RT-Link: A Time-Synchronized Link Protocol for Energy-Constrained Multi- hop Wireless Networks Anthony Rowe, Rahul Mangharam and Raj Rajkumar CMU SECON 2006

Outline Introduction and Related works Protocol Overview Protocol Enhancements Implementation Evaluation Conclusion

Introduction A MAC protocol for sensor network TDMA-based Hardware-based time synchronization Predictable lifetime (>2 years) bounded end-to-end delay

Keys for SensorNet MAC Lifetime ▫Low duty cycles ▫minimize the radio usage Delay ▫Either keep sending ▫Or co-operate very well (TDMA-based) Time synchronization is good But how to achieve with reasonable cost??

Some MACs for SensorNet Asynchronous MAC – B-MAC ▫Berkeley MAC ▫Carrier sense multiple access (CSMA) with low power listening (LPL) ▫LPL ▫Optional RTS-CTS

Some MACs for SensorNet (Cont.) Loosely Synchronous ▫S-MAC ▫local sleep-wake schedules ▫exchange synchronizing packets with neighbors Fully Synchronized ▫TRAMA – scheduled slot ▫Assume time synchronization is an orthogonal problem Time-sync protocols for SensorNet ▫Basic idea – time packet exchange ▫Suffer for link error rate (up to 50%)

Outline Introduction and Related works Protocol Overview Protocol Enhancements Implementation Evaluation Conclusion

Protocol Overview Each cycle ▫32 frames Each frame ▫start by sync pulse ▫32 5ms slots ▫Scheduled Slots (SS) – for member nodes ▫Contention Slots (CS) – for guest nodes ▫when a node is in transmission slot, its neighbors must listen to the channel ▫Every member nodes must listen to all CS So, they need central control (Gateway) Communication cycle

Protocol Operations New Node Send Hello to GW in CS Software time-sync

How to Enhance Basic idea of scheduling ▫k-hop coloring Topology Control ▫Degree high => energy consumption high ▫Gathering topology and prune nodes

Interference-free Scheduling Observations ▫Jammer-RX distance is critical Conclusion ▫Jammer has no effect beyond 2*(SRD) ▫Concurrent transmitter may be placed 3*(SRD) Stable Reception distance (SRD)

Coloring and Ordering Max concurrency vs delay sensitive Get minimum delay – NP-complete

Heuristic 1.Dijkstra’s shortest path  spanning tree 2.BFS from G, assign smallest color that satisfy 2- hop constraint 3.Replace the leaves with larger value 4.Inverts the numbers

Outline Introduction and Related works Protocol Overview Protocol Enhancements Implementation Evaluation Conclusion

Hardware Platform CMU FireFly – 8-bit Micron + IEEE

Time Synchronization – outdoor WWVB atomic broadcast (Wiki) ▫WWVB is a special NIST time signal radio station near Fort Collins, ColoradoNISTtime signalFort Collins, Colorado ▫WWVB is the station that radio-controlled clocks throughout North America use to synchronize themselves.radio-controlled clocksNorth America ▫The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic clocks located at the transmitter sitekHzatomic clocks Off-the-shelf WWVB receiver

Picture

Time Synchronization – Indoor Carrier-current AM broadcast ▫Low power AM broadcasting for campus ▫Not licensed in US ▫Use building power lines as antenna

Time Synchronization – Indoor A modified FireFly receive outdoor atomic clock Transmit to AM station A (commercial AM receiver) + (custom interface board) receive AM broadcast and sync the FireFly

Performance ▫5 nodes ▫Separate several hundred feet ▫Hard wired

TDMA Slot Mechanics

Outline Introduction and Related works Protocol Overview Protocol Enhancements Implementation Evaluation Conclusion

Energy Model – over a frame

T frame = T active + T idle Active time over a frame Minimum energy consumption Maximum energy consumption Lifetime

Lifetime Comparisons Sample Interval ▫Event generation rate Check Interval ▫Time between two consecutiv e LPL

Lifetime Comparisons

For high sample rate, hardware sync doesn’t help

Delay Simulation setup ▫Topology ▫Only leaf node generate traffic ▫Total traffic fixed to byte packet ▫Packet aggregation up to 100 bytes

Delay -- Result

Real Deployment

Conclusion Hardware time sync, nice idea Not so difficult to implement High data rate: ▫ maybe software time sync is enough Low data rate ▫could software time sync achieve similar performance with hardware one? FireFly??