Bluetooth and Sensor Networks :A Reality Check

Martin Leopold, Mads Dydensborg, Philippe Bonnet

University of Copenhagen

Hogthrob• A sensor net for sow monitoring

– Tracking in large pens– Heat period alert

• Motion detectors• Correlation between movement of sows (circles) and

their heat period

• Sensor net– Interferences– Mobile nodes + Fixed infrastructure

• Sensor nodes– 2 years lifetime– Low cost– Packaged for tough conditions

Radio technology is key.Spread spectrum radios are natural candidates.Is Bluetooth a good option?

Bluetooth and Sensor Networks:Promises and Challenges

• Off-the-shelf radio– Representative of spread-spectrum

radios (frequency hopping)– Free 2.4 GHz band– Promise to be cheap

• The Bluetooth stack is complex– Stripped down version of

Bluetooth adapted to constraints of sensor nodes?

• A Bluetooth module embeds front-end radio, baseband and MAC layer

– Are standard Bluetooth physical layer and MAC layer adapted to the sensor network regime?

• Multihop capabilities – Scatternet support has been

announced for years but was not supported at the time of our study

– How to build multihop Bluetooth-based networks?

• Bluetooth is connection-based– How to define network self-

assembly based on Bluetooth device discovery. What is the impact on performance?

• Bluetooth implements Time Division Multiplexing (TDM) at the radio level

– Can applications leverage radio-driven TDM?

Our Approach

Pragmatic ApproachBTNodes from ETH Zurich

• Atmega 128 7.32 MHz

• 128 KiB flash

• Dual-radio – Ericsson’s Bluetooth

module ROK 101 007

• Port of TinyOS to BTNodes– Development of

TinyBluetooth

• Self-Assembly Procedure• Application using Radio-

level TDM– UC Berkeley’s TinyDB on

top of TinyBluetooth

• Performance Evaluation– Intrinsic properties– Prototype properties

Bluetooth Stack

RF

Baseband

HCI

L2Cap

Profiles

Applications

Physical Bus Hardware

TinyBluetooth Stack

RF

Baseband

HCI

Physical Bus Hardware

TinyBluetooth

TinyOS Application

TinyBluetooth Stack• Asynchronous Programming

Model– HCI mapped onto tinyOS events

and commands– UART events decoupled from

HCI events

• Buffer Trading– Buffers swapped between

modules– Generic Packet type casted into

specific packet depending on event/command

• Interesting information encapsulated inside Bluetooth module

Self-Assembly Procedure

• Each node is equipped with 2 radios

• For each node– To which node to connect?

– Connect as master or slave?• 3 node configurations:

– S-S

– M-M

– M-S, S-M

Self-Assembly Procedure• Building a connection tree as a

baseline (BlueTree [Petrioli, Basagni 2002])– Each node has a radio set up as a

master, the other as a slave– Recursive connection establishment

• First slave radio is turned on.• One node is chosen as the root of

the connection tree.• Master radio turned on once a

connection is established on slave radio.

– Rely on Bluetooth device discovery and connection establishment

M

M

M

M

M

M

M

M

M

S

S

S

S

S

S

S

S

S

UC Berkeley’s TinyDB

• Push declarative queries into sensor net– Impose a hierarchical routing tree onto the

network• Divide time into epochs• Every epoch, sensors evaluate query over (i) local

sensor data and (ii) data from children nodes– Aggregate local and children data– Each node transmits just once per epoch

• Rest of the time, sensors sleep (deep microcontroller sleep)

The TDM is driven by the application: How long should sensors sleep? What if interesting data needs to be transmitted while sensors sleep?

TinyDB on top of TinyBluetooth

• Connection tree supports hierarchical routing tree.

• Radio drives TDM– Bluetooth radio in Sniff mode:

Master and Slaves agree on synchronization points (ideally once per epoch). Rest of time sensor node sleeps or senses. Microcontroller waken up on radio signal.

– Pipelined aggregation along the routing tree.

• Separated Channels– No unplanned collisions

M M

MS

S

S

S

M

M

TinyDB on top of TinyBluetooth

• Problem # 1: The sniff period is not longer than 40 secs.

• Problem # 2: When a connection is in sniff mode, the microcontroller sleeps in idle mode (which is less efficient than the power save mode according to the Atmel specs).

M M

MS

S

S

S

M

M

Code Size Breakdown

Description code bss dataSupport & TinyOS core 1180UART 0 & Interrupts 346 4

UART1 & Interrupts 292 5

hciPacket0 604 155

hciPacket1 588 155

hciCore0 1624 159

hciCore1 1590 159

Assembly Component 4796 1021 16

Total 11020 1658 16

Throughput- Point-to-point throughput ishigh!- The performance weachieve is far from the

theoretical max– UART limit is 45

Kib/sec– Junk sent by

Bluetooth module- Slave-to-master andmaster-to-slave throughputare similar- Throughput degrades forMultipoint connections

73,2 59,73 90,4

0

5

10

15

20

25

30

35

40

45

50

DM1 DH1 DM3 DH3 DM5 DH5Bluetooth Encoding

Th

rou

gh

pu

t (k

b/s

ec

)

20 bytes payload

668 bytes payload (max)

Theoretical max

• DM and DH are two encoding schemes. DM offers a lower error rate.• 1, 3 and 5 corresponds to the number of consecutive slots during which slaves and masters communicate.

1 2 3

Aggregate 38.1 25.4 19.3

Per Slave 38.1 12.7 6.4

Energy Usage Breakdown

• 50mW when idle and 250 mW when communicating

• Berkeley’s mica motes: 10 mW when idle and 160 mW when communicating

Maintaining connections is very expensive

Different sleep modes!

Self Assembly

a. Node is turned on.b. Connection on slave radioc. 1st Connection on master

radiod. 2nd Connection on master

radioe. Master radio is

discoverablef. Data packets are

transmitttedg. Disconnections on master

radioh. Disconnection on slave

radio

Application Characteristics• Throughput is high

– Best suited for applications that transmit lots of data

• Energy consumption is high (in particular connections)– Life time of applicatoin must be short (days)

– Short periods of connections

• Suited for Asynchronous In-Network Processing with radio driven TDM.

Bluetooth-Based Sensor Network well suited for short lived deploymentsWith unplanned burst of data with high throughput (images, video).

Conclusion

• Code available in TinyOS contrib directory• More info on our project home page:

http://www.distlab.dk/manatee

• This study is a baseline for:– Intel motes– 802.15.4 radios– Tailored radios relying on Bluetooth front-end

(Pico Radio)