The Problem
A US based sensor device manufacturer, wanted to design and develop a next generation wireless sensor network. Their current product suite supported a wired protocol. The target market imposed restrictions on the cost, battery life, range and size.
The Solution
A preliminary scan of available standard wireless protocols made it clear that none of them would meet the client’s constraints on cost. It was decided to develop a light weight custom protocol optimized for this application in the 915 ISM RF band. The topological requirements eliminated a simple star network configuration. A hierarchical packet routing protocol strategy was chosen for its simplicity, ability to extend the range and handle dynamic addition and deletion of nodes. The number of nodes on the network was limited to 32 including a coordinator node. The wireless protocol was designed to allow for some of the nodes which were battery powered to go to sleep for long periods of time. A central mains powered coordinator was burdened with the task of establishing the network, setting up routing for remote nodes, tracking missing nodes and disseminating sensor data. Packets were designed to be short, protected by CRC and limited in number of types. This permitted simple state machine based implementation of the protocol on the nodes, thus leaving more processor resources for the application. A protocol analyzer was built that would analyze packets in the air and ensure that the implementation adhered to the protocol specifications. A network simulator was used to test the network under various scenarios ensuring a robust behavior of the network under all the conditions. The electronics was designed around a 915 MHz modem chip along with a low-cost microcontroller which also interfaced to the sensor electronics. Frequency shift keying with Manchester encoding at low bit rates was chosen for the physical layer. A carrier senses multiple access mechanism was used to share the bandwidth amongst the nodes. The design also catered to multiple such networks coexisting in the same space.
The client was able to meet the requirements of his target market with this custom low-cost wireless protocol that met his cost, range and battery life points.