## 3 Related Work  
Although the deployment of WSNs has highly fascinated academia and industry, WSN platform has been experiencing several kinds of challenges due to many limitations and constraints. The WSN performance depends on an efficiency of the MAC protocol. The necessity of multi-featured MACprotocol is of paramount importance to handle mobility based scenarios for several real time WSN applications. The salient features of most related work are discussed. We emphasize some of the known hybrid MAC protocols. The hybrid protocol named Z-MAC is introduced that integrates the features of both TDMA and CSMA techniques [25]. In Z-MAC, CSMA is used as a baseline and TDMA resolves conflicts by scheduling the channel access. The protocol is based on the owner slot concept. Z-MAC uses novel flexible local time-frame synchronization without global synchronization. But, it requires the global clock synchronization. Z-MAC also introduces node highest priority scheme. If any node competes for accessing the channel, then the highest priority based node first gets the access to the channel. In a highly competitive environment, the node priority scheme decreases the network congestion. However, Z-MAC experiences latency issues due to the use of long preambles. Further, Z-MAC has another network adaptability problem because the nodes are tightly scheduled with each. As a result, Z-MAC decreases the throughput and increases excess energy consumption during the mobility. Advertisement-based MAC (A-MAC) hybrid protocol is introduced in [26] for controlling collision, overhearing and marginally idle-listening issue. In A-MAC, TDMA is used as baseline while CSMA improves the channel access. Each node is assigned certain number of time slots within the two-hop destination. The assigned time slots are used to transmit the data without disturbing the other nodes. A-MAC also uses an advertisement message that helps the sender to inform the neighboring nodes regarding its transmission schedule. The major advantage of A-MAC protocol is to inform the nodes in advance in order to make receiver and sender ready for data transmission. This inclusion avoids the idle listening and overhearing. However, the overhead of control packets increases the latency and consumes extra energy. Further, A-MAC is only designed for monitoring the surveillance applications, but it does not have enough support for mobility and real time communication. Speck MAC is a deviation of B-MAC protocol [27]. The Speck MAC aimed to reduce energy consumption and overhearing problem during heavy traffic. However, it consumes extra energy by sending wake-up frames [28] and also experiences excess latency. Speck MAC does not support for the real time and mobility based applications. ADC-SMAC [29] is an improved version of S-MAC that adds two new features to S-MAC. First, the node is capable to calculate its energy consumption and an average sleep time before sending synchronized packets. Second, the node adjusts the duty cycle based on network conditions then announces its schedule by sending broadcast messages to neighbor nodes. These two features reduce the energy consumption, but increase latency. Additionally, ADC-SMAC behaves poorly in mobile environments. Low-power real-time medium access control (LPRT) protocol is proposed for actuation and wireless systems using star topology [30]. The LPRT-MAC introduces the super frame concept that uses mini slots for transmission to the base station. LPRT-MAC reduces the energy consumption when coordinating with the channel. Star topology avoids the network overhead. However, the LPRT-MAC performance is limited and not suitable for long multi-hop WSNs. Additionally, it is also not compatible with other communication topologies. Based on the literature survey of hybrid MAC protocols, we conclude that the reported hybrid MAC protocols are not good candidates for mobility and real time applications under congested and heavy traffic network load.