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Medium-Access Control

Medium-Access Control. Medium Access. Radio communication: shared medium. Throughput, delay, and fairness. MAC for sensor networks: Must also be energy efficient. Contention- versus scheduled-based access. Energy efficiency through switching nodes to low-power, sleep mode.

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Medium-Access Control

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  1. Medium-Access Control

  2. Medium Access • Radio communication: shared medium. • Throughput, delay, and fairness. • MAC for sensor networks: • Must also be energy efficient. • Contention- versus scheduled-based access. • Energy efficiency through switching nodes to low-power, sleep mode.

  3. Hidden- and Exposed Terminals B C A D B A C Exposed Terminal Problem: B is supposed to transmit to A and C to D; but C senses B’s transmission and defers: waste of bandwidth. Hidden Terminal Problem: a transmits to B; C does not sense A, and also transmits to B: collision!

  4. Contention-based access performs well in low load scenarios. Examples: Packet radio (1970’s): Aloha, Slotted Aloha. CSMA, CSMA-CD (Ethernet). Collisions? Scheduled-based access are collision free. Example: TDMA. No nodes within 2 hops of each other may use the same slot. Lends itself to energy efficient approaches. Turn nodes off when they are not sending/receiving. Contention- versus Scheduled-Based Access

  5. MAC in Sensor Networks • Sensor networks are a special class of multihop wireless networks. Ad-hoc deployment. Self-configuring. Unattended. Battery powered.

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

  7. MAC Protocols • Regulate channel access in a shared medium. C No coordination (ALOHA) A X B Collision at B

  8. CSMA Listen before transmitting Stations sense the channel before transmittingdata packets. Hidden Terminal Problem S R H X Collision at R

  9. 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). RTS S CTS R H Data Backoff due to CTS ACK

  10. Sleep Scheduling Idle Listening Idle Listen Sleep Rx Rx Tx Tx Sleep Idle Listen 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.

  11. Achieving energy efficiency • When a node is neither transmitting nor 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

  12. Contention-based Channel Access Protocols: S-MAC & T-MAC

  13. 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.

  14. 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.

  15. S-MAC Operation

  16. 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.

  17. S-MAC Illustration

  18. T-MAC[Dam etal]: S-MAC Adaptive Listen

  19. T-MAC: Early Sleeping Problem

  20. 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.

  21. Scheduling-based Channel Access Protocols: TRAMA and FLAMA

  22. 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.

  23. 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.

  24. 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.

  25. Time slot organization

  26. Neighborhood-aware contention resolution (NCR)[Bao et al., Mobicom00] • 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.

  27. 13 H D 9 A I 2 F 3 C C 11 Winner 15 B G E 8 3 14 NCR - Example NCR does not elect receivers and hence, no support for radio-mode control.

  28. 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.

  29. 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.

  30. Packet Formats

  31. 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.

  32. Schedule Packet Format

  33. 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.

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

  35. Energy Savings Percentage Sleep Time Average Length of sleep interval

  36. TRAMA Limitations • Complex election algorithm and data structure. • Overhead due to explicit schedule propagation. • Higher queueing delay. Flow-aware, energy-efficient framework.

  37. 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.

  38. 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.

  39. Flow Information • Flow information characterizes application-specific traffic patterns. • Flows can be unicast, multicast or broadcast. • Characterized by source, destination, duration and rate.

  40. Example: Data Gathering Application C Fc Fd D Fb B A Sink Fe E

  41. 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.

  42. Example 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. B SYNC A C S SYNC_REQ Sink

  43. 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. B wc=1 A C S wa=3 wc=1 Sink ws=0

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

  45. FLAMA Summary • Simple algorithm that can be implemented on a sensor platform. • Significant performance improvement by application awareness.

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