From a security perspective, WLAN signal travels way too far, leaking into hallways, parking lots, and neighboring...
buildings. But from a performance perspective, received signal often falls far short of user needs and expectations. APs that easily reach 300 meters in open space can have trouble being heard through obstructions commonly found in homes and offices. This is why many WLAN owners ask: How can I predict, measure, and improve signal strength?
Predicting signal strength
Wi-Fi devices transmit 802.11 frames by radiating RF energy at a given strength, referred to as Equivalent Isotropically Radiated Power (EIRP). This output power can be reduced (attenuated) by cables and connectors, or increased by amplifiers and high-gain antennas. Maximum EIRP varies by product, subject to regulatory limits. Visit this SeattleWireless Web page to view several EIRP examples, measured in dBm (decibels) or mW (milliwatts).
When energy is radiated from the transmitter's antenna, a wavefront propagates through the air and obstacles encountered along the way. Free Space Path Loss refers to power lost as energy disperses into the air, and is a function of both frequency and distance. In other words, the farther away the receiver, the greater the loss, and thus the lower the Received Signal Strength Indicator (RSSI). Free Space Path Loss is fairly predictable and can be easily computed.
However, air isn't the only obstacle encountered by 802.11 transmissions. Radio waves are reflected, refracted, or attenuated by windows, doors, walls, and even people. Loss due to obstacle attenuation is harder to predict, but can be estimated using published metrics. For example, Airespace design notes estimate 2.4 GHz (802.11b/g) loss for drywall at 4 dB, brick wall at 8 dB, and concrete wall at 10-15 dB.
Eventually (if we're lucky!) the transmission reaches the receiver. There, results depends on the Receive Sensitivity of that device -- i.e., the minimum power required to handle arriving frames at a given data rate. Like EIRP, Receive Sensitivity varies across products; several examples can be seen on the FreeNetworks.org Web site.
Operating at a higher data rate requires a stronger signal. When signal strength declines, 802.11 devices automatically shift to a lower data rate to sustain communication -- for example, dropping from 11 to 5.5 to 2 to 1 Mbps. Due to 802.11 protocol overhead, application throughput is typically about half the data rate.
Measuring signal strength
We can see that power output, path loss, and receiver sensitivity all contribute to signal strength, and thus have a direct impact the user's WLAN experience. Your network design should take these into consideration, calculating a "link budget" that estimates the power output and components (antennas, amps) required to meet user needs.
However, pencil-and-paper predictions can only take you so far. To verify and refine your WLAN design, you'll need to take some measurements in the target environment. But you won't be measuring EIRP or path loss -- you'll be measuring observable consequences: signal, noise, errors rate, data rate, and (perhaps) application throughput.
- Noise refers to the background RF radiation present in the receiver's environment. Every environment has some noise; sources of interference, like cordless phones and Bluetooth, increase noise. Ultimately, the receiver must be able to clearly and reliably differentiate between the transmitted signal and noise.
- Signal-to-Noise Ratio (SNR) compares peak signal strength to noise. According to the CWNA Study Guide, an SNR of 22 dB or more is viable, but opinions vary. The higher the SNR, the more stable and usable the WLAN service.
- Bit Error Rate (BER) compares corrupted packets to the total number of packets received. In general, the lower the SNR, the higher the BER. When BER is high, the data rate may be dropped to reduce the number of corrupted packets. When BER is low, the data rate may be increased.
- Measuring SNR and BER and data rate can help you tune your WLAN, but there's no substitute for measuring actual application performance. On the other hand, keep in mind that application throughput is impacted by many other factors, including competition with other users, client and server speed and utilization, and upstream network latency.
Sample these values throughout your WLAN's environment, plotting measurements on a floorplan that represents your site. Ideally, your map should include physical obstacles (e.g., walls, doors, windows), AP locations, and desired service areas. Using some type of portable tool (laptop or PDA), take measurements at regular intervals and critical locations, working to identify areas where error rates are high, data rates are low, and SNR is unacceptable.
Fixing identified problems may require moving or adding APs to avoid obstacles, increasing AP power output to overcome obstacles, or using directional antennas to better focus available power. Purchasing more sensitive WLAN adapters can also help, although this is not always practical -- for example, when you must deliver WLAN service to laptops or scanners or other devices with embedded 802.11 adapters.
Useful measurement tools
There are many shareware and commercial tools available to help you measure WLAN signal strength and related metrics.
Windows XP's WLAN Connection Status panel includes a real-time signal strength meter and data rate indicator. Many WLAN adapters have vendor-specific Client utilities that provide signal meters and more. For example, see Cisco's Aironet Client Utility Site Survey and Link Test panels. Signal meters are handy for qualitative spot-checking -- for example, to verify suspected blind spots -- but you'll need something more to take discrete quantitative measurements. If you look around, you can find freely-available third-party WLAN Client programs that measure signal strength, error rate, and noise. For example, see Wavemon (Linux), WLANExpert (Win32), Wscan (Linux, BSD), and XNetworkStrength (Linux). Like Cisco's ACU, these programs are intended to measure the quality of the station's current association to a particular AP. But if you have many APs to survey, manually associating with each could become quite tedious.
Alternatively, consider using a shareware "stumbler" program to scan all channels and record vital statistics for each discovered AP. Most stumblers can display AP signal, noise, and SNR; some can record this information to a file for later use. Several freely-available examples include Aerosol (Win32), Airsnort (Linux, Win32), AP Radar (Linux, BSD), DStumbler (BSD), KisMAC (MacOS), Kismet (Linux, BSD, MacOS), iStumbler (MacOS), MacStumbler (MacOS), MiniStumbler (PPC), NetChaser (PalmOS), NetStumbler (Win32), Pocket Warrior (PPC), PrismStumbler (Linux), WaveStumbler (Linux), Wellenreiter (Linux, including Zaurus), WiFiFoFum (PPC), and WiStumbler (BSD).
Commercial WLAN analyzers usually include measurement tools that do much more than stumblers. For example, AirMagnet Handheld's Site Survey tool records AP measurements whenever there's a change in association state, signal strength, or data rate, or at specified intervals. Survey output can then be fed into AirMagnet's Surveyor product to assist with site design and plan verification. Other commercial WLAN analyzers include AirDefense Mobile, Baseband Technologies LinkFerret, BVS YellowJacket, Fluke OptiView, Network General Sniffer Wireless, Network Instruments Network Observer, TamoSoft CommView for Wi-Fi, and WildPackets AiroPeek.
These are just a few of the many tools that can be used to measure your WLAN's signal strength. Obviously, this lists ranges from software already present on your laptop to source code you'd need to compile; from freely-available code to products with hefty price tags; from rudimentary signal meters to enterprise WLAN management toolboxes. Some of these tools present raw data; others analyze that data to isolate potential performance problems (e.g., cross channel interference) and suggest remedies. Ultimately, you may want to use more than one tool and compare results -- in particular, it can be helpful to take measurements with several WLAN adapters, since receive sensitivity varies.
In the end, no tool can tell you exactly how to change your WLAN implementation to improve signal strength. But getting a handle on how well your WLAN is performing now is a necessary first step to accomplishing that goal.
About the author: Lisa Phifer is vice president of Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of data communications, internetworking, security, and network management products for nearly 20 years. She teaches about wireless LANs and virtual private networking at industry conferences and has written extensively about network infrastructure and security technologies for numerous publications. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.