## 2 Related Work
### Delay-Aware MAC Protocols  
Although LPL protocols reduce the energy consumption by reducing the idle listening periods, they may introduce longer end-to-end communication latencies as a consequence of the medium contention and of the strict duty cycles governing the activity of the nodes. Low duty cycles increase the latencies, since the nodes can transmit and receive packets only during the window of time scheduled for their duty cycle. Thus, senders may avoid the transmission if the channel is busy, and receivers may become too burdened to be able to manage the incoming traffic with their current LPL configuration. Many works in literature study whether energy-efficient duty cycles may be maintained without affecting quality of service (QoS) parameters (throughput, latency, reliability). One approach is to dynamically adjust the duty cycle based on the traffic load and/or topology information. Thus, some authors propose to increase or decrease the sleep interval under certain network conditions. For example, in BoostMAC [20] the nodes sample the medium with a periodicity of \(t\). If the sampling node finds the medium idle, it increments \(t\) by \(t_{add}\), up to a maximum \(t_{max}\); otherwise, it divides the current sampling period \(t\) by \(t_{dec}\). MaxMAC [21] uses the number of packets received as a metric to react to the network congestion. This protocol adapts to the current traffic by switching between four states (\"basic\", \"extra\", \"more-extra\", and \"CSMA\") that represent congestion levels. The transitions between states occur based on comparing the packet rate with thresholds defined for each state: if the threshold is passed, the protocol halves the sampling interval and switches to a state representing higher congestion (up to \"CSMA\"); otherwise the protocol doubles the sampling interval and switches to a state representing lower congestion (down to \"basic\"). SCP-MAC [22] uses adaptive channel polling to detect packets traveling over multiple hops. There are also other protocols that balance the load based on routing information. One example is Energy Aware Adaptive LPL (EA-ALPL) [23], which is built on top of B-MAC. It dynamically selects its own listening and transmission modes among those provided by B-MAC. The selection is done according to the duty cycle and number of descendants in the routing tree. Another approach is Adaptive Staggered sLEEp Protocol (ASLEEP) [24], which uses the number of received packets and the inter-arrival time to compute the length of the activity period in the next communication interval. In order to synchronize neighboring nodes, each node sends its own scheduling to its neighbors by means of beacon messages. Figure 1: Basic preamble sampling scheme. All the approaches discussed above search for a balance between energy and throughput requirements, and, as a side effect, may reduce communication delays. Other studies analyze the delays for different MAC protocols. For example, [25] adopts a TDMA protocol that assigns receiving slots to sleeping nodes. Under this scenario the authors investigate the delay-efficient sleep scheduling for two different communication patterns, _i.e._, when every pair of nodes is equally likely to communicate and when it is not so. Using graph theory the authors formulate an equation for computing the delays and demonstrate that the minimization of delays for each pair of nodes in the graph is NP-hard. Then, they analytically find optimal solutions for two specific topologies: trees and rings. Finally, they suggest several heuristics for other arbitrary topologies. DW-MAC [26] describes the design and implementation of a new CSMA-based protocol, which is designed for reducing the packet delivery latencies in applications that have high traffic load. In [27] the authors consider an anycast communication scenario and they propose a probabilistic model to find an optimal solution for the delay minimization problem that maximizes the network lifetime.