wirelesshart modeling and performance evaluationpcuijper/docs/qees/mfc/dsn.pdfwirelesshart modeling...

WirelessHART Modeling and Performance Evaluation

Anne Remke and Xian Wu

October 24, 2013

A. Remke and X. Wu (University of Twente) WirelessHART October 24, 2013 1 / 21

WirelessHART [www.hartcomm.org]

Introduction

Control applications

I switch from wired communication to Wireless

I reduced installation costs and increased flexibility

I can be adapted during runtime

I involves multiple communication hops

Challenge is the predictability!

Recent work by Alur, Pappas et al. introduces

I formal syntax and semantics for multi-hop WirelessHart

I different kinds of link failure models

I sufficient conditions for stability in control loop if only single link fails

Introduction

Control applications

I switch from wired communication to Wireless

I reduced installation costs and increased flexibility

I can be adapted during runtime

I involves multiple communication hops

Challenge is the predictability!

Recent work by Alur, Pappas et al. introduces

I formal syntax and semantics for multi-hop WirelessHart

I different kinds of link failure models

I sufficient conditions for stability in control loop if only single link fails

This work

Proposes to

I separate failure model from control analysis

I allows to consider the possible failure of all involved links

Computes reachability probabilities

I for certain messages, paths or networks

I evaluates influence of different parameters

I takes into account different failure models

I could be included into control analysis directly

This work

Proposes to

I separate failure model from control analysis

I allows to consider the possible failure of all involved links

Computes reachability probabilities

I for certain messages, paths or networks

I evaluates influence of different parameters

I takes into account different failure models

I could be included into control analysis directly

WirelessHART protocol

I feedback controlI field devices are source and relay nodesI gateway is routing destination

I pseudo-random frequency channel hopping and channel blacklistingI to avoid channel overlaping and to reduce interference

I network layer determines routingI upstream and downstream graph routing

I data link layer defines strict 10 ms time slots

I Time Division Multiple Access (TDMA)I for collision-free and deterministic communicationsI one transmission per slot

I MAC layer is slotted and synchronizedI series of consecutive slots forms super-frame

Radio connectivity graph

s2Plant

Wireless Control Netowork

controller

Model information flow

s2Plant

controller

Static routing

I v1 → v4 → controller, v2 → v5 → controller, v3 → v6 → v7 → controller

Communication schedule with superframe of size 7

η = (〈v1, v4〉, 〈v2, v5〉, 〈v3, v6〉, 〈v4, c〉, 〈v5, c〉, 〈v6, v7〉, 〈v7, c〉)

.A. Remke and X. Wu (University of Twente) WirelessHART October 24, 2013 7 / 21

Model information flow

s2Plant

controller

Static routing

I v1 → v4 → controller, v2 → v5 → controller, v3 → v6 → v7 → controller

Communication schedule with superframe of size 7

η = (〈v1, v4〉, 〈v2, v5〉, 〈v3, v6〉, 〈v4, c〉, 〈v5, c〉, 〈v6, v7〉, 〈v7, c〉)

.A. Remke and X. Wu (University of Twente) WirelessHART October 24, 2013 7 / 21

WirelessHART parameters

Super-frame

I all field nodes share the same super-frame of size (Fs)

I slots are specifically allocated to field devices to transmit uplink/downlink

Reporting interval

I frequency at which measurements are taken and communicated (Is)

I reduced frequency saves wireless communiciation overhead and extendslife-time of field devices

Message life cycle

I Time-to-Live (TTL) defines message life time

I TTL is decreased with each slot (uplink OR downlink)

Super-frame

Reporting interval

Message life cycle

Super-frame

Reporting interval

Message life cycle

Hierarchical path model

Information flow

I each flow periodically generates packets at source

I passes it through to its destination

Time-triggered nature of the protocol

I iterate through slots of superframe and

I schedule possible transitionsI idle slot (if communication schedule does not allow a transmission)I successful orI unsuccessful transmission

Compositional model per path

I state reflects age of the message at each hop (age1, age2, . . . , agen)

I unique initial state (1, 0, 0, . . . , 0)

I allows to include failure probabilities

Hierarchical path model

Information flow

I each flow periodically generates packets at source

I passes it through to its destination

Time-triggered nature of the protocol

I iterate through slots of superframe and

I schedule possible transitionsI idle slot (if communication schedule does not allow a transmission)I successful orI unsuccessful transmission

Compositional model per path

I state reflects age of the message at each hop (age1, age2, . . . , agen)

I unique initial state (1, 0, 0, . . . , 0)

I allows to include failure probabilities

Underlying link model for transient failures

UP DOWN

1-pfl 1-prc

I successful transmission of each bit with probability 1−BERI for WirelessHART message with L bits

I results in failure proability of pfl = 1− (1−BER)LI recovery prc = 0.9 for transient failures

For links in transient state:

[ps(t), pf (t)] = p(t) = p(0)

[1− pfl pflprc 1− prc

]tFor links in steady-state:

[ps, pf ] = [π(up), π(down)] = [prc

prc + pfl,

pflprc + pfl

Underlying link model for transient failures

UP DOWN

1-pfl 1-prc

I successful transmission of each bit with probability 1−BERI for WirelessHART message with L bits

I results in failure proability of pfl = 1− (1−BER)LI recovery prc = 0.9 for transient failures

For links in transient state:

[ps(t), pf (t)] = p(t) = p(0)

[1− pfl pflprc 1− prc

]tFor links in steady-state:

[ps, pf ] = [π(up), π(down)] = [prc

prc + pfl,

pflprc + pfl

Example DTMC for 3-hop path

Reporting interval is Is = 1, uplink frame-size Fup = 7 andcommunication schedule η = (∗, 〈n1, n2〉, ∗, ∗, 〈n2, n3〉, ∗, 〈n3, G〉)

1,-,- 2,-,- 3,-,- 4,-,- 5,-,- 6,-,- 7,-,-

3,3,- 4,4,- 5,5,- 6,6,- 7,7,-

6,6,6 7,7,7

Discard1

Predictability measures

Reachability

I probability that message generated at source reaches gateway beforeend of given reporting interval

I time difference between Tborn and Trec, which equals the age of a message

I delay distribution τ can be derived from transient distribution of the DTMC

Utilization

I indicates fraction of slots that transmitted irrespective of success

I directly relates to network communication overhead and power consumption

Reachability

Utilization

Reachability

Utilization

Typcial WirelessHART network

Real plant settings according to HART Communication Foundation

I 30% of nodes communicate directly with gateway access points

I about 50% are two hops away

I remaining 20% may be 3 or 4 hops away

Link layer availability

I WirelessHART MAC layer payload length is 127 bytes

I for BER = 1 ∗ 10−4 it follows pfl = 0.0966 and π(up) = 0.9031

Typcial WirelessHART network

Real plant settings according to HART Communication Foundation

I 30% of nodes communicate directly with gateway access points

I about 50% are two hops away

I remaining 20% may be 3 or 4 hops away

Link layer availability

I WirelessHART MAC layer payload length is 127 bytes

I for BER = 1 ∗ 10−4 it follows pfl = 0.0966 and π(up) = 0.9031

Connectivity graph of a typcial WirelessHART network

Parameters

I reporting interval is 4

I superframe size for uplink is 20

I ηa = (〈n1, G〉, 〈n2, G〉, 〈n3, G〉,〈n4, n1〉, 〈n1, G〉, 〈n5, n1〉, 〈n1, G〉, 〈n6, n2〉, 〈n2, G〉, 〈n7, n3〉, 〈n3, G〉,〈n8, n3〉, 〈n3, G〉, 〈n9, n6〉, 〈n6, n2〉, 〈n2, G〉, 〈n10, n7〉, 〈n7, n3〉, 〈n3, G〉)

Robustness against transient link failures

0 1 2 3 4 5 60

Time slot

steady−state probability when pfl=0.184

transient UP probability when pfl=0.184

steady−state probability when pfl=0.05

transient UP probability when pfl=0.05

Reachability

Table: The reachability probabilities with a link failure lasting one cycle

Path number 3 7 8 10

Hop number 1 2 2 3

Reachability (%)

without link failure 99.92 99.64 99.64 99.07

with link failure 99.51 98.30 98.30 96.28

BottleneckThe longest path with the lowest availability forms the bottleneck of the system.

Reachability

Table: The reachability probabilities with a link failure lasting one cycle

Path number 3 7 8 10

Hop number 1 2 2 3

Reachability (%)

without link failure 99.92 99.64 99.64 99.07

with link failure 99.51 98.30 98.30 96.28

BottleneckThe longest path with the lowest availability forms the bottleneck of the system.

Utilization

Table: Influence of π(up) on the utilization rate of the example network

Link availability 0.693 0.774 0.83 0.903 0.948 0.989

Utilization rate 0.313 0.297 0.283 0.263 0.25 0.24

Bad linksnot only degrade the control stability but also introduce more communicationoverhead and power consumption.

Utilization

Table: Influence of π(up) on the utilization rate of the example network

Link availability 0.693 0.774 0.83 0.903 0.948 0.989

Utilization rate 0.313 0.297 0.283 0.263 0.25 0.24

Bad linksnot only degrade the control stability but also introduce more communicationoverhead and power consumption.

Overall delay distribution

0 200 400 600 800 1000 1200 14000

delay (ms)

Expected delays with different schedules

1 2 3 4 5 6 7 8 9 100

X= 10Y= 421.409

X= 7Y= 317.9528

Schedule ηa

Schedule ηb

Reachability probabilities with Is = 2 and Is = 4

Conclusions

WirelessHart

I is able to deliver reliable service under typical industrial environments

I important to choose parameters appropriate to achieve stable control

ContributionI Tool to automatically derive the DTMC

I for a specific network,I communication schedule,I routing graph, andI reporting interval

I and to compute predictability measuresI reachabilityI delay distributionI utilization

Conclusions

WirelessHart

Conclusions

WirelessHart

Conclusions

WirelessHart

wirelesshart modeling and performance evaluationpcuijper/docs/qees/mfc/dsn.pdfwirelesshart modeling...

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