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Unique Challenges for Capillary Network
The capillary networks must adhere to the requirements of reliability, latency and connectivity imposed by innovative use-cases outlined for the fifth generation of communication infrastruc-ture [6]. Meeting these requirements especially in capillary networks is difficult owing to the atypical nature of devices.
It is envisioned that capillary networks would be used to enable M2M communication for the sensor type devices. These devices are characterized by limited power, compute and memory resources. Under these added constraints, the operational characteristic of the underlying short range radio technology used to enable the capillary network become critical to meet architec-tural needs. The technology must be able to minimize the delay, maximize networks operational lifetime while meeting the Quality of Service (QOS) requirements.
Irrespective of the enabling technology, the two aspects of which play a decisive role in delivering aforementioned requirements are: 1) How are the devices connected? Moreover, 2) How is the data transmitted over the multi-hop architecture? These can be classified as topology formation and routing aspects of the technology.
Bluetooth Technology Review
Bluetooth is a short-range communication technology aiming to replace wires over small dis-tances. It operates in 2.4 GHz range which is reserved for Industrial, Scientific and Medical (ISM) use. This free-rent policy allows endless technologies to operate in the spectrum, few to mention are Wi-Fi, ZigBee, microwave, etc. creating interference and requiring Bluetooth to be robust along with low power and low cost.
There are two types of Bluetooth devices: Basic Rate (BR) and Low Energy (LE). These devices include basic functionalities to perform device discovery, connection establishment and connection mechanism. These functionalities enable a Bluetooth device to detect neighbouring devices, connect and transfer data among them. The BR device may optionally have support for the Enhanced Data Rate (EDR), enabling faster data communication between the devices. A Bluetooth system is split into two logical entities, namely, host and controller.
Host: it is defined as a group of protocols for control and data multiplexing. It comprises of Logical Link Control and Adaptation Protocol (L2CAP) which is primarily responsible for multiplexing and de-multiplexing. Also, security and profiles are defined here enabling and dictating access to reusable device resources. It is possible to split the host application, the host, and the controller physically.
Controller: it is the radio module responsible for the reception and the transmission of pack-ets. The Bluetooth specification [14] dictates that a Bluetooth device can have one pri-mary and multiple secondary controllers. The primary controller is comprised of exactly one of BR/EDR or LE or combined BR/EDR and LE as the primary controller. In addi-tion, there can be multiple secondary controllers including an 802.11 Protocol Adaptation Layer (PAL), 802.11 Media Access Control (MAC) and Physical (PHY), and optionally Host Controller Interface (HCI). Various possible valid combinations of Bluetooth host and con-troller are represented in figure 2.1.
Physical Layer Operation
Bluetooth operates in ISM band. Depending on the controller i.e. BR/EDR or LE, a device may use 80 or 40 channels spaced at 1 MHz or 2 MHz. The first channel starts at 2042 MHz and con-tinues up to 2480 MHz. The lowest architectural layer in the Bluetooth system is the physical channel. A number of types of physical channels are defined which are characterized by the combination of a pseudo-random frequency hopping sequence, the specific slot timing of the transmissions, the access code and packet header encoding. These aspects, together with the range of the transmitters, define the signature of the physical channel.
Two devices that wish to communicate use a shared physical channel for this communica-tion. To achieve this, their transceivers must be tuned to the same Radio Frequency (RF) at the same time, and they must be within a nominal range of each other. Given that the number of RF carriers is limited and that many Bluetooth devices could be operating independently within the same spatial and temporal area there is a strong likelihood of two independent Bluetooth devices having their transceivers tuned to the same RF carrier, resulting in a physical channel collision. To mitigate the unwanted effects of the collisions each transmission on a physical channel starts with an access code that is used as a correlation code by devices tuned to the physical channel. The access code is a property of the channel and is always present at the start of every transmitted packet.
Five Bluetooth physical channels are defined. Each is optimized and used for a different purpose.
• Basic and Adapted piconet channel: they are used for communication between connected devices and are associated with a specific piconet.
• Inquiry scan channel: it is used for discovering Bluetooth devices.
• Page scan channel: it is used for connecting Bluetooth devices.
• Synchronization scan channel: it is used by devices to obtain timing and frequency infor-mation about the Connectionless Slave Broadcast physical link or to recover the current piconet clock. A Bluetooth device can only use one of these physical channels at any given time. To support multiple concurrent operations, the device uses time-division multiplexing between the chan-nels. In this way, a Bluetooth device can appear to operate simultaneously in several piconets, as well as to be discoverable and connectable.
The Bluetooth system provides a point-to-point (figure 2.2(a)) or a point-to-multipoint con-nection(figure 2.2(b)). One Bluetooth device acts as the master of the piconet, whereas the other device(s) act as a slave(s). Piconets that have common devices are called scatternet (fig-ure 2.2(c)). Each piconet has a single master. However, slaves can participate in different piconets on a time-division multiplex basis. Also, a master in one piconet can be a slave in other piconets.
Bluetooth BR/EDR Operation
The BR / EDR radio PHY uses a shaped, binary frequency modulation to minimize transceiver complexity. During typical operation, a physical radio channel is shared by a group of devices that are synchronized to a common clock and frequency hopping pattern. One device provides the synchronization reference and is known as the master. All other devices synchronized to a master’s clock and frequency hopping pattern are known as slaves. A group of devices syn-chronized in this fashion forms a piconet. This is the fundamental form of communication in the Bluetooth BR/EDR wireless technology.
Devices in a piconet use a specific frequency hopping pattern, which is algorithmically de-termined by certain fields in the Bluetooth address and clock of the master. The basic hopping pattern is a pseudo-random ordering of the 79 frequencies, separated by 1 MHz, in the ISM band. The hopping pattern can be adapted to exclude a portion of the frequencies that are used by interfering devices. The adaptive hopping technique improves co-existence with static (non-hopping) ISM systems when they are co-located. Frequency hopping takes place between the transmission or reception of packets. Bluetooth technology provides the effect of full duplex transmission through the use of a Time-Division Duplex (TDD) scheme.
Typically within a physical channel, a link is formed between a master device and slave devices. Exceptions to this include inquiry and page scan physical channels, which have no associated physical link. The link provides bi-directional packet transport between the master and slave devices, except in the case of a connectionless slave broadcast physical link. In that case, the link provides a unidirectional packet transport from the master to a potentially unlim-ited number of slaves. Since a physical channel could include multiple slave devices, there are restrictions on which devices may form a physical link. There is a link between each slave and the master. The physical links are not formed directly between the slaves in a piconet.
Bluetooth LE Operation
The LE system employs a frequency hopping transceiver to combat interference and fading and provides many Frequency Hopping Spread Spectrum (FHSS) carriers. The radio operation uses a shaped, binary frequency modulation to minimize transceiver complexity. It employs two multiple access schemes: Frequency Division Multiple Access (FDMA) and Time Division Mul-tiple Access (TDMA). Forty (40) physical channels, separated by 2 MHz, are used in the FDMA scheme. Three (3) are used as advertising channels, and 37 are used as data channels. A TDMA based polling scheme is used in which one device transmits a packet at a predetermined time, and a corresponding device responds with a packet after a predetermined interval. The physical channel is sub-divided into time units known as events. Data is transmitted between LE devices in packets that are positioned at events.
There are two types of events: Advertising and Connection events. Devices that transmit advertising packets on the advertising PHY channels are referred to as advertisers. Devices that receive advertising packets on the advertising channels without the intention to connect are called scanners. Transmissions on the advertising PHY channels occur in advertising events. At the start of each advertising event, the advertiser sends an advertising packet corresponding to the advertising event type. Depending on the kind of advertising packet, the scanner may make a request to the advertiser. LE devices may fulfil the entire communication in the case of unidirectional or broadcast type between two or more devices using advertising events.
They may also use advertising events to establish pair-wise bidirectional communication between two or more devices using data channels. Devices that need to form a connection to another device listen for connectable advertising packets. Such devices are referred to as initiators. If the advertiser is using a connectable advertising event, an initiator may make a connection request using the same advertising PHY channel on which it received the con-nectable advertising packet. The advertising event is ended, and connection events begin if the advertiser receives and accepts the request for a connection be initiated. Once a connection is established, the initiator becomes the master device in what is referred to as a piconet, and the advertising device becomes the slave device. Connection events are used to send data packets between the master and slave devices. Within a connection event, the master and slave alter-nate sending data packets using the same data PHY channel. The master initiates the beginning of each connection event and can end each connection event at any time [14].
BR/EDR Topology
Anytime a link is created using the BR/EDR controller it is within the context of a piconet. A piconet consists of two or more devices that occupy the same BR/EDR physical channel. The terms master and slave are only used when describing these roles in a piconet.
A number of independent piconets may exist nearby. Each piconet has a different physical channel (that is a different master device and an independent timing and hopping sequence). A Bluetooth device may participate concurrently in two or more piconets. It does this on a time-division multiplexing basis. A Bluetooth device can never be a master of more than one piconet. (Since in BR/EDR the piconet is defined by synchronization to the master’s Bluetooth clock it is impossible to be the master of two or more piconets.) A Bluetooth device may be a slave in many independent piconets. A Bluetooth device that is a member of two or more piconets is said to be involved in a scatternet. Involvement in a scatternet does not necessarily imply any network routing capability or function in the Bluetooth device.
Factors Affecting Network Lifetime
Given the definition of the lifetime, it is important to understand the various topological, routing and technology factors which impact the energy dissipation and thereby affecting the network lifetime. Under the purview of Bluetooth as the technology, it is important to consider following as-pects to obtain maximal network lifetime. In the current thesis, we focus on the problem of device role selection while topology adaptation and routing are mentioned for the complete-ness. We consider heterogeneous devices in the network and aim to uses their characteristics to formulate rules for assigning roles to the devices.
1. Device Role Selection: in the BLE scatternet, we have three kinds of devices as earlier outlined i.e. master, relay, and slave. It is recommended to assign these roles based on the device characteristics as master and relay nodes are used to enable inter and intra-piconet communication. Any algorithm which uses device characteristic based metric to assign these roles must be preferred over random role assignment. For example, it is optimal to assign ”master” role to a device with maximum energy. The node with second highest energy should be assigned the ”relay” role.
In the figure 2.5, an example of random and characteristics based role selections is pro-vided. The topology as depicted in figure 2.5b must be preferred over 2.5a as it allows a device with the maximum capability to be chosen as a master.
2. Topology Adaptation: as the network become operational, the energy at various nodes is consumed. The master and the relay nodes are worst affected owing to extra responsi-bility of relaying the traffic. Therefore, it is beneficial to alternate the role of the master and relay among several nodes. Thus, it is important for a topology formation algorithm to have a maintenance phase where options for suitable master and relays can be period-ically evaluated, and role can be switched if deemed necessary.
For example, let us assume two piconets with M1 and M2 as the master and R1, R2 and R3 as possible relays between them. Initially, all of them have 100% energy, and R1 is chosen as the relay. The rate of energy dissipation for a slave is 1 unit/sec and for a relay is 3 unit/sec while all the masters are connected to mains. The resulting topology is presented in figure 2.6. Let’s assume there are two algorithms, one with maintenance phase and one without, the network lifetime can be given by the following graphs in the figure 2.7. It is evident that existence of maintenance phase in the algorithm enhances the network lifetime by switching the nodes in master or relay roles.
3. Routing Algorithm: this plays a significant role as all inter-piconet communication must be routed. The routing algorithm generally performs two steps i.e. route discovery and packet forwarding along the discovered route. The methodology utilized to achieve these fundamental operations impact the network lifetime of the network.
Table of contents :
1 Introduction
1.1 Capillary Network – A Viable Option
1.1.1 Unique Challenges for Capillary Network
1.2 Motivation
1.3 Scenario
1.4 Research Question
1.5 Solution Approach
1.6 Research Methodology
1.7 Outline of the Thesis Contributions
2 Background Study
2.1 Bluetooth Technology Review
2.1.1 Physical Layer Operation
2.1.2 Bluetooth BR/EDR Operation
2.1.3 Bluetooth LE Operation
2.1.4 Bluetooth Topology
2.1.4.1 BR/EDR Topology
2.1.4.2 LE Topology
2.2 Network Lifetime
2.2.1 Factors Affecting Network Lifetime
2.3 Network Residual Energy
2.4 Topology Formation Algorithm(s)
2.4.1 WSN Topology Formation Algorithm(s)
2.4.2 Bluetooth based Topology Formation Algorithm(s)
2.5 Routing
2.6 Research Gap(s)
3 Problem Statement
3.1 Logical View of Problem
3.2 Input Parameters & Explanation
3.3 Objective
3.4 Output Parameter
4 Topology Formation considering Role Suitability (TFRS)
4.1 Role Suitability Metric (RSM)
4.2 Proposed Algorithm
4.3 Piconet Construction in Reference Algorithm
4.3.1 Limitation(s) of the Reference Algorithm
4.3.2 Piconet construction in Topology Formation considering Role Suitability (TFRS)Piconet construction in Topology Formation considering Role Suitability (TFRS)
4.4 Piconet Interconnection in Reference Algorithm
4.4.1 Limitation(s) of Reference Algorithm
4.4.1.1 Gateway Prioritization
4.4.1.2 Interconnect Rule(s) in Reference Algorithm
5 Methodology
5.1 System Design
5.1.1 Bluetooth Low Energy (BLE) Device
5.1.1.1 BLE Device Design
5.1.2 Transmission Manager
5.1.3 Channel Capacity Model
5.1.4 Power Model
5.1.5 Routing Model
5.1.6 Traffic Model
5.2 Simulation Environment
6 Performance Evaluation and Results
6.1 System Parameters
6.2 Simulation Results
6.2.1 TFRS versus Reference algorithm
6.2.2 Impact of Master Role Selection
6.2.3 Impact of Gateway Selection
6.2.4 Impact of Device-Type Ratio
6.2.5 Impact of Device Density
6.3 Limitation(s) of Simulations
7 Summary & Outlook
7.1 Discussion
7.2 Future Work
Appendix A Impact of Master Selection on Network lifetime
Appendix B Impact of Gateway Selection on Network lifetime
Appendix C Impact of Algorithms on Network lifetime
Appendix D Device Survey
D.1 Rechargeable Battery Capacity
D.2 Coin Cell Battery Capacity
