The so-called Internet of Things (IoT) aims at connecting every single object to the Internet with the purpose to automate every aspect of daily life. The IoT relies heavily on wireless low-power and lossy networks (LLNs) that collect information from the physical world and send the measurements to data aggregation and processing nodes. Most LLNs operate in the non-licensed industrial, scientific, and medical radio band, which is shared by a considerable number of systems. Coexisting wireless systems cause interference to each other, limiting their achievable performance. Multichannel communications enable frequency diversity, which in turn provides robustness against interference as well as increased network capacity. There is a considerable interest in multichannel medium access control (MAC) protocols for LLNs, including an evolving standard for the MAC layer of LLNs. In this paper, we review the latest advances in the topic and introduce a new classification framework for multichannel MAC protocols for LLNs. While our framework builds on previous review and classification studies, it adds aspects of a MAC protocol that reflect its interactions with the surrounding network stack. Seeing the resource constraints of the LLN devices, the study of such interactions-which is missing in prior classification efforts-can be the key for improving future designs. Relevant protocols published since 2006 are discussed and classified using the presented framework, including the recent multichannel MAC protocols for LLNs, such as the latest version of the IEEE 802.15.4 standard for time slotted channel hopping.

The Time-Slotted Channel Hopping (TSCH) mode of the standard IEEE 802.15.4 provides Medium Access Control (MAC) for most Low-power and Lossy Network (LLN) applications in the Internet of Things (IoT). TSCH creates a synchronized mesh network capable of delivering data packets in a reliable and timely manner. The standard defines a scan mechanism to allow nodes to detect the presence of a TSCH network and to associate to it. However, the time required to do so can be long, which causes excessive energy consumption, as nodes that are trying to associate stay awake during this process. In this paper, we present a rendezvous-based algorithm for TSCH association called PRV-TSCH that significantly reduces the time required for going through this process. The algorithm uses the principle of parallel rendezvous to allow nodes to associate to a TSCH network in a very short time. The interaction between the IPv6 Routing Protocol for LLNs (RPL) and the time to associate to the TSCH network is also studied. The results of extensive simulation experiments and a mathematical model demonstrate that PRV-TSCH outperforms the purely passive scan proposed by the standard in terms of the time and energy required to associate to a TSCH network.

Blind rendezvous strategies for dynamic spectrum access in multi-channel ad-hoc networks have recently attracted the attention of many researchers. Their work has mostly focused on the design of rendezvous strategies between two network devices which actively try to establish communication. In this paper, we extend the analysis of pair-wise strategies to multi-hop ad-hoc networks and propose strategies to implement rendezvous process in these scenarios. The upper bound of time to rendezvous for multiple users is computationally derived for different network topologies. Analytic results are validated through extensive simulations using state-of-the-art channel hopping sequences.

The Time Slotted Channel Hopping (TSCH) mode of the standard IEEE 802.15.4 provides Medium Access Control (MAC) for most Low power and Lossy Network (LLN) applications in the Internet of Things (IoT). TSCH uses a synchronized schedule with frequency diversity to allow flexible, reliable and resilient communication. The standard defines an association algorithm to allow nodes to autonomously form the network. The time required to create the network can be long, which consumes valuable energy given that nodes keep their radios on when trying to associate. In this letter, we propose a theoretical model to estimate the time required for a node to associate to one single synchronizer. This model extends and complements earlier mathematical models. Its results comply with outcomes from extensive simulation studies. This makes the proposed model a valuable tool to further investigate the time needed to form TSCH-based LLN networks.

Time slotted channel hopping (TSCH) has become the standard multichannel MAC protocol for low-power lossy networks. The procedure for associating nodes in a TSCH-based network is not included in the standard and has been defined in the minimal 6TiSCH configuration. Faster network formation ensures that data packet transmission can start sooner. This paper proposes a dynamic beacon transmission schedule over the TSCH mechanism that achieves a shorter network formation time than the default minimum 6TiSCH static schedule. A theoretical model is derived for the proposed mechanism to estimate the expected time for a node to get associated with the network. Simulation results obtained with different network topologies and channel conditions show that the proposed mechanism reduces the average association time and average power consumption during network formation compared to the default minimal 6TiSCH configuration.