Within business IT infrastructure, wireless is becoming an accepted and often expected mode of data communications. A myriad of hardware vendors and technologies exists, however. To gain maximum performance, an employed WLAN system must therefore use a solid testing procedure. For the customer, such procedures should summarize the potential performance of the device under test (DUT). They also must take into account the widely varying fading circumstances that exist in the workplace.

This article is the third of a three-part series that focuses on WLAN test methodologies. The first article addressed the value of WLAN performance testing. The second feature provided information on the proper benchmarking of WLAN devices. This last article will walk through an example test setup. It also will delve into test methodologies and review some typical results.

The Centaur Lab at the University of Georgia conducted the tests discussed in this article. It measured frame loss and throughput on a representative 802.11a WLAN access point (AP). In doing so, the following equipment was used:

• A Spirent Communications SR5500 wireless channel emulator
• A Spirent Communications SR5500 6-GHz option (block converter unit)
• A Spirent SMB-600 chassis with a LAN-3101A card
• A representative 802.11a access point
• A representative 802.11a network interface card (NIC)
• A 5.0-to-6.0-GHz circulator
• Three Murata-to-SMA-type shielded coax cables
• Two N-type-to-N-type shielded coax cables
• Three N-type-to-SMA-type shielded coax cables
• Two N-type 10-dB attenuators
• One N-type 2-dB attenuator
• A Compaq Evo N610c laptop

For a valid test to be conducted, the designer must ensure the proper setup of the test environment. Figure 1 shows a sample configuration for the above-mentioned test. The SR5500 is used to simulate the radio-frequency (RF) environment in a controlled and repeatable fashion. The fading characteristics vary from a clean channel or baseline to a heavily faded environment. In this way, the true performance of the WLAN device under test will be revealed. The SMB-600 acts as the Ethernet generator/analyzer. It will produce and verify the test traffic that will cross the network.

During the test-setup process, the AP and wireless NIC must be properly shielded. The data will then pass through the cabled network via the wireless channel emulator instead of over the air. Generally, shielding is accomplished via the cabled RF environment. If there is RF leakage, however, an RF shielded enclosure can be used.

Once the test setup is complete, the wireless channel emulator must be configured for both the correct test frequency and the desired fade-model parameters. This configuration must be consistent with the frequency or channel setting on the AP. Table 1 shows the channel-number-to-frequency relationship that should be used when configuring the wireless channel emulator for 802.11a testing. If the AP is set to channel 36, for example, the corresponding frequency of 5180 MHz must be set on the SR5500. The velocity of the DUT also must be configured. For these tests, a velocity of 1.5 km/hr was selected. This speed emulates a static to slow-moving user. The tests were conducted with an approximate −15-dBm input signal into the SR5500 6-GHz option. For the AP and NIC that were used in the test configuration, an output signal level of roughly −50 dBm was supplied from the SR5500.

A personal computer (PC) must be configured to route test traffic between a wired and wireless interface. To be used for this purpose, a laptop must be reasonably fast. It also has to provide a wired Ethernet connection and a CardBus interface for the wireless NIC under test. In this test, a Compaq PC provided the networking requirements. It also ensured that any performance-related issues observed during the baseline resulted from the wireless equipment rather than the forwarding performance of the laptop. To offer this guarantee, the laptop was required to supply sufficient processing power.

Before the PC can be used as a router between the wired and wireless interfaces, however, routing must be enabled. In Windows 2000 or XP, this means that the IPEnableRouter registry bit must be enabled. Consequently, routing will be enabled between both the wired and wireless interfaces.

Remember that the test-system setup does not end with the configuration of the test environment, applicable networking information, and necessary proper shielding. It is critical that the DUT can only communicate via the cabled "wireless" configuration through the wireless channel emulator. For a simple way to check the DUT's communication, disable one of the channels with the SR5500's TestKit graphical user interface (GUI). Verify that the communication link is interrupted. The only valid communication path will then be through the wireless channel emulator.

When the transmission path from the AP to the NIC is blocked, the laptop should disassociate from the AP almost instantaneously. The client should no longer be receiving beacon messages from the AP. When the transmission from the NIC to the AP is blocked, the client should disassociate only after a measurable period of time (on the order of a few seconds). In this scenario, the client is still receiving beacons from the AP, but the AP is not receiving any acknowledgements from the client. If either of these conditions is not met by the test setup, the setup is likely to have RF leakage problems. Once the test setup has been configured, the performance testing can begin.