Low-power Wireless Mesh Networks Research Articles

Enabling Cross-technology Communication from LoRa to ZigBee via Payload Encoding in Sub-1 GHz Bands

Low-power wireless mesh networks (LPWMNs) have been widely used in wireless monitoring and control applications. Although LPWMNs work satisfactorily most of the time thanks to decades of research, they are often complex, inelastic to change, and difficult to manage once the networks are deployed. Moreover, the deliveries of control commands, especially those carrying urgent information such as emergency alarms, suffer long delay, since the messages must go through the hop-by-hop transport. Recent studies show that adding low-power wide-area network radios such as LoRa onto the LPWMN devices (e.g., ZigBee) effectively overcomes the limitation. However, users have shown a marked reluctance to embrace the new heterogeneous communication approach because of the cost of hardware modification. In this article, we introduce LoRaBee, a novel LoRa to ZigBee cross-technology communication (CTC) approach, which leverages the energy emission in the Sub-1 GHz bands as the carrier to deliver information. Although LoRa and ZigBee adopt distinct modulation techniques, LoRaBee sends information from LoRa to ZigBee by putting specific bytes in the payload of legitimate LoRa packets. The bytes are selected such that the corresponding LoRa chirps can be recognized by the ZigBee devices through sampling the received signal strength. Experimental results show that our LoRaBee provides reliable CTC communication from LoRa to ZigBee with the throughput of up to 281.61 bps in the Sub-1 GHz bands.

Security Vulnerabilities and Countermeasures for Time Synchronization in TSCH Networks

Time‐slotted channel hopping (TSCH), which can enable highly reliable and low‐power wireless mesh networks, is the cornerstone of current industrial wireless standards. In a TSCH network, all nodes must maintain high‐precision synchronization. If an adversary launches a time‐synchronization attack on a TSCH network, the entire network communication system can be paralyzed. Thus, time‐synchronization security is a key problem in this network. In this article, time synchronization is divided into single‐hop pairwise, clusterwise, and three‐level multihop according to the network scope. We deeply analyze their security vulnerabilities due to the TSCH technology itself and its high‐precision synchronization requirements and identify the specific attacks; then, we propose corresponding security countermeasures. Finally, we built a test bed using 16 OpenMoteSTM nodes and the OpenWSN software to evaluate the performance of the proposed scheme. The experimental results showed that serious security vulnerabilities exist in time‐synchronization protocols, and the proposed countermeasures can successfully defend against the attacks.

Performance Study of Routing Protocols in ZigBee Wireless Mesh Networks

ZigBee is one of the key standards that enable low-cost and low-power wireless mesh networks. Working on top of IEEE 802.15.4 MAC/PHY standard, ZigBee defines higher layers that support reliable multi-hop wireless communications. In particular, ZigBee provides multiple routing protocols to support diverse application traffic types such as multipoint-to-point, point-to-multipoint, and point-to-point. However, there are no studies that thoroughly incorporate multiple ZigBee routing protocols. In this paper, we explore multiple unicast routing and many-to-one protocols incorporated into the ZigBee standard and compare them through extensive simulations using NS2. We discuss real-world implementation issues and introduce our implementation decisions. We also examine their suitability for different scenarios and discuss some of the improvements that can be made to the ZigBee standard routing protocols.

Adaptive Synchronization in IEEE802.15.4e Networks

Industrial low-power wireless mesh networks are shifting towards time-synchronized medium access control (MAC) protocols which are able to yield over 99.9% end-to-end reliability and radio duty cycles well below 1%. In these networks, motes use time slots to communicate, and neighbor motes maintain their clocks' alignment, typically within 1 ms. Temperature, supply voltage, and fabrication differences cause the motes' clocks to drift with respect to one another. Neighbor motes need to re-synchronize periodically through pairwise communication. This period is typically determined a priori, based on the worst case drift. In this paper, we propose a novel technique which measures and models the relative clock drift between neighbor motes, thereby reducing the effective drift rate. Instead of resynchronizing at a preset rate, neighbor motes resynchronize only when needed. This reduces the minimum achievable duty cycle of an idle network by a factor of 10, which in turn lowers the mote power consumption and extends the network lifetime. This Adaptive Synchronization is implemented as part of IEEE802.15.4e in the OpenWSN protocol stack and is validated through extensive experimentation.

Building a low cost low power wireless network to enable voice communication in developing regions

In this work, we describe our experiences in building a low cost and low power wireless mesh network using IEEE 802.15.4 technology to provide telephony services in rural regions of the developing world. 802.15.4 was originally designed for a completely different application space of non-real-time, low data rate embedded wireless sensing. We use it to design and prototype a telephony system, which we term as Lo 3 (Low cost, Low power, Local voice). Lo 3 primarily provides two use cases; (1) local and broadcast voice within the wireless mesh network, and (2) remote voice to a phone in the outside world. A Lo 3 network can cost as less as $2K, and can last for several days without power "off the grid", thus making it an ideal choice to meet cost and power constraints of rural regions. We test deployed a full-fledged Lo 3 system in a village near Mumbai, India for 18 hours over 3 days. We established voice calls with an end-to-end latency of less than 120ms, with an average packet loss of less than 2%, and a MOS of 3.6 which is considered as good in practice. The users too gave a positive response to our system. We also tested Lo 3 within our department where it can be used as a wireless intercom service. To our knowledge, Lo 3 is the first system to enable such a voice communication system using 802.15.4 technology, and show its effectiveness in operational settings.