Medium Access in Sensor Networks. Presented by: Vikram Shankar

Objective What is a MAC? Why is it needed? Study effect of MAC design on performance. Performance in terms of: Energy Efficiency Fair bandwidth allocation Throughput Efficiency All graphs and data are from [1]. S-MAC is discussed in [2].

Medium Access Control Coordinates medium access in the presence of multiple stations. Part of the Data Link Layer in ISO OSI protocol stack. May also perform local error recovery.

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.

Hidden and Exposed Terminal Problems Node Red Node Green Node Yellow Node Blue Node Red Node Green Node Blue Node Yellow Hidden Terminal Exposed Terminal

Sources of Energy Wastage Packet collisions Overhearing Idle listening (consumes 50–100% of the energy required for receiving) Control Packet Overhead

MAC Design Available design choices [1]: Listening mechanism. Random time Constant time Backoff mechanism. Fixed window. Exponential increasing window. Exponential reducing window. Contention based mechanism. RTS/CTS Rate control mechanism.

Design CSMA Schemes (Delay|Listen|Backoff) Random Delay Listening Time Backoff Mechanism ND_RANDNoRandomNone ND_RAND_FIXNoRandomFixed Window ND_RAND_EXPNoRandomExp Increase ND_RAND_REVEXPNoRandomExp Decrease ND_CONST_FIXNoConstantFixed Window ND_CONST_EXPNoConstantExp Increase ND_CONST_REVEXPNoConstantExp Decrease D_CONST_FIXYesConstantFixed Window D_CONST_EXPYesConstantExp Increase D_CONST_REVEXPYesConstantExp Decrease

Throughput Efficiency 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 achieve 75% efficiency. Offered load of 5 packets/second/nod e with slightly different start times. Channel capacity is 20.8 packets per second.

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

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.

Energy Consumption 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.

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.

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.

References [1] Alec Woo and David E. Culler, “A Transmission Control Scheme for Media Access in Sensor Networks”, Mobicom 2001, July 2001, Rome. [2] Wei Ye, John Heidemann, Deborah Estrin, “An Energy-Efficient MAC Protocol for Wireless Sensor Networks”, INFOCOM 2002, New York, June, 2002.