© 2004 Matt Welsh – Harvard University1CS263: Wireless Communicationsand Sensor NetworksMatt WelshLecture 1: Course IntroductionSeptember 23, 2004© 2004 Matt Welsh – Harvard University2Welcome to CS263!Wireless networks are everywhere ...This course is all about wireless communications●Basics of radio communication●The deep guts of how wireless LANs work●Specific standards: 802.11, Bluetooth, 802.15.4With a focus on wireless sensor networks●Exciting new technology: small, low-power,wireless devices with sensors●You will develop sensor net applicationsand test them on a real network© 2004 Matt Welsh – Harvard University3Course OverviewCourse is roughly divided into three parts:Part 1: Survey of wireless network technologies●Radio communication fundamentals, antennas, and propagation●Coding schemes, broadband, medium access control●Wireless networking standards: 802.11, Bluetooth, and 802.15.4Part 2: Research papers on wireless networks●Ad-hoc routing, TCP/IP in mobile environments●Community wireless networksPart 3: Sensor networks (about ½ of the course)●Exciting new technology: small, low-power, wireless devices with sensors●Applications, operating systems, power management●Programming models, querying, network storage, distributed algorithms●Localization, time synchronization, and security© 2004 Matt Welsh – Harvard University4Goals of this classLearn about wireless networks and sensor networksRead research papers on exciting new topicsExperiment with a real sensor networkDo a research project on your favorite topicHopefully, publish a paper on your work© 2004 Matt Welsh – Harvard University5Wireless Networking OverviewWireless Local Area Networks (WLANs)●Wide range of technologies for local, high-data-rate, wireless communications●Very different than wide area networking, e.g., cellular, GSM, CDPD, etc.Physical Layer (PHY)●How devices transmit binary data over the airwaves●Determined by frequency range, transmit power, modulation scheme, etc.Medium Access Control (MAC)●How devices share the radio channel and avoid interfering with one another●“Listen before you speak” or “Only speak at certain predetermined times”Network Layer●How multiple devices in a wireless network talk to each other●May involve devices relaying messages to each other (multihop routing)© 2004 Matt Welsh – Harvard University6Technology SpaceData rateComplexity/power/costCC1000Bluetooth802.15.4Zigbee802.11a802.11b802.11g38.4 kbps250 kbps720 kbps11 Mbps54 Mbps© 2004 Matt Welsh – Harvard University7802.11 / WiFiThe most popular Wireless LAN standardDistribution systemBasic service setAccess point© 2004 Matt Welsh – Harvard University8BluetoothShort-range, moderate data rate wireless link for personal devices●720 Kbps, 10 m rangeOne master and up to 7 slave devices in each Piconet:Master controls transmission schedule of all devices in the Piconet●Time Division Multiple Access (TDMA): Only one device transmits at a timeFrequency hopping used to avoid collisions with other Piconets●79 physical channels of 1 MHz each, hop between channels 1600 times a sec© 2004 Matt Welsh – Harvard University9IEEE 802.15.4 and ZigbeeEmerging standard for low-power wireless monitoring and control●2.4 Ghz band, 250 kbps data rateChipcon/Ember CC2420: Single-chip 802.15.4 radio transceiver, $5●1.8V supply, consumes 19.7 mA receiving, 17.4 mA transmit●Easy to integrate: Open source software drivers●All PHY and encryption in hardware●O-QPSK modulation, “plays nice” with 802.11 and Bluetooth© 2004 Matt Welsh – Harvard University10Ad Hoc RoutingHow to route messages along a complex graph of moving nodes?●Nodes are mobile wireless devices (e.g., laptops or PDAs)●When nodes move, connectivity changes...●Must avoid overheads such as flooding entire network to discover new routes© 2004 Matt Welsh – Harvard University11RoofNetMIT community wireless network●How to organize a network of rooftop 802.11 nodes to provide Internet connectivityto a large community?© 2004 Matt Welsh – Harvard University12Sensor NetworksIntegration of sensing, computation, and communication●Low-power, wireless “motes” with tiny amount of CPU/memory●Large federated networks for high-resolution sensing of environmentDrive towards miniaturization and low power●Eventual goal - complete systems in 1 mm3, MEMS sensorsWeC (1999) Rene (2000) Dot (2001)MICA (2002) Speck (2003) Telos (2004)© 2004 Matt Welsh – Harvard University13The Berkeley MICA Mote●ATMEGA 128L (7.3 Mhz 8-bit CPU)●128 KB code, 4 KB data SRAM●512 KB flash for logging●433 or 916 Mhz, 76.8 Kbps radio (100m max)●Sandwich-on sensor boards●Powered by 2AA batteriesSeveral thousand produced, used by 100s of research groups●Get yours at www.xbow.com, about $150 a popGreat platform for experimentation (though not particularly small)●Easy to integrate new sensors & actuators●15-20 mA active (5-6 days on 2 AAs)●15 μA sleeping (21 years, but limited by shelf life of battery!)© 2004 Matt Welsh – Harvard University14The Telos mote●TI MSP430 processor●128 KB code, 2 KB data SRAM●512 KB flash●CC2420 radio (2.4 Ghz, 802.15.4)●250 kbps, 100 m range●Lower power than Mica2You will get a “kit” of 3 Telos motes to experiment with© 2004 Matt Welsh – Harvard University15Sensor Network ChallengesLow computational power●Current mote processors run at < 10 MIPS●Not enough horsepower to do real signal processing●DSP integration may be possible●4 KB of memory not enough to store significant dataPoor communication bandwidth●Current radios achieve about 10 Kbps per mote●Note that raw channel capacity is much greater●Overhead due to CSMA backoff, noise floor detection, start symbol, etc.●802.15.4 (Zigbee) radios now available at 250 Kbps●But with small packets one node can only transmit around 25 kbpsLimited energy budget ●2 AA motes provide about 2850 mAh●Coin-cell Li-Ion batteries provide around 800 mAh●Solar cells can generate around 5 mA/cm2 in direct sunlight●Must use low duty cycle operation to extend lifetime beyond a few days© 2004 Matt Welsh – Harvard University16Typical ApplicationsVehicle tracking●Sensors take magnetometer readings, localize object ●Communicate using geographic routing to base station●Robust against node and radio link failuresHabitat monitoring – Great Duck Island●Gather temp, IR, humidity, and other