QoS, Security and Energy Requirements in Wireless Automation ...

QoS, Security and Energy Requirements inWireless Automation NetworksMikael BjörkbomTKK | Control Engineering

Contents• Quality of Service• What? – What service quality does a wireless automationsystem need?• How? – QoS techniques• Security• Why? – Why bother when there are communication andcomputational constraints• How? – How to do it lightweight• Energy• What? – Energy constraints• How? – How to cope with theseTKK | Control EngineeringAS-74.3199 Wireless Automation2

Introduction• QoS is needed to obtain a good wireless automationsystem• Real-time automation• Manages the network, such that the application requirementsare satisfied• Security is needed to maintain a working automationsystem• Safe, dependable automation• Energy for long lifetime of the application• Batteries, energy sources• Conserve energyTKK | Control EngineeringAS-74.3199 Wireless Automation3

QUALITY OF SERVICETKK | Control EngineeringAS-74.3199 Wireless Automation4

Introduction• “QoS is the application experienced quality of the datatransportation in terms of some parameters”• Wireless network issues Automation requirements• QoS difficult in wireless networks• Shared media, noiseWireless NetworkWirelessAutomation:- Varying delay- Packet lossTKK | Control EngineeringAS-74.3199 Wireless Automation5

• What it is notQoS in Wireless Automation• High throughput• Large data streams• Average availability• What it is• Many small packets (~10-50 bytes)• Single measurements• Control commands• Delivered on time• Preferably without loss, Constant availability• If loss• No large gaps!• Communication dependability, reliabilityTKK | Control EngineeringAS-74.3199 Wireless Automation6

Or then...• If process is slow• The dynamics of the network is negligible• Only successful packet delivery is needed• Data queries, monitoring, maintenance• For service man with handheld device• Usually delay tolerantTKK | Control EngineeringAS-74.3199 Wireless Automation7

Basic QoS Measures• Packet loss• Low bit error rate, physical level• MAC protocol• Avoid dropping packets• Avoid dropping consecutive packets• Some packets may be dropped on purpose• Old information• Redundant information• Average, Maximum Delay• Affects control performance: ω c < 1/T d• Delay Jitter• Variation in delayTKK | Control EngineeringAS-74.3199 Wireless Automation8

More QoS Measures• Continuous operation• If network down orcongested, real-timerequirements are not met• Plant must be brought toa safe state• MTBF/Failure length• Energy consumption• Node lifetime• Network lifetimeTKK | Control EngineeringAS-74.3199 Wireless Automation9

Network Related QoS Measures• Robustness to abnormal traffic• Network setup• Node join time• Node depart time• Network recovery time• Recovery to normal operation after a disruptive event• Topology changes, obstacles• Communication overhead• Protocol overhead• Routing maintenance, robustness• Issues of protocol designTKK | Control EngineeringAS-74.3199 Wireless Automation10

Sensor Network QoS Measures• Number of active sensors/nodes• Data accuracy• Time-synchronization accuracy• Network scalability• Throughput• Reliability• Interference tolerance• Security• Mobility• Coordination effectivenessTKK | Control EngineeringAS-74.3199 Wireless Automation11

Time-Synchronization• Time-synchronization of nodes used in all wirelessautomation protocols• Control system requirement• TDMA MAC• Transmission slots, reliable communication• Tight margins• ISA100.11a: 12 ms for packet and ACK• Data timestamp• Application needs to know when data was recorded• Synchronization of actions• Sensing at same time• Actuator apply command at certain timeTKK | Control EngineeringAS-74.3199 Wireless Automation12

• Depends on applicationRequired QoS• Guaranteed service prefered over best effort• Monitoring, Sensor Networks, Building automation• Lossless packet delivery• Sources to sink• Energy consumption• Military• High security• Fault tolerant• Industry automation• Real-time requirements• Computation, Transmission• No large gapsTKK | Control EngineeringAS-74.3199 Wireless Automation13

Delay Jitter – Control System Stability• Variation in delay• Problematic for control systems• Z-transform: z -d = constant delay• Jitter marginδ max• The additional (varying) delay acontrol system can tolerate• 0 ≤δ( t)≤δmax• Stable ifMagnitude (dB)151050-5-10-15-20-25-30Complementary sensitivity function and 1/(δ max ω) for δ max= 0.5KLT+PID w/o filter1/(δ max ω)KLT+PID w. filter10 -4 10 -2 10 0 10 2Frequency rad/s( ω) ( ω)( ) ( )P j C j 1< , ∀ω∈ 0, ∞+ P jω C jω δ ω1maxTKK | Control Engineering[ ]More on thisin a laterlectureAS-74.3199 Wireless Automation14

Alarm shower• A fault causes abnormal behaviour• Which causes alarms• Which causes alarms...• Large number of alarms must be communicated• Most of them are superfluous• The right must quickly be communicated• Priorization• Normal control communication must still stay operation• Guarantee stability of plant• Current standards transmit urgent data on CSMA slots• Can transmit immediately• Alarm shower -> all alarms collide?TKK | Control EngineeringAS-74.3199 Wireless Automation15

QoS Methods• Some over-provisioning• Bandwidth constraints• Redundancy• Many sensors, nodes• Multipath routing• Scheduling/Coordination of actions• Medium Access Control• TDMA, FDMA• Causes most of the communication delay• Most important for wireless control system QoSTKK | Control EngineeringAS-74.3199 Wireless Automation16

IEEE 802.11 MAC• Point Coordination Function (PCF)• AP polls other nodes, ”TDMA”• Contention Free Period• More controlled delay and delay variation• Enhancements IEEE 802.11e• Sheduling• Distributed Coordination Function (DFC)• CSMA/CA• Ad-hoc networking• Mostly usedContention free period Contention periodBeacon(CSMA/CA)TKK | Control EngineeringAS-74.3199 Wireless Automation17

802.11 Medium Access Control• Carrier Sense Multiple Access with CollisionAvoidance (CSMA/CA)• Less reliable than wired• Since own transmission covers other transmission signals• Hidden terminals problem• Request To Send (RTS) – Clear To Send (CTS)TransmitterReceiverHiddenterminalTKK | Control EngineeringAS-74.3199 Wireless Automation18

IEEE 802.11 QoS• Requested QoS• Priority• Contention – Contention free• Only with PCF, best effort with DCFTKK | Control EngineeringAS-74.3199 Wireless Automation19

IEEE 802.11 PriorityShort Inter Frame Spaces (SIFS)• Highest priority• ACK...• Transmitted earlier on network idle period• PCF IFS (PIFS)• Communication under PCF• DCF IFS (DIFS)• Lowest priority• Competing under DCFPrevious frame SIFS PIFS DIFSIdle until transmission beginsTKK | Control EngineeringAS-74.3199 Wireless Automation20

QoS extensions to IEEE 802.11• IEEE 802.11e• Hybrid Coordination Function• Service classes• Different priority classes of messages• User priority• 0-7, access priority• Traffic specification• 8-15, TSpecTKK | Control EngineeringAS-74.3199 Wireless Automation21

Bluetooth QoS• Polling• Two link types depending on prefered service• Asynchronous Connectionless• Max. 721 kbit/s asymmetric• For data• Synchronous Connection Oriented• Fixed 64 kbit/s throughput• For voiceTKK | Control EngineeringAS-74.3199 Wireless Automation22

TDMA• Used in wireless automation standards• WirelessHART, ISA100• TDMA+FDMA, (Time/Frequency slots)• Reliable and robust• Quaranteed timeslot for transmissions• Can utilize full bandwidth without collisions• Sleep periods between transmissions (saves power)• Timeslot disadvantages• Cannot send immediately• Must synchronize the wakeup and sampling such that ready tosend at own timeslot• Measurements from different times• Time-synchronization must be accurateTKK | Control EngineeringAS-74.3199 Wireless Automation23

QoS Difficulties• Multiple access media• IEEE 802.11 mostly DCF used/implemented• Distributed system• Energy, Computational constraints• The QoS protocols must be simple (signaling + computation)• Varying environment• Signal fading, interference• Nodes go to sleep, faults• => Nodes temporarily unavailable• Periodic and event driven communication• Alarm/event shower• Sometimes QoS is not metTKK | Control EngineeringAS-74.3199 Wireless Automation24

SECURITYTKK | Control EngineeringAS-74.3199 Wireless Automation25

Security• Focus on Wireless security• Communication is in the air• Anybody can listen• Anybody can interfere• In wired systems eavesdropping is harder• Automation system must operate safely• Even under malicious/unintended attacks• Generally in automation: secure access from internet• Special security requirements• Short packets• Low communication and computation overheadTKK | Control EngineeringAS-74.3199 Wireless Automation26

Security Threats• Eavesdropping• Why care, it’s only process data?• Taking control of automation system• Injektion of harmfull messages• Routing disruption• Constantly transmitting/Intentionally collide• Virtual jamming• Communication jamming / DoS attacks• Route request• Denial of sleep attackTKK | Control EngineeringAS-74.3199 Wireless Automation27

Topology related, Sensor Network• In sensor networks topology may be important• Blackhole, Wormhole• Sybil nodeABABBlackholeWormholeCABA,B,CSybil nodeY = mean(A, B, C, D)DTKK | Control EngineeringAS-74.3199 Wireless Automation28

Secure Communication• Physical security• Authentication• Semantic securityTKK | Control EngineeringAS-74.3199 Wireless Automation29

Physical security• Undetected communication• Seems like noise• Jamming/Interference resistance• Techniques• Spread Spectrum communication• Frequency hopping• UWBTKK | Control EngineeringAS-74.3199 Wireless Automation30

Authenticating• Access control, node authenticaton• Only messages from authenticated nodes are accepted• Authentication/Signing of messages is also needed• Verify that the message has not been tampered with• Authentication methods• Message Authentication/Integrity Codes (MAC/MIC)• Signing with Public / Private KeyTKK | Control EngineeringAS-74.3199 Wireless Automation31

Communication Encryption• Semantic security• Message unreadable by third party• Confidentiality: The communication is secret/encrypted• More computation is needed at the nodes toencrypt/decrypt the messages• Encryption methods• WEP, SSH, SSL, IPSec: Too heavy-weightTKK | Control EngineeringAS-74.3199 Wireless Automation32

Keys• How to manage the keys?• Keying mechanism• Create/Destroy• Distribute• Change• TinyPKKeys Benefits CostsGlobal Simple Not robust to nodecompromisePer-link/nodeIEEE 802.15.4GroupTKK | Control EngineeringGraceful degradation,No passiveparticipationGraceful degradation,Passive participationKey distributionprotocolKey distributionprotocolAS-74.3199 Wireless Automation33

TinySec• Security module for TinyOS• Optimised for motes• Low computation and communication overhead• Easy to use: add one line to the makefile• TinySec-Auth: Authentication only• TinySec-AE: Authentication and EncryptionDest AM Len Data MACTinySec-Auth packet formatDest AM Len Src Ctr Data MACTinySec-AE packet formatTKK | Control EngineeringAS-74.3199 Wireless Automation34

Message Authentication Codes• TinySec uses MAC for authentication• Secret key• Compute MAC with key• Add MAC result to the end of packet• Receiver can verify MAC using public keyTKK | Control EngineeringAS-74.3199 Wireless Automation35

Encryption• Only the data is encrypten in TinySec• No decryption needed for routing• TinySec uses Initialisation Vectors for encryption• Encryption of same message twice, different results• Othervise: easy to decrypt yes/no messages• Different IV for every packet• Message encryption initialisation with IV• Send IV with message (TinySec: packet headers)• Receiver can decrypt with public key and IVTKK | Control EngineeringAS-74.3199 Wireless Automation36

Other Security Tasks• Intrusion detection• Monitor that neighbouring nodes work as expected• Forwarding messages• Transmission pattern• Find and Isolate rogue nodesTKK | Control EngineeringAS-74.3199 Wireless Automation37

ENERGY REQUIREMENTSTKK | Control EngineeringAS-74.3199 Wireless Automation38

Power consumption and lifetime• Wireless sensor nodes equipped with limited powersource• No use in wireless if they have wired power(?)• Physical size and cost constraints directly affectenergy available• Power sources• Battery• Solar cells• RF powering• Energy harvestingTKK | Control EngineeringAS-74.3199 Wireless Automation39

Microsensors• Microelectromechanical (MEMS) sensors:etching process used to carve out tinymechanical structures from silicon• Benefits:• Small, low energy consumption• Cheap to manufacture• Read-out electronics etc. (transistors) onsame chip• Example: micromechanicalgyro/accelometer• ESP system in cars• Adaptive cruise controlA schematic of MEMS accelerometer(Ville Kaajakari)• Reduces energy requirements! Micromechanical gyro(VTI Technologies)TKK | Control EngineeringAS-74.3199 Wireless Automation

Communication and Sleeping• Sensing, data processing and communication consumepower:• Maximum energy consumed in data transmission and reception• Data processing consumes much less power→ local data processing rather than transmitting raw data• Sporadic sensing may consume less power than constant eventmonitoring• Sleep mode and wake-up radio (e.g. S-MAC)• Only wake up for a short period to communicate to save power• Node can not be contacted if radio is sleeping• QoS when node is sleeping?TKK | Control EngineeringAS-74.3199 Wireless Automation

Computation Strategy• Online vs. offline/offnetwork• Computation in nodes saves energy• But does it need much communication/synchronization of data?• Centralized computation can be more effective• Is the computation power enough• Homogeneous devices vs. network consisting of a variety ofdifferent devices• Some nodes may compute more than others or act as gatewayto long-range data communication• Affects robustness and capacity of networkTKK | Control EngineeringAS-74.3199 Wireless Automation

• MAC protocols:• SMACS and EAR:MAC Energy Examples• Wake-up during communication phase, radio off while idle• Pairwise, network-wide synchronization not necessary• Perform well in static networks• Has to wait to the wake phase to transmit• Hybrid TDMA/FDMA:• TDMA minimizes transmit on-time, but high time synch. costs• Static TDMA waste bandwidth if nothing to send• Hybrid TDMA-FDMA scheme with optimum number ofchannels to give lowest system power consumption• CSMA:• Constant listen periods (consumes power!!)• Adaptive to traffic loadTKK | Control EngineeringAS-74.3199 Wireless Automation

Sectors for Improvement• CPU• Dynamic voltage scaling and• Dynamic frequency scaling to reducepower consumption by the CPU.• Hardware• Dedicated computation hardware such asFFT circuits to optimize computationsinstead or using general purposeprocessors.• Radio• Duty cycle to power off the radio.• Reducing startup time of radio: improvephase locked loop in the frequencysynthesizer.• Transmit power control.• Signal processing• Data aggregation to reduce transmissionof redundant information.• Link layer• Bit error rate control. Coding techniques.• Adaptive error correction.• Network• Energy efficient network protocol, such asLEACH (Low Energy Adaptive ClusteringHierarchy) [7].• Efficient routing• Clustering• Software• Power aware software reducescomputations if power is scarce.• Energy efficient coded software isobtained by using instructions andalgorithms that takes less energy.• OS• Sleep modes. The OS controls thesleep/wake state, voltage and frequencyscaling and other power saving modes ofthe node.• System• Distributed computations allow longerlatency per computation, thus decreasingthe required CPU frequency.• System level coordination andoptimization.TKK | Control EngineeringAS-74.3199 Wireless Automation45

TradeoffQoSEnergySecurityTKK | Control EngineeringAS-74.3199 Wireless Automation46

Conclusions• Quality of Service• What: Delay, Packet loss...• How: Design MAC• Security• Why: Wireless automation system integrity• How: Authentication, Encryption, TinySec• Energy• What? – Limited energy and therefore limited capacity• How? – Energy sources, sleep, energy awareTKK | Control EngineeringAS-74.3199 Wireless Automation47