Medium Access in Sensor Networks.

1. Medium Access in Sensor Networks. Presented by: Vikram Shankar

2. Objective • Study effect of MAC design on performance. • Performance in terms of: • Energy Efficiency • Fair bandwidth allocation • Throughput Efficiency • All graphs and data are from: Alec Woo and David E. Culler, “A Transmission Control Scheme for Media Access in Sensor Networks”, Mobicom 2001, July 2001, Rome.

3. Characteristics of Sensor Networks: • Low energy availability. • Limited transceiver capability: • Half duplex • No collision detection • Few bits of buffering • Sporadic bursts of correlated data. • Short data packet length. • Roughly same cost for transmitting, receiving and listening. • Hidden terminal problem between every other pair of levels in network topology.

4. System Model • ATMEL 4MHz, 8 bit microcontroller running TinyOS. • 8K bytes program memory, 512 bytes data memory. • 10kbps channel using on-off keying. • Zero bit error rate.

5. Design • Listening mechanism. • Random time • Constant time • Backoff mechanism. • Fixed window. • Exponential increasing window. • Exponential reducing window. • Contention based mechanism. • RTS/CTS • Rate control mechanism.

6. Design

7. Throughput Efficiency • Offered load of 5 packets/second/node with slightly different start times. • Channel capacity is 20.8 packets per second. • Schemes with constant listen period and no random delay (ND_CONST_???) achieve highest bandwidth. • ND_CONST_??? Schemes are unstable because of repeated collisions. • As load exceeds capacity, all schemes except 802.11 achieve 75% efficiency.

8. Throughput Efficiency (Worst case scenario). All nodes are synchronized to start at the same time. In the absence of collision detection hardware, random backoff not sufficient to avoid repeated collision. ND_CONST_??? schemes have zero bandwidth utilization. ACK failure in 802.11 triggers retransmissions.

9. Lessons of Throughput Study • Backoff mechanism has no effect on performance. • Even ND_RAND, which has no backoff mechanism performs as well as others. • Randomness in pre-collision phase is essential for robustness.

10. Energy Consumption • 802.11 has worst performance because it listens even during backoff. Becomes worse when we account for ACK overhead. • Schemes with constant listen period most energy efficient (10µJ/packet). • Schemes with random listen period consume 40µJ/packet. • Delay has no effect on energy as transceiver is turned off.

11. Fairness of CSMA Schemes Three schemes are very similar. Standard deviation of approximately 0.25 packets/sec and decrease with increase in traffic. Thus, backoff mechanism has no effect on fairness at uniform load.

12. Fairness of 802.11 scheme • Standard deviation of more than 1 packet/sec. • Nodes that have earlier start time hog the channel.

13. Fairness with non-uniform load. Data normalized to bandwidth of node 10. • This time, backoff mechanism does have an effect on fairness. • Binary exponential increasing backoff has worst performance.

14. Sensor Phase Shifting.

15. Sensor Phase Shifting