QoS-aware MAC protocols for wireless sensor networks: A survey <s> PQ-MAC <s> This paper presents the design, implementation and performance evaluation of a hybrid MAC protocol, called Z-MAC, for wireless sensor networks that combines the strengths of TDMA and CSMA while offsetting their weaknesses. Like CSMA, Z-MAC achieves high channel utilization and low latency under low contention and like TDMA, achieves high channel utilization under high contention and reduces collision among two-hop neighbors at a low cost. A distinctive feature of Z-MAC is that its performance is robust to synchronization errors, slot assignment failures, and time-varying channel conditions; in the worst case, its performance always falls back to that of CSMA. Z-MAC is implemented in TinyOS. <s> BIB001 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> PQ-MAC <s> The media access control (MAC) protocol in wireless sensor networks provides a periodic listen/sleep state for protection from overhearing and idle listening. However, many scenarios and applications exist in which sensor nodes must send data quickly to destination nodes. This paper proposes the priority-based quality-of-service MAC (PQMAC) protocol for wireless sensor networks. We use data priority levels to differentiate among data transmissions, and propose a MAC protocol based on these levels. This protocol manages scheduling by adaptively controlling network traffic and the priority level. We focused on reducing the latency of the message transmission from the source to the destination. Simulation results showed that PQMAC reduces latency problems in wireless sensor networks while maintaining energy efficiency. <s> BIB002
PQ-MAC BIB002 aims to use advantageous features of both contention based and schedule based approaches and uses a hybrid scheme for medium sharing. Global clock synchronization, neighbor discovery and accordingly slot assignment are done during the setup phase and followed by the transmission phase where the real data delivery takes place. The slot assignment within the setup phase considers the two hop distance neighbor nodes and allocates different time slots based on the DRAND  algorithm and the frame size is determined by the time frame rule of the Z-MAC BIB001 protocol. Owner node of a specific transmission slot, assigned in the setup phase, has an exclusive right to send the data in it. If the owner of the slot does not have any data to send or has lower priority data, non-owners of the slot can contend for the slot based on priorities of their data. The Super Frame (SF) structure of the PQ-MAC consists of two sub frames: Data Frame (DF) which is used for data delivery and Control Frame (CF) which used for the sleeplisten schedule. An adaptive sleep-listen schedule is used for energy efficiency and synchronization between neighboring sensor nodes is provided by generating sequence of bits indicating whether the sensor node will sleep or be awake during the corresponding time slot. In Fig. 4 , the medium access prioritization mechanism is presented for three different traffic classes. Only the owner of the slot can access privileged contention windows T0, T2 and T4 while non-owners can contend during T1, T3 or T5 with respect to their traffic types. Advantages and disadvantages: The neighborhood of the sensor nodes, relay nodes or cluster heads may change frequently because of the dynamic nature of the WSNs, as mentioned earlier. Therefore, accuracy of the slot assignment performed once at the beginning of the setup phase will be obsolete during the transmission period gradually. In heavy traffic conditions, PQ-MAC behaves like a TDMA based protocol since almost all nodes will have a packet to send and use its own transmission slot. This improves the channel utilization and reduces the probability of collision significantly at the cost of tight clock synchronization.