Wireless Systems Design

WLAN Testing Gets Down To A Science

This IEEE 802.11 verification system uses programmable, multipoint, synchronized test points to capture and recreate Layer-1 PHY and Layer-2 MAC test scenarios.

When engineers think of wireless-local-area-network (WLAN) test equipment, images of bulky spectrum analyzers often come to mind. They may even picture oddly named "sniffer" programs that are used in conjunction with these analyzers. In addition, engineers may envision the use of attenuating devices that can simulate test scenarios in which a wireless device-under-test (DUT) appears to roam from one access point to another.

Roaming is critical to the smooth operation of any wireless device that switches between access points during a transmission. Yet it isn't an easy scenario to test. Timing accuracy is a big problem. Testers of wireless-LAN switches, for example, need to know precisely what steps occur as a roaming device leaves one access point and is associated to another. This simple transaction affects both the physical (Layer 1) connectivity and media-access-control (Layer 2) frame configurations. While spectrum analyzers, "sniffer" programs, and attenuators constitute one approach to WLAN testing, they fail to provide an accurate picture of many potential problems.

A new picture of WLAN test equipment is now emerging, thanks to the introduction of VeriWave's WaveTest WLAN Verification System. This hardware-software system incorporates the hallmark features of today's best wireless devices: compactness, mobility, and multi-functionality. It hails from a Portland-based startup company nestled in the heart of Oregon's Silicon Forest. VeriWave's development team consists of protocol, validation, and test experts from major vendors in the WLAN industry. The announcement of its first product—the WaveTest 1000 System—marks the company's official entrance into the rapidly growing market of IEEE 802.11 test equipment.

Why is a new type of WLAN test system needed? To answer this question, Rick Denker, VeriWave's Vice President of Marketing, suggests looking back at the early years of wired-Ethernet-LAN development: "Back in the 1980s, Ethernet throughput rates varied dramatically across vendors. The market for Ethernet systems didn't really take off until the supporting test equipment became robust enough to differentiate performance characteristics between competing Ethernet products." Once throughput and other measures could be guaranteed, corporate users bought into LANs in a big way.

A similar argument can be made for today's wireless-LAN Ethernet market. To bridge the gap between early WLAN adapters and large corporate users, equipment manufacturers must attain high levels of security, reliability, and performance. These demands, in turn, mandate the development of better wireless-local-area-network test equipment. VeriWave's WaveTest 1000 System claims to be just such a product.

TODAY'S TEST LANDSCAPE
Validating a WLAN product isn't an easy task. There are many different types of wireless devices that need to be tested. Such devices include access points, NIC boards, VoIP phones, and WLAN switches and gateways. Now, factor in the challenges that arise from evolving standards and the emergence of Bluetooth and WiMAX (IEEE 802.16) products. Those technologies may interfere with IEEE 802.11 wireless LANs. How can designers, manufacturers, or support engineers test in this diverse environment?

A multipoint 802.11 verification system like VeriWave's WaveTest can help to satisfy these diverse test requirements. The system consists of a number of intelligent test points that are connected to a computer through standard Ethernet cable (FIG. 1). They can be distributed anywhere in space around the wireless DUT.

In the WaveTest system, all of the individual test-point units can be synchronized together. This characteristic greatly enhances the timing accuracy of the entire system. Furthermore, details of the test setup plus the resulting data can be captured on the computer. Everyone in the DUT product-development cycle can therefore have access to the tests. This group might include the designers, manufacturer, quality-assurance team, and customer support. This capability also makes it possible for problems at a customer site to be captured and reconstructed in the lab, thereby providing a great diagnostic test tool.

To facilitate the use of sharable and repeatable test results, WaveTest supports standard Packet Capture (PCAP) files as well as its own proprietary log format. Using the proprietary format, which contains critical sequencing information, the user can capture network issues at a customer site and then recreate them in the developer's lab. This aspect has a myriad of uses. A wireless-LAN-component supplier, for instance, can use WaveTest to quickly demonstrate a previously sporadic network problem to a primary equipment vendor.

Each test-point unit may act like an access point, NIC, or—for that matter—any 802.11 device. A test-point unit boasts multiple processors in addition to the RF subsystem. As a result, every test point functions as a programmable protocol box (FIG. 2). Whether they do it individually or in synchronization, the test points interact with the DUT in order to verify complex protocol handshakes.

Every unit can transmit any type of legal or illegal 802.11 frame along with different types of interference patterns. At the same time, the units can receive any frame types as well as partial frames (frames with CRC errors and interference). The testers can therefore see every bit of information that passes over the air interface. All points also provide extreme timing accuracy. For example, frame streams from different test points can be synchronized to less than 50 ns.

Often, engineers want to trigger on specific events. Their goal is to capture as much data as possible once those events occur. To fulfill this need, each VeriWave test point comes with an external trigger to aid its coordination with other test equipment. A common scenario would have a test point connected to a spectrum analyzer. A trigger point would then be set up in the sequence language of the WaveTest software. The system would wait for a specific MAC address or data frame to occur. Once that event happened, a trigger would be sent to the spectrum analyzer.

For another example, check out how a common roaming scenario would be handled by the WaveTest system: First, a test scenario would be constructed using the WaveTest software program. The first portion of a suitable frame would be sent out via a test point. The next frame would be sent from a second test point for transmission. To the DUT, however, it would appear as if the station had just roamed.

A variety of spatial-testing scenarios can be constructed by using two or more test-point units. A basic configuration might consist of a DUT surrounded by three test points. The first one would merely monitor and log all of the traffic as it is seen by the DUT. The second test point would set up a specific traffic load condition for the DUT. Lastly, the third unit would perform a precise handshaking scenario with the DUT as required by all 802.11 transmissions. Other spatial-test configurations could be set up for hidden-node tests, inter-access points, and overlapping basic service sets (BSSs).

Regardless of the configuration setup, the WaveTest system offers many unique testing capabilities. It provides a level of timing accuracy that is sufficient to test all of the 802.11 timing parameters. Spatial tests can be constructed to test even the most complex Wi-Fi features like roaming. Tests also can be fully recorded and recreated for later analysis in the development lab. Finally, each test-point unit can make frames collide on the fly by causing protocol violation and frame errors wherever they are desired.

The WaveTest system is available with both standard and personal test points. The latter is a cost-reduced version of a standard WaveTest test point. The personal version maintains all of the functional capabilities and physical characteristics of the standard units. It just lacks the external triggering and synchronization features. This version is an ideal solution for hardware and software developers, who need to test out new features and functions at their desks.

The WaveTest 1000 System consists of three test points, software, user manuals, a quickstart guide, and example test sequences. The system is available now for $74,000. Additional test-point units are available for $29,000. The WaveTest Personal Test Point, which will be available in July, is offered for $18,000.

VeriWave, Inc.
9600 SW Oak St., Suite 380, Beaverton, OR 97223; (503) 473-8356, FAX: (503) 473-8351, www.veriwave.com.

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