第五章IEEE 802.11标准与Wi-Fi

©2009 Guangming Song-Southeast UniversityOutlineIEEE 802IEEE 802.11ArchitectureStandardsSecurityIEEE 802A family of IEEE standards dealing with localarea networks and metropolitan area networksIEEE 802 Protocol ArchitectureIEEE 802 Protocol Layers Compared to OSI ModelFunctions of physical layerEncoding/decoding of signalsPreamble generation/removal (for synchronization) Bit transmission/receptionIncludes a specification of the transmission medium and the topologyFunctions of medium access control (MAC) layer On transmission, assemble data into a frame with address and error detection fieldsOn reception, disassemble frame and perform address recognition and error detectionGovern access to the LAN transmission mediumFunctions of logical link control (LLC) Layer Provide an interface to higher layers and perform flow and error controlIEEE 802 Protocols in ContextSeparation of LLC and MACThe logic required to manage access to a shared-access medium not found in traditional layer 2 data link controlFor the same LLC, several MAC options may be providedMAC controlContains Mac protocol control information Destination MAC addressDestination physical attachment pointSource MAC addressSource physical attachment pointDataThe body of the MAC frameCRCCyclic redundancy checkLLC ServicesUnacknowledged connectionless service No flow-and error-control mechanismsData delivery not guaranteedConnection-mode serviceLogical connection set up between two users Flow-and error-control providedAcknowledged connectionless service Cross between the previous two servicesDatagrams are to be acknowledgedNo prior logical connection is set upIEEE working group 802.11 formed in 1990 Now the most popular and pervasive Wireless LAN standardStationAny device that contains an IEEE 802.11 conformant MAC and physical layerDistribution system (DS)A system used to interconnect a set of BSSs and integratedLANs to create an ESSAccess point (AP)Any entity that has station functionality and provides access to the distribution system via the wireless medium forassociated stationsBasic service set (BSS)Stations competing for access to shared wireless mediumIsolated or connected to backbone DS through APExtended service set (ESS)Two or more basic service sets interconnected by DSThe Wi-Fi AllianceIEEE 802.11b first gains broad industry acceptanceHow to ensure that 802.11b products from different vendors will successfully interoperate?The Wireless Ethernet Compatibility Alliance (WECA) was formed in 1999Renamed the Wi-Fi Alliance (Wireless Fidelity)The term used for certified 802.11b products is Wi-FiIEEE 802.11 ArchitectureIEEE 802.11 Protocol ArchitectureIEEE 802.11 Medium Access Control MAC layer covers three functional areas:Reliable data deliveryMedium access controlSecurityDistributed Coordination Function802.11 uses a variant of CSMACalled the Distributed Coordination Function (DCF)Access point controls when nodes can transmit.No collision detection –rather, collision avoidance (CSMA/CA)Recall CSMA:Before a node transmits, it listens for activity on the network If medium is busy, node waits to transmitAfter medium is clear, don't immediately start transmitting...Otherwise all nodes would start talking at the same time!Instead, delay a random amount of time (random backoff)DCF IllustratedIEEE 802.11 Medium Access Control LogicExponential BackoffACK-based scheme for reliabilityReceiver sends ACK after each successful transmissionSender will retransmit if no ACK is heard, after waiting for a random intervalBinary exponential backoffFirst backoff interval between [0 ... 31] time slotsIf collision occurs, new backoff interval chosen between [0 ... 63] slotsRepeat until backoff interval reaches [0...1023] slots.Why increase the backoff interval each time???Interframe Space (IFS) ValuesShort IFS (SIFS)Shortest IFSUsed for immediate response actionse.g., ACK packets from AP back to a nodePoint coordination function IFS (PIFS)Midlength IFSUsed by centralized controller in PCF scheme when using pollsDistributed coordination function IFS (DIFS)Longest IFSUsed as a minimum delay of asynchronous frames contending for access802.11 Security: WEPWireless networks are inherently a broadcast medium!It is easy to intercept transmissions between end hostsCompare to wired systems: Must physically tap into the wires Nightmare for companies: Hacker in the parking lot with a laptopWired Equivalent Privacy (WEP)Rather than provide 802.11 with a truly robust security solution, goal was to prevent “casual”snoopingProblem: WEP was developed from scratch by a closed committee, standard not readily accessible for review by researchersWEP relies on a secret key being shared by end hosts and APs Traffic between nodes is encrypted using this keyRequires key to be distributed in some fashion by system adminsMakes it very difficult to change the key later!。