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Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

1 Wireless Sensor Network 3

1.1 Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1.1 Topologies . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1.2 Metrics and constrains . . . . . . . . . . . . . . . . . . 6

1.2 Systems Challenge . . . . . . . . . . . . . . . . . . . . . . . . 7

1.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.4 Industrial application . . . . . . . . . . . . . . . . . . . . . . . 10

1.4.1 Physical network topology and logical topology . . . . 11

1.4.2 Traffic characteristic and metric . . . . . . . . . . . . . 12

2 IEEE 802.15.4 and ZigBee 14

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2 IEEE 802.15.4 features . . . . . . . . . . . . . . . . . . . . . . 15

2.3 WPAN Device Architecture . . . . . . . . . . . . . . . . . . . 17

2.3.1 IEEE 802.15.4 PHY . . . . . . . . . . . . . . . . . . . 18

2.3.2 IEEE 802.15.4 MAC . . . . . . . . . . . . . . . . . . . 19

2.3.3 Data Transfer model . . . . . . . . . . . . . . . . . . . 23

2.4 ZigBee routing . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.5 ZigBee upper layers . . . . . . . . . . . . . . . . . . . . . . . . 26

3 Routing Protocol 28

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2 The RPL protocol . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.3 DIO transmission and elegibility . . . . . . . . . . . . . . . . . 30

3.4 Data structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.5 Objective Code Points and Objective Function . . . . . . . . . 33

3.6 DAG discovery rules . . . . . . . . . . . . . . . . . . . . . . . 34

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3.7 Candidate DAG Parent States and Stability . . . . . . . . . . 35

3.8 Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4 RPL Network Simulator 37

4.1 OMNeT++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2 Mobility Framework . . . . . . . . . . . . . . . . . . . . . . . 38

4.2.1 Nodes structure . . . . . . . . . . . . . . . . . . . . . 39

4.2.2 Communication between layer . . . . . . . . . . . . . . 40

4.2.3 Network implementation . . . . . . . . . . . . . . . . . 42

4.2.4 Network parameters . . . . . . . . . . . . . . . . . . . 46

4.3 How to improve network stability . . . . . . . . . . . . . . . . 48

5 Results 50

5.1 Simultion results . . . . . . . . . . . . . . . . . . . . . . . . . 50

6 Conclusion 61

7 Appendix 63

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List of Figures

1.1 A wireless sensor networks . . . . . . . . . . . . . . . . . . . . 3

1.2 Typical industrial topology . . . . . . . . . . . . . . . . . . . . 112.1 ZigBee protocol stack . . . . . . . . . . . . . . . . . . . . . . . 15

2.2 ZigBee network topology . . . . . . . . . . . . . . . . . . . . . 16

2.3 IEEE 802.15.4 architecture . . . . . . . . . . . . . . . . . . . . 18

2.4 IEEE 802.15.4 superframe . . . . . . . . . . . . . . . . . . . . 20

2.5 CSMA/CA algorithm . . . . . . . . . . . . . . . . . . . . . . . 22

2.6 Communication from a device to a coordinator in a nonbeacon-

enabled network . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.7 Communication from a device to a coordinator in a beacon-

enabled network . . . . . . . . . . . . . . . . . . . . . . . . . . 242.8 Communication from a coordinator to a device in a beacon-

enabled network . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.9 communication from a coordinator to a device in a nonbeacon-

enabled network . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.1 OMNet++ module concept . . . . . . . . . . . . . . . . . . . 37

4.2 Simulator node stack . . . . . . . . . . . . . . . . . . . . . . . 39

5.1 nodes connected by the DAG . . . . . . . . . . . . . . . . . . 51

5.2 Experimental latency without network DIO updating . . . . . 515.3 Busy channel probability . . . . . . . . . . . . . . . . . . . . . 52

5.4 Empirical latency without network DIO updating . . . . . . . 52

5.5 Analytical latency . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.6 Best parent selected by the node number three . . . . . . . . . 54

5.7 Logical Topology variation . . . . . . . . . . . . . . . . . . . . 54

5.8 variation of varying as a function of the traffic . . . . . . . 55

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5.9 Consequence of the second method to alleviate the jitter of

the latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565.10 Best parent selected by the node three . . . . . . . . . . . . . 56

5.11 latency end to end with and without the method to alleviate

the jitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

5.12 consequence of the method on the mean and variance of the

latency end to end varying the packet rate generation . . . . . 58


latency end to end varying the number of nodes . . . . . . . . 59


latency end to end varying the transmitted power . . . . . . . 59

7.1 Markov chain model . . . . . . . . . . . . . . . . . . . . . . . 64

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Chapter 1

Wireless Sensor Network

In this chapter we give an overview on wireless sensor networks (WSNs). The

basic characteristic are summarized with particular reference to the commu-

nication protocol stack and applications of WSNs [1]. A Wireless sensor

network is composed by a large number of devices called nodes, each node is

able to share information with other devices and can cooperate to perform

advanced, control, communication and signal processing tasks. Thanks to

the availability of low cost, low power, and miniature embedded processors,

radios, and sensors, integrated on a single chip, the use of WSN is increasingvery fast.

Figure 1.1: A wireless sensor networks

The individual devices in a wireless sensor network are inherently resource

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CHAPTER 1. WIRELESS SENSOR NETWORK 4

constrained: they have limited processing speed, storage capacity, and com-

munication bandwidth, the node of a WSN must operate for long periods oftime. Since the node are wireless,minimizing the energy consumption is very

important. The nodes hardware components should be turned off most of

the time. Most of the circuits can be powered off, with a standby power of

about one microwatt. If such a device is active 1 percent of the time, its

average power consumption is just a few microwatts.

Each node in a sensor network is typically equipped with a radio transceiver

or other wireless communications devices, a small microcontroller, and an

energy source, usually a battery. In simple microcontrollers, miniaturization

increases efficiency rather than adding functionality, allowing them to oper-ate near one milliwatt while running at about 10 Mhz. This scale of power

can be obtained in many ways. Solar cells generate about 10 milliwatts per

square centimeter outdoors and 10 to 100 microwatts per square centimeter

indoors. Mechanical sources of energy, such as the vibration of windows and

air conditioning ducts, can generate about 100 microwattslow-power micro-

processors have limited storage, typically less than 10 Kbytes of RAM for

data and less than 100 Kbytes of ROM for program storage-or about 10,000

times less storage capacity than a PC has. This limited amount of memory

consumes most of the chip area and much of the power budget. Designers

typically incorporate larger amounts of flash storage, perhaps a megabyte,

on a separate chip.

The sensor purpose is measuring changes over a certain range, when the

parameters measured change, there is a voltage variation that is translated

into a binary number that may be stored or processed, and is possible to

do this with few milliwant and turning on the device on a fraction of the

time. Hance the nodes are equipped with extremely efficient Analogical to

Digital Converters (ADCs) that have an energy profile similar to the proces-

sor. Once measures are converted into bits, the information is transmitted

to other nodes, The amount of energy required to communicate wirelesslyincreases rapidly with distance, for these reasons for small devices to cover

long distances, the network must route the information hop by hop through

nodes.

However communicate with other nodes is the most expensive operation in

WNS. Each node has one or more sensing unit, and they can act as informa-

tion sources, sensing and collecting data samples from their environment. So

each sensor supports a multi-hop routing algorithm, creating multi-hop wire-


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less networks that convey data samples to other sensor nodes. Nodes can also

act as information sinks, receiving dynamic configuration information fromother nodes or external entities. Between the various nodes that form the

network you can find base stations that are one or more distinguished compo-

nents of the WSN that are equipped with much more computational, energy

and communication resources.

1.1 Main features

The resources of a wireless sensor network, while there are many activities

such as sampling sensors, processing, and streaming data that need to be

served. The network must find the best path to route information from

source to destination. The nodes abstract the physical hardware, while the

network involves a more complex protocol stack divided into layers. The

lower levels deal with the radio channel and transmit data frames. The sec-

ond layer of the stack performs error coding and channel scheduling, as well

as detecting the arrival of incoming packets and processing them into input

buffers.

A typical application that runs at the top level of the protocol stack shouldreceive and process a stream of the sensor readings, and then deliver impor-

tant notifications to the network. A second component would receive this

notification messages, maintain a routing structure, and retransmit them

along the next hop in a route to a data collection gateway. So the informa-

tion moves hop by hop along a route from the point of production to the

point of use, each node has a radio that provides a set of communication

links to nearby nodes. Reading the information received, nodes can discover

their neighbors and create a routing algorithm according to the applications

need. So it is important to determine connectivity variables to manage thenetwork and discover and adapt the network to the environment conditions.

1.1.1 Topologies

Wireless sensor networks can assume different topologies [2]. We can distin-

guish three main categories:


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Star Network A single node is used as a base station, so it can send and

receive message from other nodes. This other nodes just send and re-ceive a message from the base station and they cannot send messages to

each other. The advantage of this type of network is its simplicity, the

low power consumption and the low latency. The disadvantage of such

a network is that the base station must be within radio transmission

range of all the individual nodes, hence the network cannot be a large

one.

Mesh Network A mesh network allows for any node in the network to

transmit to any other node in the network that is within its radio

transmission range. A node wishing to send a message to a device

that is not in its communication range can use an intermediate node.

This network topology has the advantage of redundancy and scalability.

The network could be expanded adding other nodes, the disadvantage

of this type of network are the power consumption and the delivery

time increment.

Hybrid Star - Mesh Network In this network topology, the lowest power

sensor nodes are not enabled with the ability to forward messages. This

allows for minimal power consumption to be maintained. However,other nodes on the network are enabled with multihop capability, al-

lowing them to forward messages from the low power nodes to other

nodes on the network. This is the topology implemented by the up and

coming mesh networking standard known as ZigBee.

1.1.2 Metrics and constrains

Now we explore the evaluation metrics that will be used to evaluate a wireless

sensor network, the most important metrics for wireless sensor networks are

lifetime, coverage, cost, response time, temporal accuracy, security, and effec-tive sample rate, besides we have to consider that this metrics are correlated

to each other [3], [?]. Often it may be necessary to decrease performance in

one metric, such as sample rate, in order to increase another, such as life-

time. These metrics are used to describe the capabilities and performance of

a wireless sensor network.

It is essential that the nodes should operate for years so sensor nodes must be

low-power. If we use nodes in different scenarios, a wireless sensor network


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architecture must be flexible enough to accommodate a wide range of appli-

cation behaviors. In order to support the lifetime requirements demanded,each node must be constructed to be as robust as possible. In a typical de-

ployment, hundreds of nodes will have to work for years. To achieve this, the

system must be constructed so that it can tolerate and adapt to individual

node failure. Additionally, each node must be designed to be as robust as

possible. For these reasons we have also to establish set of metrics that will

be used to evaluate the performance of a device in a sensor network like com-

munication rate, power consumption, and range. The communication rate

also has a significant impact on node performance; higher communication

rates translate into the ability to achieve higher effective sampling rates andlower network power consumption. The transmission range has a significant

impact on the minimal acceptable node density. If the nodes are positioned

too far, is impossible create a network with a enough redundancy to maintain

a high level of reliability. So metrics could be very important to extend the

lifetime oh a WSN.

1.2 Systems Challenge

As mentioned WNS resources are restricted and there are multiple current ac-tivity such as sampling sensors, processing, and streaming data. The network

must find the best interconnection between nodes and routed information ef-

fectively from where it is produced to where it is used. Bur there are many

obstacles that decrement the network efficiency.

Node connection

The lowest layer of the protocol stack in WSN controls the physical radio

device. When one node transmits a signal, a set of other nodes can receive

the signal unless it is possible to distinguish it from other transmissions at

the same time. The link layer controls the channel and transmits only if the

channel is clear. If nodes dont transmit, they sample the channel to research

special symbol at the start of a packet that allows the node to synchronize.

The packet layer manages buffers, schedules packets onto the radio, detects or

even corrects errors, handles packet losses, and dispatches packets to system

or application components.


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Communication between nodes

Many protocols were developed to manage the connection between nods, one

of this is the flooding protocol in which a root node broadcasts a packet

with some identifying information. Receiving nodes retransmit the packet so

that more distant nodes can receive it. However, a node can receive different

versions of the same message from several neighboring nodes, so the network

uses the identifying information to detect and suppress duplicates. There are

many techniques to avoid connection and minimize redundant transmission.

One tool to determinate a route is dissemination. Each packet identifies the

transmitter and its distance from the root. To form a distributed tree, nodes

record the identity of a node closer to the root the network can use this

reverse communication tree for data collection by routing data back to the

root or for data aggregation by processing data at each level of the tree, nodes

may learn of potential parents by overhearing data messages. The network

continually collects statistics to reinforce the best routes. In WSN more

nodes participate in the communications, and the participants are identified

by attributes such as physical location or sensor value range. This style of

routing has been formulated as directed diffusion, a process in which nodes

express interest in data by attribute.

How reduce power

In this section some techniques are shown to minimize energy usage. As we

know, is possible to turn off the device to safe power end send data only

if there is a significant parameters variation. Another method is perform-

ing aggregation within the network, so its possible to reduce the amount

of information to transmit communication. In this case a single packet is

transmitted with a statistical summary of measurements made by the nodes

belonging to a sub-tree. Compression and scheduling also can conserve en-

ergy at lower layers. Sensor networks can avoid explicit protocol messages

by piggybacking control information on data messages and by overhearing

packets destined for other nodes. They can use prescheduled time to reduce

contention and the time the radio remains live.


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1.3 Applications

Wireless Sensor Networks have revolutionized the world of distributed sys-

tems and have enabled several new applications. The applications for WSNs

are of several kind, but we can group them in two topologies. The first

class includes entity monitoring with limited signal processing requirements.

These applications want to gather information of a relatively simple form,

such as temperature and humidity, from the operating environment. The

other classes of applications require the processing and transportation of

large volumes of complex data. This class includes heavy industrial mon-

itoring and video surveillance, where complex signal processing algorithmsare usually employed. Below some of the most common applications are

described.

Area monitoring In area monitoring, the WSN is deployed over a region

where some phenomenon are to be monitored, for example for military

purpose; a large quantity of sensor nodes could be deployed over a bat-

tlefield to detect enemy intrusion. When the sensors detect the event

being monitored the event needs to be reported to one of the base

stations, which can take appropriate action. Depending on the ex-

act application, different objective functions will require different data-propagation strategies, depending on things such as need for real-time

response, redundancy of the data.

Environmental monitoring Several WSNs have been deployed for envi-

ronmental monitoring, the vast spaces involved in such applications

requires large volumes of low cost sensor nodes that can be easily dis-

persed throughout the region, the nodes collect readings over time

across a volume of space large enough to exhibit significant internal

variation, for example WSNs are used to monitoring the microclimate

throughout the volume of redwood trees, helps form a sample of en-tire forests. The nodes are used to monitor parameters like, humidity,

temperature and other environmental data.

Motion monitoring The analysis of structural response in, for example,

bridges, buildings, and airframes, places a further requirement to use

data collected at different points in the structure in spatial-temporal

analysis. This requires establishing a common, highly accurate time

frame across nodes. Nodes share time correlated raw or processed data


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to perform the structural analysis. For example, the sensor data from

one instrument can be used as an input to a model-based analysis ateach of several other points in a structure and compared to sensor

data at those points. Researchers refine these models by using them

iteratively in normal circumstances to detect anomalies.

Habitat Study Such applications usually require the sensing and collect-

ing of bio-physical or biochemical information from the entities under

study, in many scenarios, habitat study requires relatively simple sig-

nal processing, such as data aggregation using minimum, maximum,

or average operations. The networks are designed to be equipped with

sensors for temperature, humidity, barometric pressure, and mid-range

infrared.

1.4 Industrial application

We have already seen how WSNs are used in many applications, but one of

the most important is in the industrial application [5]. Wireless, low power

field devices enable industrial users to significantly increase the amount of

information collected and the number of control points that can be remotelymanaged. The wireless network needs to have three qualities: low power, high

reliability, and easy installation and maintenance and to achieve this goals

is important to choose a suitable routing protocol. The industrial market

classifies process applications into three broad categories and six classes.

1. Safety

Class 0: Emergency action - Always a critical function

2. Control

Class 1: Closed loop regulatory control - Often a critical

Class 2: Closed loop supervisory control - Usually non critical

function

Class 3: Open loop control - Operator takes action and controls

the actuator

3. Monitoring


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Class 4: Alerting, for this class is very important to have Short-

term operational effect

Class 5: Logging and downloading or uploading

Industrial users are interested in deploying wireless networks for the mon-

itoring classes 4 and 5 and in the non-critical portions of classes 3 and 2.

Most low power and lossy network (LLN) systems in industrial automation

environments will be for low frequency data collection, sensors will have built-

in microprocessors that may detect alarm conditions that generate critical

alarm packets. Some devices will transmit a log file every day, so on this

kind of network there are alarm packets that are expected to be granted alower latency than periodic sensor data streams. Other devices will transmit

a log file every day, again with typically tens of Kbytes of data.

1.4.1 Physical network topology and logical topology

In the figure a typical physical network topology for industrial applications

is shown.

Actually there is no specific physical topology for an industrial process con-

Figure 1.2: Typical industrial topology

trol network, in one case a few hundred field devices are deployed to ensure

the global coverage using a wireless self-forming self-healing mesh network

that might be 5 to 10 hops across the backbone is many hops away. In the

opposite extreme case, the backbone network spans all the nodes and most


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nodes are in direct sight of one or more backbone router. But in the most

common case there is a backbone that spans the Wireless Sensor Networkso that any WSN node is only a few wireless hops away from the near-

est Backbone Router. WSN nodes are expected to organize into self-forming

self-healing self-optimizing logical topologies that enable leveraging the back-

bone when it is most efficient to do so. Regarding the Logical Topologies for

security, reliability, availability or serviceability reasons, it is often required

that the logical topologies are not physically congruent over the radio net-

work that is they form logical partitions of the LLN.

The aim of the network is to build proactively a set of routes between the

sensors and one or more backbone router and maintain those routes at alltime. Also, because of the lossy nature of the network, the routing in place

should attempt to propose multiple paths in the form of Directed Acyclic

Graphs oriented towards the destination.

1.4.2 Traffic characteristic and metric

In industrial application we can distinguish in four large service categories:

Event data This category includes alarms and aperiodic data reports withbursty data with bandwidth requirements. In certain cases, alarms are

critical and require a priority service from the network.

Client end Server In many case the industrial networks implement a com-

mand response protocol. The data bandwidth required is often bursty,

and the latency is based on the time to send tens of bytes over a 1200

baud link.

Bulk transfer Bulk transfers involve the transmission of blocks of data in

multiple packets where temporary resources are assigned to meet a

transaction time constraint.

The routing protocol must also support different metric types for each

link used to compute the path according to some objective function (e.g.

minimize latency) depending on the nature of the traffic.

For these reasons, the ROLL routing infrastructure has to compute and up-

date constrained routes on demand. Industrial application data flows be-

tween field devices are not necessarily symmetric. In particular, asymmet-

rical cost and unidirectional routes are common for data and alerts. The


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routing protocol must be able to compute a set of unidirectional routes with

potentially different costs that are composed of one or more different paths.For example it will find multiple paths towards a same destination, which

could be a node acting as a sink for the LLN.

In the next chapter, we study the IEEE 802.15.4 standard, which is one of

the most popular communication standards for WSNs


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Chapter 2

IEEE 802.15.4 and ZigBee

In this chapter we study one of the most popular protocols for wireless sen-

sor network: the IEEE 802.15.4. This protocol specifies the physical layer

and the medium access control layer. The ZigBee alliance has specified an

extension to IEEE 802.15.4 by adding the networking and application layer

on top of the physical and MAC layer

2.1 Introduction

The ZigBee Alliance is an association of companies working together to spec-

ify a standard for networks composed by a large number of nodes. These

networks should be reliable an self configurable with very long battery life,

secure and they should have a low cost.

The ZigBee standards includes the IEEE 802.15.4 as the physical and

MAC layer and is trying to standardize higher level applications.

ZigBee offers basically four kinds of different services: Extra Encryption, services where application and network keys imple-

ment extra 128b Advanced Encryption Service (AES) encryption

Association and authentication, which mean only valid nodes can join

to the network.

Routing protocol: for example Ad hoc On Demand Distance Vector

(AODV), a reactive ad hoc protocol has been implemented to perform

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CHAPTER 2. IEEE 802.15.4 AND ZIGBEE 15

NETWORK / SECURITY

LAYER

APPLICATION / PROFILES

PHY LAYER

MAC LAYER

APPLICATION

FRAMEWORK

ZigBee

IEEE

802.15.4

Figure 2.1: ZigBee protocol stack

the data routing and forwarding process to any node in the network.

Application Services: An abstract concept called cluster is intro-

duced. Each node belongs to a predefined cluster and can take a pre-

defined number of actions.

Hence ZigBee is used to organize the network. A node to join the network

has to ask to the coordinator for a network address, as part of the association

process. All the information in the network is routed using this address and

not the MAC address. In this step authentication and encryption procedures

are performed.

Once a node has joined to the network can send information to its brothers

through the routers which are always awake waiting for the packets. When

the router gets the packet and the destination is in its radio of signal, the

router first looks if the destination end device is awake or sleeping. In the

first case the router sends the packet to the end device, however if it is sleep-

ing, the router will bufferize the packet until the end device node gets awakeand ask for news to the router.

2.2 IEEE 802.15.4 features

IEEE 802.15.4-2006 is a standard which specifies the physical and the MAC

layer, they are designed for a low cost and low-power wireless network for


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monitoring and control applications [6]. This standard is used especially in

Wireless Personal Area Network (WPAN) and this network is composed bytwo types of device, Full Function Device (FFD) that can operate as a Per-

sonal Area Network PAN coordinator, coordinator or a device and Reduced-

Function Device (RFD) which are predisposed for simple application that

not require an high rate.

This two kind of node are use to create three topology.

Figure 2.2: ZigBee network topology

Star Topology

In this topology there is a single PAN coordinator that communicates with

other nodes, After an FFD is activated for the first time, it may establish its

own network and become the PAN coordinator, the network chooses a PAN

identifier, which is not used by any other network whereby can communicate

so each star network can operate independently.


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Peer-To-Peer Topology

Any device can communicate with any other device if they are in range that

allow them to communicate, but also in this case there is only one PAN

coordinator. This topology allows multiple hops to route messages from any

device to any other device in the network and thank to multipath routing it

is very reliable.

Cluster Tree Topology

In this topology most devices are FFDs and an RFD may connect to a cluster-

tree network at the end of a branch. Any of the FFD could be a coordinatorthat synchronizes the other devices and coordinators but only one is the

PAN coordinator which forms the firs cluster head (CLH) that is identified

with cluster identifier (CID) of zero. The PAN coordinator chooses a PAN

identifier and broadcast beacon frames to neighboring nodes. The devices

receive the bacon frame and will send a request to join in the network. If

the PAN coordinator accepts this new node, it will add the CLH as its

parent in its neighbor list and begin transmitting periodic beacons such that

other candidate devices may then join the network at that device besides the

PAN coordinator can instruct a device to become the CLH of a new clusteradjacent to the first one.

2.3 WPAN Device Architecture

Now we see how a node is structured, the definition of the network layers is

based on the OSI model; although only the lower layers are defined in the

standard, interaction with upper layers is intended, possibly using a IEEE

802.2 logical link control sublayer accessing the MAC through a convergence

sublayer.


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Upper Layers

802.2 LLC

SSCS

MAC

PHY

Pysical Medium

Figure 2.3: IEEE 802.15.4 architecture

Physical : contains the radio RF transceiver and its low-level control mech-

anism.

MAC : sublayer that provides access to the physical channel for all types of

transfer.Network Layers : provides network configuration, manipulation, and mes-

sage routing.

Application Layer: provides the intended function of a device

In the following subsection analyze in more detail the physical and the MAC

layer

2.3.1 IEEE 802.15.4 PHY

The physical layer provides two services:

The PHY data service: enables the transmission and reception of PHY

protocol data units (PPDU) across the physical radio channel.

PHY management service: interfacing to the physical layer manage-

ment entity (PLME)

It is possible choose from two different type of PHY depending on the

frequency band.There is a single channel between 868 and 868.6MHz, 10


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channels between 902.0 and 928.0MHz, and 16 channels between 2.4 and

2.4835GHz. Several channels in different frequency bands enables the abil-ity to relocate within spectrum. The standard also allows dynamic channel

selection, a scan function that steps through a list of supported channels in

search of beacon, receiver energy detection, link quality indication, channel

switching.

This layer provides three important service:

Receiver Energy Detection (ED) that estimates the received signal power

within the bandwidth of an IEEE 802.15.4 channel, this parameter is

used to decide if a signal received could be accepted as a packet but

could be also used by network layer to establish a route.

The Link Quality Indication (LQI) it is a characterization of the strength

and/or quality of a received packet. The measurement may be imple-

mented using receiver ED, a signal-to-noise estimation or a combination

of these methods.

Clear Channel Assessment (CCA) is a logical function found which de-termines the current state of use of a wireless medium in this standard

three method of CCA are implemented:

Energy above threshold: the CCA reported that the channel is

busy if the ED its over a threshold.

Carrier sense only: CCA shall report a busy medium only upon

the detection of a signal with the modulation and spreading. char-

acteristics of IEEE 802.15.4

Carrier sense with energy above threshold. CCA shall report abusy medium only upon the detection of a signal with the modu-

lation and spreading characteristics of IEEE 802.15.4 with energy

above the ED threshold.

2.3.2 IEEE 802.15.4 MAC

The MAC sublayer handles access to a shared medium and here provides

two services: the MAC management service interfacing to the MAC sub-


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layer management entity (MLME) that is responsible for MAC management

trough a collection of primitives, and the MAC data service enables the trans-mission and reception of MAC protocol data units (MPDU) across the PHY

data service.

SUPERFRAME STRUCTURE

In order to allow guaranteed time slots for low-latency applications and ap-

plications requiring a specific data bandwidth, IEEE 802.15.4 networks can

choose to synchronies their communication according to a superframe struc-

ture, its form is decided by the coordinator, but basically is divided into 16

equally sized slots and the bacon frame is set in the first slot of each super-

frame. The beacons are used to synchronize the attached devices, identify

the PAN and to describe the structure of superframe.

CAP >= aMinCAPLength CFP

SD = aBaseSuperframeDuration SO2

BI = aBaseSuperframeDurationBO

2

beacon beacon

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

aBaseSlotDurationSO

2

GTS GTS G TS Inactive period

Figure 2.4: IEEE 802.15.4 superframe

In the superframe there is inactive portion, where the coordinator dont in-

teract with its PAN and may enter a low-power mode. The active portion is

divided in :

Contention access period (CAP),where any device to communicate dur-

ing the CAP uses a slotted CSMA-CA mechanism

Contention free period (CPF) that is divided in guarantied time slots

(GTSs), The GTSs always appear at the end of the active superframe

following the CAP. The PAN coordinator may allocate up to seven of

these GTSs and a GTS can occupy more than one slot, period.


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The beacon is transmitted at the beginning of slot 0 and the CAP starts

immediately after the beacon. All frame transmitted in the CAP shall useslotted CSMA-CA to access the channel except acknowledgement or any

data frame that immediately follows the acknowledgement of a data request

command. A transmission in the CAP is complete one IFS period before the

end of the CAP, where IFS time is the amount of time necessary to process

the received packet by the PHY.

The CPF starts just after the CAP and extends to the end of the active

portion of the superframe. The length of the CFP is determined by the total

length of all of the combined GTSs, the transmission dont use CSMA-CA

and they are complete one IFS period before the end of its GTS.The CPF starts just after the CAP and extends to the end of the active

portion of the superframe. The length of the CFP is determined by the total

length of all of the combined GTSs, the transmission dont use CSMA-CA

and they are complete one IFS period before the end of its GTS.

CSMA/CA Algorithm

Usually in PAN is used Slotted CSMA/CA but if beacons are not being used

in the PAN or a beacon cannot be located in a beacon-enabled network, is

used the unslotted csma-ca. In both cases, the algorithm is implementedusing units of time called backoff periods.

In slotted CSMA/CA the backoff period boundaries of every device in

the PAN are aligned with the superframe slot boundaries of the PAN coordi-

nator so when a device wants to transmit, can do it only in the next backoff

periods instead in unslotted CSMA/CA, the backoff periods of one device do

not need to be synchronized to the backoff periods of another device.

Each device has 3 variables:

NB is the number of times that the CSMA/CA algorithm was requiredto backoff while waiting the current transmission. It is initialized to 0

before every new transmission.

CW is the contention window length, which defines the number of

backoff periods that need to be clear of activity before the transmission

can start. It is initialized to 2 before each transmission attempt and

reset to 2 each time if the channel is busy. CW is only used for slotted

CSMA/CA.


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BE is the backoff exponent, which is related to how many backoff pe-

riods a device shall wait before attempting to assess the channel.

The diagram showing the algorithm is shown in Figure.

Step 5

unitBackoffPeriods

Delay for

random (2^BE1)

backoff period boundary

Perform CCA on

BE=min(BE+1,aMaxBE)

CW=2, NB=NB+1,

Transmission failure

NB=0, CW=2

BE=aMinBE

Locate Backoff

period boundary

Success

idle?

Channel

N

Y

N

Y

N

macMaxCSMABackoffs

NB=

CW=0

N

Y

aMinBE)

BE=min(2,Battery life

extension?

Y

CW=CW1Step1

Step 3

Step 2

Step 4

Figure 2.5: CSMA/CA algorithm

Step 1: In slotted CSMA/CA, NB, CW and BE are inizializated and the

boundary of the next backoff is known.

Step 2: the MAC layer waits to complete backoff period with a casual delay

in the range 0 to 2BE - 1.

Step 3: the MAC layer request that PHY performs a CCA, than The MAC

sublayer proceed if the frame transmission, and any acknowledgement can

be completed before the end of the CAP.

Step 4: If the channel is assessed to be busy, the MAC sublayer shall in-

crement both NB and BE by one, ensuring that BE shall be no more than

aMaxBE. In slotted CSMA/CA, CW can also be reset to 2. If the value


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of NB is less than or equal to macMaxCSMABackoffs, the CSMA/CA shall

return to step 2, else the CSMA-CA shall terminate with a Channel AccessFailure status.

Step 5: If the channel is assessed to be idle, in a slotted CSMA/CA, the

MAC sublayer shall ensure that contention window is expired before starting

transmission. For this, the MAC sublayer first decrements CW by one. If

CW is not equal to 0, go to step 3 else start transmission on the boundary

of the next backoff period. In the unslotted CSMA/CA, the MAC sublayer

starts a transmission immediately if the channel is assessed to be idle.

2.3.3 Data Transfer model

Here we examine the four types of data transfer transactions allowed in IEEE

802.15.4: When a device wishes to transfer data in a nonbeacon-enabled

CoordinatorNetwork

Device

Data

Acknowledgment

(optional)

Figure 2.6: Communication from a device to a coordinator in a nonbeacon-

enabled network

network, it simply transmits its data frame, using the unslotted CSMA/CA,

to the coordinator and the acknowledgement is optional.


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CoordinatorNetwork

Device

Beacon

Data

Acknowledgment

(optional)

Figure 2.7: Communication from a device to a coordinator in a beacon-

enabled network

If a device wishes to transfer data to a coordinator in a beacon-enabled

network, it first listens for the network beacon. When the beacon is found,

it synchronizes to the superframe structure. At the right time, it transmits

its data frame, using slotted CSMA/CA, to the coordinator. There is an op-

tional acknowledgement at the end. The application transfer are controlled

by the device instead of coordinator to save power.

CoordinatorNetwork

Device

Beacon

Data request

Acknowledgment

Data

Acknowledgment

Figure 2.8: Communication from a coordinator to a device in a beacon-

enabled network

If a coordinator wishes to transfer data to a device in a beacon-enabled

network, it indicates in the network beacon that the data message is pend-

ing. The device periodically listens to the network beacon, and if a mes-

sage is pending, transmits a MAC command requesting this data, using


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slotted CSMA/CA. The coordinator optionally acknowledges the success-

ful transmission of this packet, pending data frame is then sent using slottedCSMA/CA. The device acknowledged the successful reception of the data

by transmitting an acknowledgement frame. Upon receiving the acknowl-

edgement, the message is removed from the list of pending messages in the

beacon.

CoordinatorNetwork

Device

Data request

Acknowledgment

Data

Acknowledgment

Figure 2.9: communication from a coordinator to a device in a nonbeacon-

enabled network

Finally if a coordinator wishes to transfer data to a device in a nonbeacon-

enabled network, it stores the data for the appropriate device and wait for

a request data. A device send a MAC command requesting the data, using

unslotted CSMA/CA, to its coordinator at an application-defined rate. The

coordinator acknowledges this packet. If data are pending, the coordinator

transmits the data frame using unslotted CSMA/CA. If data are not pend-

ing, the coordinator transmits a data frame with a zero-length payload to

indicate that there arent data.

2.4 ZigBee routing

The ZigBee routing algorithm is based on the notion of Distance Vector

routing, in which each ZigBee router maintains a routing table entry for the

route from a particular source to a particular destination [7]. This entry

records both a logical distance to the destination and the address of the

next router in the path to that destination.

Routes are established on-demand using a route discovery process in which


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the source device broadcasts a route request command and the destination

device sends back a route reply. After that the node can send packet on thisroute.

In most wireless network applications, there is a distinguished device, often

called an aggregator, to which every other device in the network must send

data on a regular basis. In order to prevent every device in the network from

having to discover the aggregator separately, the ZigBee PRO feature set

provides a special case of route discovery, in which a single route request

broadcast from the aggregator establishes an entry with the aggregator as a

destination in the routing tables of every router in the network.

The ZigBee introduces another kind of routing, known as source routing.Whereas in distance vector routing the routing information is stored in rout-

ing tables in the devices that participate in relaying the frame, source routing

puts the routing information in the frame itself. Thus only the originator of

the frame, in this case the aggregator, needs to maintain an entry for the

route, but this routing table entry needs to store the entire path from the

aggregator to the destination. ZigBee PRO uses a route record command,

sent from the intended destination back to the aggregator, to record the path.

Thereafter, data frames may be sent along that path using source routing.

2.5 ZigBee upper layers

In this paragraph we describe briefly the stack above the network layer.

Application layer

As we know the application layer is the highest-level layer of a protocol stack,

and defines the interface of the ZigBee system to its end users. It is composed

by two main components.

The ZigBee Device Object (ZDO) that establishs the role of a device as either

coordinator or end device but also for the discovery of new one-hop devices

on the network and the identification of the service that the node offers. It

may establish also secure links with external devices and reply to binding

requests accordingly.

The application support sublayer (APS) works as a bridge between the net-

work layer and the other components of the application layer: it keeps up-

to-date binding tables in the form of a database, which can be used to find


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appropriate devices depending on the services that are needed and those the

different devices offer. It can also routes messages across the layers of theprotocol stack.

Security services

ZigBee provides facilities to create secure communications, protecting es-

tablishment and transport of cryptographic keys, ciphering frames and con-

trolling devices. It builds on the basic security framework defined in IEEE

802.15.4. This part of the architecture relies on the correct managementof symmetric keys and the correct implementation of methods and security

policies.


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Chapter 3

Routing Protocol

In this Chapter the basic functionality of the RPL are illustrated, especially

how this protocol build up and manage the DAG

3.1 Introduction

Low power and Lossy Networks (LLNs) are a class of networks in where

nodes are constrained: LLN routers typically operate with constraints on

processing power, memory and energy [8]. These routers are interconnectedby lossy links, typically supporting only low data rates, that are usually

unstable with relatively low packet delivery rates. A network may run mul-

tiple instances of RPL concurrently. Each instance may serve different and

potentially antagonistic constraints but in this work we describe how a sin-

gle instance operates. The aim of RPL is build up Directed Acyclic Graph

(DAG). Where a directed graph has the property that all edges are oriented

in such a way that no cycles exist, all edges are contained in paths oriented

toward one or more root nodes. One of the most important factor that in-

fluence the routing protocol are the metric, in fact RPL is a distance vectorrouting protocol.

The maintenance information is added as an option - called DAG information

option (DIO) - that is periodically sent. The DIO also contains a Objective

Code Point (OCP) field, which determines what metrics and functions should

be used for a node to determine its height to build routing solution which fa-

vors energy-efficiency, latency, bandwidth, or a compound metric. The DIO

includes a measure derived from the position of the node within the DAG,

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CHAPTER 3. ROUTING PROTOCOL 29

the rank, which is used for nodes to determine their positions relative to each

other and to inform loop avoidance/detection procedures. In practice thisrouting algorithm exchanges DIO messages to establish and maintain routes.

RPL proposes a mechanism to allow data to flow from the sink outwards,

which can be necessary for controlling actuators, and reconfiguration. Along

with sending convergecast data, nodes in RPL transmit Destination Adver-

tisement Option (DAO) packet which records the sequence of nodes traversed

before reaching the sink. The sink then uses source routing to send data to

arbitrary nodes in the network, hence the node can communicate with the

sink and vice-vera. The DAG root may advertise a routing constraint used

as a filter to prune links and nodes that do not satisfy specific properties,a routing metric is a quantitative value that is used to evaluate the path

quality, also referred to as the path cost. The best path is the path with the

lowest cost with respect to some metrics that satisfies all constraints (if any)

and is also called the shortest constrained path.

3.2 The RPL protocol

The DAG starts from a node that is defined as the DAG root, ad it can act asa collection point for the data. If this node offers connectivity to an external

infrastructure such as the public Internet or IP network the DAG is called

Grounded otherwise it is called Floating.

From this nodes starts the setting up of the DAG employing a DIO, the other

nodes that received this message can decide if the DAG root is a potential

parent using the information contained in the OCP.

A node is able to decide if a emitting node could be considered as a potential

parent. After performing the validation test, the node can decide to add the

emitting node in the list of DAG parents, if so the node is joined in the DAGspecified by its DAGID. After the entrance in the DAG the nodes emits a

multicast DIO message in to expand the route outward from the DAG root.

Meanwhile the nodes can listen for the DIO message from other nodes to

expand the set of parents and augment the number of available paths.

The DIO conveys the following option:


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A DAGID used to identify the DAG as sourced from the DAG root,

the DAGID must be unique to a single DAG in the scope of the LLN,a Rank information used by nodes to determine their positions in the

DAG and also a Sequence number originated from the DAG root, used

to coordinate topology changes to avoid loops.

Objective Code Point (OCP) as described below that contains informa-

tion about the set of metrics, the objective function use to determinate

the least coast path, the function used to compute DAG Rank and

the functions used to accumulate metrics for propagation within a DIO

message

Indications and configuration for the DAG, e.g. grounded or floating,

administrative preference.

A vector of path metrics.

The decision for a node to join a DAG may be optimized according to

implementation specific policy functions on the node as indicated by one or

more specific OCP values.

For example, a node may be configured to optimize a bandwidth metric

(OCP-1), and with a parallel goal to optimize for a reliability metric (OCP-2). So there are two DAGs, with two unique DAGIDs: DAG-1 would be

optimized according to OCP-1, whereas DAG-2 would be optimized accord-

ing to OCP-2. A node may then maintain independent sets of DAG parents

and related data structures for each DAG. Note that in such a case traffic

may directed along the appropriate constrained DAG based on traffic mark-

ing within the IPv6 header.

3.3 DIO transmission and elegibilityThe transmission of the DIO is essential for the construction of the DAG,

for this reason this section is explained whenever a node sends this kind

of packet. The DIO sending is regulated by a trickle timer operating over

a variable interval. The governing parameters of this timer are configured

across the DAG and in addition to the periodic multicast message the LLN

sends the DIO to respond a Router solicitation.

In the implementation of the simulation a DIO is sent periodically to update


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the network as we described in the next chapter. We make this assumption

because for the moment we are not taking into account the detection of in-consistency and other causes which lead the variation of the trickle timer.

When an DIO message is received from a source device, the receiving node

must first determine whether or not the DIO message should be accepted for

further processing. If the DIO message is malformed the packet should be

silently discarded. Otherwise if the source device is a member of the candi-

date neighbor set, then the DIO is eligible for further processing.

If the node has sent a DIO message within the risk window then a collision

has occurred so the node do not process the DIO message. As DIO messagesare received from candidate neighbors, the neighbors can decide to choose

the advertising node as DAG parents by following the rules of DAG discovery

as will be described in the next section. When a node places a neighbor into

the DAG Parent set, the node becomes attached to the DAG through the

new parent node.

3.4 Data structure

In this paragraph the data structure employed by the RPL to built up theDAG are expleined. subsectionCandidate neighbor data structure Each node

has a data structure where nodes discovered by the Neighbors Discovery are

allocated. In this structure is possible to find the metrics related to candidate

neighbors, and is very important that the metric are stable and reliable. It

is also possible to bound the number of entries, so to determine which nodes

have to remain in the data structure there is a confidence value to order the

neighbors. subsectionDAGs data structure

For each DAG that a node is a member of, the implementation must keep

a DAG table with the following entries: DAGID

DAG Objective Code Point

A set of Destination Prefixes offered inwards along the DAG

A set of candidate DAG parents

A timer to govern the sending of DIO messages for the DAG


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DAG Sequence Number

If a node wants to join in a DAG for which no DAG data structure is instan-

tiated, then the DAG data structure is instantiated. When the candidate

DAG parent set become empty, it must be deplete and then the DAG data

structure should be suppressed after the expiration of a local timer.

Candidate parents structure

The nodes to enter in the DAG have to select the candidate DAG parents

according to the request that are contained in the OCP.

This data structure must keep a record of:

A reference to the neighboring device which is the DAG parent

A record of most recent information taken from the DAG Option last

processed from the candidate DAG parent

A state associated with the role of the candidate as a potential DAG

parent that will be further described

A DAG Hop Timer, if instantiated

A Held-Down Timer, if instantiated

DAG parents

The set of DAG parents is composed by the subset of candidate DAG parents

in the Current state.

The DAG parents are ordered according to the OCP so the protocol can

identify the preferred parents and the other DAG parents must have a rank

less than or equal to that of the most preferred DAG parent.When nodes are added to or removed from the DAG parent set, there is the

possibility that the preferred parents change so the preferred parents should

be revaluated. Any nodes having a rank greater than self after such a change

must be placed in the Held-Down state and evicted.


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3.5 Objective Code Points and Objective Func-

tion

The OCP serves to convey and control the optimization objectives used

within the DAG. The OCP contains the OF that is used to order the DAG

parents and the OF is also responsible to compute the rank of the device

within the DAG.

The OFs is fundamental for the parent selection and it will be enabled

each time an event indicates that the a potential next hop information has

changed.

If the state of candidate neighbor changes, the preferred parents could

change so the parent selection is triggered, the OF scans all the interfaces

on the device and decide if one interface is more preferred than another the

OF can also exclude an interface if not match a required criterion for an

Objective Function.

Than the OF scans all the candidate neighbors on the selected interfaces to

determine whether they are suitable as an attachment router for a DAG.

The OF computes selfs rank by adding the step of rank to that candidateto the rank of that candidate. The step of rank might vary from 1 to 16 can

is estimated as follows:

1 indicates a unusually good link.

4 indicates a normal typical link.

16 indicates a link that can hardly be used to forward any packet.

After the scanning of all the candidate neighbors, the OF selects the cur-

rent best parents and selfs rank is computed as the preferred parent rank

plus the step in rank with that parent. Other rounds of scans might be nec-

essary to elect alternate parents and siblings.


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3.6 DAG discovery rules

DAG discovery locates the nearest sink and forms a Directed Acyclic Graph

towards that sink, by identifying a set of DAG parents, it specifies also a set

of rules to be followed to avoid loop and maintain a safe DAG structure.

For this purpose the DAG Rank is very important. After selecting the pre-

ferred parents, a node is able to compute its Rank using the metrics conveyed

by the most preferred parent, the node own metric and a related function

defined by OCP.

A node that does not have any DAG parents in a DAG is the root of its own

floating DAG. Its rank is 1. This situation could happen when the node losesall of its current feasible parents. In that case, the node should remember

the DAGID and the sequence counter in the DIO of the lost parents for a

period of time which covers multiple DIO.

Whether the LLN node is attached to an infrastructure that does not sup-

port DIO, this is the DAG root of its own grounded DAG with rank 1. This

node sends the first DIO to begin the DAG construction.

After that a device is joined in the DAG and has computed its rank, it may

move in its DAG without increasing its own DAG Rank otherwise the node

has to leave the DAG. For this reason nodes must ignore DIOs that are re-

ceived from other routers located deeper within the same DAG.

A node that has selected a new set of DAG parents but has not moved yet,

because it is waiting for DAG Hop timer to elapse, become unstable and

doesnt send DIO. When a node wants to jump from its current DAG into

any different DAG has also to wait for a DAG Hop timer. This allows the

new higher parts of the DAG to move first, thus allowing stepped DAG re-

configurations and limiting relative movements but a node should not join a

previous DAG, identified by its DAGID, unless the sequence number in the

DIO has incremented since the node left that DAG.

If a node receives a DIO from one of its DAG parents, and if the parentcontains a different DAGID, indicating that the parent has left the DAG,

and if the node can remain in the current DAG through an alternate DAG

parent, then the node should remove the DAG parent which has joined the

new DAG from its DAG parent set and remain in the original DAG.

When a node detects or causes a DAG inconsistency, then the node sends an

unsolicited Router Advertisement message to its one-hop neighbors. The RA

contains a DIO that propagates the new DAG information. Such an event


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will also cause the trickle timer governing the periodic DIOs to be reset.

If a DAG parent increases its rank but the node does not wish to follow,the DAG parent should be evicted from the DAG parent set. If the DAG

parent is the last in the DAG parent set, then the node may chose to follow it.

3.7 Candidate DAG Parent States and Sta-

bility

Using the DIO message a node can build up a set of DAG parents from whichchooses the parents depending on runtime conditions. The following states

for the candidate parents are defined:

CURRENT The candidate parent is in the set of DAG parents and may

be used for forwarding traffic inward along the DAG.

HELD-UP A new DAG is discovered upon a router advertisement message,

the node gets in the DAG by selecting the source of the RA message

as a DAG parent and propagating the DIO accordingly. Before the

node jumps in the new DAG become unstable, in this phase the node

continues to listen the DIOs message from other node to discover other

candidate parents, and a new DAG Hop timer starts for all of this. The

first timer that elapses for a given new DAG clears them all for that

DAG, allowing the node to jump to the highest position available in

the new DAG.

HELD-DOWN When a neighboring node is removed from the Default

Router List, it is actually held down for a hold down timer period. A

node that is held down is not considered for the purpose of forwarding

traffic inward along the DAG toward the root. When the hold downtimer elapses, the node is removed from the Default Router List.

COLLISION A race condition occurs if 2 nodes send DIO at the same time

and then attempt to join each other In order to detect the situation,

LLN Nodes time stamp the sending of DIO, than a risk windows is

defined and any DIO received in this period introduces a risk.

So a node will forward its packet only if the parents is in a current state.

Since in our network we want to build only a DAG, for a node there is not


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Chapter 4

RPL Network Simulator

This is the main Chapter of the thesis. Here we explain how the simula-

tor is implemented. We explained the software used, the main step of the

simulation and the parameters chosen.

4.1 OMNeT++

OMNeT++ is an object-oriented modular discrete event simulator. The fea-

ture of the OMNeT++ is the capability to create modules which could com-municate each other through messages. One module contains one or more

sub-modules each of which could contain other sub-modules. The modules

are distinguished among simple or compound [9].

NETWORK

SIMPLECOMPOUND

Figure 4.1: OMNet++ module concept

A simple module is associated with a C++ file that supplies the desired be-

37


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CHAPTER 4. RPL NETWORK SIMULATOR 38

haviors that encapsulate algorithms. Compound modules are aggregates of

simple modules and are not directly associated with a C++ file that suppliestheirs behaviors. Modules communicate by exchanging messages that repre-

sent frames or packets in a network. Each message may be a complex data

structure and they may be exchanged directly between simple modules or via

a series of gates and connections, there is also the possibility to implement

self-messages which are used by a module to schedule events at a later time.

Each layer of the sensor node is represented as a Simple Module of OM-

NeT++. The layers communicate with each other through gates and each

of the layers has a reference to the Coordinator.

These Simple Modules are connected according to the layered architectureof a Sensor Node. The different layers of the Sensor Node have gates to the

other layers of the Sensor Node to form the Sensor Node stack.

In OMNeT++, NED language is used to describe the structure of a simu-

lation model, NED stands for Network Description and NEDs files lets the

user declare simple modules, and connect and assemble them into compound

modules. The user can label some compound modules as networks, self-

contained simulation models. Channels are another component type, whose

instances can also be used in compound modules.

Through the NED files is possible define modules parameters but they can

be assigned also with configuration file omnetpp.ini.

To build up a simulation we need:

NED language topology description (.ned files) which describe the mod-

ule structure with parameters, gates etc.

Message definitions (.msg files). You can define various message types

and add data fields to them. OMNeT++ will translate message defi-

nitions in C++ classes.

Simple modules sources. They are C++ files, with .h/.cc suffix.

A .ini file to specified parameters

4.2 Mobility Framework

We need a framework to support simulations of wireless and mobile networks

within OMNeT++, and for our work we choose the Mobility Framework [10].


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The core framework implements the support for node mobility, dynamic con-

nection management and a wireless channel model and provides basic mod-ules that can be derived in order to implement own modules.

One of the main features of MF is the ChannelControl module that controls

and maintains all potential connections between the nodes. An OMNeT++

connection link in the MF does not mean that the corresponding hosts are

able to communicate with each other. A host will receive every data packet

that its transceiver is potentially able to sense. The physical layer then has

to decide dependent on the received signal strength whether the data packet

will be processed or whether it will be treated as noise.

4.2.1 Nodes structure

We use MF because the 802.15.4 standard is implemented in the framework.

In this framework the node is composed by the modules shown in figure.

APP

NET

MAC

SNR

RADIO

DECIDER

Figure 4.2: Simulator node stack

A NIC module is a network interface that includes physical layer functions


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like transmitting, receiving, modulation as well as medium access mecha-

nisms. The NIC module in the MF therefore is divided into two simplemodules:

A physical layer like part (snrEval and decider) and a MAC layer

(macLayer). The snrEval module can be used to compute some snr

information for a received message whereas the decider module can

process this information to decide if a message got lost, if it has bit

errors or if is correctly received.

A medium access control (MAC) protocol coordinates actions over ashared channel. In our MAC layer implementation we decided to use

un-slotted CSMA-CA and non-beaconed mode (also called beaconless).

At a given point of the simulation as we shall see, nodes that will receive

a packet should send an acknowledgment, to send this ack the node will

use the CSMA algorithm to avoid collision.

The delay that we will obtain to send the ack will be our latency.

MAC layer decide when a simple radio being able to send or transmit on one

channel. The interface with the MAC layer is simple: the MAC can ask the

radio one of three commands (ENTER SLEEP, ENTER RX and ENTER

TX). Internally, the radio has 7 states, and three of them are steady. So the

MAC layer only cares about steady states and the internal transient states

are used to model power consumption.

In the Network layer, we have to implement a routing algorithm, as men-

tioned we want to use the RPL to find a route toward a sink node, and its

implementation is described in the next section. Finally in Application layer

where is possible define when and which packet should be sent, with which

kind of traffic the node will generate the packets.

4.2.2 Communication between layer

The handle*Msg() functions, are called each time a corresponding message

arrives and contains all necessary processing and forwarding information.

There are three different functions to handle three different kinds of message:


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HandleSelfMsg using self messages is possible implement timers in OM-

NeT++. HandleSelfMsg() is the message to handle all timer relatedthings and to initiate actions upon timeouts.

HandleUpperMsg This function is called every time a message has arrived

from an upper layer. After processing the message can be forwarded

to the lower layers with the sendDown() function, if necessary.

HandleLowerMsg For messages from lower layers it is the other way around.

After being processed they have to be forwarded to upper layers if nec-

essary. This is done by using the sendUp() function which also takes

care of decapsulation.

Convenience Functions are special funcion defined to facilitate common

interfaces and to hide inevitable interface management from the user. the

MF provides three different functions:

EncapsMsg This function is called right after a message has arrived from

the upper layers. It is responsible for encapsulation of the message

into appropriate message layer. After this the message is passed to the

handleUpperMsg() function.

SendUp is the function to be called if a message should be forwarded to

upper layers and is usually called within handleLowerMsg(). It decap-

sulates the message before sending it.

SendDown Sending messages to underlying layer is done with the send-

Down() function. Sometimes it may be necessary to provide or process

also additional meta information here.

The Message Concept

In order to provide basic functionality like encapsulating and decapsulating

messages in the Basic* modules we need to have fixed message formats for

every layer. The provided message types have the most important fields

needed for the corresponding layer. These message types with these fields

are obligatory and can only be extended but not exchanged. Here is a list of

all base message classes and their parameters


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For the physical Layer, it is used AirFrame message and contains the

following parameters: sending power, channel used to send message, idof the originator to get the position time needed to send the message

on the channel, control Information class used to pass Snr information

to the decider

Medium Access Control Message is MacPkt where we can find: destina-

tion MAC address, source MAC address and the channel for forwarding

the message.

NetwPkt is the message used in the network layer and in its field con-

tains: destination network address, source network address, sequence

number, time to life and the control Information class used to tell the

MAC protocol the address of the next hop.

Finally the ApplPkt is used to set destination application address and

source application address in the network layer.

However the framework allows to implement own message or modify an

existing message. For example to implement the protocol we need to add the

information about the Rank in the network layer, hence we should modify

the NetwPkt.

4.2.3 Network implementation

Below are listed the step to implement the simulation of the network.

Phase 1

Now decide the network topology, thus establish the nodes number and their

position through the .ini file. In the network there is a single root node that

works as a sink and collects packets from all the other nodes. We use a peer-

to-peer topology where there is a PAN coordinator, as in the star topology,

but is different because any devices can communicate with the other if they

are in the communication range.


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In our network we have only one root and the latency is the only metric

used (see the previous chapter), for this reason there is only one DAGID.Thus we dont make use of a DAGID in our software implementation. The

metric used to build the DAG and obtain the rank is the latency, so we have

to find first a method to compute the latency and get an initial value to

perform the algorithm to construct the DAG. We assume that nodes send a

broadcast packet and this packet is used to discover children, when a child

receives a message, the device can recognize the senders as a possible parent

and should send an acknowledgment to inform the sending node that has

receive the packet.

We establish that the node to send the ack should employ the CSMA algo-rithm, and this ack is used to estimate the latency to send a packet from the

child to the parent. The latency is given by the time between the instant in

which the node wants to send the ack message and the moment in which it is

sent, hence we have to consider the backoff periods and CCA timer that the

protocol uses to avoid collision. We consider the packet transmission time

negligible.

In the simulation each node sends five broadcast packets, nodes that receive

the broadcast packet (that will be possible children of the sending nodes)

have to replay sending an ack and collect the latency value, at the end of

the process a node should calculate the average latency to obtain a more

accurate value. Besides, since we want the latency value calculated on the

packets received, we have to derive the probability that a packet reaches the

destination node, and consider it in the metric formulation.

According to the RPL standard we have to quantize the latency to obtain a

metric value, called step of rank. Will be used to compute the rank of the

entering nodes and to find the OF value to select the best parent. This value

is calculated at the MAC layer and is stored in a matrix, that we call Latency

Matrix, where on the rows there is the node that sends the ack and on the

columns the receiving node. This matrix will be stored in a text file, to beused later at the network layer to decide the best parent. In a real situation

the OF value is stored in the node that should decide the parent to forward

the packet.

The process to obtain these initial values of the latency starts from the root,

which sends the number of broadcast packet established and each node that

received this message, may enter later in the network choosing a node as a

parent. When the root ceases to send its broadcast packets, only the nodes,


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that have received a packet, are able to send their packet to discover other

nodes that could enter in the network. At the end of this phase each nodeknows its neighbor and the latency to send packets to its parent. How many

packet of this kind we have to send, the sending rate and the traffic type are

decided at the application layer ned file.

Phase 2

When the discovering phase is finished the second step starts where we want

to form the DAG. The process starts from the root node that sends the first

DIO packet. We should add the necessary information changing the message

used in the network layer (NetwPkt) of Mobility Framework, especially we

should add the rank information, as we know this information is essential to

establish the position of the node into the DAG to avoid loop. According to

the protocol the root rank is 0 end the device will add this information in

the packet in the network layer.

Suppose that a node receives a DIO. The node add himself to the DAG, and

compute the value of the OF of all the candidate parents node. The parent

node with the lower OF value will be selected as a best parent and the node

that will enter in the DAG choosing its best parent will calculate its rank asthe candidate parents rank plus the step of rank. Every time a node receives

a DIO, each node will update its routing table with the address of the best

parent. When a node receives a DIO it will chooses a parent and update its

rank. Than this node will inform the other nodes about these changes by

sending its own DIO. As mentioned in Chapter 3, nodes will retransmit the

DIO packet to update continuously the DAG, in our case we simplify the

DIO transmission method and assume that it is sent with a constant interval

time.

In this way every node, after that has received a DIO packet and has joinedto the DAG, starts to send traffic packet to its best parent. This packet will

be forwarded to the corresponding best parent from the receiving node, until

the packet will reach the sink node. For this reason each node will add the

sink address as final destination address and the best parent address as next

hop address. When a node will receive a packet it will check if the packet

is addressed to itself, otherwise will search on its routing table the next hop

address to reach the destination. Clearly in many applications there may be


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multiple final destinations,which means having more sink nodes.

Latency estimation

During the transmission of the traffic packets, the latency estimation keep

running using the same method of the first phase, the purpose is to see if

there is a variation of the latency caused by the cross-traffic packets that

could influence the routing state, changing the route to achieve the sink.

Each node collects a certain number of latency value, compute the mean of

the latency and update the latency matrix. Whenever a node will retransmit

the DIO, the receiving node update the OF value and if is necessary changes

the best parent, thus finding a path that is less expensive.

In this work we use two methods to derive the latency. The first way is

experimental and it takes into account the probability that the packet is

correctly received. Since we know how many packets a node sends and if the

packet is received, this probability is easy to find, so the latency is given by

the following relation .

step of rank = experimental latency1

probability of correct reception

Then we use an empirical mode to verify the results obtained. The latencyis given by the following equation [12]:

D =

NBi=0

i

k=0

Wk + 1

2

i(1 )

1 NB+1+ L

rs,

where rs is the slot duration, is the busy channel probability, NB the maxi-

mum number of backoff, Wi is the delay window and it is initially W0 = 2BEmin

and doubled any stage until Wi = Wmax = 2BEmax, (BEmax BEmin) i

N B and L denotes the packet transmission duration measured in slots. This

model is empirical since is obtained experimentally using the simulator.

Every node knows how many times it tries to attempt to access the channel

and the number of times that it was found busy, so it easy derive this prob-

ability. How to obtain the equation above is explained in the appendix. To

have another proof of the measures we are going to compute using only the

analytical model. Knowing when a packet is sent by the node, we can com-

pute the value of X the denotes the time duration to wait before the next

transmission attempt. Using matlab simulation we derived the analytical

value of to obtain the mean latency for the sent packet.


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4.2.4 Network parameters

In this section we shown how to set up the parameters of the network.

Information about traffic

Our simulation is implemented in three steps In the first we want to find

the metrics to compute the nodes rank and build up the DAG according

to the RPL rules. We have 8 nodes and the node do not move, the node 0

is the sink node and each node after that is entered in the network, has to

send 5 packets to discover other nodes obtaining also the latency as described

in the previous paragraph. Nodes generate broadcast packets, to schedulethis packet the simulator will choose a random value uniformly distributed

between 0 and 2.

When all the nodes in the network have sent their five packets the sink node

starts the second phase sending the firs DIO packet, thus each node sends

periodically the DIO every 0.5 seconds to update the DAG periodically. After

a node sends its first DIO packet, since it entered the DAG, starts to send

traffic packet towards its best parents. The traffic model used is uniform

also in this case, and the interval chosen for the traffic is between 0,0012

and 0,075 seconds. Is not possible choose 0 as lower bound, because it couldhappens that the node will schedule the sending of the next packet before

the node has sent the previous packet. The result would be finding the node

busy and inserting the packet its queue, in this way there is the risk to have

many packets in the queue and to lose it. After the node sends the traffic

packet, we collect the latency value in a vector and every 1 second, when

this timer is expired we compute average of the vector updating the metric

in the latency matrix. The simulation ends when all the nodes have sent all

the DIO and the traffic packet.

Channel parameters

The ChannelControll is the Module to control the channel and is the central

module that coordinates the connections between all nodes, and handles dy-

namic gate creation. In this module we can set this four parameters pMax,

sat, , and carrier frequency, where pMax is maximum sending power used

for this network (in mW), sat is the minimum signal attenuation threshold

(in dBm), alpha is m

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