Some administrators might have you believe that setting up wireless access points for optimal coverage is an exact science. Let me first dispel that notion. Selecting the best spot for your wireless access points can be a little like trying to use an old radio dial to tune in a distant station. At best, it's an art that you become better at with experience. For those who have not mastered this art, this first of a two-part series will guide you through the information necessary to plan your wireless access point placement.
If your coverage area is extremely large, possibly consisting of thousands of square feet, a scaled building diagram is extremely important in the initial planning process. Hopefully, you can acquire a copy of the diagram from the building contractor or architect. If not, you may need to create one yourself.
You can do this by simply using a pencil, paper, ruler, and measuring tape, but if I had my guess, I’d say you would rather use a program such as Visio. The drawing does not have to be in great detail. The key is to have an accurate scale and a view of square footage, complete with walls, windows, and doors. This diagram will essentially turn into the project plan for your wireless access point implementation. I have provided a sample Visio diagram you can download. (For those who don't have Visio, the download includes a snapshot of the drawing you can view.)
An access point's transmission radius is called a cell. Overlapping cells using multiple access points creates seamless access as you roam from one cell to another. In a setting with overlapping cells, mobile devices and access points frequently check the strength and quality of transmission, handing off users to the access point with the strongest and highest quality signal. For that reason, it is important to not have roaming areas that extend outside the coverage of any given cell.
The size of your cell will depend largely on two factors: the type of 802.11 used and possible obstacles and interference that the radio frequency (RF) signal may encounter. We'll discuss planning for obstacles and interference later in the article. You've probably read numerous articles detailing the difference between the A, B, and G variations of 802.11. You have likely chosen a standard that you believe will work best for your budget and network requirements. If so, use your building diagram to show your anticipated cell coverage. When creating your cells, use a circumference slightly less than the maximum range for your standard, ignoring (for now) possible obstacles and interference. For example, if planning for an 802.11b implementation, I might use a range of 250 feet instead of the reported 300 feet.
Number and placement of clients
Theoretically, hundreds of clients can associate to a single wireless access point. This is typically not a good idea, since your access point will more than likely be connected to a 100-MB Ethernet cable. Depending on the network performance required by your end users, bringing the number of clients down to 15 or 20 will yield better network response.
You can adjust this number up or down based on the equipment you select and on your configuration. Simply note on your building diagram the number of clients you expect to associate in a cell. At this point, you may need to adjust your diagram to the number of cells required to provide coverage for heavily occupied areas, such as cubicle clusters and conference rooms.
As you are probably aware, RF is susceptible to signal loss resulting from the materials it has to pass through. This is known as attenuation. Typical office obstacles such as doors, windows, walls, and furniture result in attenuation.
For example, if a large bookshelf filled with books sits between two offices, you should expect some RF loss. This attenuation is in addition to the path loss resulting from simple range traveled between endpoints.
Again, use your building diagram to make appropriate annotations concerning the types of obstacles you expect to encounter. Start thinking about the minor changes required of your wireless access point placement to accommodate the anticipated attenuation caused by certain obstacles.
RF interference involves the presence of unwanted, interfering RF signals that disrupt normal system operations. Because of the 802.11 medium access protocol, an interfering RF signal of sufficient amplitude and frequency can appear as a bogus 802.11 station transmitting a packet. This causes legitimate 802.11 stations to wait for indefinite periods of time until the interfering signal goes away.
Depending on your environment, this may or may not be a problem during the planning stage. For example, healthcare environments are more susceptible to RF interference due to the numerous electronic medical devices in use. For 802.11a and 802.11g wireless LANs, microwave ovens, wireless phones, Bluetooth-enabled devices, and other wireless LANs can cause problems. The use of a spectrum analyzer (discussed in my next article) will provide a detailed report of possible interference.
However, initially, a simple walk-through in which you look around and note possible devices that may cause interference is sufficient. Also, talk to people within the facility and learn about other RF devices that might be in use. Either make plans to move these devices away from desired converge areas or plan on changes to your access point placement and/or configuration to counteract their interference. Don’t forget to make a note of these on your diagram.
Now that you have a general idea of where you may place your access points, you should start analyzing the various mounting considerations you may face. Will you be mounting the access point in the ceiling or attached to a column or maybe over a door? Consider your power and cabling needs when choosing a mounting location. If you are fortunate enough to have inline power coming from your CAT5 cable, you will simply require an Ethernet connection to your access point. If this is not the case, however, you may have to look at placing the unit close to a power outlet. Of course, this can limit your choice for optimal RF coverage. You may need to have an electrician wire some additional outlets in places where you need access points.
Environmental operability should be considered as well. These factors include temperature, moisture, and plenum requirements. For instance, your building may allow only plenum-rated equipment to be mounted in the ceiling, forcing you to either purchase plenum-rated access points or mount the access points somewhere below the ceiling.
Finally, consider the aesthetic aspect. If you can't hide your access points, at least ensure that they complement or blend into their surroundings and do not look unprofessional—especially in an office setting where customers frequently come calling.
In my next article, I will describe the proper way to perform an in-building site survey against your plans. I will also address the use of low-cost spectrum analyzers and the often-challenging task of channel assignment.