Choosing between single and multi-channel architecture
Following the completion of the site surveys, placement of the required access points (APs) on the facility blueprint should be complete. Depending upon what topology is being used, different channelization schemes can be considered. Two methods of assigning channels are used; each is uniquely suited to differing topologies. Single-channel architectures (SCAs) use a single radio frequency (RF) channel and a basic service set identifier (BSSID) throughout the system. Multi-channel architectures (MCAs) use multiple repeating channels in a set pattern.
There are several network topologies available for use, depending upon the implementation. For a small office, home office (SOHO) environment, using one AP is the obvious choice for the BSSID. Any of the 11 industrial, scientific, and medical (ISM) channels can be chosen, though it would be advantageous to use a channel that is not overlapping your neighbor’s. Using commonly available tools, the channels in use around you can be easily determined and avoided. There are three nonoverlapping channels in the ISM band-1, 6, and 11. The channels are separated by 22 Mhz and are designed to operate without interfering with one another. This is especially important if there is more than one AP in a small network.
In a network with multiple APs using the ISM band, it is required that each AP be assigned a different channel. This is an example of an MCA in its most basic form. Using multiple channels will facilitate the formation of an extended SSID (ESSID) and allows seamless roaming between APs. Figure 1 illustrates the concept using two APs using two different channels and BSSIDs:
In larger systems, with many APs over a large area, the assignment of channels to individual APs becomes something of an art. The rule of thumb is to ensure that each adjacent cell is at least two channels away from one another. Using the convention of representing coverage areas using hexagons, a larger ESSID in the 2.4 GHz ISM band is represented as shown in Figure 2 with the pattern repeating across the network.
Using the UNII band gives a designer a lot more leeway. While the ISM band allows the use of 11 channels (in the U.S.) with three nonoverlapping channels as shown above, the UNII band allows the use of 24 nonoverlapping channels. It is still good practice to allow a two-channel separation between cells as an added safeguard. As you would expect, using the UNII band affords a designer much more spectrum to work with. A UNII-3 band ESSID is in Figure 3.
Multi-channel architectures use a mesh topology or an arrangement of wired autonomous APs to accomplish the ESSID. Some systems use a centrally controlled wireless distribution system to control the operation of the connected APs. These options should be evaluated to determine suitability to the application.
An SCA consists of a single channel and a single BSSID (AP MAC address) that are used for the entire network. This is possible because of the use of multiple, lightweight APs that are actively controlled by a central WLAN controller. All communication occurs on a single channel, requiring continuous traffic management.
The advantages to an SCA include co-channel interference being eliminated; latency due to the reassociation inherent in roaming between BSSIDs is greatly reduced; and the reassociation and reauthorization process is not required. Also, the WLAN controller, instead of the roaming device, in an SCA network controls roaming. The roaming device does not know it is roaming; it is always associated with a single BSSID.
SCA systems are not prevalent due to the higher cost of equipment, configuration, and management. Having a single point of failure at the controller also makes these systems unsuitable for mission-critical applications such as process control. The loss of the central controller essentially takes down the network. SCA systems are, however, well-suited to latency sensitive applications such as voice and video delivery. Another upside of a single channel system is that the owner has complete control of the network. However, most wireless management systems also provide a high level of control, without the obvious modes of failure.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA); firstname.lastname@example.org. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, email@example.com.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Virtual and physical WLAN site surveys
WLAN design preparation and needs analysis
WLAN design basics and wireless network considerations
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.