Designed for Speed: Network Infrastructure in an 802.11N World Designed for Speed Aruba White Paper

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

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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

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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.

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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. One contributor to this issue is multipath propagation, and the best technology to counter this to date has been antenna diversity – nearly every Wi-Fi device sports two antennas, and switches between them because when one is in a multipath null, the other should still have a workable signal. The MIMO technology in 802.11n is extremely effective in reducing the effect of multipath nulls by allowing antennas to work together to recover the original signal: the effect is that the incidence and severity of signal nulls is greatly reduced, especially as a mobile client moves across the network. • Lower network costs. In a homogeneous 802.11n network, improved range and more reliable coverage can allow APs can be spaced further apart. This reduces costs in a number of ways: fewer APs, lower installation costs, possibly fewer LAN edge switch ports, and fewer outdoor APs to cover campus areas between buildings. But to date, it has been difficult to realize these gains because of the need to support legacy clients, and because it is usually more important to increase data rates than to extend range (most enterprise WLANs are designed for capacity rather than coverage).

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Benefits for the user End users benefit from higher data rates, higher throughput and more uniform coverage. For a given group of users on an access point, each will now have access to more bandwidth. The WLAN is no longer the limiting factor for application performance, overcoming a significant obstacle to un-wiring the desktop and leading eventually to the All-Wireless Workplace. One example of an application that benefits from 802.11n performance is video, both interactive and broadcast, which many organizations are beginning to use as an internal communications tool. Earlier 802.11 networks can support video, but its demands, at 1 to 4 Mbps per channel, stress 802.11a/g installations. 802.11n provides 5x the data performance, as well as incorporating features optimized for video.

Realizing these benefits with a mixed client base requires careful planning While the promise of 802.11n is great, some factors make it difficult for enterprises to realize its full benefits in the short-term.

The presence of legacy clients impairs 802.11n’s performance. The benefits outlined above are easy to achieve in a ‘greenfield’ network, where all APs and all clients are 802.11n-capable. However, very few deployments are completely new, particularly on the client side. As long as older clients exist, they will affect the performance of the network by connecting at much lower rates than 802.11n clients, effectively slowing APs to near-802.11a/g rates. Legacy devices are also unable to take advantage of the improvements in range and uniformity of coverage offered by 802.11n. If 802.11n APs are spaced farther apart to lower costs, legacy clients will likely run into more coverage problems than before.

Despite the limitations, 802.11n deployments are moving forward rapidly. A number of factors mitigate the picture painted above. Firstly, a rising percentage of the client base is 802.11n-capable. Draft-2.0 devices (equivalent to 802.11n in this context) approached 50% of the client population on some university campuses at the fall entry of 2009, and this percentage will only increase, as nearly all new PCs are now 802.11n capable. Secondly, the legacy poisoning effect is only significant at high AP loading, and the average offered traffic for most WLANs is very low: networks are dimensioned for infrequent peak loading conditions. Most of the time, an 802.11n client of an 802.11n AP will be able to see its full bandwidth.

Thirdly, network managers with specific performance needs, such as broadband video, add 802.11n APs as an additional WLAN overlay, limiting the clients on those APs to ensure they connect only at 802.11n rates. Naturally, there can be issues when users with legacy PCs find they can’t access these high-speed services, but the increased penetration of 802.11n over time is a positive trend.

Lastly, most WLAN vendors implement some kind of per-user bandwidth control, even though it is only a partial solution to the problem. As long as even a few legacy clients exist, the expected capacity improvements are limited.

When we wrote the first edition of this booklet, in mid-2007, the issue of mixed-client network performance was a significant concern. Fortunately, it has turned out to be less significant than we first feared, and many 802.11n networks are working well in mixed mode – even though with high traffic they do not deliver the data capacity of a pure 802.11n network. Time is our greatest ally, as the percentage of older PCs and other 802.11b/a/g clients fades.

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802.11n migration strategies for enterprises 802.11n includes a number of mechanisms for successful coexistence with legacy 802.11 clients and networks. As noted above, these operating modes impair the performance advantages of a greenfield 802.11n network, but they do offer complete support for older 802.11 clients. Thus, 802.11n access points can be installed before any 802.11n clients are present, and operate in 802.11a/b/g mode. It is also possible to mix 802.11n access points within an 802.11a/b/g network, replacing selected existing access points, or deploying new 802.11n access points as an overlay in parallel with an existing 802.11 network: a later section of this booklet provides more detail on migration strategies.

Network design with 802.11n For RF planning purposes, 802.11n differs from 802.11a/g in utilizing MIMO and the option of a 40 MHz channel, which provides approximately twice the data rate of a 20 MHz channel, but in doing so uses twice the RF spectrum. Because the 2.4 GHz band has a limited number of channels, the 40 MHz option is not usually deployed there, although the many other enhancements in 802.11n still provide significant performance improvements in a 20 MHz channel. However, the 40 MHz channel is popular in the 5 GHz band where channels are plentiful.

Most RF planning tools rely on modeling the reduction of signal strength over distance and across or through RF obstructions. For most purposes, this reduces to a simple calculation of dB loss at a given distance from the AP; knowing the transmit power, the data rate achievable at a given distance from the AP can be easily calculated. For instance, to plan an Aruba network, the required inputs are the dimensions of the floorplan, the minimum data rate desired and the cell overlap factor. Since the propagation characteristics for 802.11n signals are little different from 802.11a/g, existing RF planning tools such as this one need little modification. The output of the RF planning tool will be, as before, a set of suggested AP locations to satisfy the input parameters.

The new factors that must be considered are the number of antennas on both AP and client, and whether the design should account for legacy clients.

The type, placement and attitude of the antennas, particularly on the AP, can be important to 802.11n performance; most enterprise office deployments use the articulated captive antennas supplied on the AP.

Spacing the antennas by at least half a wavelength (6.25 cm for 2.4 GHz or 2.7 cm for 5.5 GHz) allows good MIMO capability: closer spacing may restrict performance.

Wired backhaul from APs & wired LAN design With 802.11a/g, the maximum data rate on the air is 54 Mbps, and actual, achievable data throughput tops out at about 26 Mbps for UDP and 21 Mbps for TCP traffic. Such a rate is easily supported on a 10/100 Mbps Ethernet connection for backhaul from the AP.

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802.11n rates, however, can move well beyond 100 Mbps, at least for peak traffic. A dual-radio, draft-2.0 access point with two spatial streams can generate up to 600 Mbps peak traffic, albeit half-duplex, and even this is too much for a single 100 Mbps Ethernet connection. The new 802.11n certification tests to three spatial streams, (IEEE 802.11n allows for up to four) which could generate 900 Mbps if both radios operate in 40 MHz channels. Thus, vendors have three options in designing APs:

• Continue with the current design, with one 10/100 Ethernet connection, realizing that at peak loads this will be the network bottleneck. In the short-term this may be acceptable: traffic peaks will be widely spaced since clients do not often connect at maximum data rates. • Use multiple 10/100 Ethernet connections to the AP. Two (or more) cable drops and two LAN edge switch ports are needed for each AP location. • Provide Gigabit Ethernet connections (10/100/1000). This comprehensively accommodates the traffic load, but may require that cabling is upgraded to Category 5e or Category 6, and that LAN edge switches provide GE ports. Thus far most infrastructure vendors, including Aruba, are providing this GE option.

Other areas of the LAN should be checked for traffic capacity but will probably not require upgrades. This is because the traffic on the upstream connection from the edge switch still represents the same aggregate number of users and applications as when clients were all wired.

Power consumption The power consumption of the early generations of draft-2.0 APs is greater than for 802.11a/g APs, exceeding 802.3af limits (12.95W maximum can be delivered to a Class 3 device under 802.11af, now properly termed clause 33 of updated 802.3-2005). This means the edge switches or in-line power injectors must support the unratified 802.3at standard. Alternatively, the AP must use a local power brick.

But the longer-term trend in 802.11n access points is to reduce power consumption, even as performance increases, and the requirement for 802.3at will not last for long. Already, the first generation of 802.11n-compliant silicon uses higher levels of integration (fewer chips in the AP), more sophisticated clock management and smaller-geometry processes to reduce power needs below the 802.3af limit, even for APs with three driven antennas.

Based on currently-available (late 2009) 802.11n technology, single-radio and dual-radio APs with up to three driven (transmitting) antenna chains should be within 802.3af limits. APs with four transmitting antennas per radio or more than two radio units will be borderline or into the 802.3at range, although over time we expect this limit to be attainable.

Implications for rogue APs and WIDS Most 802.11n enterprise infrastructure is multi-purpose, also providing RF monitoring and wireless intrusion detection (WIDS). 802.11n access points are important for identifying ‘rogue APs’ as employees adopt consumer products at home, find they work and bring them to the office. While most of these devices use beacons and other frames that can be recognized by 802.11a/g access points, some can be configured for ‘greeenfield’ operation. In this mode, the only way to reliably identify these devices is to use another 802.11n device such as an 802.11n AP in monitor mode.

To date, WIDS vendors have not found the need for enhancements to identify characteristics of 802.11n-specific attacks: as most attacks are at higher levels than the PHY, the current WIDS functionality has proven sufficient for draft-2.0 and should be adequate for 802.11n environments.

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Good-neighbor (or bad-neighbor) strategies 802.11n networks will often overlap with other 802.11 networks. In some cases the network manager will wish to cooperate with these neighbors, while in others it may be better to use non-coexisting modes, supporting higher throughput, and to ignore occasional interference. 802.11n allows a number of strategies.

Several forms of interference are possible:

• If an 802.11n network is operating in ‘greenfield’ mode, its transmissions will not be comprehensible to 802.11a/b/g APs and clients. Therefore, even though neighboring APs may be operating in the same RF channel, the transmissions from one cell (AP and clients) will appear as noise bursts to the other. This interference will work in both directions: 802.11n transmissions are likely to be affected as much as the legacy network. • An 802.11n network using a 40 MHz channel will have just this effect in the 5 GHz band. In 2.4 GHz, because the channel boundaries of 802.11n do not line up with the traditional channels used in 802.11b/g (channels 1, 6, 11), transmissions will inevitably overlap in frequency. This is one reason such 40 MHz operation at 2.4 GHz is not recommended. However, it is quite reasonable to use 802.11n in 20 MHz channels in the 2.4 GHz band. • An 802.11n network operating in mixed-mode is entirely compatible with overlapping 802.11a/b/g cells. In this mode, all transmissions are prefixed by a preamble that uses legacy modulation, allowing such networks to coexist. • Any 802.11n device, client or AP, can indicate that it is ’40 MHz intolerant’. Any device hearing this indicator must inform its AP, and the AP must indicate and execute a switch to 20 MHz operation.

Another optional mechanism, Phased Coexistence Operation (PCO) allows an AP to alternate between 20 MHz channel 40 MHz channels. The AP sets the Network Availability Vector (NAV) on the 20 MHz channels to inhibit transmissions for a time, during which it and whatever 40 MHz-capable clients it supports switch to 40 MHz operation.

Changes between ‘draft-2.0’ and ‘802.11n’ An enormous amount of work went into the IEEE 802.11 standard as it progressed from draft-2.0 (March 2007) to the final version (from draft-11.0). But none of the changes was significant enough to affect backwards- compatibility. Most of the changes were in just a few areas that were not part of the Wi-Fi Alliance’s draft-2.0 certification, such as beamforming.

This allowed the Wi-Fi Alliance to incorporate the changes from ‘draft-2.0’ to ‘802.11n’ as optional extensions. In fact, equipment that was certified as draft-2.0 is automatically qualified for the 802.11n label, with full backwards and forwards compatibility.

At the technical level, the new Wi-Fi Alliance 802.11n certification adds a number of features – as options – that were not part of the original certification. These features are:

• Short guard interval • Greenfield preamble • Transmitting with A-MPDU MAC aggregation • 40 MHz operation in the 2.4 GHz band with coexistence mechanisms • 40 MHz operation in the 5 GHz band • Transmitting in HT duplicate mode • Space-time block coding (2x1) • Transmitting up to 3 spatial streams

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A number of these options were already part of ‘draft-2.0’, and some of them were already tested in the receive direction (e.g. for A-MPDU, a device had to be able to receive correctly from a testbench device), but there was no option to test for correct transmission before.

Also, even though a number of these features were not part of the ‘draft-2.0’ certification, they were still available in commercial equipment. All enterprise WLAN vendors operate at 40 MHz channels in both 2.4 GHz and 5 GHz, for instance, and some products already support three spatial streams. The practical performance difference between ‘draft-2.0’ devices and ‘802.11n’ devices may not be uniformly significant.

Three other questions exercised the Wi-Fi Alliance as it considered the 802.11n certification:

• Whether to allow 40 MHz channels in the 2.4 GHz band. An AP using a 40 MHz channel in the 2.4 GHz band will interfere with any other 2.4 GHz equipment within range, not only Wi-Fi equipment but Bluetooth, DECT and other wireless protocols. Even with a 20 MHz channel, interference is common, but devices have the option to find unused parts of the band. The 40 MHz channel takes so much of the band that it is difficult for neighboring devices to avoid it, hence a debate over whether it should be allowed. The resolution is that a 40 MHz channel is tested and certified in the 2.4 GHz band, but only if coexistence mechanisms are also supported. The main coexistence mechanisms are the ’40 MHz intolerant’ bit, that can be broadcast by any 802.11 or other device, a periodic AP scan for other 802.11 activity, and the use of the CTS protection mechanism for frames such as ‘greenfield’ frames that would not be detected by older Wi-Fi equipment. • How to label products to inform the buyer of their capabilities and performance. Since the IEEE 802.11n standard allows a wide range of options, performance of 802.11n devices will vary. For instance, a device supporting two spatial streams in a 20 MHz channel and an 800 nsec guard interval would have a top rate of 130 Mbps, while three spatial streams in a 40 MHz channel and a 400 nsec guard interval can reach 450 Mbps. Both are 802.11n (see below for the debate on single SS APs, technically they are not recognized as 802.11n). The wide performance range caused considerable apprehension. Some equipment vendors were concerned that the 802.11n logo should not be devalued by lower-end devices: there should be a minimum threshold of performance, hence the decision to deny the 11n logo to single-stream APs. At the other extreme, many felt it important to allow higher-end vendors to market their equipment with a higher-performance label, and this gave rise to the tagline concept (see below). Still others complained that while the number of spatial streams is an indicator of performance, it is one of many factors and should not be singled out; while cynics suggested that the buying public will likely not read or understand taglines in any case, and the complexity of 802.11n features can never be successfully explained. The compromises resulting from this debate are that single- stream APs are tested but not given the ‘802.11n’ logo, while a set of three taglines defines different categories of equipment, based on functionality and, hopefully, predicting relative performance. • Should single-stream APs be allowed? This is a derivative of the previous question, but fiercely debated, as many vendors plan to build low-cost APs with only a single radio, but using 802.11n silicon. The argument why should these not be ‘802.11n’ centers on performance. Customers purchasing ‘802.11n’ labeled equipment will be expecting a level of performance – perhaps 100 Mbps. A single spatial stream AP is intrinsically limited to a top rate of 150 Mbps, whereas the second stream doubles this to 300 Mbps. Should the Wi-Fi Alliance mandate a minimum level of performance, implied by a 2 SS baseline certification? Many clients only support 1 SS, but that will not be important if they are dedicated devices such as Wi-Fi phones, where the higher performance will never be seen, so the question was whether to certify a single-stream AP. The final answer is that a single- stream AP will be certified, but it is supposed to carry an ‘802.11g’ logo and the tag line ‘with some n features’. Time will tell whether equipment vendors hew to these labeling guidelines, or find other ways to market their features and performance claims.

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• Here is the Wi-Fi Alliance lineup of taglines

Wi-Fi Alliance 802.11n certification taglines

Client Device Access Point

Capabilities Tagline Capabilities Tagline

• 1x1 client devices None • 1x1 access points With some n features (a special certification)

• Receive two-spatial streams Dual-stream n • Transmit & receive two Dual-stream n spatial streams • Transmit & receive AMPDU • Transmit & receive AMPDU • If operating at 5 GHz, support 40 MHz channel • If operating at 5 GHz, support 40 MHz channel • 2x2, 1x2 or 2x3 MIMO • 2x2 MIMO • Transmit STBC (2x1)

• Transmit & receive three Multi-stream n • Transmit & receive three or Multi-stream n spatial streams more spatial streams • Transmit & receive AMPDU • Transmit & receive AMPDU • If operating at 5 GHz, • If operating at 5 GHz, support 40 MHz channel support 40 MHz channel • 3x3, 3x4 or 4x4 MIMO • 3x3 or 4x4 MIMO • Transmit STBC (2x1)

The tagline is optional, displayed at the vendor’s discretion. It is quite possible for a device to be certified and display the ‘802.11n’ logo with a different combination of options, not lining up with any tagline. • The concept of ‘device classes’ developed for the ‘draft-2.0’ certification has effectively been abandoned. Originally there was provision for PC devices, the baseline class, and also for HH (handheld) and CE (consumer electronics) products. The rationale behind this was that not all equipment required all 802.11n features, so the full list of required features could be shortened for devices with defined purposes. Although there is some vestigial language in the 802.11n test plan for HH and CE device classes, it is unlikely that a test suite will ever be defined for them.

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Technology in 802.11n The IEEE 802.11n standard includes several key technologies and features. This section includes a brief technical treatment of all significant new technologies. The Wi-Fi Alliance 802.11n certification is a sub-set of the techniques in this section.

• High throughput PHY (physical layer): MIMO. The new PHY uses Orthogonal Frequency Division Multiplexing (OFDM) modulation with additional coding methods, preambles, multiple streams and beamforming. These features can support higher data rates, longer ranges, and a much larger range of data rates than earlier 802.11 standards. The MIMO technique that is synonymous with 802.11n belongs in this section. • High throughput PHY: 40 MHz channels. Two adjacent 20 MHz channels are combined to create a single 40 MHz channel. This simple technique, already used in some point-to-point bridges and consumer equipment, more than doubles the effective data rate under a given set of RF conditions. • Efficient MAC: MAC aggregation. Two MAC aggregation methods are supported to efficiently pack smaller packets into a larger frame. This reduces the number of frames on the air, and reduces the time lost to contention for the medium, improving overall throughput. • Efficient MAC: Block Acknowledgement. Particularly for streaming traffic such as video, a performance optimization where one acknowledgement can cover many transmitted frames, so an ack is no longer required for every frame. This technique was first introduced in 802.11e. • Power Saving: power save multi-poll. This is an extension of the APSD concept introduced in 802.11e.

Techniques for high-throughput PHY IEEE 802.11n marks a significant increase in complexity over previous versions of IEEE 802.11, giving it excellent performance, but making it increasingly difficult for the layman to understand. Several different techniques are used, and at times combined, to give the improved PHY performance. We will examine some of these techniques in this section, particularly those dealing with multiple antenna systems.

In legacy 802.11 equipment, the radio unit only drives one antenna at a time, and only receives on one antenna at a time, usually the same antenna. Although such equipment often has two antennas, the radio input and output is switched from one to the other, so only one at a time is carrying a signal. IEEE 802.11n allows multiple antennas to be used simultaneously for either or both the transmit and receive functions. Four distinct algorithms may be used, although not all at once:

• Maximum ratio combining is a receiver function, where signals received on multiple antennas, whether from one or a number of transmit antennas, can be combined to improve the signal-to-noise ratio. MRC is an antenna diversity technique that can increase range for a given data rate. • Space-time block coding can be used where there are multiple transmit antennas, regardless of the number of receive antennas. STBC is an antenna-diversity technique that improves the signal-to-noise ratio at the receiver, also increasing range for a given data rate. • Spatial division multiplexing is the technique most often associated with MIMO. Rather than increasing range, SDM sends different ‘spatial streams’ of data from each transmit antenna to each receive antenna. Since these streams carry different data, the overall data rate of the system is increased. Under good conditions, a MIMO system of two transmit and two receive antennas doubles the achievable data rate over a single- antenna system. • Transmit beamforming is a technique where signals sent to multiple transmit antennas can be phased such that the RF power at a targeted receive antenna is maximized. Although TxBF can be used with non-802.11n clients, it is most effective when the 802.11n receiver cooperates by returning a feedback message to allow the transmitter to optimize its beam.

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High Throughput PHY: Maximum ratio combining MRC is an established technique using multiple receive antennas to extract a better signal: it requires more than one receive antenna chain, but can work with a single transmit antenna. Since 802.11n equipment already features multiple independent receive chains, MRC is easily added to 802.11n products.

2 6 A B SNR (dB) 3

1 0

-3

-6 time

Figure 1

In the diagram above, each receive antenna would receive an identical signal under line-of-sight conditions. However, due to noise and multipath effects, one or both of the antennas will often receive an impaired signal. MRC allows the receiver to process both signals independently, and then combine them, weighted by the strength of each signal, to extract a more accurate replica of the transmitted data-stream than it could from a single antenna. In simple terms, when one antenna is in a ‘null’ and has a bad signal, the other is likely to have a good signal. The receiver will only suffer when all antennas have bad signals simultaneously. As the number of antennas increases, the probability of bad conditions for all of them simultaneously becomes progressively smaller.

In order for MRC (and MIMO techniques) to be effective, the receive antennas must receive different versions (distorted by noise & interference) of the original transmitted signal. Accomplishing this goal usually means separating the antennas by at least half a wavelength, in the order of 3 cm for a 5 GHz signal.

MRC is not explicitly covered in 802.11n because it can be implemented at the receiver only, with no changes at the transmit end. However, most 802.11n chip vendors now implement a form of receive diversity such as MRC.

Although receiver-feedback is not required, MRC works better if the RF channel can be ‘characterized’. In simple terms, this involves sending a known sequence of symbols from the available transmit antennas to calibrate the system. Because the receiver knows the symbols that were sent, it can determine the type of distortion introduced by the RF channel. 802.11n intrinsically provides for these known sequences in the long training fields (LTF). As MRC is only implemented in 802.11n receivers, this is part of the usual algorithm.

High Throughput PHY: space-time block coding STBC is another diversity technique for improving SNR, but is applied when the number of transmitting antenna chains exceeds the number of receive antennas. STBC uses coding to transmit different (but known) copies of the data-stream from different antennas; assuming the receiver knows the code; it will be able to extract the original data with fewer errors than when a single transmit antenna is used.

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The form of space time coding used in 802.11n is the Alamouti code. This spreads one spatial stream over two space time streams, taking a pair of data-stream bits, and performing operations on them in consecutive time intervals.

A B

Spatial stream Spatial stream Space time streams

MAC MAC etc Tx etc STBC coding Rx STBC decoding Tx

802.11n coding example: for two consecutive symbols, s1 and s2:

– In time interval t1, transmit s1 from antenna A and s2 from antenna B

– In time interval t2, transmit -s2 from antenna A and s1 from antenna B

Figure 2

When processing this sequence of two symbols from two space-time streams, the receiver is able to reconstitute the original data-stream even in the presence of channel noise and distortion. STBC uses the time dimension: consecutive symbol intervals contain the same pair of basic symbols, but modified according to the code. An efficient code such as Alamouti allows minimum complexity for the transmitter and receiver, but maximum improvement in SNR under normal channel impairments.

802.11n defines STBC codes to work with as many as 4 space-time streams. This technique can be used when the number of transmit antennas exceeds the number of receive antennas: it can also be used in conjunction with MRC. STBC requires both channel characterization at the receiver, and knowledge at the transmitter and receiver of the STBC code in use.

High Throughput PHY: Spatial division multiplexing 802.11n achieves its most dramatic improvements in data rate through the use of MIMO (Multiple Input, Multiple Output) spatial division multiplexing. SDM requires MIMO, specifically the transmitting and receiving stations must each have multiple RF chains with multiple antennas – it does not work where either station has only a single antenna chain. The diagram shows the simplest MIMO system, with two transmitting and two receiving antenna chains (2 spatial streams).

A B

MAC MAC etc Tx Rx etc Signal Processing Signal Processing Tx Rx

Figure 3

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Each antenna is connected to its own RF chain for transmit and receive. The baseband processing on the transmit side can synthesize different signals to send to each antenna, while at the receiver the signals from different antennas can be decoded individually. (We will simplify this explanation by showing only one direction of transmission, practical systems will transmit in both directions.)

Under normal, line of sight conditions, the receiving antennas all ‘hear’ the same signal from the transmitter. Even if the receiver uses sophisticated techniques to separate the signals heard at antennas 1 and 2, it is left with the same data. If the transmitter attempts to send different signals to antennas A and B, those signals will arrive simultaneously at the receiver, and will effectively interfere with each other. There is no way under these conditions to better the performance of a non-MIMO system: one might as well use only one antenna at each station. If noise or interference affects the signals unevenly, MRC or STBC techniques can restore it to a clear-channel line-of-sight condition, but in the absence of multipath, only one stream can be supported, and the upper bound on performance is a clear-channel single-stream.

A B

Figure 4

(Nearly all 802.11 stations built before 802.11n actually use two antennas. However, they do not use MIMO – the single radio unit switches from one antenna to the other, so only one is used at any time. Using two antennas in this way helps to negate the effects of multipath, as when one antenna is in a multipath ‘null’, the other is likely to have a better signal. It is generally reckoned that using antenna diversity in this way improves overall reception by perhaps 3-6 dB, although the effect is of course statistical. MIMO is different in that both antennas are driven by and receiving signals at all times, and those signals need not be identical.)

A B

Figure 5

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However, if there is sufficient RF distortion and especially multipath in the path, receiving antennas will see different signals from each transmit antenna. The transmit antenna radiates a signal over a broad arc, scattering and reflecting off various objects in the surrounding area. Each reflection entails a loss of signal power and a phase shift, and the longer the reflected path, the more delay is introduced relative to a line-of-sight signal. In the past, multipath has been the enemy of radio systems, as the receiver sees a dominant signal (usually line of sight), and all the multipath signals tend to interfere with this dominant signal, effectively acting as noise or interference and reducing the overall throughput of the system. Multipath effects also change over time, as objects in the path move, and movement of reflecting objects results in a Doppler shift of the frequency of the received signal, further complicating the mechanisms needed to counter multipath.

To understand how MIMO works, first consider the signal each receive antenna sees in a multipath environment.

b c a 2

A B

a Dominant signal at Rx antenna 1 is from path a. Path b, c and other multipath signals cause some b degradation to the dominant signal – a similar effect to higher background noise.

c Received power level time

Total throughput (per unit time) from transmitter to receiver = a b

Figure 6 In this example there are 3 multipath signals arriving at antenna 2. The strongest is signal a, and the information carried in this signal will be decoded. Other signals arrive at lower power levels, and they are time-shifted (or phase-shifted) compared to a, so it is likely they will degrade the overall signal-to-noise ratio associated with a.

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A2 2

A B A1 B1

Dominant signal at Rx antenna 1 is A1 from Tx antenna A (B1 signal and other multipath signals cause some degradation) B1

B2 Dominant signal at Rx antenna 2 is from Tx antenna B (A2 signal and Received power level other multipath signals cause A2 some degradation)

time

Total throughput (per unit time) from transmitter to receiver =

Figure 7

When multiple antennas are considered, however, MIMO offers considerable gains in throughput. The example above shows that each receive antenna receives its dominant signal from a different transmit antenna: receiver 1 tunes to transmitter A while receiver 2 uses transmitter B. When the system understands this, it can take advantage by transmitting different signals from each antenna, knowing each will be received with little interference from the other. Herein lies the genius of MIMO.

V11 U11

V21 U21 S1 in S1 out MAC MAC etc etc Signal Processing Signal Processing V12 U12

S2 in S2 out

V22 U22

Figure 8

(In practice the technique is more sophisticated, using RF channel characterization as explained earlier in this paper: it is not necessarily the case that individual signal paths can be drawn between pairs of transmit-signal receive antennas, but given that the nature of the cross coupling is known at the receiver, and that mathematical conditions for the channel are favorable, this is the overall effect.)

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The diagram above shows a more detailed explanation of MIMO implementation. At the transmit side, signal processing provides real and imaginary outputs S1in and S2in. These are then mixed with different weights V11 etc, before the signals are combined and delivered to the transmit antennas. A similar mixing function processes signals from the receive antennas using weights U11 etc. Provided the RF characteristics are known, the weights U11… can be calculated and set for optimum throughput, given the RF channel conditions.

The most favorable case would be where each transmit-receive pair operates with a completely independent RF path: a 2x2 (two transmitting and two receiving antennas) system will have double the throughput of a single-antenna 1x1 system, and a 3x3 configuration could extend to triple the throughput. IEEE 802.11n defines MIMO configurations from 2x1 to 4x4 antennas and up to four spatial streams (The Wi-Fi Alliance 802.11n certification only tests to three spatial streams).

MIMO is the most difficult aspect of 802.11n to understand: multipath (reflected RF between transmitter and receiver) is normally the enemy of performance, but with MIMO it can be used constructively. Line of sight normally gives the best performance, but with MIMO it provides just baseline data rates. (Note, however, that reflected signals are usually much weaker than primary, line-of-sight signals. Even though losing line-of-sight may allow use of more RF paths and hence the additive MIMO effect, the signal-to-noise ratio of each path may be considerably worse than previously. It is difficult to predict the relative weight of these two opposing effects.)

One key question in MIMO systems is how to tune the transmit signals at different antennas for optimum reception at the receiver. 802.11n offers different methods for this. With implicit feedback the MIMO transmitter characterizes signals from the receiver, and assumes that the channel is reciprocal – reflections and impairments operate equally in both directions. This is a reasonable approximation for most purposes, but better performance is achieved when the receiver sends explicit feedback messages to the transmitter; with these, the transmitter can accurately tune its signals for optimum reception and best signal to interference and noise ratios at the receiver.

High throughput PHY: Transmit beamforming (TxBF) Transmit beamforming is a technique that has been used in radio for many years. Beamforming allows an AP to ‘focus’ its transmission to a particular client in the direction of that client (and vice versa for a client with multiple antennas), allowing higher signal to noise ratios and hence higher data rates than would otherwise be the case.

A Normal isotropic B radiation pattern C

Directional pattern after beamforming

A A

B B

C C Received power level Transmitted power level Transmitted

time time Identical RF signals are transmitted from Transmit timing offsets are calculated to This results in constructive interference, each antenna, but very slightly offset in match the path delays of the different RF and a higher signal-to-noise ratio at the time (phase) beams, so all signals are directed at the receive antenna target in-phase

Figure 9

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By carefully controlling the time (or phase) of the signal transmitted from multiple antennas, it is possible to shape the overall pattern of the received signal, emulating a higher-gain, or directional antenna in the direction of the target. The same implicit and explicit feedback mechanisms used to characterize the MIMO channel allow beamforming.

In practice, beamforming may be used when MIMO with SDM is not effective. This is because beamforming aims to produce a single, coherent RF signal at the receiver, while SDM relies on multiple, independent signals. Also, contrasting with other ‘adaptive antenna’ or ‘beam steering’ technologies, the 802.11n, beam is not based on an indication of the direction of a client, but rather on the actual RF conditions at its antennas; hence the requirement below for explicit feedback messages.

IEEE 802.11n defines three modes for beamforming. The first is ‘implicit’, so named because it relies on the beamformee sending a sounding frame to the beamformer, which then assumes that the RF channel is reciprocal, using the weights derived from the received signals to set the transmitting beam pattern. The diagram below illustrates the implicit beamforming exchange.

Implicit feedback relies on the assumption that the RF channel is reciprocal – that the multipath reflections and attenuation from the beamformee to the beamformer are identical to the reverse direction. Experience shows this is a good assumption, and implicit beamforming benefits from simplicity – the beamformer does all the calculation, and there is no need to transmit channel state information from the beamformee to the beamformer.

But the implicit method has a significant disadvantage. Although the RF channel itself is reciprocal, the RF amplifiers in the stations’ transmit and receive antenna chains have different characteristics, over amplitude, phase and frequency. These effects are not accounted for in the implicit model; because the beamformer’s transmit chains are not used in sounding. They can be corrected with a calibration table, but this is often not practicable, as the effects change with time and temperature – they must be measured at the time of transmission, with cooperation between the beamformer and beamformee.

A B 1. Beamformer requests sounding frames

A B

1 2. Beamformee transmits sounding frames

A B

1 3. Beamformer uses channel assessment from sounding framse to form transmitted beam

Figure 10

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Thus, the implicit beamforming method is only useful if the channel state is measured before and during the transmissions - which makes it just as complicated as other forms of beamforming. This calibration effect explains why beamforming to legacy 802.11 clients is unsatisfactory in practice: legacy clients cannot participate in channel state information (CSI) calibration, so beamforming in this regard is imprecise and ineffective.

A B

1 1. Beamformer transmits sounding frames

A 2. Beamformee calculates correct B beamforming weights for the beamformer, and returns the V matrix 1

A B

1 3. Beamformer uses V matrix to form transmitted beam

Figure 11

The discussion above established that beamforming works best with explicit feedback messages from the client. IEEE 802.11n provides three methods, variations on an explicit beamforming procedure. In all cases, the beamformer first transmits sounding frames to characterize the RF channel and transmit and receive chains. On receipt of these frames, the beamformee sends information back to the beamformer, allowing it to set up the optimal beam.

In the first method, the beamformee returns raw channel state information. This is a set of complex factors indicating how each beamformee antenna hears each beamformer spatial stream. The beamformer then takes the CSI matrix and calculates the weights it must use to optimize the beam.

In the second and third methods of explicit beamforming, the beamformee calculates the ‘V’ matrix, the set of weights the transmitter should use to maximize its signal, or SNR. In the ‘explicit full matrix’ method, the full V matrix is returned to the beamformer, which can then use it for subsequent transmissions.

These are satisfactory models, except that the amount of data in the CSI or V matrix can be very large. It includes coordinates for each OFDM sub-carrier, per-transmit stream, per-receive antenna and with a reasonable degree of precision: this can be a large amount of data. In order to reduce the amount of data, IEEE 802.11n specifies an option where a matrix compression technique is used: the ‘compressed V matrix’ method. Compressing the V matrix reduces overhead on the air.

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All three IEEE 802.11n beamforming methods are complex, and as yet not well-characterized in practical networks. Experience with indoor WLANs indicates that spatial multiplexing is generally more effective than beamforming, but development organizations will continue to advance the technology.

MIMO, STBC, SDM & beamforming In this note, we use the term ‘MIMO’ (Multiple Input, Multiple Output) for any system where the transmitter has a number of antenna transmit chains (antennas that can be powered simultaneously with independent signals) and/or receiver chains. Many commentators use terms such as ‘SIMO’, ‘SISO’: we see these as degenerate forms of MIMO, but there is a brief explanation in the Appendix. A MIMO system can use a variety of techniques to improve range and/or data rate.

P1a

P1a 2 3 P1a

P1 A P1 B P1b

P1b P1b 1 C

SDM combined with antenna diversity. In this example, the path between the A-3 antenna pair has different RF characteristics from the other antenna pairs: it offers RF diversity, and carries one spatial stream. The other inter-antenna paths, B/C to 1/2, are not RF-isolated, so they cannot only be used for one further spatial stream. However, transmit beamforming or receive antenna diversity may be used to optimise this spatial stream. in this case a system with 3 transmit and 3 receive antennas nevertheless supports only 2 spatial streams.

Figure 12

The key technique associated with MIMO is ‘SDM’ (spatial division multiplexing). For our purposes, these are related terms: SDM provides a MIMO system with its superior data throughput. SDM allows transmission of multiple streams of data, enabling higher data throughput due to the multiple antenna chains.

The diagrams explain the difference between MIMO/SDM, STBC, MRC, and transmit beamforming as used in this document. While SDM is a multiplexing technique to increase overall data rate, STBC and MRC are diversity techniques that improve the signal to noise and interference ratio, SNR or SINR. These techniques can be combined under some conditions.

802.11n MIMO configurations and terminology A full specification of an 802.11n system includes a number of parameters. MIMO is often defined as MxN: e.g. 2x2, 3x3. In this case M refers to the number of transmit antennas configured, and N to the number of antennas at the receiver.

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P1a

P1a 2 P1a

P1 A P1 B P1b P1b P1b 1

MIMO with Spatial Diversity Multiplexing (SDM) and Space Time Block Coding (STBC). Independent paths between pairs of antennas allow data transmission in parallel: data packets (P1) are interleaved and mapped to different paths, where they may be encoded at a different data rate for each path, depending on RF conditions. The receiver interleaver re-builds the original packet.

P1 2

A P1 B P1

P1 1

Transmit beamforming. The transmitter sends a single stream of data, adjusting the signal from each antenna to ensure the optimal signal forms at the receiving antenna. This is used where there is little RF separation between the different inter-antenna paths, so SDM is not useful.

A P1 B P1 P1

Receive antenna spatial diversity. Working on only one transmitted signal, the receiver can use RF combining techniques on signals from different receive antennas to achieve higher signal-to-noise ratios and higher data rates.

Figure 13

Next the number of spatial streams must be specified. While for most access points this will be the same as the number of antennas, many clients, particularly where power consumption, processing or size is a concern, may have ‘asymmetric’ capabilities. IEEE 802.11n offers MIMO specifications up to 4x4, with 4 spatial streams, while the Wi-Fi Alliance 802.11n certifies up to 3 spatial streams. (The Wi-Fi Alliance ‘draft-2.0’ certification tested up to 2 spatial streams.)

Hierarchy of MIMO techniques MIMO as defined here includes diversity techniques (STBC for multiple transmit antennas and MRC for multiple receivers) and SDM where there are multiple parallel paths that can be used to increase data rate. The logical combination of these techniques in IEEE 802.11n is somewhat confusing: the diagram below shows the general relationship.

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The first decision is how many ‘spatial streams’ to use. A spatial stream in this context carries data that is independent of other spatial streams: spatial streams are combined by SDM techniques. The number of spatial streams can be no greater than the smaller of the number of transmit or receive antenna chains in the system, where the system covers a transmitting and a receiving unit. Most current 802.11n chipsets process two or three spatial streams.

Space-Time MAC MAC etc Spatial Space-Time Block Decoder Spatial etc Division Block Coder RF channel and Division Multiplexor Maximal Ratio Multiplexor Combiner

Original Split into Split again into NSTS Received signals from Re-built into NS Original data stream NS spatial streams space-time streams available antennas spatial streams data stream

Figure 14

After the number of spatial streams is defined, there may be ‘excess’ antennas at either the receiver or transmitter. For instance, take the case where an 802.11n access point sports three driven antennas, while its PC clients have two. Transmitting towards the client, the access point has an excess antenna. This transmitter chain can be used to provide better performance by implementing STBC. The STBC encoder can take one of the two spatial streams from the SDM block, and expand it to form three signals to drive the transmit antennas.

At the receiver, antenna chains are programmed for the STBC used at the transmitter, and may use whatever MRC capabilities the designer implements. In this case one would not expect any gains from MRC, as there are two antennas receiving two spatial streams. The combined system gains bandwidth from SDM, and some SNR gains from STBC: in practice, SNR may translate to a higher data-rate for a given range, or a longer range for a given data-rate, it’s the implementer’s choice.

The diagram below shows the reverse link, where the transmitter has two antenna chains and the receiver three. Now, it is not possible to use an STBC gain, as there are no ‘excess’ transmit antennas, but the MRC processing the receiver realizes from its excess antenna will provide approximately the same gain.

Space-Time MAC MAC etc Spatial Space-Time Block Decoder Spatial etc Division Block Coder RF channel and Division Multiplexor Maximal Ratio Multiplexor Combiner

Original Split into Split again into NSTS Received signals from Re-built into NS Original data stream NS spatial streams space-time streams available antennas spatial streams data stream

Figure 15

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High Throughput PHY: 40 MHz channels All earlier versions of 802.11 have used 20 MHz channels, defined in the 2.4 GHz and 5 GHz bands. Some vendors have increased throughput, particularly in point-to-point bridging applications by using two adjacent channels simultaneously, usually modulating each channel separately then combining the streams at the far end.

802.11n specifies operation in the same 20 MHz channels used by 802.11b/g in the 2.4 GHz and 802.11a in the 5 GHz bands, but adds a mode where a full 40 MHz wide channel can be used. As might be expected, this offers approximately twice the throughput of a 20 MHz channel. However, while in the 5 GHz band the channels are defined as pairs of existing 20 MHz channels, they do not line up with commonly-used 20 MHz channels in the 2.4 GHz band, as these channels are not adjacent. This means that when a 40 MHz channel is used in 2.4 GHz, it will interfere with at least one other 802.11b/g channel.

High Throughput PHY: Shorter guard interval The diagram shows how the guard interval is used in OFDM. 802.11n uses complex modulation techniques with OFDM, where blocks of input data are coded into a single OFDM symbol at RF.

a 2

A B c

Guard Inter-symbol interval interference 802.11n (also 802.11a/g) transmission is by a OFDM symbols (examples N, N+1, N+2). N N+1 N+2 Multipath increases delay spread at the receiver; the guard interval prevents b N N+1 N+2 inter-symbol interference. In this example, path b is within the guard interval while c causes inter-symbol interference. c N N+1 N+2 Received power level time

Previous 802.11 standards used a guard interval of 800 nsec. 802.11n adds an option for 400 nsec, negotiated between receiver and transmitter, for cases where the worst-caase multipath delay is low. (propagation in free-space, delay = distance x 0.3 metres/nsec, so 400 nsec is equivalent to 120 metres path difference.)

Figure 16

For best (least-error) decoding, the symbol must arrive at the receiver without any interference or noise. Previous sections of this document have shown how 802.11n uses MIMO to improve reception of multipath, but this only works symbol-by-symbol. Inter-symbol interference occurs when the delay between different RF paths to the receiver exceeds the guard interval, causing a reflection of the previous symbol to interfere with the strong signal from the current symbol: a form of self-interference.

The optional 400 nsec short guard interval in 802.11n can be used when the path difference between the fastest and slowest RF paths is less than that limit. Experience with draft-2.0 equipment has been encouraging: the shorter guard interval is usually attained in the indoor enterprise environment.

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High Throughput PHY: More subcarriers Through advances in implementation, it is now possible to squeeze more OFDM subcarriers in a 20 MHz or 40 MHz channel (each subcarrier allows more data to be transmitted over the RF channel).

Figure 17

The additional subcarriers effectively add bandwidth to the channel, allowing increased data rates for a given modulation type (see the section below on new modulation rates). The number of subcarriers is increased from 48 to 52 in a 20 MHz channel, and to 108 in a 40 MHz channel.

High Throughput PHY: New modulation rates Radio systems have to adapt to the signal and noise characteristics of the RF path, and they accomplish this by changing the modulation rate. For a given SNR (signal to noise ratio) the system will change the modulation rate to provide the best compromise between raw data rate and error rate: at any point, modulating for a higher data rate will increase the error rate and at some point the increased error rate will decrease the overall data throughput. This is a continuous decision-making process, the transmitter relying on feedback from the receiver about its SNR to adjust the transmit modulation.

While 802.11a and g specify 8 rates (6, 9, 12, 18, 24, 36, 48 and 54 Mbps), 802.11n provides many more: over 300. However, the basic set is of 8 rates:

Basic rates (Mbps) of 802.11n; 20 MHz channel; single stream; 400 nsec GI; equal modulation

MCS 0-7 7.2 14.4 21.7 28.9 43.3 57.8 65.0 72.2

(IEEE 802.11n indexes rates as ‘Modulation and Coding Schemes’ (MCS), and the MCS references are included in these tables for reference.)

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This is a set of rates for one spatial stream in a 20 MHz RF channel and with the 400 nsec guard interval and ‘equal modulation’ on all spatial streams. This basic set of rates is comparable to the 802.11b/g rates above: each rate is improved by about 20% (e.g. 18 to 21.7 Mbps) by using slightly wider bandwidth, more subcarriers and the shorter guard interval. The 72.2 Mbps rate has no equivalent in 802.11a/g: it uses 5/6 coding, a higher rate than the previous maximum of 3/4.

Other rates are generally derived as multiples of the basic rates above:

Rates (Mbps) of 802.11n 20 MHz channel; two streams; 400 nsec GI; equal modulation

MCS 8-15 14.4 28.9 43.3 57.8 86.7 115.6 130.0 144.4

Rates (Mbps) of 802.11n 40 MHz channel; single stream; 400 nsec GI; equal modulation

MCS 0-7 15.0 30.0 45.0 60.0 90.0 120.0 135.0 150.0

The 40 MHz channel allows slightly more than twice the data rate of a 20 MHz channel.

Rates (Mbps) of 802.11n 20 MHz channel; single stream; 800 nsec GI; equal modulation

MCS 0-7 6.5 13.0 19.5 26.0 39.0 52.0 58.5 65.0

The longer 800 nsec guard interval restricts data rates below the 400 nsec option.

For a given situation, the range of choices will be smaller than the tables above or below would indicate, because some of these factors are fixed for a given system:

• 20 MHz or 40 MHz channel. As discussed elsewhere in this paper, 40 MHz channels are widespread in the 5 GHz band. However, a 40 MHz channel will not often be feasible for an enterprise deployment in the 2.4 GHz band. • Spatial streams. As described above, the number of supported spatial streams cannot be larger than the number of antenna chains. But it can be smaller, because of silicon processing capabilities or because the client has fewer antenna chains than the access point. For instance, some 3x3 systems only support 2 spatial streams. Also, where there is insufficient RF path isolation between streams, even a 2x2:2SS system may not be able to support 2 diverse streams. • Guard interval. The guard interval is the time between OFDM symbols in the air. Normally it will be 800 nsec: the option is for a 400 nsec guard interval. In practice, the shorter guard interval can be used most of the time when indoors. • Convolutional coding. When data arrives at the PHY layer for transmission, it is scrambled and coded. This alters its spectral characteristics in order to achieve the best signal-to-noise ratio, and also includes built-in error correction, known as convolutional coding. The 802.11n standard includes BCC (block convolutional coding), as included in previous 802.11 standards, but also adds an option for LDPC (low density parity check) coding, which can improve effective throughput under certain RF conditions. • Modulation. All spatial streams may use the same (equal) modulation, or they may carry different (unequal) modulation and coding. An example might be where there are three spatial streams with good MIMO characteristics, but one stream has a high noise floor or a low signal level: under these conditions the weak stream would support a lower data rate than the other streams. In practice, current equipment implements equal modulation only.

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Data rate ranges (min & max) for various 802.11n system parameters

Equal Channel Guard Number Minimum Maximum Number MCS Modulation width (MHz) Interval (nsec) of streams PHY rate (Mbps) PHY rate (Mbps) of rates 0-7 YES 20 800 1 6.5 65 8

0-7 YES 20 400 1 7.2 72.2 8

8-15 YES 20 800 2 13 130 8

8-15 YES 20 400 2 14.4 144.4 8

16-23 YES 20 800 3 19.5 195 8

16-23 YES 20 400 3 21.7 216.7 8

24-31 YES 20 800 4 26 260 8

24-31 YES 20 400 4 28.9 288.9 8

0-7 YES 40 800 1 13.5 135 8

0-7 YES 40 400 1 15 150 8

8-15 YES 40 800 2 27 270 8

8-15 YES 40 400 2 30 300 8

16-23 YES 40 800 3 40.5 405 8

16-23 YES 40 400 3 45 450 8

24-31 YES 40 800 4 54 540 8

24-31 YES 40 400 4 60 600 8

Techniques to enhance the MAC

MAC layer enhancements: Frame aggregation A client (or AP) must contend for the medium (a ‘transmit opportunity’ on the air) with every frame it wishes to transmit. This results in contention, collisions on the medium and backoff delays that waste time that could be used to send traffic. 802.11n incorporates mechanisms to aggregate frames at stations, and thus reduce the number of contention events. Many tests have shown the effectiveness of reducing contention events in prior 802.11 standards. For instance, in 802.11g, a given configuration can send 26 Mbps of data using 1500 Byte frames, but when the frame length is reduced to 256 Bytes, throughput drops to 12 Mbps.

With MAC-layer aggregation, a station with a number of frames to send can opt to combine them into an aggregate frame (MAC MPDU). The resulting frame contains less header overhead than would be the case without aggregating, and because fewer, larger frames are sent, the contention time on the wireless medium is reduced.

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Two different mechanisms are provided for aggregation, known as Aggregated MSDU (A-MSDU) and Aggregated-MPDU (A-MPDU). The figure below shows the general architecture:

Applications

P1 P2 P3 MSDU (MAC Service Data Unit) P1 P2 P3

P1 P2 P3

MAC processing MAC processing MAC processing

MAC MAC MAC MAC P1 P2 P3 MPDU (MAC Protocol Data Unit) P1 P2 P3 header header header header

Aggregated MPDU format (A-MPDU) PHY layer Aggregated MPDU format (A-MPDU)

Figure 18

In the A-MSDU format, multiple frames from higher layers are combined and processed by the MAC layer as a single entity. Each original frame becomes a subframe within the aggregated MAC frame. Thus this method must be used for frames with the same source and destination, and only MSDUs of the same priority (access class, as in 802.11e) can be aggregated.

An alternative method, A-MPDU format, allows concatenation of MPDUs into an aggregate MAC frame. Each individual MPDU is encrypted and decrypted separately. Since MPDUs are packed together, this method cannot use the earlier 802.11 per-MPDU acknowledgement mechanism for unicast frames. A-MPDU must be used with the new Block Acknowledgement function of 802.11n.

In order to accommodate aggregated MAC frames, the maximum frame length accepted by the PHY is increased from 4095 in previous standards to 65535 in 802.11n.

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MAC layer enhancements: Multiple traffic ID block acknowledgement (MTBA) Earlier 802.11 standards demanded an ack frame for every unicast data frame transmitted. The new block ack feature allows a single ack frame to cover a range of received frames. This is particularly useful for streaming video and other high-speed transmissions, but when a frame is corrupted or lost, there will be a delay before a non-acknowledge is received and re-transmission can be accomplished: this is not often a problem with broadcast video, where re-transmission is often not feasible, given the time constraints of the media, but may be problematic for other real-time applications.

Block Ack covers many frames in one Ack

P1 header P3 header P2 header P1 header header Ack P1, P2, ...P4

Aggregate MPDU is a special case requiring Block Ack

P3 P2 P1 header header Ack P1, P2, ...P4

Figure 19

The format of the Block Ack is a bit-map to acknowledge each outstanding frame: it is based on a mechanism originally defined in 802.11e. The bit-map identifies specific frames not received, allowing selective retransmission of only those required.

MAC layer enhancements: Reduced inter-frame spacing (RIFS) When a station (client or AP) has a number of frames to send sequentially, it must pause between frames, seizing the medium before each transmission. This time on the air lost due to contention constitutes overhead for the overall network. Prior to 802.11n, the pause between frames transmitted by the same station was set at SIFS (single inter-frame spacing). 802.11n defines a smaller inter-frame spacing, RIFS (reduced inter-frame spacing). RIFS cannot be used between frames transmitted by different stations, and it can only be used when the station is transmitting in 802.11n HT (greenfield) mode, so all the other clients of the access point are designed for RIFS operation. It accomplishes similar goals to the MAC aggregation functions explained earlier, but arguably with less implementation complexity. 802.11n defines a RIFS interval as 2 usec, whereas SIFS is 16 usec.

MAC layer enhancements: Spatial multiplexing power save (SM power save) The basic 802.11n power save mode is based on the earlier 802.11 power save function. In this mode, the client notifies the AP of its power-save status (intention to sleep), then shuts down, only waking for DTIMs (Delivery Traffic Indication Maps) broadcast by the AP, while the AP buffers downlink traffic for sleeping stations between DTIMs.

Power save in 802.11n is enhanced for MIMO operation with SM power save mode. Since MIMO requires maintaining several receiver chains powered-up, standby power draw for MIMO devices may be considerably higher than for earlier 802.11 equipment.

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To mitigate this, a new provision in 802.11n allows a MIMO client to power-down all but one RF chain when in power save mode. When a client is in the ‘dynamic’ SM power-save state, the AP sends a wake-up frame, usually an RTS/CTS exchange, to give it time to activate the other antennas and RF chains. In ‘static’ mode, the client decides when to activate its full RF chains, regardless of traffic status.

MAC layer enhancements: Power save multi-poll (PSMP) PSMP is a new application of the existing APSD mechanism: in IEEE 802.11n it has the same extensions, scheduled- and unscheduled-PSMP (viz. S-APSD, U-APSD), even though scheduled-PSMP may never be implemented in a Wi-Fi Alliance certification.

AP 802.11n (HT)

Incoming to AP Data Multicast Data (wired side)

Data Multicast Data AP Buffers traf c for A Buffers traf c for A

A Sleep Trigger/Data Ack/Sleep

time

Figure 20

Unscheduled PSMP is the simpler mode: it is very similar to U-APSD, supporting both trigger-enabled and delivery-enabled options. Each sleep interval is considered and signaled independently, with the client determining when to wake to receive or transmit data. In the diagram above, the ‘sleep’ frame informs the AP that the client will stop receiving frames until further notice. When the client wishes to communicate, it sends a regular or trigger frame to the AP, and both parties then transmit whatever data is queued. At the end of this exchange, the client can indicate its return to sleep mode.

Scheduled PSMP is very similar to the S-APSD function introduced in 802.11e. The client requests a reservation for a T-Spec (traffic specification) from the AP, giving details of data rate, frame size, frame interval and access class (QoS priority) of the traffic streams it wishes to send and receive. The AP, once it has admitted this T-Spec, defines and a polling schedule for the client. Since there may be several clients using S-PSMP, the AP defines global PSMP SP (service period) for S-PSMP traffic, informing other stations they cannot transmit during these intervals. Once a PSMP SP is declared, the AP first transmits data in the downlink direction to all applicable S-PSMP clients during the DTT (downlink transmission time), then accepts traffic from clients during the UTT (uplink transmission time).

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A C

AP 802.11n PSMP 802.11n Not PSMP B

802.11n PSMP

Data Data Data Data Incoming to AP A B A B (wired side) Buffers traf c, no polling PSMP-DTT PSMP-UTT Buffers traf c, no polling PSMP-DTT PSMP-UTT AP Publish Data Data Data Data schedule A B A B

TSpec A request Data Data

TSpec B request Data Data

Sends & receives frames Transmit inhibited by NAV Sends & receives frames Transmit inhibited by NAV C

time

Figure 21

S-PSMP is a very efficient way to transmit streaming or periodic traffic over 802.11n: there is no contention for the medium, as everything depends on a published schedule. However, it is likely that S-PSMP will not be incorporated in Wi-Fi Alliance certifications or in products for some time.

Compatibility modes and legacy support in 802.11n One of the most difficult aspects of 802.11n is operation in the presence of earlier 802.11 technologies. Because it operates in the same bands as legacy 802.11, and is developed by the same standards bodies, and because there are already more than 100 million Wi-Fi devices in use world-wide, 802.11n is designed to support earlier forms of Wi-Fi. This includes:

• Support for legacy clients. 802.11a/b/g clients can connect to 802.11n APs. They will not be able to use 802.11n features, and their performance will be only marginally improved when connecting to an 802.11n AP; and • Awareness of neighboring or overlapping 802.11a/b/g networks. This is particularly important when using the new 40 MHz channel capability, which would impair the performance of such networks.

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As explained elsewhere in this note, working with legacy 802.11 clients and networks degrades the performance of 802.11n considerably, 802.11a/b/g clients will see very comparable performance whether they are using an 802.11a/b/g or 802.11n access point. In addition, working with legacy clients ‘poisons’ the 802.11n cell: its capacity will be severely degraded as soon as even one legacy client is present. This does not negate the need for legacy operation, but it does increase the urgency of upgrading the client population to 802.11n. The diagram below shows how introducing an 802.11a client into an 802.11n cell reduces the throughput (based on data rates alone: the overhead introduced by co-existence mechanisms will further reduce throughput).

A C 802.11n 2SS SDM 2SS SDM

AP 802.11n 52 Mbps 802.11n 104 Mbps

Data on A to AP 1KB @ 52 Mbps AP to B 1KB B to AP 1KB A to AP 1KB @ 52 Mbps B to AP 1KB A to AP 1KB @ 52 Mbps the air @ 104 Mbps @ 104 Mbps @ 104 Mbps 2N N N 2N N 2N time

Data transferred = 6KB, 3KB to/from A and 3KB to/from B ***(for a time interval of 9Nº).

A C 802.11n 2SS SDM 2SS SDM

AP 802.11n 52 Mbps 1SS 802.11n 117 Mbps B

802.11a 24 Mbps

Data on A to AP 1KB @ 52 Mbps AP to B 1KB AP to C 1KB @ 24 Mbps AP to A 1KB @ 52 Mbps the air @ 104 Mbps 2N N 4N 2N time

Data transferred = 4KB, 2KB to/from A, 1KB to/from B, 1KB to/from C ***(for a time interval of 9Nº).

*** In a time interval of 9N, ignoring contention time

Figure 22

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Greenfield, high-throughput and non-HT modes 802.11n defines three modes of client compatibility; the mode chosen depends on the extent of legacy 802.11a/b/g client support:

HT (High Throughput) Green eld format Key HT-GF-STF HT-LTF1 HT-SIG HT-LTF HT-LTF HT-LTF HT-LTF Data STF Short Training Field LTF Long Training Field SIG Signal GF Green eld Non-HT format L Legacy (e.g. pre-802.11n) L-STF L-LTF L-SIG Data HT High Throughput (e.g. 802.11n) Note: In 802.11n terminology, L = Non-HT

HT mixed format

L-STF L-LTF L-SIG HT-SIG HT-STF HT-LTF HT-LTF HT-LTF HT-LTF Data

Figure 23

• High Throughput (HT). In HT or Greenfield mode, the AP does not expect to connect to any legacy 802.11 clients, and indeed, assumes that there are none operating in the area. Apart from the beacon and some control frames at 20 MHz, no indication is available that will allow older devices to understand the remaining part of the transmission: it is all in HT-format. • Non-HT format. This is essentially legacy mode. The frames are all in 802.11a/g format (PHY and MAC), so they can be understood and decoded by 802.11a/b/g clients. This mode gives essentially no performance advantage over legacy networks, but offers full compatibility. Non-HT mode cannot be used with 40 MHz channels. • HT Mixed Format. As might be expected, this allows operation of 802.11n clients in HT mode, while legacy clients are fully supported. There is a full legacy preamble, then the option of using HT or legacy format afterwards. The preamble allows legacy clients to detect the transmission, acquire the carrier frequency and timing synchronization, and the L-SIG field allows them to estimate the length of the transmission and set their NAV.

Mixed mode is the normal setting for an 802.11n AP. It can be used in a 40 MHz channel, but to make it compatible with legacy clients, all broadcast and non-aggregated control frames are sent on the primary 20 MHz channel as defined in 802.11a/b/g, so as to be interoperable with those clients. And of course all transmissions to and from legacy clients must also be on the primary 20 MHz channel. Hence all 40 MHz operation in 802.11n is termed ‘20/40 MHz’.

Phased coexistence operation (PCO) Phased coexistence operation is designed to allow an 802.11n AP to support both 802.11n and 802.11a/g clients while being a good neighbor to older 802.11 APs operating in the area.

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A C

AP 802.11a 20 MHz 802.11n In 20 MHz or 40 MHz B

802.11a 20 MHz

Sends self-addressed Sends self-addressed Sends self-addressed Sends self-addressed Sends self-addressed CTS setting NAV to N CTS setting NAV to N CTS setting NAV to N CTS setting NAV to N CTS setting NAV to N in secondary 20 MHz in both 20 MHz in secondary 20 MHz in both 20 MHz in secondary 20 MHz AP channel channels channel channels channel

Primary 20 MHz AP and A and C AP and C AP and A and C AP and C AP and A and C channel exchange traf c C exchange traf c exchange traf c C exchange traf c exchange traf c in 802.11a T in 802.11n in 802.11a T in 802.11n in 802.11a 20 MHz channel S 40 MHz channel 20 MHz channel S 40 MHz channel 20 MHz channel (mixed mode) (HT green eld) mode (mixed mode) (HT green eld) mode (mixed mode)

Secondary 20 MHz channel C C C C C T No transmission T T No transmission T T No transmission S S S S S

N N N N N

time

Figure 24

As the diagram shows, the AP time-slices its cell, switching between 20 MHz, 802.11a/b/g compatible operation in the primary 20 MHz channel and full 40 MHz operation for 802.11n clients. To maintain order and provide the best throughput, two mechanisms are used.

Firstly, for 802.11n clients, the AP advertises a forthcoming switch of operation, allowing these clients to continue communicating in all time slices, whether 20 MHz or 40 MHz. Clearly, throughput is lower during 20 MHz time slices, but nevertheless, two-way transmission between the client and the AP can continue uninterrupted. While the diagram shows all time slices of equal length for simplicity, the AP can choose between a number of defined time intervals for each time slice.

For legacy 802.11 clients, only one of the modes of operation will be possible at any time: these clients will only operate in the primary 20 MHz channel. During time slices when the AP is operating in 40 MHz mode, these clients must be informed that they cannot transmit. This is achieved by the AP transmitting a self-addressed CTS (clear to send, see below) frame with a ‘duration’ value equal to the next time-slice duration. When clients hear this frame, they set their NAV (network allocation vector) to this value: under the rules of all 802.11 standards, they are not allowed to attempt transmission until this timer has expired.

Phased coexistence operation is also a good-neighbor policy because APs and clients in range of the AP will be able to hear the self-addressed CTS messages and set their NAV timers appropriately, avoiding one form of co-channel interference. However, 802.11a/b/g cells operating in range of a PCO cell will experience reduced capacity, as APs and clients will be inhibited from transmitting for a significant percentage of the time previously available.

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Other mechanisms for coexistence: RTS/CTS & CTS-to-self In this mode, the AP transmits extra CTS (clear to send) frames, so every data frame, whether from a client or the AP, is protected by a legacy CTS. When the traffic is generated by a station, it first sends an RTS (request to send) to the AP. The AP responds with a CTS frame in legacy format. The client is then free to transmit the data frame, while other clients in the same and neighboring cells set their NAV correctly so they do not transmit over the authorized frame, interfering with it. When the AP has traffic to send, it uses a self-addressed CTS frame to perform the same function.

AP 802.11n (HT) 20 MHz B

L = Legacy 802.11a/b/g mode) HT = High Throughput (802.11n mode) 802.11a (L) 20 MHz

AP CTS (HT) CTS (L) CTS (HT) Data frame (L) CTS (L) Data frame (L) CTS to self CTS to self

A RTS (HT) Sends data (HT) Sets NAV Receives data (HT)

Sets NAV Receives data (L) Sets NAV B

time

Figure 25

CTS-to-self makes the network a good neighbor to overlapping or adjacent legacy 802.11 networks. It also solves the ‘hidden node’ problem where different clients in a cell may not be able to hear each others’ transmissions, although, by definition they can all hear the AP and its CTS frames. However, the use of RTS/CTS further reduces the data throughput of the cell.

When 802.11b clients are detected (only applicable in the 2.4 GHz band), an option allows protection mechanisms using RTS/CTS in DSSS/CCK, the basic type of modulation used by 802.11b. This is very bandwidth-inefficient, but effective.

Other mechanisms for coexistence: 40 MHz-intolerant indication A mechanism is defined in 802.11n that allows an AP to advertise in its beacon that it prohibits 40 MHz operation in adjacent or overlapping cells. This indication can also be transmitted by a client, informing the AP that it must prohibit 40 MHz operation by all other members of its cell.

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Using 802.11n in the 2.4 GHz band The 2.4 GHz band presents special challenges for 802.11n. While in the 5 GHz band, usable channels have been defined by regulators to be 20 MHz apart, the spacing in 2.4 GHz is 5 MHz. Combined with the limited overall spectrum available in this band, this has led to customary usage where channels 1, 6 and 11 are used in a 3-channel plan (for the US: other regulators allow up to channel 13). This presents two difficulties when defining a 40 MHz channel in 2.4 GHz. First, the limited spectrum would allow only two channels in the band, one at 40 MHz and the other at 20 MHz. Second, it would not be possible to build the 40 MHz channel on two adjacent 20 MHz channels. This channel overlap is illustrated below:

Figure 26

This means that in practice, it is unlikely that 40 MHz channels will be used in the 2.4 GHz band. But it does not mean that 802.11n should not be used: there are performance improvements even when a 20 MHz channel is used, although the presence of legacy clients will reduce the realized benefits. This also illustrates one of the drivers for using 802.11n in handheld clients such as mobile Wi-Fi phones, and even in plug-in PC NIC cards where space is limited and designers are tempted to revert to 802.11g components for power consumption, size and cost reasons.

Allowing such devices to work in 802.11n mode avoids the need for the AP to fall into a coexistence mode, and greatly increase overall cell performance; even if a single antenna with a single transmit/receive chain is used. Designers of small clients such as cellphones are already attacking these challenges using techniques such as polarization and attitude diversity, but a PC with antennas embedded in the frame is likely to realize better MIMO performance than a small hand-held 802.11n device for some time to come. The Wi-Fi Alliance 802.11n certification has specific provision for single-antenna APs and clients.

20/40 MHz channel numbering in the 2.4 GHz band In 802.11n, the underlying 20 MHz channel is used as the basic concept for all channelization. Even when a 40 MHz channel is used, it is designated by a ‘primary’ or ‘control’ 20 MHz channel and a ‘secondary’ or ‘extension’ 20 MHz channel, contiguous to the primary channel. The direction of the offset for the secondary channel is designated +1 or -1. Thus, a 40 MHz channel in the 2.4 GHz band might use channels 1 and 5, and be designated 1+ (1,1), or 5- (5,-1), depending which is the primary channel. The next non-overlapping channel to the (1,1) 40 MHz channel is channel 10, as its energy extends down to 2447 MHz, a 5 MHz guard interval away from the upper bound of channel 5.

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Using 802.11n in the 5 GHz band Most 802.11n enterprise networks are deployed in the 5 GHz band, because of its generous spectrum allocation.

Figure 27

The diagram above shows the available 40 MHz channels in the various 5 GHz bands. Note that ‘Dynamic Frequency Selection’ a technique from 802.11h, is required for operation in some of these bands:

‘Draft-2.0’ silicon was generally, but not always, compliant with the DFS requirement: More recent 802.11n silicon is compliant. The large number of channels allows widespread deployment with 40 MHz channels, and also allows deployment options with parallel 802.11a and 802.11n APs at 5 GHz, allowing the 802.11n APs to load-balance legacy clients to the 802.11a cells, allowing them optimal performance at 802.11n.

The convention for numbering 40 MHz channels at 5 GHz is similar to the lower band. A 20/40 MHz channel might be designated 36+ or 40-, and would ‘bond’ channels 36 and 40. Other possible combinations are channels 44 and 48; 52 and 56, 60 and 64 in the UNII I and II bands, and others indicated in the diagram above.

Use of 20/40 MHz channels, coexistence and protection mechanisms The preceding chapter listed a large number of mechanisms that can, should or must be used when 802.11n is deployed. Both the IEEE and the Wi-Fi Alliance have gone to great lengths to ensure that new equipment does not interfere with existing Wi-Fi and other wireless devices, and that it can interoperate with earlier Wi-Fi equipment. The following is a practical guide rather than an exhaustive theoretical treatment – some less important features are omitted.

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20/40 MHz operation and fallback to 20 MHz There are two reasons to provide fallback mechanisms to control 40 MHz channels. Firstly, earlier Wi-Fi equipment cannot decode 40 MHz transmissions, so 40 MHz operation can cause interference to older devices in the vicinity. Since the newer devices can receive both 20 MHz and 40 MHz signals, they must provide accommodation where such interference is detected. Secondly, the 2.4 GHz band in particular is cramped and it may be difficult for other wireless devices to operate in the presence of a 40 MHz 802.11n AP. In both cases, whether interfering with Wi-Fi or other types of wireless, the remedy is for the access point to fall back to its primary 20 MHz channel. At that point, it may also need to use protection schemes to prevent its transmissions (those at HT rates) from interfering with older Wi-Fi devices. Several requirements in 802.11n restrict 40 MHz channel use.

When an AP first starts 40 MHz operation, it must scan all channels that may overlap with its proposed channels. It may continue scanning during subsequent operation: in enterprise WLANs, periodic scans are already used for ongoing RF management. If it finds an AP overlapping the secondary channel, it must fall back to 20 MHz operation. It should also move its primary channel to line up with existing APs.

In the 5 GHz band, an AP cannot operate if its secondary channel overlaps with another AP’s primary channel. The preferred solution is to match the other AP’s primary and secondary channels, and indeed this is the normal situation seen in enterprise 802.11n WLANs: at 5 GHz, a limited set of 40 MHz channels is used with either 100% overlap, including matching primary channels, or no overlap at all.

Also, if the AP (or any of its clients) detects a transmission with the ’40 MHz intolerant’ bit set, it must revert to a 20 MHz channel. The ‘intolerant’ bit must be in an 802.11 frame, of course, but it must be accepted even if broadcast from a nearby device that is not a client of the AP. In theory, Bluetooth devices could automatically and blindly generate such a frame, although no such behavior has been seen to date. More likely, a multi-mode Bluetooth/Wi-Fi device could generate a ’40 MHz intolerant’ frame from its Wi-Fi radio if it was using Bluetooth and detected a 40 MHz AP in the area. When used in the 2.4 GHz band, the ’40 MHz intolerant’ bit prohibits 40 MHz operation by any nearby AP anywhere in the band.

If an AP needs to move to a different channel set, it has an option to signal its clients ahead of the switch, using the ‘channel switch’ announcement. This accounts for clients in power-save and is designed so associations can be maintained in a non-disruptive switch (these mechanisms were originally developed in 802.11h for radar-avoidance in the 5 GHz band). Alternatively, the AP can switch operation to the new channels and the clients will eventually catch up and re-associate, or they may handover to a new AP.

Separate from the 20/40 MHz channel, there will be cases when an AP needs to operate (normally at 20 MHz) where either a client or another AP on the same channel is not 802.11n-capable. This is where protection mechanisms are used.

The most common scenario is to set the AP to ‘mixed mode’. Here, the beacon and parts of data frame preambles are transmitted at rates the legacy client can understand. This allows legacy clients to detect that there are 802.11 frames on the air, and the duration of the transmissions (setting the NAV for their backoff timers), even though they will not be able to decode the data contained in the frames.

Mixed mode is the most widely-used option for 802.11n APs. Its drawback, the overhead suffered from longer headers and slow transmissions to legacy clients, is often mitigated by a separate configuration preventing legacy clients from associating with the AP. This keeps the AP ‘pure-n’, and maintains the high rates, although there is still some extra overhead compared to greenfield operation. Of course, this only works in an overlay network where legacy clients can find an alternative AP, otherwise they can be orphaned.

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An alternative to mixed-mode with selective association is greenfield mode with protection mechanisms. The protection used is RTS/CTS or CTS-to-self, in the same way as an 802.11g AP provides protection for 802.11b clients.

While mixed mode provides inherent protection for 802.11a (5 GHz) and 802.11g (2.4 GHz) devices, 802.11b devices in 2.4 GHz require the ‘RTS/CTS’ or ‘CTS-to-self with DSSS/CCK’ protection mechanisms. This means 802.11b clients severely impact 802.11n performance.

When STBC is transmitted (it is a tested AP option in ‘802.11n’), 802.11n provides coexistence mechanisms when non-STBC clients are in the vicinity. The ‘dual beacon’ and ‘dual CTS’ options essentially transmit those frames twice, once with STBC and once without, so non-STBC devices can receive the beacons and set their NAV appropriately: the CTS protection mechanism works in the usual way.

New Wi-Fi alliance 802.11n certifications The new Wi-Fi Alliance 802.11n baseline certification includes two classifications only: APs and clients. Each certification includes mandatory and optional elements:

Test cases for Wi-Fi Alliance ‘802.11n’ certification (tests concerning authentication methods and WMM are omitted for clarity)

Access Points Clients Notes

Test Case Frequency Channel Mandatory/ Test Case Frequency Channel Mandatory/ Band Width Optional Band Width Optional (GHz) (MHz) and Tested (GHz) (MHz) and Tested

Basic Association 2.4 20 Mandatory Basic Association 2.4 20 Mandatory Includes check of in 802.11n in 802.11n IEs in the beacon. 5 20/40 5 20/40 Environment Environment

Ability to Receive 2.4 20 Mandatory Ability to Receive 2.4 20 Mandatory 1 and 2 Spatia 1 and 2 Spatial 5 20/40 5 20/40 Streams Streams

SM Power Save 2.4 20 Mandatory AP must sent RTS Operation before MIMO is used 5 (client in dynamic power save)

A-MDPU 2.4 20 Mandatory A-MDPU 2.4 20 Mandatory Verifies block Aggregation Aggregation when ack and AMPDU 5 20/40 5 20/40 when the AP is the STA is the received by the the Recipient Recipient AP (not WPA2- with and without enterprise) WPA2-PSK A-MSDU 2.4 20 Mandatory A-MSDU 2.4 20 Mandatory Verifies handling Aggregation when Aggregation when of long A-MSDU 5 5 20/40 the AP is the the STA is the frames. Recipient Recipient Overlapping 2.4 20 Mandatory Overlapping 2.4 20 Mandatory AP must detect, BSS – 2.4 GHz BSS – 2.4 GHz advertise and interoperate with overlapping AP. Overlapping 5 20 Mandatory Overlapping 5 20 Mandatory AP must detect, BSS – 5 GHz BSS – 5 GHz advertise and interoperate.

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Test cases for Wi-Fi Alliance ‘802.11n’ certification (tests concerning authentication methods and WMM are omitted for clarity)

Access Points Clients Notes

Test Case Frequency Channel Mandatory/ Test Case Frequency Channel Mandatory/ Band Width Optional Band Width Optional (GHz) (MHz) and Tested (GHz) (MHz) and Tested

HT-Greenfield 2.4 20 Optional and Greenfield 2.4 20 Optional and Tests Rx, Tx GF Operation Tested Operation Tested frames, using 5 5 protection with

Short GI 2.4 20 Optional and Short GI 2.4 20 Optional and Communicating Operation Tested Operation Tested with GI, non G 5 20/40 5 20/40 clients.

Overlapping BSS 5 20/40 Optional and Overlapping BSS 5 20/40 Optional and AP must sense the on the Extension Tested on the Extension Tested extension channel. Channel Channel

HT Duplicate 5 20/40 Optional and HT Duplicate 5 20/40 Optional and Both must receive Mode (MCS = 32) Tested Mode (MCS = 32) Tested HT duplicate mode. AP may transmit.

AP Concurrent 2.4 20 Optional and Concurrent Operation in 2.4 Tested operation. 5 20/40 and 5 GHz Frequency Band

AP RIFS Test 2.4 20 Mandatory STA RIFS Test 2.4 20 Mandatory AP can receive Receive Receive RIFS. 5 20/40 5 20/40

AP STBC 2.4 20 Optional and STBC Receive 2.4 20 Optional and AP can transmit Transmit Test Tested Test Tested 2x1 STBC. 5 20/40 5 20/40

A-MPDU 2.4 20 Optional and A-MPDU 2.4 20 Optional and AP can transmit Aggregation when Tested Aggregation when Tested A-MPDU 5 20/40 5 20/40 the AP is the the STA is the Transmitter Transmitter AP 20/40 MHz 2.4 20 Optional and STA 20/40 MHz 2.4 20 Optional and AP falls back to Coexistence Tested Coexistence Tested 20 MHz at the 5 20/40 5 20/40 slightest provocation.

Ability to Receive 2.4 20 Optional and Ability to Receive 2.4 20 Optional and AP can support 3 Spatial Streams Tested 3 Spatial Streams Tested 3 SS. 5 20/40 5 20/40

AP Transmitting 2.4 20 Optional and STAUT 2.4 20 Optional and AP respects client’s to STA using Tested Transmitting to Tested indication. 5 20/40 5 20/40 Supported AP using Number of Spatial Supported Streams Number of Spatial Streams

Disallow TKIP 2.4 20 Mandatory Disallow TKIP 2.4 20 Mandatory TKIP is not allowed with HT Rates with HT Rates in 802.11n. 5 20/40 5 20/40

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Migration strategies

Different paths to enterprise-wide 802.11n There are essentially three strategies to get an Enterprise WLAN to ‘802.11n’: greenfield, overlay and interspersed. The four questions the network designer must answer are where to place the new 802.11n APs, which channels to use, whether to remove some older 802.11a/b/g APs, and how to manage the client population.

As indicated earlier, a uniform 802.11n network will allow greater range and hence wider AP spacing than an 802.11a/b/g network. RF engineers insist that every site has unique conditions and usually avoid giving any figures for ‘typical’ deployments, but a rule of thumb in ‘carpeted’ enterprise buildings is that with design figures of 9-12 Mbps minimum rate and 150% cell overlap, and a ‘typical’ office layout and user population, 802.11a/g APs can be spaced at about 15-21 meters (50–70 feet). With 802.11n APs and 802.11a/b/g clients, we would expect the same guidelines to hold: legacy clients will not connect over greater ranges, or at higher data rates.

As noted throughout this paper, 802.11n offers an opportunity to increase usage of the 5 GHz band where channels are plentiful, interference is uncommon and 40 GHz channel usage is feasible. However, while 5 GHz 802.11n PC clients are readily available, specialized clients such as Wi-Fi phones, location tags or handheld bar-code readers will be confined to 2.4 GHz and 802.11b/g for at least one or two years, with few exceptions. For this reason, and because of the considerable installed base of 802.11b/g clients, enterprise network must continue to provide coverage in the 2.4 GHz band. Since WLAN vendors now marketing access points supporting two radios, this is quite feasible.

We recommend that when 802.11n coverage is planned, dual-radio access points should be used with one radio offering a 40 MHz channel at 5 GHz and the other a 20 MHz channel at 2.4 GHz. This will provide maximum performance and flexibility for a price premium over a dual radio 802.11a/b/g or a single radio 802.11n access point. One can expect high performance from the 5 GHz radio, while in the short-term the legacy clients in the 2.4 GHz band will likely inhibit overall data capacity.

Even though phones, location tags and other handheld clients will be slow to adopt 802.11n, it is important that they eventually achieve this goal. Even if they were only to support one antenna, one RF chain and a single spatial stream, the upgrade will prevent access points from dropping into legacy-compatible modes and hence will remove a serious inhibitor of performance as measured by cell capacity. In this respect it is more significant to eliminate 802.11b clients than 802.11a/g: an upgrade to 802.11n is a good time to remove 802.11b clients wherever possible.

Greenfield This strategy is not really a migration: it is building a network with new APs and new clients where there was no WLAN before. This offers an opportunity to start with all 802.11n coverage, and to ensure that clients are 802.11n-capable, or can be rapidly phased-out in favor of capable substitutes. The foremost concern with this strategy is that all clients must be 802.11n-capable; otherwise older clients will reduce the performance of the APs where they are connected.

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802.11n Access Points

802.11n coverage

Figure 28

Other aspects of a greenfield deployment include rogue detection and intrusion prevention. These functions should work successfully for all types of 802.11a/b/g/n devices.

A further consideration will be whether to cover one or both of the 2.4 GHz and 5 GHz bands. Enterprise-class PC 802.11n clients are likely to be 5 GHz-capable, so for these clients, coverage of 5 GHz only will be sufficient. However, the likelihood of requirements for 2.4 GHz-only client support (as discussed above) means it will be important for most enterprise deployments of this type to include dual-radio access points providing coverage of the 2.4 GHz band, even if this is legacy coverage.

Summary of ‘Greenfield’ enterprise WLAN recommendations:

• Confirm that 802.11n-capable clients are available for all current and foreseeable enterprise requirements. • RF planning can proceed with 802.11n range assumptions, allowing fewer APs, but only if all clients are capable of at least 2 spatial streams. Restrict existing APs to a subset of the available 5 GHz channels, and use the remainder for 802.11n. • Consider power to the APs, edge switch ports, etc. • Disable co-existence mechanisms such as PCO that support mixed-mode operation. • Enable good-neighbor mechanisms such as dual-CTS if there are other 802.11a APs in the same building, on the same channel. • Ignore ‘40 MHz-intolerant’ requests, or reject such clients. • Use dual-radio APs where the primary radio(s) is 802.11n at 5 GHz, while the second radio (can be 802.11a/b/g) runs in the 2.4 GHz band. • Remember that this scheme does not guarantee complete 2.4 GHz coverage for non-802.11n clients: there may be areas at the edge of cells where signal strength will not support an 802.11a/b/g signal, downlink or uplink.

In terms of 802.11n coexistence mechanisms, this ‘greenfield’ network can operate in ‘greenfield’ or ‘mixed’ mode: the latter is more bandwidth-efficient, but more tolerant of non-802.11n devices in the area.

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AP-overlay Here, the 802.11n network would be planned as if there were no existing network, for optimum placement of the new APs. The old and new network will be able to operate in parallel if care is taken with RF channel allocation, and when the new one is complete and all clients are 802.11n-capable, the old APs can be deinstalled or abandoned. For this scheme, it is important that the radio planning and management algorithm of the WLAN can manage 802.11a, b, g and 802.11n APs simultaneously.

802.11n Access

802.11a/g Access

802.11n

802.11a/g

Figure 29

Since 802.11n makes best use of the 5 GHz band (with 40 MHz channels), it makes an attractive overlay on an 802.11b/g network operating at 2.4 GHz: the original network can operate in the 2.4 GHz band (or even 2.4 GHz and several channels of the 5 GHz band) while the 802.11n network can use the remainder of the 5 GHz band. As for drawbacks, the 802.11n network is additive, so additional cabling, power and edge switch ports will probably be required (but it is likely these upgrades would eventually be needed, anyway).

Summary of ‘Overlay’ recommendations:

• Decide on the client migration strategy: it may be better to wait until more 802.11n clients are deployed before investing in 802.11n-capable infrastructure. • RF planning can proceed with 802.11n range assumptions, allowing fewer APs. • Use mixed mode or CTS-to-self when legacy devices are detected. • Consider power to the APs, edge switch ports, etc. • Eliminate 802.11b clients wherever possible. • Use single-radio APs if there is no future requirement for 2.4 GHz ‘Draft-2.0’ coverage. If there is an intent to eventually remove the older infrastructure, it is better to use dual-radio access points for the overlay so the second radio can be switched on later (becoming similar to the greenfield model over time, as the original infrastructure disappears).

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AP substitution In this model, existing APs are swapped one-for-one with new 802.11n APs. If every AP is replaced with 802.11n, the resulting network will have plenty of capacity – we estimate perhaps a three-fold improvement, depending on the mix of 802.11a/g and 802.11n clients – but will have more APs that would be strictly necessary.

802.11n Access 802.11a/g Access

802.11n 802.11a/g

Figure 30

A network of mixed 802.11n and 802.11a/g or even 802.11b APs can operate indefinitely: this is a possible solution where 802.11n is important for classifying rogue APs but not for capacity or range considerations. Note that even though APs can be swapped at a location, the new 802.11n AP may require different power and Ethernet connections.

Summary of ‘AP substitution’ recommendations:

• Decide on the client migration strategy: it may be better to wait until more 802.11n clients are deployed before investing in 802.11n-capable infrastructure. • Run an RF planning tool to determine which APs should be upgraded to 802.11n. • Enable mixed mode, dual-CTS, RTS/CTS, DSSS/CCK format. • Consider power to the APs, edge switch ports, etc. • Eliminate 802.11b clients wherever possible. • Use single-radio or dual-radio APs, depending on the existing infrastructure and plans for future migration.

Other considerations when planning an upgrade Points to bear in mind when planning an upgrade strategy:

• 802.11n APs will require Gigabit Ethernet connections if the backhaul link is not to become a bottleneck. This may in turn require upgrading cables to Category 5e or Category 6 cable (older Category 5 cable is not rated for Gigabit Ethernet). However, performance will be significantly increased even without the GE upgrade. • Also, GE ports must be available on LAN edge switches in wiring closets. • Since 802.3af power-over-Ethernet is not sufficient for many new 802.11n APs (assuming at least 2 antennas and RF chains at 5 GHz, a dual-radio AP will draw at least 15W), 802.3at should be supported by edge switches or mid-span POE equipment, or local power bricks may be required. (New silicon and AP technology will likely avoid this requirement in the future.)

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• AP mounting and positioning guidelines may differ. With earlier 802.11 APs, it was important to provide as clear a line of sight as possible between the AP and the client, so hiding APs behind metal pipes or air conditioning ducts was not recommended. Experience to date with 802.11n has shown that in-building deployment provides plenty of multipath opportunities for MIMO. • Opportunities to mount APs ‘in the workspace’ – on walls, partitions and on furniture, but not in the ceiling plenum space – increase with 802.11n, as the non-line-of-sight characteristics are favorable. This may mitigate some of the installation issues mentioned above: if GE drops already exist in some offices, 802.11n APs can be located to take advantage of these, and local power for the AP via a wall brick may be a reasonable option. Conclusion

This note has two goals: to explain the technology and features of the new 802.11n standard, and to examine its implications for the design of enterprise Wi-Fi networks, in terms of likely product development cycles.

It is clear that 802.11n represents a significant leap forward in technology and performance for enterprise networks. Uniform ‘greenfield’ 802.11n networks will be able to offer much higher capacity and longer range than current WLANs, and there are potential savings in terms of fewer access points to cover a given area.

However, this note has also identified several issues that require attention when adopting 802.11n in enterprise networks. These include infrastructure requirements such as LAN edge switch ports, cabling and power, and the installed base of 802.11a/b/g clients and WLANs that must be considered in any migration strategy. Experience with 802.11n draft-2.0 equipment has shown that eliminating 802.11b clients is often the most significant contributor to performance during 802.11n migration.

In the pages above, we have discussed the facts of 802.11n, along with our best estimates of future activity. When and how to migrate to 802.11n is an important decision: the information here is intended to assist the enterprise network manager in formulating an optimum upgrade strategy.

Perhaps the most significant aspect of the Wi-Fi Alliance’s new ‘802.11n’ certification is that it is fully backwards and forwards compatible: equipment certified to ‘draft-2.0’ is automatically eligible for the ‘802.11n’ label. Newer equipment may include more options and offer higher performance, and this is reflected in the different tag-lines the Wi-Fi Alliance has approved, to be used with the ‘802.11n’ logo. Concerns that early products could be made obsolete by the final certification have proven unfounded.

802.11n offers very real and exciting benefits: it will eventually change the way we build and operate enterprise wireless LANs. Already, organizations with particular needs have found uses for the increased bandwidth. Universities in particular are beginning to take the video-over-IP networks they have already installed in recent years, and delivering both in-house video content and live TV signals over WLANs in residence halls and across campus.

As a consequence, organizations of all types are embarking on a cycle to retire wired LAN switch ports, consolidate and de-commission switches, reducing ongoing maintenance expenditure while recycling a fraction of the savings into a stronger 802.11n WLAN. This ‘right-sizing’ is a virtuous spiral, as denser 802.11n coverage in turn reduces the requirement for wired Ethernet ports. But it is only possible because the increased bandwidth capabilities of 802.11n allow the WLAN to take on nearly all traffic previously handled over wires – the gateway to eventual adoption of the All-Wireless Workplace.

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Appendix

Note on expected ‘real-world’ cell capacity with 802.11n

Whereas an 802.11a/g access point can support a top data rate of 54 Mbps, 802.11n can stretch to 600 Mbps (with four spatial stream configurations, although these are not yet available). This is important, as a Wi-Fi cell (the area covered by a single AP) shares all bandwidth: it’s very similar to the original wired Ethernet, where all stations shared a single segment of cable, and had to co-ordinate transmissions to avoid collisions. Three effects limit the effective capacity of an 802.11 cell (examples are taken from 802.11g, but should be equivalent form 802.11n):

• Actual data rates. While specifications quote maximum data rates, these are only achieved under the best radio conditions. The distance from AP to client, RF obstructions such as walls, furniture, people, and interfering RF transmissions all limit the achievable data rate. Thus, a client 20 meters from the AP in an office environment might only support a data rate of 12 Mbps rather than the advertised peak of 54 Mbps. If all clients connect at this rate, the raw capacity of the cell becomes 12 Mbps. • Contention loss. Since the wireless medium is shared, stations wishing to send data must contend for temporary control of the medium. This contention takes time that is then not available for data traffic, lowering the capacity of the cell. Contention depends on the number of clients and the length of frames sent: the more clients and the shorter the frame, the less the effective capacity of the cell. For instance, while an 802.11g cell may achieve throughput of 26 Mbps with 1500 Byte frames, capacity drops to around 12 Mbps with 256 Byte frames. The MAC aggregation function in 802.11n should reduce losses due to contention, but it will not be effective for all types of traffic. • Legacy support. All 802.11 systems are designed to support older 802.11 clients. Thus an 802.11g AP will support 802.11b clients. However, there is a cost associated with this. When even a single legacy client joins a cell, all other clients and the AP must indicate traffic is present, using data rates the older client can understand. For 802.11g/b compatibility, this means using RTS/CTS at slower rates, considerably increasing overhead and decreasing cell capacity. And of course, when the legacy station transmits or receives, it does so at lower data rates, reducing the effective capacity of the cell. Client design for 802.11n is challenging, as it is difficult on many devices to find the space to mount extra antennas, the extra RF chains and processing require more board area and power, and of course 802.11n silicon will command a price premium for several years over 802.11a/g silicon. Not least, NIC cards and embedded clients are complex to design: it was only a year or two ago that Wi-Fi phones moved from 802.11b to 802.11g, and few are yet 5 GHz-capable.

All these effects serve to reduce the effective capacity of a cell. Thus, while 802.11n advertises rates to 600 Mbps, the expected capacity of an 802.11n cell is between 100 and 200 Mbs, and it could certainly be less if clients connect over long distances, transmit short frames, or there are legacy 802.11a/b/g clients present. However, this is still an increase of 5x to 7x over 802.11a/g technology.

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Here are our estimates of ‘reasonable’ expectations for data rates in an enterprise 802.11n deployment (this assumes ‘greenfield’ deployment with no legacy 802.11a/b/g clients or APs). Interpret this table as a comparison of the achievable data rate of 802.11n with that of 802.11a or 802.11g at the same distance from the access point. Alternatively, it can be read as the capacity of a cell (with all-802.11n clients) when compared with a cell of the same radius.

Channel width Guard Interval Range of PHY Performance relative Spatial Streams (MHz) (nsec) rates (MHz) to 802.11a/g

1 (2x1, 3x1) 20 800 7.2 – 72.2 1x – 1.5x

1 (2x1, 3x1) 40 800 13.5 – 135 1.5x – 2x

2 (2x2) 40 800 27 – 270 2x – 4x

3 (3x3) 40 800 40.5 – 405 4x – 6x

In addition to the PHY effects above, the many MAC enhancements in 802.11n will increase the throughput, and hence the capacity of a cell. Analysis of these effects is challenging, as they are extremely dependent on the data patterns. Some of the enhancements are aimed specifically at streaming media such as video, but there has been speculation that they may actually reduce the throughput of other types of traffic such as voice or file transfers.

Forms of MIMO

There are already more than enough terms and acronyms in this document. We have deliberately avoided introducing more than were necessary to explain 802.11n. But the reader may have encountered ‘SIMO’ and other terms. The figure below shows that these are just degenerate forms of MIMO with a single antenna chain at one or the other end of the connection:

SISO Tx Rx Single input, single output

MISO Tx Rx Multiple input, single output Tx

SIMO Rx Tx Single input, multiple output Rx

MIMO Tx Rx Multiple input, multiple output Tx Rx

Figure 31

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Channel estimation

As explained above, all the techniques used for MIMO require the receiver to estimate the channel characteristics. It does this by receiving a sequence of known signals. The diagram below explains how this is done, both in general matrix algebra and with an example. This is for a 2x2 system.

TA 2 R2 hA2 = 0.7

A hB2 = 0.5 B TB R1 hA1 = 0.3 h = 0.9 B1 1

To determine the channel characteristics, known symbols are sent from antennas A and B. If enough known symbols are sent, it is possible to construct these equations:

R1 = [ TA x hA1 ] + [ TB x hB1 ] Algebraic form R1 = 0.3TA + 0.9TB Numerical example R2 = [ TA x hA2 ] + [ TB x hB2 ] R2 = 0.7TA + 0.5TB

Which allows the inverse relationship to be determined:

TA = [ R1 x qA1 ] + [ R2 x qA2 ] Algebraic form T1 = -1.04RA + 1.88RB Numerical example TB = [ R1 x qB1 ] + [ R2 x qB2 ] T2 = 1.46RA + -0.63RB

Now, for every received symbol (RA and RB in this case), the original transmitted symbol can be determined.

Figure 32

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Glossary of terms used in this note

802.11 is replete with acronyms, and 802.11n has contributed more than its share to the lexicon. The following is a subset of terms used in the standard, but should cover all terms used above.

A-MPDU Aggregate MAC protocol data unit

A-MSDU Aggregate MAC service data unit

AP Access point

CTS Clear to send

DTT Downlink transmission time (Power-save multi-poll)

HT High throughput

HT-GF-STS High throughput greenfield short training symbol

HT-STF High throughput long training field

HT-SIG High throughput signal field

HT-STF High throughput short training field

LDPC Low density parity check

L-LTF Non-high throughput (= legacy) long training field

L-SIG Non-high throughput (= legacy) signal field

L-STF Non-high throughput (= legacy) short training field

LTF Long training field

MC Mobility Controller (an Aruba product controlling access points)

MCS Modulation coding scheme

MIMO Multiple input, multiple output

MISO Multiple input, single output

MRQ Modulation coding scheme request

MTBA Multiple traffic ID block acknowledgement

NAV Network Allocation Value

PCO Phased coexistence operation

PSMP Power save multi-poll

RF Radio frequency

RIFS Reduced inter-frame spacing

RTS Request to send

SISO Single input, single output

SM Spatial multiplexing

SNR Signal to noise ratio

STBC Space time block code

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STBC/SM Space time block code/spatial multiplexing

TRQ Training request

TxBF Transmit beamforming

UTT Uplink transmission time (Power save multi-poll)

WLAN Wireless Local Area Network

References

The IEEE is the fount of 802.11 standards. While it does not publish standards until final ratification, much useful information may be found here: http://grouper.ieee.org/groups/802/11/. In particular, there is a current estimate of schedules (http://www.ieee802.org/11/Reports/802.11_Timelines.htm).

The information in this paper was taken from and checked against the IEEE document ‘P802.11n Draft 11.0’.

The Wi-Fi Alliance certification is ‘802.11n’. The WFA web site is here: http://wi-fi.org/

About Aruba Networks, Inc.

Aruba Networks is a leading provider of next-generation network access solutions for the mobile enterprise. The company’s Mobile Virtual Enterprise (MOVE) architecture unifies wired and wireless network infrastructures into one seamless access solution for corporate headquarters, mobile business professionals, remote workers and guests. This unified approach to access networks enables IT organizations and users to securely address the Bring Your Own Device (BYOD) phenomenon, dramatically improving productivity and lowering capital and operational costs.

Listed on the NASDAQ and Russell 2000® Index, Aruba is based in Sunnyvale, California, and has operations throughout the Americas, Europe, Middle East, Africa and Asia Pacific regions. To learn more, visit Aruba at http://www.arubanetworks.com. For real-time news updates follow Aruba on Twitter and Facebook, and for the latest technical discussions on mobility and Aruba products visit Airheads Social at http://community. arubanetworks.com.

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