Wireless Networks

1. Wireless Networks Lecture 35 MAC Protocols for WSN Part II Dr. Ghalib A. Shah

2. Outlines • Challenges in WSNs. • Attributes of MAC Protocol • Overview of MAC protocols • Energy Efficiency in MAC • Proposed Routing Protocol • S-MAC • T-MAC • DS-MAC • Traffic Adaptive MAC • DMAC • Contention-Free MAC

3. Last Lecture • Introduction to WSN • Applications of WSN • Factors Influencing Performance of WSN • Power consumption, fault tolerance, scalability, topology, cost • Architecture and Communication Protocols

4. Research Directions • Topology Control • Coverage • Data Aggregation • Temporal/Spatial Correlation • Localization / Synchronization • Energy Efficient Data Dissemination • QoS Framework • Network Monitoring and Management • How to integrate WSNs into NGWI ?

5. Simulation for Sensor Networks Simulation provides : • Controlled , Reproducible testing environment • Cost – effective alternative • Means to explore and improve design space

6. TinyOS • The role of any operating system (OS) is to promote development of reliable application software by providing a convenient and safe abstraction of hardware resources. • Wireless sensor networks (WSNs) are embedded but general-purpose, supporting a variety of applications, incorporating heterogeneous components, and capable of rapid deployment in new environments • An open-source development environment • A programming language and model (NesC) • TOSSIM for simulating TinyOS • TinyDB for Sensor DB in TinyOS

7. Introduction • Important attributes of MAC protocols • Collision avoidance • Basic task — medium access control • Energy efficiency • Scalability and adaptivity • Number of nodes changes overtime • Latency • Fairness • Throughput • Bandwidth utilization

8. C A B Hidden terminal: A is hidden from C’s CS Overview of MAC protocols • Contention-based protocols • CSMA — Carrier Sense Multiple Access • Ethernet • Not enough for wireless (collision at receiver) • MACA — Multiple Access w/ Collision Avoidance • RTS/CTS for hidden terminal problem • RTS/CTS/DATA

9. Overview of MAC Protocols • Contention-based protocols (contd.) • MACAW — improved over MACA • RTS/CTS/DATA/ACK • Fast error recovery at link layer • IEEE 802.11 Distributed Coordination Function • Largely based on MACAW • Protocols from voice communication area • TDMA — low duty cycle, energy efficient • FDMA — each channel has different frequency • CDMA — frequency hopping or direct sequence

10. Dominant in sensornets Energy Efficiency in MAC Design • Energy is primary concern in sensor networks • What causes energy waste? • Collisions • Control packet overhead • Overhearing unnecessary traffic • Overemitting • Long idle time • bursty traffic in sensor-net apps • Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten)

11. Energy Efficiency in MAC Design • TDMA vs. contention-based protocols • TDMA can easily avoid or reduce energy waste from all above sources • Contention protocols needs to work hard in all directions • TDMA has limited scalability and adaptivity • Hard to dynamically change frame size or slot assignment when new nodes join • Restrict direct communication within a cluster • Contention protocols easily accommodate node changes and support multi-hop communications

12. Listen Sleep Listen Sleep Listen C A B D S-MAC: Periodic Listen & Sleep • Frame • Duty cycle • (Listen Interval / Frame Length) • Frame schedule • Nodes are free to choose their listen/sleep schedule • Requirement: neighboring nodes synchronize together • Exchange schedules periodically (SYNC packet) • Synchronization period (SP) • Nodes communicate in receivers scheduled listen times

13. S-MAC: Coordinated Sleeping (1) Frame Schedule Maintenance • Choosing a schedule • Listen to the medium for at least SP • Nothing heard, choose a schedule • Broadcast a SYNC packet (should contend for medium) • Following a schedule • Receives a schedule before choosing/announcing • Follows the schedule • Broadcast a SYNC packet • Adopting multiple schedules • Receives a schedule after choosing/announcing • Can discard the new schedule; or • Follow both the schedules – suffer more energy loss

14. Listen Sleep for SYNC for RTS for CTS Receiver S-MAC: Coordinated Sleeping (2) Maintaining Synchronization • Clock drifts – not a major concern (listen time = 0.5s – 105 times longer than typical drift rates) • Need to mitigate long term drifts – schedule updating using SYNC packet (sender ID, its next scheduled sleep time – relative); • Listen is split into 2 parts – for SYNC and RTS/CTS • Once RTS/CTS is established, data sent in sleep interval

15. ListenR ListenON RTS DATA Sender CTS ACK Receiver Wakes up even though it is not the correct listen-interval Sleep (based on RTS) Overhearing nodes (ON) Sleep (based on CTS) S-MAC: Coordinated Sleeping (3) Adaptive Listening – Low-duty cycle to active mode * Overhearing nodes – wakeup at the end of the current transmission (duration field in RTS/CTS) Not all receiver’s next-hop nodes can hear the transmission, if adaptive

16. Normal S-MAC Sleep Active Sleep Active Sleep Active Drawbacks of S-MAC • Active (Listen) interval – long enough to handle to highest expected load • If message rate is less – energy is still wasted in idle-listening • S-MAC fixed duty cycle – is NOT OPTIMAL • High Latency

17. Active Active Active Sleep Sleep TA TA TA T-MAC: Preliminaries • Adaptive duty cycle: • A node is in active mode until no activation event occurs for time TA • Periodic frame timer event, receive, carrier sense, send-done, knowledge of other transmissions being ended • Communication ~= S-MAC/802.11 • Frame schedule maintenance ~= S-MAC

18. T-MAC: Choosing TA • Requirement: a node should not sleep while its neighbors are communicating, potential next receiver • TA > C+R+T • C – contention interval length; • R – RTS packet length; • T – turn-around time, time bet. end of RTS and start of CTS; • TA = 1.5 * (C+R+T);

19. Prons • Performs better under variable traffic load • Cons • Higher overheads than SMAC to maintain variable wakeup schedule. • Unfairness and unpredictable delay.

20. Dynamic Sensor-MAC (DSMAC) • TMAC improves the latency in SMAC at cost of complexity. • DSMAC provides simple solution to static duty cycle. • All nodes start with same duty cycle. • If one-hop latency is observed higher by receiver, it doubles its duty cycle • Nodes share their one-hop latency values with neighbors during SYNC period. • The transmitter also doubles its duty cycle if the destination reported higher one-hop latency. • This change will not affect the schedule of other neighbors.

21. DSMAC Schduling

22. Traffic-Adaptive MAC (TRAMA) • Time is divided into random-access and scheduled-access (transmission) periods. • The random-access period is used to establish two-hop topology information • MAC layer can calculate the transmission duration needed, which is denoted as SCHEDULE_INTERVAL • the node calculates the number of slots for which it will have the highest priority among two-hop neighbors • The node announces the slots it will use as well as the intended receivers for these slots with a schedule packet. • the node announces the slots for which it has the highest priority but it will not use • The schedule packet indicates the intended receivers using a bitmap whose length is equal to the number of its neighbors

23. Advantages • Higher percentage of sleep time and less collision probability are achieved, as compared to CSMA-based protocols. • Since the intended receivers are indicated by a bitmap, less communication is performed for the multicast and broadcast types of communication patterns, compared to other protocols. • Disadvantages • Transmission slots are set to be seven times longer than the random-access period. This means that without considering the transmissions and receptions, the duty cycle is at least 12.5 percent (idle time),

24. DMAC • Supports convergecast communication model, • Data-aggregation tree is formed from sources to sink node. • It is an improved slotted ALOHA algorithm. • Slots are allotted according to the level of tree from leaf to root. • It incurs low latency but no collision avoidance for nodes at same level

25. DMAC A minimum period u consists of one packet tx and rx. Wakeup period in three is skewed depending on depth d. so du is the wakeup time Node at higher layer will be in rx state when lower layer nodes are in tx state Nodes on path wakeup sequentially to forward packet to next hop: low latency with efficient energy consumption

26. 1 2 3 i  1 2 3 i  1 2 3 4  1 2 3 4  Contention-Free MAC protocols for Wireless Sensor Networks • Asynchronous Slot Assignment • Each node locally discretizes its local time. • The number of slots in a time frame, called the frame size and denoted by , is set to 22. • Having the same frame size at all nodes ensures that overlapping time slots remain the same.

27.   Nonready Ready ASAND – Basic Approach Select random slot  Report conflicts between neighbors Transmit beacon at  Listen for  slots Conflict YES NO Obtain slot 

28. u w v ASAND – Conflict Reporting • The 2-hop neighbors u and v are unaware that they have selected conflicting time slots (their transmissions collide on w). • Having observed a collision in its local time t, node w transmits at time t+, creating a spurious conflict with both u and v. • This is called conflict reporting essentially reduces a conflict between hidden terminals to a conflict between neighbor nodes. • After t+, u and v will be forced to select new slots

29. Summary • Challenges in WSNs. • Attributes of MAC Protocol • Overview of MAC protocols • Energy Efficiency in MAC • Proposed Routing Protocol • S-MAC • T-MAC • DS-MAC • Traffic Adaptive MAC • DMAC • Contention-Free MAC