Ensuring Wireless USB Quality Assurance

Copyright © Ellisys. All rights reserved. Published in the November 2006 issue of RF Design Magazine EWT.

These five steps can help manufacturers ensure a positive experience for users of products based on Wireless USB.

Introduction

As a technology, USB enjoys an excellent reputation. With millions of USB devices sold, users are confident that they can connect any USB peripheral to their PC and it will immediately work. Users will expect this same ease of operation and high level of interoperability from Wireless USB.

Manufacturers need to deliver on these expectations to ensure that their products are successful. Customer satisfaction with Wireless USB will be shaped by a positive user experience enabled by interoperability and flawless performance. This means manufacturers must carefully test their products both during development and production

This article describes five specific steps that can play an important role in any Wireless USB quality assurance program.

Step 1: Design in interoperability from the start

A major source of technological risk when working with emerging technology is that specifications are subject to misinterpretation. Clever developers “design in” interoperability from the very beginning by testing prototypes against a protocol analyzer instead of the much less reliable method of connecting two prototypes back-to-back.

Back-to-back testing saves the purchase of an analyzer, but costs more in the long run since it takes longer to find and fix protocol errors. And a high-quality Wireless USB analyzer has been validated against many products from major industry players. This means that the analyzer can automatically detect errors and display any misinterpretation of the Wireless USB specification -- avoiding common interoperability errors and improving the chances of passing certification tests.

Wireless USB protocol is innately different from USB 2.0, so developers need an interface tailored to Wireless USB with multiple views that show all protocol levels. As shown in Figure 1, developers will also want to be able to display how the Wireless USB protocol is transmitted over Ultrawideband to verify low-level detail such as timing.

Some analyzers also interpret and display Wireless USB protocol in the same way as USB 2.0. This lets developers quickly learn and understand the new protocol thanks to their previous knowledge.

Figure 1 - Ultrawideband and corresponding Wireless USB decode views from an Ellisys WiMedia Explorer 300 Analyzer

Step 2: Reproduce error scenarios to ensure successful resolution

Nothing is harder than resolving intermittent errors without the proper tools. When this happens, the usual testing procedure is to capture data until an intermittent error occurs and try to analyze what happened. Then the developers need to design a fix. But how do they ensure that the fix really worked? If the problem doesn't re-occur, most people simply assume everything is fine.

There is a better way. If your protocol analyzer has traffic generation capabilities, create a script from the captured traffic to reproduce the error scenario. Replay the scenario as many times as required to fully understand and correct the problem.

To further stress test the device, modify the script to test corner cases and error recovery mechanisms.

Step 3: Optimize Wireless USB device battery life

Wireless USB has a sophisticated power-management scheme designed to enable portable devices to maximize battery life. Unfortunately this power-management scheme is too complex to explain here, but interested readers can refer to our “Migrating to Wireless USB” white paper available from www.ellisys.com.

In short, successful power management requires powering down the radio as much as possible, thereby decreasing the average power consumption and extending battery life. However, a poor RF design or improper protocol implementation can cause the opposite to occur, powering on the radio for excessive retransmissions due to data and protocol errors.

Shown in Figure 2 is a view of Wireless USB Micro-scheduled Management Command (MMC) timing. This visualization permits developers measure timing and performance to help understand which parameters can be improved.

Figure 2 - InstantTiming view from an Ellisys WiMedia Explorer 300 Analyzer

Step 4: Characterize the overall transmission quality

A Wireless USB link is theoretically more prone to errors than a USB link using a shielded cable. Not only are longer packet lengths preferred in wireless medium to optimize throughout and reduce overhead, but Wireless USB will be used by peripherals such as digital camcorders that exchange large amounts of data. The longer the packet length, the greater the probability of a data error. These two factors -- a higher inherent data error rate and long packet lengths -- combined present a serious engineering challenge that can negatively influence usability.

RF performance can be affected by many design factors, including the antenna, RF chip and PBC layout. Designers need a reliable setup to understand the impact of their tweaks. A traffic generator is of great help for this purpose.

Connect the device under test to a traffic generator and exchange a known sequence of frames between the units. Then verify that the frame error rate is in the acceptable range. Repeat the test at different data rates to characterize the RF transmission quality and your device's performance.

Since the expected error rate increases with distance, use the same setup with a variable attenuator connected between the Ultrawideband traffic generator and its antenna. Increase the attenuation to simulate increasing the distance between the two units.

Step 5: Validate the proper operation of production units

Manufacturers' production test beds will need to be upgraded with UWB transmission testing. It is obviously impractical to validate production units by using a manual procedure such as manipulating a digital camera to upload a photo.

A more efficient alternative is to program a traffic generator to produce a known scenario suitable for your product. This sequence is then used to verify the proper operation of production units. You will need a traffic generator with a programming interface in order to integrate it into your existing setup.

Conclusion

Quality gurus know that QA is a philosophy that encompasses product development through production. The five steps described above will help developers and QA engineers alike ensure that their new Wireless USB products successfully pass certification tests and are happily accepted by customers. In turn, high-quality USB peripherals with outstanding wireless capabilities will create greater opportunities for manufacturers.

For More Information

To learn more about designing Wireless USB interfaces, read our free white paper called “Migrating to Wireless USB”.

Product Information

	With the WiMedia Explorer 300 Analyzer Ellisys introduce a protocol analysis solution that captures, analyses and verifies traffic transmitted over-the-air in both the WiMedia ultrawideband and Wireless USB protocols. Analysis and display software enables developers to choose a dedicated window to focus on the protocol layers of their choice. They show transfers, transactions and packets simultaneously on screen, giving them split second understanding of bus activity. Developers can use powerful search functionality to filter out unwanted data, quickly locate essential information they need to speed up their development work and accelerate time to market.
	The WiMedia Explorer 300 Generator is the world's first frame generator for WiMedia Ultrawideband and Wireless USB protocols. It helps verify product and component reliability by generating reproducible traffic, timing and error scenarios. Containing a specialized processor designed specifically for WiMedia and Wireless USB protocols, the WiMedia Explorer 300 Generator enables you to emulate Wireless USB hosts and devices as well as various WiMedia equipments. Based on the same proven hardware as the Ellisys WiMedia Explorer 300 Analyzer, the WiMedia Explorer 300 Generator provides a powerful new test methodology to maximize the quality of your protocol implementation and surpass your Ultrawideband R&D challenges.
	The USB Explorer 200 Professional Edition is the only protocol analyzer currently on the market that decodes all USB-defined classes. USB class decoding enables embedded software, drivers and applications developers to view USB protocol subsystem layers easily, enabling them to instantly improve quality and lower production costs of their USB peripherals.

Find out more about Ellisys products at www.ellisys.com.

About Ellisys

Ellisys is a Test and Measurement company committed to the design and timely introduction of advanced protocol analysis solutions for USB devices, Wireless USB and Ultrawideband. Developers have been using Ellisys' USB products and solutions for more than five years with great success. With the consumer electronics market moving toward wireless technology Ellisys proves once again its commitment to the developers' community. By providing WiMedia and Wireless USB early adopters with the right tools at the right time Ellisys enables these promising markets to grow in a secure and confident manner, and ensures a rapid and wide acceptance of these technologies. For more information, please visit www.ellisys.com.

Ellisys, the Ellisys logo and USB Explorer are trademarks of Ellisys sàrl, which may be registered in some jurisdictions. All other logos or trademarks are the property of their respective owners