Hierarchical Power Management in Disruption Tolerant Networks with Traffic-Aware Optimization

In this paper, the author investigates power management in DTNs with high randomness in the node mobility. Author presents a hierarchical power management framework, in which nodes control two radio interfaces to discover contacts. In addition, provides traffic aware approximation algorithms to save energy while discovering enough contacts to deliver the traffic load in the network. Simulation results from two mobility models show that generalized power management mechanism could achieve better energy efficiency than mechanisms relying only on one radio for contact discovery. In addition, when the traffic load can be predicted, approximation algorithms helps nodes to save significant amount of energy while handling the expected traffic load. Finally, the additional information allows the one-radio architecture to save equivalent energy to the two-radio architecture, overcoming the disadvantage of not having the additional radio.

A wake up scheme for sensor networks: Achieving Balance betwwen energy saving and end to end delay.

The energy consumption of wireless sensors is very critical since often their energy supply comes from a small battery with limited lifetime. Some techniques like STEM (Sparse Topology Energy Management) make use of a low power wake up channel in addition to a high power data channel. The idea is to minimize energy consumption by turning off the data channel when it is not used. However there is a drawback, the low power wake up channel reduces the maximum range. Also, this technique (STEM) has a considerable latency on the wake up process.
This paper proposes a technique named PTW(Pipelined Tone Wakeup) that also incorporates 2 radios, but for the wakeup channel, a tone is transmitted for a period of time long enough for all neighbors to be recognized, so all of them are awakened, but only one of them receives the notification packet, the awakened nodes that do not receive the notification packet will go back to sleep after a timeout expiration. The energy saving is achieved by passing the duty of the wakeup from the receiver to the sender. In this case, the receiver is alternating between sleep and tone monitoring where the listening period is shortened and the sleeping period is increased compared to STEM.
The end to end delay improvement is achieved by overlapping the data transmission time (data channel), to the awakening time (wakeup channel). So during this process, a node will be receiving data and sending the wake up tone at the same time.

DMAC - An Adaptive Energy-Efficient and Low-Latency MAC for Data Gathering in Sensor Networks

In general, MAC protocols for sensor networks utilize activation/sleep duty cycles so as to improve energy efficiency and prolong network lifetime. However, this results in the data forwarding interruption problem, whereby nodes on a multihop path to the sink are not all awake when the data delivery is in progress. This results in sleep latency. The paper proposes DMAC, which eliminates the sleep latency by basing the active/sleep schedule of a node on its depth on the forwarding tree. Hence nodes wake up sequentially as data moves up the forwarding tree. The paper also proposes mechanisms to adapt the duty cycles based on the current traffic conditions in the network to further reduce sleep latency and improve energy efficiency. Moreover, they propose a data prediction mechanism and the use of more to send (MTS) packets to reduce the latency in scenarios where when a node has more data to send than it could possibly send in the current cycle, or when there is channel contention from its siblings. Though the approach works well for links which are pretty reliable, the delay in packet delivery increases significantly when the channel is noisy, since if a packet is lost during transmission, it can only be re-sent during the next cycle. The authors compare the performance of DMAC with SMAC and a MAC in which nodes are ON at all times. The results show that DMAC has lesser latency and is more energy efficient compared to SMAC.