2/17/20051 Guest lecture for CS113, UCLA Medium Access Control in Wireless Sensor Networks Wei Ye USC Information Sciences Institute

2/17/2005Guest lecture for CS113, UCLA2 Outline Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary

2/17/2005Guest lecture for CS113, UCLA3 Introduction to MAC The role of medium access control (MAC) –Controls when and how each node can transmit in the wireless channel Why do we need MAC? –Wireless channel is a shared medium –Radios transmitting in the same frequency band interfere with each other – collisions –Other shared medium examples: Ethernet

2/17/2005Guest lecture for CS113, UCLA4 Where Is the MAC? Network model from Internet A sublayer of the Link layer –Directly controls the radio –The MAC on each node only cares about its neighborhood Application layer Transport layer Network layer Link/MAC layer Physical layer End-to-end reliability, congestion control Routing Per-hop reliability, flow control, multiple access Packet transmission and reception

2/17/2005Guest lecture for CS113, UCLA5 What’s New in Sensor Networks? A special wireless ad hoc network –Large number of nodes –Battery powered –Topology and density change –Nodes for a common task –In-network data processing Sensor-net applications –Sensor-triggered bursty traffic –Can often tolerate some delay Speed of a moving object places a bound on network reaction time

2/17/2005Guest lecture for CS113, UCLA6 Next… Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary

2/17/2005Guest lecture for CS113, UCLA7 Primary MAC Attributes Collision avoidance –Basic task of a MAC protocol Energy efficiency –One of the most important attributes for sensor networks, since most nodes are battery powered Scalability and adaptivity –Network size, node density and topology change

2/17/2005Guest lecture for CS113, UCLA8 Other MAC Attributes Channel utilization –How well is the channel used? Also called bandwidth utilization or channel capacity Latency –Delay from sender to receiver; single hop or multi-hop Throughput –The amount of data transferred from sender to receiver in unit time Fairness –Can nodes share the channel equally?

2/17/2005Guest lecture for CS113, UCLA9 Energy Efficiency in MAC Design Energy is primary concern in sensor networks What causes energy waste? –Collisions –Control packet overhead –Overhearing unnecessary traffic –Long idle time bursty traffic in sensor-net apps Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten) Dominant factor

2/17/2005Guest lecture for CS113, UCLA10 Classification of MAC Protocols Schedule-based protocols –Schedule nodes onto different sub-channels –Examples: TDMA, FDMA, CDMA Contention-based protocols –Nodes compete in probabilistic coordination –Examples: ALOHA (pure & slotted), CSMA

2/17/2005Guest lecture for CS113, UCLA11 Next… Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary

2/17/2005Guest lecture for CS113, UCLA12 Scheduled Protocols: TDMA Time division multiple access –Divide time into subchannels –Advantages No collisions Energy efficient — easily support low duty cycles –Disadvantages Difficult to accommodate node changes Requires strict time synchronization

2/17/2005Guest lecture for CS113, UCLA13 Master-slave configuration –The master node decides which slave can send by polling the corresponding slave –Only direct communication between the master and a slave –A special TDMA without pre-assigned slots –Examples IEEE infrastructure mode (CPF) Bluetooth piconets Scheduled Protocols: Polling

2/17/2005Guest lecture for CS113, UCLA14 Scheduled Protocols: Bluetooth Wireless personal area network (WPAN) –Short range, moderate bandwidth, low latency –IEEE (MAC + PHY) is based on Bluetooth Nodes are clustered into piconet –Each piconet has a master and up to 7 active slaves – scalability problem –The master polls each slave for transmission –CDMA among piconets –Multiple connected piconets form a scatternet Difficult to handle inter-cluster communications

2/17/2005Guest lecture for CS113, UCLA15 By Sohrabi and Pottie –Have a pool of independent channels Frequency band or spreading code Potential interfering links select different channels –Talk to neighbors in different time slots –Sleep in unscheduled time slots –Looks like TDMA, but actual multiple access is accomplished by FDMA or CDMA Any pair of two nodes can talk at the same time –Low bandwidth utilization Scheduled Protocols: Self-Organization

2/17/2005Guest lecture for CS113, UCLA16 Scheduled Protocols: LEACH Low-Energy Adaptive Clustering Hierarchy — by Heinzelman, et al. –Similar to Bluetooth –CDMA between clusters –TDMA within each cluster Static TDMA frame Cluster head rotation Node only talks to cluster head Only cluster head talks to base station (long dist.) –The same scalability problem

2/17/2005Guest lecture for CS113, UCLA17 Next… Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary

2/17/2005Guest lecture for CS113, UCLA18 ALOHA –Pure ALOHA: send when there is data –Slotted ALOHA: send on next available slot –Both rely on retransmission when there’s collision CSMA — Carrier Sense Multiple Access –Listening (carrier sense) before transmitting –Send immediately if channel is idle –Backoff if channel is busy non-persistent, 1-persistent and p-persistent Contention Protocols: Classics

2/17/2005Guest lecture for CS113, UCLA19 Hidden terminal problem –CSMA is not enough for multi-hop networks (collision at receiver) CSMA/CA (CSMA with Collision Avoidance) –RTS/CTS handshake before send data –Node c will backoff when it hears b’s CTS Contention Protocols: CSMA/CA ab c Node a is hidden from c’s carrier sense

2/17/2005Guest lecture for CS113, UCLA20 Contention Protocols: MACA and MACAW MACA — Multiple Access w/ Collision Avoidance –Based on CSMA/CA –Add duration field in RTS/CTS informing other node about their backoff time MACAW –Improved over MACA –RTS/CTS/DATA/ACK –Fast error recovery at link layer

2/17/2005Guest lecture for CS113, UCLA21 Contention Protocols: IEEE IEEE ad hoc mode (DCF) –Virtual and physical carrier sense (CS) Network allocation vector (NAV), duration field –Binary exponential backoff –RTS/CTS/DATA/ACK for unicast packets –Broadcast packets are directly sent after CS –Fragmentation support RTS/CTS reserve time for first (fragment + ACK) First (fragment + ACK) reserve time for second… Give up transmission when error happens

2/17/2005Guest lecture for CS113, UCLA22 Contention Protocols: IEEE (cont.) Power save (PS) mode in IEEE DCF –Assumption: all nodes are synchronized and can hear each other (single hop) –Nodes in PS mode periodically listen for beacons & ATIMs (ad hoc traffic indication messages) –Beacon: timing and physical layer parameters All nodes participate in periodic beacon generation –ATIM: tell nodes in PS mode to stay awake for Rx ATIM follows a beacon sent/received Unicast ATIM needs acknowledgement Broadcast ATIM wakes up all nodes — no ACK

2/17/2005Guest lecture for CS113, UCLA23 Contention Protocols: IEEE (cont.) Unicast example of PS mode in DCF

2/17/2005Guest lecture for CS113, UCLA24 Contention Protocols: Tx Rate Control By Woo and Culler –Based on a special network setup A base station tries to collect data equally from all sensors in the network –CSMA + adaptive rate control –Promote fair bandwidth allocation to all sensors Nodes close to the base station forward more traffic, and have less chances to send their own data –Helps in congestion avoidance

2/17/2005Guest lecture for CS113, UCLA25 Contention Protocols: Piconet By Bennett, Clarke, et al. –Not the same piconet in Bluetooth –Low duty-cycle operation — energy efficient Sleep for 30s, beacon, and listen for a while Sending node needs to listen for receiver’s beacon first, then CSMA before sending data –May wait for long time before sending

2/17/2005Guest lecture for CS113, UCLA26 Contention Protocols: PAMAS PAMAS: Power Aware Multi-Access with Signalling — by Singh and Raghavendra –Improve energy efficiency from MACA –Avoid overhearing by putting node into sleep –Use separate control and data channels RTS, CTS, busy tone to avoid collision Probe packets to find neighbors transmission time –Increased hardware complexity Two channels need to work simultaneously, meaning two radio systems.

2/17/2005Guest lecture for CS113, UCLA27 Contention Protocols: ZigBee Based on IEEE MAC and PHY –Three types devices Network Coordinator Full Function Device (FFD) –Can talk to any device, more computing power Reduced Function Device (RFD) –Can only talk to a FFD, simple for energy conservation –CSMA/CA with optional ACKs on data packets –Optional beacons with superframes –Optional guaranteed time slots (GTS), which supports contention-free access

2/17/2005Guest lecture for CS113, UCLA28 Contention Protocols: ZigBee (cont.) Low power, low rate (250kbps) radio MAC layer supports low duty cycle operation –Target node life time > 1 year

2/17/2005Guest lecture for CS113, UCLA29 Next… Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary

2/17/2005Guest lecture for CS113, UCLA30 Case Study 1: S-MAC By Ye, Heidemann and Estrin Tradeoffs Major components in S-MAC –Periodic listen and sleep –Collision avoidance –Overhearing avoidance –Massage passing Latency Fairness Energy

2/17/2005Guest lecture for CS113, UCLA31 Coordinated Sleeping Problem: Idle listening consumes significant energy Solution: Periodic listen and sleep Turn off radio when sleeping Reduce duty cycle to ~ 10% (120ms on/1.2s off) sleep listen sleep Latency Energy

2/17/2005Guest lecture for CS113, UCLA32 Coordinated Sleeping Schedules can differ Prefer neighboring nodes have same schedule — easy broadcast & low control overhead Border nodes: two schedules or broadcast twice Node 1 Node 2 sleep listen sleep listen sleep Schedule 2 Schedule 1

2/17/2005Guest lecture for CS113, UCLA33 Coordinated Sleeping Schedule Synchronization –New node tries to follow an existing schedule –Remember neighbors’ schedules — to know when to send to them –Each node broadcasts its schedule every few periods of sleeping and listening –Re-sync when receiving a schedule update Periodic neighbor discovery –Keep awake in a full sync interval over long periods

2/17/2005Guest lecture for CS113, UCLA34 Coordinated Sleeping Adaptive listening –Reduce multi-hop latency due to periodic sleep –Wake up for a short period of time at end of each transmission CTS RTS CTS  Reduce latency by at least half listen t1 t2

2/17/2005Guest lecture for CS113, UCLA35 Collision Avoidance S-MAC is based on contention Similar to IEEE ad hoc mode (DCF) –Physical and virtual carrier sense –Randomized backoff time –RTS/CTS for hidden terminal problem –RTS/CTS/DATA/ACK sequence

2/17/2005Guest lecture for CS113, UCLA36 Overhearing Avoidance Problem: Receive packets destined to others Solution: Sleep when neighbors talk –Basic idea from PAMAS (Singh, Raghavendra 1998) –But we only use in-channel signaling Who should sleep? –All immediate neighbors of sender and receiver How long to sleep? –The duration field in each packet informs other nodes the sleep interval

2/17/2005Guest lecture for CS113, UCLA37 Message Passing Problem: Sensor net in-network processing requires entire message Solution: Don’t interleave different messages –Long message is fragmented & sent in burst –RTS/CTS reserve medium for entire message –Fragment-level error recovery — ACK — extend Tx time and re-transmit immediately Other nodes sleep for whole message time Fairness Energy Msg-level latency

2/17/2005Guest lecture for CS113, UCLA38 Implementation and Experiments Platform: Mica Motes Topology: 10-hop linear network S-MAC saved a lot of energy compared with a MAC without sleep Message inter-arrival period (S) Energy consumption (J) 10% duty cycle without adaptive listen No sleep cycles 10% duty cycle with adaptive listen Energy consumption at different traffic load

2/17/2005Guest lecture for CS113, UCLA39 Case Study 2: B-MAC Another low-power MAC for sensor networks B-MAC design considerations –Simplicity: based on simple CSMA –Configurable options –Minimize idle listening –Based on model of periodic sensor data transfer B-MAC components –CSMA without RTS/CTS –Optional Low-power listening (LPL) –Optional ACK

2/17/2005Guest lecture for CS113, UCLA40 Low-Power Listening Determine channel status by quick sampling –Very low overhead on wake-up Joe Polastre, et al., SenSys’04

2/17/2005Guest lecture for CS113, UCLA41 Low Duty Cycle with LPL Nodes periodically sleep and perform LPL Nodes do not synchronized on listen time Sender uses a long preamble before each packet to wake up the receiver Shift most burden to the sender

2/17/2005Guest lecture for CS113, UCLA42 Comparison of S-MAC and B-MAC S-MACB-MAC Collision avoidanceCSMA/CACSMA ACKYesOptional Message passingYesNo Overhearing avoidanceYesNo Listen periodPre-defined + adaptive listenPre-defined Listen intervalLongVery short Schedule synchronizationRequiredNot required Packet transmissionShort preambleLong preamble Code size6.3KB4.4KB (LPL & ACK)

2/17/2005Guest lecture for CS113, UCLA43 Next… Introduction to MAC MAC attributes and trade-offs Scheduled MAC protocols Contention-based MAC protocols Case studies Summary

2/17/2005Guest lecture for CS113, UCLA44 MAC Design for Sensor Networks MAC protocols can be classified as scheduled and contention-based Major considerations –Energy efficiency –Scalability and adaptivity to number of nodes Major ways to conserve energy –Low duty cycle to reduce idle listening –Effective collision avoidance –Overhearing avoidance –Reducing control overhead

2/17/2005Guest lecture for CS113, UCLA45 Scheduled vs. Contention Protocols Scheduled Protocols Contention Protocols CollisionsNoYes Energy efficiencyGood Need improvement Scalability and adaptivity BadGood Multi-hop communication DifficultEasy Time synchronization Strict Loose or not required

2/17/2005Guest lecture for CS113, UCLA46