MAC Protocols for Sensor Networks Paper Presentation of selected topics in EE206A Based on the work of Alec Woo, David Culler, Wei Ye, Join Heidemann, and Deborah Estrin Slides by Dayou Qian and Xiaolong Huang

Outline The characteristics of sensor networks Issues of MAC protocols for sensor networks Proposed solutions for MAC protocols An adaptive rate control scheme S-MAC (Sensor-MAC) Bibliography

The characteristics of sensor networks Tight constraints Computation power Storage Primary functions Sample the environment for sensory information Propagate data back to the infrastructure Traffic pattern Little activity in lengthy period Intensive traffic in short time Highly correlated traffic End to end flows are required to be fair Base station Sensor Multihop

The characteristics of sensor networks Networking component stack for sensor networks Complete TinyOS application component graph

Issues in MAC protocols for sensor networks Energy efficiency Fairness of end to end flows Related work IEEE802.11 High energy consumption when the nodes are in the idle mode CSMA To improve the energy consumption by avoiding overhearing among neighboring nodes TDMA No contention-introduced overhead and collisions Not easy to manage the inter-cluster communication and interference Not easy to dynamically change its frame length and time slot assignment PAMAS Power off radio when not actively transmitting and receiving packet

An adaptive rate control scheme The metrics for evaluation of a sensor network MAC protocol Fair bandwidth allocation from each node to the base station over multihops is desired. Energy efficiency is desired, including energy spent in listening for the channel and all packets transmissions and forwarding. Design Carrier sensing reduce the time for carrier sensing Backoff backoff in application layer is desired other than in MAC layer Contention RTS-CTS only during high traffic load, while CSMA during low traffic load Bidirectional multihop network (parenting structure) helps to reduce ACK packets

An adaptive rate control scheme Rate control mechanism Monitor route through traffic and original traffic Application data rate , Probability of transmission , actual data originate rate Increment by a constant , when application traffic is congested Monitor: Multiply by a factor , when application traffic is not congested Monitor: , because dropping route through traffic is more expensive Hidden terminal problem solved by parenting If a node hears its parent transmit at time , assume its grandparent will start at time The node restrains from transmission from time to

An adaptive rate control scheme Simulation settings Various CSMA schemes with different settings on: random delay, listening time, backoff scheme Single cell topology Small data packets 30 bytes CSMA versus IEEE 802.11 Results on throughput Randomness improves the throughput performance

An adaptive rate control scheme Results on energy consumption IEEE 802.11 is worst in terms of listening energy consumption due to carrier sensing during backoff time The most efficient schemes are those CSMA with constant listening time and random delay Results on fairness Different backoff scheme is insignificant for fairness

An adaptive rate control scheme Results on application phase shift Better fairness is provided Conclusion CSMA mechanism should incorporate random delay, constant listen period with radio powered down during backoff period

An adaptive rate control scheme Multihop simulation settings Multihop environment with bidirectional link Results on fairness Fairness is achieved by ARC

An adaptive rate control scheme Results on energy efficiency Conclusion The adaptive rate control scheme together with the new CSMA mechanism provides an effective medium access control without explicit control packets, which is effective in achieving fairness while maintaining good aggregate bandwidth with reasonable energy efficiency

S-MAC (Sensor-Networks) Trade Off Energy efficiency Node-level fairness & Latency Basic Scheme: Periodic Listen and Sleep Choosing schedule The node randomly choose a time to go to sleep. The node receives and follows its neighbor’s schedule by setting its schedule to be the same. If the node receives a different schedule after it select its own schedule, it adopts both schedules.

S-MAC (Sensor-Networks) Maintaining Schedule To update schedule by sending a SYNC packet periodically

S-MAC (Sensor-Networks) Collision Avoidance Virtual carrier sense checking the keeping silent time recorded in the NAV (network allocation vector) Physical carrier sense Listening to the channel for possible transmission RTS/CTS exchange Overhearing Avoidance Let interfering nodes go to sleep after they hear an RTS or CTS packet

S-MAC (Sensor-Networks) Message passing Only one RTS packet and one CTS packet are used To avoid large control overhead and long delay ACK would be sent after each data fragment To avoid fragment loss or error To Prevent hidden terminal problem After the neighbor node hears the RTS and CTS, it will go to sleep for the time that is needed to transmit all the fragments (using the duration field)

S-MAC (Sensor-Networks) Energy Saving vs. Increasing latency (Multi-hop Network) Carrier sense delay Determined by the contention window size Backoff delay Because the node detects another transmission or the collision occurs Transmission delay Determined by channel bandwidth, packet length and the coding scheme adopted Propagation delay Determined by the distance between the sending and receiving nodes Processing delay Depends on the computing power of the node and the efficiency of in-network data processing algorithms Queuing delay Depends on the traffic load Sleep delay Caused by the nodes periodic sleeping

S-MAC (Sensor-Networks) Testbed Used Rene Motes as the development platform and testbed TinyOS 3 working modes: receiving, transmitting and sleep Topology used in the experiment 3 MAC modules on the mote and TinyOS platform Simplified IEEE802.11 DCF Message passing with overhearing avoidance The complete S-MAC

S-MAC (Sensor-Networks) The energy consumption result on the source nodes A and B When the traffic is heavy (the inter-arrival time<4s), S-MAC achieves energy saving mainly by avoiding overhearing and efficiently transmitting a long message When the traffic is light, the periodic sleep plays a key role for energy savings

S-MAC (Sensor-Networks) The percentage of time that the source nodes are in the sleep mode The S-MAC protocol adjusts the sleep time according to the traffic patterns The Module of message passing with overhearing avoidance spend more and more time in idle listening when the traffic is light

S-MAC (Sensor-Networks) Energy consumption in the intermediate nodes When the traffic is light, S-MAC still outperforms 802.11 MAC Reason1:S-MAC has synchronization overhead of sending and receiving SYNC packets Reason2:S-MAC introduces more latency and actually uses more time to transmit the same amount of data

S-MAC (Sensor-Networks) Conclusion This new MAC protocol has very good energy conserving compared with IEEE802.11. This new MAC protocol has the ability to make trade-offs between energy and latency according to the traffic conditions.

Bibliography The contents of the slides are based on Alec Woo and David Culler. A transmission control scheme for medium access in sensor networks. Mobicom 2001 Wei Ye, Join Heidemann, and Deborah Estrin. A energy-efficient MAC protocol for wireless sensor networks. Infocom 2002