As 802.11a products began shipping months ago, more and more companies have
been taking advantage of 802.11a’s superior performance. 802.11a radios transmit
at 5GHz and send data up to 54Mbps using OFDM (orthogonal frequency division
multiplexing). The results have been very good. 802.11a products deliver excellent
performance.
Inside 802.11a
Before discussing benefits and implications of 802.11a, let’s take a look at
how 802.11a devices operate.
802.11a defines one of several different 802.11 Physical Layers (PHYs). The
actual name of 802.11a is the "High Speed Physical Layer in the 5GHz band,"
commonly referred to as the "OFDM PHY." Another popular PHY of course
is 802.11b, which most companies have been installing for the past couple years.
Others include 802.11 FHSS (frequency
hopping spread spectrum) and 802.11 IR (infrared).
No matter which 802.11 PHY you deploy, the MAC (medium access control) Layer
is the same. The MAC
Layer manages and maintains communications between 802.11 radio NICs and
access points by coordinating access to a shared radio channel. The MAC Layer
is actually a program that runs on a processor; whereas, the PHY involves digital
communications circuitry and an RF (radio frequency) modulator to prepare data
for transmission over the air medium.
The 802.11a PHY is quite different than 802.11b, which uses direct sequence
spread spectrum (DSSS). 802.11a specifies the use of OFDM to support higher
data rates.
OFDM divides the data signal across 48 separate sub-carriers to provide transmissions
of 6, 9, 12, 18, 24, 36, 48, or 54Mbps of which 6, 12, and 24Mbps are mandatory
for all products. For each of the sub-carriers, OFDM uses PSK (phase shift
keying) or QAM (quadrature amplitude modulation) to modulate the digital signal
depending on the selected data rate of transmission. In addition, four pilot
sub-carriers provide a reference to minimize frequency and phase shifts of the
signal during transmission. This form of transmission enables OFDM to operate
extremely efficiently, which leads to the higher data rates, and minimize the
affects of multi-path
propagation.
The operating frequencies of 802.11a in the U.S. fall into the national information
structure (U-NII) bands: 5.15-5.25GHz, 5.25-5.35GHz, and 5.725-5.825GHz. Within
this spectrum, there are twelve, 20MHz channels, and each band has different
output power limits. The Code of Federal Regulations, Title 47, Section 15.407,
regulates these frequencies in the U.S.
OFDM is becoming very popular for high speed transmission. In addition to being
selected as the basis for the 802.11g PHY, OFDM is the basis for the European-based
HiperLAN/2
wireless LAN standards. In fact the 802.11a PHY is very similar to the HiperLAN/2
PHY. In addition, OFDM has also been around for a while supporting the global
standard for asymmetric digital subscriber line (ADSL).
802.11a Benefits and Implications
The following are benefits of 802.11a:
- Higher performance. By far the top reason
for choosing 802.11a is the need to support higher end applications involving
video, voice, and the transmission of large images and files. In addition,
802.11a does a superior job of supporting densely populated areas of users
having lower bandwidth needs, such as surfing the Internet. 802.11a can deliver
data rates up to 54Mbps and there’s enough room in the 5GHz spectrum to support
up to 12 access points operating in the same area without causing interference
between access points. This equates to 432Mbps (12 X 54Mbps) total data rate
performance. Even the upcoming 802.11g standard, which will deliver 54Mbps
data rates in the 2.4GHz band doesn’t come close to the performance of 802.11a.
With 802.11g, the same problem exists as with 802.11b: You have only three
non-overlapping channels for setting access point frequencies, which severely
limits capacity.
- Less RF
interference. The growing use of 2.4GHz cordless phones and Bluetooth
devices is crowding the radio spectrum within many facilities. This significantly
decreases the performance of 802.11b wireless LANs. Cordless phones wreak
enough havoc to cause companies to either ban the use of the phones or not
install wireless LANs. The use of 802.11a operating in the relatively un-crowded
5GHz band avoids this interference. Of course non-802.11 devices will eventually
occupy the 5GHz band as well; however, there’s much more room with 12 non-overlapping
channels to limit interference with the other devices.
The following are drawbacks of 802.11a:
- Less range. The superior performance of 802.11a offers excellent
support for bandwidth hungry applications, but the higher operating frequency
equates to relatively shorter range. Even with this limitation, however, 802.11a
can sometimes deliver better performance than 802.11b at similar ranges from
the access point. For example at ranges of 100 feet, 802.11a may deliver 24Mbps,
but 802.11b devices at the same range are operating at 5.5Mbps.If you’re planning to deploy 802.11b networks for 11Mbps throughout the
facility, it’s very likely that you can install 802.11 access points at
the same locations and still achieve 6 to 12Mbps data rates. As a result,
you can install approximately the same number of 802.11a access points as
802.11b and likely have similar performance. When needs for higher performance
occur in the future, you can add more access points to increase the coverage
to 54Mbps throughout the facility. This approach enables you to grow into
a longer term, higher performing solution while spreading the costs over
time. - Limited interoperability. 802.11a doesn’t talk to 802.11b. For example,
an end user equipped with an 802.11a NIC will not be able to connect with
an 802.11b access point. The 802.11 standard offers no provisions for interoperability
between the different physical layers. The solution to this problem is multimode
radio cards that support multiple 802.11 PHYs, such as 802.11a/b, 802.11a/g,
etc. These cards should be available on the market by the end of 2002. As
a result, an 802.11a/b radio within an end user device will automatically
sense whether the access point is 802.11a or 802.11b and then communicate
accordingly.Likewise, an access point can also deploy a dual 802.11a/b solution, enabling
interoperability with end user devices equipped with either an 802.11a or
802.11b radio. In the meantime, however, you can still install 802.11a wireless
LANs. This assumes, however, that you’re able to implement 802.11a radios
in the user devices. Some devices today, such as bar code scanners, come
equipped with 802.11b cards that you can’t easily change. - Higher prices. The current list prices of 802.11a products are approximately
30 percent higher than 802.11b, but the price gap should close over the next
couple years. The higher price today, nevertheless, causes some companies
to install 802.11b in order to lower initial costs. The problem is that the
primary migration path for these companies to deliver higher data rates in
the future will be to upgrade their 802.11b access points to 802.11g. It’s
not clear when 802.11g products will be available, though, because the standard
still requires major work before IEEE ratification and FCC approval takes
place. 802.11a is available today and operates in a much less crowded part
of the spectrum that includes higher capacity. 802.11a is clearly a better
long term solution, especially when future performance needs are not very
well known. It’s better to pay a little more now for a better solution rather
than a lot more later to replace hardware.
Of course the your decision on which 802.11 PHY to support depends on requirements
of your specific wireless LAN application. Based on the benefits, I highly recommend
using 802.11a unless requirements dictate otherwise. It’s always better to have
too much performance rather than not enough, especially for large numbers of
users and higher end applications.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs
(SAMs, 2001), and regularly instructs workshops on wireless LANs.
