WHITE PAPER Delivering Breakthrough Performance with 802.11ac 26601 Agoura Road, Calabasas, CA 91302 Tel: 818.871.1800 Fax: 818.871.1805 www.ixiacom.com 915-6107-01 Rev. A, April 2012
Table of Contents Overview... 4 The Blessing and Curse of Backward Compatibility... 5 How 802.11ac Achieves Gigabit+ Performance... 6 Boosting Raw Bandwidth... 6 Real Information Superhighway... 7 Individual Client Channel Optimization... 7 Meeting 802.11ac Technical Challenges... 7 Layer 1 Testing... 8 Layer 2 Testing...10 Testing Multiple Layers Concurrently...12 Conclusion...12
Overview The next generation of the 802.11 standard known as IEEE 802.11ac promises to finally break the wireless Ethernet gigabit barrier. This technology will deliver higher bandwidth while retaining better quality of experience (QoE) for end users, and is expected to be adopted rapidly into all markets: residential, enterprise, carrier and large venue. The next generation of the 802.11 standard known as IEEE 802.11ac promises to finally break the wireless Ethernet gigabit barrier. Digital-content consumption is on a steep rise, with video content expected to reach more than 70 percent of global traffic. The growth in video content and increased reliance on wireless networks is putting stress on existing 802.11a/b/g/n networks. As a result, users are prone to experience deteriorated performance, choppy videos and slower load times. 802.11ac is positioned to overcome the digital content challenge on wireless networks. 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 can 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. All 802.11 revisions to date have focused on increasing transport speeds, which lead to higher traffic delivery rates and ultimately to faster response times as experienced by the end user. 802.11n brought with it the advances of MIMO (multiple-in, multiple-out) to deliver traffic over multiple spatial streams, and packet aggregation. MIMO delivered marked improvements in physical transport rates, enabling more bits/second to be transmitted than ever before across WiFi. Packet aggregation delivered equally impressive improvements in transport experience, allowing devices to send more data once they had gained access to the wireless media. 802.11ac continues in this thread by preserving aggregation techniques, advancing the physical transport rates yet again, and introducing the concept of parallel transport into WiFi through a technique known as Multi-User MIMO (MU-MIMO), where multiple client devices are receiving packets concurrently. While this may seem like a minor technical detail on the surface, it marks the first time in the WiFi timeline that directed traffic can be delivered to multiple client devices simultaneously, which 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 WiFi connected device per person, techniques to handle the rapid growth of client devices are at a premium. 802.11ac uses 256-QAM and 80 MHz bandwidths to achieve the next level of performance, making the demands on the radio systems in terms of signal fidelity, sensitivity and noise the most demanding to date. However, the realities of deployment strategies require compatibility with existing legacy and 802.11n client devices. It will be common for an 802.11ac access point to carry on conversations with multiple 802.11a phones, 802.11n tablets, and 802.11ac laptops. Each of these different devices supports a variety of features including QoS, power save, and multicast. New 802.11ac chipsets must simultaneously excel at delivering blistering performance to other 802.11ac devices, while gracefully interoperating with a multiplicity of previous devices. This paper examines the challenges that must be met to deliver an industry-leading 802.11ac solution by first identifying the challenges of managing legacy 802.11 solutions, examining the latest technologies that 802.11ac exploits to deliver breakthrough performance, and highlighting techniques for chip developers and AP designers to speed their products to market. 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. 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. 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. 1 st Generation 3 rd Generation 5 th Generation IEEE 802.11 Data Rate: 2 Mbps IEEE 802.11g/a-Wi-Fi starts to become ubiquitous Data Rate: 54 Mbps IEEE 802.11ac Data Rate: Up to 3.6 Gbps First solution is <1.8 Gbps. 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2 nd Generation IEEE 802.11b Data Rate: 11Mbps 4 th Generation IEEE 802.11n Data Rate: Up to 600 Mbps. Most common is 150 Mbps Enhanced range due to use of MIMO
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. This latest generation of 802.11 requires a rethinking of how the technology is developed and tested to include a much more holistic view through the product development lifecycle. To increase the physical-layer transport rate, 802.11ac makes use of a higher rate encoding scheme known as 256-QAM which transmits 33% more data than the 64-QAM used in the 802.11n standard. How 802.11ac Achieves Gigabit+ Performance 802.11ac uses a variety of advancements in order to achieve the targeted performance. The new specification addresses the need for performance improvement through three primary initiatives: increasing the raw bandwidth, enabling multiple flows to use the medium concurrently, and optimizing performance to specific clients. Boosting Raw Bandwidth To increase the physical-layer transport rate, 802.11ac makes use of a higher rate encoding scheme known as 256-QAM which transmits 33% more data as the 64-QAM used in the 802.11n standard. Signal-to-noise ratios that worked for 802.11n are no longer sufficient for the higher speeds in 802.11ac because the difference in detectable signal level is now significantly smaller. To further increase the amount of data transported per second, channel bonding approaches made popular in 802.11n have been taken further to provide 80 MHz-, and ultimately, 160 MHz-wide channels. Increasing channel bandwidth allows for more data to be transmitted simultaneously out of the same antenna. Legacy versions of the 802.11 devices commonly used 20 MHz channels. When using 802.11n, users could select between 20 MHz or 40 MHz channel operation. These wider channel bandwidths and the need for proper channel separation mean that 802.11ac can only be used in the 5.0 GHz band (where more non-interfering bandwidth is available). Note that dual-band APs will still be produced, but the 2.4 GHz band will be limited to 802.11bgn and will not be able to be configured for 802.11ac. The use of multiple spatial streams is also a key factor in the 802.11ac equation. The 802.11n standard accommodates for up to four spatial streams to achieve a maximum of 600 Mbps of performance, although most access points today use only three spatial streams for a maximum PHY rate of 450 Mbps. The 802.11ac standard allows for up eight spatial streams. Early products will use three or four spatial streams, but increased numbers of spatial streams are expected in future solutions. Finally, packet aggregation is present in 802.11n as well, but is worth a mention in this discussion because it is often the single biggest performance multiplier on a per-transmission basis. To understand the importance of aggregation, it is critical to
realize that the amount of overhead required to obtain a chance to transmit a frame and acknowledge its transmission is often much longer than the time required to actually transmit the useful data. With aggregation, once a high performance device obtains its transmit opportunity, the transmitter strings multiple frames together, and transmits them in succession without having to reacquire the medium. Through this process, the overhead cost of the time to obtain and acknowledge each additional frame is saved. Real Information Superhighway To support more users, 802.11ac has moved away from only allowing one 802.11 device to transmit at a time. Multiuser multiple-input, multiple-output technology, or MU-MIMO for short, allows an access point to transmit data to multiple client devices on the same channel at the same time. In previous versions of 802.11 whenever the access point transmitted data, all of the traffic at any given instance in time was being directed to a single client. As a consequence, if a set of devices included a mix of fast and slow client devices the fast traffic was often delayed substantially by the transmissions to the slower clients much like traffic being delayed behind a slow driver on a single-lane road. In contrast, MU-MIMO allows a single AP to transmit to multiple client devices at the same time. MU-MIMO works by directing some of the spatial streams to one client and other spatial streams to a second client. In the previous analogy, this is much like driving down a four lane freeway. Traffic is only significantly delayed when all of the lanes are blocked, so overall throughput can be much higher. There are a number of permutations to this basic principle, but MU-MIMO is critical to performance improvements in environments with high client counts. Individual Client Channel Optimization The final major performance boost comes from technologies that optimize the communications when speaking to a specific client. The first major enhancement is a concept known as transmit beamforming, (TxBF for short). The reflections and attenuations, common during the transmission of 802.11 signals, have a significant performance impact on overall network performance. With TxBF, the access point communicates with the client devices to determine the types of impairment that are present in the environment. Then the access point precodes the transmitted frame with the inverse of the impairment such that when the next frame is transmitted and transformed by the medium, it is received as a clean frame by the client. Since no two clients are in the same location, TxBF needs to be applied on a client-by-client basis and constantly updated to reflect the changing environment. A critical component of this feature is the fact that the Access Point and client device are participating in a controlled handshake between themselves, each sharing information to the other about the propagation channel that exists between them. Delivering breakthrough 802.11ac demands that developers stretch their designs both in complexity and precision, and requires a rethinking of the traditional approach to testing. Meeting 802.11ac Technical Challenges Delivering breakthrough 802.11ac demands that developers stretch their designs both in complexity and precision, and requires a rethinking of the traditional approach to testing. Traditionally, the RF section is verified using one set of equipment, and then the upper 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 layers of the protocol stack. Without this coordination, it would be difficult to exercise these functions and to quickly pinpoint performance issues to a specific function of the hardware from the RF layer all the way up to the application
switching. Furthermore, new 802.11ac chipsets and system designs must be verified to work flawlessly with legacy and 802.11n client devices, meaning that entire system validation must be accomplished in a fraction of the time of the 802.11n development cycles. Meeting the advanced challenges and timelines of 802.11ac, the traditional bring-up techniques are not fast enough, nor are they capable of providing the breadth of coverage required. Up to this point, qualification of RF subsystems have been performed within the limitations of the test equipment, and system-level testing has been performed using best-effort software tools and product adaptations. 802.11ac brings the promise of moving Wi-Fi into the limelight as a trusted and capable communication protocol, and will require equipment and rigor to match. Designers are required to deliver performance advances in phase noise performance, noise floor, modulation accuracy virtually every dimension that impacts digital modulation performance. The new generation of testing should be able to decode every frame in real-time and determine each frame s RF characteristics, as well as their frame-level performance, and generate every frame without limitation in real-time to adequately test receiver performance. Previous approaches use a digitized data record approach for both generation and analysis, creating or capturing what are known as I/Q files, and equipment typically adapted from the general-purpose RF domain. This results in equipment being capable of a single spatial stream, and able to generate or capture a small fraction of the frames required to perform testing. To meet the need, the approach needs to be able to generate and analyze all frames in real-time to the limit of the specification, tightly integrate RF and MAC functionality in 802.11ac, and include integral, real-time channel emulation to address Tx beamforming performance Layer 1 Testing 802.11ac brings changes to layer 1 that are extremely challenging for radio designers: 246-QAM and wider bandwidths. Designers are required to 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 require better performance than was sufficient for 802.11n, and characterization must include all new 802.11ac rates plus legacy/802.11n rates. Best-inclass 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 legacy frames, 802.11n frames and 802.11ac frame in various combinations, and must be representative of the actual diversity, rate, and complexity that actual devices will experience in the field. 1400 Data Rate in Mbps 1200 1000 800 600 400 11ac (3-antenna) 11n (1- antenna) 11n (3-antenna) 200 0 0 5 10 20 30 40 50 60 70 80 90 100 Distance in Meters 1 Room 2 Rooms 3+ Rooms
To test these highly-integrated and high performance 802.11ac radio systems on a timeline that meets the aggressive market pace requires two critical components: the ability to make RF measurements on every frame transmitted at line rate and the ability to generate any desired frame quickly and without limitation. When it comes to testing transmit functions, using I/Q analyzers means that a performance assessment is being made on a small sampling of frames. Because these analyzers trigger only on power, it is statistically improbable that the test engineer will ever see frames that are faulty unless the fault exists on virtually every frame. Power supply issues, clock interference issues, and decoupling problems that impact the fidelity of the transmitted signal randomly or at a low repetition rate are missed. Positive verification of quality comes only when RF measurements are made at line rate on every frame transmitted. When it comes to receiver testing, the same principle applies. Receivers tested with I/Q signal generators see only a small sampling of frames, and almost no diversity from one frame to the next. Positive verification of quality comes when receivers are subjected to continuous frames at line rate, with real-life variation from frame to frame. The traditional I/Q approach will not work for 802.11ac because it does not provide for real-time analysis of every frame transmitted, nor can it subject receivers to realistic and continuous flows of frames. The traditional I/Q approach will not work for 802.11ac because it does not provide for real-time analysis of every frame transmitted, nor can it subject receivers to realistic and continuous flows of frames. Even worse, it requires a very experienced engineer to create and maintain the frame library and impairments. The amount of time it takes to download and play these files limits the number of tests that can be run. As most projects are constrained by relatively fixed schedules, the lengthy process to execute these tests in regression severely limits the number of tests that can be run. Finally, the limited record length of the memory buffers mean that some of the longest, most challenging, and frames most important to performance, aggregate frames, cannot be tested. There is a strong need to be able to create the variety of frame conditions quickly and easily, and to extend coverage to include the much longer aggregate frames possible in 802.11ac.
The tables below list the measurements required before the radio transmit and receive performance can be declared complete. The difference in quality and performance between 802.11ac devices will be a function the approach used: thorough and complete, or sampled. Key 802.11ac RF Transmitter Tests 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 QA phases. Power Frequency Spectral Area Modulation I/Q Key 802.11ac RF Receiver Tests Area Tone Generation Frame Generation Modulation Impairments Layer 2 Testing Measurements Average, peak, power spectral density, power peak excursion, power-on / power-down Center frequency tolerance, Symbol clock frequency tolerance, preamble frequency error, RF carrier suppression Tx spectrum mask, Spectral flatness, Tx center frequency leakage, CCDF, Occupied Bandwidth Constellation error, EVM, Transmitter modulation accuracy Gain mismatch, Phase mismatch Frequency, amplitude Permutations Frame sequences, rate, length, encoding a/b/g/n/ac PHY rates, preamble, FEC, etc. Frequency offset, pre/post encoder bit errors, channel models, etc. Optimizing the layer 1 performance of a design is not sufficient to ensure a highperformance 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 QA phases. 802.11ac is even more stateful than its predecessors, and therefore include a great deal of complexity at the MAC layer. Underperformance can be due to any number of potential causes including: slow ACK response times, poorly designed aggregation algorithms, internal buss limitations, poor rate adaptation algorithms, poor AP selection algorithms, power save implementations, poorly implemented legacy protection schemes, and so on. 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: Make sure a single client is able to reliably connect / disconnect Perform benchmark tests using a single client to understand any basic system bottlenecks in the upstream and downstream direction. Enable more features on the single client to ensure that the basic functions work properly (aggregation, power save, IPv4 and IPv6, QoS, etc.)
If 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. Test with mixed-mode clients to make sure that the introduction of legacy devices will not adversely impact your 802.11ac solution. All of the above tests should be run under cabled conditions. The goal is to eliminate stray RF effects so that any unexpected test results can be isolated to the system design and not be attributed to RF issues. Because the layer 1 testing is already complete and the performance is known, there will be a very high degree of confidence that the root cause of any identified issues must be in the MAC layer. Being able to see both the RF and MAC results at the same time greatly improves problem identification and resolution. It is also critical to understand that laptop-based test solutions are simply not adequate for testing these solutions. An 80 MHz 4x4 802.11ac solution should be capable of delivering 1.7 Gbps of traffic. Any test solution used to assess performance must be able to demonstrate that it achieves the maximum performance at all frame sizes. The risk of using any solution that is incapable of achieving these rates is that the measured performance may be the limited performance levels of the test system instead of the 802.11ac device under test. Further, dedicated hardware-based solutions also enable you to see capture files of problematic sequences with a variety of PHY and MAC layer statistics. After completing the functional and performance testing, it is useful to perform system level testing using a mix of TCP-based traffic and real application traffic flows. This testing is also conducted in a wired configuration and is meant to help the designers to tune the features and algorithms in the solution 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. The last two steps in the process are to run the system level tests while simultaneously varying the individual RF channels of each client. First, the test should be run in a cabled environment with a mix of clients in a variety of locations. Note that it is necessary to create a wide variety of conditions without necessarily using numerous client devices in a physical layout. The cost and repeatability of performing this testing prohibits using actual facilities. Test solutions that integrate the channel impairment capability with the traffic gene ration and analysis are ideal because the impairment function is synchronized with the frame transmission. IEEE members developed a set of channel impairment models that can be used, along with adjustments to client power levels, to virtually create a layout and conduct this testing. Test solutions that integrate the channel impairment capability with the traffic generation and analysis are ideal because the impairment function is synchronized with the frame transmission. The last step is rerunning the same tests over the air in an RF-isolated environment to eliminate any unintended interference. This ensures that the integrated system, including antennas, cabling, etc., continue to perform as expected.
Testing Multiple Layers Concurrently Again, much more comprehensive testing can be accomplished much faster if you can test at multiple layers of the protocol stack concurrently. With wireless technologies, the underlying medium is likely to experience issues that will affect the performance of the overall system. The challenge is to know when the performance degradation is 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 are almost always attributed 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. 802.11ac holds the promise of gigabit+ performance that will enable much broader adoption in key target markets such as enterprise, residential video, and carrier hot spots. In order to expediently 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 gigabit+ performance that will enable much broader adoption in key target markets such as 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: Ensuring graceful migrations from existing deployed solutions by providing backward compatibility Delivering high performance RF transmission and receive performance with a wide variety of signals Maintaining high performance to multiple clients under the channel conditions that will exist in real deployments Providing the high reliability and feature robustness to enable enterprise and carrier grade 802.11 adoption 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.
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