Understanding wireless network architecture
By Patrick McCurdy, Phoenix Contact -- Plant Engineering, 2/1/2009 1:00:00 AM
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Sections:
Point to point Star/point to multi-point Repeater Mesh |
One of the initial considerations when planning an industrial wireless implementation is the scope and sophistication of the network. Many times simple objectives such as communication to a single remote asset can be achieved with “wire replacement” technology in a very straight forward and easy to deploy approach.
On the other end of the sophistication scale, the objective may be to network large numbers of critical process assets with the systems needed to increase plant efficiencies and meet regulatory challenges. The topology or layout of the wireless network is one aspect of this issue that needs to be considered.
The topology of a data network, whether it is wired or wireless, generally describes the structure or layout of the data communication devices. In detail it can describe the physical and logical map of the way various intelligent devices, generically called nodes, are laid out and communicate to each other. In a wired network, the topology is in part determined by the location of the nodes and what – if any – network components are used. In the case of wireless networks, the topology can be a function of numerous factors including the wireless modulation technology, the logical capabilities of the network components, the geographical topology and distance between nodes and the protocols that are supported by the wireless devices.
Point to point
A common early implementation of wireless technology in industrial applications was the simple monitoring application where wire or conduit was replaced with a wireless link. These applications typically involved bridging some distance that was considerable enough to make wiring impractical, or at least not cost effective.
You might think of the wired equivalent of this as a 4-20 mA installation or an RS-232 connection. In this example, you are communicating between one fixed field device such as a flow or level transmitter and a control or monitoring location. This is a simple implementation of wireless that can be very fast and easy to deploy in an industrial space and offer significant savings when considering expensive costs of wiring and conduit in some industrial environments.
Star/point to multi-point
To facilitate more sophisticated applications such as Supervisory Control and Data Acquisition (SCADA), a wireless point to multipoint or star configuration is commonly deployed. In the classic implementation of this configuration, there is a star cluster in which the wireless “access point” or “master” is at the center of the topology and manages the flow of information to it on a polled basis with remote sites.
Many traditional SCADA installations that use wireless technology as a form of telemetry to communicate to remote sites use this “master/slave” configuration. This implementation can also be compared to a wired installation of an RS-485 bus network. In more modern wireless Ethernet networks such as industry standard IEEE 802.11a/b/g, it is also common to deploy a star topology with the “master,” in this case typically described as an access point and the slave devices known as clients.
Repeater
Repeaters are often included in wireless networks to extend the reach of the network to areas that are either too far for the wireless technology to reach in one hop or areas where the RF path may be obstructed due to geographical topology or other physical objects.
Mesh
The newest topology often discussed in the industrial wireless market is mesh networking. In the truest implementation of a mesh network, each node functions as a both an end device and as a network forwarding (or repeating) device. It allows self-healing, continuous connections and reconfiguration around broken or blocked paths by hopping from node to node until the data reaches the designated destination.
If a node in an 802.11 network is lost, using industrial WLAN technology, the network will “heal” itself by redirecting the flow of data in the broken path to the closed path. Not all WLAN technology is capable of this mesh functionality.
Wireless sensor networks (WSNs) broadly describe IEEE 802.15.4-based technology such as the WirelessHART and the pending ISA100 standards. In the case of WSN, great importance is placed on power conservation. Toward this end, the devices are typically low power devices. Therefore, to achieve the low power goals and still have reliable communication across the physical boundaries of an industrial facility, the technology needs to be capable of this type of node-to-node routing.
