RealTime IT News

Identifying Access Point Installation Locations

It's difficult for those without radio frequency (RF) experience to determine the optimum location for access points. Without paying close attention to the factors that contribute to RF propagation, you'll end up with spotty coverage, inadequate performance, and frustrated users. As we've discussed in a previous tutorial, you should perform an RF site survey to identify the number and location of access points. Let's take a closer look now at what you should consider when making decisions regarding access point installation locations.

Requirements First

As the basis for making decisions about these locations, take into account application requirements, which are generally attributes of the solution that you don't want to (or can't) change. Have a clear understanding of the areas where users will require access to the wireless LAN. This helps you scope out the coverage needed from the access points and where you should place them. If users don't need access from the parking garage, then you don't need to install any access points there.

However, whether or not users need access from the lunch room will very likely impact the placement of access points. You should know precisely where users will operate to make decisions on access point locations during the site survey. In order to cover the lunch space, you may need to position an access point closer or within the applicable area, which could then require the addition of another access point to cover an adjacent portion of the facility.

Performance requirements provide some insight into placement. If users need 11Mbps throughout the facility, access points will need to be close together (e.g., 100 feet). When performing the RF site survey, your goal will be to ensure that access points are placed in a manner where the edge of the access point's propagation overlaps the propagation of the adjacent access point. On the other hand, if users must have connectivity regardless of data rate you could allow the placement of access points much farther apart (e.g., 500 feet) because the propagation overlap can be held at a lower data rate.

Facility construction plays a role in positioning of access points as well. In general, choose a location that enables the access point antenna to have maximum line-of-sight propagation with the users. For example, don't hide the access point in a broom closet in a far-reaching corner of the building. Instead, make sure that there's a clear path between the access point antenna and the users, free from obstructions such as overheat vents, office partitions, walls, etc., that offer attenuation . Also, choose a location that maximizes the propagation pattern of the antenna over areas of the facility where users will reside (not outside or in non-user areas).

Some companies may have policies related to the dicor of the facility. If this is the case, fully understand where and how you can mount the access points and antennas. Ideally, you should mount the access points (more importantly the antennas) as high as possible, unless there are obstructions. The higher vantage point increases the horizontal range of the RF signal.

If policies dictate that WLAN hardware must be completely out of sight, then you'll probably need to mount the access points above the ceiling. This is doable in most offices because of the use of false ceilings to hide electrical wires and ventilation ducts. In warehouses and manufacturing facilities, you'll probably be free to expose the hardware, but you may be limited to installing the access points on pillars or overhead beams. Keep in mind too that your selected mounting location will need to accommodate data cabling and possibly electrical connections, assuming you're not using Power-over-Ethernet .

Consider Configurations

When negotiating access point locations, think about configuration attributes, such as signaling method (i.e., 802.11a or 802.11b), transmit power and antenna type. These configurations affect range; therefore, use them to your advantage when tweaking the positioning of the access points.

802.11b access points generally offer greater range than 802.11a, mainly because 802.11b operates using lower frequencies (2.4GHz instead of 5GHz band). As a result, the use of an 802.11a network requires access points to be closer together (e.g., 100 feet) as compared to 802.11b (e.g., 500 feet). Keep this in mind when positioning the access points.

Enterprise class access points generally offer several transmit power settings, which affect range (i.e., coverage) of the RF signals. The maximum transmit power for 802.11b is 100mW, which provides the greatest range. In most cases, base the selection of access point location with the highest transmit power.

Some applications, especially those requiring support for a large density of users and high throughput, need a large number of access points in a relatively small area. In order to accomplish this and avoid inter-access interference, try using a lower transmit power. Of course, this will require the access points to be closer together.

Similar to transmit power, the antenna type affects the positioning of access points. The antenna affects range, and it also affects the pattern of the propagated signal. For example, an omni-directional antenna broadcasts horizontally in all directions. The use of omni-directional antennas provides widespread coverage, which are best for applications in office complexes, warehouses, homes, etc. The access points in these cases need to have fairly equal spacing, assuming the construction and layout of the facility is fairly uniform.

A directional antenna focuses the RF signal more in one direction than others (thus increasing the range more in that direction). To cover relatively long, narrow areas, such as airport concourse or convention center hallways, a directional antenna may make most sense to minimize the number of access points. In these environments, the access point should be located near one end of the long corridor to focus most of the signal in the right direction. This technique can make use of only one access point instead of several access points having omni-directional antennas.

As you begin to deploy more and more wireless LANs, you'll quickly find that locating access points is more of an art rather than a cut-and-dry procedure. In fact, you eventually develop a knack for predicting the propagation of radio waves in various scenarios. Don't let that, however, rule out the need for an RF site survey to verify your conclusions!

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001) and offers computer-based training (CBT) courses on wireless LANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

802.11 Planet Conference Need to do your own site survey soon? Join us at the 802.11 Planet Conference & Expo, Dec. 3-5 in Santa Clara, CA. Jim Geier will be leading several workshops, including RF Site Survey Basics: Equipment and Techniques for Maximizing WLAN Performance.