# An Adaptive Back-Off Mechanism for Wireless Sensor Networks
## 1 Introduction
Several protocols have also been proposed to minimize packet latency, such as demand wakeup MAC (DW-MAC) [10] and multi-divided deliver MAC (MDD-MAC) [11]. DW-MAC introduces a low overhead scheduling algorithm that allows nodes to wake up on demand during the sleep period. MDD-MAC proposes a rotation method between sending and receiving data during the active period. The reinforcement learning mechanism [12] is introduced to MAC protocols in WSNs to avoid idle listening and overhearing. Compressed handshake MAC (CH-MAC) [13] is presented to reduce unnecessary and redundant handshake frame so as to reduce the end-to-end delay and energy consumption. CH-MAC is a synchronized duty cycle MAC protocol. Similar to S-MAC, each cycle in CH-MAC is divided into the active period and the sleep period. The active period also consists of two periods: data and avoid. In the data period, a node with pending data contends for a wireless channel using a four-way handshake as in S-MAC. However, the RTS and CTS are replaced by a T frame. Data communication is initiated by an intended receiver node. Receiver will send a T frame with an upcoming request if the medium is idle. Upon receiving the T frame, the sender node will then reply with a data frame, which will be acknowledged by a receiver's ACK. Once the sender node receives the ACK, it will go to sleep. By reducing the handshake time, CH-MAC can shorten the time during which a pair of nodes occupy the medium and can improve the end-to-end delay. Besides, CH-MAC can also decrease the transmission energy consumption by cutting down the RTS packet transmission. Furthermore, CH-MAC sets up an initiative sleeping mechanism. In the avoid period, after finishing the data transmission, a node goes to sleep immediately and wakes up when the next hop neighbor finishes its data transmission. This method will slightly reduce energy consumption and solve the hidden terminal problem. As mentioned above, many S-MAC-based protocols are proposed to further improve the performance in WSNs. It is important to find an appropriate target to make an improvement. It is worth noting that the back-off mechanism of S-MAC is quite simple. The size of the contention window in S-MAC is fixed, and all nodes share the same contention window sizes whether they have successfully obtained the channel or not. When the WSN is under light load, the mechanism is fairly efficient because, under this circumstance, the probability of data collision is quite small, and a fixed-size but narrow contention window can decrease the transmission delay. Thus, a short delay and a relatively high throughput are acquired. However, when the network load is heavy, the common fixed-size contention window makes nodes and their neighbors in the network more likely to send data at the same time. Under this circumstance, the fixed-size contention window becomes the bottleneck leading to a serious collision and aggravating the efficiency of wireless channel access. To cope with this performance deterioration, the back-off mechanism of S-MAC should be emphatically improved in order to guarantee the network performance under heavy load. In this paper, we propose a well-designed MAC protocol adopting an adaptive contention window which has higher adaptability in a heavy-load network. Based on the contention condition, conflict probability is significantly controlled by altering the back-off window size. Ultimately, higher throughput and energy efficiency are obtained. The remaining portions of this paper are organized as follows. In Section 2, we present a literature review of the related work. In Section 3, our proposed MAC protocol is shown in detail. In Section 4, the simulation results based on network simulator version 2 (NS-2) are given out. Finally, we conclude with a summary and some directions for future work.