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04/19/2304/19/23 Cpre 458/558: Real-Time Systems (G. Manimaran)Cpre 458/558: Real-Time Systems (G. Manimaran) 11
Wireless Communication in Wireless Communication in Industrial NetworksIndustrial Networks
Kavitha BalasubramanianTeaching Assistant, CprE 458/558
04/19/2304/19/23 Cpre 458/558: Wireless Communication in Industrial NetworksCpre 458/558: Wireless Communication in Industrial Networks 22
AgendaAgenda
IntroductionIntroduction Existing Wireless StandardsExisting Wireless Standards Non real time applicationsNon real time applications Soft real-time applicationsSoft real-time applications Hard Real-time applicationsHard Real-time applications Techniques for improving reliabilityTechniques for improving reliability The FutureThe Future
04/19/2304/19/23 Cpre 458/558: Wireless Communication in Industrial NetworksCpre 458/558: Wireless Communication in Industrial Networks 33
IntroductionIntroduction
Immense development of wireless Immense development of wireless communication technology for consumer communication technology for consumer electronicselectronics
Also finding its way into industrial setupAlso finding its way into industrial setup Salient features of communication system Salient features of communication system
in industrial applicationin industrial application• Part of production facilityPart of production facility• Loss of production costly in comparison to the Loss of production costly in comparison to the
communication systemcommunication system• Focus on reliability, predictability and fault Focus on reliability, predictability and fault
tolerancetolerance
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Introduction …Introduction …
How wireless is accepted?How wireless is accepted?• Technology should have clear benefits Technology should have clear benefits
and is reliable so as to justify and is reliable so as to justify investmentinvestment
• Globally accepted standards has led to Globally accepted standards has led to mass chip productionmass chip production
• Low prices for complex productsLow prices for complex products• Some of these are used in the industrySome of these are used in the industry
55
Motivation for wireless industrial networksMotivation for wireless industrial networks
Reduced installation, Reduced installation, reconfiguration and maintenance reconfiguration and maintenance costscosts
Easy access to machines for Easy access to machines for diagnostic or programming diagnostic or programming purposespurposes
Improved coverage of the factory Improved coverage of the factory floorfloor
Eliminates adverse effects due to Eliminates adverse effects due to damage of cablingdamage of cabling
Desire to save on cablingDesire to save on cabling
Globally accepted standards Globally accepted standards leading to mass production and leading to mass production and reduced pricesreduced prices
66
Industrial ApplicationsIndustrial Applications
Wireless Industrial Applications
time
Non Real -
Hard Real -Time
Remote ControlMachine Health Monitoring
System ConfigurationInternet Connectivity
Control LoopsMachine-to-machine communication
Events RegistrationMeasurements
Media-
Rea
lS
oft
time
77
WorkloadWorkload
Application Requirements
Meeting DeadlinesPredictability and ReliabilityGuaranteed packet delivery Guaranteed delivery timesPrioritizing messages
Message Characteristics
Periodic traffic with deadlinesAcyclic packets (alarms) with bounded latencyShort packets (order of kilobytes)
88
Industrial wireless network marketIndustrial wireless network marketS
HO
RT
<
R
AN
GE
>
L
ON
G
LOW < DATA RATE > HIGH
PAN
LAN
TEXT GRAPHICS INTERNET HI-FI AUDIO
STREAMINGVIDEO
DIGITALVIDEO
MULTI-CHANNELVIDEO
Bluetooth1
Bluetooth 2
ZigBee
802.11b
802.11a/HL2 & 802.11g
99
Existing protocols- comparisonExisting protocols- comparison
Feature 802.11 Bluetooth Zigbee
Interference from other devices
-- Avoided using frequency hopping
Dynamic channel selection possible
Optimized for Multimedia, TCP/IP and high data rate applications
Cable replacement technology for portable and fixed electronic devices.
Low power low cost networking in residential and industrial environment.
Energy Consumption High Low (Large packets over small networks)
Least (Small packets over large networks)
Voice support/Security Yes/Yes Yes/Yes No/Yes
Type of Network / Channel Access
Mobile / CSMA/CA and polling
Mobile & Static / Polling Mostly static with infrequently used devices / CSMA and slotted CSMA/CA
Bit error rate High Low Low
Real Time deadlines ??? ??? ???
1010
Challenges and Spectrum of SolutionsChallenges and Spectrum of Solutions
Wireless Challenges
AttenuationFadingMultipath dispersionInterferenceHigh Bit Error rateBurst channel errors
Application Requirements
Reliable deliveryMeet deadlinesSupport message priority
Antenna Redundancy
Cooperative diversity
ARQ
Error Correction Codes
Modulation Techniques
Transmitter Design
Existing SolutionsExisting Solutions
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Existing Wireless StandardsExisting Wireless Standards 802.11802.11
• Oldest and most matureOldest and most mature• Spread spectrum techniques for PHY layerSpread spectrum techniques for PHY layer
Direct Sequence Spread spectrum with Differential Direct Sequence Spread spectrum with Differential Binary Phase Shift Keying or Differential Quadrature Binary Phase Shift Keying or Differential Quadrature Phase Shift KeyingPhase Shift Keying
Frequency Hopping Spread Spectrum with Gaussian Frequency Hopping Spread Spectrum with Gaussian Frequency Shift KeyingFrequency Shift Keying
• Split intoSplit into 802.11a – 5GHz ISM band, Up to 2 Mbps802.11a – 5GHz ISM band, Up to 2 Mbps 802.11b – 2.45GHz ISM band, Up to 11 Mbps802.11b – 2.45GHz ISM band, Up to 11 Mbps 802.11e – Supports QoS802.11e – Supports QoS
• Uses CSMA/CA MAC (contention based) with Uses CSMA/CA MAC (contention based) with dynamic packet length up to 4096 bytes longdynamic packet length up to 4096 bytes long
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Existing Wireless Standards …Existing Wireless Standards … HiperLAN/2HiperLAN/2
• Support for soft real-time applications like Support for soft real-time applications like mediamedia
• Uses Time Division Duplex scheme (Contention Uses Time Division Duplex scheme (Contention free)free)
• Static packet length of 54 bytesStatic packet length of 54 bytes• Better suited for real-time applications because Better suited for real-time applications because
of time deterministic MACof time deterministic MAC• OFDM modulationOFDM modulation
Multicarrier modulation schemeMulticarrier modulation scheme Adapts bit-rate based on channel conditionsAdapts bit-rate based on channel conditions Handles performance reduction due to multipath Handles performance reduction due to multipath
fading radio channelsfading radio channels
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Existing Wireless Standards …Existing Wireless Standards … BluetoothBluetooth
• Low-complexity low-cost solution for short-range Low-complexity low-cost solution for short-range wireless communicationswireless communications
• Intended for cable replacement and adhoc connections Intended for cable replacement and adhoc connections of consumer devicesof consumer devices
• Instead of creating protocols in all layers of the protocol Instead of creating protocols in all layers of the protocol stack for each application, profiles existsstack for each application, profiles exists
• Subset of protocols serving a certain applicationSubset of protocols serving a certain application• Uses FHSS with hopping frequency of 1600 hops/secUses FHSS with hopping frequency of 1600 hops/sec• Also supports soft real-time applications like multimediaAlso supports soft real-time applications like multimedia
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Non real-time applicationsNon real-time applications Remote Control:Remote Control:
• Used for remote control of overhead cranesUsed for remote control of overhead cranes• Dependent on security Dependent on security • Uses long code words to initiate remote control actionUses long code words to initiate remote control action
Machine health monitoring:Machine health monitoring:• Accurate information about the status of a processAccurate information about the status of a process• Locally on demand or over a wireless network to a Locally on demand or over a wireless network to a
control roomcontrol room• For local information, use PDA or laptop that connects to For local information, use PDA or laptop that connects to
sensors or actuatorssensors or actuators• Plant equipped with access point for communication with Plant equipped with access point for communication with
the control room at the same timethe control room at the same time• Using Bluetooth is a cheap way of achieving local health Using Bluetooth is a cheap way of achieving local health
monitoringmonitoring
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Non real-time applications …Non real-time applications … System Configuration and Information Exchange: System Configuration and Information Exchange:
• Information is downloaded to a target deviceInformation is downloaded to a target device• Higher demand for fault toleranceHigher demand for fault tolerance• Use File Transfer Protocol with secure deliveryUse File Transfer Protocol with secure delivery• Profiles in BTProfiles in BT
Object Exchange protocol includes a file transfer protocolObject Exchange protocol includes a file transfer protocol TCP/IP over Bluetooth transport protocolsTCP/IP over Bluetooth transport protocols
• Fault ToleranceFault Tolerance With Error detection/correction, there is a probability that the With Error detection/correction, there is a probability that the
errors go undetected (depends on type of coding method and the errors go undetected (depends on type of coding method and the code rate)code rate)
Repeat message a number of times and use majority votingRepeat message a number of times and use majority voting For small configuration changes, value is uploaded into the device For small configuration changes, value is uploaded into the device
and echoed back. A mismatch will initiate a re-transmissionand echoed back. A mismatch will initiate a re-transmission Configured device can echo the changed value a number of times – Configured device can echo the changed value a number of times –
if correct value is received all the times, there is a very little if correct value is received all the times, there is a very little chance that the configured value is wrong chance that the configured value is wrong
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Non real-time applications …Non real-time applications … Internet Connectivity:Internet Connectivity:
• Used for surveillance, supervision and health monitoring Used for surveillance, supervision and health monitoring of devicesof devices
• Demands soft real-time guarantees if applications like Demands soft real-time guarantees if applications like voice and video need to be supportedvoice and video need to be supported
• E.g. Distributed supervision of heat and ventilation E.g. Distributed supervision of heat and ventilation systemsystem
• Heat/ventilation system has an embedded web servers Heat/ventilation system has an embedded web servers that runs scriptsthat runs scripts
• Personnel can supervise and configure the system using Personnel can supervise and configure the system using a PC with a web browsera PC with a web browser
• Can use 802.11 or BT depending on the bandwidth Can use 802.11 or BT depending on the bandwidth requirementsrequirements
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Soft real-time applicationsSoft real-time applications
QoS Parameters include QoS Parameters include • deadline : delay constraints of the deadline : delay constraints of the
applicationapplication• probability of correct delivery within the probability of correct delivery within the
deadline: bit error rate of the deadline: bit error rate of the communication channelcommunication channel
• Jitter of the delayJitter of the delay
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Soft real-time applications …Soft real-time applications …
Event Registration:Event Registration:• Time-stamp an event with high time Time-stamp an event with high time
resolutionresolution• Transmission of the event from device Transmission of the event from device
that registered the event is not so that registered the event is not so critical i.e. requires only a reasonable critical i.e. requires only a reasonable response time.response time.
• E.g. Events don’t control anything but E.g. Events don’t control anything but should be logged at the right timeshould be logged at the right time
• Requires clock synchronizationRequires clock synchronization
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Soft real-time applications …Soft real-time applications … Measurement:Measurement:
• Measures a physical process, timestamp the sample Measures a physical process, timestamp the sample values and transmit sequence of values to the uservalues and transmit sequence of values to the user
• Present course of events in a correct way so that the Present course of events in a correct way so that the time stamps can reconstruct the processtime stamps can reconstruct the process
• Notion of global correct time is importantNotion of global correct time is important
• Requires clock synchronization; precision demands is Requires clock synchronization; precision demands is dictated by the granularity of the measured valuesdictated by the granularity of the measured values
• For e.g. Geological or industrial wireless sensor system For e.g. Geological or industrial wireless sensor system with sensors collecting data and transmitting them to with sensors collecting data and transmitting them to base station or control roombase station or control room
• No delay constraints in transmission but measures and No delay constraints in transmission but measures and timestamps done in real-time to reconstruct course of timestamps done in real-time to reconstruct course of events of the physical processevents of the physical process
• High accuracy and synchronized clocksHigh accuracy and synchronized clocks 802.11 : 150ms; BT: 20us802.11 : 150ms; BT: 20us
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Soft real-time applications …Soft real-time applications … Media:Media:
• Applications like voice and video transferApplications like voice and video transfer• Delay and loss rate constraints based on comfort provided by Delay and loss rate constraints based on comfort provided by
application to the userapplication to the user• BT:BT:
Supports voice channelsSupports voice channels Range of 10mRange of 10m Establishes adhoc connections: As soon as 2 or 3 people are within the Establishes adhoc connections: As soon as 2 or 3 people are within the
range, a communication channel is establishedrange, a communication channel is established• HiperLAN/2:HiperLAN/2:
Supports guaranteed media streamsSupports guaranteed media streams Slow control loop:Slow control loop:
• Control loops used in process control of slow or non-critical operationsControl loops used in process control of slow or non-critical operations• Low sample rate which are not affected by a few samples being lostLow sample rate which are not affected by a few samples being lost• Delay constraint based on comfort demandsDelay constraint based on comfort demands• E.g. heat control and ventilation system E.g. heat control and ventilation system • No time stamping necessary but low jitter requirementNo time stamping necessary but low jitter requirement• HiperLAN/2 or IEEE 802.11e is suitableHiperLAN/2 or IEEE 802.11e is suitable
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Hard real-time applicationsHard real-time applications Result presented too late is disastrous to the Result presented too late is disastrous to the
systemsystem One missed deadline will led to a disaster and One missed deadline will led to a disaster and
cannot be toleratedcannot be tolerated E.g. control loopsE.g. control loops
Most distributed real-time systems assume fault-Most distributed real-time systems assume fault-free operation of the processing unit and free operation of the processing unit and communication channel that is not truecommunication channel that is not true• Error probability cannot be neglected for wireless Error probability cannot be neglected for wireless
channel and its function cannot be guaranteed at all channel and its function cannot be guaranteed at all timestimes
• Errors are sporadic in nature for wireless while for wired, Errors are sporadic in nature for wireless while for wired, permanent errors caused by wire or connector damage permanent errors caused by wire or connector damage dominatesdominates
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Hard real-time applications …Hard real-time applications … Parameters:Parameters:
• DeadlineDeadline• Probability to deliver within the deadlineProbability to deliver within the deadline
If we quantify the probability for If we quantify the probability for communication channels, we can analyze communication channels, we can analyze the error probability of the entire system the error probability of the entire system and then employ risk analysis methods for and then employ risk analysis methods for calculating failure characteristics of the calculating failure characteristics of the systemsystem
Problem: Quantify Probability to deliver Problem: Quantify Probability to deliver within the deadline for certain system within the deadline for certain system conditionsconditions
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Techniques for improving reliability Techniques for improving reliability of wireless channelof wireless channel
Based on radio signalsBased on radio signals Transmitted wave interferes with the surrounding Transmitted wave interferes with the surrounding
environment creating multiple waves hitting the receiver environment creating multiple waves hitting the receiver antennaantenna
Waves delayed with respect to each otherWaves delayed with respect to each other For some conditions, there is destructive interference at the For some conditions, there is destructive interference at the
receiver antenna causing signal attenuation. This is called receiver antenna causing signal attenuation. This is called fadingfading
Fading causes bursts of errors in wireless channelFading causes bursts of errors in wireless channel Fast fading occurs when the transmitter or the receiver Fast fading occurs when the transmitter or the receiver
move fast. Slow fading occurs in stationery situations. move fast. Slow fading occurs in stationery situations. FEC: Add redundant information to the bit stream that helps FEC: Add redundant information to the bit stream that helps
receiver correct channel induced bit errors. Used for fast receiver correct channel induced bit errors. Used for fast fading channelsfading channels
ARQ: Retransmit entire packets of data when the receiver ARQ: Retransmit entire packets of data when the receiver cannot decode the packet. Used for slow fading channelscannot decode the packet. Used for slow fading channels
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Techniques for improving reliability Techniques for improving reliability of wireless channelof wireless channel
Soft decision decodingSoft decision decoding• Efficiency of decoding depends on chosen codewordEfficiency of decoding depends on chosen codeword• At receiver, the decoder tries to decode the code words At receiver, the decoder tries to decode the code words
into information symbolsinto information symbols• Separate symbols as much as possible to minimize Separate symbols as much as possible to minimize
probability that decoder interprets one symbol for probability that decoder interprets one symbol for another. These are called maximum distance codes. E.g. another. These are called maximum distance codes. E.g. Reed Solomon codes.Reed Solomon codes.
• In soft decision decoding, the distance between the code In soft decision decoding, the distance between the code word and symbols is calculated as the Euclidian distance word and symbols is calculated as the Euclidian distance rather than the hamming distance. No information is rather than the hamming distance. No information is discarded during the coding process.discarded during the coding process.
• Turbo coding is a recursive scheme that iterates a Turbo coding is a recursive scheme that iterates a number of cycles to enhance the probability of number of cycles to enhance the probability of successful decodingsuccessful decoding
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Techniques for improving reliability Techniques for improving reliability of wireless channelof wireless channel
Deadline dependent coding:Deadline dependent coding:• Uses FEC and ARQ to improve Bit Error Uses FEC and ARQ to improve Bit Error
RateRate• Have a number of re-transmissions Have a number of re-transmissions
before the deadline; retransmissions before the deadline; retransmissions have different coding rate depending on have different coding rate depending on the remaining time to the deadlinethe remaining time to the deadline
• Tradeoff between throughput and how Tradeoff between throughput and how much redundant information is needed much redundant information is needed to secure the communication linkto secure the communication link
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Techniques for improving reliability Techniques for improving reliability of wireless channelof wireless channel
From release time to deadline, there is a transmission time window. The 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 aim is to transmit the information within this window and succeed with a certain probabilitycertain probability
Send predefined number of transmissions. Therefore no need to wait for Send predefined number of transmissions. Therefore no need to wait for ACK. Decoder keeps packet even if decoding fails and performs additional ACK. Decoder keeps packet even if decoding fails and performs additional processing such as majority voting. Use different codes for different processing such as majority voting. Use different codes for different packets to conserve energy.packets to conserve energy.• Fading nature of channel will give error bursts. ARQ isolates the error burstsFading nature of channel will give error bursts. ARQ isolates the error bursts• Decoder keeps information for future use enhancing the decoder efficiency.Decoder keeps information for future use enhancing the decoder efficiency.
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Ultra wide-Band radioUltra wide-Band radio Transmitter sends Gaussian pulses with duration less than a Transmitter sends Gaussian pulses with duration less than a
nanosecond. This is called a Gaussian monocyclenanosecond. This is called a Gaussian monocycle
Monocycle is a wide bandwidth signal with center frequency Monocycle is a wide bandwidth signal with center frequency and bandwidth dependent on the width of the pulseand bandwidth dependent on the width of the pulse
Modulation done in time domain by introducing different Modulation done in time domain by introducing different delays between pulses for different symbolsdelays between pulses for different symbols
Signal not vulnerable to multipath fading to the same Signal not vulnerable to multipath fading to the same extent as signals with carrier frequencyextent as signals with carrier frequency
The bandwidth allows the transmitter to send at a very low The bandwidth allows the transmitter to send at a very low output power. Hence it does not interfere with already output power. Hence it does not interfere with already allocated bandsallocated bands
Fear of interference with GPS signals because of their Fear of interference with GPS signals because of their relatively low powerrelatively low power
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The FutureThe Future Wireless Technologies will play an important role even in Wireless Technologies will play an important role even in
safety critical industrial applicationssafety critical industrial applications
First step is to implement non-critical applications and get First step is to implement non-critical applications and get wireless technology accepted in the industrial sectorwireless technology accepted in the industrial sector
For safety critical applications, more research is requiredFor safety critical applications, more research is required
New wireless technologies with higher throughput makes it New wireless technologies with higher throughput makes it possible to use complex coding schemes possible to use complex coding schemes
These coding schemes handle the insecure wireless media These coding schemes handle the insecure wireless media in a predictable and secure wayin a predictable and secure way