Tackling wireless LAN test

Independent methods exist for testing local area networks (LANs) and for testing radio devices. Complete testing of a wireless LAN (WLAN) device, however, requires more than the sum of testing those individual elements. Yet, only now are devices becoming available to explore the interactions of a device's radio link and its network behavior at the system level.

Traditional testing of WLAN devices followed a two-step approach. One step was to evaluate the device from the network side with protocol and logic analyzers. The other step involved testing the RF side using vector signal and spectrum analyzers along with signal generators, oscilloscopes, and a host of other equipment. The many pieces of equipment needed for complete testing made it difficult to test devices in a full network configuration.

Test equipment vendors have been working to consolidate the list on the RF side. Earlier this year, Agilent Technologies (Palo Alto, CA) released the N4010A wireless connectivity test set. The device uses software modules to incorporate signal and vector analysis functions as well as signal generation to form a more self-contained RF test package.

Similarly, Anritsu (Morgan Hill, CA) uses software for its spectrum analyzers to combine several WLAN RF test abilities in one instrument. LitePoint (San Jose, CA) has two products: the IQview for development and production use and the IQflex aimed at addressing high-volume manufacturing test. The company also offers software for creating custom signal waveforms with the instruments to test response to impaired waveforms.

Still, these instruments are aimed at directly stimulating a device under test (DUT) with a single signal. Testing the device in a multi-signal environment to simulate real-world conditions has required the use of many instruments. Coordinating the signals from these instruments to create repeatable test conditions has been difficult at best. Further, these instruments only tackle the RF testing and do not address data protocol analysis.

The situation changed recently with the introduction of system-level test instruments from Azimuth Systems (Acton, MA) and VeriWave (Portland, OR). Both companies have products that provide protocol analysis and also allow the coordinated generation of multiple test signals for evaluating WLAN devices under a range of conditions. These instruments simplify the creation of test scenarios such as:

• hidden node—where two devices can communicate with a common access point but cannot detect each other,
• overlapping basic service sets—where a node has multiple sets available to it simultaneously,
• roaming—where a node can dynamically switch access points during transmission, and
• rate adaptation—where a node
• can alter its data rate based on received signal strength to optimize its performance.

The two companies have different approaches to the testing, however. The Azimuth W-Series test platform provides a shielded enclosure for the DUT, feeding it controlled signals through a cable system. The test signals come from a bank of generator modules under the common control of a computer system that specifies the function of each module, coordinates signal activity, and manages the combination of the RF outputs using programmable attenuators. This allows the system to mimic a variety of traffic patterns and signal conditions, including roaming, in a repeatable configuration without the need for a shielded room.

The VeriWave WaveTest system uses an open-air approach in conjunction with PC-controlled units called

The WaveTest Test Point works in teams to provide a coordinated set of signals for testing wireless LAN devices under installed network conditions.

These latest devices help round out the testing of WLAN devices by addressing network issues during the test. They do not, however, provide measurement of the RF parameters, although the WaveTest system can generate triggering signals to RF test equipment to coordinate such measurements. WLAN testing is consolidating its tools, but there is still a bit of a two-step going on.