The communications world is becoming increasingly dependent on wireless local-area networks (WLANs). New applications based on 802.11 standards are rapidly expanding into corporate, residential, Wireless-Internet-Service-Provider (WISP), and hot-spot applications. As with all new technology, the initial novelty eventually wears off. At that point, it becomes increasingly important to know how products will respond and perform in their intended environments. As the technology moves into the mainstream, a demand is created for more thorough and accurate testing.

The need already exists for throughput, latency, errored packet performance, and hundreds of other wired-based performance tests. Now, there also is a need to test the characteristics that are unique to wireless networks. These characteristics include: radio-signal impairments (fading, interference, Doppler effect, reflections, multiple paths, etc.); roaming hand-off delays (when a user passes from one access point to another); security; power management; and Media Access Control (MAC) -layer collision avoidance. Whether it is standard performance testing or unique WLAN network functionality, however, all WLAN testing must be done against repeatable, real-world radio-frequency (RF) characteristics. This is the only way to ensure valuable test results.

Compared to the testing of cable-based networks, the testing of wireless-network systems presents new challenges. For example, wireless-network performance can degrade significantly over time due to environmental changes in the signal broadcast area. WLAN service providers and IT managers must therefore perform network acceptance testing when they are turning over a newly installed network to their customers. To make sure network performance does not degrade below acceptable levels, they also must execute ongoing service-level testing.

This article is the first of a three-part series that will introduce WLAN testing requirements, test tools, and some of the significant issues facing design engineers, test engineers, and IT managers. Testing methodologies will be the focus of the second article. The third article will conclude with actual comparative test results provided by CENTAUR Laboratories at the University of Georgia.

As a standard WLAN RF signal radiates through its environment, it bounces off various obstructions like walls, file cabinets, and other reflective surfaces. As a result, multiple signal paths arrive at the receiver. Each of those paths is subject to independent delay and loss (FIG. 1).

When individual signal paths from the transmitter arrive at the receiver at different times, relative path delay occurs. The initial signal's net effect on the arrival time is to spread it out in time. In a digital system, this causes the received symbols to overlap. Called inter-symbol interference, this condition impairs the receiver's ability to successfully lock onto the signal and decode the desired waveform. This problem can cause a loss of data or the need for retransmissions, resulting in increased latency. The amount of relative path delay varies based on the environment (residential, business office, or commercial) and the application.

The receiver also is subject to relative path loss. Here, the individual signal paths arrive at the receiver with different absolute-power levels. These differences result from the power that is lost when the waveform reflects off of a physical obstruction.

The signal strength of an individual waveform will diminish according to the distance traveled. The loss of signal strength should follow the 1/d² law, where d is the distance between the transmitter and the receiver. In actuality, the loss will be much greater (between 1/d³ to 1/d⁶). This is mainly due to the variety of reflective surfaces in WLAN environments.

The characteristics of these multiple paths are variable and fairly complex. It is thus desirable to have a standard way to simulate them in a repeatable fashion. If the tester is unable to simulate the same or similar multipath conditions over an entire test plan, it is very difficult to determine the actual performance of the device. It also becomes nearly impossible to conduct performance-comparison analysis from one product release to the next. For this reason, RF channel models (also known as fade models) are used.

A channel model is designed to simulate the signal characteristics of different frequencies. It also simulates different environments, such as offices, residences, or factories. The effects of each environment can be fairly complex. The channel or fade models attempt to generalize the complexities and establish a typical or average behavior for the channel in the environment in question.

A key parameter for any channel model is RMS delay spread. This is a measure of how the path delays are spread about the mean. Different environments will have different signal-reflection characteristics. As a result, they will have different RMS delay spreads. For example, the types of obstructions encountered in a warehouse and in an office complex will likely differ in delay and signal strength, resulting in different channel profiles. The channel profile specifies the number of independent paths, their relative path delay, and their signal strength. It is therefore extremely important to choose an appropriate channel model when testing for a specific type of environment (FIG. 2).

Over the last few years, a number of studies have been done on indoor fade models for WLAN applications. To date, several models are in use. But most of these were developed primarily for computer simulation. In some cases, it can be difficult to implement them in a real-world test environment with actual hardware. Here are some fade models that are currently in use for WLAN applications:

Exponentially Decaying Rayleigh Fade Model
This model, proposed by Naftali Chayat, is recommended in the 802.11 specification. It models a path delay and loss profile that follows a standard exponential decay. (Note that this proposed model became the baseline model for the comparison of modulation methods during the development of 802.11a.)