WLAN Interference : Identify the Source & Implement a Solution
Article by Stephen Snow
Interference is typically interpreted as other sources of RF energy that compete with an existing implementation. Nevertheless, it’s important to note that everything in an environment, from bookshelves to microwave ovens, can potentially cause signal degradation. It’s also important to understand that different materials impact RF energy in different ways -- each creating its own “signature” challenge when installing or maintaining a WLAN.
When deploying or maintaining a fully functioning and robust wireless local area network (WLAN), careful consideration must be given to ensure that interference is avoided. If possible, the ideal situation is to identify potential sources of interference during the site survey process.
There are numerous materials and technologies that can negatively impact a wireless system. Because interference is a problem every WLAN engineer and user will face at one time or another, I’d like to devote this column to the most egregious sources of RF interference and provide some suggestions on how to ensure your WLAN’s performance is not compromised.
2.4 GHz Devices
As WLANs continue to gain in popularity, individuals and businesses often find their wireless network impacted by other, adjacent, wireless networks. Another 2.4 GHz wireless LAN operating in the same area can cause performance issues since WLAN technology is a shared media. When two systems attempt to send data at the same time, failed transmissions can occur. This may require both systems to resend data in order to successfully get information through, compromising the efficiency of the network.
Whether the technology is being used in the same location, or if a neighbor has RF energy that bleeds into your facility or campus, it will likely cause problems. In this situation, you may have to work closely with neighbors to make certain that both systems can work simultaneously without creating interference for one another. To guard against interference, as well as security issues, you may also need to perform another survey to ensure that your RF energy is contained (as much as possible) within your coverage area.
GHz Phone Systems
Today, 2.4 GHz phone systems are a hot item. Unfortunately, 2.4 GHz cordless phone systems and WLANs negatively affect one another. At some point, the 2.4 GHz cordless phone coverage will cause the WLAN user to loose their connection with the access point. This is unacceptable and troubling, especially in an environment where business is conducted.
The co-existence of a 2.4GHz WLAN and mobile phone system is a stumbling block. Users of 2.4GHz phone systems may have to change to a 900 MHz phone to accommodate the wireless network. In practical terms, it’s smarter to replace a $200 cordless phone than $10,000 worth of networking equipment.
Florescent Lighting & Microwave Ovens
There is a new breed of fluorescent lighting that operates in the 2.4GHz band. This technology will make it very difficult to mount wireless LAN equipment at ceiling height (where the lighting is located) without creating issues. Similar to a microwave oven, this lighting uses 2.4 GHz RF to excite electrons.
Additionally, an older microwave oven that is not sealed properly will leak RF energy and potentially cause interference (as well as a health risk). If your office encounters what appears to be WLAN interference at approximately noon everyday, you may find the problem to be co-workers firing up a microwave located in close proximity to an access point.
These devices compete with a WLAN and must be addressed. Potential solutions are to locate the equipment away from one another or even change the technology being used. Try and avoid the obvious; don’t set an access point on top of a microwave oven. Changing channels is not a viable solution since 2.4 GHz lighting and microwaves use the entire 2.4 GHz spectrum.
Bluetooth Devices
Keep in mind that Bluetooth devices also share the 2.4 GHz band. If people bring Bluetooth technology into a facility with a wireless network, it may create issues for WLAN users. Because 2.4GHz wireless LANs and Bluetooth are incompatible, you may want to make employees aware that roaming the office with personal Bluetooth devices will compromise the operation of the wireless network.
Again, since 2.4 GHz is a shared spectrum, neglecting to take into consideration interference created by microwaves, cordless phone systems, satellite systems, lighting systems, and Bluetooth devices can be a serious oversight.
Environmental Issues
Everything in your environment has the potential to affect a RF signal. There are materials or objects that can hinder or block your signal by absorbing the energy, as well as objects that can block your signal by reflecting the radio waves. It is important to know your environment so that you can work around potential issues and take advantage of others.
Interference By Absorption
There are all types of materials that may interfere with the propagation of the RF signal by absorbing energy. Typically, items with high water content or very dense objects will absorb radio waves. Examples of absorptive sources include paper, wood, dog food and kitty litter. When these items absorb RF energy, it can become difficult to achieve the coverage required for your application. Also, if your environment changes, such as in a warehouse with fluctuating stock levels, you should expect to see different results in the coverage of your wireless network.
There is no “easy” solution for this type of scenario. A fluctuating warehouse environment may require the installation of additional access points to accommodate high inventory levels. When inventory levels become depleted, the system will probably provide more coverage than needed. And this could be a double-edge sword since too much coverage, during low inventory levels, can create additional performance issues. A more complex solution may be required.
Interference By Reflection
Materials can also reflect radio waves. Typically, metal objects create a bounce effect. Trying to provide coverage through a lead lined wall in a hospital x-ray room will create difficulties. While these types of environments create RF shadows, where coverage is not provided, a savvy site survey engineer can design and install a wireless network that operates effectively alongside these common causes of interference.
Understanding your environment is essential to installing and maintaining a high-performance WLAN. Environments filled with materials that absorb the RF signal may require higher gain antennas and use more access points to achieve the desired coverage. Environments that create a great deal of bounce, such as a warehouse or manufacturing plant, may require directional antennas to minimize bounce affects. In fact, materials that reflect RF energy can also be used to your advantage. You can sometimes use the bounce to cover hard to reach areas and provide coverage in a sector that may have required an additional access point. However, intentionally bouncing RF energy to obtain desired coverage should only be attempted by engineers with a great deal of RF and antenna knowledge.
Regardless of the source of interference, with the proper planning, a high performance wireless LAN with the coverage, functionality and throughput requirements is attainable. Identifying the common culprits of interference and building solutions into your wireless LAN during the site survey process is your best bet for success.
Steven Snow is Vice President of Technical Services for GigaWave Technologies, a leading provider of innovative training services, curriculum development and certification programs for the dynamic and rapidly expanding WLAN industry. Based out of San Antonio, Texas, GigaWave focuses solely on WLAN education.