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Air Time: 11a or 11g: Which One Is Best for You?

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.

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