The University of Iowa. Copyright© 2005 A. Kruger 1 Introduction to Wireless Sensor Networks Medium Access Control (MAC) 21 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 Contention-Based MAC Protocols Channel are not divided, but shared channel allocated on-demand Advantages –Scale easily across node density and load –More flexible (no need to for clusters, hierarchies) peer-to-peer directly supported –Don’t require fine-grained synchronization as in TDMA Major disadvantage –Inefficient use of energy

The University of Iowa. Copyright© 2005 A. Kruger 4 Review: Energy Efficiency in MAC Protocols Question – what causes energy waste from a MAC perspective? –Collision –Idle listening –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 5 Examples of Contention MAC Protocols Carrier Sense Medium Access (CSMA) –Central idea – listen (carrier sense) before transmitting Variants –Non-persistent CSMA If medium busy, wait random time and try again –1-Persistent CSMA If medium busy, keep listening and transmit when medium becomes free –p-Persistent CSMA If medium busy, transmit with probability (1-p). If medium free transmit with probability p.

The University of Iowa. Copyright© 2005 A. Kruger 6 Hidden Terminal Problem in CSMA abc Node a, b, and c can only hear their immediate neighbors When node a send to b, c is unaware of a, its carrier sense indicates carrier free Node c starts transmitting Packets from a and c collide at b

The University of Iowa. Copyright© 2005 A. Kruger 7 CSMA/CA - Carrier Sense Collision Avoidance Establish a brief handshake between sender and receiver before sending data –Sender sends Request-to-Send (RTS) packet to intended receiver –Receiver replies with Clear-to-Send (CTS) packet –Only then does transmitter send data –RTS-CTS packets announce to neighbors –Node c hears CTS packets from b to a, and does not transmit –Does not eliminate collisions, but collisions are now mostly (brief) RST a b c

The University of Iowa. Copyright© 2005 A. Kruger 8 Variations MACA – RST and CTS packets indicate size on data so other nodes know how long to back off MACAW – adds an Acknowledge ACK packet –RTS-CTS-DATA-ACK IEEE –CSMA/CA, MACA, and MACAW => Distributed coordination function (DCF) + enhancements

The University of Iowa. Copyright© 2005 A. Kruger 9 Energy Conservation in Contention Protocols Basic idea, put radio to sleep with it is not used This makes it difficult for nodes to communicate Beacons Coordinate sleeping (frames)

The University of Iowa. Copyright© 2005 A. Kruger 10 Beacons (IEEE & Piconet) One node periodically broadcast beacon (all participate) Beacon synchronizes all nodes After each beacon, ad hoc traffic indication message (ATIM). All nodes are awake during ATIM Then CSMA Assumption: all nodes can hear each other. Generalizing to multi-hop is not easy

The University of Iowa. Copyright© 2005 A. Kruger 11 PAMAS and Channel Probing Two channels –Data –Control Upon wakeup, probe control channel for activity related to destination node –Neighbor answer probe? Yes, go back to sleep Probing eliminated interference with transmission in data channel More complex to implement (two channels) PAMAS does not reduce idle listening.

The University of Iowa. Copyright© 2005 A. Kruger 12 S-MAC Specifically designed for WSN Reduces energy from all major sources –Idle listening, collision, overhearing and control overhead Course-grained sleep/wakeup cycle Each node has its own wakeup-listen (communicate), sleep schedule Schedules are shared with neighbors

The University of Iowa. Copyright© 2005 A. Kruger 13 S-MAC Scheduling All nodes choose their own schedules Share schedules with neighbors Node then schedule transmissions during listen times of neighbors If a wants to send to b, it just waits for b’s listen cycle to start

The University of Iowa. Copyright© 2005 A. Kruger 14 S-MAC Scheduling Nodes periodically broadcast SYNC packets to synchronize clocks S-MAC encourages neighbors to adopt identical schedules –When it configures itself, a node listens for a synchronization period, and adopts the first schedule it hears –Nodes periodically does neighbor discovery, by listing for an entire frame 1-10% Duty cycle

The University of Iowa. Copyright© 2005 A. Kruger 15 S-MAC Data Transmission Contention happens only during listen interval S-MAC puts a duration field in each packet (some other protocols have it only at the start) –Nodes that don’t have medium access, know how long to sleep even if they try to gain medium access in the middle of an ongoing conversation Application-Level Message Passing –Only one RTS and CTS for all fragments –Each fragment has ACK (which also contains duration) –Reserves medium –Burst mode –Aids in-network processing in WSNs (how?) Variation Adaptive Listening –Rather than wait until the next scheduled listen interval, nodes wake up immediately after RTS-CTS-DATA-ACK

The University of Iowa. Copyright© 2005 A. Kruger 16 S-MAC Pros and Cons Cons –Relatively Complex (especially since CSMA is often touted as being simple…) –Increased Latency Pros –Periodic sleep provides excellent energy performance at light loads –Adaptive listen adjusts to traffic to achieve same performance as no-sleep at heavy load

The University of Iowa. Copyright© 2005 A. Kruger 17 S-MAC Energy Performance Heavy Load Light Load Mica motes

The University of Iowa. Copyright© 2005 A. Kruger 18 S-MAC Latency Performance

The University of Iowa. Copyright© 2005 A. Kruger 19 Energy-Time (Js)/byte Heavy Load Light Load

The University of Iowa. Copyright© 2005 A. Kruger 20 Review Questions Briefly describe channel probing in MAC protocols (e.g., the PAMAS MAC protocol). List disadvantages True or false – Without adaptive listening latency in S-MAC is linear with the number of hops Contrast application-level message passing with MAC fragments. Explain why this is relevant in WSNs Explain how application-level message passing is implemented in S- MAC True or false – CSMA is an example of a contention-based MAC protocol Explain the difference between non-persistent, 1-persistent, and p- persistent CSMA What is the hidden-terminal problem as it relates to CSMA in WSNs What are RTS and CTS packets? Explain how handshaking is used to reduce collisions in CSMA What are “beacons” as it relates to CSMA Explain the advantage of adding the message duration to each packet in S-MAC What does “S-MAC” stand for?