Overcome challenges in wireless sensor networks

Sensor networks for monitoring and control are not a new concept. Technologies for both wired and proprietary wireless implementations have been in existence for a number of years; however, the adoption of each has been different. Wired implementations have found widespread use owing to their simplicity and low cost nature. On the other hand, the use of proprietary wireless networks has historically been limited to just a few niche applications.

Over the last few years, the emergence of standards-based solutions for wireless communications has dramatically changed the market's perception of the technology. Of the many benefits that a standards-based solution offers, a key advantage is interoperability. For example, GSM, WLAN, Bluetooth and DECT have all found success within their market segments as they have allowed for end users to mix products and develop systems independent of vendor.

The emergence of standards such as IEEE802.15.4 and ZigBee, which are targeted at RF applications requiring low data rate, long battery life, and secure networking, have equally changed the market's perception of wireless technologies for sensor networks. Developers started studying and performing technology evaluation of these standards in 2005, but in the last 12 months, there has been a significant progression to real-life product development. In July 2006, the ZigBee Alliance announced that 10,000 developer kits had been sold, and now the first ZigBee products are being deployed. While the market is still in its infancy, there is significant global interest in the technology, which means that widespread adoption is sure to follow. The key catalyst for this growth, as with previous wireless standards-based solutions such as Bluetooth and WLAN will be the availability of integrated chip solutions at a price-point that the market can afford.

Key Issues
As each new wireless standard emerges, there is a consistent trend of primary and secondary issues that needs to be addressed. Once the market need is considered (the communication between devices in a sensor network) primary issues tend to be cost, performance, and quality. Secondary issues tend to be power and security. Cellular network evolution is a good example: once the market and prime needs were addressed, handset power consumption became a major product feature and security became a key concern. The top issues specific to wireless sensor networks are: cost, transmission time, interference and environment, and security.

Cost
In the case of IEEE802.15.4 and ZigBee, the critical price for low end products will be sub $2 for a single chip solution that comprises microcontroller, transceiver, and on-chip memory. This is the point at which large-scale wireless sensor networks will start to be deployed in volume, and, at this price, the choice of wireless control over wired for many applications will become compelling.

A key factor in the cost of wireless microcontrollers is the amount of memory required (Figure 1). Developers of sensor networks and related products will be expecting to have access to a range of chips or wireless microcontrollers with optimized memory footprints to meet the needs of a diversity of applications. Equally, the requirements for larger applications development such as gateway products, third party network layer development, and support of the enhanced ZigBee-Pro, indicate that there is a need for the availability of much larger memory footprints; greater than 250kB in some instances.

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1. A typical mesh network showing the memory requirements at each point.