## 1 Introduction  
A Wireless Sensor Network (WSN) [1] is an ad-hoc network specialized in environmental monitoring that is composed of autonomous, cooperating, small-sized nodes connected through wireless links, and a special node, the sink, that can forward data from the nodes to external users. Each node in a sensor network is equipped with a processor, one or more sensing units, and a radio transceiver. It is powered by an embedded battery [2], which is, in general, the sole source of energy. This means that the lifetime of the nodes is limited, and there is great emphasis on the efficient use of energy to prolong battery life. Among the low energy strategies in WSN, the approaches operating at the MAC layer are without doubt the most numerous. It is well known [3] that the radio transceiver is the most energy-consuming component of a node. The MAC layer is responsible for managing this component by scheduling the times when the node must turn its radio on and off. This makes the MAC layer the main responsible component for saving energy. This idea is the basis of preamble sampling MAC protocols, which periodically toggle the radio from sleep to active state to check the channel for incoming packets. If there is no activity on the channel the radio is shut off to keep the activity period short and thus save energy. For this approach to work well, the time when the radios are activated must match for each pair of communicating nodes. Otherwise, if a node transmits a packet later than the originally scheduled time, the receiving node may not be listening, which would cause the loss of the packet followed by retransmission. Conversely, a node may listen to the channel when no one is sending (idle listening). To solve this problem, preamble sampling protocols use some form of synchronization among 1-hop neighboring nodes. Most of these strategies are based on sending long preambles or check intervals. The first approach ensures that the preamble can be detected in the receiver node in order to maintain the radio on as long as necessary to receive the data. The second approach consists of computing a check interval that is shared for neighboring nodes and which enables the nodes to synchronize the times at which they switch their radios on and off. Both these approaches may negatively affect communication latencies. Specifically, nodes may experience delays in the transmission and reception of the packets as a consequence of the medium contention and of the strict activity periods. Moreover, in these protocols the radio scheduling is generally static, which means that it is not updated in the presence of communication delays [4; 5; 6; 7; 8]. This results in packet loss and packet retransmission, which increases the energy consumption. This problem becomes worse in multi-hop scenarios, where the communication delays increase as they are propagated along the path from the sender to the sink. In this paper we propose Cross Layer Adaptation of Check intervals (CLAC), an approach that is concerned with the trade-off between communication delays and energy consumption which occurs in preamble sampling protocols based on check intervals. The main goal of CLAC is to reduce the energy consumption of the nodes without significantly increasing delay, by adjusting appropriately the nodes scheduling. CLAC exploits information extracted from the application, routing, and MAC layers to estimate the communication delays that affect a packet traveling along a multihop path. It then uses this estimation to recompute and update the value of the check intervals at each hop such that each node can adapt to the expected packet arrival time according to the accumulated delays along the paths. We have implemented and evaluated CLAC using the TinyOS/MicaZ platform on top of the BoX-MAC datalink protocol and Collection Tree Protocol (CTP) routing protocol. As compared to CTP/Box-MAC, CLAC improves the network lifetime of the communicating nodes without increasing the end-to-end delay and without additional packet loss.