Networking

03:00 PM
Dave Molta
Dave Molta
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802.11n 100Mbps WLANs: Some Day

The 802.11n standard, which calls for WLANs with performance of at least 100 Mbps, is in its very early stages of development and, given the immense complexities, I'll be shocked

As if there isn't enough confusion out there concerning the alphabet soup of wireless standards, the last thing we need is the VP of Broadcom's enterprise computing division talking about products based on standards that are about as mature as an eighth-grader. A Broadcom spokesperson was subsequently quoted as stating the company had "no plans to preemptively launch products that could possibly undermine the strong standards-based industry that we've helped build."

So what's really going on here? The 802.11n standard, which calls for WLANs with performance of at least 100 Mbps, is in its very early stages of development and, given the immense complexities, I'll be shocked if it is real before 2006. The technical hurdles are immense. Unlike previous 802.11 standards, which have defined the system's physical layer data rate, 11n defines performance as effective throughput. When you account for overhead from interframe spacing, packet preambles, MAC headers, contention and acknowledgments, today's 802.11 products have typically achieved TCP throughput of about half the data rate (5.5 Mbps for 11b and about 27 Mbps for 11a). Further, this roughly 4x improvement in throughput must be achieved without decreasing effective range. Applying Shannon's capacity limit theory using today's modulation systems, the maximum possible data rate using existing 802.11 channels is 133 Mbps. Even if the underlying MAC design were radically altered, it would appear insufficient to achieve the stated goal of 100 Mbps throughput.

One solution to this problem is to use multiple RF channels, thereby doubling the available bandwidth. Both Broadcom and Atheros have used this channel-bonding technique in proprietary implementations to boost the performance of their systems. But it is far from ideal: Throughput typically increases by less than 30 percent; it makes dense AP deployment much more difficult; it reduces effective transmission range; and it makes legacy system coexistence much more difficult. Overall, it's a poor solution.

Most experts I have spoken with agree that newer, more spectrally efficient wireless modulation technologies are far more preferable than using larger channels. It's highly likely that some form of MIMO (multiple-input multiple-output) technology will form the basis for future 11n standards. But today's MIMO implementations, while promising, fail to meet the 100 Mbps throughput standard, and they also can be expected to suffer from other deficiencies normally associated with first-generation implementations.

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