Wireless Communication in Industrial Networks

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					            Wireless Communication in
                Industrial Networks
               Kavitha Balasubramanian
               Teaching Assistant, CprE 458/558
               Dept. of Electrical and Computer Engineering
               Iowa State University, Ames, IA 50011


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                          Agenda
   Introduction
   Existing Wireless Standards
   Non real time applications
   Soft real-time applications
   Hard Real-time applications
   Techniques for improving reliability
   The Future

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                      Introduction
   Immense development of wireless
    communication technology for consumer
    electronics
   Also finding its way into industrial setup
   Salient features of communication system
    in industrial application
      • Part of production facility
      • Loss of production costly in comparison to the
        communication system
      • Focus on reliability, predictability and fault
        tolerance
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               Introduction …
   How wireless is accepted?
      • Technology should have clear benefits
        and is reliable so as to justify
        investment
      • Globally accepted standards has led to
        mass chip production
      • Low prices for complex products
      • Some of these are used in the industry

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       Existing Wireless Standards
   802.11
      • Oldest and most mature
      • Spread spectrum techniques for PHY layer
               Direct Sequence Spread spectrum with Differential
                Binary Phase Shift Keying or Differential Quadrature
                Phase Shift Keying
               Frequency Hopping Spread Spectrum with Gaussian
                Frequency Shift Keying
      • Split into
               802.11a – 5GHz ISM band, Up to 2 Mbps
               802.11b – 2.45GHz ISM band, Up to 11 Mbps
               802.11e – Supports QoS
      • Uses CSMA/CA MAC (contention based) with
        dynamic packet length up to 4096 bytes long
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    Existing Wireless Standards …
   HiperLAN/2
      • Support for soft real-time applications like
        media
      • Uses Time Division Duplex scheme (Contention
        free)
      • Static packet length of 54 bytes
      • Better suited for real-time applications because
        of time deterministic MAC
      • OFDM modulation
               Multicarrier modulation scheme
               Adapts bit-rate based on channel conditions
               Handles performance reduction due to multipath
                fading radio channels

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    Existing Wireless Standards …
   Bluetooth
      • Low-complexity low-cost solution for short-range
        wireless communications
      • Intended for cable replacement and adhoc connections
        of consumer devices
      • Instead of creating protocols in all layers of the protocol
        stack for each application, profiles exists
      • Subset of protocols serving a certain application
      • Uses FHSS with hopping frequency of 1600 hops/sec
      • Also supports soft real-time applications like multimedia




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            Non real-time applications
   Remote Control:
      • Used for remote control of overhead cranes
      • Dependent on security
      • Uses long code words to initiate remote control action
   Machine health monitoring:
      • Accurate information about the status of a process
      • Locally on demand or over a wireless network to a
        control room
      • For local information, use PDA or laptop that connects to
        sensors or actuators
      • Plant equipped with access point for communication with
        the control room at the same time
      • Using Bluetooth is a cheap way of achieving local health
        monitoring

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     Non real-time applications …
   System Configuration and Information Exchange:
      •     Information is downloaded to a target device
      •     Higher demand for fault tolerance
      •     Use File Transfer Protocol with secure delivery
      •     Profiles in BT
            Object Exchange protocol includes a file transfer protocol
             
          TCP/IP over Bluetooth transport protocols

      • Fault Tolerance
          With Error detection/correction, there is a probability that the
            errors go undetected (depends on type of coding method and the
            code rate)
          Repeat message a number of times and use majority voting
          For small configuration changes, value is uploaded into the device
            and echoed back. A mismatch will initiate a re-transmission
          Configured device can echo the changed value a number of times
            – if correct value is received all the times, there is a very little
            chance that the configured value is wrong


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     Non real-time applications …
   Internet Connectivity:
      • Used for surveillance, supervision and health monitoring
        of devices
      • Demands soft real-time guarantees if applications like
        voice and video need to be supported
      • E.g. Distributed supervision of heat and ventilation
        system
      • Heat/ventilation system has an embedded web servers
        that runs scripts
      • Personnel can supervise and configure the system using
        a PC with a web browser
      • Can use 802.11 or BT depending on the bandwidth
        requirements


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            Soft real-time applications
   QoS Parameters include
      • deadline : delay constraints of the
        application
      • probability of correct delivery within the
        deadline: bit error rate of the
        communication channel
      • Jitter of the delay



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     Soft real-time applications …
   Event Registration:
      • Time-stamp an event with high time
        resolution
      • Transmission of the event from device
        that registered the event is not so
        critical i.e. requires only a reasonable
        response time.
      • E.g. Events don’t control anything but
        should be logged at the right time
      • Requires clock synchronization
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     Soft real-time applications …
   Measurement:
      • Measures a physical process, timestamp the sample
        values and transmit sequence of values to the user
      • Present course of events in a correct way so that the
        time stamps can reconstruct the process
      • Notion of global correct time is important
      • Requires clock synchronization; precision demands is
        dictated by the granularity of the measured values
      • For e.g. Geological or industrial wireless sensor system
        with sensors collecting data and transmitting them to
        base station or control room
      • No delay constraints in transmission but measures and
        timestamps done in real-time to reconstruct course of
        events of the physical process
      • High accuracy and synchronized clocks
               802.11 : 150ms; BT: 20us

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     Soft real-time applications …
   Media:
      • Applications like voice and video transfer
      • Delay and loss rate constraints based on comfort provided by
        application to the user
      • BT:
                Supports voice channels
                Range of 10m
                Establishes adhoc connections: As soon as 2 or 3 people are within the
                 range, a communication channel is established
      • HiperLAN/2:
                Supports guaranteed media streams
   Slow control loop:
      •     Control loops used in process control of slow or non-critical operations
      •     Low sample rate which are not affected by a few samples being lost
      •     Delay constraint based on comfort demands
      •     E.g. heat control and ventilation system
      •     No time stamping necessary but low jitter requirement
      •     HiperLAN/2 or IEEE 802.11e is suitable


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            Hard real-time applications
   Result presented too late is disastrous to the
    system
   One missed deadline will led to a disaster and
    cannot be tolerated
   E.g. control loops
   Most distributed real-time systems assume fault-
    free operation of the processing unit and
    communication channel that is not true
      • Error probability cannot be neglected for wireless
        channel and its function cannot be guaranteed at all
        times
      • Errors are sporadic in nature for wireless while for wired,
        permanent errors caused by wire or connector damage
        dominates

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    Hard real-time applications …
   Parameters:
      • Deadline
      • Probability to deliver within the deadline
   If we quantify the probability for
    communication channels, we can analyze
    the error probability of the entire system
    and then employ risk analysis methods for
    calculating failure characteristics of the
    system
   Problem: Quantify Probability to deliver
    within the deadline for certain system
    conditions
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 Techniques for improving reliability
        of wireless channel
   Based on radio signals
   Transmitted wave interferes with the surrounding
    environment creating multiple waves hitting the receiver
    antenna
   Waves delayed with respect to each other
   For some conditions, there is destructive interference at the
    receiver antenna causing signal attenuation. This is called
    fading
   Fading causes bursts of errors in wireless channel
   Fast fading occurs when the transmitter or the receiver
    move fast. Slow fading occurs in stationery situations.
   FEC: Add redundant information to the bit stream that
    helps receiver correct channel induced bit errors. Used for
    fast fading channels
   ARQ: Retransmit entire packets of data when the receiver
    cannot decode the packet. Used for slow fading channels
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 Techniques for improving reliability
        of wireless channel
   Soft decision decoding
      • Efficiency of decoding depends on chosen codeword
      • At receiver, the decoder tries to decode the code words
        into information symbols
      • Separate symbols as much as possible to minimize
        probability that decoder interprets one symbol for
        another. These are called maximum distance codes. E.g.
        Reed Solomon codes.
      • In soft decision decoding, the distance between the code
        word and symbols is calculated as the Euclidian distance
        rather than the hamming distance. No information is
        discarded during the coding process.
      • Turbo coding is a recursive scheme that iterates a
        number of cycles to enhance the probability of
        successful decoding


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 Techniques for improving reliability
        of wireless channel
   Deadline dependent coding:
      • Uses FEC and ARQ to improve Bit Error
        Rate
      • Have a number of re-transmissions
        before the deadline; retransmissions
        have different coding rate depending on
        the remaining time to the deadline
      • Tradeoff between throughput and how
        much redundant information is needed
        to secure the communication link
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    Techniques for improving reliability
           of wireless channel
    From release time to deadline, there is a transmission time window. The
     aim is to transmit the information within this window and succeed with a
     certain probability

    Send predefined number of transmissions. Therefore no need to wait for
     ACK. Decoder keeps packet even if decoding fails and performs additional
     processing such as majority voting. Use different codes for different
     packets to conserve energy.
      •     Fading nature of channel will give error bursts. ARQ isolates the error bursts
      •     Decoder keeps information for future use enhancing the decoder efficiency.




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            Ultra wide-Band radio
   Transmitter sends Gaussian pulses with duration less than a
    nanosecond. This is called a Gaussian monocycle
   Monocycle is a wide bandwidth signal with center frequency
    and bandwidth dependent on the width of the pulse
   Modulation done in time domain by introducing different
    delays between pulses for different symbols
   Signal not vulnerable to multipath fading to the same
    extent as signals with carrier frequency
   The bandwidth allows the transmitter to send at a very low
    output power. Hence it does not interfere with already
    allocated bands
   Fear of interference with GPS signals because of their
    relatively low power




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                         The Future
   Wireless Technologies will play an important role even in
    safety critical industrial applications

   First step is to implement non-critical applications and get
    wireless technology accepted in the industrial sector

   For safety critical applications, more research is required

   New wireless technologies with higher throughput makes it
    possible to use complex coding schemes

   These coding schemes handle the insecure wireless media
    in a predictable and secure way



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