Wireless sensor networks (WSNs) have expanded in the last decade. Sensors are deployed in an environment to control the data generated within that area, without any wiring. Data is sensed by the deployed devices (often called “motes”) and sent wirelessly to a central node that connects to the Internet or other wide-area network (WAN). These environments are usually geographically and spatially limited to a very specific area. The motes are typically able to reach the central node directly, and these scenarios are called one-hop networks. On the other hand, if there are nodes that cannot connect directly to the central node, some nodes must forward the messages. Network infrastructures based on trees, for instance, have been proposed to overcome this limitation. However, if motes are moving or more challenging interconnections between nodes are required, the network infrastructures have to be more complex. Usually, these infrastructures require some intermediary motes to forward the messages from further motes to other motes that cannot be reached directly; these scenarios are known as multi-hop networks.
Multi-hop networks require messages to interchange between motes in order to maintain the network’s stability and functionality. The aggregation of all these command and configuration messages is the control plane, and the grouping of all the messages containing the rest of the information (either directly from the sensors or computed by the processors of the motes) is called the data plane. These two planes may use the same networking technology and connection band. For instance, two different connections (sockets) can be created using the same technology and sharing the same physical medium. Homogeneous or in-band control planes use one single hardware for communication with several different logical connections, which simplifies the required hardware (although the software is more complex). On the other hand, all the advances in technology have allowed the integration of multiple different communication systems into one system. Heterogeneous or out-of-band control planes use heterogeneous communication technologies to support the dual plane strategy (in which the communication in one plane does not affect the other one)
This paper deals with multi-hop heterogeneous networks in which the data and control planes are separate. These networks use different communication technologies to carry out the out-of-band management of the data and control planes.
The authors provide a large simulation using TOSSIM, the well-known TinyOS simulator, to compare the in-band and out-of-band approaches for both distributed and centralized network control. They use the collection tree protocol (CTP) for the comparison of the distributed and centralized network protocol to achieve multi-hop minimum-cost-routing tree-based on dynamic link quality. The distributed version is called collection tree protocol (CTP), which is an in-band distributed approach. The out-of-band version is called CTP-SDCP (separation of data and control planes). The data plane is Zigbee in both versions, and the control plane used in CTP-SDCP is a low-power WAN (LPWAN) enabled node. This node is able to reach all the data plane motes in a one-hop star network.
Simulations show that the CTP-SDCP outperforms the in-band distributed CTP version. The out-of-band version (CTP-SDCP) provides better results in terms of network convergence and resilience to link noises and node faults.
Furthermore, for the control plane, the authors tested two different LPWANs: LoRaWAN and a sub-GHz Texas Instruments (TI). Their protoype, the LoRaCP node, includes “a LoRaWAN shield and a Raspberry Pi (RPi) 3 Model B single-board computer.” The LoRaCP controller is a gateway composed of a Raspberry Pi 3 Model B and an iC880A board capable of receiving “frames over all LoRa channels simultaneously.” The TI sub-GHz radio used to create the second hardware prototype for the out-of-band control plane management was based on the CC1352R TI SimpleLink multiprotocol wireless microcontroller (MCU).
The authors propose an enhancement of the LoRa protocol, called LoRaCP, to efficiently transmit network reports of the data plane. They use up to six LoRaWAN concurrent uplinks channels; five of them use time-division multiple access (TDMA) and one of them uses ALOHA to send urgent frames. They set the maximum time slot length to fulfill the LoRaWAN duty cycle requirements imposed by the official regulators of the different regions. To reduce the amount of messages sent by the control plane, a negative acknowledgment (NAK) procedure is followed.
In summary, this paper presents both simulated and real implementations of a multi-hop WSN with separate data and control planes and very limited power consumption. It is a worthwhile read.