Medium Access Protocol for the EYES sensor nodes Lodewijk van Hoesel

energy efficient sensor networks 2 Contents Current EYES sensor node design Network structure MAC protocol Future work

energy efficient sensor networks 3 Current sensor node design Processor: TI MSP-430F μA for 1 Mips 16 bit 2 Kb RAM 60 Kb ROM Built-in AD converter Supports power save modes Transceiver: RFM TR Kbps data rate Amplitude Shift Keying (ASK) No mixers are used; SAW filters Very low standby power consumption Analog RSSI (AD converter necessary)

energy efficient sensor networks 4 RF power consumption in the current sensor node design RFM TR1001 worst case power consumption: Transmit20.0 mJ/s Receive14.4 mJ/s Dormant15.0 µJ/s Battery capacity 3.6 1Ah12.96 kJ 180 h 250 h 27 y

energy efficient sensor networks 5 Network structure Wireless network nodes are capable of: Measuring physical conditions Relaying messages from other nodes Routing decides main function: Sensor node (mainly measuring; passive or event driven) Relay node(mainly relaying messages; passive)

energy efficient sensor networks 6 Communication types Sensor node  Sensor node Route discovery, clustering, neighborhood discovery, localization Unicast or Multicast Relay node  Sensor node Requests for data, signaling messages Unicast or Multicast Sensor node  Relay node Sensor data Unicast Relay node  Relay node Backbone traffic Unicast OFTEN RARELY

energy efficient sensor networks 7 MAC protocol: CSMA, CDMA, FDMA, TDMA,... ?? Carrier Sense Multiple Access Carrier Sense Multiple Access (CSMA) High collision rate Transmissions of sensor data occurs in groups due to physical event horizon Requires constant channel monitoring Code Division Multiple Access Code Division Multiple Access (CDMA) Requires signal processing of analog received signal Not supported by current hardware design Requires constant channel monitoring Frequency Division Multiple Access Frequency Division Multiple Access (FDMA) Requires multi-channel receiver Not supported by current hardware design Requires constant channel monitoring Time Division Multiple Access Time Division Multiple Access (TDMA) Does not require constant channel monitoring Requires synchronization between nodes

energy efficient sensor networks 8 Overview of the proposed MAC protocol for the EYES sensor nodes Communication Request (CR) Claim a timeslot (for nodes that join the network) Notify the need for data communication to the owner of the timeslot Traffic Control (TC) Owner of the timeslot transmits its schedule Data Contains a data packet

energy efficient sensor networks 9 Communication Request Section Contains requests to timeslot owner Node Announcement (NA) Request to listen to TC in specified timeslot Request to Receive (RTR) Useful to ask data from passive sensor nodes Request to Send (RTS) Useful for event driven nodes and relay nodes Collisions can occur Timeslot owner should detect collisions Retry in data section (CSMA/cd based) Direct transmission of (abbreviated) request +data Only useful for RTS

energy efficient sensor networks 10 Traffic Control Section TC Schedule Indicates to which TCs this node is listening New nodes in the network: “AND” all schedules, a timeslot is free at “zero” Acks/Nacks of request in CR Contains a “collision detected” flag Timeslot owner listens to data section for retries NA is acknowledged implicitly in TC schedule Multicast flag Gives the node the option for multicast transmissions Request for Any (RFA) Gives the node the option to request for a reply of any node that has the specified information available Random node respond in data section (CSMA/cd)

energy efficient sensor networks 11 Data Section TC indicates whether Data Section is: Used Data transmission as indicated in TC Free May be used for transmissions agreed on higher protocol layers “Slotted” CSMA/cd usage i.e. 8 possible start times for transmissions Priority of the data section usage: 1.Multicast transmissions (discards any requests in CR except NA) 2.Request to Send (RTS) 3.Request to Receive (RTR) 4.Requests for any (RFA) data type 5.Higher protocol layers (CSMA/cd)

energy efficient sensor networks 12 Energy consumption (transceiver only) with current MAC design: Overhead Receive CR (18 bytes) in own timeslot: 1.25 ms 18 µJ/frame TC of 4 other nodes: 5.00 ms 72 µJ/frame Transmit Own TC (18 bytes): 1.25 ms 25 µJ/frame Sleep Remainder of the frame: ms 15 µJ/frame 130 µJ/frame Lifetime of a sensor node:3 y 60 d +

energy efficient sensor networks 13 Conclusion: MAC protocol power consumption Minimize the number of transmissions Do useful transmissions Each transmission should reach its sink Do not discard packets Add own sensor readings to relayed data Group transmissions to a neighbor Compress data Minimize the number of transmitted TCs Minimize the number of receptions Minimize the number of received TCs Use wakeup schemes

energy efficient sensor networks 14 Timeslot Ownership Not all nodes need to own a timeslot Event driven nodes may redeem their right to own a timeslot Saves energy Eases scalability of the network Can participate in: Do RTS/RTRs in CR sections Listen to omnicast messages Receive data on higher layers negotiated times

energy efficient sensor networks 15 Current state of MAC implementation Finalizing OS radio control functions MAC implementation Work in progress No higher layer negotiated communication No CSMA/cd retries of collisions in CR To do: physical layer Measurements to determine: Best DC balancing scheme Preamble usage for UART synchronization Transmission power control “Raw” bit error rate

energy efficient sensor networks 16 Ideas for MAC protocol communication demonstration Coffee status Measure weight of coffee can Determine coffee machine status; coffee ready?? Mobility test Node controls an electric car Determine direction of a beacon and drive to it Room temperature and light Rapport temperature and light in a number of rooms

energy efficient sensor networks 17 New technologies: Transceiver front-ends Antenna design Multiple antennas Increases gain Allows spatial aiming of beam On chip antenna Reduces losses in matching circuits Active antenna’s Reduces the need of matching circuits and amplifiers IC design New component designs Down scale components Silicon On Anything technology Reduces losses caused by parasitic capacitances of components Down scale operation voltage

energy efficient sensor networks 18 New technologies: Modulation techniques QPSK, 16 QAM or 64 QAM Modulate multiple bits per transmitted symbol Less energy per transmitted bit Depends strongly on radio channel properties Operation can be tuned to different radio frequency in future Impulse radio Ultra low power Range: >16 km using 250 µW Extremely large RF bandwidth Signal is –almost- not disturbed by the radio channel Large data rates Transmissions can occur virtually at same instance Harder to demodulate High level of security

energy efficient sensor networks 19 Lifetime of the sensor node

energy efficient sensor networks 20 Wakeup methods Dormant mode is far more energy efficient In literature wakeup is no part of MAC MAC synchronization is lost! Methods for waking nodes: Beacon wakeup call to a specific node Tonewakeup all nodes in proximity Location of wakeup calls: Data channel Wakeup channelimplies use of a multi-channel transceiver Dormant listening states: Periodically Continuousimplies use of an ultra low power receiver

energy efficient sensor networks 21 Wakeup methods: Example I Parameters: Beacon, data channel, periodical After wakeup, node needs hooks to synchronize with MAC No other communications during wakeup call possible Nodes can not be woken during other transmissions

energy efficient sensor networks 22 Wakeup methods: Example II Parameters: Beacon, wakeup channel, periodical No interference with other data communications Easier synchronization with MAC Drawback: multi-channel radio or multiple radios

energy efficient sensor networks 23 Wakeup schemes: Conclusion Prolongs lifetime of sensor network Enlarges network latency Wakeup call length should consume far less than 36s in 1h Beacon or tone? Spatial density Common activity rate Data or wakeup channel? Costs of multi-channel/multiple transceivers Bandwidth necessary in data channel Periodically or continuously listening? Availability of very low power receiver New technologies?

energy efficient sensor networks 24 Future work Add physical layer to OMNET++ model Power consumption transceiver Using measurements Timing constraints transceiver 868 MHz radio channel propagation Design wakeup system Use simulations to verify total network lifetime Evaluate new technologies for use in EYES

energy efficient sensor networks 25 Comparison of transceivers RFM TR1001Infineon TDA5250Chipcon CC1020 Data rate115.2 Kbps64 Kbps153.6 Kbps ModulationASKASK, FSKASK, FSK, GFSK Receiver sensitivity-85 dBm, 4.8 mA (BER ) -109 dBm, 9.5 mA (BER ) -119 dBm, 17 mA (BER ) Transmit power11 dBm, 12 mA9 dBm, 12 mA12 dBm, 24 mA Standby current5 µA5 nA0.2 µA RSSIAnalogDigital Interface2 wireI²C or 3-wire4-wire Modes of operationExternalExternal\ Programmable Frequency hoppingNoYes