Abstract
Sensor networks are one of the most powerful technologies applied to control and supervising systems. They are present in almost all environments, so we can find them in different industrial, medical, security and/or home applications. In addition, the use of wireless technologies applied to sensor networks improves the final system. Examples given are the enlargement of the coverage area and the low deployment costs. Nowadays a combination of both items is used in many common implementations of a wide variety of applications. This chapter is focused on the use of sensor networks applied to disabled and elderly people. We will study the wireless technologies most used for this purpose and we will show a survey with the benefits when they are applied.
TopWireless Technologies
Four main wireless technologies are applied to wireless sensor networks. Depending on the environment where it is going to be used, the type of use, the infrastructure conditions and/or the coverage extension needed, one of them (or a combination of two of them) can result the best option to develop the system.
Bluetooth is an open specification for wireless networks which is based on radiofrequency. It operates in the Industrial Scientific Medical (ISM) frequency band (2.4 GHz) forming wireless personal area networks (WPAN) (IEEE Std 802.15.1-2005, 2005). It has low energy consumption and its cost is quite low. Bluetooth controls its interference, and the susceptibility to the interference, by using spread spectrum modulation. It uses a frequency-hopping spread spectrum signaling method (FHSS). Bluetooth was formalized in IEEE 802.15.1 standard in its version 1.2. This technology uses a small area network without infrastructure (piconets). Nodes share a physical channel with a clock and a unique sequence of jumps in the same piconet. In Bluetooth, different channels can coexist. While a master can only belong to one piconet, any other device can belong to several piconets at the same time. This overlap is denominated scatternet (dispersed network), although there are not defined routing capacities among them. This technology appeared to be used for the devices interconnection like computers, mp3, PDAs, etc. at a distance of about 10m, although lasts products in the market achieve 100m. of coverage radius. Furthermore, this coverage range can be longer with higher antenna gain and even longer distances can be gotten using signal repeaters.
Key Terms in this Chapter
Frequency-Hopping Spread Spectrum: FHSS is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels. In order to synchronize the transmitter and the receiver a pseudorandom sequence known by both is used. Bluetooth has been designed to operate in noisy radio frequency environments. That noise produces interferences in the system, being allocated in some random frequencies. By switching among frequencies the impact of noise results highly decreased.
Ad-Hoc Network: An ad-hoc network is a local area network (LAN) that is built spontaneously as devices connect and based on the communication between all the members without the need of a network controller to coordinate the flow of messages in the network.
IEEE 802 Standard: An IEEE 802 standard is a group of rules that specifies how to implement a local or metropolitan area network. That rules are determined by the Institute of Electrical and Electronics Engineers (IEE), and there are many different standards, each one defining one communications system. So, IEEE 802.11 standard is dedicated to wireless local area networks (WLAN) and 802.15 defines wireless personal area networks.
Piconet: Also called Bluetooth personal area network, a piconet is a network of devices connected ad hoc using Bluetooth technology. A piconet is formed when at least two devices connect, and can support up to eight devices.
Industrial Scientific Medical (ISM) Frequency Bands: The ISM radio bands are a group of frequency ranges originally reserved internationally for the use of RF electromagnetic fields for industrial, scientific and medical purposes other than communications. Due to the initial potential use of these frequencies, all equipments must be protected from interferences at ISM bands. This interference protection is the reason for which nowadays several of these frequencies are used by unlicensed communications devices, like cordless phones.
Scatternet: A group of independent and non-synchronized piconets that share at least one common Bluetooth device.
Routing Protocol: A routing protocol is a set of rules which function is to determinate what is the correct way and order for information to be sent in a computer network. As a computer software, it is executed and the transmitter obtain as a result in which direction data have to be sent in order to arrive to the desired receiver (the data can go through several equipments before get the destination).