## 1 Introduction  
The limitation of energy due to the limited battery capacity of sensor nodes is a fundamental problem in Wireless Sensor Networks (WSNs). Communication protocols for WSNs, including routing and MAC layer protocols should thus be designed energy-efficiently. Traditional wireless MAC protocols such as IEEE 802.11 are not suitable for this purpose since in these protocols nodes are required to stay awake to listen to the medium, even when the network becomes idle. This inefficient idle-listening mechanism wastes substantial energy [1, 2]. Nowadays, many methods introduce duty-cycling mechanisms into MAC designs for WSNs to achieve low energy consumption. In the duty-cycling approach, each node periodically experiences an active state and a sleeping state. When in the active state, a node listens to the radio channel for possible transmissions, whereas in the sleeping state, it turns off its radio to save energy. Each node establishes and maintains a schedule to indicate when it should wake up or sleep based on the synchronization requirements among neighboring nodes. S-MAC [3] is a typical synchronized duty cycle MAC protocol for WSNs. In S-MAC, each node maintains a fixed listening/sleeping schedule. The listening interval is divided into two parts, namely _SYNC_ and _DATA_. The _SYNC_ part is for synchronization among neighboring nodes using SYNC packets, and the _DATA_ part is for data transmission using the RTS/CTS handshake mechanism as in 802.11. Although S-MAC is energy efficient, it may introduce significant packet delivery latency, since a packet can only be forwarded to a 1-hop distance in each operational cycle. The improved S-MAC with adaptive listening [4] can improve latency by delivering packets up to a 2-hop distance in each cycle. But the latency is still significant and the use of adaptive listening can significantly increase energy consumption. Several protocols have been proposed to mitigate packet delivery latency without sacrificing energy efficiency of duty-cycling (e.g., RMAC [5], T-MAC [6] and DW-MAC [7]). Take RMAC for example. Similar to S-MAC, RMAC divides the operational cycle of a sensor node into three periods: _SYNC_, _DATA_ and _SLEEP_. But the difference lies in the fact that RMAC delivers a pioneer frame (PION) over multiple hops during the _DATA_ period to set up a multi-hop schedule for subsequent data forwarding during the _SLEEP_ period. Therefore, this approach can forward a data packet over several hops within a single cycle, thus improving delivery latency. Note that the PION frame has a dual function, one is to request communication like RTS, and the other is to confirm a request like CTS. This PION relaying process continues until either the PION frame has reached the final destination or the current _DATA_ period has ended. Although the improved duty-cycling mechanisms mitigate the delivery latency problem, they were designed independently without considering routing. For example, RMAC assumes a routing protocol has been deployed over it to provide the routing information it needs. But the application of a routing protocol would cause significant performance degradation of these MAC protocols. This paper designs and evaluates a new cross-layer duty cycle MAC protocol with data forwarding supporting a pipeline feature (P-MAC). P-MAC divides the whole network into several groups around the sink, each with different grades. At the network layer, each sensor node identifies its grade according to its logical hop distance to the sink and simultaneously establishes a sleep/wakeup schedule based on its grade information. The sinkis in grade zero and the lower a node's grade, the fewer hops it needs to send packets to the sink. The wakeup period of a node is divided into two parts, the first one is used for receiving data from the upper grade node, and the second one is used for sending the received data to the lower grade node. Those nodes in the same grade keep the same schedule, which is staggered with the schedule of the nodes in the adjacent grade.