Wireless sensor networks are a dynamic technology impacting multiple application areas. Success in applying the technology is based on employing sound engineering principles and using a structured process in order to address the issues which are common to all designs as well as those that relate to the specific application.
Traditional wireless architectures are based on point-to-point or point-to-multipoint approaches. More recently mesh designs have emerged offering the potential of more reliable, more easily expandable and more flexible solutions.
There are a number of wireless mesh protocols in the wireless mesh sensor network domain. The common denominator is the IEEE 802.15.4 specification which defines the lowest layers in the protocol stack. The higher layers in the stack are defined by individual vendors who have focused on meeting specific performance targets, or by bodies responsible for applications in specific vertical markets e.g. ISA, Wireless Hart in the process industries. In some cases consortia of companies have grouped together to define a protocol that is then marketed as a ‘standard’, the prime current example of which is ZigBee which is widely available but is not a ratified standard.
Wireless mesh networks communicate with each other through routers and a gateway to form a mesh network. Most mesh networks use end nodes which are the primary sensing devices which relay their signals through one or more router nodes ( which may also act as sensors) and each router forwards the signal finally to a gateway device. All the mesh devices relay signals for each other, so if a given sensor is out of range of the gateway, other members of the network carry data.
Factors influencing range include transmit power and receive sensitivity, antenna design, operating frequency, building topology and obstructions
Devices operating at 2.4 GHz appear to offer the bandwidth required to build-in protocol features to achieve the optimum combination of range and reliability for internal applications. Additionally the ISM band at 2.4 GHz provides the benefit of a worldwide standard whereas other frequencies have regional restrictions on their use.
Network architecture is determined by the application location type. One approach is based on classifying building types as – commercial, light industrial or heavy industrial. Each type has common features in terms of building structure, material and space usage which affect the overall approach to network design.