmitted at 6 MbpsPLCP parity bit field: used for error checking of headerPLCP tail field: used as a pad to “byte” align the packetPayload service field: to sync circuitry in NIC and AP25Copyright 2005 John Wiley & Sons, Inc802.11a Packet Layout>>>>>> Figure 7.7 goes here26Copyright 2005 John Wiley & Sons, Inc802.11a Data TransmissionSimilar to 802.11b; spreads its transmission over a wider spectrumEach of 12 channel’s bandwidth = 20 MHzBroken into 52 separate channels: 312.5 KHz each, plus guard bands48 channels for data (sent across all channels in parallel using Orthogonal Frequency Division Multiplexing (OFDM)4 channels for control27Copyright 2005 John Wiley & Sons, IncOFDM Versions6-Mbps version of .11aGroups data into sets of 24 data bitsConverts each group into an OFDM symbol of 48 bitsPattern chosen enables some error correctionTransmit each symbol in one of 48 sub channels using BPSK sent at 250 KHz24 data bits x 250 KHz  6 Mbps9-Mbps versionGroups data into sets of 36 bitsTransmits each symbol using BPSK36 data bits x 250 KHz  9 Mbps28Copyright 2005 John Wiley & Sons, IncOFDM Versions (Cont.)12-Mbps versionGroups data into sets of 48 bitsTransmit OFDM symbol using QPSK (2 bits per symbol)48 bits x 250 KHz x 2 bits  12 Mbps18-Mbps versionGroups data into sets of 72 bits; uses QPSK72 bits x 250 KHz x 2 bits  18 Mbps24-Mbps versionGroups data into sets of 96 bitsTransmit OFDM symbol using QAM (4 bits per symbol)96 bits x 250 KHz x 4 bits  24 Mbps29Copyright 2005 John Wiley & Sons, IncOFDM Versions (Cont.)36-Mbps versionGroups data into sets of 128 bits; uses QAMTransmit OFDM symbol using QPSK (2 bits per symbol)128 bits x 250 KHz x 4 bits  36 Mbps48-Mbps versionGroups data into sets of 192 bitsTransmit OFDM symbol using 64-QAM (6 bits per symbol)192 bits x 250 KHz x 6 bits  48 Mbps54-Mbps versionGroups data into sets of 216 bits; uses 64-QAM216 bits x 250 KHz x 6 bits  54 Mbps30Copyright 2005 John Wiley & Sons, IncOFDM Versions (Cont.)>>>>>Fig 7.8 goes here31Copyright 2005 John Wiley & Sons, IncIEEE 802.11gDesigned to combine advantages of 802.11a and 802.11bOffers higher data rates (up to 54 Mbps) in 2.4 GHz band (as in .11b) with longer rangesBackward compatible with 802.11b.11b devices can interoperate with .11g APsPrice to pay: when an .11g AP detects an .11b device, it prohibits .11g devices from operating at higher speedsUses the same topology as .11bProvides 3-6 channels (depending on configuration)54 Mbps rate obtained within 50 meter range32Copyright 2005 John Wiley & Sons, Inc802.11g Media Access ControlAlmost the same media and error control protocols as .11bSimilar packet layout, exceptPreambles and headers transmitted at slower speeds (up to a maximum of 11 Mbps)Payload transmitted at higher speeds (up to a max of 54 Mbps)Data Transmission in the Physical LayerSame techniques in .11a and .11bUses PSK, QPSK, and CCK to provide .11b ratesUses BPSK, QPSK, and QAM to provide .11a rates33Copyright 2005 John Wiley & Sons, Inc802.11g Media Access ControlAlmost the same media and error control protocols as .11bSimilar packet layout, exceptPreambles and headers transmitted at slower speeds (up to a maximum of 11 Mbps)Payload transmitted at higher speeds (up to a max of 54 Mbps)Data Transmission in the Physical LayerSame techniques in .11a and .11bUses PSK, QPSK, and CCK to provide .11b ratesUses BPSK, QPSK, and QAM to provide .11a rates34Copyright 2005 John Wiley & Sons, IncBluetooth (IEEE 802.15)A standard for Wireless Personal Area Network (WPAN)Provides networking in a very small areaUp to 10 meters (current generation) Up to 100 meters (next generation)Includes small (1/3 of an inch square) and cheap devices designed toReplace short distance cabling between devicesKeyboards, mouse, handsets, PDAs, etcProvides a basic data rate of 1 MbpsCan be divided into several voice and data channelsUses Frequency Shift Keying (FSK) for data transmission (1 bit per symbol)35Copyright 2005 John Wiley & Sons, IncBluetooth TopologyUses the term “piconet” to refer to a Bluetooth networkConsists of 8 devicesA “master” device controlling other devices, “slaves”Acts like an APSelects frequencies and controls accessAll devices in a piconet share the same frequency range36Copyright 2005 John Wiley & Sons, IncBluetooth Media Access ControlUses Frequency Hopping Spread Spectrum (FHSS)Available frequency range (2.4000-2.4835) divided into 79 separate 1-MHz channelsA data burst transmitted using one channel, next data burst uses the next channel, and so on.Channels changed based on a sequence and established by the slave and the master prior to the data transfers1,600 channel change per secondAlso used to minimize interferenceA noisy channel avoided eventuallyNot compatible with 802.11bPotential interference problems (especially if many Bluetooth devices present close to .11b devices)37Copyright 2005 John Wiley & Sons, IncBluetooth Packet Formats>>>Figure7.9 goes here38Copyright 2005 John Wiley & Sons, IncBluetooth Packet Fields/SubfieldsAccess Code: to sync the sender and receiverPreamble: Alternating 1’s and 0’sSync Bytes: bit patterns based on addresses and packet typesTrailer: Alternating 1’s and 0’sHeader: for address and error controlAddress: Slave’s addressType: Payload’s type (e.g., data, control etc.)Flow Control: 1 means continue, 0 means to stopARQ ACK/NAK: 1 means ACK, 0 means NACKSequence Number: packet number used for ARQHeader Error Check: CRC-8 for the header39Copyright 2005 John Wiley & Sons, IncBluetooth Packet Fields/SubfieldsPayload HeaderLogical Channel: whether the payload has a data or control frameFlow Control: same as before (for a another software)Length: Payload’s length in bytesFuture Use: ReservedPayload:Format depends on the type of data transmittedPayload trailerCRC-16 error check code40Copyright 2005 John Wiley & Sons, IncInfrared Wireless LAN Require line of sight (LOS) to work (less flexible) Main advantage: reduced wiringusually mounted in fixed positions to ensure they will hit their targetsNew version: diffuse infrared, Operates without a direct LOS by bouncing the infrared signal off of wallsOnly able to operate within a single room and at distances of only about 50-75 feet41Copyright 2005 John Wiley & Sons, IncEffective Data Rates in WLANsMaximum speed in bits the hardware layers can provideDepends on Nominal data rate, Error rate, Efficiency of data link layer protocol, and Efficiency of MAC protocol Error plays a greater role in WLANsSignificant impact of interference on performanceCauses frequent retransmissions, thus lower data rates42Copyright 2005 John Wiley & Sons, IncData Link Protocol EfficiencyFactors involved:Typical WLAN overhead: 51-bytes (with a short preamble) Packet size: Data packets: assume a 1500-byte for full lengthControl packets: ACK/NAK packetsTransmission rates:Overhead bits transmission speedsPayload transmission speedsAssuming a mix of short and full length packets85% average efficiency for 802.11b75% average efficiency for 802.11a and 802.11g 43Copyright 2005 John Wiley & Sons, IncMAC Protocol EfficiencyUses a controlled approach (PCF)Imposes more fixed delays initially when traffic is lowDue to PCF’s permission based proceduresAllows response time delays increases slowly up to 85-90% of capacity>>>>>>>>Figure 7.11 goes here44Copyright 2005 John Wiley & Sons, IncEffective Rate for a Computer* 802.11b85% efficiency x 85% capacity x 11 Mbps = 9.6 Mbps2 users: 9.6 Mbps / 2  4.8 Mbps per user10 users: 9.6 Mbps/10  960 Kbps per user802.11a75% efficiency x 85% capacity x 54 Mbps = 34.4 Mbps2 users: 34.4 Mbps / 2  17.2 Mbps per user10 users: 34.4 Mbps/10  3.4 Mbps per user802.11g75% efficiency x 85% capacity x 54 Mbps = 34.4 Mbps2 users: 34.4 Mbps / 2  17.2 Mbps per user10 users: 34.4 Mbps/10  3.4 Mbps per user* Under perfect conditions45Copyright 2005 John Wiley & Sons, IncEffective Rate EstimatesFigure 7-12 goes here46Copyright 2005 John Wiley & Sons, IncCosts802.11bDecreasing cost of NICs and AP s802.11a and gNewer technologies, higher costsComparison with wired Ethernets(cost of .11b AP) = (cost of 10/100Base-T switch)(cost of .11b NIC) = $20 + (cost of 10/100Base-T NIC)No cost for cabling and its deployment in WLANWired Ethernet cable deployment cost: $50 - $400Cheapest to install during construction of buildingFor new buildings Wired LANs are less expensiveDo not forget the need for mobility !!47Copyright 2005 John Wiley & Sons, IncBest Practice RecommendationsAdopt 802.11gWill replace 802.11b and .11aPrices of .11g NICs and APs coming downWireless vs. Wired802.11g ~ 10Base-TSimilar data rates for low traffic environmentWhen mobility important  802.11gUsing WLAN as overlay network (over wired LAN)WLANs installed In addition to wired LANsTo provide mobility for laptops, etc.,To provide access in hallways, lunch rooms, etc., 48Copyright 2005 John Wiley & Sons, IncPhysical WLAN DesignMore challenging than designing a traditional LANPlacement of APs: Locations chosen to:Provide coverageMinimize potential interferenceBegins with a site survey to determineFeasibility of desired coverageMeasuring the signal strength from temporary APsPotential sources of interferenceMost common source: Number and type of wallsLocations of wired LAN and power sourcesEstimate of number of APs required 49Copyright 2005 John Wiley & Sons, IncPhysical WLAN DesignBegin locating APsPlace an AP in one cornerMove around measuring the signal strengthPlace another AP to the farthest point of coverageAP may be moved around to find best possible spotAlso depends on environment and type of antennaRepeat these steps several times until the corners are coveredThen begin the empty coverage areas in the middleAllow about 15% overlap in coverage between APsTo provide smooth and transparent roamingSet each AP to transmit on a different channel50Copyright 2005 John Wiley & Sons, IncPhysical WLAN DesignFigure 7.13 goes here51Copyright 2005 John Wiley & Sons, IncMultistory WLAN DesignMust includeUsual horizontal mapping, andVertical mapping to minimize interference from APs on different floorsFigure 7.14 goes here52Copyright 2005 John Wiley & Sons, IncWLAN SecurityEspecially important for wireless networkAnyone within the range can use the WLANFinding a WLANMove around with WLAN equipped device and try to pick up the signalUse special purpose software tools to learn about WLAN you discoveredWardriving – this type reconnaissanceWarchalking – writing symbols on walls to indicate presence of an unsecure WLAN53Copyright 2005 John Wiley & Sons, IncTypes of WLAN SecurityService Set Identifier (SSID)Required by all clients to include this in every packetIncluded as plain text Easy to breakWired Equivalent Privacy (WEP)Requires that user enter a key manually (to NIC and AP)Communications encrypted using this keyShort key (40-128 bits)  Easy to break by “brute force”Extensible Authentication Protocol (EAP)WEP keys created dynamically after correct loginRequires a login (with password) to a serverAfter logout, WEP keys discarded by the serverWi-Fi Protected Access (WPA) – new standardA longer key, changed for every packet54Copyright 2005 John Wiley & Sons, IncImproving WLAN PerformanceSimilar to improving wired LANsImproving device performanceImproving wireless circuit capacityReducing network demand55Copyright 2005 John Wiley & Sons, IncImproving WLAN PerformanceSimilar to improving wired LANsImproving device performanceIf 802.11g widely deployed, replace 802.11b cards with .11g cards (may be the cause for slow performanceBy high-quality cards and APsImproving wireless circuit capacityUpgrade to 802.11gReexamine placement of APsCheck sources of interference (other wireless devices operating in the same frequencies))Use different type of antennasReducing network demandw56Copyright 2005 John Wiley & Sons, IncImproving WLAN Device PerformanceIf 802.11g widely deployed, replace 802.11b cards with .11g cards May be the cause for slow performanceBy high-quality cards and APsBetter designStronger signals,Longer ranges57Copyright 2005 John Wiley & Sons, IncImproving Wireless Circuit CapacityUpgrade to 802.11gRe-place APsFewest walls between AP and devicesCeiling or high mounted to minimize obstaclesOn halls, not in closetsRemove sources of interference Other wireless devices operating in the same frequenciesBluetooth devices, cordless phones, etc.Use different type of antennasDirectional antennas in smaller range to get stronger signals (faster throughput)58Copyright 2005 John Wiley & Sons, IncReducing WLAN DemandNever place a serve in a WLANDoubles the traffic between clients and serverSince all communications ii through the AP Locate the server in the wired part of the network (ideally with a switched LAN)Place wired LAN jacks in commonly used locationsIf WLAN becomes a problem, users can switch to wired LAN easily59Copyright 2005 John Wiley & Sons, IncImplications for ManagementWLANs becoming common placeAccess to internal data, any time, any placeBetter protection of corporate networksPublic access through WLAN hotspotsCompetition with cell phone technologiesNew cell phone technologies (faster, longer ranges)Drastic price drops of WLAN devicesWidespread Internet access via multiplicity of devices (PDAs, etc,)Development of new Internet applicationsNew companies created; some old ones out of businessDrastic increase in the amount of data flowing around60Copyright 2005 John Wiley & Sons, IncCopyright 2005 John Wiley & Sons, Inc. 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