Analysis of Security Threats, Requirements, Technologies and Standards in Wireless Sensor Networks
Lecture Notes in Computer Science
As sensor networks are more and more being implemented in real world settings, it is necessary to analyze how the different requirements of these real-world applications can influence the security mechanisms. This paper offers both an overview and an analysis of the relationship between the different security threats, requirements, applications, and security technologies. Besides, it also overviews some of the existing sensor network standards, analyzing their security mechanisms. and their
... rity mechanisms. Consequently, we will provide an overview of these different standards, focusing on their security capabilities. Later, we will analyze not only how these standards compare with each other, but also if they offer support for implementing the previously mentioned security mechanisms. Overview of Wireless Sensor Networks Base Station Sensor Nodes Cluster Cluster Head Fig. 1. An overview of the architecture of WSN The ability to perceive the physical world is not inherent to the nature of computer systems: they are tightly tied to the realm of the abstract. The existence of sensor hardware tries to build a bridge between the abstract world and the physical world. These sensors are devices that can measure a physical quantity (e.g. temperature, humidity) and convert it into a digital signal. Using these sensors, computer systems ranging from the simplest washing machine to the Large Hadron Collider (a particle accelerator located at the European Organization for Nuclear Research (CERN) ) can acquire and process information coming from the physical world. This ability to "feel" the world is usually embedded in the design of a computer system, e.g. sensors in a washing machine are integrated within the system from the initial design. However, it would be particularly interesting to make this ability available as an off-the-shelf component. As a result, any computer system, regardless of its design, could be able to perceive the physical world. Such is the task of Wireless Sensor Networks, or WSN. The structure of a wireless sensor network can be seen in Figure 1 . A wireless sensor network is composed by two types of devices: sensor nodes, and base stations. The sensor nodes, also known as motes or simply nodes, are small and constrained devices that have the ability to "feel", "think", "talk", and "subsist". They can "feel", because they can sense the physical features of their surrounding (e.g. temperature, humidity, radiation, vibration) using hardware sensors. They can "think", because although they are highly constrained in both computational power and memory, they are capable of processing information on their own. They can "talk", because they are equipped with wireless transceivers, and can collaborate towards a common goal. Finally, they can "subsist" because they are in most cases powered by batteries, and can survive in their deployment field for more than a year if their internal operations are optimized. Regarding the base station, it is a more powerful device that usually behaves as an interface between the services provided by the sensor nodes (the "data acquisition network") and the users of the network (the "data dissemination network"). Normally, the base station collects all the information coming from the sensor nodes and stores it for later use. Also, it can issue control orders to the sensor nodes in order to change their behaviour. While it would seem that wireless sensor networks are highly dependent of the existence of this base station, the architecture of the network is not centralized. Sensor nodes can operate in a decentralized fashion, managing themselves without accessing to the base station. A powerful simile that can be used to illustrate the structure of wireless sensor networks is to consider them as "living beings". The sensor nodes behaves as "cells", since they all belong to the same body (WSN), are usually loaded with the same "DNA" (program), and cooperate unselfishly towards a common goal. On the other hand, the base station could be considered as the "brain", since it receives and processes all the physical information coming from the "cells", and can also send control information to them. Note that, in terms of communications, the "cells" also behave as "nerves", since they transmit (wirelessly) the information sensed by other "cells" to the "brain". The services offered by wireless sensor networks can be classified into four major categories: monitoring, alerting, provisioning of information "on-demand", and actuating. As for the first case, sensor nodes can continuously monitor certain features of their surroundings (e.g. measuring the ambient noise level) and timely send such information to the base station. Secondly, sensor nodes can check whether certain physical circumstances (e.g. a fire) are occurring, alerting the users of the system when an alarm is triggered. In the third case, the network can be queried about the actual levels of a certain feature, providing information "on-demand". Finally, sensor nodes can be able to change the behaviour of an external system (e.g. an irrigation system) according to the actual state of the context (e.g. humidity of the soil) 1 . Due to the computational capabilities of the sensor nodes, it is possible to reprogram the network during its lifetime, or even use it as a distributed computing platform under specific circumstances. Finally, wireless sensor networks can be organized in a completely distributed way (flat configuration), but they can also implement levels of hierarchy (hierarchical configurations). In flat configurations, all the nodes contribute in the decision-making process and participate in the internal protocols, like routing. Conversely, in hierarchical configurations the network is divided into clusters or group of nodes. Inside a cluster all organizational decisions, like data aggregation, are made by a single entity called cluster head. It should be noticed that it is also possible to have a combination of the two previous configurations into 1 These type of sensor networks are also known as Wireless Sensor and Actuator Networks (WSAN).