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New wireless networking standards can simplify the implementation of and add flexibility to industrial control systems. The first standard is IEEE 802.15.4 is a low data rate, wireless standard that allows industrial control systems to communicate using wireless sensors and switches. The second standard is the ZigBee standard. ZigBee is based on 802.15.4, but also provides specifications for network security and application profiles that guarantee interoperability between equipment from different vendors.
The ZigBee standard is a superset of the 802.15.4 standard. In other words, a ZigBee-certified application must conform to both the ZigBee standard and the 802.15.4 standard, while an 802.15.4 application may or may not adhere to the ZigBee standard. The distinction is important because an 802.15.4 network is simpler to implement than a ZigBee network, and because the 802.15.4 standard is complete, while the ZigBee standard continues to evolve. Some of ZigBee's higher-level application layers have not even been defined yet. Thus, any application that must be deployed now will probably have to be deployed as an 802.15.4 network, with ZigBee capability added later when the profile for the application has been ratified.
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. The application framework and profiles provide specifications at the application level that guarantee interoperability between equipment from different vendors. In addition, the ZigBee network layer specification defines networking topologies, specifically "mesh" networks.
Network Topologies
There are several 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.
Figure 2: A mesh network.
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.
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. This capability makes mesh networks ideal for large building control systems or wide area sensing. Mesh networks are by far the most difficult 802.15.4/ZigBee networks to design and implement. Building a mesh network from scratch is a complex process. Since mesh networking is defined as part of the ZigBee network layer specification, anyone wanting to deploy a mesh network should stick with the ZigBee standard. (Figure 2 " ZigBee Mesh Network)
If the application does not require a "mesh" networking capability or does not need interoperability with equipment from other vendors, ZigBee may not be necessary. In the case of industrial control systems, interoperability may actually be a disadvantage that compromises network security. Since the vast majority of industrial control systems can be accommodated using a simpler tree network, developers may prefer to go with 802.15.4 and a proprietary network layer, rather than waiting for the ZigBee standard to be ratified.
Network Integration
In most wireless networking applications, such a wireless LAN, WiFI, Bluetooth, etc, the network is the primary application. In industrial control, the primary application is some kind of process or flow control application. The network is the vehicle by which the processes and equipment communicate. When deploying ZigBee or 802.15.4 functionality in an industrial control system, two separate applications must be developed in parallel: the primary control application and the supporting networking application.
Most industrial control engineers are not, and should not have to become RF experts in order to add 802.15.4 functionality to their systems. Fortunately, most 802.15.4 vendors, including Atmel, TI, Freescale, Jennic and Ember, offer fairly complete system-level solutions that already integrate the radio, controller, collateral software and development tools. The designer does not have to become an RF expert.
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