Analysis and Tradeoffs

1. THE ARMSTRONG PROJECT Design Priorities in Power-constrained WSNs Hidden Transmitter Problem Idle Channel Detection Problem • Minimize wasted energy from packet collisions. • Minimize energy spent due to signaling overhead. • Be adaptive to variable traffic patterns. • Maximizing throughput is a secondary concern. • CSMA fails when a transmission arrives at the transmitter after it has sensed the channel, determined it to be clear, and is switching from receive mode to transmit mode. • Transmissions outside the carrier sense range of the transmitter cannot be detected, but can still interfere at an intended receiver. Analysis and Tradeoffs Overhead and Throughput expressions. max(q) is the largest quantum integer required. Sleep duty cycle for non-intended recipient of a packet in a given slot. D is the number of quanta. Suppression area comparison with RTS/CTS solution Packet delay comparison with TDMA solution. max(q) is linear in the number of contenders (M). Averages over 5 randomly generated topologies • Two Causes of Packet Collisions in CSMA • Node separation is d and transmission radius is d * sqrt(2); arrows indicate the intended receiver. • (a.) – (c.) represent collision scenarios resulting from the idle channel detection delay (t). • (d.) and (e.) are hidden transmitter collision scenarios. qMAC: Collision-free Wireless Channel Access • Intelligent assignment of channel access opportunity eliminates all packet collisions, while: • avoiding the overhead of RTS/CTS packets. • requiring NO out-of-band signaling (single transceiver solution). • avoiding the rigidity of TDMA data slot assignment. • Solution: add structure to CSMA • Observation: • The smallest time scale at which a node • can interact with the network is limited • by t. We term t the time quantum of • the radio communication system. • Requirements: • Neighbor Discovery: a light-weight beaconing scheme is used; each node learns its two-hop neighborhood. • 2. Time Synchronization guaranteeing worst-case clock skew, tskew. • 3. Access Quantum Assignment: using a distributed coloring algorithm each node iis assigned a t–length access quantum qi that is unique within a contention region. Coloring constraints are: • a. , • b. , • where and are the nodes in the one-hop and two-hop neighborhoods, respectively, of node i. • Why It Works: • The minimum two quanta distance between one-hop neighbors guarantees all transceivers have enough time to sample and react to channel activity. • The minimum four quanta distance between nodes exactly two-hops away allows one-hop neighbors of transmitter ito warn nodes hidden from i that a transmission is already reserved for the current slot. Structuring Contention-based Channel Access in Wireless Sensor Networks Shane B. Eisenman† and Andrew T. Campbell‡†Electrical Engineering, Columbia University ‡Computer Science, Dartmouth College Channel Access Decision Flow and Example With support from Army Research Office (ARO) under Award W911NF-04-1-0311 More information on The Armstrong Project, including publications, technical reports, and source code from http://comet.columbia.edu/armstrong.