Two new standards, ZigBee and IEEE 802.15.4, now offer the ability to implement wireless industrial control applications. IEEE 802.15.4 is a low data rate, wireless standard that allows industrial control, embedded control and other systems to communicate without wires.
Sensors, switches and servo controllers can be located anywhere in the industrial environment and communicate reliably with the control application. IEEE 802.15.4 covers the basic RF part of the network: the physical layer and the media access control layer.
ZigBee covers the network layer, application framework and application profiles that that guarantee interoperability between equipment from different vendors. In addition, the ZigBee network layer specification defines networking topologies, specifically "mesh" networks.
The ZigBee standard is a superset of the 802.15.4 standard and specifies the network security layer, application framework and application profiles so that ZigBee-certified equipment from different vendors will interoperate seamlessly. A ZigBee-certified application must conform to both the ZigBee standard and the 802.15.4 standard.
It is also possible to have a functioning 802.15.4 wireless network that is not a ZigBee network. In fact, if the application does not require interoperability and will not be employing a complex mesh-type network, it may be preferable to implement a non-ZigBee 802.15.4 network because it will be much easier to deploy.
Network Topologies
There are basically three types of network configurations that can be implemented under the 802.15.4 and ZigBee umbrella: point-to-multipoint (star) networks, tree networks and mesh networks.
Point-to-multipoint (star) networks are typically used for low cost gaming or entertainment center control.
They are the simplest to implement and require the least amount of code for setup and control. They are typically limited in the quantity of nodes and coverage. Tree networks are more appropriate for applications such as access or industrial control sensing.
Since they allow more nodes, they can cover a larger area than point-to-multipoint networks. However, tree networks may suffer from latency effects that can cause unacceptable data delays for critical applications.
Tree networks also may be subject to critical node failure and cause system failure. Tree networks also usually need larger amounts of code to implement than multipoint systems.
Figure1: A ZigBee tree network.
Mesh networks represent the highest level of 802.15.4/ZigBee configuration and require the most network level code. Mesh networks route data dynamically creating the most efficient path among a multiple of network nodes.
The ability to route data among multiple paths provides mesh networks with a "self healing" capability. If a node in the path fails for any reason, the network identifies a new path using other nodes.
