Medium Access Protocol for the EYES Medium Access Protocol for the EYES sensor nodessensor nodes

Lodewijk van Hoesel

hoesel@cs.utwente.nl

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ContentsContents

Current EYES sensor node design

Network structure

MAC protocol

Future work

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Current sensor node designCurrent sensor node design

Processor: TI MSP-430F149560 μA for 1 Mips16 bit2 Kb RAM60 Kb ROMBuilt-in AD converterSupports power save modes

Transceiver: RFM TR1001115.2 Kbps data rateAmplitude Shift Keying (ASK)No mixers are used; SAW filtersVery low standby power consumption Analog RSSI (AD converter necessary)

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RF power consumption in the RF power consumption in the current sensor node designcurrent sensor node design

RFM TR1001 worst case power consumption:

Transmit 20.0 mJ/s

Receive 14.4 mJ/s

Dormant 15.0 µJ/s

Battery capacity

3.6 V @ 1Ah 12.96 kJ

180 h

250 h

27 y

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Network structureNetwork 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)

Source

To sink

Source

To sink

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Communication typesCommunication types

Sensor node Sensor nodeRoute discovery, clustering, neighborhood discovery, localizationUnicast 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

OFTENOFTEN

RARELYRARELY

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MAC protocol: CSMA, CDMA, MAC protocol: CSMA, CDMA, FDMA, TDMA,... ?? FDMA, TDMA,... ??

Carrier Sense Multiple AccessCarrier Sense Multiple Access (CSMA)High collision rate

Transmissions of sensor data occurs in groups due to physical event horizonRequires constant channel monitoring

Code Division Multiple AccessCode Division Multiple Access (CDMA)Requires signal processing of analog received signal

Not supported by current hardware designRequires constant channel monitoring

Frequency Division Multiple AccessFrequency Division Multiple Access (FDMA)Requires multi-channel receiver

Not supported by current hardware designRequires constant channel monitoring

Time Division Multiple AccessTime Division Multiple Access (TDMA)Does not require constant channel monitoringRequires synchronization between nodes

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Overview of the proposed MAC Overview of the proposed MAC protocol for the EYES sensor protocol for the EYES sensor

nodesnodes

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

DataContains a data packet

1 s

Frames:

t

n n+1

t1/16 s

Time slots:

s s+1 s+2

Channels:

t

CR DataTC

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Communication Request SectionCommunication 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 occurTimeslot owner should detect collisionsRetry in data section (CSMA/cd based)

Direct transmission of (abbreviated) request +dataOnly useful for RTS

DATACR TC

SD DATACollision detected

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Traffic Control SectionTraffic Control SectionTC Schedule

Indicates to which TCs this node is listeningNew nodes in the network: “AND” all schedules, a timeslot is free at “zero”

Acks/Nacks of request in CRContains 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 availableRandom node respond in data section (CSMA/cd)

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Data SectionData Section

TC indicates whether Data Section is:Used

Data transmission as indicated in TC

FreeMay be used for transmissions agreed on higher protocol layers“Slotted” CSMA/cd usagei.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 type5. Higher protocol layers (CSMA/cd)

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Energy consumption (transceiver Energy consumption (transceiver only) with current MAC design: only) with current MAC design:

OverheadOverhead

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: 992.50 ms 15 µJ/frame

130 µJ/frame

Lifetime of a sensor node:Lifetime of a sensor node: 3 y 60 d3 y 60 d

+

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Conclusion: MAC protocol power Conclusion: MAC protocol power consumptionconsumption

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

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Timeslot OwnershipTimeslot 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

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Current state of MAC Current state of MAC implementationimplementation

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 schemePreamble usage for UART synchronizationTransmission power control“Raw” bit error rate

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Ideas for MAC protocol Ideas for MAC protocol communication demonstrationcommunication 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