# An Energy Efficient MAC Protocol for Wireless Passive Sensor Networks
## 2 Related Work
### Several Existing MAC Protocols  
There are several existing MAC protocols for WSNs, including the S-MAC protocol, the Timeout MAC (T-MAC) protocol, the Data gathering tree-based MAC (D-MAC) protocol, and the Berkeley MAC (B-MAC) protocol. The S-MAC protocol works with a periodic sleep/listening mechanism and a low duty cycle in order to reduce the energy consumption of nodes. Each node independently controls its working state and should be in sleep state as much as possible. When a node wakes up, it will determine whether to send or receive data packets after listening to the channel. Neighboring nodes consult with each other to have a common sleep/listening mechanism, so they will sleep and listen at the same time. The idle time of nodes is reduced, and the transmission delay is reduced through the adaptive traffic mechanism [11]. The neighboring node of the communication node does not sleep immediately after the communication but keeps listening for some time. If the node receives the RTS frame during this period, it can receive the data packets without waiting for the next listening period. Therefore, the transmission delay of the data packets is reduced. The T-MAC protocol is proposed based on the S-MAC protocol. The S-MAC protocol cannot adaptively change duty cycle according to the current network load. The T-MAC protocol solves this problem: the scheduling period is divided into active states with variable length and sleep states. The idle time can be reduced by dynamically changing the duration of the active states. However, the disadvantage of the T-MAC protocol is that random sleep leads to nodes sleeping early and the delay increases relative to the delay of the S-MAC protocol [12]. In view of the problems of the S-MAC and T-MAC protocols, the D-MAC protocol is proposed. According to the data gathering tree formed by the nodes forwarding data, the D-MAC protocol adopts the staggered wakeup scheduling mechanism to divide a period into receiving time, sending time, and sleeping time. The scheduling of each node has different offsets and the sending time of the lower nodes corresponds to the receiving time of the upper nodes. Ideally, the data can be transmitted continuously from the source node to the sink node so the sleep delay can be eliminated.  
However, the D-MAC protocol also has its disadvantage. Nodes need strict clock synchronization and the protocol is not suitable for mobile nodes and larger WSNs [13]. The B-MAC protocol has the biggest difference. It adopts a clock asynchronization mechanism so the sleep scheduling has more initiative and the dependence on the accuracy of clock synchronization is reduced. B-MAC protocol uses the Low Power Listening (LPL) mechanism. The nodes have the same duty cycle but do not need to be synchronized. When the nodes wake up from sleep, LPL is used for channel sampling. If the preamble sequence is detected, it means the node will receive data packets. The node will be synchronized by the preamble sequence and then receive data. The sending nodes must send a long preamble sequence before sending data packets so that the receiver can be synchronized. However, the B-MAC protocol has advantages only when network traffic is low [14].