Wireless Sensor Networks (WSNs) are becoming common in use, with a vast diversity of applications. Due to its resource constraints, it is hard to maintain Quality of Service (QoS) with WSNs. Though they contain a vast variety of applications, at the same time they are also required to provide different levels of QoS, for various types of applications. A number of different issues and challenges still persist ahead to maintain the QoS of WSN, especially in critical applications where the accuracy of timely, guaranteed data transfer is required, such as chemical, defense, and healthcare. Hence, QoS is required to ensure the best use of sensor nodes at any time. Researchers are trying to focus on QoS issues and challenges to get maximum benefit from their applications. With this chapter, the authors focus on operational and architectural challenges of handling QoS, requirements of QoS in WSNs, and they discuss a selected survey of QoS aware routing techniques by comparing them in WSNs. Finally, the authors highlight a few open issues and future directions of research for providing QoS in WSNs.
TopIntroduction
WSN is a wireless network consisting of small nodes with sensing, computation, and wireless communications capabilities (Al-Karaki & Kamal, 2004), operating in an unattended environment, with limited computational and sensing capabilities. Normally nodes routs data back to the base station BS (Akyildiz, Su, Sankarasubramaniam, & Cayirci, 2002a). Data transmission is usually a multi- hop, from node to node toward the base station or gateway. Sensor nodes are equipped with small, often irreplaceable batteries with limited power and computation capacity. Important concern is the network lifetime and QoS as nodes run out of power, congestion may caused, the connectivity decreases and the network can finally be partitioned and become dysfunctional (Akyildiz, Su, Sankarasubramaniam, & Cayirci, 2002b),(Bhardwaj & Chandrakasan, 2002),(Ettus, 1998), which directly involved to reduce the QoS of network. The minimum energy routing problem linked to QoS has been addressed in (Rodoplu & Meng, 1999),(Estrin, Govindan, Heidemann, & Kumar, 1999), (Hac, 2003), (Wood & Stankovic, 2010),(Sadek, Su, & Liu, 2007), (Sadek, Su, & Liu, 2010). The service is bound to the data and to the application QoS vs. QoI (Quality of Information), QoSu (Jeong, Sharafkind, & Du, 2009) (Quality of Surveillance) and data delivery can be continuous, event-driven, query-driven, or hybrid. An important concern is the QoS of network. QoS model can be defined as under, what the users have required from the network. User demands for guaranteed data transfer timely, guaranteed bandwidth in case of image and video data, data accuracy timely in case of any critical applications such as defense or health application. Resulting sensor nodes’ network must satisfy requirements of users as shown in figure 1 with QoS simple model .
Figure 1. QoS model of WSN (Bhuyan, et al., 2009)
TopQuality of Service (QoS) always has very key role for all types of applications in networks, including conventional, wireless ad hoc and wireless sensor network. QoS routing is performed usually through resource reservation in a connection oriented communication in order to meet the QoS requirement for each individual connection. While couple of different mechanisms have been proposed for routing QoS constrained image and video type of data in wire based network (Lee et al, 1995)(Wang & Crowcraft, 1996)(Ma & Steenkiste, 1997)(Zhang, et al. 1993)(Crowley, et al., 1998), they cannot be directly applied to wireless sensor network, because of its different architecture, structure and resource constraints. Therefore several new protocols have been proposed for QoS routing in wireless networks taking the dynamic nature of the network into account (Querin & Orda, 1997)(Chen & Nahrstedt, 1999)(Srivakumar, et al., 1998)(Lin, 2000)(Zhu & Corson, 2002). Some of the proposed protocols consider the imprecise state information while determining the routes (Querin & Orda, 1997)(Chen & Nahrstedt, 1999).