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Title: System Architecture of Networked Sensor Platforms


1
System Architecture of Networked Sensor Platforms
2
Introduction
Wireless sensor networks (WSN) consists of group
of sensor nodes to perform distributed sensing
task using wireless medium. Characteristics-
low-cost, low-power, lightweight - densely
deployed - prone to failures - two ways of
deployment randomly, pre-determined or
engineered Objectives- Monitor
activities- Gather and fuse information -
Communicate with global data processing unit
3
Introduction
  • Recent sensor networks research involves almost
    all the layers and can be categorized into the
    following three aspects Akyildiz2002,
    Elson2002
  • Energy Efficiency
  • small devices, limited amount of energy,
    essential to prolong system lifetime
  • Scalability
  • deployment of thousands of sensor nodes,
    low-cost
  • Locality
  • smallest networks cannot depend on having global
    states

4
Why Sensor Platforms?
  • Traditional mechanisms of exploring the network
    (analysis and simulation) are not satisfied for
    exploring such a large-scale, dynamic and
    resource-constrained networks due to their
    difficulties to modeling every aspect of the
    system as a whole
  • For example, energy consumption model of the
    hardware platforms, including sensing,
    computation and communication, is not fully
    considered and overly-simplified assumptions have
    been made
  • Application-specific property of WSN makes the
    existing research mechanisms even harder to
    obtain meaningful results
  • Therefore, the demand to build a platform is
    increasing e.g., Berkeleys motes and MANTIS

5
Why Sensor Platforms?
  • Compared to analysis and simulation techniques,
    designing a system platform has the following
    advantages
  • Provides genuine executive environment various
    proposed algorithms can be exactly evaluated
    good way to examine existing design principles
    and discover new ones under different
    configurations
  • More attention can be focused on the
    application-layer
  • A real system platform can accelerate the pace of
    research and development

6
General WSN System Architecture
  • Constructing a platform for WSN falls into the
    area of embedded system development which usually
    consists of developing environment, hardware and
    software platforms.
  • Hardware Platform
  • Consists of the following four components
  • a) Processing Unit
  • Associates with small storage unit (tens of kilo
    bytes order) and
  • manages the procedures to collaborate with other
    nodes to carry out the
  • assigned sensing task
  • b) Transceiver Unit
  • Connects the node to the network via various
    possible transmission
  • medias such as infra, light, radio and so on

7
General WSN System Architecture
  • Hardware Platform
  • c) Power Unit
  • Supplies power to the system by small size
    batteries which makes the
  • energy a scarce resource
  • d) Sensing Units
  • Usually composed of two subunits sensors and
    analog-to-digital
  • Converters (ADCs). The analog signal produced by
    the sensors are
  • converted to digital signals by the ADC, and fed
    into the processing unit
  • e) Other Application Dependent Components
  • Location finding system is needed to determine
    the location of sensor
  • nodes with high accuracy mobilizer may be needed
    to move sensor
  • nodes when it is required to carry out the task

8
General WSN System Architecture
  • Hardware Platform

Figure 1 The components of a sensor node
Akyildiz2002
9
General WSN System Architecture
  • Software Platform
  • Consists of the following four components
  • a) Embedded Operating System (EOS)
  • Manages the hardware capability efficiently as
    well as supports
  • concurrency-intense operations. Apart from
    traditional OS tasks such as
  • processor, memory and I/O management, it must be
    real-time to rapidly
  • respond the hardware triggered events,
    multi-threading to handle
  • concurrent flows
  • b) Application Programming Interface (API)
  • A series of functions provided by OS and other
    system-level components
  • for assisting developers to build applications
    upon itself

10
General WSN System Architecture
  • Software Platform
  • c) Device Drivers
  • A series of routines that determine how the upper
    layer entities
  • communicate with the peripheral devices
  • d) Hardware Abstract Layer (HAL)
  • Intermediate layer between the hardware and the
    OS. Provides uniform
  • interfaces to the upper layer while its
    implementation is highly dependent
  • on the lower layer hardware. With the use of HAL,
    the OS and
  • applications easily transplant from one hardware
    platform to another

11
General WSN System Architecture
  • Software Platform

Figure 2 The software platform for WSN
12
General WSN System Architecture
  • System Development Environment
  • Provides various of tools for every stage of
    software development over
  • the specific hardware platform
  • a) Cross-Platform Development
  • Generally, an embedded system unlike PC and does
    not have the ability
  • of self-development. The final binary code run on
    that system, termed as
  • target system, will be generated on the PC,
    termed as host system, by
  • cross-platform compilers and linkers, and
    download the resulted image
  • via the communication port onto the target system

13
General WSN System Architecture
  • System Development Environment
  • Provides various of tools for every stage of
    software development over
  • the specific hardware platform
  • b) Debug Techniques
  • Due to the difficulties introduced by
    cross-platform development mode,
  • the debug techniques become critical for the
    efficiency of software
  • production. For this reason, many chips on the
    system provide the
  • on-chip debugger, such as JTAF, to reduce the
    development time.

14
Berkeley Motes Hill 2000
  • Motes are tiny, self-contained, battery powered
    computers with radio links, which enable them to
    communicate and exchange data with one another,
    and to self-organize into ad hoc networks
  • Motes form the building blocks of wireless sensor
    networks
  • TinyOS TinyOS, component-based runtime
    environment, is designed to provide support for
    these motes which require concurrency intensive
    operations while constrained by minimal hardware
    resources

Figure 3 Berkeley Mote
15
Berkeley Motes Hill 2000
  • Hardware Platform
  • Consists of
  • micro-controller with internal flash program
    memory
  • data SRAM
  • data EEPROM
  • a set of actuator and sensor devices, including
    LEDs
  • a low-power transceiver
  • an analog photo-sensor
  • a digital temperature sensor
  • a serial port
  • a small coprocessor unit

16
Berkeley Motes Hill 2000
  • Hardware Platform

Figure 4 The schematic for representative
network sensor platform
17
Berkeley Motes Hill 2000
  • Hardware Platform
  • The processing unit
  • MCU (ATMEL 90LS8535), an 8-bit architecture with
    16-bit addresses
  • provides 32 8-bit general registers and runs at
    4 MHz and 3.0 V
  • has 8 KB flash as the program memory and 512
    Bytes of SRAM as the data memory
  • MCU is designed such that the processor cannot
    write to instruction memory the prototype uses a
    coprocessor to perform this function
  • the processor integrates a set of timers and
    counters which can be configured to generate
    interrupts at regular time intervals
  • three sleep modes idle (shuts off the
    processor), power down (shuts off everything, but
    the watchdog and asynchronous interrupt logic
    necessary to wake up), power save (keep
    asynchronous timer on)

18
Berkeley Motes Hill 2000
  • Hardware Platform
  • The sensing units
  • contains two sub-components photo sensor and
    temperature sensor
  • photo sensor represents an analog input device
    with simple control lines which eliminate power
    drain through the photo resistor when not in use
  • temperature sensor (Analog Devices AD7418)
    represents a large class of digital sensors which
    have internal A/D converters and interface over a
    standard chip-to-chip protocol (the synchronous
    two-wire I2C protocol with software on the
    micro-controller synthesizing the I2C master over
    general I/O pins. There is no explicit arbiter
    and bus negotiations are carried out by the
    software on the micro-controller

19
Berkeley Motes Hill 2000
  • Hardware Platform
  • The transceiver unit
  • consist of an RF Monolithics 916.50 MHz
    transceiver (TR1000), antenna, and a collection
    of discrete components to configure the physical
    layer characteristics such as signal strength and
    sensitivity
  • operates in an ON-OFF key mode at speeds up to
    19.2 Kbps
  • control signals configure the radio to operate in
    either transmit, receive, or power-off mode
  • the radio contains no buffering, so each bit must
    be serviced by the controller on time
  • the transmitted value is not latched by the
    radio, so the jitter at the radio input is
    propagated into the transmission signal

20
Berkeley Motes Hill 2000
  • Hardware Platform
  • The transceiver unit is an Energizer CR2450
    lithium battery rated at
  • 575 mAh
  • The other auxiliary components include
  • The coprocessor
  • represents a synchronous bit-level device with
    byte-level support
  • MCU (AT09LS2343, with 2KB instruction memory, 128
    bytes of SRAM and EEPROM) that uses I/O pins
    connected to an SPI controller where SPI is a
    synchronous serial data link, providing high
    speed full-duplex connections (up to 1 Mbit)
    between peripherals
  • the sensor can be reprogrammed by transferring
    data from the network into the coprocessors 256
    KB EEPROM (24LC256)
  • can be used as a gateway to extra storage by the
    main processor

21
Berkeley Motes Hill 2000
  • Hardware Platform
  • The other auxiliary components include
  • The serial port
  • represents a synchronous bit-level device with
    byte-level controller support
  • uses I/O pins that are connected to an internal
    UART controller
  • in transmit mode, the UART takes a byte of data
    and shifts it out serially at a specified
    interval
  • in receive mode, it samples the input pin for a
    transition and shifts in bits at a specified
    interval from the edge
  • interrupts are triggered in the processor to
    signal completion of the events

22
Berkeley Motes Hill 2000, TinyOS
  • Hardware Platform
  • The other auxiliary components include
  • Three LEDs
  • represent outputs connected through general I/O
    ports they may be used to display digital values
    or status
  • Software Platform
  • based on Tiny Micro-Threading Operating System
    (TinyOS) which is designed for resource-constraine
    d MEMS sensors
  • TinyOS adopts an event model so that high levels
    of concurrency can be handled in a small amount
    of space
  • A stack-based threaded approach would require
    that stack space be reserved for each execution
    context

23
Berkeley Motes Hill 2000, TinyOS
  • Software Platform
  • A complete system configuration consists of a
    tiny scheduler and a graph of components
  • A component has four interrelated parts a set of
  • a set of command handlers
  • a set of event handlers
  • an encapsulated fixed-size frame
  • Bundle of simple tasks
  • tasks, commands and event handlers execute in the
    context of the frame and operate on its state
  • each component declares the commands it uses and
    the events it signals
  • these declarations are used to compose the
    modular components in a per-application
    configuration

24
Berkeley Motes Hill 2000, TinyOS
  • Software Platform
  • the composition process creates layers of
    components where higher-level components issue
    commands to lower-level components and
    lower-level components signal events to the
    higher-level components
  • Frames
  • fixed-size and statistically allocated which
    allows us to know memory requirements of a
    component at a compile time -- prevents overhead
    associated with dynamic allocation
  • Commands
  • non-blocking requests made to lower level
    components
  • typically, a command will deposit request
    parameters into its frame and conditionally post
    a task for later execution

25
Berkeley Motes Hill 2000, TinyOS
  • Software Platform
  • Commands
  • can invoke lower commands, but it must not wait
    for long
  • must provide feedback to its caller by returning
    status indicating whether it was successful or
    not
  • Event handlers
  • Invoked to deal with hardware events, either
    directly or indirectly
  • The lowest level components have handlers
    connected directly to hardware interrupts which
    may be external interrupts, timer events, or
    counter events
  • An event handler can deposit information into its
    frame, post tasks, signal higher level events or
    call lower level commands

26
Berkeley Motes Hill 2000, TinyOS
  • Software Platform
  • Event handlers
  • in order to avoid cycles in the command/event
    chain, commands cannot signal events
  • both signals and events are intended to perform a
    small, fixed amount of work, which occurs within
    the context of their components state
  • Tasks
  • perform the primary work
  • atomic entities with respect to other tasks, run
    to completion and can be preempted by events
  • can call lower level commands, signal higher
    level events, and schedule other tasks within a
    component

27
Berkeley Motes Hill 2000, TinyOS
  • Software Platform
  • Tasks
  • run-to-completion semantics make it possible to
    allocate a single stack that is assigned to the
    currently executing task which is essential in
    memory constrained systems
  • allows to simulate concurrency within each
    component, since tasks execute asynchronously
    with respect to the events
  • must never block or spin wait, otherwise, they
    will prevent progress in other components
  • Task scheduler
  • Utilizes a bounded size scheduling data structure
    to schedule various tasks base on FIFO,
    priority-based or deadline-based policy which is
    dependent on the requirements of the application

28
Berkeley Motes Hill 2000, TinyOS
Software Platform
Figure 5 The schematic for the architecture of
TinyOS
29
MANTIS Abrach 2003
  • MANTIS (MultimodAl system for NeTworks of In-situ
    wireless Sensors) provides a new multi-threaded
    embedded operating system integrated with a
    general-purpose single-board hardware platform to
    enable flexible and rapid prototyping of wireless
    sensor networks
  • the key design goals of MANTIS are
  • ease of use, i.e., a small learning curve that
    encourages novice programmers to rapidly
    prototype sensor applications
  • flexibility such that expert researchers can
    continue to adapt and extend the
    hardware/software system to suit the needs of
    advanced research

30
MANTIS Abrach 2003
  • MANTIS OS is called MOS
  • MOS selects its model as classical structure of
    layered multi-threaded operating systems which
    includes multi-threading, preemptive scheduling
    with time slicing, I/O synchronization via mutual
    exclusion, a standard network stack, and device
    drivers
  • MOS choose a standard programming language that
    the entire kernel and API are written in standard
    C. This design choice not only almost eliminates
    the learning curve, but also accrues many of the
    other benefits of a standard programming
    language, including cross-platform support and
    reuse of a vast legacy code base. C also eases
    development of cross-platform multimodal
    prototyping environments on X86 PCs

31
MANTIS Abrach 2003
  • Hardware Platform
  • MANTIS hardware nymphs design was inspired by
    the Berkeley MICA an MICA2 Mote architecture
  • MANTIS Nymph is a single-board design,
    incorporating the micro-controller, analog sensor
    ports, RF communication, EEPROM, and serial ports
    on one dual-layer 3.5 x 5.5 cm printed circuit
    board
  • the Nymph is centered around the AMTEL
    ATmega128(L) MCU, including interfaces for two
    UARTs, an SPI bus, an I2C bus, and eight
    analog-to-digital converter channels. It provides
    additional 64KB EEPROM external to MCU in
    addition to 4KB EEPROM included in MCU
  • the unit is powered either by batteries or an AC
    adapter, and a set of three on-board LEDs are
    included to aid in the debugging process. It is
    designed to hold a 24mm diameter lithium ion coin
    cell battery (CR2477), but any battery in the
    range of 1.8V to 3.6V can be connected

32
MANTIS Abrach 2003
  • Hardware Platform
  • in order to facilitate rapid prototyping in
    research environment, the Nymph has solderless
    plug connections for both analog and digital
    sensors, which eliminates the external sensor
    board for many applications
  • each connector provides lines for ground, power
    and sensor signal, allowing basic sensors such as
    photo sensors or complex devices such as infrared
    an ultra sounds receivers to be connected easily
  • the Chipcon CC1000 radio was chosen to handle
    wireless communication. It supports four carrier
    frequency bands (315, 433, 868, and 915 MHz) and
    allows for frequency hopping which is useful for
    multi-channel communication. It is one of the
    lowest power commercial radios and allows MOS to
    optimize the radio to further reduce the power
    consumption

33
MANTIS Abrach 2003
  • Hardware Platform
  • for additional modules, the Nymph includes JTAG
    interface which allows the user to easily
    download code to the hardware. This addition
    eliminates a need for separate programming board,
    simplifying the process of reprogramming the
    nodes while reducing the cost of overall system.
    As added benefit, the JTAG port allows the user
    to single-step through code on the MCU and also
    supports the remote shell
  • the Nymph uses one of the UARTs to supply a
    serial port (RS232) for connection to a PC while
    the second one is used as an interface to the
    optional GPS unit
  • MAX3221 RS232 serial chip is used and may be set
    in three different power saving modes
    power-down, receive only and shut down

34
MANTIS Abrach 2003
  • Hardware Platform

Figure 6 MANTIS Nymph
35
MANTIS Abrach 2003
  • Software Platform
  • MANTIS OS (MOS) adheres to classical layered
    multi-threaded design
  • top application and API layers show a simple C
    API which promotes the ease of use,
    cross-platform portability, and reuse of a large
    installed code base
  • in lower layers of MOS, it adapts the classical
    OS structures to achieve small memory footprint
  • System APIs
  • MANTIS provides comprehensive System APIs for I/O
    and system interaction
  • the choice of C language API simplifies
    cross-platform support and the development of a
    multimodal prototyping environment

36
MANTIS Abrach 2003
  • Software Platform
  • System APIs
  • since MANTIS System API is preserved across both
    physical sensor nodes as well as virtual sensor
    nodes running on X86 platforms, the same C code
    developed for MANTIS sensor Nymphs with AMTEL MCU
    can be compiled to run on X86 PCs with little or
    no alteration
  • Kernel and Scheduler
  • design of MOS kernel resembles classical
    UNIX-style schedulers
  • The services provided are subset of POSIX
    threads, most notably priority-based thread
    scheduling with round-robin semantics within a
    priority level
  • binary (mutex) and counting semaphores are also
    supported
  • the goal of the kernel design is to implement
    these familiar services in an efficient manner
    for resource-constrained environment of a sensor
    node

37
MANTIS Abrach 2003
  • Software Platform
  • Network Stack
  • focused on efficient use of limited memory,
    flexibility, and convenience
  • implemented as one or more user-level threads
  • different layers can be implemented in different
    threads, or all layers in the stack can be
    implemented in one thread
  • the tradeoff is between performance and
    flexibility
  • designed to minimize memory buffer allocation
    through layers
  • the data body for a packet is common through all
    layers within a thread
  • the headers for a packet is variably-sized and
    are pre-pended to the single data body
  • designed in a modular manner with standard APIs
    between each layers, thereby allowing developers
    easily modify or replace layer modules

38
MANTIS Abrach 2003
  • Software Platform
  • Device Drivers
  • Adopts the traditional logical/physical
    partitioning with respect to device driver design
    for the hardware
  • The application developer need not to interact
    with the hardware to accomplish a given task

39
MANTIS Abrach 2003
  • Software Platform

Figure 7 MANTIS OS Architecture
40
MANTIS Abrach 2003
  • System Development
  • application developers need to be able to
    prototype and test applications prior to
    distribution and physical deployment in the field
  • during deployment, in-situ sensor nodes need to
    be capable of being both dynamically reprogrammed
    and remotely debugged
  • in order to facilitates these issues, MANTIS
    identifies and implements three key advanced
    features for expert users of general-purpose
    sensor systems
  • multimodal prototyping environment
  • dynamic reprogramming
  • remote shell and commander server

41
MANTIS Abrach 2003
  • System Development
  • Multimodal Prototyping Environment
  • Provides a framework for prototyping diverse
    applications across heterogeneous platforms
  • A key requirement of sensor systems is the need
    to provide a prototyping environment to test
    sensor networking applications prior to
    deployment
  • Postponing testing of an application until after
    its deployment across a distributed sensor
    network can incur severe consequences
  • MANTIS prototyping environment extends beyond
    simulation to provide larger framework for
    development of network management and
    visualization applications as virtual nodes
    within a MANTIS network
  • MANTIS has property of enabling an application
    developer to test execution of the same C code on
    both virtual sensor nodes and later on in-situ
    physical sensor nodes

42
MANTIS Abrach 2003
  • System Development
  • Multimodal Prototyping Environment
  • Seamlessly integrates virtual environment with
    the real deployment network such that both
    virtual and physical nodes can co-exit and
    communicate with each other in the prototyping
    environment
  • Permits a virtual node to leverage other APIs
    outside of the MANTIS API, e.g., a virtual node
    with the MANTIS API could be realized as a UNIX
    X windows application that communicates with
    other rendering or database APIs to build
    visualization and network management applications

43
MANTIS Abrach 2003
  • System Development
  • Multimodal Prototyping Environment

Figure 8 Multimodal prototyping integrates both
virtual and physical sensor nodes across
heterogeneous X86 and AMTEL sensor platforms
44
MANTIS Abrach 2003
  • System Development
  • Dynamic Reprogramming
  • Sensor nodes should be remotely reconfigurable
    over a wireless multi-hop network after being
    deployed in the field. Since sensor nodes may be
    deployed in inaccessible areas and may scale to
    thousands of nodes, this simplifies management of
    the sensor network
  • MOS achieves dynamic reprogramming in several
    granularities re-flashing the entire OS
    reprogramming of a single thread and changing of
    variables within a thread
  • Another useful feature would be the ability to
    remotely debug a running thread. MOS provides a
    remote shell that enables a user to login and
    inspect the sensor nodes memory
  • MOS includes two programming modes (simpler and
    more advanced) in order to overcome the
    difficulty of reprogramming the network

45
MANTIS Abrach 2003
  • System Development
  • Dynamic Reprogramming
  • The simpler programming mode is similar to that
    used in many other systems and involves a direct
    communication with a specific MANTIS node
  • On a Nymph, this would be accomplished via a
    serial port the user simply connects the node to
    a PC and opens the MANTIS shell
  • Upon reset, MOS enters a boot loader that checks
    for communication from the shell. At this point,
    the node will accept a new code image, which is
    downloaded from the PC over the direct
    communication line
  • From the shell, the user has the ability to
    inspect and modify the nodes memory directly as
    well as spawn threads and retrieve debugging
    information including thread status, stack fill,
    and other statistics from OS
  • The boot loader transfers control to the MOS
    kernel on command from the shell, or at a startup
    if the shell is not present

46
MANTIS Abrach 2003
  • System Development
  • Dynamic Reprogramming
  • The more advanced programming mode is used when a
    node is already deployed, and does not require
    direct access to the node
  • The spectrum of dynamic reprogramming of in-situ
    sensor networks ranges from fine grained
    reprogramming to complete reprogramming
  • MOS has a provision for reprogramming any portion
    of the node up to and including the OS itself
    while the node is deployed in the field
  • This is accomplished through the MOS dynamic
    reprogramming interface

47
MANTIS Abrach 2003
  • System Development
  • Remote Shell and Commander Server
  • MOS includes the MANTIS Command Server (MCS)
    which is implemented as an application thread
  • From any device in the network equipped with a
    terminal, the user may invoke the command server
    client (also referred to as the shell) and log in
    to either a physical node (e.g., on a Nymph or
    Mica board) or a virtual node running as a
    process on a PC
  • MCS listens on a network port for commands and
    replies with the results, in a manner similar to
    RPC
  • The shell gains the ability to control a node
    remotely through MCS

48
MANTIS Abrach 2003
  • System Development
  • Remote Shell and Commander Server
  • The user may alter the nodes configuration
    settings, run or kill programs, display the
    thread table and other OS data, inspect and
    modify the nodes data memory, and call arbitrary
    user-defined functions
  • The shell is powerful debugging tool since it
    allows the user to examine and modify the state
    of any node, without requiring physical access to
    the node

49
References
  • Abrach 2003 H. Abrach, S. Bhatti, J. Carlson,
    H. Dai, J. Rose, A. Sheth, B. Shucker, J, Deng
    and R. Han, MANTIS System Support for MultimodAl
    NeTworks of In-Situ Sensors, 2nd ACM
    International Workshop on Wireless Sensor
    Networks and Applications (WSNA 2003), September
    2003.
  • Akyildiz 2002 I. F. Akyildiz, W. Su, Y.
    Sankarasubramaniam, and E. Cayirci, A Survey on
    Sensor Networks, IEEE Communications Magazine,
    Vol. 40, No. 8, pp. 102-114, August 2002.
  • Elson 2002 J. Elson and K. Romer, Wireless
    Sensor Networks A New Regime for Time
    Synchronization, First Workshop on Hot Topics in
    Networks (Hotnets-I), Princeton, USA, October
    2002.
  • Hill 2000 J. Hill, R. Szewczyk, A. Woo, S.
    Hollar, D. Culler, and K. Pister, System
    Architecture Directions for Networked Sensors,
    Architectural Support for Programming Languages
    and Operating Systems (ASPLOS) 2000
  • TinyOS TinyOS a component-based OS for the
    networked sensor regime.
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