A Transmission Control Scheme for Media Access in Sensor Networks Alec Woo, David Culler (University of California, Berkeley) Special thanks to Wei USC for some of the ppt slides

Sensor Net Traffic Characteristics Sampling of environment for sensory information Propagation of time series information to infrastructure Duty cycle low until abnormal event sensed Periodic sampling creates a high amount of highly correlated traffic

Sensor Networks Sensing and Radio capabilities Limited storage, processing power Limited ENERGY Small packet size due to low bandwidth Multi-hop topologies Route-thru traffic at sensor nodes Bi-directional connectivity (in the face of multipath interference and short irregular transmission ranges)

Related Work IEEE & MACAW –Last-hop wireless, single cell scenario –Peer to peer communications –Single-hop base station interaction within a cell Bluetooth –Centralized TDMA within piconet –Time synchronization requirement Home RF –Combo of contention control protocol for synchronization data traffic and centralized TDMA for synchronous voice –No multihop support

Evaluation Metrics Fairness in sensor data coverage –Original vs. route-thru traffic –But, route-thru traffic already has network resources invested in it Energy Efficiency –Aggregate bandwidth per unit of energy –Fairness and high channel utilization are at odds

Fairness Fairness in bandwidth allocation –Each node generates roughly the same amount of data –Example N nodes in a multihop network All data are sent to 1 base station Ideally, the base station should equally receive 1/N portion of data from each node

Fairness Fairness in bandwidth allocation –If nodes 4, 5, 6 generate too many packets, congestion may happen at node 1. –Node 1 has no more bandwidth for its own data. Base station

Sensor Network Platform ATMEL 4MHz 8 bit microprocessor –8K program memory –512 bytes data memory Single channel RF transceiver –Operating at 916MHz –10kbps using on-off-keying encoding Variety of Sensors –Temperature, photo, etc. TinyOS – event-based operating system –30 byte messages

Design Listening Mechanism –CSMA effective when all nodes can hear each other –Periodic listening to conserve energy –CD requires additional circuitry –Problematic for periodic and synchronized traffic –Solution: introduce random delay Backoff mechanism –Also can help to break the synchronization by introducing a phase shift Contention based mechanism –Control packets, like RTS/CTS/ACK, are large overhead if packets are short (up to 40% overhead)

Design Rate control mechanism –MAC controls the rate of originating data of a node S * p, where p = [0, 1], S is application transmission rate –Without any MAC control packets –A node periodically tries to inject a packet If successful, linearly increase transmission rate p = p +  If unsuccessful, multiplicatively decrease rate p = p * 

Design Rate control mechanism without control packets 1. How does a node know whether its transmission is successful or not? If my parent routes the same packet to my grandparent, I know my transmission is successful Success relies on symmetric links and implicit ACKs 2. How do we deal with collision without RTS/CTS? Constantly tune transmission rate. If I just transmitted a packet, restrain further transmission for duration x, which is the processing time at my parent.

Simulation Environment Simple home-grown simulator –Simple radio propagation model (not specified) –Zero bit error rate

Simulation Analysis of CSMA schemes

Simulation Single-hop topology

Simulation Settings –Channel capacity 10 kbps –Packet length 30 bytes –Node transmission rate 5 packet/s –16 bit CRC for error detection –Highly synchronized traffic – all nodes start at the same time

Simulation Results – Single Hop Schemes with randomness built into delay or listening achieve both fairness and stable channel utilization. Overall, the best CSMA schemes are those that have constant listen period with random delay of transmission

Simulation – Multihop Scenario Topology and fairness results

Simulation – Multihop Scenario Energy efficiency

Simulation – Multihop Scenario Aggregate bandwidth

Conclusions Adaptive rate control can effectively achieve fairness of bandwidth allocation Energy efficient – no control packets More collisions than RTS/CTS schemes Lower aggregate bandwidth and yield than RTS/CTS schemes

Critique Link symmetry assumption Many to one routing assumption No aggregation Control overhead not so large if per ADU Weak multihop results Can we trust their simulator? Extraordinarily sparse references