mac scheduling in industrial wireless cell-based mesh

MAC SCHEDULING IN INDUSTRIAL

WIRELESS CELL-BASED MESH SENSOR

NETWORKS

Imran Yousaf

Master’s Degree Thesis 2011

EMBEDDED SYSTEMS

Postadress: Besöksadress: Telefon: Box 1026 Gjuterigatan 5 036-10 10 00 (vx) 551 11 Jönköping

MAC SCHEDULING IN INDUSTRIAL

WIRELESS CELL-BASED MESH SENSOR

NETWORKS

Imran Yousaf

This thesis work has been carried out at the School of Engineering, Jönköping

University within the subject area of embedded systems. The work is a part of

the Master programme in Electrical Engineering. The author takes full responsibility for opinions, conclusions and findings

presented.

Examiner: Youzhi Xu

Supervisor: Youzhi Xu

Scope: 30 credits (second cycle)

Date: December 07, 2011

Abstract

iii

Abstract

Undergoing developments for the adaptation of Wireless Sensor Networks (WSNs) in

automation industry is creating several research questions. The usage of wireless

technologies in industrial environments requires real-time reliability and security.

Many standards for WSNs and Industrial Wireless Sensor Networks (IWSNs) have

been developed. The list includes WirelessHART [4], ISA100.11a [10], Zigbee [16],

IEEE 802.15.4, and IEEE 802.15.4a etc.

Recently a new network topology “Cell-Based Mesh Networks” [12] has been

developed in an effort to make IWSNs feasible for large scale deployments in process

automation industries. Cell-Based Mesh Network topology inherits the qualities of

star-mesh topology and mesh topology, and also offers many additional features.

Reliability of an IWSN significantly depends on the real-time scheduling of the

network. During this thesis work a scheduling mechanism has been developed by

exploiting the features of a Cell-based Mesh Network. The idea of superframes’

further partitioning into time-cells of 100ms duration is presented, these 100ms time-

cells guarantees interlocking data transmission [9] within 100ms. The closed loop

control data [9] transmission is guaranteed by assigning dedicated time-slots. The

performance evaluation of the presented scheduling mechanism is done by

considering two case studies i.e. oil-industry and paper & pulp industry.

Keywords

iv

Keywords

Wireless Sensor Networks (WSN)

Industrial Wireless Sensor Networks (IWSN)

Wireless Sensor and Actuator Networks (WSAN)

WirelessHART

Cell-Based Mesh Networks

MAC Scheduling Algorithms

Real-time scheduling for IWSN

Network Topologies for WSNs

MAC Scheduling in WirelessHART

Acknowledgements

v

Acknowledgements

First of all, I would like to thank Allah almighty for all his blessings that he bestows

on us.

I would like to thank my supervisor Prof. Youzhi Xu, he has been very encouraging

and helpful for me during the whole thesis work. I really liked the way he explains the

research problems and involves the student in such a way that he can easily come up

with new ideas and innovations.

I would also like to thank our programme coordinator Alf Johansson for his guidance

and encouragement throughout the master programme and to all my teachers who

have given me invaluable knowledge during this master programme and to all my

colleagues.

Finally, I would like to thank my parents and family for their prayers, support, and

love throughout my whole life.

Contents

vi

Contents

Abstract ......................................................................................... iii

Keywords ....................................................................................... iv

Acknowledgements ........................................................................ v

Contents ........................................................................................ vi

List of Abbreviations ................................................................... viii

1 Introduction ............................................................................. 1

1.1 WIRELESS SENSOR NETWORKS .............................................................................................. 1 1.2 INDUSTRIAL WIRELESS SENSOR NETWORKS ........................................................................... 2 1.3 THESIS OBJECTIVES AND TASKS ............................................................................................. 3 1.4 THESIS STRUCTURE ............................................................................................................... 3

2 Background .............................................................................. 4

2.1 CELL-BASED MESH NETWORKS ............................................................................................. 4 2.2 WIRELESSHART PROTOCOL .................................................................................................. 5 2.3 SCHEDULING IN WIRELESSHART......................................................................................... 10

2.3.1 Location of Scheduling Algorithm in WirelessHART ................................................... 10 2.3.2 Requirements for Scheduling ...................................................................................... 10 2.3.3 Performance Measurers for MAC Scheduling Algorithms ........................................... 11

2.4 STATE-OF-THE-ART SCHEDULING MECHANISMS .................................................................. 11 2.4.1 Optimal Branch-and-Bound (B&B) Scheduling .......................................................... 11 2.4.2 Conflict-aware Least Laxity First (C-LLF) Scheduling ............................................... 12 2.4.3 Communication Schedule Defined in [1] .................................................................... 12

3 Design and Implementation ................................................. 14

3.1 PROBLEM FORMULATION ..................................................................................................... 14 3.1.1 Payload ..................................................................................................................... 14 Monitoring and Supervision ..................................................................................................... 14 Closed Loop Control ............................................................................................................... 14 Interlocking ............................................................................................................................. 15 Network Management .............................................................................................................. 15 3.1.2 Network Topology ...................................................................................................... 15 3.1.3 Superframe ................................................................................................................ 15 3.1.4 Scheduling ................................................................................................................. 16 Time-Slots ............................................................................................................................... 16 Dedicated ................................................................................................................................ 16 Shared ..................................................................................................................................... 16 Reserved.................................................................................................................................. 16 Unused .................................................................................................................................... 16

3.2 SCHEDULING ALGORITHM .................................................................................................... 16 3.2.1 Flow Diagram ........................................................................................................... 19

3.3 IMPLEMENTATION................................................................................................................ 21

4 Performance Evaluation ........................................................ 24

4.1 CASE-1: OIL INDUSTRY ........................................................................................................ 24 4.1.1 Schedule for Cell-1 .................................................................................................... 27 4.1.2 Schedule for Cell-2 .................................................................................................... 29

4.2 CASE-2: PAPER AND PULP INDUSTRY .................................................................................... 32 4.2.1 Schedule for Cell-1 .................................................................................................... 35 4.2.2 Schedule for Cell-2 .................................................................................................... 37 4.2.3 Schedule for Cell-3 .................................................................................................... 39

Contents

vii

5 Conclusions and Future Work .............................................. 41

5.1 CONCLUSION ....................................................................................................................... 41 5.2 FUTURE WORK .................................................................................................................... 41

6 References .............................................................................. 42

7 Appendices ............................................................................. 44

7.1 APPENDIX A STATE-OF-THE-ART NETWORK TOPOLOGIES ..................................................... 44 7.1.1 Star Topology ............................................................................................................ 44 7.1.2 Mesh Topology .......................................................................................................... 44 7.1.3 Star-Mesh Topology ................................................................................................... 44 7.1.4 Hub-and-Spoke Topology ........................................................................................... 45

7.2 APPENDIX B SCHEDULE TABLES OF CASE-2 .......................................................................... 45

List of Abbreviations

viii

List of Abbreviations

WSN Wireless Sensor Network

IWSN Industrial Wireless Sensor Network

HART Highway Addressable Remote Transducer Protocol

DLL Data Link Layer

B&B Optimal Branch-and-Bound Scheduling

C-LLF Conflict-aware Least Laxity First

TDMA Time Division Multiple Access

MAC Media Access Control

ASN Absolution Slot Number

DSSS Direct Sequence Spread Spectrum

ISM Industrial, Scientific, and Medical radio bands

OSI Open Systems Interconnection

Introduction

1

1 Introduction

The developments in wireless communication technologies have proven to be part of

various aspects of automation that include buildings, home, shipboard, transportation

systems, industry etc. Every environment has various distinct requirements for the

deployment of a wireless communication system e.g. for a temperature control system

in a building, the coverage area should be large in contrast to an industrial process

control system where the timing requirements are stringent though coverage area can

also be large in this case but not so important. Such wireless communication has been

made possible with the help of wireless sensor networks (WSNs). Research and

development at a large scale is undergoing to take this evolutionary step in the area of

automation. Many WSN standards have been developed though there are still lots of

research problems that need to be solved since WSNs are targeting a wide range of

environments.

This thesis work is related to industrial WSNs, that has stringent requirements for

wireless communication. Recently a new network topology called ‘Cell-Based Mesh

Networks’ [12] has been developed for industrial WSNs. In this thesis report a mac

layer scheduling mechanism for Industrial Wireless Cell-Based Mesh Networks has

been proposed.

1.1 Wireless Sensor Networks

A wireless sensor network (WSN) consists of spatially distributed autonomous

sensors to monitor physical or environmental conditions, such as temperature, sound,

vibration, pressure, motion or pollutants and to cooperatively pass their data through

the network to a main location [8]. Each of the sensor nodes of a WSN is equipped

with a radio transceiver, a small microprocessor and a number of sensors. The sensor

nodes are capable of forming the network autonomously and wirelessly communicate

with a central control, the data transmission can be in either direction. An abstract

picture of a WSN is shown in Fig 1.1a.

Figure 1.1a A Wireless Sensor Network

Introduction

2

Wireless sensor networks are bringing a revolutionary change in a wide variety of

application areas. These areas include;

Environmental monitoring

Healthcare

Critical industrial areas

Warehouse and supply chain

Military surveillance

Building automation etc.

Let’s take the example of environmental monitoring; a WSN can be deployed in a

forest that has caught fire, to monitor the further consequences and temperature in the

surroundings. The deployment can be done via an airplane since the sensor nodes are

autonomous so these will create a network autonomously and transmit data to the

central control to help the fire fighters to cope with the situation in a more efficient

way.

1.2 Industrial Wireless Sensor Networks

An industrial wireless sensor network can be defined as a WSN that can fulfil

stringent requirements on security and real-time reliability. Mostly the IWSNs are

centrally controlled and the dominate data traffic is sensed data from sensors to the

central control and actuation data to the actuators from the central control. A typical

industrial wireless sensor network is depicted in Fig 1.2a.

Figure 1.2a A typical Industrial Wireless Sensor Network [4]

These days, developments are on the go to make large scale deployment of wireless

communication technologies possible in industrial environments. Recently new

Introduction

3

standards have been developed that are particularly targeting industrial automation

domain, these include WirelessHART [4], ISA 100.11a [10] etc.

1.3 Thesis Objectives and Tasks

The main objective of this thesis project is to design and implement a feasible

scheduling algorithm/mechanism for Industrial Wireless Cell-Based Mesh Networks.

Although, the target IWSNs can be based on WirelessHART, ISA-100 or other

similar standards based on IEEE 802.15.4 but in this thesis work we will stick to the

WirelessHART standard for design and implementation details. More precisely

WirelessHART standard will be leveraged by cell-based mesh network topology for

network routing and for communication scheduling a feasible mechanism will be

proposed. Performance evaluation of the proposed scheduling algorithm will be for

one cell of a cell-based mesh network, the complete network’s performance

evaluation is out of scope of this thesis work.

1.4 Thesis Structure

The remainder of this thesis has been structured as follows. In Chapter 2, the

significance of scheduling in WSNs will be described by introducing Cell-Based

Mesh Networks and taking the case of WirelessHART protocol for scheduling. The

design and implementation of the proposed scheduling algorithm is described in

Chapter 3 and the performance evaluation is done in Chapter 4. The thesis report is

concluded in Chapter 5, some possible future work related to this project is also given

there.

Background

4

2 Background

This chapter is meant for describing the significance of scheduling in WSNs. Firstly,

Cell-Based Mesh Networks and WirelessHART protocol are introduced, and then the

significance of communication schedule in WirelessHART and State-Of-The-Art

Scheduling Mechanisms are given in this chapter.

2.1 Cell-Based Mesh Networks

State-of-the-art network topologies1 for IWSNs include star topology, mesh topology,

star-mesh topology, and hub-and-spoke topology, but IWSNs require a more suitable

network topology that can fulfil the stringent requirements of IWSNs that include

security, reliability, real-time, and low power consumption. Mostly the IWSNs are

centrally controlled and the dominate data traffic is sensed data from sensors to the

central control and actuation to the actuators from the central control. Cell-based mesh

network topology has been developed to provide a feasible solution for IWSNs.

Figure 2.1a A Cell-Based Mesh Network

The basic architecture of a Cell-based mesh network is depicted in Fig 2.1a. Its basic

elements include field devices, backbone routers, a gateway, a security manager, and

a network manager. The network is built up by linking multiple cells or clusters via

backbone to the network manager. Each cell has a backbone router that works as

cluster head, responsible for providing basic links to each field device with few hops

one or two and maximally 3 hops in the cell. Redundant routes are also available in

the form of intra-cell and inter-cell links [12]. Two frequency channels will be utilized

by each cell.

By inheriting the properties of a multiple-star-clusters network and a mesh network, a

cell-based mesh network provides a solution with significant improvements. The main

advantageous properties [12] are as follows:

i. High network reliability 1State-of-the-art network topologies are described in Appendix A.

Background

5

ii. Reduced retry times

iii. Improved network size and scalability

iv. Low latency

v. Simplicity

vi. High time synchronization accuracy

vii. High failure tolerance

2.2 WirelessHART Protocol

WirelessHART protocol is part of the HART 7 (www.hartcomm.org) standard. HART

was developed in the late 80’s. Initially HART communications protocol offered a

simple two-communications by using 4-20mA signals. After more than two decades

HART has evolved from its simple two-communication capability to the wired and

wireless communication capabilities with extensive features that includes security,

unsolicited data transfers, event notifications, block mode transfers, and advanced

diagnostics. The evolution of HART standard is depicted in Fig 2.2a. The most recent

version of HART standard is version 7 or simply HART 7. Along with many other

new features HART 7 introduced wireless mesh networking in the form of

WirelessHART communication protocol.

Figure 2.2a Evolution of HART standard [3]

WirelessHARTTM

protocol is designed for communication within Wireless Sensor

Networks (WSNs). The main target markets for WirelessHART communication

protocol are real-time process measurement and control applications. WirelessHART

is the first global wireless communication standard approved by IEC [4] which claims

to provide simple, reliable, and secure communication. A typical WirelessHART

based network is shown in Fig 2.2b. In addition to wireless communication capability,

WirelessHART also offers all the capabilities of the wired HART protocol.

Background

6

Figure 2.2b A typical WirelessHART network [1]

The architecture of HART protocol is shown in Fig 2.2c in comparison to the OSI

model. The architecture of the WirelessHART protocol can be seen in the right side of

Fig 2.2c, since it is a part of HART protocol.

The physical layer of WirelessHART protocol is based on the IEEE 802.15.4-2006

2.4GHz DSSS physical layer. The WirelessHART protocol operates in the 2400-

2483.5MHz license-free ISM band with a data rate of up to 250 Kbits/s. Two adjacent

channels in WirelessHART have 5MHz gap, and the channels are numbered from 11

to 26 [18].

The data link layer of WirelessHART is not compatible with IEEE 802.15.4-2006

DLL, rather it defines its own time-synchronized DLL. WirelessHART defines a strict

10ms time-slot and utilizes TDMA technology to provide collision free and

deterministic communications [17]. The concept of superframe is introduced to group

a sequence of consecutive time slots. Note a superframe is periodical, with the total

length of the member slots as the period. All superframes in a WirelessHART

network start from the ASN (absolution slot number) 0, the time when the network is

Background

7

first created. Each superframe then repeats itself along the time based on its period. In

WirelessHART, a transaction in a time slot is described by a vector:

{frame id, index, type, src addr, dst addr, channel offset}

Figure 2.2c The architecture of HART communication protocol [1]

where frame id identifies the specific superframe; index is the index of the slot in the

superframe; type indicates the type of the slot (transmit/receive/idle); src addr and dst

addr are the addresses of the source device and destination device, respectively;

channel offset provides the logical channel to be used in the transaction. To fine-tune

the channel usage, WirelessHART introduces the idea of channel blacklisting.

Channels affected Figure 2. WirelessHART Data Link Layer Architecture by

consistent interferences could be put in the black list. In this way, the network

administrator can disable the use of those channels in the black list totally. To support

channel hopping, each device maintains an active channel table. Due to channel

blacklisting, the table may have less than 16 entries. For a given slot and channel

offset, the actual channel is determined from the formula:

ActualChannel = (ChannelOffset + ASN) % NumChannels

The actual channel number is used as an index into the active channel table to get the

physical channel number. Since the ASN is increasing constantly, the same channel

offset may be mapped to different physical channels in different slots. This shows that

WirelessHART offers channel diversity and enhanced communication reliability.

Along with strict 10 ms timeslot, network-wide time synchronization, channel

hopping, channel blacklisting, WirelessHART DLL also offers industry-standard

AES-128 ciphers and keys.

The architecture of WirelessHART DLL is shown in Fig 2.2d. The WirelessHART

DLL has six major modules i.e.

Background

8

Interfaces

The interfaces between layers are meant for providing service primitives to the

corresponding higher layers e.g. the interface between the MAC and physical layer

defines the service primitives provided by the physical layer.

Timer

Timer is a fundamental module in the WirelessHART standard since WirelessHART

offers centralized network control/management. Its main purpose is to keep those

10ms timeslots in synchronization.

Communication Tables

Each network device maintains a collection of tables in the data link layer. The

information regarding the communication configurations and scheduling mechanism

is kept in the superframe table and link table. The neighbor table is meant for keeping

the list of neighbor nodes that the device can reach directly and the graph table is used

to collaborate with the network layer and record routing information.

Figure 2.2d WirelessHART data link layer architecture [3]

Link Scheduler

The link scheduler is meant for determining the next timeslot to be serviced based on

the schedule mechanism that is kept in the superframe table and link table. The

scheduler is complicated by such factors as transaction priorities, the link changes,

and the enabling and disabling of superframes. Every event that can affect link

scheduling will cause the link schedule to be re-assessed.

Message Handling Module

Background

9

The functionality of the message handling module is to buffer packets from the

network layer and physical layer separately.

State Machine

The state machine in the data link layer consists of three primary components: the

TDMA state machine, the XMIT and RECV engines. The TDMA state machine is

meant for executing the transaction in a slot and synchronizing the timer clock. The

XMIT and RECV engines deal with the hardware directly, which send and receive

packets over the transceiver [3].

The network layer and transport layer provide secure and reliable end-to-end

communication for network devices. The overall design of the network layer and

transport layer is depicted in Fig 2.2e.

Figure 2.2e WirelessHART Network Layer [3]

The basic elements of a typical WirelessHART network include: Field Devices,

Handhelds, a gateway and a network manager. WirelessHART is centrally controlled

by the network manager. Network manager is responsible for configuring the

network, scheduling and managing communication between WirelessHART devices.

To support the mesh communication technology, Each WirelessHART device is

required to be capable of forwarding packets on behalf of other devices since it is a

requirement for mesh communication technology. There are three routing protocols

defined in the WirelessHART standard i.e. source routing, graph routing, and

superframe routing [3].

The application layer is the highest level layer in WirelessHART standard that

provides infrastructure for building applications for WirelessHART based networks.

In the WirelessHART standard, the communication between the devices and gateway

is based on commands and responses. The responsibilities of the application layer

Background

10

include parsing the message content, extracting the command number, executing the

specified command, and generating responses [3].

2.3 Scheduling in WirelessHART

Since the data link layer (DLL) in WirelessHART is time-synchronized.

WirelessHART defines a strict 10ms time slot and utilizes TDMA technology to

provide collision free and deterministic communications. Transactions happen in

these 10ms time slots which are further divided as depicted in Fig 2.3a, each division

of the 10ms timeslot is used for a specific purpose. Infrastructure for time-

synchronization is defined in the WirelessHART standard but it is not defined that

which device should do the transaction in which time slot through which

communication channel? Thus a schedule (or scheduling algorithm) is required to

assign time slots of a communication channel for particular transactions for particular

devices.

Figure 2.3a WirelessHART slot timing

2.3.1 Location of Scheduling Algorithm in WirelessHART

The main scheduling algorithm resides in the network manager by using which the

network manager defines schedules. These schedules are broadcasted to the field

devices by using control and configuration messages. Field devices maintain these

schedules using link tables and superframe tables.

2.3.2 Requirements for Scheduling

The construction of the communication schedule is subject to several practical

constraints in WirelessHART networks [1]:

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 250ms

(2−2

sec) to 8min and 32sec (29sec).

Background

11

2.3.3 Performance Measurers for MAC Scheduling Algorithms

Different performance measures are defined in literature for evaluating MAC

scheduling in WirelessHART mesh-networks. Some of these are:

Schedulable Ratio [2]

Buffer Size [2]

Scheduling Success Ratio [1]

Network Utilization [1]

Power Consumption

Packet Loss Rate or Delivery Rate

Throughput

Average and Maximum Delay

Power Consumption

2.4 State-Of-The-Art Scheduling Mechanisms

There are various scheduling mechanisms developed for WirelessHART network that

are given below, there might have also been developed other scheduling mechanisms

for the same purpose but I could have found these up till August 2011.

Optimal Branch-and-Bound (B&B) Scheduling [2]

Conflict-aware Least Laxity First (C-LLF) Scheduling [2]

Communication Schedules defined in [1]

Multihop multi-channel scheduling for wireless control in WirelessHART

networks [5]

Deadline-constrained transmission scheduling and data evacuation in

WirelessHART networks [6]

Optimal link scheduling and channel assignment for convergecast in linear

WirelessHART networks [7]

In this report the first three scheduling mechanisms/algorithms will be described since

these are the latest one and seem to be interesting but the details for any of the above

mentioned scheduling mechanism can be seen in the given references.

2.4.1 Optimal Branch-and-Bound (B&B) Scheduling

The B&B scheduling algorithm exploits the necessary condition established in

Theorem 2 (which states that for a set of flows F, let be the set of unscheduled

transmissions at slot s. If these transmissions are schedulable, then

[2]) to effectively discard infeasible branches in the

search space. The B&B scheduling algorithm guarantees to find a schedule whenever

a feasible one exits that makes it optimal and complete. The algorithm made decision

at every node by estimating an upper bound of the laxity of the schedule that the node

Background

12

may lead to. The laxity of a packet is “its remaining time slots minus its remaining

number of transmissions”, and the laxity of a schedule is “the minimum laxity among

all packets”. According to Theorem 2, for transmissions to be scheduled on or after

slot s, following is an upper bound (UB) of its schedule’s laxity:

(2.4a)

A global lower bound (LB) of schedule laxity 0 is maintained by the search. Two

decisions are made i.e. unschedulable or may be schedulable by computing UB at a

node (using Equation 2.4a). If UB < LB at a node, it is guaranteed that this node will

not lead to any feasible schedule and, hence, is discarded without further

consideration. And if UB LB, then this node may lead to a feasible solution and,

hence, is expanded further. The algorithm terminates as soon as it finds a feasible

complete schedule that meets all deadlines. If the original problem is infeasible, the

algorithm will also terminate as soon as it determines that this is the case [2].

2.4.2 Conflict-aware Least Laxity First (C-LLF) Scheduling

Conflict-aware least laxity first (C-LLF) scheduling is suitable for dynamic

environments where network topology changes frequently. In traditional Least Laxity

First (LLF) have been developed without taking, conflicts between transmissions, into

consideration and these are effective in end-to-end real-time scheduling over wired

networks, but in WSNs e.g. in WirelessHART based networks transmission conflicts

are highly expected that can easily decrease the effectiveness of the communication

schedule. Moreover, different nodes experience different degree of conflicts as

different nodes have different number of neighbors in a routing graph. The gateway

and the nodes with high connectivity in the routing graph tend to experience

significantly higher degrees of conflicts. Hence, scheduling algorithms for

WirelessHART networks must be cognizant of conflicts between transmissions [2].

Based on this key insight into the WirelessHART networks, an efficient scheduling

policy called Conflict-aware Least Laxity First (C-LLF) have been developed. It uses

conflict-aware laxity of every released transmission as the decision variable. The

conflict-aware laxity of a transmission is determined by considering the length of time

windows in which the transmission must be scheduled as well as the potential

conflicts that the transmission may experience in these windows. That is, the approach

combines LLF and the degree of conflicts associated with a transmission. Thus, it can

schedule a transmission while the remaining ones are likely to retain the necessary

condition established in Theorem 2 (which states that for a set of flows F, let be the

set of unscheduled transmissions at slot s. If these transmissions are schedulable, then

[2]). Specifically, the algorithm identifies some critical

time windows in which too many conflicting transmissions have to be scheduled,

thereby determining the criticality of each released transmission. Criticality of a

transmission is quantified by its conflict-aware laxity. Transmissions exhibiting lower

conflict-aware laxity are assessed to be more critical. C-LLF gives the highest priority

to the transmissions exhibiting lower conflict-aware laxity [2].

2.4.3 Communication Schedule Defined in [1]

This scheduling mechanism is based on the routing graphs that are defined in [1]. This

scheduling technique allows multiple devices to compete for the retry links to the

Background

13

same device, and split the traffic from one device among all its successors, thus

reduces the bandwidth allocation on each of them. By designing the communication

schedules on the successors so that their combination has the same communication

pattern as the original device, the global communication schedule is then spliced into

sub-schedules and distributed to the corresponding devices. These sub-schedules work

together and guarantee that the periodic process/control data between devices and the

Gateway can be forwarded through multi-hops in a timely manner.

The design philosophy for constructing this communication schedule is to spread out

the channel usage in the network as much as possible and to apply the Fastest Sample

Rate First policy1 (FSRF) to schedule the devices’ periodic publishing and control

data.

1Fastest Sample Rate First Policy (FSRF) ~ Rate Monotonic.

Design and Implementation

14

3 Design and Implementation

It is clear that optimized scheduling plays a significant role in enhancing the

reliability, network throughput, latency, and delay etc. of an IWSN but there is still

need of actual industry requirements analysis more precisely what is currently

required in the industry. Thus before designing the scheduling algorithm a detailed

requirement analysis has also been carried out by mainly focussing on process

automation industry.

Table 3a Typical Requirements for IWSNs in Process Automation Domain [9]

The process automation industry has mainly three types of data that need to be

communicated over an IWSN i.e. monitoring and supervision, closed loop control,

and interlocking and control. The wireless communication requirements for these data

types are tabulated in Table 3a.

3.1 Problem Formulation

The scheduling problem is formulated by defining the following properties for an

industrial wireless sensor network.

3.1.1 Payload

The payload of packets can contain four types of data i.e.

Monitoring and Supervision

For this type of data transmission, only sensors are involved and no actuators involved

means that only uplink communication is expected. Monitoring and supervision

sensors would be the 30% of the total number of sensor nodes. The update rate for

these sensors would be greater or equal to 1 sec. Allowed data loss is 1 packet/sec and

in case of data loss the allowed delay is 250ms or less. This type of data transmission

has the lowest priority.

Closed Loop Control

For this type of data transmission both uplink and downlink communication is

involved. These sensors and actuators would be the 70% of the total number of sensor

nodes out of which 52% would be the sensors and 18% would be the actuators. The


15

update rate for these sensors would be greater or equal to 500ms. Allowed data loss

delay is 250ms or less. This type of data transmission has the middle priority.

Interlocking

This type of data transmission would be event driven and would involve the closed

loop control actuators. Allowed data loss delay is 100ms or less. This type of data

transmission has the highest priority.

Network Management

In this data the information of network configuration and management is involved that

will be broadcasted time to time as required by the network manager.

The properties of the above mentioned data types are summarized in Table 3.1a.

Table 3.1a Properties of IWSN Data Traffic

Data Type % of total

Nodes

Sensors Actuators Update

Frequency

Data Loss Delay Priority

Monitoring &

Supervision

30 30% 0% 1 sec or greater

1 packet/sec 250 ms 4th

Closed Loop

Control

70 52% 18% 500 ms or

greater

very low 250 ms 3rd

Interlocking - - 18% or

less

event driven ideally no 100 ms 2nd

Network

Management

100 82% 18% event driven - - 1st

3.1.2 Network Topology

Routing is done according to the Cell-based Mesh Network topology. A routing table

would be available for building the schedule which contains routing details and link

cost for each link. Update rate of each node is also appended in the routing table for

the purpose of building the schedule. Several routing tables for different scenarios

have been generated on the basis of experimentation and mathematical models of

Cell-based Mesh Networks which are presented in [11]. Each cell will have one

backbone router and it should have a maximum of 30 sensors nodes in total.

3.1.3 Superframe

Superframe is defined as a group of consecutive time-slots with a fixed period i.e. the

total time taken by these time-slots. For instance in case of WirelessHART each time-

slot is 10 ms long, so if we consider a superframe of 25 time-slots then the period of

the superframe would be 250 ms.


16

3.1.4 Scheduling

Time-Slots

In this scheduling solution four types of 10ms duration time-slots in the superframe

are considered i.e.

Dedicated

These time-slots will be assigned for closed loop data transmission.

Shared

These time-slots will be assigned for monitoring & supervision data and

retransmissions of closed loop data.

Reserved

These time-slots will be used for broadcast data and network management.

Unused

These time-slots can be assigned for inter-cell communication or for any other

purpose.

3.2 Scheduling Algorithm

The scheduling is designed in such a way that it will always guaranty the periodic

closed loop data transmissions and event-driven interlocking data transmissions; this

is accomplished by defining reserved and shared time-slots with a period of 100ms.

For instance if a data packet couldn’t reach its destination within the dedicated time-

slot then the network manager can notify other nodes in the upcoming reserved slot of

the superframe about the retransmission and a shared slot or an unused slot will be

assigned for this retransmission; and in the event of interlocking first the network

manager will notify all the nodes using the reserved slots and then notify the actuators

in question using a reserved slot, a shared slot, or an unused slot within 100ms.

It is assumed that each cell can use only two channels at the maximum and there

would be four types of time-slots i.e. dedicated, shared, reserved, and unused. One

superframe will be assigned to each channel in the cell and the period of each

superframe would be the maximum update rate from the closed loop sensors. Thus

each cell would have two superframes working at the same time. Following is given a

scheduling algorithm that can be used to build a schedule for one cell; same procedure

can be repeated for all the cells in the network. The steps of the algorithm are;

1. Find the maximum update rate out of the closed loop control sensors; this

would be the period of the superframes.

2. Extract information of all the nodes related to one cell say Cell#01 from the

routing table.


17

3. Take the first generation of sensors in the cell; these sensors are directly

connected to the backbone router. This can be done using the routing table.

4. Initialize the first superframe with period that was found in step-1 by assigning

the first time-slot and one time-slot after every 90ms as reserved slot in the

superframe. The reason for having these time-slots with a period of 100ms is

to cope with the stringent requirement of the event-driven interlocking data

transmission which must be transmitted within 100ms. These time-slots can

also be used to notify for any retransmissions of closed loop data.

5. Assign two time slots after every reserved slot as shared slot in the

superframe. Normally these shared slots will be used for monitoring and

supervision data but in case of packet loss these can be assigned for

retransmissions of closed loop data as required by the network manager.

6. After doing steps 4 and 5 we have actually created 100ms time-cells within the

superframe, and each time-cell contains reserved, dedicated, shared, and may

also contain unused slots.

7. Assign dedicated time slots to the closed loop sensors according to rate

monotonic priority assignment (Rate monotonic is used here because it is

optimal [13] and static policy). Also assign dedicated time slots to the

monitoring sensors that are in the route of closed loop sensors.

8. While assigning time-slots to the routing nodes make sure that each of its

closed loop sensor child nodes is assigned one time-slot for transmission1 from

the routing node to the backbone router or to the parent node in case of second

superframe. In case of second superframe an additional time-slot to each

closed loop sensor child node will also be assigned to complete the

communication link up till first generation.

9. For more optimization, group the sensors and actuators that belong to closed

loop control with same update rate. Assign time slots to the sensors first then

to the actuators with the same update rate (here it is assumed that there is no

delay for the control loop processing and transmission between backbone

router (BR) and the network manager/gateway). In case of the control loop

processing delay and BR – Gateway transmission delay; the actuators will be

assigned at least two time slots after the sensors of the same update rate (in

this way we can give room of 20ms for processing and BR – Gateway

transmission).


18

RES

ERV

ED

SHA

RED

SHA

RED

RES

ERV

ED

SHA

RED

SHA

RED

RES

ERV

ED

SHA

RED

SHA

RED

RES

ERV

ED

SHA

RED

SHA

RED

FIRST SUPERFRAME

SECOND SUPERFRAME

Figure 3.2a Basic structure of superframes in a cell

10. The first superframe is done.

1The word transmission is used just for explaining purpose, the schedule is valid for both uplink as well as downlink communication.

11. Now take all the remaining generations of sensors in the cell, and initialize the

second superframe by assigning two shared slots as done in the first

superframe but the reserved slots will come just after the shared slots in this

superframe. The reserved slots are assigned after the shared slots to make sure

that a packet transmitted in a reserved slot of superframe can be transmitted to

higher generations by a routing node of first generation using the reserved slot

of second superframe within 100ms.

12. Repeat steps 6 – 9 to get the second superframe ready.

This scheduling mechanism is optimized for maximum 3 hops since in a Cell-based

Mesh network the maximum number of hops in a cell is 3. This algorithm can be used

to build schedule for more than 3 hops but the event-driven interlocking data

transmissions would not be guaranteed within 100ms in that case. The basic structure

of the two superframes in a cell is depicted in Fig 3.2a.


19

3.2.1 Flow Diagram


20


21

BR

M2

A4 M5

S3

S6 S7

Generation 1

Generation 2

Generation 3

M8

Figure 3.2b The hierarchy of nodes in a cell

To analyze the above mentioned scheduling algorithm let’s take the example of a cell

presented in Fig 3.2b. BR is the backbone router through which every node in the cell

communicates with the network manager. M is for monitoring nodes, S is for closed

loop sensor nodes, and A is for closed loop actuator nodes. In first superframe S3 will

be allocated a dedicated time-slot, M8 will not be assigned dedicated time-slot since

it’s a monitoring sensor but M2 will be assigned 3 time-slots since it has three closed

loop control child nodes i.e. A4, S6, and S7. In the second superframe A4 will be

assigned only one time-slot but M5 will be assigned four time-slots out of which two

meant for transmissions from child nodes to M5 and two meant for transmissions

from M5 to parent node i.e. M2.

3.3 Implementation

The implementation of the algorithm is done using LabVIEW [15] development

environment. The application program for generating the schedule is based on a GUI.

The GUI (top level VI1) reads the routing table from a .csv file, then generates the

schedule by using the given scheduling algorithm and outputs the schedule in the form

of two superframes. The GUI of the application is shown in Fig 3.3a. 1VI (Virtual Instrument) refers to a program that is written using LabVIEW programming language.


22

Figure 3.3a GUI for Calculating the Schedule

The VI hierarchy of the GUI based application program is shown in Fig 3.3b. The VI

hierarchy presents the dependencies of VIs among each other. The top level VI calls

three subVIs, the first subVI reads the routing table from a given .csv file. The other

two subVIs are meant for calculating the two superframes. The superframes are output

in the form of 2-D arrays. Each row of the array defines one time-slot and the columns

in the row have information about that particular time-slot. The first column is ‘slot

type’, the second is ‘source node’, and the third is ‘destination node’. Six types of

time-slots are defined within the scope of application program, i.e.

Unused = 0 (numeric representation)

Reserved = 1

Shared = 2

Dedicated = 3

TxFailure = 4 (this time-slot can be used to keep track of transmission

failures)

Routing = 5 (this time-slot is used by nodes for broadcast data)


23

Figure 3.3b VI hierarchy of the GUI based Application Program

The application program has been tested by testing and analyzing each of its sub-part

(SubVI). Superframes for two different cells were generated for testing purpose, and

then analyzed by comparing with the proposed scheduling algorithm and the routing

table and found no errors. Thus, this application program is reliable for doing the

performance analysis of the proposed algorithm.

Performance Evaluation

24

4 Performance Evaluation

The real-time scheduling problem for a WirelessHART network is NP-hard [2], so

heuristic approach will be used to analyze the reliability of the proposed scheduling

algorithm. Performance evaluation is done by considering two case studies from the

industry. For each case study, first the details about the cells of the scenario in

question are presented then the schedule is presented in tabular form. Afterwards the

throughput is calculated using the following equation;

Throughput =

The transmission failure is predicted using Poisson process that is given by the

following equation [14].

The value of is set to 0.25 for transmission failures.

4.1 Case-1: Oil Industry

In this case the area in question is in which there are 60 Sensor nodes in

total. The area is further divided into two cells and each cell is covered by one

backbone router i.e. BR1 and BR2 as shown in Fig 4.1a. Each cell will be utilizing

two frequency channels. Normally each backbone router should be responsible for

routing data of 30 sensor nodes.

Thus, most of the time each cell will have;

30% of 30 equals to 9 monitoring and supervision sensor nodes.

52% of 30 equals to 15 closed loop control sensor nodes.

18% of 30 equals to 6 closed loop actuator nodes.

The routing information for this scenario is given in Table 4.1a. The table header is

pretty straight forward except ‘A, S, or M’ column, this is meant for describing the

type of the sensor node i.e. A for closed loop actuators presented by numeric 1, S for

closed loop sensors presented by numeric 2, and M for monitoring sensors presented

by numeric 3 in the table.


25

Figure 4.1a Distribution of sensor nodes in Case-1 [11]

Table 4.1a Routing table for Case-1

Node# Cell # Direct father

Generation# A, S, or M

Route cost

Update Period (ms)

9 1 36 2 2 2 500

10 1 67 2 3 2 1000

11 2 46 2 2 2 500

12 1 36 2 2 2 500

13 1 1 1 2 1 500

14 1 67 2 2 2 500

15 2 57 2 3 2 1000

16 1 1 1 2 1.02 500

17 2 60 2 2 2 500

18 2 2 1 2 1 500

19 2 43 2 2 2 500

20 2 28 2 1 2 500

21 1 48 2 2 2 500

22 2 2 1 3 1.12 1000

23 1 13 2 2 2 500

24 2 2 1 3 1 1000

25 1 64 3 2 3 500

26 2 28 2 3 2 1000

27 2 57 2 1 2 500

28 2 2 1 3 1 1000

29 1 48 2 2 2 500

30 2 2 1 3 1 1000

31 1 62 2 2 2 500

32 2 60 2 2 2 500

33 2 46 2 2 2 500


26

34 1 67 2 1 2 500

35 2 50 2 2 2 500

36 1 1 1 2 1 500

37 1 36 2 3 2 1000

38 1 62 2 1 2 500

39 2 60 2 1 2 500

40 2 18 2 2 2 500

41 1 48 2 3 2 1000

42 2 2 1 1 1 500

43 2 2 1 2 1 500

44 1 1 1 2 1 500

45 1 1 1 3 1.29 1000

46 2 2 1 2 1 500

47 1 1 1 1 1 500

48 1 1 1 2 1 500

49 1 16 2 1 2.02 500

50 2 2 1 2 1 500

51 2 60 2 1 2 500

52 1 67 2 3 2 1000

53 2 22 2 3 2.12 1000

54 2 50 2 3 2 1000

55 1 62 2 1 2 500

56 1 13 2 3 2 1000

57 2 2 1 2 1 500

58 2 50 2 2 2 500

59 1 67 2 3 2 1000

60 2 2 1 2 1 500

61 1 16 2 3 2.02 1000

62 1 1 1 1 1 500

63 2 30 2 3 2 1000

64 1 36 2 2 2 500

65 2 28 2 2 2 500

66 1 36 2 2 2 500

67 1 1 1 3 1 1000

68 2 50 2 1 2 500


27

4.1.1 Schedule for Cell-1

A detailed picture of the links in Cell-1 is shown in Fig 4.1b. After having all the

details about the Cell it’s time to build a schedule for it. The schedule is generated

using the GUI application program described in the previous chapter.

Figure 4.1b The spanning tree of Cell-1 of Case-1 [11]

The schedule is for Cell-1 is presented in Table 4.1b in the form of two superframes.

The first superframe will work in channel no. 1 and the second will work in channel

no. 2 of the cell.

Table 4.1b Schedule for Cell-1

Slot No.

SUPERFRAME 1 SUPERFRAME 2

Slot Type

Source Node Destination Node

Slot Type


1 1 0 0 3 9 36

2 2 0 0 2 0 0

3 2 0 0 2 0 0

4 3 44 1 1 0 0

5 3 13 1 3 12 36

6 3 13 1 3 14 67

7 3 16 1 3 21 48

8 3 16 1 3 23 13

9 3 36 1 3 29 48

10 3 36 1 3 31 62

11 1 0 0 3 34 67

12 2 0 0 2 0 0

13 2 0 0 2 0 0

14 3 47 1 1 0 0


28

15 3 36 1 3 38 62

16 3 36 1 3 49 16

17 3 36 1 3 55 62

18 3 36 1 3 25 64

19 3 48 1 3 64 36

20 3 48 1 3 64 36

21 1 0 0 3 66 36

22 2 0 0 2 0 0

23 2 0 0 2 0 0

24 5 0 0 1 0 0

25 3 48 1 0 0 0

26 3 62 1 0 0 0

27 3 62 1 0 0 0

28 3 62 1 0 0 0

29 3 62 1 0 0 0

30 3 67 1 0 0 0

31 1 0 0 0 0 0

32 2 0 0 2 0 0

33 2 0 0 2 0 0

34 5 0 0 1 0 0

35 3 67 1 0 0 0

36 0 0 0 0 0 0

37 0 0 0 0 0 0

38 0 0 0 0 0 0

39 0 0 0 0 0 0

40 0 0 0 0 0 0

41 1 0 0 0 0 0

42 2 0 0 2 0 0

43 2 0 0 2 0 0

44 5 0 0 1 0 0

45 0 0 0 0 0 0

46 0 0 0 0 0 0

47 0 0 0 0 0 0

48 0 0 0 0 0 0

49 0 0 0 0 0 0

50 0 0 0 0 0 0


29

The performance of the generated schedule can be analyzed from the plots presented

in Fig 4.1c and Fig 4.2d. In Fig 4.1c the successful packet transmissions are plotted

against required time-slots for these transmissions. In Fig 4.1d the Cell’s throughput is

plotted against required time-slots for the successful transmissions.

Figure 4.1c Successfully Transmitted Packets in Cell-1 of Case-1

Figure 4.1d Throughput in Cell-1 of Case-1


A detailed picture of the links in Cell-2 is shown in Fig 4.1e. The schedule for Cell-2

is given in Table 4.1c and the results are shown in Fig 4.1f and Fig 4.1g.


30

Figure 4.1e The spanning tree of Cell-2 in Case-1 [11]

Table 4.1c Schedule for Cell-2

Slot No.


Slot Type


Slot Type


1 1 0 0 3 11 46

2 2 0 0 2 0 0

3 2 0 0 2 0 0

4 3 42 2 1 0 0

5 3 18 2 3 17 60

6 3 18 2 3 19 43

7 3 28 2 3 27 57

8 3 28 2 3 32 60

9 3 43 2 3 20 28

10 3 43 2 3 33 46

11 1 0 0 3 35 50

12 2 0 0 2 0 0

13 2 0 0 2 0 0

14 5 0 0 1 0 0

15 3 46 2 3 39 60

16 3 46 2 3 40 18

17 3 46 2 3 51 60

18 3 50 2 3 65 28


31

19 3 50 2 0 0 0

20 3 50 2 0 0 0

21 1 0 0 3 58 50

22 2 0 0 2 0 0

23 2 0 0 2 0 0

24 5 0 0 1 0 0

25 3 50 2 0 0 0

26 3 57 2 3 68 50

27 3 57 2 0 0 0

28 3 60 2 0 0 0

29 3 60 2 0 0 0

30 3 60 2 0 0 0

31 1 0 0 0 0 0

32 2 0 0 2 0 0

33 2 0 0 2 0 0

34 5 0 0 1 0 0

35 3 60 2 0 0 0

36 3 60 2 0 0 0

37 0 0 0 0 0 0

38 0 0 0 0 0 0

39 0 0 0 0 0 0

40 0 0 0 0 0 0

41 1 0 0 0 0 0

42 2 0 0 2 0 0

43 2 0 0 2 0 0

44 5 0 0 1 0 0

45 0 0 0 0 0 0

46 0 0 0 0 0 0

47 0 0 0 0 0 0

48 0 0 0 0 0 0

49 0 0 0 0 0 0

50 0 0 0 0 0 0


32

Figure 4.1f Successfully Transmitted Packets in Cell-2 of Case-1

Figure 4.1g Throughput in Cell-2 of Case-1

4.2 Case-2: Paper and Pulp Industry

In this case the area in question is in which there are 100 Sensor nodes

in total. The area is further divided into three cells and each cell is covered by one

backbone router i.e. BR1, BR2, and BR3 as shown in Fig 4.2a. Each cell will be

utilizing two frequency channels. Normally each backbone router should be

responsible for routing data of 30 sensor nodes but in this case Cell-1, Cell-2 and

Cell-3 have 29, 42, and 29 nodes respectively. To focus on the results of the generated

schedule, the schedule tables for each Cell in this case are given in Appendix B. For

each cell first the distribution the nodes is depicted then result plots are shown.


33

The routing information for this scenario is given in Table 4.2a.

Figure 4.2a Distribution of sensor nodes in Case-2 [11]

Table 4.2a Routing table for Case-2

Node# Cell # Direct father

Generation# A, S, or M

Route cost

Update Period (ms)

9 2 17 2 2 2 500

10 1 104 2 2 2 500

11 3 90 2 2 2 500

12 2 91 2 3 2 1000

13 1 1 1 3 1 1000

14 3 71 3 3 3 1000

15 2 82 2 1 2 500

16 2 108 2 2 2 500

17 2 2 1 1 1 500

18 2 92 2 1 2 500

19 1 1 1 1 1 500

20 3 86 3 2 3 500

21 2 108 2 3 2 1000

22 1 19 2 3 2 1000

23 3 93 2 3 2 1000

24 2 2 1 2 1 500

25 1 19 2 2 2 500

26 2 100 2 2 2 500

27 2 57 2 3 2 1000

28 1 1 1 1 1 500

29 3 66 2 2 2.19 500

30 3 93 2 1 2 500

31 1 28 2 2 2 500


34

32 2 91 2 1 2 500

33 2 84 3 2 3 500

34 3 105 3 2 3 500

35 3 3 1 3 1 1000

36 1 1 1 2 1 500

37 1 52 2 3 2 1000

38 3 63 2 2 2 500

39 2 2 1 2 1 500

40 1 1 1 3 1 1000

41 2 108 2 3 2 1000

42 2 39 2 3 2 1000

43 1 19 2 1 2 500

44 1 36 2 3 2 1000

45 1 19 2 3 2 1000

46 1 19 2 2 2 500

47 1 13 2 2 2 500

48 2 91 2 2 2 500

49 1 104 2 1 2 500

50 1 62 2 2 2.07 500

51 2 103 3 2 3 500

52 1 1 1 3 1 1000

53 2 16 3 2 3 500

54 2 39 2 2 2 500

55 3 63 2 2 2 500

56 3 63 2 3 2.47 1000

57 2 2 1 3 1 1000

58 2 108 2 3 2 1000

59 3 93 2 2 2 500

60 2 2 1 2 1 500

61 2 91 2 2 2 500

62 1 1 1 2 1.07 500

63 3 3 1 2 1 500

64 1 94 2 3 2 1000

65 2 82 2 2 2 500

66 3 3 1 3 1.19 1000

67 1 13 2 2 2 500

68 3 78 2 2 2 500

69 1 52 2 1 2 500

70 3 90 2 3 2 1000

71 3 93 2 3 2 1000

72 2 82 2 2 2 500

73 1 104 2 2 2 500

74 1 13 2 3 2 1000

75 2 108 2 1 2 500


35

76 1 28 2 2 2 500

77 2 88 3 2 3 500

78 3 3 1 2 1 500

79 3 55 3 2 3 500

80 2 92 2 3 2 1000

81 2 108 2 2 2 500

82 2 2 1 2 1 500

83 3 106 2 1 2 500

84 2 82 2 2 2 500

85 2 15 3 3 3 1000

86 3 35 2 1 2 500

87 3 78 2 2 2 500

88 2 24 2 2 2 500

89 1 1 1 2 1.03 500

90 3 3 1 3 1 1000

91 2 2 1 2 1 500

92 2 2 1 3 1 1000

93 3 3 1 3 1 1000

94 1 1 1 2 1 500

95 2 91 2 3 2 1000

96 2 80 3 3 3 1000

97 3 106 2 1 2 500

98 3 106 2 2 2 500

99 2 57 2 2 2 500

100 2 2 1 1 1 500

101 3 93 2 2 2 500

102 1 19 2 2 2 500

103 2 91 2 1 2 500

104 1 1 1 2 1 500

105 3 93 2 1 2 500

106 3 3 1 2 1 500

107 2 91 2 2 2 500

108 2 2 1 3 1 1000


A detailed picture of the links in Cell-1 is shown in Fig 4.2b and the results are shown

in Fig 4.2c and Fig 4.2d.


36

Figure 4.2b The spanning tree of Cell-1 in Case-2 [11]

Figure 4.2c Successfully Transmitted Packets in Cell-1 of Case-2


37

Figure 4.2d Throughput in Cell-1 of Case-2


A detailed picture of the links in Cell-2 is shown in Fig 4.2e and the results are shown

in Fig 4.2f and Fig 4.2g.

Figure 4.2e The spanning tree of Cell-2 in Case-2 [11]


38

Figure 4.2f Successfully Transmitted Packets in Cell-2 of Case-2

Figure 4.2g Throughput in Cell-2 of Case-2


39


A detailed picture of the links in Cell-3 is shown in Fig 4.2h and the results are shown

in Fig 4.1i and Fig 4.1j.

Figure 4.2h The spanning tree of Cell-3 in Case-2 [11]

Figure 4.2i Successfully Transmitted Packets in Cell-3 of Case-2


40

Figure 4.2j Throughput in Cell-3 of Case-2

Throughput is around 0.8 for different number of transmissions in almost every cell of

the presented case studies. Thus the proposed scheduling mechanism is suitable for

these industrial scenarios. Interestingly it also worked fine when the number of nodes

was 42 in Cell-2 of Case-2 and the successful packet transmissions crossed 400 by

utilizing 600 time-slots.

Conclusions and Future Work

41

5 Conclusions and Future Work

5.1 Conclusion

The results calculated in the previous chapter have proven that the proposed

mechanism is workable for different industrial scenarios by keeping the prominent

feature of transmitting interlocking data within 100ms. If we analyze the delays in the

different cases, we will reach the point that the delay is always less than or equal to

maximum update period of the closed loop control data i.e. the period of the

superframes. Latency is also less than or equal to the period of superframes.

It has been concluded that the proposed scheduling algorithm should be adopted for

the Cell-Based Mesh Networks, to have meet the stringent timing requirements of

IWSNs.

5.2 Future Work

Dynamic simulations of the proposed scheduling algorithm would be a beneficial

future work before experimenting it on a real-time test bed. Since both the Cell-Based

Mesh Network topology and the scheduling mechanism for it, are recently developed,

so it would be really handy if we develop a simulator that can do real-time simulations

and also analyze dynamic behavior of the network.

I had given it a try during my thesis work to develop a basis for such simulator using

National Instruments’ LabVIEW [15] development environment because using

LabVIEW real-time, we can develop real-time applications that can run on a real-time

target and the development time is considerably low as compared to other

programming languages. A GUI has been designed in an effort to start working on it

but due to shortage of time and more emphasis on developing a reliable scheduling

mechanism, it could not be completed. The designed GUI is shown in Fig 5.2a.

Figure 5.2a GUI Design for a Simulator

These are some of the efforts towards achieving the actual goal i.e. to develop

infrastructure for a reliable and secure Industrial Wireless Sensor Network.

References

42

6 References

[1] Song Han, Xiuming Zhu, Aloysius K. Mok, Deji Chen, Mark Nixon. “Reliable

and Real-time Communication in Industrial Wireless Mesh Networks”. The

University of Texas at Austin, Department of Computer Sciences. Report #TR-10-

34 (regular tech report). October 7th, 2010.

[2] Abusayeed Saifulah, Chenyang Lu, You Xu, and Yixin Chen, “Real-time

scheduling for WirelessHART networks”, in RTSS, 2010.

[3] Deji Chen, Mark Nixon, and Aloysius Mok. (2010) “WirelessHART™, Real-

Time Mesh Network for Industrial Automation”, Springer. ISBN 978-1-4419-

6046-7.

[4] “WirelessHART”,

http://www.hartcomm.org/protocol/wihart/wireless_technology.html (Acc. June

2011).

[5] Gabriella Fiore, Valeria Ercoli, Alf J. Isaksson, Krister Landernäs, and Maria

Domenica Di Benedetto, “Multihop multi-channel scheduling for wireless control

in WirelessHART networks”, in ETFA, 2009.

[6] Pablo Soldati, Haibo Zhang, and Mikael Johansson, “Deadline-constrained

transmission scheduling and data evacuation in wirelessHART networks”, in

Technical Report TRITA-EE 2008:060, 2008.

[7] Haibo Zhang, Pablo Soldati, and Mikael Johansson, “Optimal link scheduling and

channel assignment for convergecast in linear wirelessHART networks”, in

Technical Report TRITA-EE 2009:018, 2009.

[8] “Wireless Sensor Network”,

http://en.wikipedia.org/wiki/Wireless_sensor_network (Acc. November 2011).

[9] Johan Åkerberg, Mikael Gidlund, and Mats Björkman, “Future Research

Challenges in Wireless Sensor and Actuator Networks Targeting Industrial

Automation”, 2011.

[10] ISA100 http://www.isa.org/isa100 (Acc. Nov 2011).

[11] Yinchun Shen, “A Simulation Study of Cell-Based Mesh Network Topology and

Routing for Industrial Wireless Applications”, Master Thesis, Jönköping

University, December 2011.

[12] Youzhi Xu, Mikael Gidlund, Dong Yang, Wei Shen, and Tingting, “Robust

Routing in Cell-Based Mesh Networks”, Invention Disclosure Document, January

2011.

[13] Joel Goossens. “Scheduling of Hard Real-Time Periodic Systems with Various

Kinds of Deadline and Offset Constraints”. PhD Thesis, Universite Libre de

Bruxelles, 1999.

[14] Poisson Process, http://en.wikipedia.org/wiki/Poisson_process (Acc. November

2011).

[15] “LabVIEW Development Environment of National Instruments”,

http://www.ni.com/labview (Acc. November 2011).

[16] “Zigbee”, http://www.zigbee.org (Acc. November 2011).

References

43

[17] HART Communication Foundation, “TDMA Data Link Layer Specification”,

HCF_SPEC-075, Revision 1.1, May 17, 2008.

[18] HART Communication Foundation, “2.4GHz DSSS O-QPSK Physical Layer

Specification”, HCF_SPEC-065, Revision 1.0, September 01, 2007.

Appendices

44

7 Appendices

7.1 Appendix A State-of-the-Art Network Topologies

7.1.1 Star Topology

A typical star network topology is shown in Fig 7.1a. In a star network all the field

devices are connected directly to the gateway. It offers lower latency, simple

architecture and higher reliability.

M2

A4

M5

S3

S6

S7

M8

GATEWAY

Fig 7.1a Star Topology

7.1.2 Mesh Topology

In case of a mesh topology network all the nodes in the network must have the

capability to source and sink packets. The communication link is completed up till the

gateway by some of the nodes in the network that are directly connected to the

gateway. Mesh topology offers large area coverage but at the cost of increased no. of

hops which adds complexity to the routing and scheduling algorithms, lower

reliability, and higher latency.

7.1.3 Star-Mesh Topology

In case of star-mesh topology the network is divided into clusters and each cluster has

a head which is directly connected to the gateway. This topology is efficient than

mesh topology since it has lower no. of hops by the use of cluster heads, although the

failure of the cluster head will make the whole cluster fail. A typical star-mesh

network topology is shown in Fig 7.1b.

Appendices

45

Fig 7.1b Star-Mesh Topology

7.1.4 Hub-and-Spoke Topology

Hub-and-spoke topology is similar to star-mesh topology but in hub-and-spoke

topology only backbone routers can be cluster heads as compared to star-mesh

topology where any node can become a cluster head [12].

7.2 Appendix B Schedule Tables of Case-2

Table 7.2a Schedule for Cell-1

Slot No.


Slot Type


Slot Type


1 1 0 0 3 10 104

2 2 0 0 2 0 0

3 2 0 0 2 0 0

4 3 89 1 1 0 0

5 3 13 1 3 25 19

6 3 13 1 3 31 28

7 3 19 1 3 47 13

8 3 19 1 3 49 104

9 3 19 1 3 50 62

10 3 19 1 3 67 13

11 1 0 0 3 43 19

12 2 0 0 2 0 0

13 2 0 0 2 0 0

14 5 0 0 1 0 0

Appendices

46

15 3 19 1 3 69 52

16 3 28 1 3 46 19

17 3 28 1 3 73 104

18 3 28 1 3 102 19

19 3 36 1 3 76 28

20 3 52 1 0 0 0

21 1 0 0 0 0 0

22 2 0 0 2 0 0

23 2 0 0 2 0 0

24 5 0 0 1 0 0

25 3 62 1 0 0 0

26 3 62 1 0 0 0

27 3 94 1 0 0 0

28 3 104 1 0 0 0

29 3 104 1 0 0 0

30 3 104 1 0 0 0

31 1 0 0 0 0 0

32 2 0 0 2 0 0

33 2 0 0 2 0 0

34 5 0 0 1 0 0

35 3 104 1 0 0 0

36 0 0 0 0 0 0

37 0 0 0 0 0 0

38 0 0 0 0 0 0

39 0 0 0 0 0 0

40 0 0 0 0 0 0

41 1 0 0 0 0 0

42 2 0 0 2 0 0

43 2 0 0 2 0 0

44 5 0 0 1 0 0

45 0 0 0 0 0 0

46 0 0 0 0 0 0

47 0 0 0 0 0 0

48 0 0 0 0 0 0

49 0 0 0 0 0 0

50 0 0 0 0 0 0

Appendices

47

Table 7.2b Schedule for Cell-2

Slot No.


Slot Type


Slot Type


1 1 0 0 3 9 17

2 2 0 0 2 0 0

3 2 0 0 2 0 0

4 3 60 2 1 0 0

5 3 17 2 3 15 82

6 3 17 2 3 16 108

7 3 24 2 3 53 16

8 3 24 2 3 16 108

9 3 24 2 3 18 92

10 3 39 2 3 26 100

11 1 0 0 3 32 91

12 2 0 0 2 0 0

13 2 0 0 2 0 0

14 5 0 0 1 0 0

15 3 39 2 3 48 91

16 3 57 2 3 54 39

17 3 82 2 3 61 91

18 3 82 2 3 75 108

19 3 82 2 3 81 108

20 3 82 2 3 33 84

21 1 0 0 3 65 82

22 2 0 0 2 0 0

23 2 0 0 2 0 0

24 5 0 0 1 0 0

25 3 82 2 3 88 24

26 3 82 2 3 77 88

27 3 91 2 3 72 82

28 3 91 2 3 84 82

29 3 91 2 3 84 82

30 3 91 2 3 88 24

31 1 0 0 3 99 57

32 2 0 0 2 0 0

33 2 0 0 2 0 0

34 5 0 0 1 0 0

35 3 91 2 3 51 103

36 3 91 2 0 0 0

37 3 91 2 0 0 0

Appendices

48

38 3 92 2 3 103 91

39 3 100 2 3 103 91

40 3 100 2 3 107 91

41 1 0 0 0 0 0

42 2 0 0 2 0 0

43 2 0 0 2 0 0

44 5 0 0 1 0 0

45 3 108 2 0 0 0

46 3 108 2 0 0 0

47 3 108 2 0 0 0

48 3 108 2 0 0 0

49 0 0 0 0 0 0

50 0 0 0 0 0 0

Table 7.2c Schedule for Cell-3

Slot No.


Slot Type


Slot Type


1 1 0 0 3 11 90

2 2 0 0 2 0 0

3 2 0 0 2 0 0

4 5 0 0 1 0 0

5 3 35 3 3 29 66

6 3 35 3 3 30 93

7 3 63 3 3 79 55

8 3 63 3 3 59 93

9 3 63 3 3 68 78

10 3 63 3 3 83 106

11 1 0 0 3 38 63

12 2 0 0 2 0 0

13 2 0 0 2 0 0

14 5 0 0 1 0 0

15 3 66 3 3 55 63

16 3 78 3 3 55 63

17 3 78 3 3 86 35

18 3 78 3 3 20 86

19 3 90 3 3 86 35

20 3 93 3 3 87 78

21 1 0 0 3 97 106

22 2 0 0 2 0 0

23 2 0 0 2 0 0

24 5 0 0 1 0 0

Appendices

49

25 3 93 3 3 98 106

26 3 93 3 3 34 105

27 3 93 3 0 0 0

28 3 93 3 0 0 0

29 3 106 3 3 101 93

30 3 106 3 3 105 93

31 1 0 0 3 105 93

32 2 0 0 2 0 0

33 2 0 0 2 0 0

34 5 0 0 1 0 0

35 3 106 3 0 0 0

36 3 106 3 0 0 0

37 0 0 0 0 0 0

38 0 0 0 0 0 0

39 0 0 0 0 0 0

40 0 0 0 0 0 0

41 1 0 0 0 0 0

42 2 0 0 2 0 0

43 2 0 0 2 0 0

44 5 0 0 1 0 0

45 0 0 0 0 0 0

46 0 0 0 0 0 0

47 0 0 0 0 0 0

48 0 0 0 0 0 0

49 1 0 0 3 11 90

50 2 0 0 2 0 0

mac scheduling in industrial wireless cell-based mesh

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