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Comparing 802.11a, b, and g: Channels and Interference

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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 hardware off your network.

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