Should I deploy 802.11a on my wireless LAN?

That's a question that frustrates me immensely. But it's also one that I'm happy to field, as I have on many instances over the past couple of years.

If you are referring to a home Wi-Fi network, the answer is almost always no. The only caveat relates to users in multiple-dwelling units, where finding an available 2.4 GHz channel that won't interfere with your network is challenging. On most home networks, the broadband Internet connection is usually the source of performance bottlenecks so even 802.11b is typically adequate. And because 11a operates at 5 GHz, its RF absorption characteristics are high, which means it won't make it through very many walls before degrading into RF mush. In the home environment, where user density is usually quite low, maximizing coverage is the primary design objective. In most cases, using an 802.11g wireless router in conjunction with a modern wireless-equipped computer will be adequate. If your home is very large, you may want to consider a high-gain antenna on your access point (AP) or MIMO-based product offering to provide better coverage.

If you are referring to an enterprise network, or any network where you might expect high user density, the answer is a resounding yes. That's because high density translates into high contention and high contention translates into reduced performance. Although it's true that most enterprise vendors allow you to co-locate multiple 11g APs on non-overlapping channels and load-balance between them, such a design greatly complicates the channel planning model and makes it difficult to provide full coverage throughout a facility without introducing co-channel interference, which occurs when multiple APs and clients operating on the same channel overlap coverage areas. Co-channel interference is a sinister enemy, often lurking almost invisibly in the background but then raising its ugly head as more users or time-sensitive applications are added to the network. Today, many network managers aren't even aware of its existence because users are currently accustomed to glitches on wireless networks. But they won't be in the future.

One common approach to solving this problem is to use a micro or pico-cell architecture by reducing the power output of the APs, thereby shrinking the APs' coverage radius. Although such an approach can sometimes yield favorable results, it doesn't help much if your APs are whispering and your clients are yelling at full volume. Even though the APs themselves may not interfere with each other, clients in adjacent cells may interfere with APs with which they are not associated. OK, so why not turn down the power on the clients? That's not an irrational approach, but there are two problems. First, there is no standards-based mechanism that allows an AP to instruct a client to turn down its radio power level (Cisco offers this capability with CCX, but it is only supported on Cisco infrastructure). Beyond that practical limitation, turning down the client radio power level also decreases the RF link budget, which may mean that clients connect at a lower data rate. If you try to increase the power level to provide connections at, say 12 Mbps, that will increase the RF propagation range, which in turn causes co-channel interference and contention with adjacent cells, even at relatively low signal levels. There's just no easy solution to this problem.

The biggest mistake network managers make lies in assuming that they need to make a choice between 11a and 11g. That's just not the case. Yes, it's true that the cost of a dual-band, dual-radio enterprise-class AP can be 20 percent higher than an 11g model. But with the cost of APs falling, that's not much of a price premium, especially when you consider that installation costs usually far exceed equipment costs. Some argue that because the cell radius of 11a is smaller, they will need to install additional APs. That's true, but there's no reason you have to provide 11a coverage everywhere in your facility. Since 11a-only NICs are no longer sold, users can always fall back to 11g if they are outside of 11a coverage. Even if only 20 percent of your users connect via 11a that translates into improved performance for every single user because it reduces contention in both bands.

Another common mistake lies in looking at the installed base of laptop users and concluding that because only a fraction have dual-band radios it won't pay to add support for 11a. If that's the case in your organization, then someone in purchasing is asleep at the wheel. With a price premium of $10 or so, almost every organization should provision dual-band radios on their clients. Fortunately, notebook manufacturers are increasingly installing 11ag NICs on enterprise-class systems by default, so the extra cost is invisible.

Once 11a client penetration increases, your life will be a lot easier. Channel planning at 5 GHz is a whole lot easier because there are many more non- overlapping channels, which allows you to virtually eliminate the effects of co- channel interference. If no contention is tolerable, as might be the case if you are implementing wireless VoIP, you can also choose to implement WLAN infrastructure from vendors like Meru and Extricom, which utilize sophisticated RF scheduling algorithms that not only eliminate contention but also make installation planning much easier.

So remember, for enterprise Wi-Fi, think 11a AND 11g, not 11a OR 11g.