Analysis: 802.11n -- The n Factor

In most enterprises, Ethernet plus a little Wi-Fi equals network access. Sure, industries like healthcare and education thrive on WLANs, but more commonly, Wi-Fi is a convenience, deployed to provide mobile access to information in public spaces.

Proponents of the emerging 802.11n standard say that dynamic is poised to change. They argue that the speed and range improvements in 11n will let organizations relegate wired Ethernet to the core and distribution layers of the network. Wi-Fi will emerge as the main access medium. And of course, this newfound mobility will forever alter the way people work, increasing productivity.

If Ethernet is a conventional org structure, they say, Wi-Fi is the matrix. The big question for IT: Is this a compelling vision of the future or just another round of industry hype? Upward of 200 million Wi-Fi chipsets were sold in 2007, according to InStat. If you care to bet that those numbers will decline in coming years, you'll find plenty of action. Meanwhile, the 802.11n standard will likely be ratified by the Institute of Electrical and Electronics Engineers in mid- to late 2008, and 11n will soon make up the vast majority of Wi-Fi chipset and system sales.

You do the math.Still, time marches on. At issue for most enterprises is not whether they'll deploy 802.11n, but when and how.

Standards Watch

Most organizations won't consider adopting a network technology before standards are fully ratified. Experienced network professionals are content waiting for second-generation 802.11n offerings, thanks very much, while launching a few pilots.

This time, though, first adopters looking for competitive advantage won't be putting too much on the line—there are important factors in play here that mitigate risk. First, while early phases of 802.11n standards development produced plenty of design debate, it's been more than a year since most vendors have coalesced around a framework developed by a consortium of wireless chip developers and equipment makers. While the initial design on which the 11n Draft 1 specification was developed had several notable deficiencies, the Draft 2 spec is reasonably solid. The IEEE has responded to thousands of formal comments related to both drafts, and any changes implemented between now and the final standard will likely be addressable through firmware updates. Even the worst-case scenario, in which a change is made that can't be corrected in firmware, won't render draft products inoperable with those based on the final spec. At worst, we'll see minor problems related to performance or battery efficiency.

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The second factor mitigating risk is the Wi-Fi Alliance's decision to move forward with certification based on the Draft 2.0 specification. More than 100 products, mostly for small and home offices, have been ratified. While it's true that the Wi-Fi Alliance certifies only a subset of standards interoperability, it provides a de facto seal of approval that carries great weight with consumers, enterprise IT professionals, and equipment manufacturers.
Impact Assessment
Click to enlarge in another windowFinally, because the consumer market has been flooded with 802.11n Draft 2 offerings, momentum argues against changes that would diminish interoperability. Now that prices for many consumer-grade 802.11n Draft 2 products have fallen below the magic $100 barrier, few will choose access points based on earlier standards.

The Client Factor

History has taught us that there's a powerful relationship between the wireless technology people install in their homes and the systems installed in enterprises. The two must be compatible. For many years, we've seen one simple principle predict trends in enterprise Wi-Fi adoption: Follow the client.While early WLANs relied on PC Card radios, it wasn't until Wi-Fi was embedded into client devices that the market began to take off. Users don't want to deal with add-on network interfaces. More important, by embedding wireless cards in mobile devices, we achieve cost economies while also enhancing overall system performance and efficiency.

Intel's Centrino is most emblematic of this trend, and the chipmaker has already delivered a Centrino 11n offering. In fact, if your organization buys higher-end enterprise-class notebook computers for employees, it's likely that these systems will ship with integrated 802.11n support. However, if you try to save money by buying consumer notebooks, even high-end ones, you should explicitly request 11n support—assuming it's even available.

While adoption patterns vary, three-year replacement cycles for notebooks are common. Thus, it's unlikely that most organizations will have integrated 11n support in more than half of clients before 2009.This phased changeover makes sense, in part because there's a natural aversion to pre-standard products, and in part because design and implementation of enterprise- class 802.11n is complex. As far as smartphones and wireless VoIP devices are concerned, 11n is still on the drawing board.Sea ChangeCisco Systems' decision to jump in early with a Draft 2.0-compliant access point—the Cisco 1250—has helped legitimize the 11n market. Second-tier vendors such as Meru Networks and Colubris Networks also have moved aggressively into 11n, hoping to appeal to early adopters. But IT professionals must understand that supporting 11n isn't nearly as simple as slapping a new radio inside an old AP model. A range of design and implementation challenges exist, in many cases forcing network engineers to re-examine their approaches to wireless deployment.

The central opportunity—and challenge—associated with 11n is higher performance. With APs capable of offering at least five times greater throughput, users are sure to be impressed ... provided your overall architecture can support the additional traffic. Older designs may need to be reconsidered, and many vendors are making fundamental changes to their system architectures.

At the simplest level, access points will now need faster wired backhaul interfaces. Aggregate wireless performance on a dual-band, dual-radio 11n AP will easily exceed 100 Mbps, so most enterprise APs will benefit from Gigabit Ethernet uplinks. To take full advantage of improved performance, you may need to upgrade access-layer switches. Even greater challenges exist with respect to powering these new 11n APs. Current draws exceed those in 802.11af power-over-Ethernet AP designs, no matter what vendors claim.

Wireless controller capacity is also a significant concern, prompting a number of vendors to introduce new multitier wireless switches. While many enterprises streamline deployment and management by centralizing controller functionality at the core layer—Cisco's WISM-equipped Catalyst 6500 is a prime example— the higher performance of 11n may lead some network architects to reconsider a design in which all traffic is carried back to the core for processing. Many vendors will simply beef up their centralized controllers to support higher speeds, but some, including Colubris, Meru, Nortel, and Trapeze, are touting new enterprise Wi-Fi architectures that distribute WLAN controller functionality out to the distribution layer, even to the edge switch or access point.

Now, while such a design may have merit, changes in system architecture also introduce risk. Vendors may make fundamental changes in how and where packets are processed, for example. In the centralized-controller model, identification and remediation of bugs is much easier than is the case where controller intelligence is distributed. As the architecture becomes more complex, the potential for problems increases. In the end, it's likely that even vendors advancing a distributed design will need to also support higher speed centralized controllers for customers that prefer the relative simplicity of such setups.While controller capacity and wireless/wired integration challenges are notable, even more thorny issues relate to achieving interoperability between 802.11n and older protocols like 11g without sacrificing performance. Our coexistence testing of Draft 2.0 802.11n and 802.11g, both running at 2.4 GHz, suggests that aggregate throughput decreases by about 40%.

Vendors like Meru and Extricom, which employ sophisticated scheduling algorithms to allocate capacity to clients, will likely do a good job managing 11g/11n coexistence at 2.4 GHz. However, most organizations will find that 11n provides an ideal opportunity for segmentation: Legacy traffic can continue to operate at 2.4 GHz, while 11n traffic is moved to 5 GHz. That strategy lets IT more easily leverage the 40-MHz channels supported by 11n while still taking advantage of the much greater 5-GHz capacity.

In the past, organizations that were contemplating segmentation of traffic between 2.4-GHz 802.11g and 5-GHz 802.11a had to deal with the diminished transmission range at 5 GHz, forcing many to deploy expensive and complex microcell architectures. Because 802.11n significantly improves Wi-Fi's transmission range, 5-GHz deployments are now much more practical. In fact, our early testing suggests that 5-GHz 802.11n performance far exceeds that of 2.4-GHz 802.11g at every distance up to 130 feet. It's conceivable that many organizations will find it possible to upgrade their large-cell 802.11g deployments to dual-band 802.11n without having to relocate access points.

Performance Expectations

So what do you get for all this network rejiggering? The initial 802.11n design spec called for throughput in excess of 100 Mbps, three to four times what you'll achieve with current 11a and 11g. Published performance tests of consumer-grade 802.11n products conducted by Craig Mathias of the Farpoint Group show that most products approach, or even exceed, that threshold. In the enterprise market, Cisco has released early performance tests of its new 802.11n Aironet 1250AP operating with a Lenovo T61 notebook and an integrated Intel Centrino 4965AGN wireless adapter. Using 40-MHz channels, Cisco measured peak TCP throughput of about 147 Mbps. Throughput with standard 20-MHz channels was about 89 Mbps. A baseline 802.11a/g test turned in throughput of 23 Mbps.Although we haven't yet brought a full set of enterprise- class 802.11n APs into our lab, we've spent the past several months running Apple's Airport Extreme through a battery of tests, achieving peak throughput of 137 Mbps with a MacBook 5 GHz 11n client. Our decision to focus on Apple's offering was predicated on three factors. First, Apple has a reputation for high-quality product design, and our briefings demonstrate that the company has an in-depth understanding of Wi-Fi. Second, the Airport Extreme is a dual-radio 11n AP, providing support for 2.4-GHz and 5-GHz operation. Finally, the product is based on the Atheros Wi-Fi chipset, a popular platform for enterprises. (For more details of our preliminary performance testing, see story below.)

Jump Or Hold?

We have no doubt that 802.11n will emerge as the predominant WLAN platform by 2009. Organizations considering a sizable deployment of Wi-Fi access points are thus understandably concerned that going with 802.11a/g WLANs in 2007 or 2008 means choosing legacy technology over a more strategic, powerful alternative. At the same time, jumping too quickly poses considerable risk, not so much because the final standard may change, but because you're buying first-generation devices. This risk increases where vendors are touting new controller architectures to complement their new APs.Our advice: If your existing WLAN serves your needs, stretch its life span as long as you can. Few organizations are bandwidth constrained on their WLANs, so instead of spending time and effort on upgrades, most should re-evaluate their wireless security postures.

For greenfield deployments, the calculus is tougher. Unless compelling ROI can be demonstrated, we recommend stretching WLAN deployment out over a longer timeframe, say two to three years, using a mix of 11a/g and 11n APs. This won't eliminate the trade-offs between 11a/g and 11n, but you'll be in a better position to realize the full benefits of mature 802.11n offerings.

There's little doubt that second- generation 11n products, likely available by late 2008 or early 2009, will offer significant enhancements. A longer-term phased deployment may prove the smartest approach, provided it doesn't hamper application and mobility plans. Under this strategy, a limited number of 802.11a/g access points can be installed alongside a limited number of 11n APs where performance requirements dictate higher speeds and suitable client devices are available.

Apple Ups the Performance Ante for 802.11n

Downstream TCP Throughput (megabits per second)

Plans are under way to begin testing of enterprise 802.11n offerings later this year. However, for the past six months, we've been testing Apple's Airport Extreme 802.11n wireless LAN gateway in conjunction with Apple's MacBook Pro 802.11n and, more recently, a Fujitsu Lifebook E8410 laptop with an integrated Intel Centrino 4965AGN wireless interface. Although The Airport Extreme is targeted at the consumer and small office/home office market, it shares much in common with enterprise-class APs, including a dual-radio, Atheros chipset. This product has allowed us to test the throughput and range characteristics of 802.11n in the 5 GHz band, which we consider to be the most appealing portion of unlicensed spectrum for enterprise deployments.We conducted our most recent round of testing in early October, in an office building on the periphery of the Syracuse University campus. This building was selected because of its geographic and RF isolation. Throughout our testing, we used a spectrum analyzer to verify that the air was clear, ensuring that our performance results were not contaminated by RF interference. The building construction included a combination of concrete cinder-block and sheet-rock walls. While not a worst-case scenario for wireless performance, we would expect products to exhibit better rate-versus-range characteristics in more modern buildings.

We tested the Airport Extreme (version 7.2.1) using a 20 MHz channel in the 2.4 GHz band (Apple does not support 40 MHz channels at 2.4 GHz) and a 40 MHz channel in the 5 GHz band. We used IxChariot's High Performance Throughput tests with 10 megabyte file size. Client computers included an Apple MacBook Pro with integrated 802.11agn (firmware version 1.1.8.5) and a Fujitsu Lifebook E8410 Windows laptop with Intel's 4965AGN (version 11.1.1.11). Multiple iterations of upstream and downstream performance tests were run at four different locations with results averaged:

Results of our tests were illuminating and provide guidance to enterprise IT professionals as they begin to think about deploying 802.11n networks. As Table 1 shows, the highest performance—111.2 Mbps—was turned in by an Intel Centrino client communicating with the Airport Extreme AP using 40 MHz channels at 5 GHz. This represented approximately 5.5 times the throughput of our 11a baseline. Also notable is the fact that at locations 110 feet from the AP with two intervening cinder-block walls, the 5 GHz 802.11n implementations far exceeded the performance of our baseline 802.11g test system. In fact, even at 110 feet from the AP, the Intel/Apple combination was more than twice as fast as any 802.11g product we've ever tested.

802.11n: The Technology Foundation

For most of the 100+ year history of radio communications, the phenomenon of multi-path interference and fading has been a thorn in the sides of RF engineers everywhere. As radio signals bounced off walls and other structures, they diminished transmission range and produced those dreaded dead spots. The magic of MIMO (multiple input, multiple output) radio technology changed all that, turning multi-path into our friend and delivering greater performance and range. The end result is a faster and more robust radio technology that meets the needs of even the most demanding network users.MIMO's spatial multiplexing is the most significant improvement in the 802.11n standard. By using multiple radio/antenna chains and advanced digital signal processing, engineers have demonstrated near linear performance increases. The number of radios and antennas used on clients and APs will vary. Design tradeoffs include cost, physical space and power consumption. We expect most initial client implementations to have MIMO designs that use two radio chains, while APs will have either two or three radio chains. Note that dual-band offerings actually have separate radios for each band, though many MIMO processing components are shared between frequency bands. The 11n standard supports four.

While MIMO represents the most significant architectural advancement in 802.11n, the standard includes additional physical-layer feature enhancements designed to boost performance. The most notable improvement is support for 40 MHz radio channels, which have twice the theoretical capacity of existing 802.11 radio channels. Transmit beamforming will allow 802.11n APs to dynamically focus their transmission patterns, providing rate-versus-range benefits for both 11n and legacy clients. Likewise, maximal ratio combining (MRC) is expected to enhance radio receive sensitivity, further improving range.

In addition to physical-layer enhancements, 802.11n also improves the Layer 2 media access control standard. New capabilities include frame aggregation, block acknowledgements, and reduced inter-frame spacing, all designed to reduce overhead and enhance throughput. In addition, several improvements have been made to 802.11 power management, though it's not yet clear whether this will lead to extended battery life since MIMO's multiple radio chains will require more power to operate.Headline: Who's Hot On 11n

Some enterprise WLAN vendors have better 802.11n stories to tell than others. Here's our breakdown of the major players:

>> Aruba Networks: Aruba has not made any product announcements related to 802.11n but is assuring customers that its controllers will have the horsepower to support 11n.

Aruba's 802.11n whitepaper, released in May, provides valuable insights into the issues involved with migrating to 11n. Still, we're puzzled by its claims that its controllers will support as many 11n APs as 11a/g APs.

>> Cisco Systems: Cisco introduced the Aironet 1250 802.11n AP in September; it's the first enterprise-class AP to receive Wi-Fi Alliance certification. The company also increased the performance of its Catalyst 6500 WiSM wireless controller module.Some were surprised that Cisco jumped into the 11n market before final ratification of the standard. Its modular radio architecture affords customers investment protection in the event that the final spec changes, while also providing deployment flexibility. We see Cisco's aggressive move into this market as partly a response to customers pushing for an 11n offering, partly a hedge against competitors stealing some market share if it waited until the standard was approved.

>> Colubris Networks: Colubris was one of the first enterprise WLAN vendors to announce an 11n offering, and it leads the industry in 11n hype.The Colubris Intelligent Mobility Solution has always been more distributed than its competitors, a design that the company hopes to leverage with its 11n offerings. While Colubris' architecture may provide enhanced scalability under certain traffic profiles, the company's marketing claims—that 11n is faster than 100 Mbps Ethernet and that customers can reduce their backbone traffic by up to 98% using its system—are misleading at best.

>> Meru Networks: Meru has moved aggressively to introduce an 802.11n product and is heavily promoting the 11n implementation plans of New York's Morrisville State College.

While most enterprise WLAN vendors are targeting their 11n offerings at 5 GHz operation, with 2.4 GHz reserved for legacy 11g clients, Meru's architecture provides greater flexibility for running both 11g and 11n at 2.4 GHz. Its three-tier controller architecture has potential for increased scalability, but it may also add even more complexity to an setup that is already quite challenging to configure.

>> Motorola/Symbol Technologies: Motorola has been relatively quiet about 802.11n plans, in part because it's still in the process of digesting its acquisition of Symbol Technologies.Motorola gained significant assets in the WLAN arena through its acquisition of Symbol, and while we expect the company to make some future moves into the 11n space, it has made no formal announcements of products.

>> Trapeze Networks: Trapeze announced an 802.11n AP in September, and a spokesperson speculated that 802.11n APs could make up 15% of total AP sales by the end of 2007.

Trapeze has been pushing its Smart Mobile architecture, which offloads data-plane forwarding from centralized controllers to distributed controllers, even to edge APs themselves. According to Trapeze, this architecture will enable more-scalable 11n deployments. However, such a claim depends entirely on the traffic profile of a given enterprise network. To cover all bases, Trapeze has also announced a high-end centralized controller, the MX-2800, with a touted capacity of 28 Gbps.