Abstract
Wireless Sensor Network (WSN) consists of many tiny, autonomous sensor nodes capable of sensing, computation and communication. The main objective of IEEE 802.15.4 based WSN standard is to provide low cost, low power and short range communication. Providing QoS in WSN is a challenging task due to its severe resource constraints in terms of energy, network bandwidth, memory, and CPU. In this chapter, Quality of Service (QoS) performance evaluation has been carried out for IEEE 802.15.4 networks based WSN star and mesh topology using routing protocols like AODV, DSR and DYMO in QualNet 4.5 simulator. Performance evaluations metrics like Packet Delivery Ratio (PDR), throughput, average end to end delay, energy per goodput bit, network lifetime of battery model and total energy consumption which includes transmission, reception, idle and sleep mode were considered for both the topology. From the simulation studies and analysis, it can be seen that on an average DSR and DYMO performs better than AODV for different traffic load rates.
Top1. Introduction
Wireless Sensor Networks(WSN) have gained world-wide attention in recent years due to the advances made in wireless communication, information technologies and electronics field (Akyildiz, Su, Sankarasubramaniam, & Cayirci, 2002; Yick, Mukherjee & Ghosal, 2008).The concept of wireless sensor networks is based on a simple equation: Sensing + CPU + Radio = Thousands of potential applications (Hill, 2003) . It is an “In situ” sensing technology where tiny, autonomous and compact devices called sensor nodes or motes deployed in a remote area to detect phenomena, collect and process data and transmit sensed information to users via radio frequency (RF) channel.
WSN term can be broadly sensed as devices range from laptops, PDAs or mobile phones to very tiny and simple sensing devices. At present, most available wireless sensor devices are considerably constrained in terms of computational power, memory, efficiency and communication capabilities due to economic and technology reasons. That’s why most of the research on WSNs has concentrated on the design of energy and computationally efficient algorithms and protocols, and the application domain has been confined to simple data-oriented monitoring and reporting applications. WSNs nodes are battery powered which are deployed to perform a specific task for a long period of time, even years.
The basic block diagram of a wireless sensor node is presented in Figure 1. It is made up four basic components: a sensing unit, a processing unit, a transceiver unit and a power unit. A MICAZ mote is shown in Figure 2.
Figure 1.
Architecture of a wireless sensor node
Figure 2. Moog Crossbow, n.d. - •
Sensing Unit: Sensing units are usually composed of two subunits: sensors and analog to digital converters (ADCs). Sensor is a device which is used to translate physical phenomena to electrical signals. The analog signals produced by the sensors based on the observed phenomenon are converted to digital signals by the ADC and then fed into the processing unit.
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Processing Unit: The processing unit mainly provides intelligence to the sensor node. The processing unit consists of a microprocessor, which is responsible for control of the sensors, execution of communication protocols and signal processing algorithms on the gathered sensor data. For example, the processing unit of a smart dust mote prototype is a 4 MHz Atmel AVR8535 micro-controller with 8 KB instruction flash memory, 512 bytes RAM and 512 bytes EEPROM. TinyOS operating system is used on this processor, which has 3500 bytes OS code space and 4500 bytes available code space.
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Transceiver Unit: The transceiver unit enables wireless communication with neighbouring sensor nodes and the outside world. It consists of a short range radio which usually has single channel at low data rate and operates at unlicensed bands of 868-870 MHz (Europe), 902-928 MHz (USA) or near 2.4 GHz (global ISM band). The Chipcon’s CC2420 is included in the MICAZ mote that was built to comply with the IEEE 802.15.4 standard (802.15.4, 2006) for low data rate and low cost wireless personal area networks.
Key Terms in this Chapter
Beacon: A beacon is a packet sent by PAN coordinator to inform other devices its presence and readiness.
Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA): Before accessing the wireless medium, nodes usually senses the wireless medium activity. If it is not busy, the nodes can send packets, otherwise nodes will have to wait for a random backoff delay to avoid collision.
Non-Beacon Enabled Mode: A network in which the PAN coordinator does not transmit beacons is known as a Nonbeacon network.
Wireless Sensor Network: A Wireless Sensor Networks (WSN) consisting of spatially distributed tiny, autonomous and compact devices called sensor nodes or motes are deployed in a remote area to detect the phenomena, collect and process data and transmit sensed information to users.
The Beacon-Enabled Mode: In this mode, beacons are periodically sent by the PAN or Coordinator to synchronize nodes that are associated with it, and to identify the PAN.
Network Lifetime: This is defined as the maximum duration of time during which deployed sensors have the capability of monitoring the phenomena of interest.
Quality of Service: The collective effect of service performance which determines the degree of satisfaction of a user of the service
In-Situ Sensing: This is a type of sensing, where sensing is done close to the phenomena of interest. Example: Wireless sensor network.
Reduced Functional Device (RFD): End devices can only acts a slave. They do not have the capability of relaying the messages. They can only communicate to network coordinator. These devices are designed for high energy saving and typically battery powered.
Star Topology: In star Topology, PAN Coordinator will communicate with other devices.
Full Functional Device (FFD): This device acts as a Personal Area Network (PAN) Coordinator (Master) or Coordinator. It is responsible for setting up the networks by sending beacon messages and routing packets in the network.
Mesh Topology: In Mesh Topology, all Coordinators will communicate among themselves as well as with the end devices.