IMPERIA network protocol stack for wireless sensor systems
IBM's Smarter Planet initiative aims at using the "intelligence" of a world full of networked sensors within objects, the infrastructure, and the environment. By collecting data and analyzing it in powerful backend systems running sophisticated algorithms, every person, service, or organization can become an active part of a digital environment — digital and smart. An important enabling technology for this vision are efficient communications protocols for Wireless Sensor Networks (WSNs) for industrial installations. In such networks, hundreds to thousands of sensor nodes are distributed over large areas of facilities to monitor the correct operation of various machinery, to observe whether dangerous events occur, and to alert persons to critical situations that may occur and help them manage such events efficiently.
Our Intelligent, Manageable, Power-Efficient, and Reliable Internetworking Architecture (IMPERIA) for sensor networks exhibits the following characteristics:
- It is intelligent and manageable:
Designing low-power network protocols that are manageable even in large-scale deployments is a big challenge. Although a large number of protocols have been reported in research, their suitability for real physical networks still needs to be proven. We believe that centrally controlled network protocols can achieve superior performance and better manageability than purely distributed solutions. Therefore in IMPERIA we opt for a centralized networking architecture in which the overall knowledge of the network topology including the status of all sensor nodes and the wireless links are monitored and supervised by a centralized network controller (NC).
Having this global network knowledge, an IMPERIA NC is able to compute the required routing information and corresponding node configuration data based on any implementable algorithm, even the most sophisticated ones. Furthermore, the configuration data for each node can be individually adapted according its traffic and performance requirements.
A major advantage of the centralized architecture is also that it minimizes the functionality required on the wireless nodes, thus making them implementable on low-cost devices. The deployment and installation of a new network is supported by an efficient automatic topology discovery procedure, which includes discovery of the wireless nodes and probing of the quality of the links between them.
- It is power-efficient: A main driving design point of our architecture is low power consumption, meaning that an IMPERIA WSN should achieve a long operating lifetime even with all the wireless nodes powered only by batteries. This requirement leads to the selection of "Time Division Multiple Access (TDMA)" as the main media access mechanism. In a TDMA system, time is divided into periodic frames, each containing a certain number of slots. Every node is assigned one or more slots per frame, during which it can send and/or receive messages to/from other nodes. Thus nodes need to activate their radio transceivers only during their assigned slots and can sleep for the rest of the time. With a proper slot assignment algorithm, energy wasted due to idle listening, overhearing, and transmissions can be avoided by ensuring that only one sender and the corresponding receivers are active at a given time.
- It is reliable: The IMPERIA NC adapts the network if nodes are lost or new nodes are added. Through a centralized management of the network dynamics, a history and statistics can be accumulated, which in turn allows more intelligent decisions regarding how to adapt to changes. Although the centralized IMPERIA NC may be seen as a single point of failure, its functions are only required during the setup and configuration phase of a WSN as well as for adaptation. If the IMPERIA NC fails, the TDMA information can be extracted from the running network, thus allowing a replacement or restart of the IMPERIA NC. WSNs with large numbers of nodes are split into multiple clusters with local management, but with global optimization for better manageability, reliability, and performance. If cluster heads fail, the network is reconfigured to deliver data to different sinks.
- It provides internetworking with other networks: IMPERA uses the publish/subscribe messaging system MQTT-S as communication middleware to provide seamless internetworking with other networks such as the Internet, mobile phones, or WSNs based on other technologies.
Figure 1 shows the overall architecture of an IMPERIA WSN. It comprises a large number of battery-powered wireless nodes spread over a wide geographical area. Two types of nodes are distinguished: sensor nodes and relay nodes. Sensor nodes are equipped with sensors for sensing the environment, whereas relay nodes are required to ensure the hop-by-hop forwarding of the data generated by the sensors nodes to the gateway. The gateway itself consists of (i) a base station (BS) for communicating with the wireless nodes, (ii) a cluster network controller (CNC), and (iii) an MQTT-S broker for the transfer of sensor data to backend applications. Depending on the number of wireless nodes, the entire wireless network may be split into multiple clusters for better manageability and performance. In this case, global network control (GNC) is required for the coordination of multiple CNCs.
The IMPERIA architecture has been prototyped to demonstrate its implementation feasibility. Furthermore its operation and performance were verified under real conditions in multiple field tests.
- C. Lombriser, U. Hunkeler, and H.L. Truong, "Centrally controlled clustered wireless sensor networks," IBM Research Report, RZ 3811, November 2011.