RT-Link: A Time-Synchronized Link Protocol for Energy-Constrained Multi-hop Wireless Networks

RT-Link: A Time-Synchronized Link Protocol for Energy-Constrained Multi-hop Wireless NetworksAnthony Rowe, Rahul Mangharam and Raj RajkumarCMUSECON 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 goodBut 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 tree2.BFS from G, assign smallest color that

satisfy 2-hop constraint3.Replace the leaves with larger value4.Inverts the numbers

Outline

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

Hardware Platform• CMU FireFly – 8-bit Micron +

IEEE802.15.4

Time Synchronization – outdoor • WWVB atomic broadcast (Wiki)

▫WWVB is a special NIST time signal radio station near Fort Collins, Colorado

▫ WWVB is the station that radio-controlled clocks throughout North America use to synchronize themselves.

▫ The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic clocks located at the transmitter site

• 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

Energy Model – over a frame

• Tframe = Tactive + Tidle

Active time over a frame

Minimum energy consumption

Maximum energy consumption

Lifetime

Lifetime Comparisons• Sample Interval

▫ Event generation rate• Check Interval

▫ Time between two consecutive LPL

Lifetime Comparisons

Lifetime Comparisons

• For high sample rate, hardware sync doesn’t help

Delay• Simulation setup

▫ Topology

▫ Only leaf node generate traffic▫ Total traffic fixed to 1000 1-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??