1. B-MAC: Versatile Low Power Media Access for Wireless Sensor Networks SenSys ’ 04

2. Reminder: Proposal Presentation on Monday, March 5 Prepare powerpoint slides to motivate your project and show the overall approach to tackling the problem Come at least 10 minutes before the class begins –Give your slides to the tech staff –Lean how to use the presentation tool etc. Teams & presentation order: –Mike & Vic (+ Brian) –Chris & Yan –Dahee & Chao –Surabh –Mehmet

3. Design goals of B-MAC Low power operation Effective collision Simple implementation, small code & RAM size Efficient channel utilization at low & high data rates Reconfigurable by network protocols Tolerant to changing RF/Networking conditions Scalable to large numbers of nodes

4. Small, configurable MAC Export control to higher services to support wide variety of WSN workloads –WSNs are supposed to support various applications S-MAC (discussed in the previous class) is more than a link layer protocol –Drawbacks of S-MAC Scalability: A node may have to remember many schedules and wake up accordingly MAC layer may not be the best place for sleep scheduling & synchronization

5. Adaptive, reconfigurable MAC Adaptive bidirectional interface for WSN applications –Reconfigure the MAC protocol based on the current workload –Identify the best parameters for an arbitrary low power WSN applications at compile or run time & estimate the application ’ s lifetime

6. B-MAC Design Really simple –CSMA via CCA (Clear Channel Assessment) & backoff –Low power listening via Preamble –Acknowledgment Enable/disable anything above and allow to build anything on top of the configured B-MAC!

7. B-MAC Interfaces

8. Clear Channel Assessment Effective collision avoidance Find out whether the channel is idle –If too pessimistic: waste bandwidth –If too optimistic: more collisions Key observation –Ambient noise may change significantly depending on the environment –Packet reception has fairly constant channel energy Software approach to estimating the noise floor

9. Take a signal sample when the channel is assumed to be free –Right after a packet is transmitted or when no valid data is received –Take exponential moving average (EMA) of the median signal strength Works as a low pass filter Smoothed idle signal level S m (t) = a * S(t) + (1 - a) * S m (t-1) –S m (t): EMA at time t –S(t-1): Signal strength of ambient noise at t –S m (t-1): EMA at time t-1 –It contrasts to common threshold-based methods in which only a single sample is taken –Resilient to time-varying ambient noise

10. CCA vs. Threshold techniques Idle Threshold: waste channel utilization CCA: Fully utilize the channel since a valid packet could have no outlier significantly below the noise floor CCA dynamically adjusts threshold CCA finds channel busy/idle status with high accuracy

11. CCA can be turned on/off –If turned off, a schedule-based protocol, e.g., S- MAC, can be implemented atop B-MAC If turned on, initial channel backoff when sending a message B-MAC does not set the backoff time, but signals an event to the higher service that sent the packet The higer level service may return an initial backoff time or ignore the event – If ignored, use a short random delay

12. Low Power Listening: Preamble Sampling Sender Receiver Preamble Send data Preamble sampling Active to receive a message S R |Preamble| ≥ Sampling period Preamble is not a packet but a physical layer RF pulse –Minimize overhead

13. Optional link layer ACK If enabled, ACK is sent immediately after receiving a unicast packet Overall, B-MAC is easier to implement than S-MAC –No RTS/CTS Is this always good? or  ? We know RTS/CTS can reduce hidden/exposed node problem You may have to implement RTS/CTS on your own... Simple but not very friendly –No synchronization No need for a schedule table in S-MAC But periodic sleep & wake-up is a good approach to energy saving

14. Modeling Lifetime Monitoring applications E = E sleep + E listen + E d + E rx + E tx Given #nodes in the neighborhood, BMAC can estimate the network lifetime Lifetime t l = 1/E * C batt * V * 60 * 60

15. Derivation of Lifetime E d = t d * c data * V where t d = t data * r E tx = t tx * c txb * V where t tx = r * (L preamble + L packet ) * t txb E rx = t rx * c rxb * V where t rx ≤ n * r * (L preamble + L packet ) * t rxb –r * sum_i=1^n ( children (i) + 1 ) –L preamble ≥ t i / t rxb

16. Derivation of Lifetime (Cont’d) E sample = 17.3 uJ E listen ≤ E sample * 1/t i t listen = (t r_init + t r_on + t rx /t x + t sr ) * 1/t i t sleep = 1 – t rx – t tx – t d – t listen E sleep = t sleep * c sleep * V Lifetime t l = 1/E * C batt * V * 60 * 60

17. Network Parameters Scientists may determine the physical location of the nodes & ideal sampling rate –Compute the parameters to get the best lifetime that B- MAC can achieve –Best LPL check interval is the lowest line at a given network density in the following graph If n = 20, 50ms check interval is optimal If n=60, 25ms is best

18. Experiments Compare BMAC to S-MAC & T-MAC –T-MAC is similar to S-MAC, but a receiver goes to sleep if it does not receive any message –B-MAC & S-MAC: implemented in TinyOS –TMAC: simulated in Matlab

19. TinyOS Implementation B-MAC does not need timestamp

20. Packet transmission time vs. Checking frequency More frequent checking of the radio –Shorter transmission time –More energy consumption LPL check interval

21. Channel utilization Place n nodes equidistant from a receiver Increase n to increase load BMAC relies on higher level services to send data according to the traffic pattern Consider hidden node problem too For example, after a packet is sent to the parent, nodes in the same cell wait for a certain amount of time for the parent to forward the packet up the tree  Always work? More difficulty for developing sensing applications?

22. Energy per byte

23. End-to-end latency

24. Contention-Free Protocols

25. Classic Protocols TDMA (Time Division Multiple Access) –A node can sleep when it is not its turn to send or receive FDMA (Frequency Division Multiple Access) CDMA (Code Division Multiple Access) No node within two hops can use the same slot to avoid the hidden node problem

26. Optimal channel assignment Achieve contention-free communication using the minimum number of channels The problem of assigning a minimum number of channels for an arbitrary graph is NP-hard –Develop efficient heuristics –Centralized approaches do not scale

27. Stationary MAC and Startup Local synchronization only Starting phase –Handshaking on a common control channel –Each link utilizes a unique random frequency or CDMA frequency hopping code –Assume there are sufficiently many frequencies or codes –Periodically use the slot

28. BFS/DFS-based scheduling Breadth-first or depth-first traversals of a data gathering tree Every single node is given a slot BFS might provide more chances for aggregation DFS may transmit individual data more quickly Global synchronization required

29. Reservation-based synchornized MAC (ReSync) TDMA is not flexible enough to allow the traffic from each node to change over time ReSync provides more flexibility Each node maintains an epoch based on its local time (or can be synchronized with nearby neighbors) –Select a regular time in each epoch to send a short intent message Probability of collisions is low since the intent is very short –Listen long enough to learn when the neighbor is sending the intent –The intended receiver wakes up at the corresponding time to receive the message –No RTS/CTS Data transmissions are scheduled randomly

30. Traffic-adaptive medium access (TRAMA) Distributed TDMA for flexible & dynamic scheduling of time slots Divide time epochs into a set of short signaling slots followed by a set of longer transmission slots Key components –Neighbor protocol (NP) –Schedule exchange protocol (SEP) –Adaptive election algorithm (AEP)

31. Neighbor Protocol Nodes exchange one-hop neighbor info during the random access signaling slots –Ensure the slots are long enough to allow all nodes to get consistent two-hop neighbor info

32. Schedule exchange protocol Each node publishes its schedule during the last winning slot in each epoch Use bitmaps to indicate the intended unicast or multicast recipients Sleep when not required to transmit or receive

33. Adaptive election algorithm Hash function based on node IDs and time –Ensure there ’ s a unique ordering of node priorities within any two-hop region at each time –A node transmits iff it has the highest priority among the two hop neighbors at the moment –Sophisticated slot reuse protocol

34. Summary MAC protocols in WSNs –Arbitration for access to the wireless channel –Energy conservation B-MAC is a nice building block for diverse applications and workloads –TDMA, for example, can be built on top of it Energy savings –Preamble –S-MAC like periodic sleep & wake-up –Adaptive listening in S-MAC & T-MAC –D-MAC & DESS try to minimize the E2E delay, while using sleep modes TDMA –No idle listening –Higher complexity distributed algorithms or tight synchronization required

35. Questions?