A sensor network is a computer network of different sized sensor nodes. These may even be as small as a dust particle. The communication of the sensor nodes takes place by radio between the individual sensors, where they poll their surroundings and forward the gathered data. Accordingly, this is known as node-to-node communication.
Some time ago, a sensor network was announced with the name “Smart Dust”. The idea behind this was to reduce sensors with certain functions to the size of a dust particle (“intelligent dust particle”).
Sensor networks are continuously in the further development phase. Originally, sensor networks were only developed as an early warning system for the military, but nowadays other application fields for the sensor networks can also be considered, e.g. the monitoring of nature reserves for forest fires etc.
Various physical values such as temperature, air pressure, torque, brightness etc. can be registered via sensors. These data then are transferred to actuators, whereby they can then be controlled.
The sensor networks differ from local networks (LAN), WLAN and mobile phone networks in terms of the lower transferable data quantity.
Communication of the Sensor Nodes
The sensor networks form ad-hoc networks in order to enable communication between the sensor nodes. Ad-hoc networks do not have a fixed infrastructure between the devices, but are meshed. This means that the sensor nodes are connected with one or more neighboring sensor nodes. In order to transfer the data to a specific target, they must be handed over from node to node. This is called multi-hop communication.
However, these networks are considerably more unsafe than fixed installation computer networks. This is because, during the multi-hop communication, individual nodes may fail or be added.
Network Protocols and their Tasks
Network protocols should transfer the data in the sensor networks as fast as possible and keep the energy consumption of the sensors as low as possible. The energy consumption is controlled using targeted sleep times or through targeted use of the radio equipment of the sensor nodes, when this is required.
The behavior of the nodes is recorded in the network protocol as follows:
- Initialization- During the initialization phase, the sensor nodes detect their neighboring sensors and through this build up the meshed network. A good network topology guarantees success when routing.
- Daily routine – The daily routine describes the change between the active times and the sleep times of the sensor nodes.
- Communication scheme – The communication scheme describes the data transfer path between the sensor nodes. Fault-free data exchange must be guaranteed.
- Routing – The routing specifies the path to be followed when data are transferred. With this, care must be taken that all of the sensor nodes are used evenly in order to prevent the one-sided loading of the network.
Different Network Protocols
Depending on the application area of sensor networks, different network protocols are required. The most important ones are described as follows:
Media access protocols
Media access control (MAC) plays an important role with these protocols, as this is where work is performed using air terminals. Basically, the task of the MAC is to regulate the common medium of the sensor nodes (air), so that communication between the nodes can be enabled.
Energy consumption is to be kept to a minimum through the targeted switching on and off of the radio module and the decision as to when data are to be transmitted (and when not). This is known as duty cycling.
This effect is generated through two methods: Random access with carrier check and time multiplexing.
Low-power listing is used for random access with carrier check.
With this, checking is performed in order to find out whether the medium is busy or not. When the medium is busy, the radio remains activated to in order to enable data exchange. If the medium is not busy, the radio module is switched off immediately.
With time multiplexing (Time Division Multiple Access = TDMA), a schedule is created for when which sensor node must send or receive. This can reduce the energy consumption considerably, but the creation of such a schedule is very time-intensive. Examples for such protocols include Sensor Media Access Control (S-MAC) and Timeout Media Control (T-MAC).
In particular, the routing protocols address the issue as to how the data are to reach their destination as quickly and with as little effort as possible.
Here, geographic routing procedures play an important role.
In particular, the Greedy Perimeter Stateless Routing in Wireless Networks (GPSR) network protocol transfers data not to specific names, but to coordinates. With this protocol version, there is steady switching between the greedy strategy and the perimeter mode. This is done so that data packets in a suboptimal network topology never get stuck in a kind of dead end. With the greedy strategy, the data are transferred directly to the destination.
However, in the perimeter mode, the transmitted data circle around the destination. Geographic hash tables expand the GPSR to include a safety component. Sent data are transferred to multiple sensor nodes so that the data are saved at different nodes, even in the case of node failure.
However, the geographic routing procedures are only possible when there are a sufficient number of sensor nodes. This way, accurate positioning can be performed.
The protocol optimized for Sensor Networks is the RPL.
Synchronization of the Sensor Nodes
Synchronization of the sensor nodes is required for some communication protocols e.g. S-MAC, as otherwise the measurement data are falsified.
The synchronization of the sensor networks depends on various factors such as the transmission time, the access time, the velocity of propagation and the reception time. The synchronization procedures should be adapted to the communication procedure, as here in particular, the access time varies.