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1. Wireless Sensor Network:
This chapter gives an introduction to field of sensor networks and its advancement from traditional wired technique to wireless. Background information about Wireless Sensor Network (WSN) concept, different wireless sensor networks available and sensor node architecture that can be used have been briefly discussed. After comparison, a suitable communication technique (MiWi) is selected that would best suite the requirements of this project. Brief discussion on how communication acts as a medium for exchanging information in our electronic system is provided. At the end current progress is also depicted with the help of experimental results
Chapter summary: "This chapter provides brief information about Wireless Sensor Network (WSN) concept, the potential benefits of sensor technology in domestic house, machinery, structures etc. The concept of each sensor node and communication of different sensor nodes with central hub is also been discussed briefly''
Sensors incorporated into domestic house, environment, machinery or structures coupled with efficient delivery of sensed information, could result in incredible benefits to the society. Some of the potential benefits include: efficient utilization of electricity, enhanced homeland security, conservation of natural resources, improved manufacturing productivity, fewer catastrophic failures and improved emergency response. Nevertheless, barriers to widespread use of sensors in home, environment and machinery still remain because enormous amount of lead and fibre optic wires are prone to breakage or connection failures. Bundles of long wire cause significant installation and long term maintenance cost hereby limiting the number of sensors that can be installed and dropping the overall quality of the data.
Wireless sensor networks (WSN) can eliminate these costs, ease installation and eliminate the need of connectors.
1.1.1. Wireless sensor network concept
A Wireless Sensor Network (WSN) consist of spatially distributed independent sensors that monitors environmental and physical conditions such as light, temperature, humidity, sound, pressure, motion etc.
1.2. Wireless Sensor Network Architecture
''Chapter summary: There are a number of different topologies for radio communications networks. An overview of the network topologies that apply to wireless sensor networks are outlined below. In the later part of the chapter; justifications provided for using Star-mesh network topology in this project.
1.2.1. Star Network (This topology will not used)
In computer networking, star topology is the most commonly used network topology in which a single base station (central hub) can send and/or receive a message to various sensor nodes. Sensor nodes in this topology can only send or receive a message from the single base station; they are not allowed to transmit messages to each other.
In the above star model:
There is a central hub/base-station and the other sensor nodes can directly communicate only to the central hub. Such approach determines following networking features:
- Network is simple in set up and deployment.
- Data forwarding is possible only by central hub.
Advantage of the star topology is the simplicity of adding sensor nodes to the network and the ability to keep the remote node's power consumption to a minimum.
The primary disadvantage of the star topology is that the Central hub is a single point of failure. If the central hub stops working the entire network would fail as a result of the hub being connected to every node on the network. In such a network the central node must be within radio transmission range of all the individual nodes. Hence, this type of topology is not as robust as other networks because of its dependency on a single node to manage the network.
1.2.2. Mesh Network (This topology will not used)
Mesh topologies are multi-hopping systems in which all wireless sensor nodes are identical and communicate with each other to hop data to/from the sensor nodes and base station. Wireless sensors can also pass commands directly to each other in a mesh network, avoiding the need to communicate with each other through a base station. In addition, if a node wishes to transmit a message to another node which is out of transmission range then it can use intermediate nodes to forward the message to the desired node.
In the event of error, if an individual sensor node fails, the network will automatically reconfigure itself around the failed node; that is, a remote node can still communicate to any other node in its range which can transmit the message to the desired location. Moreover the range of this network is not necessarily limited by the range between the single nodes; it can be extended by adding more number of nodes to the existing system
This network topology has the advantage of redundancy and scalability.
High power consumption: The higher duty ratio of a mesh network (since sensor nodes need to always listen for messages or for changes in the prescribed routes through the mesh), is the principal reason for the high power requirements.
High Latency: Additionally, as the number of communication hops to a destination increases, the time to deliver the message also increases, especially if low power operation of the nodes is a requirement.
1.2.3. Hybrid Star- Mesh Network / Tree topology (Used)
A hybrid between the star and mesh network provides for a robust and versatile communications network, while maintaining the ability to keep the wireless sensor nodes power consumption to a minimum.
In this network topology, the base station communicates both with the low power sensor nodes (end device) and high power sensor nodes (mainly router to extend the range of the network). This type of network topology can be also seen as a tree like structure where base-station is the root of the tree, high power sensor node (router) as the branches of the tree and lower power sensor nodes ( end-device/sensors) as the leaves of the tree. In this tree network, all of the messages sent through the network follow the path of the tree structure.
Minimum power consumption can be maintained as the end devices can be kept on sleep mode when they are not active. Moreover the range of the network can be extended with the help of routers unlike the star topology
The Central hub/base-station is a single point of failure and if the hub stops working the entire network would fail as a result of hub being directly connected to the other nodes on the network.
1.2.4. Wireless sensor network Node Architecture
Each node is a sensor network is generally integrated with a transceiver module (wireless communication device), a microcontroller and an energy source, commonly a battery.
In the above diagram: Sensor Node
- Sensor/Actuators: Sensors are used to collect measurements and actuators to perform the desired operations.
- Micro-controller: Processes received data, prepares data for transmission and executes required networking and application specific tasks. Determines power consumption and software limitations.
- Transceiver module: Needed to receive and transmit data over the air. Selection of these components determines transmission range and strongly influences power consumption.
- Networking and application software: Specifies networking protocols and application functionality. Implemented networking features determine functional flexibility on the application level
- Power supply: Is required to power the node (usually a battery)
An ideal wireless sensor consumes very low power, is smart, efficient and software programmable which is capable of fast data acquisition, reliable and accurate over long term costs little to purchase and install, and requires no real maintenance.
1.2.5. WSN Technology Requirements:
According to its concept and target applications the wireless sensor network technology should meet the following requirements:
- Low cost and small size devices
- Low power consumption
- Unlicensed radio band
- Scalability: Support large number of nodes
- Flexibility: Simple deployment and network extension
- Low data rate is sufficient
- Data and network security support
Among existing wireless technologies, WSN is the only one that targets simple communication with low data rates and low power consumption.
1.3. Different types of wireless sensor networks:
The physical radio layer defines the operating frequency, modulation scheme, and hardware interface of the radio to the system. There are many low power proprietary low power radio integrated circuits that are appropriate choices for the radio layer in wireless sensor networks, including those from companies such as Atmel, MicroChip, Micrel, Melexis, and ChipCon. If possible, it is advantageous to use a radio interface that is standards based. This allows for interoperability among multiple companies networks. A discussion of existing radio standards and how they may or may not apply to wireless sensor networks is given below.
IEEE802.11 is a standard that is meant for local area networking for relatively high bandwidth data transfer between computers or other devices. The data transfer rate ranges from as low as 1 Mbps to over 50 Mbps. Typical transmission range is 300 feet with a standard antenna; the range can be greatly improved with use of a directional high gain antenna. Both frequency hopping and direct sequence spread spectrum modulation schemes are available. While the data rates are certainly high enough for wireless sensor applications, the power requirements generally preclude its use in wireless sensor applications.
Bluetooth (IEEE802.15.1 and .2)
Bluetooth is a personal area network (PAN) standard that is lower power than 802.11. It was originally specified to serve applications such as data transfer from personal computers to peripheral devices such as cell phones or personal digital assistants. Bluetooth uses a star network topology that supports up to seven remote nodes communicating with a single basestation. While some companies have built wireless sensors based on Bluetooth, they have not been met with wide acceptance due to limitations of the
Bluetooth protocol including:
- Relatively high power for a short transmission range.
- Nodes take a long time to synchronize to network when returning from sleep mode, which increases average system power.
- Low number of nodes per network (<=7 nodes per piconet).
- Medium access controller (MAC) layer is overly complex when compared to that required for wireless sensor applications.
The 802.15.4 standard was specifically designed for the requirements of wireless sensing applications.
The standard is very flexible, as it specifies multiple data rates and multiple transmission frequencies.
The power requirements are moderately low; however, the hardware is designed to allow for the radio to be put to sleep, which reduces the power to a minimal amount. Additionally, when the node wakes up from sleep mode, rapid synchronization to the network can be achieved. This capability allows for very low average power supply current when the radio can be periodically turned off. The standard supports the following characteristics:
- Transmission frequencies, 868 MHz/902-928 MHz/2.48-2.5 GHz.
- Data rates of 20 Kbps (868 MHz Band) 40 Kbps (902 MHz band) and 250 Kbps (2.4 GHz band).
- Supports star and peer-to-peer (mesh) network connections.
- Standard specifies optional use of AES-128 security for encryption of transmitted data.
- Link quality indication, which is useful for multi-hop mesh networking algorithms.
- Uses direct sequence spread spectrum (DSSS) for robust data communications.
It is expected that of the three aforementioned standards, the IEEE 802.15.4 will become most widely accepted for wireless sensing applications. The 2.4-GHz band will be widely used, as it is essentially a worldwide license-free band. The high data rates accommodated by the 2.4-GHz specification will allow for lower system power due to the lower amount of radio transmission time to transfer data as compared to the lower frequency bands.
MiWi™ and MiWi P2P are proprietary protocol stacks developed by Microchip for short-range wireless networking applications based on the IEEE 802.15.4™ wireless personal area network (WPAN) specification. The MiWi protocol stacks offer a small foot-print alternative to the standard based ZigBee® compliant protocol stack and are optimized for low-power, low data rate, cost sensitive application.
- Lowest-cost fully functional network protocol for IEEE 802.15.4 transceivers
- Ultra small memory footprint proprietary protocols
- Simple, low cost solution for customers who do not need interoperability with other ZigBee devices
- No Fees or certification required
- Protocol stacks are provided FREE of charge when used with a Microchip's PIC® microcontrollers and the MRF24J40 transceiver/transceiver module.
The ZigBee™ Alliance is an association of companies working together to enable reliable, cost-effective, low-power, wirelessly networked monitoring and control products based on an open global standard. The ZigBee alliance specifies the IEEE 802.15.4 as the physical and
MAC layer and is seeking to standardize higher level applications such as lighting control and HVAC monitoring.
It also serves as the compliance arm to IEEE802.15.4 much as the Wi-Fi alliance served the IEEE802.11 specification. The ZigBee network specification, to be ratified in 2004, will support both star network and hybrid star mesh networks. As can been seen in Figure 22.4.1, the ZigBee alliance encompasses the IEEE802.15.4 specification and expands on the network specification and the application interface.
This chapter provides an introduction to MiWi protocol stack developed by Microchip technology. The Physical layer and medium access layer similar to IEEE 802.15.4 have also been discussed briefly. For completeness, the document also introduces several aspects of wireless networking, as well as key features of IEEE 802.15.4.This chapter also explains .... More over the advantages of using this wireless technology has also been discussed briefly.
MiWi Star/Mesh and MiWi P2P are wireless protocols stack developed by Microchip Technology. The key feature of this technology is that it uses small, low power digital radios based on the IEEE 802.15.4 standards for wireless personal area network (WPANs). MiWi is an upcoming technology having its application in areas such as industrial monitoring and control, home and building automation, remote control, gamming, environmental monitoring because of its low data transmission rates and short distance cost constrained networks. The MiWi protocols are supported on certain Microchip PIC and dsPIC microcontrollers. When developing for these platforms, proprietary SDKs and hardware development tools, such as the ZENA wireless packet sniffer are used.
MiWi protocol has the following features:
- Lowest-cost fully functional network protocol for IEEE 802.15.4 device types
- Support for the 2.4 GHz spectrum through the MRF24J40 transceiver
- Simple, low cost solution for customers who do not need interoperability with other ZigBee devices
- Portable across PIC16, PIC18, PIC24 and dsPIC33 devices
- Easily programmable with the help of MPLAB, requires basic knowledge of C language
- No Fees or certification required
- Very small memory footprint proprietary protocols
1.4.3. MiWi: MAC and PHY layer:
MiWi protocol stack is based on MAC and PHY layers of IEEE 802.15.4 standards and is altered for network development in the 2.4 GHz band
Medium Access Control sub-layer (MAC)
MAC sub-layer is responsible for reliable communication between two devices over direct physical link (without intermediate nodes). The key functions of the MAC layer include:
- Data framing and validation of received frames
- Device addressing
- Channel access management
- Device association and disassociation
- Sending acknowledgement frames
Physical layer (PHY)
Physical layer provides means for bit stream transmission over the physical medium.
The key responsibilities of PHY are:
- Activation and deactivation of the radio transceiver
- Frequency channel tuning
- Carrier sensing
- Received signal strength estimation (RSSI & LQI)
- Data coding and modulation
- Error correction
Details on functionality of each layer are given below
Functionality of PHY Layer:
Physical layer is the lowest level in communication model and is responsible for data bit stream transmission/reception over the physical medium. The IEEE 802.15.4 defines three frequency bands of operation: 2.4 GHz, 915 MHz and 868 MHz in which each of the frequency bands offers a fixed number of channels and has different maximum bit rate.
Functionality of MAC Layer:
MAC layer defines mechanisms for direct (single hop) communication between two devices. Such single hop data exchange is possible only within transmission range of participating pair of nodes. Key MAC layer responsibilities are described below.
- Data framing
- Device addressing:
- Channel access management: CSMA-CA
- Device Association/Disassociation
- Upon higher layer requests MAC layer performs device association and disassociation (enters/leaves network).
Communication on MAC layer is packet based. It means that data to be sent is encapsulated into a MAC frame that is passed to RF transceiver. A node shall accept only frames destined for it and upon their reception frames are checked on errors that could have occurred during transmission and corrected if possible.
Each device is identified by unique 64 bits long MAC layer address that is used by sender as destination for the packets sent on the MAC layer.
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is well known "listen-before-send" principle for managing access to single physical channel among multiple devices. It ensures reliable communication and provides efficient usage of limited channel bandwidth.
1.4.4. MiWi Device types:
IEEE 802.15.4 defines devices based on their overall functionality. There are basically two device types shown in Table below
The MiWi protocol defines three types of MiWi protocol devices: PAN Coordinator, Coordinator and End Device. The MiWi Wireless Networking Protocol Stack functionality helps to determine the type of IEEE functionality that the device requires. The MiWi protocol device types and their relationship to IEEE device types are shown in Table below:
1.4.5. MiWi Network Topology Used in this Project: Star-Mesh
The above Star-mesh/cluster tree topology is used in this project and can be thought of as extending the Star-topology. From the above diagram it's seen that theirs only one PAN-coordinator/base-station which forms the network, allocates network addresses etc. Coordinator is added to extend the range of the network to the sensor nodes which are out of the network coverage. Moreover Coordinator itself can also perform monitoring/control functions. End devices (RFD/FFD) are linked with the PAN coordinator or coordinator to send and receive signal to the base station.
This network acts like a Tree like structure where signals sent/receive follow the pattern of: PAN Coordinator (root of tree) to Co-ordinator (branches of tree) to End device (Leaves of tree)
Hence star-mesh topology would be ideal for this project as there will be only one central hub (PAN coordinator) and nodes can be kept on sleep to minimize power consumption as Energy consumption is the key concern of this project because most of the energy is consumed by communication rather than sensor/micro-controllers etc.
1.4.6. Justification for using Star-Mesh Network Topology:
A hybrid between the star and mesh network/Tree topology provides for a robust and versatile communications network.
Taking example of the previous house module let analysis the benefits of using star-mesh network topology. The base-station has a limited network range and can communicate with sensor/actuator nodes within its network range to perform the desired operation (Eg: Assuming network range is limited to living room, kitchen, and bedroom 1). To extend the range of this network to the other rooms of the house a router is used which improves the network range of the wireless sensor network thereby allowing the sensor nodes of the other rooms (Eg: Bedroom 2) to send and receive messages from the base station.
Moreover unlike mesh networking, sensor nodes in tree topology can stay in sleep mode (when not in operation) thus consuming less energy and saving power. As most of the energy is consumed by communication in wireless sensor network, it is important to consider the energy consumption for building an effective system.
Thus a good choice would be to use a Star-mesh network topology.