Wireless LANWireless LANsEvolutionMedia AccessIEEE 802.11RequirementsIEEE 802.11 Protocol ArchitectureTopologyLayered Protocol ArchitectureLow Layer Protocol StackPHY LayerFHSSDSSSCarrier Sense Multiple Access (CSMA appropriateness?)Carrier SensingMAC LayerMAC SublayerReliable Data DeliveryHidden Terminal ProblemExposed Terminal ProblemBusy ToneRTS/CTS dialogRTS/CTS DialogSlide 24Access ControlDistributed Coordination Function (DCF)IEEE 802.11 Medium Access Control LogicIEEE 802.11 DCFIEEE 802.11 DCFBackoff IntervalDynamic Contention WindowPriority-based Access Provisioning802.11 CSMA/CAPoint Coordination Function (PCF)MAC Frame FormatFrame Control FieldIEEE 802.11 Management SublayerRegistrationHandoffHandoff procedure in IEEE 802.11Power ManagementSecurityWireless LANWireless LANs•Evolution and Technology•IEEE 802.11•Bluetooth•Zigbee and IEEE 802.15Evolution•Early experiences (1970-72): IBM, HP, Motorola–Abandoned due to limited performance and unavailability of frequency bands•Early challenges:–Complexity and cost–Bandwidth–Coverage–Interference–Frequency administration•Emergence of unlicensed bands–Release of Industrial, Scientific and Medical (ISM) bands in 1985•Applications: military, home and enterprise networks, mobile networks, teetherless accessMedia Access•Media in wireless networks is shared and is scarce – access must be controlled•Observations:–Contention is at the receiver, not at the sender – makes the carrier sense approach inappropriate–Unlike Ethernet, congestion is location-dependent–The media access protocol should propagate congestion information explicitly rather than having each device learn about congestion independently–Media access protocol should propagate synchronization information about contention periods, so that all devices can contend effectivelyIEEE 802.11•Standardization group formed in 1990, first standards completed in 1997•IEEE 802.11 is the first WLAN standard; only one to secure a market•802.11b: PHY layer supports 11 Mbps using CKK (complementary code keying) technology•802.11a: PHY layer supports 54 Mbps using OFDM•Uses CSMA/CA for contention data•Supports both infrastructure as well as ad hoc modesRequirements•Single MAC to support multiple PHY layers•Mechanism to support multiple overlapping network•Provisions to handle interference•Mechanism to handle hidden terminals•Privacy and access controlLogical link controlPoint coordination function (PCF)Distributed coordination function (DCF)2.4-Ghz frequency-hopping spread spectrum 1Mbps 2Mbps2.4-Ghz direct sequence spread spectrum 1Mbps 2MbpsInfrared 1Mbps 2Mbps5-Ghz orthogonal FDM 6, 9. 12. 18, 24, 36, 48, 54 Mbps2.4-Ghz direct sequence spread spectrum 5.5 Mbps 11 MbpsContention-free serviceContention serviceMAC layerIEEE 802.11 Protocol ArchitectureIEEE 802.11 IEEE 802.11a IEEE 802.11bTopologyBasic Service Set (BSS)BSSBSSAn Extended Service Set (ESS)Infrastructure NetworkAd hoc NetworkLayered Protocol Architecture•MAC sublayer is responsible for access mechanisms and fragmentation/reassembly•MAC management is responsible for roaming in Extended Service Set (ESS), power management, association/dissociation/reassociation/ process for registration connection management•PHY management: decides on channel tuning–Physical Layer convergence protocol (PLCP): carrier sensing and forming packets–Physical Medium Dependent (PMD): modulation and coding techniques for signaling•Station management: coordination of interaction between MAC and PHY layersLow Layer Protocol StackMAC ManagementData Link LayerStation ManagementPHY ManagementLLCMACPLCPPMDPhysical LayerPLCP: Physical Layer Convergence ProtocolPMD: Physical Medium DependentPHY Layer•When the MAC protocol data unit (MPDU) arrive at the PLCP layer, a header is attached that is designed specifically for the PMD•The PLCP packet is then transmitted by the PMD according to specification of the signaling techniques•IEEE 802.11 defines three PLCP packet formats:–FHSS (frequency hopping spread spectrum)–DSSS (direct sequence spread spectrum)–DFIR (diffused infrared)FHSS•PMD hops over 78 channels of 1 MHz each in the center of 2.44 GHz ISM bands•Each BSS can select one of the three patterns of 26 hops:– (0, 3, 6, 9, …, 75)– (1, 4, 7, 10, …, 76)– (2, 5, 8, 11, …, 77)•IEEE 802.11 specifies specific random hopping pattern for each of these frequency groups that facilitates multivendor interpretability•Multiple Basic Service Set (BSS) can co-exist in the same area by up to three APs using different frequency groupsDSSS•DSSS communicates using non-overlapping pulses at 11 Mcps•The ISM band at 2.4 GHz is divided into 11 overlapping channels spaced at 5 MHz•A PHY layer management sublayer of AP covering a BSS can select one of the choices•Because of wider bandwidth, DSSS provides a better coverage and a more stable signalCarrier Sense Multiple Access (CSMA appropriateness?)•Carrier sense provides information about potential collision at the sender, but not at the receiver•Since the receiver and sender are not co-located, carrier sense does not provide adequate information for collision avoidance – interference at the sender does not imply interference at the receiverCarrier Sensing•Carrier sensing in IEEE 802.11 is performed physically or virtually•PHY sensing is through the clear channel assignment (CCA) signal produced by PLCP•CCA is generated by sensing detected bits or by checking the RSS•Virtual carrier sensing is done based on a network allocation vector (NAV) – more laterMAC Layer•MAC Sublayer:–Defines the access mechanisms and packet formats•MAC Management:–Defines roaming support in the ESS, power management and securityMAC Sublayer•Reliable data delivery•Access mechanisms–Contention-based•CSMA/CA–Contention-free•RTS/CTS•Point Coordination Function (PCF)Reliable Data Delivery•High degree of unreliability and large timers for retransmissions used in higher layers motivates to deal with errors at the MAC layer•Each transmission is followed by an ACK as an atomic unit. Retransmission is done if the ACK is not received•RTS/CTS exchangeHidden Terminal ProblemABXNode X finds that the mediumis free, and transmits a packetNo carrier ≠OK to transmitA is transmitting a packet to BExposed Terminal ProblemA is transmitting a packet to BX can not transmit to Y, eventhough it will not interfere at