Date: 12/12/2012

IEEE 802.11ac Wi-Fi: Trends, Technologies and Reliability

The boom of mobile devices, broadening uses for Wi-Fi and increasing acceptance of all things wireless, are all adding to consumer and enterprise hunger for more and more speed. The next generation of the 802.11 Wi-Fi standard, known as IEEE 802.11ac, promises to finally break the wireless Ethernet gigabit barrier. This technology delivers higher bandwidth while attaining better quality of experience (QoE) for end-users.

The latest of Wi-Fi innovation, the IEEE 802.11ac (still in draft form) is sometimes known as VHT (very high throughput) to distinguish it from 802.11n, which has the HT (high throughput) moniker.

A final standard is expected from the 802.11ac Working Group in late 2012 and final approval from the 802.11 Working Group is expected in 2013.

Trends and Technologies:

The IEEE estimates that 802.11ac will become common by 2015, but all indications are that this will come about much sooner. As was common with previous Wi-Fi standards, we are already seeing pre-standard versions of 802.11ac products. Chipsets are becoming available from a number of vendors, including Broadcom, Marvel, Qualcomm, Intel, Mediatek, Quantenna, and Redpine. Access points are available using these chipsets from Buffalo, Cisco, TrendNET, Netgear, and Belkin. These initial products generally feature 1.3 Gbit/s of total throughput using three simultaneous streams of 433.3 Mbit/s.

The user-visible changes that are expected from 802.11ac, with respect to 802.11n and earlier standards, include:

1. Transfer speed – between 433 Mbit/s and 3.46 Gbit/s per station (STA) and up to 6.93 Gbit/s per access point (AP).

2. Range – although the theoretical maximum range is only slightly extended, performance at all distances will be improved.

3. · Multi-user performance – true simultaneous transmission from the AP to multiple STAs.

Usage Trends Requiring Higher Performance

Digital-content consumption is on a steep rise, with video content expected to reach more than 70 percent of global traffic by 2015. The growth in video content and increased reliance on wireless networks is putting stress on older Wi-Fi networks based on 802.11a/b/g/n standards. As a result, users are prone to experience deteriorated performance, choppy videos, and slower load times. Residential video streaming, data syncing between mobile devices, and data backup will be some of the first applications for 802.11ac’s faster speeds. Consumers and enterprises will be able to stream digital media between devices faster, and simultaneously connect more wireless devices. Carriers will deploy the new technology to offload traffic from congested 3G and 4G-LTE cellular networks, and in dense operator hotspots 802.11ac will supply better performance to more users.


While this may seem on the surface like a minor technical detail, 802.11ac marks the first time in the Wi-Fi timeline that directed traffic can be delivered to multiple client devices simultaneously. This has significant impact on delivery of content any location with multiple users (especially where content is revenue-generating or critical). Large venues, hotspots, enterprises, and even home video delivery stand to experience improved per-user experience. At a time when college campus IT managers are reporting that network users are now averaging more than one Wi-Fi connected device per person, techniques to handle the rapid growth of client devices are at a premium.

Technological Advancements
802.11ac represents a major step up in technology – in some areas by a factor of ten. Improvements relate to transport rate, signal fidelity, noise handling, and accuracy. The new network will reportedly offer numerous advantages over the current 802.11n protocol. The wireless connection speed with Wi-Fi 802.11ac will not only be much quicker than its predecessors, but will also provide better range and improved reliability, as well as superior power consumption.

The Blessing and Curse of Backward Compatibility
Any engineer will tell you that the fastest, most reliable way to deliver a new technology is to eliminate any requirement to interoperate with previous technologies. On the other hand, any user will express frustration when forced to completely abandon satisfactorily functioning solutions and move to newer solutions.

The designers of the 802.11 standards clearly sided with the end users – a pretty good decision from a market acceptance point of view. Adoption of 802.11 has continued to experience healthy growth even though there have been four major revisions to the base protocol and numerous options added since inception. In fact, each new release of a major 802.11 solution is met with excitement from the user community, in large part because adopting the latest technology does not immediately force users to upgrade their entire network.

Predictably, this unrelenting focus on backwards compatibility has created quite a challenge for 802.11 device manufacturers. One of the biggest frustration for developers and users of 802.11 is that it can be extremely difficult to identify the root cause of development problems. For example, when an application performs poorly, it is often hard to determine if it is due to an environmental, client, or network issue. The various devices in an 802.11 network are highly correlated so an issue in one area quickly ripples through to many other areas. Developers have lacked an effective means to assess the total picture from the RF to the application layer. IEEE 802.11ac makes this problem significantly more challenging. In addition to being deployed into an existing environment with ten years’ worth of previous releases, 802.11ac makes use of advanced technologies that are substantially more complex and demanding than previous versions.

Reliability
Wi-Fi Performance in the Real World

We have all had our problems with Wi-Fi in the past: unable to connect to a network, unable to connect to the Internet, poor throughput, dropped connections, lack of range, … The reasons for these and other problems are myriad.

Present-day Wi-Fi access points need to implement four separate standards at the same time: 802.11a/b/g/n. 802.11a/b/g are mature now, but 802.11n implementations are still maturing. To that mix we now add 802.11ac. 802.11ac is new, which means that it will undergo a typical progression: pre-standard devices, standards-compliant devices, devices that incorporate real-world experiences, and a continuing progression of more and more capable 802.11ac devices. The large number of bandwidth, antenna, and performance options both for the APs and STAs will generate hundreds of products with different combinations of feature, capabilities, and performance.

Part of the complexity is due to the number of APs present. In today’s world, it is commonplace to find four or more APs within reach in business, home, and retail environments. Each of these access points is competing for bandwidth and for time to use that bandwidth. The AP that you are using might find itself with a very limited set of reliable channels to operate in. Where the same channels are in use by multiple APs, interference can affect both availability and throughput.

Meeting 802.11ac Technical Challenges
Delivering breakthrough 802.11ac performance demands that Wi-Fi developers stretch their designs in terms of both complexity and precision. Similarly, 802.11ac requires a rethinking of the traditional approach to testing.

Traditionally, the radio frequency (RF) component of an AP or STA is verified using one set of equipment, and then higher layer functions are tested using a second set of tools. The overall technical complexity and the introduction of new technologies, such as transmit beamforming, demand coordination and control between the different software mechanisms. Without coordination, it would be difficult if not impossible to exercise these functions.

Current test equipment is only capable of generating a single spatial stream, and only able to generate or capture a small fraction of the frames required to perform testing.

Next generation test products must be able to decode every frame in real-time in order to calculate each frame’s RF characteristics and frame-level performance. To meet 802.11ac needs, an approach is needed to generate and analyze all frames in real-time to the limit of the specification, tightly integrating RF and layer 2 MAC functionality. It must also include integral, real-time channel emulation to address transmit beam forming performance.

Layer 1 Testing
802.11ac brings changes to layer 1 that are extremely challenging for radio designers: 256-QAM and 160 MHz bandwidths. Wi-Fi designers must deliver performance advances in phase noise performance, noise floor, modulation accuracy – virtually every dimension that impacts digital modulation performance. Verifying the performance of transmitters and receivers requires better performance than was sufficient for 802.11n. Characterization must include all new 802.11ac rates plus 802.11a/n rates. Best-in-class performance means verifying transmit and receive performance of literally hundreds of frame definitions, varied by modulation rate, frame length, bandwidth, frequency, channel model, and power level. The testing must include 802.11a/n/ac frames in various combinations, and must be representative of the actual diversity, rate, and complexity that actual devices will experience in normal operation.

Layer 2 Testing
Optimizing the layer 1 performance of a design is not sufficient to ensure a high-performance 802.11ac solution. Many of the performance limitations inherent in previous versions of 802.11 are the result of insufficient stress being placed on the devices during design and quality assurance phases. 802.11ac is even more stateful than its predecessors, requiring a highly complex MAC layer implementation. Underperformance can be due to any number of causes including slow ACK response times, poorly designed aggregation algorithms, internal buss limitations, poor rate adaptation algorithms, poor AP selection algorithms, power save implementations, and poorly implemented legacy protection schemes.


As with RF testing, the approach to layer 2 functionality testing is to start simple and gradually add more complexity. In this case, that means progressing through a series of steps, such as:

1. Ensure a single client is able to reliably connect and disconnect.

2. Perform benchmark tests on a single client to understand any basic system upstream or downstream bottleneck.

3. Enable more features on the single client to ensure that these functions work properly in a single client environment. These features include aggregation, power save, IPv4 and IPv6, QoS, and SU-MIMO.

· When testing an access point solution, benchmark with multiple clients to make sure that the system can achieve the expected performance under ideal conditions at scale.

4. Test with mixed-mode clients to make sure that the introduction of legacy devices will not adversely impact 802.11ac operation.

These tests should be run under cabled conditions so as to eliminate stray RF effects. In this way unexpected test results can be isolated to the system design, rather than be attributed to RF issues.

It is important to note that laptop-based test solutions are simply not adequate for testing 802.11ac solutions. An 80 MHz 4x4 802.11ac solution should be capable of delivering 1.7 Gbps of traffic. Any performance test solution must be able to demonstrate that it achieves the maximum throughput at all frame sizes.

After completing functional and performance testing, it is necessary to perform system level testing using a mix of TCP-based traffic and real-world application traffic flows. This testing is also conducted in a wired configuration and is meant to help tune features and algorithms to optimize application performance. In addition to enterprise networking, 802.11ac solutions are critical to delivering wireless video in carrier hotspot, residential, and enterprise deployments. For 802.11ac, it is necessary to ensure that the combination of the legacy protection, power save, QoS, and coexistence behaviors are optimized to deliver high quality video.

Testing Multiple Layers Concurrently
Significantly more comprehensive testing can be accomplished much faster if multiple layers of the protocol stack can be used concurrently. With wireless technologies, the underlying medium is likely to have issues that will affect the performance of the overall system. The challenge is to know when performance degradations are due to an RF design issue, an upper layer protocol issue, or the result of impairment in the RF spectrum.

To date, the disciplines of RF and MAC design and test are largely treated as separate functions. This arrangement has resulted in many RF issues discovered later during the MAC layer testing, which are almost always attributable to a medium impairment problem. In testing of production-released devices it is common to encounter performance issues that are caused by RF design issues or interactions between layers 1 and 2 caused by a product’s design and implementation.

In order to address these issues, a single solution should enable testing of each layer individually – but also allow visibility into metrics from both the RF layer and the upper layers at the same time. This approach allows rapid identification and isolation of any discovered issues, and provides the confidence that a product is really ready for production release. By using a common set of tools, productive communication is facilitated between these two functions and test results are easily duplicated.

Conclusion
802.11ac holds the promise of per-client gigabit+ performance that will enable much broader adoption in key target markets, including enterprise, residential video, and carrier hot spots. To realize the performance and density promise, chip and hardware developers must navigate some significant technical challenges including:

1. Ensuring graceful migrations from existing deployed solutions by providing backward compatibility.

2. Delivering high performance RF transmission and receive performance with a wide variety of signals.

3. Maintaining high performance to multiple clients under real-world channel conditions.

4. Providing the high reliability and feature robustness to enable enterprise and carrier grade 802.11ac adoption.

5. Ensuring that the key application traffic, most notably video, can be delivered with quality.

In addition to meeting all of the technical hurdles, developers are also expected to deliver projects on shortened schedules while striving to improve quality. 802.11ac represents a significant opportunity and challenge to the industry, one that demands a rethinking of the traditional approach to development and test.

Author: Dave Schneider, Principal Technologist, Ixia