Tips for Extending Access Point Coverage

By Jim Geier | Posted October 07, 2004
A common scenario in installing a wireless LAN is finding that that radio coverage of an access point isn't quite good enough. For example, a school may install an AP in a wing of classrooms, and students and teachers have trouble maintaining association with the AP from one particular classroom. In this type of situation, you could try moving the AP to better cover the area, but then you may disrupt access for users in other areas.

Certainly one solution to this problem is to add more hardware in the problem area; however, this requires you to purchase another AP and install the cabling. In addition, you might need to form a distribution system that includes an Ethernet switch in order to connect the access points.

Before installing more AP to increase radio frequency (RF) coverage, consider the following methods for extending the range:

Adjust the transmit power
Access points by default usually have their transmit power set to the highest value. In some cases, though, an administrator may have switched the transmit power to a lower level, possibly to avoid spilling radio waves outside the facility. In order to maximize RF coverage, ensure that the transmit power is set to the highest value. It only takes a couple minutes to do this, and it might solve the coverage problem without extra costs.

Relocate the access point
In many cases where coverage falls short, you can simply move the access point a few feet (possibly more) and fully cover the area. The only cost involved with this approach is your time, assuming there's enough slack in the distribution system cabling. Sometimes you might need to run a longer cable from the switch to the access point — a good reason to leave some extra length on access point cables when installing them.

Utilize higher gain antennas
The factory-default antennas that come with an access point usually have low gain (around 2dB). If the access point has removable antennas, then replacing the default antennas with higher gain omni-directional or even directional patch antennas boosts range significantly. Most of these higher gain antennas effectively add 6dB to the system, which equates to a four fold increase in signal power.

Even though that doesn't exactly multiple the range by four, it does make a big difference in range. For example, I've seen range increase by twenty five percent after simply replacing the antenna with a higher gain omni-directional. For a cost of around ten dollars each, antenna upgrades are extremely cost-effective. Keep in mind that some access points don't have removable antennas, which of course blocks this approach.

Consider RF amplifiers
Companies such as RFLinx and Maxim sell RF amplifiers that install between the antenna and the access point. The amplifiers, which cost a few hundred dollars each, add a whopping 20dB of gain to the RF signal. This packs quite a punch, which dramatically extends range. Just be sure to comply with effective isotropic radiated power (EIRP) rules, though, when adding amplification (and antennas). In addition most regulatory agencies, such as the Federal Communications Commission (FCC) in the U.S., require certification beyond what the access point manufacturer receives when the wireless LAN includes additional components such as amplifiers.

Install a repeater
A wireless LAN repeater, from companies such as D-Link and Buffalo Technology, is stand-alone hardware that you place within the area with poor coverage. The repeater must be within range of the AP and set to the same channel. It listens to the selected channel and retransmits traffic on the same channel. For example, the repeater may receive a data frame sent by the AP. It will then retransmit the data frame. Users within the area will then be able to receive the frame. Because repeaters duplicate the traffic sent over a common channel, they reduce the overall throughput by fifty percent. This could be a problem if you're already taxing the capacity of the AP.

Also, a repeater requires electrical power, which might be costly to install. Since a repeater doesn't connect to the distribution system, power-over-Ethernet (PoE) isn't an option. Hopefully one of these approaches will help you successfully extend the range of your AP. If not, then you'll likely need to purchase and install more APs to get the coverage you need.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the books, Wireless LANs (SAMs) and Wireless Networks - First Step (Cisco Press).

This article originally appeared on Wi-Fi Planet.com.

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