## 2 Related Work  
The original duty cycle MAC protocols for WSNs (e.g., S-MAC [3,4]) introduce significant end-to-end delivery latency. Many researchers have proposed other scheduled MAC schemes to mitigate this problem without sacrificing the energy-efficiency of the duty-cycling mechanism. These approaches can be approximately divided into two categories: synchronous and asynchronous, according to their synchronization requirements. Synchronous schemes (e.g., S-MAC [3], T-MAC [6], RMAC [5] and DW-MAC [7]) require synchronization among neighboring nodes, ensuring that they can cooperate for communication, whereas asynchronous schemes (e.g., B-MAC [8], WiseMAC [9], X-MAC [10] and RI-MAC [11]) allow each node to establish and maintain its own schedule independently, usually using preamble schemes. T-MAC [6] follows S-MAC using synchronization and virtual clustering schemes. It dynamically ends the active part of the listen/sleep duty cycle to further save energy when there are no packets to receive. However, the higher delivery latency problem still exists in T-MAC, since a data packet can only be forwarded a 1-hop distance within a single cycle. After becoming aware of this latency problem, the designers of S-MAC mitigated the problem by introducing the adaptive listening technique [4]. In the improved S-MAC with adaptive listening, if a node overhears an RTS or CTS, it won't go to sleep when the _SLEEP_ period begins but instead will keep awake for a short time. Thereby, if this node is the next-hop node, it can immediately receive data from its neighbor instead of waiting for the next cycle. Thus, a packet can be delivered up to a 2-hop distance within a single cycle. However, the improvement is minor and this technique also consumes more energy, because many neighboring nodes need to keep awake during adaptive listening, but only one of them will be the next hop. RMAC uses a PION frame to reserve a channel over several hops during the _DATA_ period, then, it transmits a data packet through the reserved channel during the _SLEEP_ period. Therefore, the data packet can be forwarded across multiple hops within a single cycle, which not only reduces delivery latency significantly but also handles traffic contention efficiently. The recently proposed PRMAC [12] inherits this advantage and exploits it further by using PION to schedule multi-hop transmission of multiple data packets, enabling multiple packets to be transmitted over multiple hops within a single cycle, thus ensuring that PRMAC can respond to traffic load changes better than RMAC. Unlike the above synchronous duty cycle MAC protocols, asynchronous schemes [8-10,13] using the preamble sampling technique [14] were introduced into the MAC layer design for WSNs. The basic idea of this strategy is that prior to data transmission, a sender transmits a long enough preamble lasting at least as long as the receiver's sleep period. The receiver periodically wakes up and checks for activity on the channel. If a preamble is detected, the receiver keeps awake long enough to receive the data, otherwise it goes back to sleep. RI-MAC [11] differs from this original asynchronous strategy since it uses the receiver-initiated mechanism, which is similar to that proposed in [15] for general wireless networks, to achieve better performance, in which the sender keeps active until the receiver explicitly informs the sender when to start data transmission by sending a short beacon frame. Hybrid approaches with channel polling scheme, such as SCP [16] and LEMR [17] have also been proposed. Compared with these duty cycle MAC protocols, P-MAC fully integrates routing into a wake-up scheduling algorithm. In P-MAC, the whole network is divided into several grades, which are used to guide data transmission. The schedules between two adjacent grades are staggered so that data can be transmitted continuously to the sink in a pipeline fashion, thus ensuring that the delivery latency is more acceptable. Actually, this pipeline scheduled pattern scheme for reducing latency is not original.