# A Hybrid Scheme for Disaster-Monitoring Applications in Wireless Sensor Networks
## 2 Related Work
### Contention-Based MAC  
The random multiple access method with collision avoidance is a fundamental contention-based mechanism for WSNs, and many protocols have been presented and studied, such as the S-MAC, B-MAC, and PMAC protocols [16; 17; 18]. To achieve energy efficiency, S-MAC reduces idle listening using a periodic duty cycle scheme [16]. Within an active period, the sensor nodes of the S-MAC transmit messages according to the IEEE 802.11 standard. The S-MAC protocol uses the CSMA/CA mechanism and request-to-send (RTS)/ clear-to-send (CTS) to maintain collision avoidance. S-MAC undergoes a high delay because packets are queued at a node until the next active period.  
To mitigate uncertain delays between source and sink nodes, the RT-MAC protocol overcomes the false blocking problem when RTS/CTS packets are exchanged, and maximizes spatial channel reuse [19]. However, the interference caused by multiple communication streams still affects the RT-MAC protocol. To obtain high throughput and low delay performance while considering the data input rate, the MaxMAC protocol takes advantage of an added wake-up scheme [20]. However, MaxMAC cannot respond accurately to traffic changes. In the ENCO protocol, optimization of the contention window size approximation for the number of contending nodes is used to reduce the contention delay [21]. ENCO requires extra computational overhead to optimize contention window sizes. Based on the IEEE 802.15.4 unslotted CSMA/CA scheme, QoS-MAC sets different priorities with differential services for data traffic to obtain QoS support, and this protocol is applied to the vital distribution monitoring of smart grids [22]. However, the QoS-MAC suffers from severe collisions when the traffic data rate increases. Based on data prioritization and delay estimation methods, DRX was designed for smart grid applications, and the MAC layer parameters responded to the delay requirements of the application and network conditions [23]. In DRX, the packet delivery ratio is reduced when the number of nodes increases, because the number of collisions increases.  
PW-MAC is an energy-efficient MAC protocol based on an asynchronous duty cycle [24]. The sender of the PW-MAC wakes up and turns on its radio immediately before the intended receiver wakes up. The sensor nodes in PW-MAC compute their wake-up times using their pseudorandom wake-up schedule generator, which reduces radio collisions caused by multiple concurrent traffic flows. However, PW-MAC addresses the real-time  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Protocol** & **Main Scheme** & **Computation** & **Control** & **Adaptivity to** & & **Time Synchronization** \\  & & **Overhead** & **Overhead** & **Changes** & **Precision** \\ \hline TDMA-W & Graph-coloring, wakeup slot & Slot assignment & High & Moderate & High & High \\ ArDez & Rendezvous-based scheme & Rendezvous period & High & Good & Moderate & Low \\ DMAC & Staggered wakeup, data prediction & Data-gathering tree & Low & Rather poor & Right \\ CSMA- & CSMA initializing, STDMA data & Frequent slot assignment & Rather high & Poor & Moderate & High \\ E-hybrid & Contention period, reserved slots & Slot assignment & Rather high & Poor & Low & High \\ WirelessHART & Time-synchronized TDMA/CSMA & Slot assignment & High & Poor & Low & High \\ Z-MAC & LPL, adaptability to contention level & Slot assignment & Low & Poor & Moderate & Moderate \\ TCS & Token cycle scheduling & Slot scheduling & Moderate & Moderate & Low & High \\ MMAC-HR & Multichannel, hopping reservation & Hopping reservation & High & Poor & Low & Low \\ DSP & Multichannel, fast and slow hopping & Multiple rendezvous & High & Poor & Low & Low \\ \hline \hline \end{tabular}
\end{table}
Table 2: _Cont._bursting of traffic. CSMA/WSD is a variant of CSMA that implements weak signal detection to mitigate energy consumption [25]. Although this approach allows the neighbor node to detect a weak signal and gain more throughput by performing a loss diagnosis, CSMA/WSD requires a dense deployment of sensor nodes.