WLAN Design

Building a wireless LAN is no easy job. For starters, you must assess your infrastructure's RF environment and map out access point placement to help ensure good performance. Don't be

January 16, 2004

10 Min Read
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It all starts with a wireless site survey, where you assess and map out your wireless infrastructure's RF (radio frequency) environment and the placement of your access points to ensure your WLAN performs well. Many handy tools are available to help you in this process, from portable WLAN hardware toolkits to software packages that give you detailed views of coverage areas at your sites, such as Trapeze Networks' RingMaster and the Ekahau Site Survey utility. But these tools can't substitute for knowing just what your wireless users need to get their jobs done.

Mapping It Out

Throughput is a major consideration for your wireless deployment. Consider what types of traffic--e-mail and Web traffic or speed-hungry ERP (enterprise resource planning) or CAD (computer-aided design) applications--will ride across your WLAN most often. Do you need the 54-Mbps speeds of 802.11a and 802.11g, or will 802.11b's 11 Mbps suffice? Either way, network speeds diminish significantly as users wander farther from their access points, so install enough access points to support not only all your users but the speeds at which they need to connect.

A WLAN's advertised speed doesn't exactly correlate to its real-world speed, either. Unlike a switched Ethernet network, a WLAN is a shared medium, much like the hub model of older Ethernet LANs, dividing available throughput rather than providing dedicated speeds to each connected device. This limitation (along with the 50 percent overhead associated with transmitting data over the airwaves) makes throughput planning on your wireless network challenging. It may be tempting to calculate the number of access points you need to install based on the number of users and their minimum bandwidth requirements, but that can limit you down the road when you need more capacity. Overprovision instead. Keep in mind, too, that though 802.11b is getting all the attention these days, 11a soon will become the high-performance WLAN standard of choice, so your infrastructure should support it from day one, or at least be upgradable to it in the near future.

Security is also a major concern, though it typically doesn't factor directly into where you place your access points or how you configure them. Some security solutions diminish throughput with their authentication and encryption methods. It's best to adjust your power outputs and antenna orientation to limit leakage beyond your borders, whether or not you add security.Once you understand what users need and expect from your WLAN, you can determine the proper placement of your access points based on things like RF behavior, coverage and interference. Interference, for example, can become a problem for some organizations. Although it's easy to get caught up hunting down offending microwaves, cordless phones and Bluetooth devices, it's more common to get interference from other access points in your network, and even from outside networks. 802.11b and 11g, for example, offer the same three nonoverlapping channels in their 2.4-GHz band, which makes planning for a dense deployment or working around the interference of your neighbor's WLAN difficult.

Ideally, Channels 1, 6 and 11 in a 2.4-GHz environment should never be adjacent to the same channel, so they won't interfere with one another. But that's not realistic. You need a healthy amount of cell overlap to let users roam--20 percent to 30 percent is best--but if your site has more than one floor, there will be some bleedover from floor to floor even if you use high-gain antennae. 802.11a's 12 nonoverlapping channels can greatly alleviate channel-allocation headaches.

The 5-GHz band, meanwhile, has little non-WLAN interference, and you'll encounter few neighboring 802.11a access points because the standard hasn't yet seen the popularity of 11b or the recent growth of 11g.

Know how your WLAN's RF signals propagate in your environment: The lower the frequency and the slower the transmission speed of your wireless network, the farther the effective range. Because of the greater RF signal propagation at lower frequencies and the increased sensitivity to signal-to-noise ratios by modulation schemes at higher speeds, a 2.4-GHz 802.11b signal at 1 Mbps will travel significantly farther than a 54-Mbps signal from 5-GHz 802.11a equipment.

Besides the wave-propagation characteristics of different RF bands and variations in throughput, your WLAN's range is limited by free-space path loss and attenuation. Free-space path loss, which is more of an issue in open or outdoor environments, is the expanding and dispersing of radio signals as their wave fronts broaden, preventing them from being heard by receiving antennae. Attenuation is more common in indoor installations: This is the decrease in amplitude, or weakening of RF signals as they pass through walls, doors and other obstacles. This is why WLANs don't perform well around dense materials like concrete. Even 2.4-GHz signals, which are more resilient than 5-GHz signals when facing this type of physical interference, still experience some RF problems.Multipath, the phenomenon whereby a signal is reflected and then echoed, can also wreak havoc. On rare occasions, it boosts amplitude at the receiver, causing a power boost. Most often, however, it weakens the received signal or cancels it out entirely. Then you get areas with little or no RF coverage where there should be adequate signal propagation. You can prevent multipath by removing or relocating interfering objects, such as cabinets and network-equipment racks, and increasing access-point density or power output.Site-survey tools streamline the layout process significantly. RF modeling software such as Trapeze Networks' RingMaster can help automate the process of deciding initial placement of access points by automatically determining access point location and coverage based on input floor plans. Other tools, like Network Instruments' Observer, provide information about your RF environment from the laptop or handheld running the software. Hybrid tools like Ekahau's Site Survey record this same RF data, along with your location, for a systemwide view of your wireless network. Whichever tools you use, you'll still need to perform a manual site survey for the tasks your survey tool doesn't cover.

A conventional site survey includes manual tasks for properly provisioning your WLAN and the location of your access points. Start with a floor plan of the wireless site. This helps you plan cable-run distances and power outlet proximity. Once you're familiar with the environment, estimate the location of each access point. Keep in mind, however, that the final installation location will likely be different because RF propagation depends on many factors, from your environment to the type of antennae you use. Access points with omnidirectional antennae are often best in central locations because they radiate outward. More focused, higher-gain antennae, such as patch antennae, tend to be off to the side of the area you intend to cover. You may need a combination of the two types of antennae.

Planning tools like Trapeze Networks' RingMaster determine access-point locations, channel assignments, power output settings and other configuration attributes. They use parameters such as user density and throughput for criteria. The downside is that you must still assign preset attenuation levels to the building materials--concrete exterior walls and metal doors, for example--in your CAD-based floor plans unless this information is already included in them. The trade-off with RingMaster is that it's built mainly for Trapeze's own wireless switches and access points, though you can input power levels and antenna gains of your non-Trapeze access points so RingMaster can include those as well.

Next, you need to verify and document the coverage area of your access points. To do this, use the site-survey utility that comes with your client WLAN card (assuming the vendor bundled one), or the utilities that come with advanced monitoring tools, such as Network Instrument's Observer and Air Magnet's Laptop and Handheld WLAN analyzers.

When you're ready to place and verify access points, site-survey hardware kits from vendors such as TerraWave Solutions, complete with an access point, antenna options and a battery pack, can streamline the manual on-site survey process significantly. This makes it easier to walk around and test your WLAN's coverage.Can You Hear Me Now?

After setting up the access point, slowly walk the floor of your facility with laptop or PDA in hand. If you move too fast, you can get inaccurate readings. Take note of three key numbers that the tool registers: signal strength, SNR (signal-to-noise ratio) and connection speed. You may need to repeat this test regularly if your facility experiences fluctuations in inventory or personnel, or if you have modular walls or other large RF obstacles, to ensure consistent wireless coverage.

Signal strength is represented in many ways, but the most common is a percentage--signal levels as low as 20 percent, for instance, typically suffice for passing data. But try to design your WLAN to support a minimum signal of 30 percent throughout so your cards don't have trouble passing data. This percentage can vary slightly from WLAN card to card, and cards will vary in sensitivity. So try to survey with a card that will be the most common one in your WLAN. If your WLAN will consist of a mix of WLAN cards, use your lowest-end card in the survey to ensure that everyone gets adequate coverage.

Once your WLAN is installed and online, conduct regular site surveys--and do one whenever you add new wireless hardware. Perform surveys every six months or so, depending on the frequency and likelihood of RF changes in your WLAN. Ekahau, a Wi-Fi positioning and survey software company, has a new site-survey utility that can ease the verification process. The Ekahau Site Survey tool graphically represents your 802.11b coverage, inferring throughput based on signal strength. It also recommends locations for new access points.

As long as you provision your WLAN for high-user density and the proper throughput, your wireless network will be set to keep up with additional wireless-enabled devices and usage as your business gets more mobile. But if you don't properly provision your WLAN, it will fall flat and cost you more in the end.Jesse Lindeman is the lab manager at the Center for Emerging Network Technologies at Syracuse University. Write to him at [email protected].

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802.11b: 2.4 GHz; supports speeds up to 11 Mbps and provides three nonoverlapping channels.

802.11a: 5-GHz UNII (Unlicensed National Information Infrastructure) band; supports speeds up to 54 Mbps. Although capable of only two-thirds the range of 11b, 11a offers up to 12 nonoverlapping channels.

802.11g: 2.4 GHz and backward-compatible with 11b. Supports speeds up to 54 Mbps, but with a greater range than 11a (75 percent of 11b's). Operates with the same three nonoverlapping channels and experiences the same RF (radio frequency) interference as 11b. Throughput suffers when both 11b and 11g clients are present.Hidden Node: Two clients associated with the same access point can't hear each other to determine when the medium is free for transmission. To resolve the problem, reduce the power output of the AP so the clients can hear or enable RTC/CTS (request to send/clear to send), a high-overhead mechanism in APs that confirms the medium is clear before any data is sent.

Interference: Competing RF (radio frequency) signals in the same band as your WLAN equipment--coming from other WLAN infrastructure or non-networking devices, such as microwaves and cordless telephones.Multipath: Reflection of radio waves where the receiver gets multiple instances of the same signal, usually weakening or canceling it. Can be lessened by removing or moving offending objects, such as metal cabinets.

Near/Far: A client closer to an AP drowns out the signals of distant clients operating at lower power levels. To fix the problem, adjust power levels or increase AP density.

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