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Hack 2 802.11a: The Betamax of the 802.11 Family

802.11a offers more channels, higher speed, and
less interference than other protocols, but it still just
isn't popular.

According
to the specifications available from the IEEE (at http://standards.ieee.org/getieee802/), both
802.11a and 802.11b were ratified on September 16, 1999. Early on,
802.11a was widely touted as the "802.11b
killer," as it not only provides significantly
faster data rates (up to 54 Mbps raw, or about 27 Mbps actual data),
but also operates in a completely different spectrumthe

5 GHz UNII
band. It uses an
encoding technique
called Orthogonal Frequency Division Multiplexing (OFDM).

While the promises of higher speeds and freedom from interference
with 2.4 GHz devices made 802.11a sound promising, it came to market
much later than 802.11b. It also suffers from
range problems: at the same power and
gain, signals at 5 GHz appear to travel only half as far as signals
at 2.4 GHz, presenting a real technical hurdle for designers and
implementers. The rapid adoption of 802.11b only made matters worse,
since users of 802.11b gear didn't have a clear
upgrade path to 802.11a (as the two are not compatible). As a result,
802.11a still isn't nearly as ubiquitous or
inexpensive as 802.11b, although client cards and
dual-band access points (which
essentially incorporate two radios, or a single radio with a
dual-band chipset) are coming down in price.

Pros

Very fast data rates: up to 54 Mbps
(raw radio rate), with some vendors providing 72 Mbps or faster with
proprietary extensions.

Uses the much less cluttered (for now, in the U.S.) UNII band, at 5.8
GHz.

Cons

As of this writing, 802.11a equipment is still more expensive on
average than 802.11b or 802.11g.

Most 802.11a client devices are add-on cards, and the technology is
built into relatively few consumer devices (specifically laptops).

802.11a PCMCIA cards require a 32-bit
CardBus slot, and won't work in older devices.

Cards and APs with external
antenna connectors are hard to find,
making distance work difficult.

Upgrading from 802.11b can be painful, as 5.8 GHz radiates very
differently from 2.4 GHz, requiring a new site survey and likely more
APs.

Limited range compared to 802.11b and 802.11g, at the same power
levels and gain.

Internal
802.11a antennas tend to be quite
directional, making them sometimes annoyingly sensitive to proper
orientation for best results.

Recommendation

The Wi-Fi alliance (http://www.weca.net/) tried to call 802.11a
"Wi-Fi5," but the name never stuck.
These devices are also sometimes confusingly labeled
"Wi-Fi," just like the completely
incompatible 802.11b. Be sure to look for the
specification's real name (802.11a) when purchasing
gear.

802.11a can be significantly faster than 802.11b, but achieves
roughly the same throughput as 802.11g (27 Mbps for 802.11a, compared
to 20-25 Mbps for 802.11g). 802.11a would be ideal for creating
point-to-point links, if devices with external antenna connectors
were more readily available. Many people tout
OFDM's ability to cope with
reflections caused by obstacles (called
multipath)
as a good reason to use 802.11a, but 802.11g uses the same encoding
while achieving greater range at the same power and gain. Some
consider the shorter range of 802.11a to be a security advantage, but
this can lead to a false sense of security. See the introduction to
Chapter 6, as well as [Hack #81] for more details.

Keep in mind that the 54 Mbps data
rate is the theoretical maximum, and frequently is only achieved when
in very close proximity to the AP. The speed scales back sharply as
your distance from the AP increases, and suffers dramatically when
separated by a wall or other solid obstacle. It is a very good idea
to perform a site survey complete with throughput testing to
determine whether 802.11a is suitable for your intended location.

It is probably a bad idea to build an 802.11a-only network unless you
are already committed to using only 802.11a gear. If you want to
allow guests to use your network, it is a very good idea to at least
incorporate a few dual-band APs (or perhaps a dedicated 802.11g AP),
as guest users are more likely to bring 802.11b or 802.11g gear with
them.