Comparing 802.11a, b, and g: Channels and
Interference
Comparing 802.11a, b, and g: Channels and Interference
Because wireless network products use radio waves for the
"physical" transmission medium, you need to consider other devices
that produce radio waves in the same spectrum that IEEE 802.11b devices use. For
example, the most common device, which is present in the home, many offices, and
many public places, is the microwave oven. Yes, these devices use radio waves to
heat your food, and they have a metal grating surrounding them that is supposed
to prevent microwave transmission from emanating outside the box. However, if
that were true you wouldn't see those warnings saying you shouldn't be
close to one if you have a pacemaker and there wouldn't be a market for
inexpensive devices you can purchase to measure leakage from a microwave oven.
Microwave ovens do leak microwave signals and these can interfere with
IEEE 802.11 devices.
The good news is that microwave ovens aren't typically operating
continuously. However, you still should consider them a source of interference
that can dramatically slow wireless communications. Another source of
interruption to wireless networks operating in the 2.4GHz radio spectrum is
other consumer devices, such as 2.4GHz portable telephones, as well as camera
devices that can be used to transmit video back to your PC. Consider this when
deciding whether to use a wireless network that uses the same radio spectrum.
Note that the newer 5.8GHz wireless telephones will not interfere with 802.11b,
802.11g, and Bluetooth devices because these operate in the 2.4GHz frequency
band.
802.11b/g Channels
Although 802.11b and 802.11g use the 2.4GHz frequency band for signaling, the
frequency is divided up into 11 channels for use in US and Canada (some
countries allow as many as 14 channels). Table 1 shows the channel frequencies
supported in the US and Canada. The effective width of each signal is about
11MHz either side of the nominal frequency.
Table 1 - US/Canada 802.11b/g Channel Frequencies
Channel
|
Nominal Frequency (MHz)
|
Minimum (MHz)
|
Maximum (MHz)
|
1
|
2412
|
2401
|
2423
|
2
|
2417
|
2405
|
2428
|
3
|
2422
|
2411
|
2433
|
4
|
2427
|
2416
|
2438
|
5
|
2432
|
2421
|
2443
|
6
|
2437
|
2426
|
2448
|
7
|
2442
|
2431
|
2453
|
8
|
2447
|
2436
|
2458
|
9
|
2452
|
2441
|
2463
|
10
|
2457
|
2446
|
2468
|
11
|
2462
|
2451
|
2473
|
Channels 1, 6, and 11 are recommended because there is a lower potential for
interference from other 802.11b/g APs when these channels are used. If you need
only a single AP to provide coverage for your location, use one of these
channels. If you need to set up multiple APs to cover your location, you should
use two or all three of these channels. Studies by Cisco Systems suggest that
throughput drops because of interference if you attempt to use more than three
different channels in a multiple-AP scenario.
Proprietary Extensions to 802.11b
There are two main factors that are encouraging the replacement of 802.11b
wireless networks with 802.11g or 802.11a-based networks:
- Network speed
- Network security
The maximum data rate supported by 802.11b-based wireless networks is a
relatively slow 11Mbps. In practice, the actual throughput could be half that
value or less due to distance between the AP and client devices, obstructions
weakening radio signals, and the additional overhead of handshaking and
security.
In an attempt to improve the performance of 802.11b-based hardware, some
manufacturers rolled out proprietary extensions to 802.11b networks. Some of
these include D-Link (AirPlus Enhanced; 22Mbps), U.S. Robotics (22Mbps); SMC
(Barricade Turbo 22Mbps); Alloy (22Mbps). Most of these products were based on
the Texas Instruments TI ACX100 chipset, and almost all of them are now
discontinued.
The main problem with using proprietary extensions to a standard wireless
technology is that all APs and clients must support the same standard, or the
network will run at standard speeds only. In practice, this means that you must
usually purchase APs and client hardware from the same vendor. Making an
across-the-board change is often not practical in terms of cost, and is not
practical if many of your PCs use built-in standards-based wireless network
adapters, as many notebook computers, PDAs and Smartphones now do.
Generally, most 802.11-based network hardware supports only first-generation
wireless security, Wireless Equivalent Privacy (WEP). Unfortunately, WEP is not
nearly as secure as newer standards; it can easily be hacked. Some 802.11b
hardware can be upgraded to WPA standards. If you want the superior security of
WPA on a mixed 802.11b/802.11g or 802.11b/802.11a network, you must upgrade your
802.11b clients to WPA if possible, or replace your hardware. Generally, you
would use 802.11g hardware as a replacement for 802.11b, because both use the
same 2.4GHz frequency and can interconnect with each other natively.
802.11a Signal Modulation
One of the advantages of 802.11a over 802.11b is the method of signal
modulation it uses. 802.11a uses a signaling method called orthogonal
frequency-division multiplexing (OFDM) for almost all data rates.
OFDM transmits multiple narrowband data streams at different frequencies
selected to avoid crosstalk (interference). This method is much different than
the DSSS (spread-spectrum) method used by 802.11b wireless networks. Because
most 802.11a networks are indoors, OFDM is a perfect choice because it provides
higher data rates than DSSS and minimizes the effects of multi-path propagation
on signal quality and throughput.
Multi-path propagation takes place when radio signals are reflected on their
way between sender and receiver. Radio waves can be reflected by metal office
furniture, structural elements, and other features common in office buildings.
This causes errors and requires retransmission. Multi-path propagation has a big
impact on the performance of 802.11b because DSSS is very susceptible to this
type of interference. However, OFDM is not affected very much by multi-path
propagation.
Because OFDM signal modulation provides better data rates, signal quality and
throughput, it is used by both 802.11a and 802.11g. Note that 802.11g hardware
is also compatible with DSSS, and switches to DSSS (and thus 11Mbps or lower
data rates) when connecting with 802.11b hardware.
802.11a Channels
Given the fact that 802.11a has the same 54Mbps maximum data rate as 802.11g,
but cannot interoperate with 802.11g unless dual-mode network adapters or APs
are used, what is the most compelling reason to use 802.11a network hardware? In
a word, channels. As I mentioned previously, 802.11b (and 802.11g) networks
offer 11 channels, but only three channels (1,6, and 11) do not overlap with
each other. In a large-scale building or campus-wide installation, the ability
to use only three channels can reduce real-world throughput and make avoiding
interference from other 802.11b or 802.11g-based wireless networks
difficult.
Unlike 802.11b/g wireless networks, 802.11a wireless networks have eight
non-overlapping channels. In other words, you can choose any combination of
channels for a multi-AP environment without interference with each other. And,
even if your installation is next to another 802.11a installation, it's
going to be relatively easy to choose channels that are not in use by other
nearby networks to avoid interference.
Table 2 lists the channels supported by 802.11a wireless networks in North
America. Note that Asian locations support fewer (and different) channel
combinations, and that Europe has been slow to standardize support for 802.11a
hardware.
Table 2 - 802.11a Channels in North America
Channel
|
Frequency (MHz)
|
Category
|
Maximum Power Level
|
Usage
|
36
|
5180
|
U-NII Low Band
|
40mW
|
Indoor
|
40
|
5200
|
U-NII Low Band
|
40mW
|
Indoor
|
44
|
5220
|
U-NII Low Band
|
40mW
|
Indoor
|
48
|
5240
|
U-NII Low Band
|
40mW
|
Indoor
|
52
|
5260
|
U-NII Medium Band
|
200mW
|
Indoor
|
56
|
5280
|
U-NII Medium Band
|
200mW
|
Indoor
|
60
|
5300
|
U-NII Medium Band
|
200mW
|
Indoor
|
64
|
5320
|
U-NII Medium Band
|
200mW
|
Indoor
|
149
|
5745
|
U-NII High Band
|
800mW
|
Outdoor
|
Proprietary Extensions to 802.11a
Just as some vendors created proprietary extensions to 802.11b to improve
performance, some vendors of 802.11a hardware have developed clients and APs
claiming throughput of up to 108Mbps. Most of these products use Atheros Super
AG chipsets
(http://www.atheros.com),
which use techniques such as channel bonding ("turbo" mode) and
special bursting techniques to achieve faster speeds with both 802.11a and
802.11g clients.
As with proprietary extensions to 802.11b standards, the biggest performance
boost is seen when all network clients support the same proprietary extensions.
Tests suggest that the presence of even one 802.11b client on a dual-mode
network is enough to disable special features and reduce network speed to
standard levels. Thus, if you're thinking of trying a so-called 108Mbps
solution for 802.11a or 802.11g, be sure to keep 802.11b
