QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS-aware MAC protocols for WSNs <s> Wireless sensor networks are appealing to researchers due to their wide range of application potential in areas such as target detection and tracking, environmental monitoring, industrial process monitoring, and tactical systems. However, low sensing ranges result in dense networks and thus it becomes necessary to achieve an efficient medium-access protocol subject to power constraints. Various medium-access control (MAC) protocols with different objectives have been proposed for wireless sensor networks. In this article, we first outline the sensor network properties that are crucial for the design of MAC layer protocols. Then, we describe several MAC protocols proposed for sensor networks, emphasizing their strengths and weaknesses. Finally, we point out open research issues with regard to MAC layer design. <s> BIB001 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> QoS-aware MAC protocols for WSNs <s> This paper presents a new prioritized medium access control protocol (PSIFT) that improves the performance of event-driven wireless sensor networks by suppressing the redundant reports of sensor nodes in proximity. This suppression of similar reports results in low collision probability, shorter delay and accurate detection of events by the sink. PSIFT is a CSMA-based MAC protocol that provides traffic differentiation by varying the inter frame space and contention window size for each traffic class. We also introduce the new report suppression mechanism based on the broadcast nature of wireless transmission in PSIFT. This mechanism works very efficiently in large-scale, event-driven wireless sensor networks. Simulation results show that the new PSIFT MAC protocol can bring significant improvement in delay performance of the networks. When the sensing range grows up to transmission range, PSIFT can reduce the latency as much as three times in delivering reports as compared to the existing IEEE802.11 MAC protocol. <s> BIB002
As emphasized in the previous sections, MAC layer of the architecture stack plays a key role in QoS provisioning. There are numerous WSN MAC protocols in the literature BIB001  but few of them take QoS support into account. Since sensor nodes are battery-powered devices, the main motivation of the almost all of the proposed MAC protocols is energy-awareness. However, there is an increasing necessity for efficient QoS-aware MAC protocols parallel to the increasing application fields such as health care, surveillance and process control. In this section, QoS-aware MAC protocols in the literature will be surveyed along with their advantages and disadvantages. We will start with the protocols employing service differentiation and continue with application specific ones. Then, protocols providing indirect support to QoS provisioning will be mentioned. Comparison and classification of the existing QoS-aware MAC protocols for WSNs will conclude this section. 6.1. Protocols with differentiated services 6.1.1. PSIFT PSIFT BIB002 is a QoS-aware MAC protocol designed for event-driven applications and it is based on the SIFT protocol  , which exploits the spatial correlation property of WSNs. SIFT assumes that the first R of N reports of a detected event are the most important part of the messaging and have to be relayed with low latency. R reports will be sufficient for the sink node to accurately identify the event and elimination of redundancy decreases both probability of collision and latency. Authors proposed two methods ''Explicit ACK'' and ''Implicit ACK'' for suppressing the unnecessary redundant reports by utilizing the broadcast nature of the wireless medium. PSIFT is a Carrier Sense Multiple Access (CSMA)-based MAC protocol and provides traffic differentiation by varying the inter frame space (IFS) and contention window (CW) size for each traffic class, as shown in Fig. 2 . Traffic classes are prioritized in a dynamic manner based on the traversed number of hops, i.e. the higher number of hops traversed, the higher level of priority that a packet has. Advantages and disadvantages: Although PSIFT might be a sensible choice for event-driven applications, it is nearly impossible to be used in any other type of applications. Besides, removal of redundancy may result in unreliable data delivery since identification of reports belonging to separate events will be an issue to be solved. Report suppression mechanism decreases the traffic load in the network and leads to mostly idle sensor nodes. This advantage of the PSIFT must be utilized to decrease the energy consumption of the network by integrating a sort of sleeplisten schedule.