The University of Iowa. Copyright© 2005 A. Kruger 1 Introduction to Wireless Sensor Networks Medium Access Control (MAC) 17 February 2005

The University of Iowa. Copyright© 2005 A. Kruger 2 Organizational Monday 4:30-5:20Room 4511 SC Thursday12:30-1:20Room 3220 SC Please note that the room numbers are different for Mondays and Thursdays. Monday5:20-6:20Room 1126 SC Thursday1:30-2:30Room 1126 SC OtherBy appointmentRoom 523C SHL Class Website Class Time Office Hours

The University of Iowa. Copyright© 2005 A. Kruger 3 Medium 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 )

The University of Iowa. Copyright© 2005 A. Kruger 4 MAC 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?)

The University of Iowa. Copyright© 2005 A. Kruger 5 MAC 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 utilization

The University of Iowa. Copyright© 2005 A. Kruger 6 Energy 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 433 MHz and 1mW Can be a dominant factor in WSN energy consumption

The University of Iowa. Copyright© 2005 A. Kruger 7 Energy 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 leave

The University of Iowa. Copyright© 2005 A. Kruger 8 TDMA Overview Channel 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)

The University of Iowa. Copyright© 2005 A. Kruger 9 TDMA for WSNs Often 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 consumption

The University of Iowa. Copyright© 2005 A. Kruger 10 TDMA 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 propagate

The University of Iowa. Copyright© 2005 A. Kruger 11 Examples 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 station

The University of Iowa. Copyright© 2005 A. Kruger 12 Scheduled 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 8

The University of Iowa. Copyright© 2005 A. Kruger 13 Bluetooth

The University of Iowa. Copyright© 2005 A. Kruger 14 Bluetooth Power Classes Power Class Max Output Power Min Output Power Range mW (20dbm) 1mW~100 m mw (4dbm) 0.25mW+10m 31mW (0dbm)0.25mW1m

The University of Iowa. Copyright© 2005 A. Kruger 15 Energy Conservation in Scheduled MAC Protocols Collision free No need for idle listening TDMA naturally support low-duty cycle operation

The University of Iowa. Copyright© 2005 A. Kruger 16 Review 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 scheduled MAC protocols. True or false – in WSN where low-power, short range radio transmission is used, idle listing contributes very little to the energy consumption budget of a node Briefly explain the term idle listening in WSNs One major issue in TDMA MAC protocols that of limited scaling. Explain in a 5-6 sentence paragraph. Describe the LEACH MAC protocol. Describe the Sohrabi & Pottie MAC protocol. What is the fundamental purpose of a MAC protocol?