Designed for Speed Aruba White Paper Designed for Speed: Network Infrastructure in an 802.11n World Designed for Speed Aruba White Paper Table of Contents Summary 4 Introduction 4 Benefits for the enterprise 5 Benefits for the user 6 Realizing these benefits with a mixed client base requires careful planning 6 802.11n migration strategies for enterprises 7 Network design with 802.11n 7 Wired backhaul from APs & wired LAN design 7 Power consumption 8 Implications for rogue APs and WIDS 8 Good-neighbor (or bad-neighbor) strategies 9 Changes between ‘draft-2.0’ and ‘802.11n’ 9 Technology in 802.11n 12 Techniques for high-throughput PHY 12 High Throughput PHY: Maximum ratio combining 13 High Throughput PHY: space-time block coding 13 High Throughput PHY: Spatial division multiplexing 14 High throughput PHY: Transmit beamforming (TxBF) 18 MIMO, STBC, SDM & beamforming 21 802.11n MIMO configurations and terminology 21 Hierarchy of MIMO techniques 22 High Throughput PHY: 40 MHz channels 24 High Throughput PHY: Shorter guard interval 24 High Throughput PHY: More subcarriers 25 High Throughput PHY: New modulation rates 25 Techniques to enhance the MAC 27 MAC layer enhancements: Frame aggregation 27 MAC layer enhancements: Multiple traffic ID block acknowledgement (MTBA) 29 MAC layer enhancements: Reduced inter-frame spacing (RIFS) 29 MAC layer enhancements: Spatial multiplexing power save (SM power save) 29 MAC layer enhancements: Power save multi-poll (PSMP) 30 Aruba Networks, Inc. 2 Designed for Speed Aruba White Paper Compatibility modes and legacy support in 802.11n 31 Greenfield, high-throughput and non-HT modes 33 Phased coexistence operation (PCO) 33 Other mechanisms for coexistence: RTS/CTS & CTS-to-self 35 Other mechanisms for coexistence: 40 MHz-intolerant indication 35 Using 802.11n in the 2.4 GHz band 36 20/40 MHz channel numbering in the 2.4 GHz band 36 Using 802.11n in the 5 GHz band 37 Use of 20/40 MHz channels, coexistence and protection mechanisms 37 20/40 MHz operation and fallback to 20 MHz 38 New Wi-Fi alliance 802.11n certifications 39 Migration strategies 41 Different paths to enterprise-wide 802.11n 41 Greenfield 41 AP-overlay 43 AP substitution 44 Other considerations when planning an upgrade 44 Conclusion 45 Appendix 46 Note on expected ‘real-world’ cell capacity with 802.11n 46 Forms of MIMO 47 Channel estimation 48 Glossary of terms used in this note 49 References 50 About Aruba Networks, Inc. 50 Aruba Networks, Inc. 3 Designed for Speed Aruba White Paper Summary The IEEE 802.11n standard and Wi-Fi Alliance 802.11n certification herald a new world for enterprise wireless networks. 802.11n brings significantly higher data rates and more reliable coverage than previous Wi-Fi technology: it represents a significant upgrade in performance. The first Wi-Fi Alliance 802.11n draft-2.0’ certification, dating from June 2007, was based on a snapshot of the then-unfinished IEEE 802.11n specification. Over the next two years, many millions of ‘draft-2.0’ products were shipped, and in enterprise WLANs ‘draft-2.0’ already accounted for 30% of all access points shipped in 2Q2009, according to the Dell‘Oro Group and Aruba Networks. In September 2009, 802.11n passed the second milestone in its rollout, as the IEEE concluded its ratification of the standard, and the Wi-Fi Alliance released its new ‘802.11n’ certification program. The new certification accepts all previous ‘draft-2.0’ products as compliant, so all equipment that was certified as ‘draft-2.0’ can immediately use the ‘802.11n’ Wi-Fi logo. The new version adds a number of features, but they are all optional. Those of us who questioned whether the final certification would render ‘draft-2.0’ devices obsolete have been proved wrong. Those who understandably insisted on waiting for a final specification can now move ahead. The accumulated experience with ‘draft-2.0’ equipment has allowed us to update this booklet – first published in September 2007 – to include experience gained from real-world deployments of ‘draft-2.0’ equipment, and to give a more concrete view of what 802.11n means for enterprise networking. We can already see, for instance: • 802.11n offers 5x to 7x the performance of 802.11a/g; • The indoor environment offers sufficient multipath that multi-spatial-stream transmission is the norm rather than the exception; and • MAC aggregation contributes significantly to throughput for many applications. It is still true that the performance benefits are only fully realized in a legacy-free environment, as even a few older (802.11a/b/g) clients on an access point can drastically reduce overall performance compared to a uniform 802.11n network. Fortunately, the PC vendors long ago standardized on ‘draft-2.0’ capabilities, and the installed base of enterprise clients now comprises a significant and growing percentage of high-performance PCs. Indeed, Aruba Networks’ university customers report that by the fall 2009 entry, the penetration of 802.11n-capable clients already approached the 50% mark. Also, the migration to 802.11n poses some challenges in network design. For best performance, LAN edge switch ports and cabling to the access points require an upgrade to Gigabit Ethernet – even more important now the 802.11n certification extends to higher data rates, and 802.11n overlays may be necessary if high-speed services are to be assured. But the concern that 802.11n access points were power-hungry – many on the market still exceed the 802.3af Power over Ethernet limits – is transient. From early 2009, all newly-designed dual-radio enterprise 802.11n access points are likely to comply with 802.3af. 802.11n migration strategies still require careful planning, but the constraints are becoming less restrictive. In this paper, we will explain the advanced technology introduced in the ‘final’ 802.11n certification, allowing enterprise network managers to understand its benefits and to plan their own upgrade strategies. Introduction Wi-Fi technology has carved a path of ever-increasing performance from the earliest pre-802.11 standards through 802.11b to 802.11a/g, with peak data rates rising from 2 Mbps to 54 Mbps. The latest set of innovations is a package known as 802.11n. Aruba Networks, Inc. 4 Designed for Speed Aruba White Paper We refer in this paper to two key documents that shape the industry, and will endeavor to maintain consistency in using these terms: • ‘IEEE 802.11n’ is a technical standard developed by the IEEE, (formerly named the Institute of Electrical and Electronic Engineers) 802.11 working group. The ‘draft 2.0’ milestone was attained in March 2007, and IEEE 802.11n was finally completed and ratified in September 2009. The earlier draft was the basis for the Wi-Fi Alliance ‘draft-2.0’ certification, but it was never formally published, and is now superseded by the ‘final’ 802.11n standard. • Meanwhile ‘802.11n’ is an interoperability certification developed by the Wi-Fi Alliance, a trade association of companies interested in promoting 802.11 products (‘Wi-Fi’ is a Wi-Fi Alliance brand). 802.11n is a certification awarded by the Wi-Fi Alliance to indicate a product has passed a set of tests that ensure it will inter-operate with other ‘802.11n’ products. For this certification, the Wi-Fi Alliance took parts of the IEEE 802.11n standard, and developed a series of tests involving a testbed of early 802.11n-compliant equipment: this certification tests only a subset of the full IEEE 802.11n functionality. • The history of ‘IEEE 802.11n draft 2.0’ and the ‘draft-2.0’ certification are included in this document because of the large amount of installed ‘draft-2.0’ equipment. Although this is fully interoperable with new equipment, as has been the case throughout the history of Wi-Fi, newer 802.11n products include options enabling better performance than ‘draft-2.0’ devices. Benefits for the enterprise 802.11n includes a number of complex technological advances which are explained in detail later in this paper. Many of these features have already demonstrated astonishing performance improvements in ‘draft-2.0’ equipment, and 802.11n enables even greater performance. • Increased capacity. 802.11n enables increased data rates, improving the usable data capacity of an access point from perhaps 15-20 Mbps with 802.11a/g to 150-300 Mbps (see appendix for more analysis). Draft-2.0 equipment already demonstrates a 5x – 7x improvement in application throughput over 802.11a/g, and ‘802.11n’ allows for a 50+% increase over draft-2.0. Given that this capacity will be spread over a number of simultaneous users, performance will match or exceed that of a wired 100 Mbps Ethernet connection, the standard for desktop connectivity. • Improved range. An 802.11g connection from AP to client can usefully extend up to 60 meters in open, unobstructed areas but this range drops to only 20 meters in office environments. 802.11n increases this through multiple-input, multiple-output (MIMO) techniques which involve driving multiple antennas on the access point and the client. The use of MIMO improves the connection data rate for a given range, and somewhat extends the range at the edge of a cell, useful if a network is designed for coverage rather than capacity. • More uniform ‘reliable’ coverage. Coverage in Wi-Fi networks can be spotty. A user may have a good signal in one location, but moving the client a short distance, stepping in front of it, or even opening a door across the room can affect the received signal strength, moving the client into a coverage ‘null’ and reducing performance.