Reliable and Real-time Communication in Industrial Wireless MeshNetworks

Song Han, Xiuming Zhu, Aloysius K. MokUniversity of Texas at Austin{shan, xmzhu, mok}@cs.utexas.edu

Deji Chen, Mark NixonEmerson Process Management{deji.chen, mark.nixon}@emerson.com

Abstract— Industrial wireless mesh networks are deployed inharsh and noisy environments for process measurement and controlapplications. Compared with wireless community networks, theyhave more stringent requirements on communication reliability andreal-time performance. Missing or delaying of the process data bythe network may severely degrade the overall control performance.In this paper, we abstract the primary reliability requirements intypical wireless industrial process control applications and definethree types of reliable routing graphs for different communicationpurposes. We present efficient algorithms to construct them anddescribe the recovery mechanisms. Data link layer communicationschedules between devices are further generated based on thesegraphs to achieve end-to-end real-time performance. We have builta complete WirelessHART communication system and integratedour solutions into its Network Manager. We demonstrate throughextensive experiment results that our algorithms can achieve highlyreliable routing, improved communication latency and stable real-time communication in large-scale networks at the cost of modestoverheads in device configuration.

I. Introduction

Wireless process control has been increasingly recognizedas an important technology in the field of industrial processmanagement [1], [2], [3], [4], [5], [6], [7], [8]. Several industrialorganizations such as ISA [9], HART [10] and ZigBee [11] havebeen actively pushing the application of wireless technologies inindustrial automation. However, compared with wireless commu-nity networks, the industrial control environment is harsher andnoisier and thus has more stringent requirements on reliable andreal-time communication. Missing or delaying the process datamay severely degrade the quality of control. The shifting wirelesssignal strength with time and location, the mobility of the controldevices and power limitation due to battery usage make the prob-lem even worse. Accordingly, network management techniquesadapted for industrial wireless mesh are critical.

WirelessHART [12] is the first global wireless communica-tion standard approved by IEC, and it is specifically designedfor process measurement and control applications. Unlike thedecentralized control adopted by wireless ad-hoc or peer-to-peernetworks, it advocates explicit and centralized network manage-ment. The standard pushes the complexity of ensuring reliableand real-time communication to a centralized Network Manager,but it provides little guidance on how to meet the demandingdesign goals. This paper attempts to bridge the gap and shall ex-plore efficient approaches for forming a WirelessHART network,managing reliable graph routing, allocating network resourcesand constructing data link layer communication schedules.

In a typical WirelessHART network, each device has a desig-nated sample rate to publish its process data to the Gatewaythrough multi-hop transmissions. In the other direction, theNetwork Manager sends the control data back to the devicesperiodically. To help relay different types of data, the standarddefines three types of communication graphs. The network sharesone broadcast graph for propagating common control messages

and one uplink graph for devices to publish process data. Ifneeded, each device further has a unique downlink graph fromthe Network Manager for forwarding specific control messagesto itself. Although several research works have been devotedon the design of data link layer scheduling in WirelessHARTnetworks [8], [13], [14], [15], how to satisfy the enforced strictreliability requirements on the routing graphs and construct datalink layer communication schedules on top of them is still achallenging problem and has not received sufficient attentions.

In this paper, we abstract the reliability requirements for packetrouting defined in WirelessHART standard. We present efficientalgorithms to construct these reliable graphs and describe therecovery mechanisms. These algorithms are designed to maintainthe maximum number of reliable nodes in the graphs whileachieving good network latency. To improve the scalability ofthe downlink graphs in large-scale networks, we further proposean extension on the standard to replace the single downlink graphwith a sequence of ordered local graphs. These local graphs workas reusable building blocks in constructing downlink graphs fordifferent destinations thus greatly reduce the overall overhead indevice configuration.

Based on these routing graphs, the data link layer communica-tion schedule is further constructed. Our approach allows multipledevices to compete for the retry links to the same device, andsplit the traffic from one device among all its successors, thusreduces the bandwidth allocation on each of them. By designingthe communication schedules on the successors so that theircombination has the same communication pattern as the originaldevice, the global communication schedule is then spliced intosub-schedules and distributed to the corresponding devices. Thesesub-schedules work together and guarantee that the periodicprocess/control data between devices and the Gateway can beforwarded through multi-hops in a timely manner.

We have conducted extensive experiments to evaluate theperformance of the proposed algorithms. We have also built acomplete WirelessHART communication system, and integratedour network management solutions into the Network Manager.We are deploying this system to a large-scale manufacturingfactory to achieve factory automation.

The remainder of this paper is organized as follows. Sec-tion II briefly describes the WirelessHART network architecture.Section II reviews the previous works on reliable routing andreal-time scheduling in WirelessHART networks. Section IVpresents the fundamental synchronization mechanism applied inWirelessHART networks. The details of reliable graph routingand communication schedule construction in WirelessHART aredescribed in Section V and Section VI. Section VII presentsour design and implementation of the complete WirelessHARTcommunication system. Section VIII summarizes our experimentresults. We conclude the paper and discuss the future works inSection IX.

OSI Layer

Application

Presentation

Session

Transport

Network

Data Link

Physical

HART

Command Oriented. Predefined Data Types and

Application Procedures

Auto-Segmented transfer of large data sets, reliable

stream transport, Negotiated Segment sizes

Power-Optimized Redundant Path,

Mesh to the edge Network

A Binary, Byte Oriented, Token

Passing, Master/Slave Protocol

Secure, Time Synched TDMA/

CSMA, Frequency Agile with ARQ

Simultaneous Analog & Digital

Signaling 4-20mA Copper Wiring

2.4 GHz Wireless, 802.15.4 based

radios, 10dBm Tx Power

Wired FSK/PSK & RS 485 Wireless 2.4 GHz

Fig. 1. The architecture of HART communication protocol

II. WirelessHART Architecture

Traditional wireless standards for office and manufacturingautomation such as ZigBee [11] and Bluetooth [16] are notdesigned to meet the stringent timing and security requirementsof industrial control. The WirelessHART standard is specificallytargeted to solve these problems and provide a complete solutionfor real-time process control applications.

Figure 1 illustrates the architecture of the HART protocolaccording to the OSI 7-layer communication model. As a part ofthe HART protocol, the architecture of WirelessHART protocolis shown on the right side of Fig. 1. At the bottom of itscommunication stack, WirelessHART adopts IEEE 802.15.4-2006 [17] as the physical layer. On top of that, WirelessHARTdefines its own time-synchronized data link layer. Some notablefeatures of WirelessHART data link layer include strict 10 mstimeslot, network-wide time synchronization, channel hopping,channel blacklisting, and industry-standard AES-128 ciphers andkeys. The network layer supports self-organizing and self-healingmesh networking techniques and uses source routing and graphrouting. In this way, messages can be routed around interferencesand obstacles and greatly improve the network performance innoisy and harsh environments. WirelessHART distinguishes itselffrom other public standards by maintaining a central NetworkManager. The Network Manager is responsible for maintainingup-to-date routes and communication schedules for the network,thus guaranteeing the reliable and real-time network communi-cations.

Fig. 2 shows a typical topology of a WirelessHART meshnetwork. All WirelessHART nodes support the basic mesh nodefunctionalities, including routing capability. The basic node typesof a WirelessHART network are:• Network Manager: It is responsible for configuring the

network, scheduling and managing communication amongWirelessHART devices. It is implemented in software thatresides in the Gateway or the Host.

• Gateway: It connects Host applications with field devices.It is responsible for data caching and query processing.

• Access Point: It is attached to the Gateway and providesredundant paths between the wireless network and theGateway.

• Router: It is deployed in the network to improve networkcoverage and connectivity.

• Field Device: It is attached to the process plant and collectsdata. It could be a sensor or an actuator.

• Handheld: It is a portable WirelessHART-enabled computer

Fig. 2. A typical topology of a WirelessHART mesh network

used to configure devices, run diagnostics, and performcalibrations.

• Adapter: It is a bridge device between the wireless meshnetwork and traditional wired HART devices.

III. RelatedWorks

In this section, we summarize previous works in the literatureon achieving reliable routing in wireless networks, and describerecent works on link and channel scheduling in WirelessHARTnetworks to achieve end-to-end real-time communication.

A. Reliable Routing in Wireless Networks

The reliable graph routing defined in WirelessHART standardis essentially a multipath routing approach which has beenextensively studied in wireless networks, and recognized as anefficient approach for improving the routing reliability [18], [19],[20], [21], [22], [23]. In [20], node-disjoint and braided multipathschemes are proposed to provide energy efficiency and resilienceagainst node failures. SMR [21] is a multipath version of DSR.It is designed to utilize multipath concurrently by splitting trafficonto two maximally disjoint routes. AOMDV [22] is a multipath,loop-free extension to AODV. It ensures that alternate paths atevery node are disjoint, therefore achieves path disjointness with-out using source routing. AODVM [23] is another extension toAODV for finding multiple node-disjoint paths. It also proposesan infrastructure to include deployment of reliable nodes whichcan route on multiple paths. This infrastructure can increasethe number of node-disjoint paths between the source and thedestination especially when they are far apart.

Most of these works focus on identifying multiple node oredge-disjoint paths to improve the routing reliability. However,to deal with much harsher and noisier industrial control envi-ronments, the WirelessHART standard defines more stringentrequirements on routing reliability. Each intermediate node onthe routing graph must have at least two neighbors to forwardthe traffic to the destination. For this reason, the works in theliterature cannot be directly applied in WirelessHART networks,and new routing algorithms have to be designed.

Source

Destination

TsCCAOffset TsCCA

TsRxTx

TsTxOffset TsError

TsMaxPacket TsRxAckDelay

TsAckWait

TsRxOffset TsRxOffset TsTxAckDelay TsAck

Fig. 3. Timing of a WirelessHART timeslot

B. Real-time Scheduling in WirelessHART Networks

Since the standard was ratified in 2007, several researchworks have been devoted to the link scheduling and channelassignment problems in WirelessHART networks to achieve end-to-end real-time communication [13], [14], [15]. In [14], [15],the convergecast scheduling problem is studied in linear networktopologies. They formulate the problem as a mixed integerlinear programming problem, and design algorithms based ondifferent assumptions on devices’ buffering capability. [13] con-siders a more general WirelessHART network model includingarbitrary network topology and multi-path routing. It formulatesthe sensor-to-actuator real-time flow scheduling problem andproves that it is NP-hard. Based on a necessary condition forschedulability in WirelessHART networks, it proposes an optimalscheduling algorithm based on a branch-and-bound technique. Apractical scheduling policy called Conflict-aware Least LaxityFirst algorithm is also proposed to achieve better scalability andhandle network dynamics.

However, all these aforementioned works assume that thenetwork layer routes have already been provided and focus ondata link layer scheduling. The relationship between the routesand the data link layer schedules are not thoroughly studied.In our work, we present a general framework for networkmanagement in WirelessHART networks. We shall study howto achieve reliable graph routing for different communicationparadigms in WirelessHART network and further construct a datalink layer communication schedule based on them. Our solutioncan be easily integrated into the Network Manager, so that thesetup of an operational WirelessHART network is simple andprompt.

IV. Time Synchronization inWirelessHART Networks

WirelessHART is a TDMA-based network protocol and everycommunication in it is time-synchronized. The basic time unitof communication activity is a fixed-length timeslot that iscommonly shared by all network devices. The timeslot providesthe time base for scheduling process data transmission. Theduration of a timeslot defined in WirelessHART is 10 ms which issufficient for sending or receiving one packet per channel and theaccompanying acknowledgement, including guard-band times fornetwork-wide synchronization. The specific timing requirementinside a WirelessHART timeslot is depicted in Figure 3. Precisetime synchronization is critical to the operation of networksbased on time division multiplexing. Since all communicationhappens in timeslots, the network devices must have the samenotion of when each timeslot begins and ends, with minimalvariation. Several mechanisms are applied in WirelessHARTfor time synchronization. In a WirelessHART network, timepropagates outwards from the Gateway.

When a new device joins a WirelessHART network initially,it has no idea what the current time is. For each incoming MAClayer packet, the device records Ta, the time when the packet’sfirst bit arrives. Because of the strict timeslot structure, the devicecan derive the start of the next timeslot, T , from the packet’sarrival time according to the following formula where TsTxOffsetis the offset in the slot to start the preamble transmission.

T = Ta + 10ms − TsTxOffset

Synchronization happens not only in the device join process,but also during a node’s normal operation. A receiving nodealways compares the start time of the incoming MAC layer packetand the expected arrival time measured on its own clock. Thedifference is the drift between their clocks. The receiver includesthe difference in the time adjustment field of the correspondingACK packet. Each node is designated a time source node.Whenever a node receives an ACK from its time source, itwill adjust its clock based on the time adjustment field. If thesender is the time source of the receiver, the receiver adjustsits clock directly from the time difference value. Together, theseadjustments provide the network-wide time synchronization inWirelessHART mesh networks.

V. Reliable Graph Routing

In this section, we present the details how we define andachieve the reliable routing in a typical wireless industrial meshnetwork like WirelessHART. We first describe the primary rout-ing approaches adopted in WirelessHART in Section V-A. Sec-tion V-C abstracts the reliability requirements on packet routing,defines three types of reliable graphs for different communicationpurposes, and describes their properties. We discuss the difficul-ties in achieving completely reliable routing in Section V-D. Thealgorithmic details to construct these routing graphs are presentedin Section V-E, Section V-F, Section V-G and Section V-H. Wedescribe the recovery mechanisms in Section V-I.

A. Source Routing and Graph Routing

Two primary routing approaches are defined in the Wire-lessHART standard: graph routing and source routing. Whenusing graph routing, a network device sends packets with a graphid in the network layer header along a path to the destination.All devices on the way to the destination must be pre-configuredwith graph information that specifies the neighbors to which thepackets may be forwarded.

With source routing, to send a packet to its destination, thesource includes in the network layer header an ordered list ofdevices through which the packet must travel. As the packetis routed, each routing device utilizes the next network deviceaddress from the packet header to determine the next hop to use.Since packets may go to a destination without explicit setup ofintermediate devices, source routing requires knowledge of thecomplete network topology.

Since the source routing approach only establishes a fixedsingle path between the source and destination, any link or nodefailure will cut off their communication. For this reason, sourcerouting is mostly used for diagnostics purposes in industrialwireless networks. In this paper, we will focus on the graphrouting approach and investigate how to achieve reliable routingin the network. Based on different communication purposes, thereare three types of routing graphs defined in a WirelessHARTnetwork, and Figure 4 illustrates an example.

G

A 1 A 2

2 1 3

4 5

G

A 1 A 2

2 1 3

4 5

G

A 1 A 2

2 1 3

4 5

G

A 1 A 2

2 1 3

4 5

( a ) Original network topology ( b ) Uplink graph

( c ) Broadcast graph ( d ) Downlink graph to Dev 3 and Dev 4

G Gateway A Access Point i Device with specific ID i

Fig. 4. Three types of routing graphs

Uplink graph: It is a graph connecting all devices upwardto the Gateway. It is used to propagate devices’ process dataperiodically to the Gateway. Different devices may have differentsample rates.Broadcast graph: It is a graph connecting the Gateway down-ward to all devices. It is used to broadcast common configurationand control messages to the entire network.Downlink graph: It is one per device. It is the graph from theGateway to each individual device. The unicast messages fromthe Gateway and the Network Manager to each device traversethrough this graph.

Based on these graphs, the Network Manager can furthergenerate the corresponding sub-routes for each device. Only afterthe routes are constructed and downloaded to every device, canthe network communication schedule be generated, which weshall elaborate in Section VI. When devices initially join into thenetwork, they carry with them a list of neighbor entries includingthe received signal strength information. The Network Manageruses this information and the periodic health reports from thedevices to construct and maintain the global network topology.

B. Notations

This section summarizes the notations to be used throughoutthe paper. Given the original network topology G(V, E), we useg to denote the Gateway, VAP to denote the set of Access Pointsand VD to denote the set of devices. We have {g}∪VAP∪VD = V .For each node i ∈ VD∪VAP, we use e+i and e−i to denote its set ofoutgoing edges and incoming edges. We use δ+i and δ−i to denoteits outgoing and incoming degree. GB(VB, EB) and GU(VU , EU)are used to represent G’s reliable broadcast graph and uplinkgraph. The reliable downlink graph for node v ∈ V is denoted byGv(Vv, Ev).

C. Reliability Requirements and Reliable Graphs

Compared with wireless community networks, industrial wire-less mesh networks have a much higher demand on the routingreliability to tolerate node and link failures. In this section, weabstract the reliability requirements defined in WirelessHARTstandard using the concept of (k,m)-reliability in packet routing.Notice that here we assume that the Gateway and Access Pointsare all connected through wire and reliable, so in the following

of the paper, the reliability requirements only apply to wirelessdevices.

Definition V.1: Given a directed graph G(V, E), a node v ∈ Vsatisfies the (k,m)-reliability if and only if δ−v ≥ k and δ+v ≥ m.There is no constraint on δ−v or δ+v if k = 0 or m = 0.

Based on this definition, we now give the definitions of theaforementioned three reliable routing graphs and present theirimportant properties.

Definition V.2: Given a directed graph G(V, E), a directed acyclicgraph GB(VB, EB) (VB = V and EB ⊆ E), is a reliable broadcastgraph if the (2, 0)-reliability holds on every node in V−{g}−VAP.

GB requires that each device has at least two parents fromwhich it can receive the broadcast messages. This significantlyincreases the chance for the broadcast messages to be propagatedto the entire network. GB has the following property.

Property V.1: Each device in GB has at least two paths from g.Proof: According to the definition of GB, ∀v, v ∈ V−{g}−VAP, ithas two different parent nodes. There are two cases on v’s parentnode u. In the first case, u is an Access Point. It is obvious thatthere exists one path g → u → v. In the second case, u is adevice. We perform the same analysis on u and find its parents.As GB is acyclic, this process can be repeated and terminateswhen it reaches an Access Point. Thus there exists a path g →· · · → u→ v. Because v has two different parent nodes, there areat least two paths from g to v in GB. �

Different from the broadcast graph, the uplink graph is usedby the devices to forward their process data to the Gateway witha required sample rate. It is considered reliable if and only if foreach device in the network except the Access Points, it has twochildren to forward its packet to the Gateway. In cases wherethe communication between the device and one of its childrenis broken, the process data can still be delivered to the Gatewaythrough the alternative child.

Definition V.3: Given a directed graph G(V, E), a directed acyclicgraph GU(VU , EU) (VU = V and EU ⊆ E), is a reliable uplinkgraph if the (0, 2)-reliability holds on every node in V−{g}−VAP.

Property V.2: Each device in GU has at least two paths to g.Proof: The proof is similar to the proof for Property V.1.�

Property V.3: GB and GU both have no less than 2 Access Points.Proof: Assume that there is only one Access Point p in GB. andv is a node with an incoming edge from p in GB. As p is theonly Access Point, node u, the other parent node of v is a device.We repeat this analysis on u and it is obvious that at least one ofu’s parents is still a device. This process will be repeated untila cycle is formed because the number of devices in the networkis finite. This is a contradiction with the definition of GB. So GB

has no less than 2 Access Points. The proof for GU is similar. �

The broadcast graph and uplink graph are global graphs sharedby the entire network. However, to support the transmissionof configuration and control messages to a specific device v,a unique downlink graph Gv(Vv, Ev) from the Gateway andNetwork Manager to v is also required. Gv is defined to bereliable only if (0, 2)-reliability holds on each intermediate node.

Property V.4: Gv(Vv, Ev) contains at least one directed cycle.Proof: Assume there is no cycle in Gv. Consider the node uwhich has a direct edge to v in Gv. According to the definition

of Gv, intermediate node u has another non-v child w. Repeatthis analysis on w, and w also has a non-v child. This processcan be repeated and finally form a cycle. �

Property V.4 states the existence of directed cycles in Gv. Toguarantee the prompt delivery of the downlink messages, wemust avoid arbitrary cycles in Gv which will generate infiniteloops in packet forwarding. Thus in its definition, we restrictthat there is only one cycle of length 2 in Gv and require thatevery node on the cycle must be the destination’s parent. Oncethe packet reaches such nodes, it will be directly forwarded tothe destination which is required by the standard. This will avoidany cyclic transmission and unnecessary delay.

Definition V.4: Given a directed graph G(V, E), a directed graphGv(Vv, Ev) (Vv ⊆ V and Ev ⊆ E), is a reliable downlink graphfrom g to v if 1) v is the only sink and g is the only source inGv; 2) (0, 2)-reliability holds on each intermediate node in Gv;and 3) there is only one cycle of length 2 in Gv, and each nodeon the cycle has a direct edge to v.

D. Difficulties in Achieving Completely Reliable Graphs

The major barrier to construct reliable routing graphs is theunderlying network connectivity. Better network connectivity willobviously lead to a higher chance for constructing completelyreliable graphs. In this section, we evaluate the relationship be-tween the network connectivity and the success ratio to constructthese reliable graphs. In our experiments, we vary the networkconnectivity by changing the edge success probability p andFigure. 5 summarizes our results. We observe that with 150devices in the experiments, the success ratio drops quickly alongwith the decrease of p. When p is 0.8, the success ratio is around80% for downlink graphs and above 95% for both GB and GU .However, when p drops to 0.5, we can barely construct reliabledownlink graphs and the success ratios for GB and GU are onlyaround 40%.

Under the same experiment settings, Figure. 6 shows thepercentage of reliable nodes in the incomplete reliable graphs. Weobserve that the percentage of reliable nodes in incomplete GU

and GB are always above 95% and this percentage for downlinkgraphs is also larger than 75% even when the edge successprobability drops to 0.5. Figure. 7 further evaluates the impactof the network density on the success ratio. We vary the size ofthe network from 75 to 150 and fix the edge success probabilityat 0.8. As expected, The results show that the network densityhas a great impact on network connectivity, and lower networkdensity will lead to poor success ratio.

Based on these results, we conclude that the success ratiofor constructing reliable routing graphs is closely related tothe underlying network connectivity. In many scenarios, it isimpossible to achieve the completely reliable graphs. For thisreason, we shall allow violations of the reliability requirementsin the routing graphs and instead focus on designing algorithmsto construct graphs with the maximum number of reliable nodes.In the following of the paper, we will still use GB, GU and Uv torepresent the broadcast graph, uplink graph and downlink graphfor node v even though they may not be completely reliable.

E. Constructing Reliable Broadcast Graph

In a broadcast graph, we say that a node i is reliable if andonly if δ−i ≥ 2. Let S B = {i | δ−i ≥ 2, i ∈ V}, and we want to max-imize |S B| when we construct the reliable broadcast graph GB.Furthermore, to reduce the energy consumption in propagating

Alg 1 Constructing Reliable Broadcast Graph GB(VB, EB)1: // G(V, E) is the original graph2: Initially VB = g ∪ VAP and EB contains all links from g to VAP.3:4: while VB , V do5: Find S ′ ⊆ V − VB: ∀v ∈ S ′, v has at least two edges from VB6: if S ′ , ∅ then7: for all node v ∈ S ′ do8: Sort its edges eu,v from VB according to hu9: Choose the first two edges eu1 ,v and eu2 ,v

10: hv =hu1+hu2

2 + 111: end for12: Choose the node v with min hv13: Add v to VB and add eu1 ,v and eu2 ,v to EB14: else15: Find S ′′ ⊆ V − VB: ∀v ∈ S ′′, v has one edge eu,v from VB16: if S ′′ , ∅ then17: for all node v ∈ S ′′ do18: hv = hu + 119: Calculate nv, the # of its outgoing edges to V − VB20: end for21: Choose the node v with maximum nv, break tie using hv22: end if23: else24: return FAIL;25: end if26: end while27: return SUCCESS;

broadcast messages to the entire network and improve networklatency, we also hope to minimize the average number of hopsfrom the Gateway to each node. For node i, we denote its averagenumber of hops from the Gateway by hi and use Pi to representits parents in GB. We have:

hi =

∑k∈Pi

hk

|Pi|+ 1 (1)

We present a greedy algorithm (Alg. 1) to achieve these twogoals in constructing GB. In our approach, we maintain a set VB

to record the explored nodes and VB is initialized as {g} ∪ VAP.The explored edges are maintained in EB which is initialized toinclude the edges from g to each Access Point. In the algorithm,we incrementally select one node v from V − VB. In each loop,we first find S ′, the set of reliable nodes in V −VB (Line 5). Foreach node v in S ′, we sort its incoming edges from VB accordingto their averaged number of hops from the Gateway in ascendingorder. We choose the first two edges and calculate hv accordingto Eq. 1. We choose the node in S ′ with the minimum hv andadd it to VB. If there is no reliable node available in V − VB, wewill instead find S ′′, the set of nodes with exact one edge fromVB (Line 15). We choose the node in S ′′ with the maximumnumber of outgoing edges to V − VB to maximize the chanceto find reliable nodes in the next round. This process continuesuntil all nodes in V are explored. Otherwise an error will bereported (Line 24). This will trigger the Network Manager toexecute appropriate recovery actions. The worst-case complexityof the algorithm is

|V |∑k=|VAP |

(|E| + (|V | − k) · lg(|V | − k)) = O(|V |3)

F. Constructing Reliable Uplink Graph

The construction of a reliable uplink graph GU(VU , EU) issimilar to that of GB(VB, EB). Essentially, we only need toreverse all edges in the original graph G(V, E), construct GB

0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0Su

cces

s R

atio

Edge Success Probability

Uplink Graph Broadcast Graph Downlink Graph

Fig. 5. Success ratio vs. Edge success probability

0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.000.75

0.80

0.85

0.90

0.95

1.00

Perc

enta

ge o

f Rel

iabl

e N

odes

Edge Success Probability

Uplink Graph Broadcast Graph Downlink Graph

Fig. 6. Percentage of reliable nodes

0.0

0.2

0.4

0.6

0.8

1.0Downlink GraphBroadcast Graph

Succ

ess

Rat

io

75 nodes 100 nodes 125 nodes 150 nodes

Uplink Graph

Fig. 7. Success ratio vs. Network density

and then reverse all its edges back. We define GR(V, ER) tobe the reversed graph of G(V, E), and the greedy algorithm toconstruct GU(VU , EU) is summarized in Alg. 2 and its worst-case complexity is O(|V |3).

Alg 2 Constructing Reliable Uplink Graph GU(VU , EU)1: // G(V, E) is the original graph, GR(V, ER) is the reversed graph2: Construct GR(V, ER)3: Construct GB(VB, EB) from GR(V, ER) by applying Alg. 14:5: if VB = V then6: // Construct GU by reversing all edges in GB7: GU (VU , EU ) = GR

B(VB, ERB)

8: else9: // the network topology is disconnected

10: return FAIL;11: end if12: return SUCCESS;

G. Constructing Reliable Downlink Graph

The construction of the reliable downlink graph Gv(Vv, Ev)for a given node v in G(V, E) only involves part of the nodesin G(V, E) and it is more complicated because of the existenceof cycles as shown in Property V.4. Furthermore, according toDefinition V.4, we want to have exactly one cycle in Gv oflength 2 and restrict it to be between the two parents of v. Ouroptimization goals in constructing Gv are similar to that of GB andGU . We hope to maximize the number of nodes in the network tohave reliable downlink graphs and for each downlink graph, wewant to minimize its average number of hops from the Gateway.

Alg. 3 summarizes the framework of our approach. In thealgorithm, we construct the reliable downlink graph for eachnode in the network. For the Access Point, its downlink graphconsists of the Gateway g, itself and the edge from g to itself.We maintain S , a set of nodes whose reliable downlink graphshave already been constructed (Line 1). We incrementally find aneligible node v in V − S to construct Gv where three constraintsin Table I are applied and v has the minimum hv as calculatedin Line 17. Constraint C1 and C2 are to satisfy the reliabilityrequirements in Gv and Constraint C3 is to make sure that wecan remove the internal cycles in the constructed Gv. If such aneligible node cannot be found, we will instead choose the nodethat has two parents from S with the minimum average latencyto the Gateway (Line 20). If every node in V − S only has oneparent from S , we choose the one with the minimum averagelatency (Line 27 - 37).

C1: v has at least two parents u1 and u2 in SC2: u1 and u2 form a directed cycleC3: u2 (u1) has at least one parent from the cycle in Gu1 (Gu2 )

TABLE IThree constraints in constructing reliable downlink graphs

Alg. 4 describes how we construct Gv based on its parents(u1 and u2)’ reliable downlink graphs Gu1 and Gu2 . We firstmerge Gu1 ,Gu2 , v and edges among u1, u2 and v together (Line4). We maintain S , the set of explored nodes in Gv and initializeit as {g, v, u1, u2}. We construct Gv in a bottom-up manner byincrementally selecting a node i ∈ Vv−S which has two outgoingedges to S in G and has the minimum hi (Line 6-30). This processcontinues until either all nodes in Vv are explored or VAP has twooutgoing edges to S (Line 7 - 10). Finally, we remove all nodesin Vv − S and their corresponding edges from Gv (Line 32 - 34).If there is no node available to have two outgoing edges to S inG, we choose the node with the minimum hi (Line 20 - 29).

H. Constructing Scalable Reliable Downlink Graph

The algorithms proposed in Section V-G strictly comply tothe WirelessHART standard and construct one downlink graphfor each individual node. However, this approach is not scalable.When a device is multi-hop away from the Gateway, its downlinkgraph has to traverse multiple intermediate devices but cannotreuse their downlink graph information. This will introduceunnecessarily high configuration overhead in the network. Toachieve reliable downlink graph routing in large-scale wirelessnetworks, in this section we propose to extend the currentdownlink route from a single graph to a sequence of ordered localgraphs, and we call this approach Sequential Reliable DownlinkRouting (SRDR). Instead of constructing a completely new graphfrom Gateway to device v, SRDR lets each node only keepa small local graph to maintain the reliable routing from itsparents. The reliable downlink graph to a given node can beconstructed by assembling the intermediate nodes’ local graphstogether based on a given order. These local graphs can be takenas building blocks in constructing downlink graphs for differentdestinations, thus existing device configurations can be reused.This will significantly reduce the overall configuration overheadand improve the downlink routing scalability.

Extension: To support sequential reliable downlink routing, weneed two extensions in the current WirelessHART standard. First,as depicted in Figure 8, we use the reserved bits (Bits 4-3) of thecontrol byte in the network layer header to indicate, when set,the presence of the sequential downlink routing fields, and we

Alg 3 Constructing Reliable Downlink Graphs in G(V, E)1: Let S be the set of nodes with downlink graphs constructed2: Initially S = g ∪ VAP and Gg = ({g},∅)3: Initially for each AP i in S , set Gi = ({g ∪ i}, {eg,i})4:5: while S , V do6: Find S ′ ⊆ V − S : ∀v ∈ S ′, v has at least two edges from S7: // S r is the reliable node set in S ′, initially S r = ∅8: if S ′ , ∅ then9: for all node v ∈ S ′ do

10: for all edge pair (eu1 ,v, eu2 ,v) from S do11: if C 1 ∧ C 2 ∧ C 3 then12: S r = S r ∪ {v}13: end if14: hu1 ,u2 = (hu1 + hu2 )/215: end for16: Choose the edge pair (eu1 ,v, eu2 ,v) with min hu1 ,u217: hv = hu1 ,u2 + 118: end for19: if S r , ∅ then20: Add node v in S r with min hv to S21: else22: Add node v in S ′ with min hv to S23: end if24: // construct Gv: hu1 ,u2 is the min among all edge pairs to v25: ConstructDG(G, Gu1 , Gu2 , v);26: else27: Find S ′′ ⊆ V − S : ∀v ∈ S ′′, v has one edge eu,v from S28: if S ′′ , ∅ then29: for all node v ∈ S ′′ do30: hv = hu + 131: Calculate nv, the # of v’s outgoing edges to V − S32: end for33: Add v to S with maximum nv, break tie using hv34: ConstructDG(G, Gu1 , null, v);35: else36: return FAIL;37: end if38: end if39: end while40: return SUCCESS;

7 6 5 4 3 2 1 0

Control Byte

7 : Destination Address 6 : Source Address

5 : reserved

1 : 1 st source route

4 : 1 st sequential graph route

2 : Proxy route 0 : 2 nd source route

3 : 2 nd sequential graph route

Control TTL ASN

Snippet Graph

ID Dest Addr

Source Addr

Proxy Route

Sequential Graph Route or Source Route

Network Layer Header

Payload

Expended Routing Information

Fig. 8. The extension of the network layer header in WirelessHART to supportsequential reliable downlink routing

use the source routing option field to store the ordered graph list;Second, the routing module is enhanced to support SRDR. Whenthe packet arrives at the intermediate node, the routing modulewill retrieve the earliest graph ID in the graph list and verify ifthe current node is the sink of this specific graph. If it is, weremove this graph ID from the graph list and route this packeton the next earliest graph. This process continues until we reachthe final destination or the routing fails. In the latter case, wewill remove this graph ID and try the next earliest graph ID ifit has the corresponding edges. Otherwise, alarm messages willbe sent to the Network Manager and appropriate actions shall betaken.

Alg. 5 summarizes the framework of SRDR. In the algorithm,given the original graph G, we construct the reliable downlink

Alg 4 ConstructDG (G(V, E), Gu1 (Vu1 , Eu1 ), Gu2 (Vu2 , Eu2 ), v)1: Let S contain explored nodes in Gv(Vv, Ev): S = {g, v, u1, u2}2:3: // Construct Gv: Merging Gu1 , Gu2 , v, and edges among v, u1, u24: Gv(Vv, Ev) = Gv(Vu1 ∪ Vu2 ∪ {v}, Eu1 ∪ Eu2 ∪ {eu1 ,v, eu2 ,v, eu1 ,u2 , eu2 ,u1 })5:6: while S , Vv do7: if VAP has two outgoing edges to S in G then8: S = S ∪ VAP9: break;

10: end if11: for all node i ∈ Vv − S do12: Sort i’s outgoing edges to S in descending order of hi13: end for14:15: Find S ′ ⊆ Vv − S : ∀v ∈ S ′, v has at least two edges to S16: if S ′ , ∅ then17: Add node i with min hi to S18: Add first two edges from i to S to Gv if they don’t exist19: Remove all other edges from Ev20: else21: Find S ′′ ⊆ Vv − S : ∀v ∈ S ′′, v has one edge to S22: if S ′′ , ∅ then23: Add i with min hi to S24: Add the edge from i to S to Gv if it doesn’t exist25: else26: return FAIL;27: end if28: end if29: end while30:31: for all node i ∈ Vv − S do32: Remove i from Vv and corresponding edges from Ev33: end for34: return SUCCESS;

G

A 1 A 2

2 1 3

4 5

( a ) Original network topology ( b ) Downlink graph : g 2 Sequential route for Dev 2 : g 2

G

A 1 A 2

2 1 3

4 5

( c ) Downlink graph : g 3 Sequential route for Dev 3 : g 3

G

A 1 A 2

2 1 3

4 5

( d ) Downlink graph : g 1 Sequential route for Dev 1 : g 2 , g 1

G

A 1 A 2

2 1 3

4 5

( e ) Downlink graph : g 4 Sequential route for Dev 4 : g 2 , g 1 , g 4

G

A 1 A 2

2 1 3

4 5

( f ) Downlink graph : g 5 Sequential route for Dev 5 : g 2 , g 5

G

A 1 A 2

2 1 3

4 5

( b )

Fig. 9. Examples of the sequential reliable downlink routes

( b ) Downlink graph : g 5 Sequential route for Dev 5 : g 2 , g 5

G

A 1 A 2

2 1 3

4 5

( c ) Standard downlink graph for Dev 5

G

A 1 A 2

2 1 3

4 5

( a ) Downlink graph for Dev 2

G

A 1 A 2

2 1 3

4 5

Fig. 10. Standard approach vs. Sequential reliable downlink routing (SRDR)

C1: v has at least two parents u1, u2, and they form a cycle.C2: u1 is u2’s parent in u2’s local downlink graph.C3: u2 (u1) has at least one parent from the cycle in Gu1 (Gu2 )

TABLE IIThree constraints in constructing scalable reliable downlink graphs

route (an ordered graph list) for each node in the network. For theAccess Point, its downlink route contains only one local graphwhich consists of the Gateway g, itself and the edge betweenthem. We maintain S , a set of nodes whose downlink routeshave already been constructed (Line 1). We incrementally find aneligible node v in V −S to construct its downlink route Rv wherethree constraints in Table II are applied and v has the minimumhv as calculated in Lines 14-26. Constraint C1 is to find v’s localdownlink graph gv = ({u1 ∪ u2 ∪ v}, {eu1,u2 , eu2,u1 , eu1,v, eu2,v}); Ifconstraint C2 is satisfied, v’s downlink route Rv can be simplyderived as Rv = Ru2 → gv; Constraint C3 presents another wayto construct the reliable downlink route for v if u1 and u2 areindependent. If an extra edge e can be found from the cycle inGu1 to u2 or from the cycle in Gu2 to u1, it will be added intogv, and Rv can be derived as Ru1 → gv or Ru2 → gv. If such aneligible node cannot be found, we will instead choose the nodethat has two parents from S with the minimum average latencyto the Gateway (Line 18). If every node in V − S has only oneparent from S , the one with minimum average latency will bechosen (Line 28 - 40). Alg. 6 gives the details how we constructRv.Example V.1: Figure 9 illustrates an example for constructing thereliable downlink routes for devices in a WirelessHART network.Figure 9(a) gives the original topology of the network. We firstinclude node 2 and node 3 into the explored node set S . Thedotted lines in Figure 9(b) and Figure 9(c) show their localdownlink graphs. When adding node 1 into S , as A1 and node 2are already in S and they satisfy the constraints C1 ∧ C2, R1 isderived as g2 → g1. We have the similar operations when addingnode 4 into S and R4 = g2 → g1 → g4. However, when we addnode 5 into S , node 2 and node 3 are independent. As we havea link between A1 and node 3, constraints C1 ∧ C3 are satisfied.The dotted links in Figure 9(f) shows g5, and the downlink routeof node 5, R5 is g2 → g5.

The next example compares the standard approach in Wire-lessHART with sequential reliable downlink routing (SRDR).Example V.2: Figure 10 compares SRDR with the standardapproach in WirelessHART. The downlink graphs for node 2under both approaches are the same (Figure 10(a)). The downlinkroute for node 5 in our approach is R5 = g2 → g5, and g5 isshown in Figure 10(b). In SRDR, the downlink routing fromthe Gateway to node 5 can leverage the local routing graph inintermediate node (node 2) while only a local graph in node5 is needed. However, the standard approach has to constructa completely new graph from the Gateway to node 5 which isshown in Figure 10(c). Comparing Figure 10(b) and Figure 10(c),the standard approach requires 3 extra links to achieve the reliabledownlink routing. This overhead will increase dramatically whenthe destination is far away from the Gateway.

Optimization: In the basic SRDR, the routing is performedstrictly according to the sequence in the ordered graph list.However, as each node can keep graph information to multipledestinations, we can take advantage of the “shortcut” to furtherimprove the network latency. We call this approach SRDR-OPT.When a packet arrives at an intermediate node i, instead of using

Alg 5 Constructing Sequential Reliable Downlink Routes1: Let S be the set of explored nodes with downlink route constructed2: Initially S = g ∪ VAP3: Initially for each AP i in S , set Gi = ({g ∪ i}, {eg,i}) and Ri = Gi4:5: while S , V do6: Find S ′ ⊆ V − S : ∀v ∈ S ′, v has at least two edges from S7: // S r is the reliable node set in S ′, initially S r = ∅8: if S ′ , ∅ then9: for all node v ∈ S ′ do

10: for all edge pair (eu1 ,v, eu2 ,v) from S do11: hu1 ,u2 = (hu1 + hu2 )/212: end for13: Find Pv, set of edge pairs of v satisfying C1 ∧ (C2 ∪ C3)14: if Pv , ∅ then15: S r = S r ∪ {v}16: Choose (eu1 ,v, eu2 ,v) from Pv with min hu1 ,u217: else18: Choose (eu1 ,v, eu2 ,v) from S with min hu1 ,u219: end if20: hv = hu1 ,u2 + 121: end for22: if S r , ∅ then23: Add v in S r with min hv to S24: else25: Add v in S ′ with min hv to S26: end if27: ConstructDG(G, u1, u2, v);28: else29: Find S ′′ ⊆ V − S and ∀v ∈ S ′′, v has one edge eu,v from S30: if S ′′ , ∅ then31: for all node v ∈ S ′′ do32: hv = hu + 133: end for34: Add v to S with min hv35: Gv = ({u ∪ v}, {eu,v})36: Rv = Ru → Gv37: else38: return FAIL;39: end if40: end if41: end while42: return SUCCESS;

the earliest graph ID, SRDR-OPT searches the ordered graph listbackward and finds the first graph ID that is stored in its routingtable. The packet then will take the “shortcut” and be forwardedon this graph. If this forwarding is successful, at the destinationof this selected graph, all the preceding graph IDs in the orderedgraph list including the current ID will be removed. Otherwise,node i will choose the next available graph ID backward in theordered graph list and repeat this process. The following exampleshows the advantage of SRDR-OPT.Example V.3: In Figure 11, we are routing packets from node sto node 4 and R4 is g2 → g3 → g4. In node 2, it contains therouting information for both graph g3 and g4. It contains edges2 → 3 and 2 → 1 on g3 and edges 2 → 4 and 2 → 3 ong4. When a packet arrives at node 2 with an ordered graph listg3 → g4 in the network layer header (g2 is removed at node 2),node 2 will take the “shortcut” and try to forward the packet ongraph g4 to node 4. Only if both edges on graph g4 are broken,node 2 will forward the packet on graph g3 and try the edge2 → 1 instead. Under this worse-case scenario, the packet willforwarded to node 4 through s→ 2→ 1→ 3→ 4.

I. Maintaining Reliable Routing Graphs with Network Dynamics

The algorithms presented in the previous subsections constructthe reliable routing graphs in ideal scenarios where network

Alg 6 ConstructDG (G, u1, u2, v)1: Let Eδ be the set of edges among u1, u2 and v2: if u1, u2 satisfy C1 ∧ C2 then3: Gv = G({u1, u2, v}, Eδ)4: if u1 is u2’s parent in Gu2 then5: Rv = Ru2 → Gv6: else7: Rv = Ru1 → Gv8: end if9: else if u1, u2 satisfy C1 ∧ C3 then

10: if u1 has an edge e from u2’s parents in Gu2 then11: Gv = G({u1, u2, v}, Eδ ∪ e)12: Rv = Ru2 → Gv13: end if14: if u2 has an edge e from u1’s parents in Gu1 ∧ (hu2 < hu1 ) then15: Gv = G({u1, u2, v}, Eδ ∪ e)16: Rv = Ru1 → Gv17: end if18: else19: if eu1 ,u2 and eu2 ,u1 both exist then20: Gv = G({u1, u2, v}, Eδ)21: Rv = (hu1 < hu2 ) ? Ru1 → Gv : Ru2 → Gv22: else if there is neither eu1 ,u2 nor eu2 ,u1 then23: Gv = (hu1 < hu2 ) ? G({u1, v}, {eu1 ,v}) : G({u2, v}, {eu2 ,v})24: Rv = (hu1 < hu2 ) ? Ru1 → Gv : Ru2 → Gv25: else if eu1 ,u2 exists then26: Gv = G({u1, u2, v}, Eδ)27: Rv = Ru1 → Gv28: else29: Gv = G({u1, u2, v}, Eδ)30: Rv = Ru2 → Gv31: end if32: end if

S 2 4

1 3

g 2 g 3 g 4

S 2

1

g 2

2 4

3

g 4

2

1 3

g 3

Fig. 11. An example of the SRDR optimization

devices work properly after joining the network. Although theindustrial wireless mesh is usually quite stable after deployment,network devices may experience various failures and need tobe reset. Wireless links can also be blocked by interferenceand become temporarily or permanently unavailable. All thesescenarios require the Network Manager to recover the routinggraphs to maintain the reliability requirements. Furthermore,corresponding adjustments on the communication schedules arealso necessary along with these routing graph modifications.

In WirelessHART networks, network abnormalities and statis-tics are reported to the Network Manager through a set of networkmaintenance commands. These commands are summarized inTable III. Command 779 summaries the communication statisticsof a specific device; Command 780 and 787 report the signalstrengths of a device’s neighbors; Command 788, 789 and 790 aretriggered once a path failure or routing failure is detected in thenetwork. These commands are carried in normal messages andpublished to the Network Manager. Based on this information,the Network Manager will update the network topology, adjustthe routing graphs and communication schedules if necessary toreach a good balance between the reliability and recovery cost.

Our current heuristics to recover GB consists of two steps.We first find G′B(V ′B, E

′B) , the sub graph of GB where all nodes

in V ′B are reliable after the topology changes. In the secondstep, we replace GB with G′B and repeat Alg. 1 to incrementallyadd nodes to GB. This process repeats until either all the nodesare included in GB or disconnected nodes are identified. Themechanism to reconstruct GU is similar to that of GB. Designingefficient algorithms to reconstruct Gv to each node v is morechallenging and will be addressed in our future works.

VI. Communication Schedule and ChannelManagementTypical wireless industrial process control applications take the

approach that devices specify their requirements in communica-tion bandwidth and the Network Manager allocates necessaryresources such as timeslots, to maintain the periodic sensing-control loop between the Network Manger and devices. In thesensing phase, the devices publish their process data to theGateway through the uplink graph based on their specific samplerates; In the control phase, the Network Manager generatescontrol messages and sends them back to each individual de-vice on its downlink graph. The Network Manager maintainsa global communication schedule for transmitting these processand control data and distributes the sub-schedule to each effecteddevice.

The construction of the communication schedule is subject toseveral practical constraints in WirelessHART networks:• The maximum number of concurrent active channels is 16.• Each device can only be scheduled to TX/RX once in a slot.• Multiple devices can compete to transmit to the same device

simultaneously (in shared timeslot).• On a multi-hop path, early hops must be scheduled first.• The practical sample rates are defined as 2n sec (−2 ≤ n ≤ 9)

from 250 ms (2−2 sec) to 8 min and 32 sec (29 sec).Our design philosophy for constructing the communication

schedule is to spread out the channel usage in the network asmuch as possible and to apply the Fastest Sample Rate Firstpolicy (FSRF) to schedule the devices’ periodic publishing andcontrol data.

We use the concept of superframe to group a sequence ofconsecutive timeslots and represent the communication patternfor a given sample rate. We define two types of superframes: datasuperframe and management superframe. The data superframe isused to support data transmissions between the devices and theGateway while the management superframe is used to supportexchanging network management messages. The number ofdata superframes is decided by the number of different samplerates existing in the network. Notice that there can be multipledevices having the same sample rate, thus a data superframe willrepresent the periodic behavior of multiple devices.

We maintain a global matrix M to keep track of the currentslot/channel usage in the network. Each entry in the matrix,Mi, j

represents the slot usage at timeslot i on channel j, and it has fourtypes: unused, exclusive, shared and reserved. An unused entrycan be allocated to any pair of devices if there is no communica-tion conflict; An exclusive entry is one occupied by two devicesfor dedicated communication; Reserved entries are managed bythe Gateway or the Network Manager for maintenance purposes;Finally a shared entry allows multiple devices to compete fortransmitting to the same device simultaneously. For instance, inour system, we allow 5 simultaneous transmissions on a sharedtimeslot. We also maintain several other important data structuresfor constructing the communication schedule. They include one

Command FunctionalityCommand 779 Report device communication statisticsCommand 780 Report neighbor health listCommand 787 Report neighbor signal levelsCommand 788 Path down alarmCommand 789 Source route failure alarmCommand 790 Graph route failure alarm

TABLE IIISummary of network maintenance commands

data superframe Fi per sample rate ri and a global managementsuperframe Fm. Here we use li to denote the length of Fi. Foreach node v, we maintain a schedule Sv to record its ownslot/channel usage. The length of M and Sv are both equalto the maximum length among the existing superframes. Theseschedules will be distributed to the devices to achieve end-to-endreal-time communication.

Alg 7 Constructing Data Communication Schedule1: Sort device sample rates in ascending order: r1 < r2 < . . . < rk.2: Identify the set of nodes with each sample rate: N1,N2, . . . ,Nk.3: Initialize the schedule for each node as ∅4:5: for all ri from r1 to rk do6: Generate the data superframe Fi7: for all node v ∈ Ni do8: // Schedule primary and retry links for publishing data9: ScheduleLinks(v, g, GU , Fi, 0, Exclusive);

10: ScheduleLinks(v, g, GU , Fi,li4 , Shared);

11:12: // Schedule primary and retry links for control data13: ScheduleLinks(g, v, Gv, Fi,

li2 , Exclusive);

14: ScheduleLinks(g, v, Gv, Fi,3li4 , Shared);

15:16: if all link assignments are successfully then17: continue;18: else19: // Defer bandwidth request from node v20: return FAIL;21: end if22: end for23: end for24: return SUCCESS;

We present the framework of constructing the data communi-cation schedule in Alg. 7. The construction of the managementschedule follows the same approach and is omitted here. Inthe algorithm, we apply the (FSRF) policy in scheduling datatransmissions. The construction is based on the reliable graphswe introduced in Section V. For each device v, in its sensingphase, it allocates the primary and retry links along the uplinkgraph GU to the Gateway (Line 9 - 10); In the control phase, theNetwork Manager sends the control messages back and allocatesthe primary and retry links along the downlink graph Gv (Line 13- 14). The ScheduleLinks(u, v, G, F , t, o) function is described inAlg. 8. It allocates every link on the paths from u to v on graphG one by one in a depth-first manner. It allocates the earliestavailable timeslot ti from t for each link and updates M, F andeach effected node’s schedule accordingly. If we cannot find aslot in [t, lF ] to accommodate all the allocations, the NetworkManager will defer the bandwidth request from the correspondingdevice until enough bandwidth resources are available (Line 19- 20 in Alg. 7).

Notice that a device v is typically multi-hop away from theGateway, and it has multiple paths to the Gateway due to theproperty of reliable graph routing. However, if we allocate the

required communication bandwidth for device v on each hopalong all its paths to the Gateway, most of the allocated links willbe wasted because in each end-to-end transmission, only one pathwill be picked. This will severely degrade the schedulability ofthe network schedule. To address this problem, as shown in Alg. 8(Line 17 - 33), when the device has two successors to forward themessages, we reduce the transmission rate between v and eachof its successors to half of the original sample rate, and schedulethe links on the corresponding superframe F ′(lF ′ = 2 · lF ). Wedetermine the timeslot offset of these links in F ′ to make surethat their combinations will form a communication pattern thesame as the original sample rate.

Alg 8 ScheduleLinks(u, v, G, F , t, o)1: // u and v are the source and destination of the communication2: // G is the routing graph and F is the superframe3: // t is the earliest slot to be allocated and o is the link option4:5: Identify data superframe F ′ with lF ′ = 2lF6: for all node i ∈ Successor(u) do7: Identify the schedule Su and Si for node u and i8: if i is the only successor of u then9: Identify the earliest slot from t with a channel c to:

10: Allocate entries Mk·lF +ti ,c(k = 0, 1, ...) on M11: Allocate the slots k · lF + ti on Su and Si12: Allocate slot ti on F13:14: if All allocations are successful then15: ScheduleLink(i, v, G, F , ti, o);16: end if17: else18: if i is the first successor then19: Identify the earliest slot from t with a channel c to:20: Allocate entries Mk·lF ′+ti ,c on M21: Allocate slots k · lF ′ + ti on Su and Si22: Allocate slot ti on F ′23: else24: Identify the earliest slot from t in M with a channel c to:25: Allocate entries Mk·lF ′+lF +ti ,c on M26: Allocate slots k · lF ′ + lF + ti on Su and Si27: Allocate slot lF + ti on F ′28: end if29:30: if All allocations are successful then31: ScheduleLink(i, v, G, F ′, ti, o);32: end if33: end if34: if No feasible allocations available then35: return FAIL;36: end if37: end for38: return SUCCESS;

VII. System ImplementationWe have built a complete WirelessHART communication

system to verify the correctness and efficiency of our networkmanagement techniques. We are deploying the system in a large-scale manufacturing factory to collect sensor data from testingdevices, and achieve factory automation. Figure 12 depicts theabstract architecture of our system which has five major compo-nents: the WirelessHART mesh network, Gateway, Access Point,Network Manager and Host applications. These components inour system are shown in Figure 13, and their design details willbe presented in the following sections.

A. WirelessHART Mesh NetworkOur WirelessHART mesh network is formed by two types

of devices. Rosemount [24] sensors and the Freescale devices

Network Manager

Security Manager

Gateway

Access Point Access Point Access Point

WirelessHART Mesh Networks

Host Applications

OPC Server

TCP Sockets

Serial Port Connection

IEEE 802 . 15 . 4 Radio

Fig. 12. Architecture of the complete WirelessHART communication system

Fig. 13. The major components in the system

with the stack that developed by ourselves [3]. All these devicescomply to the WirelessHART standard, thus have no problem tointeroperate with each other. They join into the network throughthe standardized procedure [25]. The Network Manager organizesthese devices into a multi-hop reliable mesh and configures themwith corresponding routes and communication schedules. Oncethe devices are correctly configured, they begin to exchangemanagement and data messages with the Network Manager andGateway.

B. Gateway Design

The Gateway works as a server responsible for communicatingwith the Network Manager, processing the requests from the Hostapplications, collecting and caching data from all devices in thenetwork. Its architecture is illustrated in Figure 14 and has thefollowing major components:

Physical Connections: The Gateway provides a serial portconnection to each attached Access Point. The Gateway talkswith the Network Manager through a socket connection formessage exchange. It also provides one or more Host Interfacesto backbone networks (e.g., the plant automation network) toreceive the queries and send the responses back.

Real-time Database and Query Processor: The core parts ofthe Gateway are a real-time database and a query processor. Thedatabase provides data caching for burst mode, event notification,and common HART command responses. The query processorprocesses the queries from Host applications. If the requesteddata are already cached and still valid, they are returned im-mediately to the Host applications. This reduces network traffic

Access Point Access Point

Receive Thread & Queue Receive Thread & Queue Transmit Queue Transmit Queue

Serial Port Communication

Destination : NM

Invalid Addresses : Dropped

Security Manager

Security Manager

Gateway

Command Processor

Command Processor

Network Manager Data Caching Data Caching

Network Layer

Gateway Configuration

To Field Devices

TCP / IP

TCP / IP

Destination : Gateway

Destination : Gateway and Devices Cached Response upon Read / Write Confirmation , Burst

Mode , Event Notification

Host Host

Host Interface

Fig. 14. The architecture of the Gateway

and improves the Host application’s responsiveness. Otherwise,the query processor will generate the request messages and sendthem to the corresponding devices. The return response data arecached in the Gateway and sent back to the Host applications.

Time Source: The Gateway maintains a time source module formaintaining network-wide time synchronization. It will notifyall the devices in the network and let them synchronize withthe Gateway through the approaches discussed in Section IV.In our system, the actual time source is a designated AccessPoint instead of the Gateway. This Access Point will periodicallyupdate the accurate time to the Gateway and Network Manager.

C. Access Point Design

The Access Point is a bridge between the mesh network andthe Gateway. There could be multiple Access Points attached tothe Gateway providing load balancing and reliable graph routing.Each Access Point goes through the same join procedure asa normal device to authenticate itself and establish a securedconnection with the Network Manager. As shown in Fig. 15, Thecommunication stack on the Access Point is extended from thatof the device by adding an extra UART module. The messagesreceived from the mesh will be forwarded to the UART moduleand sent to the Gateway. In the other direction, the messagesfrom Gateway/Network Manager will be sent through the serialport and put into the network layer queue in the Access Point.

If a network includes multiple Access Points, then they mustbe synchronized. In our design, except the designated timesource, all other Access Points will be instructed by the NetworkManager to scan the physical channels the same way as whena normal node joins the network. A normal node will sendout join request message to a neighbor after synchronization.These Access Point directly send the join request to the NetworkManager through the Gateway. Afterwards the Network Managerconfigures them just like it configures the original time source.

D. Network Manager Design

The core of a WirelessHART mesh network is the NetworkManager. It is responsible for authenticating the devices, formingthe network, allocating network resources and scheduling processdata transmissions. We have described the detailed algorithmicissues in Section V and Section VI for generating routing graphsand constructing communication schedules. Here we describe ourimplementation of the Network Manager and how we integrateour network management solutions into it. Figure 16 shows

Application Layer Application Layer

Data Link Layer Data Link Layer

Physical Layer Physical Layer

Network Layer Network Layer

PIB PIB

Access Point Access Point

Serial Port Connection Serial Port Connection

UART UART

UART UART Gateway Gateway

Fig. 15. The architecture of the Access Point

Gateway

Security Manager

Command Processor

Communication Tables

Topology

Network Layer

GUI

TCP Socket Communication

Network Manager

Access Control

Fig. 16. The architecture of the Network Manager

the architecture of the Network Manager which has four majorcomponents:

Command Processor: The application layer of the Wire-lessHART standard is command-oriented. The WirelessHARTdevices and the Network Manager interact by exchanging com-mand requests and command responses. The command processorin the Network Manager processes the commands from thedevices, updates the network topology and triggers the algorithmsif necessary to reconstruct the routing graphs and communicationschedules.

Network Topology and Communication Tables: In the Net-work Manager, the network topology is maintained in a directedgraph structure. All the algorithms for constructing routinggraphs and allocating network resources are conducted in thegraph and the results are maintained in a set of communicationtables. Interested readers are referred to [3] for their details.

Security Manager and Access Control: WirelessHART is a se-cure wireless communication protocol and it provides encryptionand authentication in both the data link layer and network layer.The main task of the security manager is to manage various keyinformation for the devices. It takes charge of the device joinauthentication and updates the key information in the networkperiodically for protection purpose. The access control modulemaintains a list of pre-approved devices together with their validjoin keys. Only the devices on the list can be admitted into thenetwork by providing the correct join keys.

Visualizer: Our visualizer is implemented based on the JUNGlibrary [26]. It provides the user a straight-forward way to observethe network topology, the routing graphs, the device communi-

Fig. 17. The topology of the WirelessHART network under deployment

cation schedules, and the exchanged messages. Any update onthem will also be reflected in the visualizer in real-time. With thevisualizer, users can identify problematic network topology andbottlenecks limiting network throughput and perform appropriateadjustments. [27] gives an example of a WirelessHART networkwith two Access Points and 50 field devices. It also showsthe communication schedules which are generated based theproposed algorithms and each device’s bandwidth usage.

We note that our Network Manager design is not only for theWirelessHART communication systems. It is also a generic sim-ulator for wireless mesh networks and allows the users to specifyany network topology either through reading in a topology fileor configuring it manually. It provides the user a platform todesign their algorithms, exercise them on the specified topologyand evaluate their performance.

E. System under DeploymentWe are deploying our system in a manufacturing factory to

help achieve factory automation. The factory has 4 floors, andeach floor has around 20 trollies. Each trolly can carry up to16 motherboards under test and each board is attached witha watchdog. All the watchdogs in a trolly are connected toa controller through I2C bus, and they publish their samplingdata (60 bytes) to the controller every one minute. Previously,the testers have to manually check each trolly and identifythe malfunctioning boards. To achieve factory automation, weare integrating their testing equipments with our WirelessHARTcommunication system. We attach our sensor board to each trollyand connect it to the I2C controller. The samples from thewatchdogs will be forwarded from the controller to our boardand transmitted to the Access Point. To improve the networkconnectivity, we deploy one Access Point in each floor. TheGateway and Network Manager are installed on the third floorand all the Access Points are connected to them through ethernet.Fig. 17 shows the topology of the system under deployment.Once the system is set up, the tester can monitor the status ofall motherboards under test simply through the Gateway. Thiswill save a large amount of manpower, and speed up the testingperiod.

VIII. Performance EvaluationThis section summarizes the major results from our simulations

to evaluate the performance of our algorithms. Our simulation

model and parameter settings are described in Section VIII-A.Section VIII-B compares our algorithms in constructing reliablerouting graphs to traditional approaches. Section VIII-C evaluatesthe performance of our approach for constructing communicationschedules. The results show that our approaches can achievehigher routing success rates, better end-to-end communicationlatency while incurring only modest configuration overheads ondevices.

A. Simulation Model and Parameters

In the simulations, we assume open field, line-of-sight experi-mental scenarios. The simulation area is fixed at 450 m × 450 mand the default device communication distance is 100 meters witha 0 dBm transmitter. We assume that there is no edge betweena pair of nodes if they are not in each other’s communicationrange. Otherwise, an edge exists with an edge success probabilityp that is varied from 0.0 to 1.0. The size of the network is variedfrom 50 to 150 to evaluate the effect of network density on thealgorithm’s performance. We disable a given portion of links inthe network to evaluate the reliability of the constructed routinggraphs and this percentage is varied from 0% to 95%.

B. Performance of Reliable Routing Graphs

We conducted a series of experiments to evaluate the per-formance of the reliable broadcast graph GB, reliable uplinkgraph GU and reliable downlink graph Gv for each individualnode v. Since essentially GU is the reversed version of GB,its performance is similar to that of GB. For this reason, theexperiment results of GU are omitted here.

We compare our approach for constructing GB with twobaseline methods. The first method constructs a single broadcasttree using breadth-first search and the second method generatesthe max-reliable broadcast graph. In the latter method, whena node is chosen to be added to the broadcast graph, all itsincoming edges from the current broadcast graph are also added.Different from this method, our approach only chooses thefirst two incoming edges of the chosen node with minimumlatency, and thus achieve a good balance between the routingreliability and the configuration overhead. In this paper, theconfiguration overhead is defined as the average number of linksto be configured per node. It is an important performance metricbecause wireless sensors’ memory is limited and configuringlarge number of links in the network will severely hurt theschedulability of the communication schedule.

The first experiment compares the configuration overheadintroduced by these three approaches. In the experiments, wevary the size of the network from 50 to 150 nodes and evaluateits impact. Figure 18 summarizes our results. As expected,we observe that the configuration overhead of the max-reliableapproach is much higher than the other two and it increaseslinearly along with the increase of the network density. On theother hand, the overhead in our approach and the broadcast treesolution is much low and stable. The overhead in our approach isalways below 2 links per node, and it is closer to the performanceof the broadcast tree when the network density is low. Thisobservation is mainly because when the network density is low,it is difficult for many nodes to find two parents in the networkthus has only one link in the broadcast graph.

In the second experiment, we first construct the broadcastgraphs based on these three approaches with 100 nodes in thenetwork. We then gradually increase the percentage of failed linksin the network from 0% to 95%. We measure the reliability of

these three approaches and apply the recovery mechanisms wediscussed in Section V-I on them. We compare their recoveryoverhead in terms of number of changed links. Figure 19 showsthat along with the increased percentage of failed links in thenetwork, the reliability of the broadcast tree drops quickly andwhen half of the links die, only around 25% nodes are reachablefrom the Gateway. Our approach performs much better. Withthe same percentage of failed links, around 55% of nodes arestill connected. Among all three approaches, the max-reliablebroadcast graph has the best performance as a tradeoff of its poorscalability and much higher configuration overhead. In figure 19,we also show a curve of the reachability for the broadcast graphsafter the recovery. As the recovery mechanisms are all based onthe same underlying network topology, all three approaches havethe same reachability after reconstruction. This in turn verifiesthe correctness of our recovery mechanisms.

Figure 20 and Figure 21 compare the recovery overhead amongthese approaches. Figure 20 shows the overhead to resume theconnectivity of the broadcast graphs while Figure 21 furthershows the overhead to recover their reliability properties. Weobserve from Figure 20 that the broadcast tree always has theheaviest recovery overhead while the max-reliable broadcast treehas the minimum because of its best reliability. The performanceof our approach sits between them. However, Figure 21 showsthat to recover the reliability property, our approach needs toadd more links than the other two alternatives. The reason isthe broadcast tree has no reliability requirement while the max-reliable approach has already added most of the links in theconstruction stage thus its recovery overhead is relatively smaller.

In the third experiment, we evaluate the performance of thetwo proposed approaches for constructing reliable downlinkgraphs, the standard approach as defined in WirelessHARTstandard RDG(standard) and the sequential reliable downlinkrouting approach (SRDR). we compare them with two baselinemethods. The first method finds a single shortest path from theGateway to the destination, while the second one constructsa two node-disjoint path and can tolerate one link or nodefailure. Figure 22 summarizes the comparison of the routingreliability among these four approaches. It clearly shows thatthe single path approach always has the worst performance. Onthe other hand, RDG(standard) maintains the best reliability andalways outperforms the two node-disjoint path method more than30%. SRDR is around 8% worse than RDG(standard) in routingreliability. This is because the downlink graphs constructed underRDG(standard) have more redundant links. As a tradeoff, asshown in Figure 23 and Figure 24, RDG(standard) introducesa much higher configuration overhead. The average numberof nodes in the constructed graphs is 2 times and 1.2 timeslarger than that of the single shortest path approach and twonode-disjoint path approach respectively. Furthermore, as eachnode under RDG(standard) has two outgoing edges, the averagenumber of links in the constructed graphs is even higher. Asshown in Fig. 24, it is around 5.5 times and 2.8 times larger thanthat of the single shortest path approach and two node-disjointpath approach respectively. However, SRDR only introduces verylimited configuration overhead because it only constructs localgraphs and these local graphs can be further reused for assem-bling the downlink routes to different destinations. Its averagenumber of nodes is the lowest among all the four approachesand its average number of links is only slightly higher that ofthe single shortest path approach and around 33% lower than thetwo node-disjoint path approach. In sum, SRDR achieves a good

0

2

4

6

8

10

75 15050Avg

Num

ber o

f Lin

ks p

er N

ode Broadcast Tree

2-Reliable Broadcast Graph Max-Reliable Broadcast Graph

Number of Nodes100 125

Fig. 18. Configuration overhead in broadcast graphs

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Per

cent

age

of R

each

able

Nod

es

Percentage of Failed Links

2-Reliable BCast Graph Broadcast Tree Max-reliable BCast Graph Reconstructed BCast Graph

Fig. 19. Reachability in broadcast graphs

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

10

20

30

40

50

60

70

80

Num

ber o

f Lin

ks

Percentage of Failed Links

2-Reliable Del Tree Del Max Del 2-Rreliable Add Tree Add Max Add

Fig. 20. Recovery overhead to regain connectivity

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00

10

20

30

40

50

60

70

80

90

100

110

120

Num

ber o

f Lin

ks

Percentage of Failed Links

2-reliable Del Tree Del Max Del 2-reliable Add Tree Add Max Add

Fig. 21. Recovery overhead to regain reliability

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0P

erce

ntag

e of

Rea

chab

le N

odes

Percentage of Failed Links

SRDR RDG(standard) Single Shortest Path 2 Node-Disjoint Path

Fig. 22. Reachability in downlink graph

0

1

2

3

4

5

6

7

8

9

10

11

12

Ave

rage

# o

f Nod

es

Number of Nodes in the Network

SRDR RDG(standard) Single Shortest Path 2 Node-Disjoint Path

50 75 100 125 150

Fig. 23. Average # of nodes per downlink graph

0246810121416182022242628

Ave

rage

Num

ber o

f Edg

es

Number of Nodes in the Network

SRDR RDG(standard) Single Shortest Path 2 Node-Disjoint Path

50 75 100 125 150

Fig. 24. Average # of edges per downlink graph

50 60 70 80 90 100 110 120 130 140 150

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

4.2

Ave

rage

Lat

ency

Number of Nodes in the Network

SRDR SRDR-OPT RDG(standard) Single Shortest Path 2 Node-Disjoint Path

Fig. 25. Average latency vs. Network size

60 80 100 120 140 160 180 2001

2

3

4

5

6

7

8

9

10

11

Ave

rage

Lat

ency

Communication Range

SRDR SRDR-OPT RDG(standard) Single Shortest Path 2-Node disjoint Path

Fig. 26. Average latency vs. Communication range

balance between high routing reliability and low configurationoverhead.

We also evaluate the performance of the optimization mecha-nism SRDR-OPT which is proposed in Section V-H, and measureits improvement on average latency in two different scenarios.In the first scenario, we fix the devices’ communication range at100m and increase the number of nodes in the network from 50 to150. The results is shown in Fig. 25. We observe that SRDR hasa much higher average latency compared with RDG(standard).This is because when constraint C2 is satisfied, SRDR choosesthe node with larger latency as its parent in constructing downlinkgraph while RDG(standard) take both and its latency is calculatedas their average plus one. The performance of SRDR-OPT issimilar to RDG(standard) because the shortcuts are taken in theoptimization. Obviously, the single shortest path approach alwayshas the lowest latency. In the second scenario, we fix the numberof nodes in the network at 150 and vary the communicationrange of the devices from 50m to 200m. As shown in Fig. 26,the average latencies of all the four approaches decrease withthe increase of the communication range, and consistent with the

observations in the first scenario, SRDR has a great improvementon the average latency when the optimization mechanism isapplied.

C. Construction of Communication Schedules

Our approach for constructing the communication schedulehas two unique features. First, we split the traffic from a deviceamong all its successors by reducing the bandwidth requirementon each successor. The communication schedules on the suc-cessors are carefully designed so that their combination has thesame patten as the original device. Second, we use the conceptof shared timeslot to allow multiple devices to compete forcommunicating with the same device simultaneously. This isespecially useful for the links that are allocated for retry purposeand it can significantly improve the network throughput.

In this section, we evaluate the performance of these twofeatures by comparing our approach with three baseline methods.The basic methods either lack one of the features or both ofthem. For simplicity, we only show our experimental results onscheduling process data from devices to the Gateway on the

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

20 23 262-1

Suc

cess

Rat

io

Sampling Rate

Half-Rate w/ Shared Links Half-Rate w/o Shared Links Same-Rate w/ Shared Links Same-Rate w/o Shared Links

2-2 21 22 24 25 27 28

Fig. 27. Success ratio vs. Sample rate

0.00

0.05

0.10

0.15

0.20

0.25

0.30 Half-Rate w/ Shared Links Half-Rate w/o Shared Links Same-Rate w/ Shared Links Same-Rate w/o Shared Links

Net

wor

k U

tiliz

atio

n

20 23 262-1

Sampling Rate2-2 21 22 24 25 27 28

Fig. 28. Network utilization vs. Sample rate

uplink graph. Scheduling control data on the other direction issimilar, and thus is omitted here. Two performance metrics aredefined for this experiment. The first metric is the schedulingsuccess ratio which measures the percentage of nodes that cansuccessfully allocate the required bandwidth along its paths tothe Gateway; The second metric is the network utilization whichmeasures the percentage of entries in matrix M that are alreadyallocated for communication. Our results are summarized inFigure 27 and Figure 28 respectively.

In Figure 27, we compare the scheduling success ratio bydeploying 50 nodes in the network and varying the device samplerate from 250 ms to 4 min and 16 sec (each device has thesame sample rate). We observe that by halving the bandwidthrequirement on a device’s successors (if it has two successors),the success ratio can be greatly improved. The improvementis more than 25% when the sample rate is 2 sec and is evenhigher when the sampling is faster. Figure 27 also shows that byapplying the shared timeslot, the success ratio can be increased by5% and this improvement is consistently shown in our experimentresults until the sample rate is low enough that the schedulingsuccess ratio approaches 100%. Figure 28 shows that when theapproaches have a similar scheduling success ratio, our approachhas a much lower network utilization, and this will further helpinclude more devices into the network. When the sample rate isfast, our approach has a higher network utilization because inthese scenarios, the success ratio for other approaches is so poorthat a very limited number of devices can successfully allocatetheir required bandwidth along its path to the Gateway.

IX. Conclusions and FutureWork

In this paper, we study the problem of how to achievereliable and real-time communication in industrial wireless meshnetworks. Taking WirelessHART network as an example, weabstract the reliability requirements in typical wireless indus-trial process control applications and present the algorithms for

constructing three types of reliable routing graphs for differentcommunication purposes. Based on these routing graphs, wedescribe how we construct the communication schedule in thenetwork and highlight our approach’s unique features. We presentthe architecture of a complete WirelessHART communicationsystem that we have built and we have performed extensivesimulations to evaluate the performance of our algorithms.

In ongoing and future work, we are deploying our systemin a large-scale manufacturing factory, so that we can eval-uate the performance of our network management techniquesin real industrial environments. We shall continue to look formore efficient approaches for constructing routing graphs andcommunication schedules to maximize the power saving inWirelessHART networks, and study their corresponding recoverymechanisms.

References[1] Andreas Willig, “Recent and emerging topics in wireless industrial

communications: A selection,” IEEE Trans. on Industrial Informatics, 2007.[2] Dick Caro, Wireless Networks for Industrial Automation, ISA Press, 2004.[3] J. Song, S. Han, A. K. Mok, D. Chen, M. Lucas, M. Nixon, and W. Pratt,

“WirelessHART: Applying wireless technology in real-time industrial pro-cess control,” in RTAS, 2008.

[4] Rajeev Alur, Alessandro D’Innocenzo, Karl H. Johansson, George J.Pappas, and Gera Weiss, “Modeling and analysis of multi-hop controlnetworks,” in RTAS, 2009.

[5] Gera Weiss, Rajeev Alur, Alf J. Isaksson, and Karl H. Johansson, “Scalablescheduling algorithms for wireless networked control systems,” in CASE,2009.

[6] Joonas Pesonen, Haibo Zhang, Pablo Soldati, and Mikael Johansson,“Methodology and tools for controller-networking codesign in Wire-lessHART,” in ETFA, 2009.

[7] Shahid Raza, Adriaan Slabbert, Thiemo Voigt, and Krister Landernas,“Security considerations for the wireless hart protocol,” in ETFA, 2009.

[8] Gabriella Fiore, Valeria Ercoli, Alf J. Isaksson, Krister Landernas, andMaria Domenica Di Benedetto, “Multihop multi-channel scheduling forwireless control in WirelessHART networks,” in ETFA, 2009.

[9] “ISA,” http://www.isa.org/.[10] “HART communication,” http://www.hartcomm.org.[11] “ZigBee Alliance,” http://www.zigbee.org.[12] “WirelessHART,” http://www.hartcomm.org/protocol/wihart/

wireless_technology.html.[13] Abusayeed Saifulah, Chenyang Lu, You Xu, and Yixin Chen, “Real-time

scheduling for WirelessHART networks,” in RTSS, 2010.[14] Pablo Soldati, Haibo Zhang, and Mikael Johansson, “Deadline-constrained

transmission scheduling and data evacuation in wirelesshart networks,” inTechnical Report TRITA-EE 2008:060, 2008.

[15] Haibo Zhang, Pablo Soldati, and Mikael Johansson, “Optimal linkscheduling and channel assignment for convergecast in linear wirelesshartnetworks,” in Technical Report TRITA-EE 2009:018, 2009.

[16] “Bluetooth,” www.bluetooth.com/bluetooth.[17] “IEEE 802.15.4 WPAN Task Group,” www.ieee802.org/15/pub/TG4.

html.[18] Stephen Mueller, Rosep. Tsang, and Dipak Ghosal, “Multipath routing in

mobile ad hoc networks: Issues and challenges,” Performance Tools andApplications to Networked Systems, 2004.

[19] Sasan Adibic Mohammed Tariquea, Kemal E. Tepeb and Shervin Erfanib,“Survey of multipath routing protocols for mobile ad hoc networks,” Journalof Network and Computer Applications, vol. 32, no. 6, 2009.

[20] Deepak Ganesan, Ramesh Govindan, Scott Shenker, and Deborah Estrin,“Highly-resilient, energy-efficient multipath routing in wireless sensor net-works,” SIGMOBILE Mob. Comput. Commun. Rev., vol. 5, no. 4, 2001.

[21] S.J.Lee and M. Gerla, “Split multipath routing with maximally disjointpaths in ad hoc networks,” in ICC, 2001.

[22] Mahesh K. Marina and Samir R. Das, “On-demand multipath distancevector routing in ad hoc networks,” in ICNP, 2001.

[23] Z. Ye, S.V. Krishnamurthy, and S.K. Tripathi, “A framework for reliablerouting in mobile ad hoc networks,” in INFOCOM, 2003.

[24] “Rosemount,” http://www.emersonprocess.com/Rosemount/.[25] S. Han, J. Song, X. Zhu, A. K. Mok, D. Chen, M. Nixon, W. Pratt, and

V. Gondhalekar, “Wi-HTest: testing suite for diagnosing WirelessHARTdevices and networks,” in RTAS, 2009.

[26] “Java Universal Network/Graph Framework,” jung.sourceforge.net/.[27] “An example of network manager visualizer,” www.cs.utexas.edu/

˜shan/WH-example.pdf.