In smart factory, many IoT devices detect events and process them. Most of these IoT devices need to operate wirelessly, so they use batteries and are configured to use wireless networks. However, these wireless factory networks are prone to be interfered by many environmental factors. Therefore, smart factories require an energy-efficient network of wireless IoT devices even in such harsh environments. RPL is a distance vector IPv6 routing protocol designed for Low-Power and Lossy Networks such as factory networks. However, RPL has a problem of high energy consumption due to frequent parent changes in a lossy network. In this paper, we propose ADP-RPL, which is an improved protocol of RPL to solve the aforementioned problem. The proposed ADP-RPL deals with the problem that RPL selects unstable nodes with weak link quality as the preferred parent just considering the current link quality. In order to avoid these unstable nodes, ADP-RPL reflects the deterioration of current link quality from the previous link quality and the network stability of potential parents. In this way, the child node maintains the routing stably by avoiding unstable nodes when selecting the parent node. In addition, ADP-RPL adaptively reflects the threshold of MRHOF. In the existing RPL, child nodes are affected by more frequent parent changes than their parents using the same threshold regardless of the value of rank. ADP-RPL limits the parent change using a larger threshold value for the neighbor with a higher rank. This adaptive threshold reduces energy consumption due to frequent parent changes. In Cooja simulator, we implement ADP-RPL on Contiki OS and show that it outperforms the existing RPL in the viewpoint of energy consumption.

Software defined networking controller platforms support clustering architectures to provide scalability and availability to large-scale carrier-grade networks. OpenDaylight, a well-known open source project, provides a clustering and distributed datastore architecture. The datastore is distributed into shards such that a subset of the shards can be located in any cluster member. To guarantee strong consistency in the datastore so all shard replicas have the same value, only the shard leader is responsible for accepting data updates. Even though OpenDaylight supports a distributed shard leader election algorithm, elected leaders are not distributed over the cluster members due to a lack of centralized control. Since update requests for the datastore are inherently concentrated in the leader, this results in unbalanced CPU usage of the controller cluster, as well as degradation of throughput in the distributed datastore. In this paper, we propose a shard leader distribution algorithm that maximizes the throughput of the distributed datastore by distributing the shard leaders to cluster members as evenly as possible. The shard leader distribution algorithm restricts the maximum number of leaders that a cluster member may have by monitoring the status of the shards within the cluster members. In the performance evaluation, we prove that shard leader distribution significantly improves the throughput of the datastore by load balancing data update requests to the cluster.

An increasing number of recent experimental works have demonstrated that the supposedly secure channels in the Internet are prone to privacy breaking under many respects, due to packet traffic features leaking information on the user activity and traffic content. We aim at understanding if and how complex it is to obfuscate the information leaked by packet traffic features, namely packet lengths, directions, and times: we call this technique traffic masking. We define a security model that points out what the ideal target of masking is, and then define the optimized traffic masking algorithm that removes any leaking (full masking). Further, we investigate the tradeoff between traffic privacy protection and masking cost, namely required amount of overhead and realization complexity/feasibility. Numerical results are based on measured Internet traffic traces. Major findings are that: 1) optimized full masking achieves similar overhead values with padding only and in case fragmentation is allowed, and 2) if practical realizability is accounted for, optimized statistical masking attains only moderately better overhead than simple fixed pattern masking does, while still leaking correlation information that can be exploited by the adversary.

The ZigBee tree routing is widely used in many resource-limited devices and applications, since it does not require any routing table and route discovery overhead to send a packet to the destination. However, the ZigBee tree routing has the fundamental limitation that a packet follows the tree topology; thus, it cannot provide the optimal routing path. In this paper, we propose the shortcut tree routing (STR) protocol that provides the near optimal routing path as well as maintains the advantages of the ZigBee tree routing such as no route discovery overhead and low memory consumption. The main idea of the shortcut tree routing is to calculate remaining hops from an arbitrary source to the destination using the hierarchical addressing scheme in ZigBee, and each source or intermediate node forwards a packet to the neighbor node with the smallest remaining hops in its neighbor table. The shortcut tree routing is fully distributed and compatible with ZigBee standard in that it only utilizes addressing scheme and neighbor table without any changes of the specification. The mathematical analysis proves that the 1-hop neighbor information improves overall network performances by providing an efficient routing path and distributing the traffic load concentrated on the tree links. In the performance evaluation, we show that the shortcut tree routing achieves the comparable performance to AODV with limited overhead of neighbor table maintenance as well as overwhelms the ZigBee tree routing in all the network conditions such as network density, network configurations, traffic type, and the network traffic.

Wireless Sensor Networks (WSNs) enable the surveillance and reconnaissance of a particular area with low cost and less manpower. However, the biggest problem against the commercialization of the WSN is the limited lifetime of the battery-operated sensor node. Taking this problem into account, a mobile sink is deployed as a robot, vehicle or portable device to only activate the sensor nodes that are interesting to the sink and leaves other nodes deactivated for. This can considerably extend the lifetime of the sensor nodes compared to existing power management algorithm of using all nodes. However, in this environment, the mobility of the sink raises new issues of energy efficiency and connectivity in communications. To solve these issues, we propose a DRMOS (Dynamic Routing protocol for Mobile Sink) method that includes a designated wake-up-zone to make sensor nodes prepare for an incoming sink. The shape of the wake-up-zone is dynamically changing to reflect the past moving patterns of the sinks. Moreover, we present the extensive simulation results and recommend parameters for practical use of DRMOS from the simulation analysis.

A body sensor network (BSN) is a network of sensors that sense vital signs of a human body. The vital signs can be divided into two categories based on significance to the life: critical and non-critical signs. The critical vital signs, or real-time messages (RTMs), should be transmitted within the deadlines, and they must not be collided. On the other hand, the non-critical signs, or non-real-time messages (NRTMs), are required to be delivered with the best effort. In this paper, an on-demand MAC (OD-MAC) is proposed for BSN to provide real-time transmission, collision avoidance, and energy efficiency. The protocol is developed on IEEE 802.15.4 standard, and some specifications are modified to support those requirements. The OD-MAC changes the superframe structure dynamically to schedule RTMs and NRTMs. Those messages request bandwidth, and the network coordinator adaptively changes beacon interval and allocates slots to the messages. For evaluating the performance, duty cycle according to required utilization, delivery ratio, and energy efficiency are analyzed with simulations using NS2.

In this paper, we propose the cluster-based hierarchical time synchronization protocol (CHTS) for wireless sensor networks (WSN). To couple the network with real world tightly, we provide a network-wide time synchronization through the introduction of abstractions for cluster and hierarchy. Generally the error of time synchronization has the tendency of being accumulated as the hop count along the synchronization path increases in multi-hop wireless sensor network. Thus we cluster the randomly deployed nodes to decrease the hop count along the synchronization path. We then apply different optimal synchronization mechanisms to the cluster heads and cluster members for the sake of power efficiency through an explicit broadcast manner. We prove the performance enhancements with simulation comparisons.

We propose a real-time message scheduling algorithm which is applied to schedule periodic real-time messages in IEEE 802.15.4 for LR-WPAN(Low Rate Wireless Personal Area Network). The standard allows GTSs (Guaranteed Time Slots) in the optional use of a superframe structure in a beacon-enabled network to be used to exchange real-time messages. To utilize these features of the standard efficiently, a proper message scheduling algorithm is needed. The proposed off-line message scheduling algorithm which is based on a distance constrained scheduler generates the standard specific parameters including BO, SO, and GTS information to schedule the given message set. The algorithm is evaluated by simulation and the guaranteed time service using the schedule is implemented and evaluated on CC2420DB which is a prototyping platform including an IEEE 802.15.4 compliant transceiver of Chipcon AS.