1. Ubiquitous Resources Abstraction
using a File System Interface
on Sensor Nodes
Till Riedel, Christian Decker
TecO, University of Karlsruhe
Institut for Telematics
Telecooperation Office (TecO)
www.teco.edu

2. Outline
 Analysis
 Design of a Resource Abstraction
 Evaluation
 Applications
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3. Embedded Sensor Devices in
Ubicomp
Motes Particles Telos
 Limited computing power, 8-bit microcontroller
 Few kilobytes … 512 kilobytes of Flash memory
 Customized radio protocols
 Battery powered
 Extensible by various sensors
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4. Analysis
 Big variety of sensors
 Growing APIs
 Experiences with developers
 Hesitate to develop code for „new“ sensors
 Stick to the stuff they know
 Use example program
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5. Ubicomp Development –
State-of-the-Art
 Lightweight OS (e.g. TinyOS)
 OS shields resources, e.g. sensors
 No direct access
 Communication through events -> event dispatching
 Library based access models
 Abstract access functions
 Direct access, virtually no overhead
 But… shielding/abstraction causes still confusion
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6. File System for Sensor Nodes
Design principles Application
 Developer in the center
 Support developer to Name File System Type
File System
follow the simplest way Space Operations System
 Easy-to-understand
interfaces -> generic Direct Resources Mediated Resources
Detection
Actuators
unication
Sensors
Memory
Volume
Comm-
Battery
Status
Shock
Audio
...
Support in two ways
 Uniform representation of
all resources
 Uniform access model Hardware
Microphone
Frontend
Memory
Radio
LEDs
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7. File System Interface
Operation Explanation
size_t read Reads n data bytes from the resource identified by fd
(int fd, void* buf, size_t n) to buf; returns number of bytes or -1 if error occurred
size_t write writes n data bytes from buf to the resource fd; returns
(int fd, void* buf, size_t n) number of bytes or -1 if an error occurred
int open(char* resource_path) Returns a descriptor for the resource; -1 if it is not
valid.
int close(int fd) Frees the descriptor of a resource; -1 if fd is not valid
int getType(int fd) Returns the type of a resource fd; -1 if fd is not valid.
int mount Creates a resource in the name space. Type and
(char* resource_path, int type, function pointers to their specific read and write
(*pFunc) read, (*pFunc) operations are given. -1 is returned if the
write) resource_path already exists.
int umount(char* resource_path) Removes a resource; -1 is returned if it is not valid.
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8. Namespace and Ressources
Mount Table
name type read* write*
Name Space Table 0 audio 6 readMic setRate
0 /dev 1 audiovolume . calcVol setRange
1 /light ... ... 3 ... ...
2 /audio n-1 light readLight nop
3 /usr n [free]
4 /file1
5 /subdir
6 /file2
7 /audiolib Storage Table
8 /audiovolume name type read* write*
9 [free] 0 file1
writeFile
readFile
1 file2.frag
...
1
2047 [free]
2048 file2
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9. File System – Access Model
Name space
 Hierarchical
 Textual representation of fd= o p en(”/ com /rf”)
resources read write state typ e com rf
 Combine freely resource fil desc rip tor
e rf d river
and specific behavior
Streams write(fd ,...)
 Access functions “read”, “write”
read write state typ e
 Sequential access
 enforced by state rf_ read rf_ write 0 21 3
 Pass data between streams
-> Stacking of streams
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10. Hardware
 TecO Particle Computer
 18f6720 PIC microcontroller
 128KB FlashROM, 4KB RAM,
512KB external Flash
Resources
 Memory – 512KB Flash
 Power supply – AAA battery
 Wireless communication
 Sensors – acceleration, light, force, temperature, audio,
ball switch
 Actuators – LEDs, speaker
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11. Optimization
 Paralallize write operations
(Flash Buffer, Flash, EEPROM)
13 ms Flash, 4 ms EEPROM, assynchronously programmable
 Mimimize writes on Flash/EEPROM
only 50.000-100.000 erase/write cycles per page
 Minimize read times for sensor devices
support relatively high sampling rates / bytewise sampling
 Minimize RAM consumption
only 4K of RAM available
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12. Performance
Access overhead
cycles PIC18F6720
Table look up function 15 cycles 3 µs
Dereferencing state 4 cycles 0.8 µs
Passing Parameters 10 cycles 2 µs
Function pointer call 26 cycles 5.2 µs
Accessing Parameters 10 cycles 2 µs
Writing the buffer 15 cycles 3 µs
Returning 13 cycles 2.6 µs
Overhead of file system read 93 cycles 18.6 µs
Overhead of simple Library call 26 cycles 5.2 µs
Relative overhead 67 cycles 13.2 µs
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13. Performance
Memory Consumption
 File library
 2 byte on flash per resource (129 per page)
 12 byte per file descriptor (3 byte state)
 Non-Persistent ressources
▫ 6 byte + name in RAM
+ Library Code in internal flash
▫ about 12 KB (10%)
▫ 6K without persistent storage
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14. Integration of persistent storage
 Simple flash FS
 Sequential append-only files
 Non-blocking operation
 Uncontrolled extrusion
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15. Application - Integration
Compiled
FTP Proxy Program
 Seamless transfer of data
between backend infrastructure PC
and sensor nodes
 Composition of name spaces
using URI scheme
ste;1;33
^C
FTP Proxy
Network
 Programming is file copy
 Sensor logging is downloading
Bridge
Particle
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16. POSIX interface
Applications can be easily ported
libc standard I/O
 fprintf
 stdout
compliant C Programming Environment
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17. Application - Shell
File system as interactive runtime environment
“cat audiovolume“ Application
Command
Shell
ParticleFS
Reources cat
audio
audioVolume
Direct Resources Mediated Resources
Hardware
Microphone
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18. Application - Shell
File system as interactive runtime environment
 Standard output as context
 Popping stack elements from pipe stack
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19. Asynchronous Interaction
Service
/service/
result
Client
Simple IPC mechanism
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20. Dynamic Loadable Modules
 No linking of function calls needded
 Specify fixed location for
 file descriptor table
 open
 mount
Functionality instantly available after mount
Hot code replacement
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21. Thank you.
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