Analysis of adaptive multi-hop time synchronization in large wireless sensor networks

Time synchronization in wireless sensor networks is important for sensing services that require clock accuracy but use sensors without accurate hardware clocks. Pairwise synchronization is a common technique for two nodes to synchronize their clocks; a leading protocol that extends it across a larger network is AMTS. The challenge with AMTS and other protocols is that synchronization may incur significant communication overhead and more importantly reduce battery lifetime. Although AMTS has been analyzed with small networks, in this paper we analyze its performance in chain, grid and tree topology networks, identifying the trade off between synchronization period and energy consumptio

iACK: Implicit acknowledgements to improve multicast reliability in wireless sensor networks

Reliability in IP-multicast forwarding becomes useful while operating in lossy environment of wireless sensor networks. RPL is a protocol for forming a multicast tree in sensor networks for multicast data forwarding. Trickle multicast uses RPL to provide reliability, however it has a high overhead and delay, especially when a radio duty cycle is used on the sensors. Stateless Multicast Forwarding (SMRF) optimizes for the radio duty cycle, reducing delay but also reducing reliability. This paper proposes iACK, a retransmission scheme on top of SMRF that uses implicit acknowledgements present in wireless broadcast to determine which packets a node should retransmit. We have implemented iACK in ContikiOS. Results show that iACK delay is about 5 times less than Trickle (and close to SMRF), and packet delivery ratio is about 80% (compared to between 20% and 60% for Trickle and SMRF). With a slight increase in memory requirements, iACK offers a valuable tradeoff compared to existing protocols