Toward a structured approach to wireless sensor network design

Many wireless sensor networks are based on proprietary standards for wireless networking, but the recent trend has been increasingly towards the standardization of low-power wireless communication. ZigBee offers a standard for wireless sensing and control that is based on a well-known 802.15.4 specification. While the 802.15.4 document describes PHY and MAC layers of the protocol stack only, ZigBee builds on top of 802.15.4 to offer a network and application layer specifications.

Figure 1: ZigBee builds on top of 802.15.4 to offer a network and application layer specifications.

Among the numerous benefits of ZigBee is the mesh protocol for multi-hop routing and data delivery, a security specification and a set of provisions for application-level interoperability. Generally speaking, ZigBee provides embedded application developers with a higher-level abstraction for managing the network and interfacing with other nodes.

Figure 2: ZigBee mesh network topology.

Although ZigBee is referred to throughout the paper, many of the paper's insights and conclusions would work equally well in context of other standards that use an 802.15.4 MAC and PHY. In order to avoid any further confusion, the rest of the paper will assume that our target design involves a multi-hop network using a mesh routing protocol and a 802.15.4-compatible modulation scheme and a medium access protocol. The paper also assumes basic familiarity with the ZigBee and 802.15.4 specification.

Network organization and size
Network organization and size is perhaps the most critical design choice that very often informs and guides the rest of the design process. It can also have a sobering effect as large networks are typically more difficult to design and maintain. Fortunately, there are approaches to implementing and maintaining very large networks without paying the penalty for doing so.

The state of the art for ZigBee-based networks are networks between 300 and 500 nodes. This may not seem like much, but if one considers that all of these nodes operate on the same physical channel, send data to each other at the same time, route data on each other's behalf and try at the same time to preserve overall network integrity (by sending periodic control messages), it should seem like a very loud and congested network. Note also that 802.15.4 on which ZigBee standard is based uses a CSMA/CA (Carrier sense multiple access/collision avoidance) protocol, i.e. no two nodes in the earshot of each other can speak at the same time. If they do, both will fail and both will retry at a later time. If the network is already congested the retries produce a cascading transmission failure where more and more nodes try to access the air only to find it busy.

In fact, one of the major challenges in growing the networks past a few hundred nodes in size is effectively managing network congestion (the other is optimizing system resources used to store internal stack state at runtime). The following subsections outline three alternative strategies for addressing the congestion issue.