Wireless LAN Faces Test Challenges

To become accepted in the corporate environment, wireless LAN must add features and performance, placing severe requirements on the test equipment that will debug and verify these networks.

By Dr. Thomas Alexander, Rick Denker, and Gerard Goubert

Wireless Local Area Networks (WLANs) are gaining tremendous popularity among early adopters. Now, WLANs are being challenged to provide more sophisticated features in order to evolve into a widespread corporate technology. To bridge the gap between early WLAN adopters and large corporate users, equipment manufacturers will need to attain higher levels of security, reliability and performance. Validating that enterprise-class equipment standards are being met will require sophisticated new test equipment.

WLANs today resemble the early days of cell phones, whose first users were driven by the great convenience, and whose later users demanded much more sophisticated features and reliability: the transparent roaming, compact equipment, and reasonable prices we enjoy today. Early adopters of WLANs have also been attracted by the convenience, as well as ease of installation, of wireless networks. As a result, WLANs are experiencing tremendous market growth, even in today’s tough economy. Worldwide unit shipments are forecast to reach as many as 25 million per year by 2005 [source: Micrologic Research], translating to $5.2 billion in sales annually [source: Instat]

Now, WLANs are on the verge of transitioning from early adoption status to a widespread corporate technology. WLAN technology will need to gain some additional attributes, however, to make that transition successfully. To understand what WLANs will need to add, it is informative to consider the earlier evolution of Ethernet into a widespread corporate technology.

Ethernet’s early throughput rates varied dramatically among vendors, interoperability was not guaranteed, and the user experience was unpredictable. Hence early adopters used Ethernet primarily in engineering or research applications, until performance validation test equipment emerged. The first dedicated Ethernet test products were introduced in the early 1990s, and quickly proved indispensable for building demonstrably better Ethernet equipment and robust and interoperable networks.

This dedicated test equipment quickly became a requirement for all IC and equipment vendors developing Ethernet products for the corporate market. Within two years of test equipment introduction there was a drastic improvement in the throughput and interoperability of Ethernet equipment. Ethernet is now the de facto standard in enterprise networking.

WLAN Needs New Test Equipment

WLAN technology is at the same turning point. Addressing and solving the key issues that now keep WLAN out of business networks requires complex test equipment that can identify the problems and provide the detailed information needed to debug them. Unfortunately, the test equipment currently available does not provide the capability needed, and WLAN has attributes that make appropriate test equipment difficult to develop.


Figure 1: Current Range of WLAN Test Environments

Current test equipment evolved from the test needs of wired LANs. Wireless LANs, however, have many attributes that distinguish them from their wired equivalents and complicate test. Compared to wired LANs, for example, the wireless protocols are more sophisticated. The environment at the customer site influences the performance of wireless LANs whereas it has no effect on wired equipment. Further, the range of wireless product characteristics is broader than for wired products, and the protocols are rapidly evolving because the industry is, relatively speaking, in its infancy. This last attribute is particularly challenging for test equipment, as test must keep up with (and ahead of) new standards and protocols.

Many of these complications relate to the freedoms of, and corresponding lack of control over, the wireless connection. For instance, there are a number of specific behaviors that WLANs implement, unseen in wired networks, that have a significant impact on test requirements. The behaviors that test equipment must contend with include:

  • Getting Connected-Stations can query for Access Points (APs) (Probe Requests), and an AP can announce services supported (Beacons). These services are typically present in higher-layer protocols for wired networks, but WLANs need to implement them at the media access control (MAC) layer. Further, wireless devices need to determine which AP to use (Association), and the network needs to ensure that the user is valid (Authentication). Any device within range can hear a WLAN data transmission; hence the WLAN protocols need special features to deal establishing a valid connection.
  • Dealing with Non-Standard Connections-Multiple Basic Service Sets (BSS) can be available to a station at the same time (Overlapping BSS). Also, stations may both be in range of an AP, but not within range of each other (Hidden Node).
  • Dealing with Mobility-Mobile devices need optimizing power dissipation modes (Power Management) to extend battery life. Also, the data rate may need to adapt to optimize performance based on the current signal strength received by the radio (Rate Adaptation). In addition, mobility means that stations can dynamically switch between APs during a conversation or transmission (Roaming), a behavior similar to the cellular concept of roaming between coverage areas of base stations.

In addition to having these new behaviors, wireless networks have a different operating environment to contend with. As a result, WLAN test equipment must work in a variety of environments and be able to test many different situations. Some of the considerations are:

  • Influence of customer site-Operation of WLAN equipment is affected by a building’s structure. Commercial environments may vary from the wall-less cubicle farm to old buildings with heavy concrete walls between offices. Other field situations-home, high-and low-rise apartments, hospitals, warehouses, manufacturing centers-each pose their own challenges. As a result, equipment should be tested for the potential environments where it will be used, not just in a lab environment
  • Lack of isolation- WLANs cannot be easily isolated from each other. This means that even the WLANs installed in neighboring businesses can affect performance. One common source of interference is another WLAN client or access point. Packing too many clients into a confined space (e.g., a conference room) can lead to serious interference issues. Equipment needs to be designed and developed to take this into account

Another challenge for test equipment is that WLAN product characteristics are very broad, and the protocols are evolving rapidly. Test equipment needs to be developed to accommodate this great variation. Some of the variables in WLAN products include:

  • Equipment Variety-Access Points, NIC Cards, VoIP phones, WLAN switches, and WLAN gateways, are among just a few of the kinds of WLAN equipment - and each differs in significant ways in what is most important. For instance, portable devices such as laptops, handhelds, and phones, must conserve power, so they will focus on power management. VoIP phones, in order to maintain voice quality in real-time, require tighter Quality of Service (QOS) parameters than data transmissions. Business equipment needs the support of security protocols (WEP, WPA, 802.11i, 802.1X). Test equipment must handle these variations in significance.
  • Rapidly evolving protocols-In 1990 the IEEE established a working group on wireless LAN and in 1997 the 802.11 standard was approved. Two years later the IEEE approved two Project Authorization Requests for higher rate extensions, the 802.11a and 802.11b standards. Today there is also 802.11g and work on the even higher rate 802.11n, along with several related activities, such as wireless QOS (802.11e), and wireless security (802.11i, 802.1X). The standard for 802.11 is evolving at an accelerating pace, and test equipment must keep up.

As complex as all this seems, it is only the beginning. In the next year WLAN IC and WLAN system developers will find testing issues further exacerbated by the introduction of new equipment standards, the introduction of advanced product architectures, and the emergence of Bluetooth and WiMAX (802.16) products that can cause interference. All of these issues point to the need for advanced validation test equipment.

The needs of WLAN test equipment are many. To fully understand what 802.11 testers need to do, start by examining the range of test environments that can be used. “Test environment” in this case means the setup or environment into which the device(s) being tested and the test equipment are placed. For instance, the device under test (DUT) and tester may be completely enclosed in a single large shielded box, or the DUT and the tester may be placed at selected locations within an actual building. See Figure 1 for a comparison of test environments.

Each environment has its advantages, weaknesses, and uses. With all the different attributes available, it is clear that complete testing will require a combination of test environments. Situations will arise where a test developer will need to choose one environment over another. For example, antenna tests may require the full accuracy of a Faraday cage. Because protocol testing at Level 2 and above is not affected by the antenna, it may be tested by cable, chamber or cage all roughly equivalently, and the test developer can choose whichever is most convenient.

Note, too, that some approaches may mix two of the above environments to gain the advantages of both. For instance, a test vendor may choose to combine a cabled setup with a shielded test chamber, to offset the issues caused by poorly shielded DUTs in a cabled configuration. There is no single environment, however, that is considered “optimal” for WLAN testing. Due to the impact of RF effects upon the MAC-layer protocol, and vice versa, users must make tradeoffs between repeatability, accuracy, cost, convenience and end-user performance, when selecting a suitable test methodology. Test equipment vendors therefore need to accommodate all the different types of test environments, and cannot confine themselves to a single type; a one-size-fits-all environment does not exist.

Developers seeking to acquire test equipment for a WLAN project, then have many elements to consider. Some are operational. The technical attributes of test equipment need especially careful consideration in light of WLAN’s special needs. Test equipment should provide answers to problems encountered while diagnosing malfunctioning devices or systems, as well as drive such devices to their limits while still accurately measuring their capabilities and responses. The complex nature of the WLAN environment means that WLAN test equipment must provide far more comprehensive and detailed traffic generation and analysis capabilities (See Box: WLAN Test Equipment Requirements) than can be achieved by simply utilizing off-the-shelf LAN hardware with load generation software.

Toward the Future

Given all of these issues and difficulties, WLAN test can almost seem an intractable problem. Fortunately, a new generation of testing tools, architected for wireless, is starting to become available. Such tools will be critical to WLANs addressing widespread corporate applications, and for handling the more complex requirements of future WLAN technologies.

When making an equipment purchase, designers will need to examine the ability of the equipment to meet the needs that have been discussed. There are also several new technologies that are on the near horizon, including smart antennas, higher bandwidth standards, new features, and advanced switching architectures, and the testing needs for these should also be considered in a test equipment purchase.

WLAN Test Equipment Requirements

Accurate testing of WLAN devices and systems mandates complex and purpose-built hardware that has been designed “from the ground up” specifically for WLAN scenarios. Some of the unique requirements for WLAN suggest that the test equipment:

  • Provide all information seen on the air - One of the key requirements of WLAN test equipment is that it must be capable of accurately capturing and presenting to the user all information that is sent to or emitted by a device under test. This includes not just error-free frames, but also frames with errors, partial frames, and interfering signals. Without such information, it is frequently impossible for a user to determine exactly why a device is malfunctioning, or why a particular sequence of transactions took place.
  • Be repeatable - Repeatability is another significant requirement that is placed on test equipment. Repeatability here implies that a specific test, if run over and over again under a constant set of environmental conditions, will yield the same outcome every time. This is extremely difficult to achieve unless great care is taken, while designing the test equipment, to ensure that it can generate test stimuli in a highly controllable manner. For WLAN testing, repeatability necessitates precise control of not only factors such as RF signal strength and attenuation, but also packet timing and protocol handshakes. To achieve full repeatability, the test equipment must be capable of controlling and measuring signals at accuracy substantially better than one symbol time.
  • Accurately measure timing and signal strength - Accuracy of recording is another key attribute of test equipment, and is essential in order to diagnose obscure faults or measure the responses of devices under test at extremes of the protocol. As in the case of repeatability, accuracy must be achieved at both the RF signal strength level (i.e., transmitted and received power and signal characteristics) as well as the bit timing level.
  • Share tests and results across distances - In today’s multi-site development environment, where teams are spread over different locations, the ability to share information between sites and between departments is critical. For instance, validation engineers must communicate problems found during qualification testing to firmware development engineers in a simple and efficient manner, and manufacturing engineers need to send characterization and QA data back to development engineers in order to improve manufacturing processes and raise product quality. Test equipment is essential to this kind of interdisciplinary communication. WLAN test equipment, therefore, must support the effective sharing of tests and test results between teams, and enable tests to be replicated accurately at different physical locations.
  • Handle all testing environments - As part of the need to support a broad-based, interdisciplinary approach to WLAN testing, the test equipment should also lend itself to being used in all types of test environments, including Faraday cages, RF chambers, cabled, and open air. Fully shielded testing (cages, chambers, and cabled) is useful for cases with a high density of users; open-air environments are essential for QA departments that need to completely characterize the behavior of the products being developed under “real-world” conditions. It is also extremely desirable that results obtained under one type of environment be easily portable to another type of environment, preferably even using exactly the same test equipment for both. This not only simplifies the company’s validation and testing strategy, but also considerably speeds up test development.
  • Handle both lab bench and customer site - Because certain issues will only be manifested at the customer site, companies must be able to extend the reach of their test equipment from the laboratory to the customer site. Problems that are only visible at specific customer sites need to be diagnosed quickly and easily, and be brought back to the lab for replication and detailed analysis.
  • Be spatially aware - An attribute required by WLAN test equipment that is unnecessary for wired LAN test equipment is spatial awareness. WLANs are inherently spatially aware, in that their behavior is considerably influenced by their position within the physical environment and their location with respect to other devices. WLAN protocols take this into account and hence add to the spatial nature of WLAN systems. To fully test a WLAN device or system, therefore, test equipment must take these spatial characteristics into account.

As an example, consider the common WLAN issue of roaming. Roaming occurs when a WLAN client physically moves from the neighborhood of one access point to another. As the signal strength received by one access point decreases and that received by the other increases, the client de-associates with the first access point and re-associates with the second new access point in order to maintain its network connectivity.

Testing roaming, however, requires that the test equipment be capable of emulating the characteristics of the environment as well as simulating the effect of moving a client from one location to another. In particular, precise and repeatable testing of roaming necessitates highly accurate control of signal characteristics and timing. Essentially, the test equipment must cause a “virtual” movement in space in a highly controlled manner while generating traffic in an accurate and repeatable manner.

Other instances of situations requiring spatial awareness are common WLAN-specific effects such as hidden nodes, the “near/far” problem, coherent and non-coherent interference, diversity antenna switching, etc. The WLAN protocol attempts to compensate for many of these issues with specific functions and capabilities. Complete testing of a WLAN device, therefore, mandates that the traffic patterns corresponding to these effects be precisely generated.

For instance, fully testing diversity antennas requires that the test equipment be set up in some sort of real or simulated multi-path environment, and suitable traffic patterns generated to exercise the device features supporting diversity reception. In many cases (e.g., integrated diversity antennas), complete testing of these features can only be carried out in an open-air environment, with the test equipment placed at fixed and known locations within this environment.

  • Handle new switching architectures - The new generation of WLAN “switches” being introduced into the marketplace imposes special requirements on test equipment. WLAN switching is fundamentally a spatial phenomenon; for instance, one version of switching uses electronically steered beams that are focused on specific WLAN clients, and follow these clients as they move about.

Clearly, testing WLAN switches is fundamentally different from testing wired LAN switching technologies. Unless the test equipment is capable of spatial awareness and can generate and receive traffic in a spatially distributed fashion; any attempt to test such equipment will be superficial and incomplete. As more companies bring various types of WLAN switching technologies to market, therefore, the requirements placed on test equipment will rise correspondingly.

  • Handle combinations of Layer 1 and Layer 2 - Finally, WLAN test equipment must be capable of straddling the “Layer 1 / Layer 2 boundary”. This implies that the test equipment must be able to address problems that occur as an interaction between the physical layer protocol and the MAC (or higher) layer protocols. For instance, consider the issue of testing an adaptive summing diversity receiver being used for a WLAN access point. Such a receiver may make use of information at the bit stream or even the packet layer to adaptively adjust a set of weights within the PHY; however, the combination of firmware, packet-level protocols and RF effects makes this a difficult problem to address with physical layer test equipment alone. Instead, it is necessary to utilize test equipment that is capable of simultaneously handling the needs of testing the device at the bit level, the packet level, the MAC level and even the driver level. Without such capabilities, the user may be reduced to solving problems by guesswork, or may have to resort to expensive ad-hoc test setups.

Several critical areas of WLAN protocols and technology require the issue of combined Layer 1 and Layer 2 testing. For instance, roaming is a Layer 2 function that is triggered by a Layer 1 effect (a reduction in signal strength), that may have consequences even at the Layer 3 level depending on the security protocol in use. A similar problem manifests itself in Voice over IP (VoIP) technologies, but along a different dimension. VoIP testing requires accurate measurement of timing variations in order to estimate the worst-case latency of a voice channel. Layer 1 effects such as interference bursts causing retransmissions and back-off may cause these timing variations. It is hence necessary for the test equipment to provide the user with both visibility and control of the data stream at multiple levels, and not focus merely on a specific protocol layer while sacrificing controllability or observability at other layers. ❏
Dr. Thomas Alexander is the Chief Technology Officer at VeriWave, Inc. Tom has extensive experience in the networking and computation industries. Prior to VeriWave, he was the chief architect for Ethernet products for PMC-Sierra. He is active in communications standards being the editor for the IEEE 802.3ae 10G Ethernet standard, and the Chief Editor, IEEE 802.17. He has invented widely-applied Ethernet products and has a history of founding and guiding successful companies.

Rick Denker is the Vice President of Marketing. Rick has broad experience in high technology product marketing and business development. Prior to VeriWave, he was the Director of Product Marketing for WeSync.com, which was acquired by Palm. He has held senior marketing positions at Synopsys, and PMC-Sierra.

Gerard Goubert is the Manager of the Wireless consortium for UNH-InterOperability Lab. Gerard has many years of experience in a variety of networking technologies. He currently manages the Bridge Functions, Voice over X, and Wireless consortiums for the University of New Hampshire InterOperability Lab (UNH-IOL). UNH-IOL is the leading independent test lab for interoperability and conformance testing.