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An Integrated approach to developing sensor network solutions Presented by Richie John Thomas 08/27/04 Introduction • Paper on the development work on sensor networks at Computer and Network Architecture Lab. Of the Swedish Institute of Computer Science • System core – ESB Sensor Hardware running the Contiki OS – Contiki emulation/simulation enviornment for development • Communication Stack – Adaptive energy efficient MAC – TCP/IP layer optimized for resource constrained devices – allows system to be connected to internet system Hardware Platform • ESB (Embedded sensor board) – Texas Instruments MSP 430 low power micro controller – RF monolithics TR 1001 single chip RF transceiver – Collection of sensors • • • • • • Light- visible light Passive infra red-movement Temperature Vibration-movement of sensor board Microphone-ambient noise level Infra red sender and receiver – MSP 430 has 60 kb flash ROM and 2kb RAM – 32 kb EEPROM provides addl. Persistent sec. storage – RF transceiver operates at 868 MHz and supports rates upto 115.2 kbps – Board has two external Connectors • RS 232 port – for communication with PC • JTAG interface – code downloading and debugging – MSP 430 for low power appln. – Provides sleep modes awakened by interrupts from internal timers or sensors – Supports selective rewriting of internal flash ROM – TR 1001 RF transceiver • Baseband transmission with either amplitude shift keying or on-off keying • Provides half duplex bit level access to physical radio medium – Higher level mechanisms (MAC protocol processing, data encoding, time multiplexing) should be done in s/w – Transceiver connected to one of MSP 430 UART-Bit shifting in h/w rather than s/w – UART causes interrupt only after full 8 bit received as against MICA motes where interrupt for each incoming bit The embedded sensor board The Contiki OS • Flexible- allows individual programs and services to be dynamically loaded and unloaded in a running system. • Based on event based concurrency model • But also provides preemptive multithreading • Event based systems have lower resource requirements and well suited for sensor networks • Allows cryptographic computations as it can be run on a separate thread • Allows dynamic reprogramming of n/w behavior – due to service layer • Conceptual layer providing service discovery and run-time dynamic service replacement • Portability makes it trivial to run Contiki as a user level process under different PC OS • Appln. pgms developed in simulator can be directly run and compiled on the sensor h/w MAC Layer • Plays key role in energy efficiency and quality of service • MAC layer under development – Energy efficient TDMA-like structure overlaid on CSMA based collision avoidance protocol – Asynchronous – Meet requirements on size, complexity and cost and deployment in extreme environment with variable h/w stability – Lightweight – No traffic overhead- foregoing synchronization – Scalable for multihop sensor n/w-no centralized coordination used – Provide good best effort QoS – Energy efficiency • Asynchronous power save protocol • Based on the observation if node awake for just over half of the time is awake interval will overlap with that of each of its neighbors • Nodes can determine available transmission window of neighbors • Node sleeps when no transmission – Flow adaptation • Phase adjustment used to increase effective capacity of a region and reduce latency • Node adjust its phase to avoid sending data when there are high levels of contention or interference • Sequence of nodes forming a path can adjust their phase to minimize intra path interference TCP/IP for Sensor Networks • This requirement for network management, calibration, diagnostics, debugging • Possible to connect network directly to Internet • Sensor data is transmitted using UDP/IP but for administrative tasks reliable unicast connections required • TCP/ IP used • Individual nodes can be addressed and necessary reprogramming of sensors performed • Also for debugging and diagnostic tasks requiring reliable connectivity to a specific sensor • uIP has been developed with size of few kb and few hundred bytes of RAM – not only on ESB but variety of 8 and 16 bit processors • Spatial IP addressing – Each node uses its spatial location to construct its IP address – The spatial IP address only denotes the location and not single identifiable node – If node replaced new node given same IP address as replaced node – Nodes aware of their spatial location neither require central server or communication between nodes for address assignment • Distributed TCP Caching – Packet loss result in heavy overhead due to TCP end to end ack. and retransmission scheme – Poor performance in energy consumption and throughput – DTC cache TCP segments in network and perform local retransmissions – Nodes are allowed to cache only one segment – Nodes attempt to identify and cache segments not received by next hop • The segment lost i.e. for which no ack. has been received is locked in cache • DTC has to respond to lost packets more quickly to avoid end-to-end transmissions • DTC uses ordinary TCP mechanisms to detect packet loss • Analytical and simulation results indicate that DTC increases TCP performance • DTC currently being implemented in ESB nodes using Contiki simulator Applications • Building security – Unwarranted motion in the secured building notified via GSM and security personnel logs into the building network to obtain status – Two functions for sensor nodes- motion detectors and backbone nodes – Motion detectors in rooms and backbone nodes along corridor – Motion detectors has direct comm. path with at least one backbone node and each backbone node had contact with one other backbone node • One backbone node equipped with external interface device • Alarm from motion detector to its backbone node and from there to its back bone node • Eventually all backbone nodes have info. abt. entire state of network • Security team with mobile backbone node to scan the information • Uses spatial IP addressing but mobile backbone node has fixed IP address from another n/w to differentiate it from other backbone nodes • Marine monitoring – Used to study water temp. and salinity – Sensors attached to a buoy takes measurements at known depths – These connected as fixed network as communication expensive – Above waterline on the buoy is a full function ESB – These collect data from fixed n/w below and transfer over wireless interface to gateway node – From here by GPRS to marine sciences center – This gateway can also be used to transport data to sensors for reprogramming, debugging and monitoring – This exemplifies usefulness of being able to manage nodes directly via TCP/IP protocols • HVAC Monitoring – Explore feasibility of instrumenting a residential complex to improve the efficiency of its HVAC – Temperature and vibration sensors of ESB are used – IP based sensor accommodated into the Ethernet of the energy control room