## 2 Related Work
### Medium Access Control Protocol in WSNs  
Energy-efficiency is one of the most important performance metrics in WSNs because sensor nodes typically have a very limited energy supply and are expected to operate independently over a long time-period. This is a significant differentiator between research in WSN MAC and WLAN MAC. Because the traffic load in WSNs is typically low, an effective method to reduce the (idle listening) duty cycles of nodes is of prime importance. S-MAC is a classic CSMA-style protocol for sensor networks [11]. Nodes are coarsely time synchronized and transmit using a fixed wake-up schedule in a multi-hop network to save energy. S-MAC enables the nodes to run at duty cycles of \(1-10\%\). T-MAC improves the performance of S-MAC by using an adaptive timer that can adjust the wake-up schedule of a node dynamically based on traffic fluctuations [12]. A Low Power Listening (LPL) node periodically and briefly samples the channel for the presence of radio transmissions [13]. The node powers down immediately if no transmission is detected. Otherwise, the node keeps listening until a short transmission start symbol is detected. B-MAC [14] is the LPL implementation in TinyOS [8] and has a well-defined interface that allows applications to control sleep intervals, back-off window size, and power down policy within the MAC. It uses an advanced clear channel assessment (CCA) algorithm to handle random channel noise. As an alternative to transmitting a long preamble, which creates overhearing overhead for the neighboring nodes, X-MAC [15] sends out a strobe of small packets, which includes the destination node address. This enables faster power-down reducing the listening duty-cycles for unintended recipients. F-MAC [16] sends each packet as a number of frame-lets. A carefully chosen frame-let frequency of each node can guarantee collision-free transmission of at least one frame-let. CSMA-style protocols provide good system performance, such as throughput, when the traffic load is low. However, when the network approaches a congested state, the performance of CSMA-style protocols degrades dramatically. Therefore, a class of MAC protocols based on TDMA [2, 10, 17, 18, 19] have been proposed. The conventional TDMA protocols require a centralized entity to assign transmission slots, which is not feasible for large scale sensor networks. Luster [2] uses LiteTDMA, a low-power cluster-based MAC protocol, that limits network topology to a single hop. While TDMA-style MAC protocols can provide better network throughput when the traffic load is high, the complexity of the protocol and the requirement of fine-grained time-synchronization are major disadvantages. Z-MAC [18] is a hybrid CSMA and TDMA scheme. Transmission time slots are assigned to individual nodes in Z-MAC. However, rather than completely owning a time slot as in conventional TDMA, the owner node of the time slot gets transmission precedence by using a smaller competition window than the other nodes. SCP-MAC [10] addresses the long preamble problem of LPL by introducing scheduled channel polling to LPL. Nodes are fine-grained time synchronized in SCP-MAC. Instead of sending out a long preamble, a node sends a very short \"tone\" to wake up receivers. Therefore, SCP-MAC can lower node duty cycle to less than 1% -- a factor of 10 reduction compared with both S-MAC and B-MAC. While a number of adaptive mac algorithm exist for wireless sensor networks they are inherently different by design in that most adapt to varying traffic patterns. For example Demand Wakeup MAC (DW-MAC) [20], uses a low-overhead scheduling algorithm that allows nodes to wake up on demand during their sleep period. This adaptively increases the effective channel capacity during an operational cycle as traffic load increases. Convergent MAC (CMAC) [21] avoids synchronization overhead but still supports low latency. By avoiding communication when there is no traffic, CMAC allows for very low-duty cycles.