Presentation at the Second International Workshop on the Web of Things (WoT 2011), San Francisco, CA, USA. http://www.webofthings.org/wot/2011/

1. Connecting Things to the Web using
Programmable Low-power WiFi Modules
Benedikt Ostermaier, Matthias Kovatsch, Silvia Santini
{ostermaier | kovatsch | santinis}@inf.ethz.ch
Sunday, 12 June 2011 Institute for Pervasive Computing / Distributed Systems Group

2. Internet of Things
Now
Optimized LoWPANs WiFi infrastructure
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 2

3. Platform: Roving RN-131G
 Programmable low-power WiFi module
 44 MHz 32-bit RISC CPU
 128 kB RAM, 2 kB non-volatile RAM
 2 MB ROM, 8 MBit flash
 Built-in IEEE 802.11b/g transceiver with on-board antenna
 Rates of up to 54 Mbit/s
 WEP, WPA-PSK and WPA2-PSK RN-131
 Interfaces
 UART, SPI, SDIO, RFID
 10 GPIOs, 8 analog sensor I/Fs
 Runs eCos with
lwIP TCP/IP stack
37 mm / 1.46 in
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 3

4. Platform: Low-power
 Current consumption
 212 mA when active (max. TX)
 4 µA when sleeping
 Hardware support
 Power Management Unit
 Wake-up on sensor or timer events
 Sleep mode
 2 kB battery-backed RAM
 Short wake-up time
(~35 ms until connected)
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 4

5. Connecting to Things
Physical connection
electronic attached
Batteries
Power supply
Light switch Chair sensor sleeping
vs.
always on
Grid
Power outlet Room sign
prototype vs. ad-hoc
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 5

6. Connecting Things to the Web
REST: maps nicely
to physical resources
Single-threaded
Web server
HTTP callbacks
(Webhooks)
RESTful API for sensors, ,
actuators, configuration
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 6

7. Putting Things Together
http://lightswitch
/
/position
/callback
http://poweroutlet
/
/power
/consumption
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 7

8. HTTP Callbacks on Sensor Events
POST /power HTTP/1.1
Host: poweroutlet
Content-Length: 4
Content-Type: text/plain
Connection: close
Referer: http://lightswitch/position
User-Agent: WiFiNode 0.4
Date: Sat, 01 Feb 2011 12:45:26 GMT
Last-Modified: Sat, 01 Feb 2011 12:45:26 GMT
X-SensorEvent-Count: 12
X-Last-Uptime: 88
true
http://lightswitch/ http://poweroutlet/power
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 8

9. Z
Energy Efficiency Z
Z
 Up to 212 mA when active
 Maximize sleep time
Z
 Only report events
 Optimize wake-up and uptime
Z
 Problem: Availability
 Sleeping for sensors only
 Heartbeats
 Poll configuration updates
 Setup with /sleep resource
or “double-click”
Button “sensor“ prototype
Matthias Kovatsch Connecting Things to the Web using Programmable Low-power WiFi Modules 9

10. 450
Evaluation
400
Wake-up Cycle
350
1. Enable CPU and RAM
Average uptime [ms]
300
2. Boot application Other
250 Callback
3. Find AP and connect*
DNS
4. Run DHCP* 200
DHCP
5. Run DNS* 150 WiFi
6. Open TCP socket 100
7. Perform HTTP callback
50
8. Close TCP socket
0
9. Go to sleep Office Home1 Home2
* can be optimized
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11. Results
Reed and PIR sensor
 Battery-powered sensors
 >99% data reception rate without retries
 ~300‘000 Callbacks with two AAA batteries
 ~8 years with 100 callbacks per day
 Grid-powered actuators
 ~24 ms round trip time (~19 ms for Apache)
 Real-time support
WiFi Plogg prototype
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12. Conclusions and Outlook
 Sufficient battery life for event-based sensing
 WiFi infrastructure greatly reduces deployment costs
 However, may also introduce some hard-to-trace problems
 Hardware support for real energy savings
 Future Work
 RFID bootstrapping
 JavaScript programming
Subtle notification
device prototype
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