1. UNIVERSITY OF CALIFORNIA SANTA CRUZ Energy-Efficient Channel Access Protocols Venkatesh Rajendran venkat@soe.ucsc.edu

2. UC Santa Cruz 2 Introduction  Sensor networks are a special class of multihop wireless networks. Ad-hoc deployment. Self-configuring. Unattended. Battery powered.

3. UC Santa Cruz 3 Network Architecture Sink Thousands of nodes. Short radio range (~10m). Event-driven. Hierarchical deployment. Fault tolerance. Information Processing

4. UC Santa Cruz 4 MAC Protocols  Regulate channel access in a shared medium. A C B Collision at B X No coordination (ALOHA)

5. UC Santa Cruz 5 CSMA Listen before transmitting Stations sense the channel before transmitting data packets. SRH X Collision at R Hidden Terminal Problem

6. UC Santa Cruz 6 CSMA with Collision Avoidance  Stations carry out a handshake to determine which one can send a data packet (e.g., MACA, FAMA, IEEE802.11, RIMA). SRH RTS CTS Data ACK Backoff due to CTS

7. UC Santa Cruz 7 Idle Listening Tx Rx Idle Listen Tx Rx Sleep Sleep Scheduling Medium Access  Energy-efficient channel access is important to prolong the life-time of sensor nodes.  Conventional media-access control protocols waste energy by collisions, and idle listening.  Radios have special sleep mode for energy conservation.

8. UC Santa Cruz 8 Achieving energy efficiency  When a node is neither transmitting or receiving, switch to low-power sleep mode.  Prevent collisions and retransmissions.  Need to know Tx, Rx and when transmission event occurs. Scheduled-based(time-slotted) MAC Protocols

9. UC Santa Cruz 9 Contention-based Channel Access Protocols: S-MAC & T-MAC

10. UC Santa Cruz 10 S-MAC [Ye etal] Features  Collision Avoidance Similar to 802.11 (RTS/CTS handshake).  Overhearing Avoidance All the immediate neighbors of the sender and receiver goes to sleep.  Message Passing Long messages are broken down in to smaller packets and sent continuously once the channel is acquired by RTS/CTS handshake. Increases the sleep time, but leads to fairness problems.

11. UC Santa Cruz 11 S-MAC Overview  Time is divided in to cycles of listen and sleep intervals.  Schedules are established such that neighboring nodes have synchronous sleep and listen periods.  SYNC packets are exchanged periodically to maintain schedule synchronization.

12. UC Santa Cruz 12 S-MAC Operation

13. UC Santa Cruz 13 Schedule Establishment  Node listens for certain amount of time.  If it does not hear a schedule, it chooses a time to sleep and broadcast this information immediately.  This node is called the ‘Synchornizer’.  If a node receives a schedule before establishing its schedule, it just follows the received schedule.  If a node receives a different schedule, after it has established its schedule, it listens for both the schedules.

14. UC Santa Cruz 14 S-MAC Illustration

15. UC Santa Cruz 15 T-MAC [Dam etal] : S-MAC Adaptive Listen

16. UC Santa Cruz 16 T-MAC: Early Sleeping Problem

17. UC Santa Cruz 17 T-MAC/S-MAC Summary  Simple contention-based channel access with duty cycle-based sleeping.  Restricting channel contention to a smaller window  negative effect on energy savings due to collisions.  Requires schedule co-ordination with one hop neighbors.

18. UC Santa Cruz 18 Scheduling-based Channel Access Protocols: TRAMA and FLAMA

19. UC Santa Cruz 19 TRAMA: TRaffic-Adaptive Medium Access  Establish transmission schedules in a way that: is self adaptive to changes in traffic, node state, or connectivity. prolongs the battery life of each node. is robust to wireless losses.

20. UC Santa Cruz 20 TRAMA - Overview  Single, time-slotted channel access.  Transmission scheduling based on two-hop neighborhood information and one-hop traffic information.  Random access period Used for signaling: synchronization and updating two-hop neighbor information.  Scheduled access period: Used for contention free data exchange between nodes. Supports unicast, multicast and broadcast communication.

21. UC Santa Cruz 21 TRAMA Features  Distributed TDMA-based channel access.  Collision freedom by distributed election based on Neighborhood- Aware Contention Resolution (NCR).  Traffic-adaptive scheduling to increase the channel utilization.  Radio-mode control for energy efficiency.

22. UC Santa Cruz 22 Time slot organization

23. UC Santa Cruz 23  Each node maintains two-hop neighbor information.  Based on the time slot ID and node ID, node priorities are calculated using a random hash function.  A node with the highest two-hop priority is selected as the transmitter for the particular time slot. Neighborhood-aware contention resolution (NCR)[Bao et al., Mobicom00]

24. UC Santa Cruz 24 A C D B E F 3 2 15 8 9 11 C Winner G H I 13 3 14 NCR does not elect receivers and hence, no support for radio-mode control. NCR - Example

25. UC Santa Cruz 25 TRAMA Components  Neighbor Protocol (NP). Gather 2-hop neighborhood information.  Schedule Exchange Protocol (SEP). Gather 1-hop traffic information.  Adaptive Election Algorithm (AEA). Elect transmitter, receiver and stand-by nodes for each transmission slot. Remove nodes without traffic from election.

26. UC Santa Cruz 26 Neighbor Protocol  Main Function: Gather two-hop neighborhood information by using signaling packets.  Incremental neighbor updates to keep the size of the signaling packet small.  Periodically operates during random access period.

27. UC Santa Cruz 27 Packet Formats

28. UC Santa Cruz 28 Schedule Exchange Protocol (SEP)  Schedule consists of list of intended receivers for future transmission slots.  Schedules are established based on the current traffic information at the node.  Propagated to the neighbors periodically.  SEP maintains consistent schedules for the one-hop neighbors.

29. UC Santa Cruz 29 Schedule Packet Format

30. UC Santa Cruz 30 Adaptive Election Algorithm (AEA)  Decides the node state as either Transmit, Receive or Sleep.  Uses the schedule information obtained by SEP and a modified NCR to do the election.  Nodes without any data to send are removed from the election process, thereby improving the channel utilization.

31. UC Santa Cruz 31 Simulation Results Delivery Ratio Synthetic broadcast traffic using Poisson arrivals. 50 nodes, 500x500 area. 512 byte data. Average node density: 6

32. UC Santa Cruz 32 Energy Savings Percentage Sleep TimeAverage Length of sleep interval

33. UC Santa Cruz 33 TRAMA Limitations  Complex election algorithm and data structure.  Overhead due to explicit schedule propagation.  Higher queueing delay. Flow-aware, energy-efficient framework.

34. UC Santa Cruz 34 TRAMA Summary  Significant improvement in delivery ratio in all scenarios when compared to contention- based protocols.  Significant energy savings compared to S- MAC (which incurs more switching).  Acceptable latency and traffic adaptive.

35. UC Santa Cruz 35 Flow-aware Medium Access Framework  Avoid explicit schedule propagation. Take advantage of application.  Simple election algorithm to suit systems with low memory and processing power (e.g., 4KB ROM in Motes).  Incorporate time-synchronization, flow discovery and neighbor discovery during random-access period.

36. UC Santa Cruz 36 Flow Information  Flow information characterizes application-specific traffic patterns.  Flows can be unicast, multicast or broadcast.  Characterized by source, destination, duration and rate.

37. UC Santa Cruz 37 Example: Data Gathering Application A C E B D Sink Fb Fc Fd Fe

38. UC Santa Cruz 38 Flow Discovery Mechanism  Combined with neighbor discovery during random-access period.  Adapted based on the application. for data gathering application, flow discovery is essentially establishing the data forwarding tree.

39. UC Santa Cruz 39 Example S Sink initiates neighbor discovery, flow discovery and time synchronization. Broadcasts periodic SYNC packets. Potential children reply with SYNC_REQ. Source reinforces with another SYNC packet. Once associated with a parent, nodes start sending periodic SYNC broadcasts. A B C Sink SYNC SYNC_REQ

40. UC Santa Cruz 40 Election Process  Weighted election to incorporate traffic adaptivity.  Nodes are assigned weights based on their incoming and outgoing flows.  Highest priority 2-hop node is elected as the transmitter.  A node listens if any of its children has the highest 1- hop priority. Can switch to sleep mode if no transmission is started. S A B C Sink ws=0 wc=1 wa=3

41. UC Santa Cruz 41 Simulation Results (16 nodes, 500x500 area, CC1000 radio, grid topology, edge sink) Delivery Ratio Queueing Delay

42. UC Santa Cruz 42 FLAMA Summary  Simple algorithm that can be implemented on a sensor platform.  Significant performance improvement by application awareness.