## 2 QoS Requirements  
As [PERSON]'s law continues, it is envisioned that WSANs will become pervasive in our daily lives, for example, in our homes, offices, and cars. They promise to revolutionize the way we understand and manage the physical world, just as Internet transformed how we interact with one another. Ultimately, they will be connected to the Internet in order to achieve global information sharing [8]. This technical trend is driving WSANs to provide QoS support because they have to satisfy the service requirements of various applications.  
From an end user's perspective, real-world WSAN applications have their specific requirements on the QoS of the underlying network infrastructure [4]. For instance, in a fire handling system, sensors need to report the occurrence of a fire to actuators in a timely and reliable fashion; then, the actuators equipped with water sprinklers will react by a certain deadline so that the situation will not become uncontrollable. It is intuitive that different applications may have different QoS requirements. For instance, for a safety-critical control system, large delay in transmitting data from sensors to actuators and packet loss occurring during the course of transmission may not be allowed, while they may be acceptable for an air-conditioning system that maintains the temperature inside an office.  
Although QoS is an overused term, there is no common or formal definition of this term. Conceptually, it can be regarded as the capability to provide assurance that the service requirements of applications can be satisfied. Depending on the type of target application, QoS in WSANs can be characterized by reliability, timeliness, robustness, availability, and security, among others. Some QoS parameters may be used to measure the degree of satisfaction of these services, such as throughput, delay, jitter, and packet loss rate. There are many other QoS parameters worth mentioning, but these four are the most fundamental [9; 10; 11; 12].  
* _Throughput_ is the effective number of data flow transported within a certain period of time, also specified as bandwidth in some situations. In general, the bigger the throughput of the network, the better the performance of the system is. Those nodes that generate high-speed data streams, such as a camera sensor node used to transmit images for target tracking, often require high throughput. In order to improve the resource efficiency, furthermore, the throughput of WSAN should often be maximized.
* _Delay_ is the time elapsed from the departure of a data packet from the source node to the arrival at the destination node, including queuing delay, switching delay, propagation delay, etc. Delay-sensitive applications usually require WSANs to deliver the data packets in real-time. Notice that real-time does not necessarily mean fast computation or communication [5,12]. A real-time system is unique in that it needs to execute at a speed that fulfills the timing requirements.
* _Jitter_ is generally referred to as variations in delay, despite many other definitions. It is often caused by the difference in queuing delays experienced by consecutive packets.
* _Packet loss rate_ is the percentage of data packets that are lost during the process of transmission. It can be used to represent the probability of packets being lost. A packet may be lost due to e.g. congestion, bit error, or bad connectivity. This parameter is closely related to the reliability of the network.