Introduction to Wireless Sensor NetworksOrganizationalMedium Access Control (MAC) IntroductionMAC AttributesSlide 5Energy Efficiency in MAC ProtocolsEnergy Efficiency in Mac ProtocolsTDMA OverviewTDMA for WSNsTDMA Disadvantages for WSNsExamples of Scheduled ProtocolsScheduled ProtocolsBluetoothBluetooth Power ClassesEnergy Conservation in Scheduled MAC ProtocolsReview Question1The University of Iowa. Copyright© 2005A. KrugerIntroduction to Wireless Sensor Networks Medium Access Control (MAC)17 February 20052The University of Iowa. Copyright© 2005A. KrugerOrganizationalMonday 4:30-5:20 Room 4511 SCThursday 12:30-1:20 Room 3220 SCPlease note that the room numbers are different for Mondays and Thursdays.Monday 5:20-6:20 Room 1126 SCThursday 1:30-2:30 Room 1126 SCOther By appointment Room 523C SHL Class Websitewww.engineering.uiowa.edu/~ece195/2005/Class TimeOffice Hours3The University of Iowa. Copyright© 2005A. KrugerMedium Access Control (MAC)Introduction•One Approach (Be nice – share)–Avoid interference by scheduling nodes on sub-channels•TDMA (Time-Division Multiple Access) •FDMA (Frequency-Division Multiple Access)•CDMA (Code-Division Multiple Access)•Another Approach (Compete/contend)–Don’t pre-allocate transmission, compete => probabilistic coordination–ALOHA (Transmit. Collision? Yes, discard packet, retransmit later)–Carrier Sense (IEEE 802.11)4The University of Iowa. Copyright© 2005A. KrugerMAC Attributes•Basic task of MAC protocol–Collision avoidance/minimization•Energy efficiency–MAC layer controls radio. Radio often consume most energy•Scalability and adaptivity–Nodes join, exit, rejoin, die, move to different location–Good MAC should accommodate such changes•Channel utilization–Very important in cellular or wireless LAN–Often secondary in WSNs (Why?)•Latency•Throughput•Fairness–Important in traditional cellular/wireless LAN, less important in WSNs (Why?)5The University of Iowa. Copyright© 2005A. KrugerMAC Attributes•For WSNs, most important attributes of a good MAC are–Effective collision avoidance–Energy Efficiency–Scalability and adaptivity•Other attributes are normally secondary–Fairness–Latency–Channel utilization6The University of Iowa. Copyright© 2005A. KrugerEnergy Efficiency in MAC Protocols•Motivation – Energy efficiency is very important in WSNs.•Question – what causes energy waste from a MAC perspective?–Collision•Collided packets are discarded, retransmission require energy•Not a big issue in scheduled (TDMA, CDMA, FDMA) MAC protocols, but an issue in contention MAC protocols.–Idle listening•Long distance (500 m or more) Tx energy consumption dominates, but in short-range communication Rx energy consumption can be close to Tx energy consumption•MICA2 idle:receiving:transmission ratio at 1 mW is 1:1:1.41 @ 433 MHz and 1mW•Can be a dominant factor in WSN energy consumption7The University of Iowa. Copyright© 2005A. KrugerEnergy Efficiency in Mac Protocols–Overhearing•When a node receives packets that are destined for another node–Control packet overhead•Sending, receiving, listening, all consumes energy–Adaptation•Reconfiguring when nodes join leave8The University of Iowa. Copyright© 2005A. KrugerTDMA OverviewChannel is divided into N slots (a frame)–Each node gets a time slot–It only transmits in its time slot–It only need listen during its time slot–Frame my be static – fix number of slots–Frames may be dynamically adjusted–Frequently used in cellular communications (i.e., GSM)9The University of Iowa. Copyright© 2005A. KrugerTDMA for WSNsOften used in WSNs•Typically, nodes communicate with base station•Major advantage of TDMA – supports low-duty-cycle operations on nodes –Large frames–Nodes only have to listen in their time slot•Low duty-cycle =>low energy consumption10The University of Iowa. Copyright© 2005A. KrugerTDMA Disadvantages for WSNs•Cluster paradigm (analogous to cell phones)•One node is selected as the cluster head and acts as base station•Nodes communicate only with head •Direct peer-to-peer communication not energy efficient–Nodes must listen on all time slots, reducing energy•Inter-cluster communication requires other MAC protocols•Most important issue is limited scaling–When a new nodes joins the base station must reallocate slots and adjusting the frame size–This can take time and energy to propagate11The University of Iowa. Copyright© 2005A. KrugerExamples of Scheduled Protocols•Sohrabi & Pottie–Homework/Exam (page 79 in book)•Low-Energy Adaptive Clustering Hierarchy (LEACH)–Organize nodes into cluster hierarchies–TDMA within each cluster–Nodes only talk to node head–Position of head is rotated among nodes depending on remaining energy–Node then uses long-range/high-power communication to base–Nodes don’t need to know global topology–Nodes don’t need control information from base station12The University of Iowa. Copyright© 2005A. KrugerScheduled Protocols•Bluetooth–Designed for PAN, but attractive for WSNs–Bluetooth organizes itself into clusters, piconets–Frequency-hopping CDMA is used to handle inter-cluster interference–Within cluster, TDMA MAC protocol–Master-slave approach. Cluster head (master), other nodes are slaves.–Master uses polling to decide which salve can transmit–Only communication between master and one or more slaves are possible–Maximum number of nodes in a cluster is 813The University of Iowa. Copyright© 2005A. KrugerBluetooth14The University of Iowa. Copyright© 2005A. KrugerBluetooth Power ClassesPower ClassMax Output PowerMin Output PowerRange1100 mW (20dbm)1mW ~100 m+210 mw (4dbm)0.25mW +10m3 1mW (0dbm) 0.25mW 1m15The University of Iowa. Copyright© 2005A. KrugerEnergy Conservation in Scheduled MAC Protocols•Collision free•No need for idle listening•TDMA naturally support low-duty cycle operation16The University of Iowa. Copyright© 2005A. KrugerReview Question•True or false – With respect to WSN MACs, effective collision avoidance is less important than fairness.•Describe and elaborate briefly (three sentences) what we mean by the term “latency” as an attribute of a WSN MAC. •True or false – channel utilization is a crucial attribute of as WSN MAC•True or false – energy waste from collisions are less in contention MAC protocols than in scheduled MAC protocols.•List three