Ubiquitous Resources Abstraction using a File System Interface on Sensor Nodes

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

Outline

 Analysis
 Design of a Resource Abstraction
 Evaluation
 Applications

TecO KuVs FG System Software für Pervasive Computing 2005, October 12, 2005 2

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


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


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


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

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.


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


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


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


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


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


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


Integration of persistent storage

 Simple flash FS
 Sequential append-only files
 Non-blocking operation
 Uncontrolled extrusion


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


POSIX interface
Applications can be easily ported

libc standard I/O
 fprintf

 stdout

compliant C Programming Environment


Application - Shell
File system as interactive runtime environment
“cat audiovolume“ Application
Command
Shell

ParticleFS

Reources cat
audio

audioVolume

Direct Resources Mediated Resources
Hardware

Microphone


Application - Shell
File system as interactive runtime environment
 Standard output as context
 Popping stack elements from pipe stack


Asynchronous Interaction
Service

/service/
result

Client

Simple IPC mechanism


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


Thank you.


Ubiquitous Resources Abstraction using a File System Interface on Sensor Nodes

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