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.