Deployments of 802.11a deployments have been relatively sluggish, given that the technology's has better flexibility for designing around interference. It has 12 to 23 non-overlapping channels in the 5 GHz range, compared to three in the 2.4 GHz range used by 802.11b and 11g.
As real-time multimedia traffic, including voice, joins mainstream enterprise Wi-Fi LANs, interference avoidance will become a larger issue for network operators.
As you likely know, 802.11a and 802.11b both became standards in 1999, but 11 Mbit/s 802.11b networks flourished first. There are both regulatory and technical reasons.
5GHz spectrum is not global
One is that use of RF spectrum is consistent throughout the world in the 2.4 GHz band for 802.11b (and, now, 802.11g); however, the 5 GHz rules for 802.11a have been less uniform.
"802.11a RF is legal in about 25 percent of my 45 countries," an architect for a global company who reads these columns observed by e-mail earlier this month. "We have mobility concerns that trump technical performance," so he chose to deploy 802.11b/g instead of 11a. Other readers have written to say they have found few 802.11a client choices available.
Both situations have been improving. On the regulatory side of things, the World Radiocommunications Conference harmonised some worldwide 5 GHz spectrum for 802.11a use in 2003. The FCC added spectrum in the "middle band" - 5.150 GHz to 5.350 GHz and 5.47 GHz to 5.725 GHz - in 2004, putting North America's 802.11a 5GHz spectrum use on a par with other major regions of the world.
"Virtually every country has approved some section of the 5 GHz band for use by WLAN products," says Bill McFarland, CTO at WLAN chipmaker Atheros, who adds that his company's chips can "tune to the entire band."
Regulations still vary
Still, product compliance regulations from country to country differ (power levels, antenna types allowed, whether hazardous materials can be used in product manufacturing). It helps having the frequency bands better in line, but the different rules make 802.11a devices more difficult for vendors to ship (and their customers to deploy) globally.
On the client-availability side, Intel is converting its Centrino mobile product line to dual-band 5 GHz/2.4 GHz products; it announced its Intel PRO/Wireless 3945ABG Network Connection for supporting all 802.11 client types at the Consumer Electronics Show in January.
Meanwhile, for those seeking additional enterprise-class 802.11a client options, here are some links to Wi-Fi Alliance-certified 802.11a client products:
Varying rules for using 5 GHz spectrum across different countries mean that vendors must ship 802.11a-compliant Wi-Fi products to different places with different capabilities turned on and off (for example, 802.11h in Europe). But the industry now seems more willing to do it than it once was - if perhaps for no other reason than to set the stage for the faster standard, 802.11n.
Worldwide compliance has been one thorn in the side of 802.11a Wi-Fi technology, which is finding a home in mesh backhaul networks and, as more dual-band and tri-mode access points and clients ship, might see more action in the coming two years.
Products are improving
In addition to the global progress of regulatory matters, vendors have also improved their products. Traditionally, 802.11a products have consumed much more power than their 802.11b/g counterparts, for example, but vendors "all continue to push power levels on chipsets down," says Chris McGugan,
senior director of the wireless infrastructure division at Wi-Fi company Symbol Technologies. McGugan says Symbol will bring a flash 802.11a/b/g offering to market this year for users requiring more 802.11a options.
Meantime, current-generation 802.11a technology supports coverage ranges much closer to those of 802.11b than they once were. Even so, remaining range inequalities bring up design issues, particularly in using the generous number of channels available in 802.11a (and the forthcoming option to 802.11n) for real-time multimedia traffic.
For example, Intel cites the following indoor ranges for the Intel PRO/Wireless 3945abg Network Connection, announced at the Consumer Electronics Show, as typical:
- 802.11a: 40 feet @ 54 Mbit/s / 300 feet @ 6 Mbit/s
- 802.11b: 100 feet @ 11 Mbit/s / 300 feet @ 1 Mbit/s
- 802.11g: 100 feet @ 54M bit/s / 300 feet @ 1 Mbit/s
From one perspective, at the full theoretical maximum speeds, the bit per second per foot of 802.11a beats that of 802.11b: while the 11a network is only radiating 40 percent as far, it is transmitting 500 percent as fast. And network designers get four to
seven times as many channels to use to design around interference as a big bonus.
On the other hand, with the range of 802.11a not even half that of 802.11b (or, more importantly, 802.11g, which has equal capacity) an 802.11a network would likely require more access points and more handoffs between access points. Until the
fast-roaming specification (802.11r) and technology becomes solid, this could result in more overall coverage holes, which would be a negative for voice and other real-time sessions.
The proposed high-speed 802.11n draft standard currently under review by the IEEE specifies use of the 5 GHz band as an option, though it will be difficult to get around using it and still find 40 MHz channel bandwidth to use, as required to achieve 802.11n's 100 Mbit/s-plus speeds.
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