An Integrated approach to developing sensor network

solutionsPresented 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