802.11n: How fast? How far? How soon?

The recently adopted 802.11n Wi-Fi specification promises to bring an exciting boost in bandwidth and range to wireless networks. Offering up to a 10x increase in performance and a 3x enhancement of range, this new standard promises to open up an exciting new world of applications for both the home and the office. Interest is rapidly building for a new generation of Gigabit Ethernet WAN routers, DSL/cable gateways, PC/laptop network cards, wireless HDTV adapters and IP set-top boxes.



First, silicon hit the market in early spring of this year. Three of the leading network IC developers, Marvell Semiconductor, Broadcom Corporation and Atheros Communications, introduced new silicon to support this emerging, nextgeneration standard. Vendors are generally offering two-chip sets, which combine an RFIC with a baseband chip.



Marvell, for instance, supplies a single transceiver along with a combined baseband/MAC. Marvell has a unique architecture where the baseband/MAC IC includes a Foreceon CPU, which is Marvell’s own ARM-compatible processor.Some controversy arose when all three chip vendors introduced 802.11n ICs immediately after the IEEE agreed on a compromise draft proposal for the standard and months before formal balloting for the final standard. While there is still time before a final standard emerges (toward the second half of 2007), there is a high degree of confidence among the vendors that the current offerings with some software/firmware upgrades will be able to interoperate with the devices based on the final standard at fairly high data rates.



FASTER, FARTHER

While the industry waits for the IEEE’s 802.11n Working Group to iron out its differences, the large majority of users want to know what kind of performance and range they expect with this new generation of equipment. The draft standard, as it is currently written, employs multiple-input, multipleoutput (MIMO) technology, which uses multiple data streams in parallel through the wireless channel, to boost data rates up to 600Mbps. With each radio channel carrying different modulated digital information, these MIMO schemes can multiply throughput without using more radio spectrum.



The initial 802.11n network equipment based on these early chipsets uses any of three antenna configurations: 2 × 2, 2 × 3 or 3 × 3. For example, the initial product offering from Netgear which is based on ICs from Marvell, uses two antennas on the transmit sid and three on the receive side. Other antenna configurations are under development.



Overall, early equipment designs are demonstrating significantly better throughput than earlier generation 802.11g equipment. At shorter distances of less than 50 feet, these initial systems are performing between 150Mps and 200Mbps in so-called “encumbered” environments such as inside a building with interior walls and other structures. At longer ranges out to 300 and 400 feet, performance declines to 20Mbps to 30Mbps.



Interoperability between network gears designed by different vendors is always a major concern when any new technology is introduced. The 802.11n is no exception. In this instance, however, retailers, who have a large stake in the product development process, have pressured silicon vendors to address basic connectivity issues early in the product development cycle. Accordingly, the leading silicon vendors who have already introduced silicon have worked with their competitors to tune their devices so that each vendor’s device can recognize and talk to competing vendors’ devices. Interoperability between different vendors is typically classified into three levels.



INTEROPERABILITY LEVELS

Level-1 interoperability is the ability of different implementations to talk to one another at legacy rates (meaning 11g). Throughput at Level-1 is typically around the 20Mbps range.



Level-2 interoperability is the ability of different implementations to talk to one another using the MIMO PHY rates, and the throughput at this level is typically approximately 50Mbps.



Level-3 interoperability is the ability of different implementations to talk to one another using all the enhancements in 802.11n. The throughput at this level is typically approximately 100Mbps (Figure 1). Firmware/software upgrades that achieve Level-2 interoperability between vendors have already been posted. Full Level-3 firmware/ software should be available from the various vendors in the next few weeks.



Over the long term, leading 802.11n silicon vendors and the Wi-Fi Alliance plan to develop a suite of interoperability tests that will allow OEMs to demonstrate their products’ compliance. Industry participants expect this test suite to be available sometime in 2007.



MINIMIZING INTERFERENCE

One of the major challenges 802.11n equipment developers face is minimizing interference with legacy equipment. One way developers can maximize throughput is by running in greenfield mode which crowds out all legacy transmissions. Instead, most vendors are currently optimizing their equipment to run in a harmonious mode, which supports coexistence between the two technologies. While the draft 802.11n specification calls for mandatory use of a single 20MHz channel, for example, it also offers the option to use a second 20MHz channel to maximize data transfer rates. When using the extended 40MHz modes, it is important that the network does not clobber legacy networks in either of the two 20MHz channels.



Vendors are already predicting that future retail products will offer an automatic mechanism that will evaluate traffic patterns within the wireless environment and then determine which mode will offer the best option. Marvell, for example, addresses co-existence with its “wireless harmony” mode that is being tuned to ensure fair operation in the presence of legacy networks. Similarly, while the 802.11 draft standard supports operation in both the 2.4GHz and the 5GHz bands, initial products are primarily focused on the 2.4GHz band where current generation 802.11g products primarily operate. As more mature 802.11n products move to the 5GHz band, interference issues with legacy products will diminish.



CONCLUSION

Whatever track product developers take, the ability to transmit hundreds of megabits of data per second will open up exciting new applications for wireless networking. Service providers are already expressing keen interest in the technology’s ability to transmit multiple streams of high-definition TV around the home from a DSL or cable gateway. The opportunities for Gigabit Ethernet wireless routers look equally promising. The development work completed by today’s early adopters will lay the foundation for these exciting new 802.11n markets.





About the author

Mahesh Venkatraman is Technical Marketing Manager at Marvell Semiconductor Inc. For more information, contact Marvell Semiconductor, 5488 Marvell Lane, Santa Clara, CA 95054; (408) 222- 2500; www.marvell.com.