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> Focus - WLAN Technology
Step up on spectrum

Given potential spectrum interference issues with 802.11b, should you consider moving to 802.11a on 5 GHz? by Dr Seamus Phan

For most parts, the 802.11b specification has thrived on the fact that the radio spectrum which it rides on—the 2.4-GHz band—is unlicensed. This meant that vendors, installers and even DIY types can build WLAN networks without needing to get clearance from regulatory bodies, and consequently avoid paying exorbitant radio spectrum license fees.
But latitude is a double-edged sword. Plying in an unregulated spectrum means that interference from other devices becomes a potentially disastrous prospect.

Interfering signals
For example, three signal-emitting devices which are household staples and can potentially trip up an 802.11b network are alarm systems, microwave ovens and cordless phones.

Surveillance cameras equipped with on-board antennae, including audio transmitters, can be mounted at any corner within the home. Often, the cameras are secured at the corner of each possible entry point within the home, and provide a spread pattern overlapping all connected intersections. Such wireless cameras usually come in three common frequencies: 400 MHz, 900 MHz and 2.4 GHz.

900 MHz is illegal in most parts of Asia as it interferes with the licensed GSM 900 MHz spectrum. 400 MHz is too low a frequency and can be interfered with even the likes of remote control toys. Therefore, the only useful form of wireless camera surveillance systems are in the 2.4 GHz range.

Microwave ovens also use the 2.4 GHz frequency range, although most models have sufficient shielding. At the same time, 802.11b WLANs are usually used in study rooms which are normally situated away from the kitchen, where microwave ovens are found. Because of the concrete walls found in most modern architectures, the range of 802.11b WLANs can be drastically reduced. If you observe the range indicator for the WLAN, you may find that your effective range is reduced to nearly half if your base station needs to reach an adapter separated by two thick concrete walls. Therefore, it is unlikely that the microwave oven in the kitchen will interfere too much with the WLAN in your study room, often separated by at least two, if not three, concrete walls. If your study is situated upstairs, and your kitchen downstairs, it is also likely that the degree of separation is by atleast 2 walls (including the ceiling of the kitchen).

Cordless phone also come in 400-MHz, 900-MHz and 2.4-GHz models. Again, 900-MHz models are banned in most parts of Asia because they interfere with the legal GSM 900 MHz frequency range. 400-MHz models usually have low sound fidelity and are not favored. This leaves the 2.4-GHz model, which is often touted by vendors to yield "superb" sound quality. Since you do need to use your cordless phones around the home, including in your study, there will be possible significant interference issues.

Another potentially phone-related interference culprit is Bluetooth. Though not widely used, it is used by a few cellular phone vendors to wirelessly-connect associated gadgets together. Today, most cellular phones use wired hands-free kits to allow users to talk without holding the cellular phone close to their ears. Ericsson has a Bluetooth-enabled wireless headset that communicates with your cellular phone within 30 ft. These headsets can potentially present interference issues should there be users using 802.11b WLANs in the same vicinity.

Interference prevention

If you need to wire up your home or small office (or workgroup) with 802.11b WLAN, be aware that devices in the same frequency range should preferably be excluded from the vicinity.

You may choose NOT to use 2.4 GHz cordless phones, Bluetooth gadgets (which are showy but not that useful anyway), and microwave ovens in the same room. For phones, you can use corded phones, or use good quality speakerphones or conference rooms to free up your hands for your keyboard instead.

If you must have surveillance systems, install a hard-wired setup instead. Incidentally, hard-wired surveillance and alarm systems do have an added benefit—they can run from line power with a battery backup, rather than rely on battery only. Although most surveillance and alarm systems that run from battery sources (typically 9V DC) can last at least six months to a year, it is still safer to have a powered setup with a battery backup. Also, failed batteries can give false positives, creating havoc for you to travel home to disable the alarm.

As for your 802.11b WLAN, you can also turn on Interference Robustness, if your base station presents such a choice in its setup. With Interference Robustness, essentially the base station limits its transmission range with a stronger signal to recognize your network traffic.

You can also cycle through the available channels to test which channel works best even if you do have other conflicting 2.4 GHz devices around.

Moving up
It is with the issue of interference that the 802.11a IEEE specification came about, and became adopted as the next generation of WLAN infrastructure technology.

With the 5 GHz Unlicensed National Information Infrastructure (UNII), FCC regulations for 15.407 became more specific. Therefore, the upper band from 5.725 to 5.825 GHz is intended for outdoor fixed broadband wireless access devices requiring higher power to reach longer ranges. The lower 200 MHz band is intended for indoor, lower-powered WLANs in the 5.15 to 5.35 GHz. The specification also states that all UNII devices must be high data rate communication devices, specifically excluding low-rate devices such as cordless phones, Bluetooth gadgets, microwave ovens, surveillance systems, and so on.
How 802.11a modulation works is through orthogonal frequency division multiplexing (ODFM) to reduce the multipath effect whereby the transmitted radio signal is reflected from walls, fixtures and indoor objects, rather than reaching the receiver in a single direct path. This reduces the effective range, but also reduces interference from other broadcasting signals at the same time.

There is also a technical advantage of the number of usable channels for 802.11a versus 802.11b. 802.11b technology has only three usable, non-overlapping channels for use in data networking environments, which can present proximity interference from adjacent base stations from other subnets. On the other hand, 802.11a has more usable non-overlapping channels (8 in the USA, 19 in the European Union and four in Japan), which means that there will be even fewer instances of proximity interference from neighboring base stations belonging to other companies with no business participating in your own setup.

Hold your horses
However, not all is green light for the cutting-edge 802.11a WLAN technology yet. Even in Japan, there is some resistance for widespread adoption of 802.11a. Likewise, in many parts of the world, including Asia-Pacific, 802.11a coincides with the primary use of the 5 GHz band by radar and satellite applications.

There is a Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) specification under review by spectrum regulatory boards and hopefully, these will smooth the path for universal adoption and acceptance of the 802.11a WLAN technology, especially in more regulated markets in Asia-Pacific.

Beyond regulatory and governmental resistance issues, 802.11a technology is still quite expensive compared to traditional 802.11b WLAN technologies, often at 50 percent or so higher retail prices. The lack of backward compatibility with 802.11a, especially for corporations that have already implemented partial or even complete 802.11b infrastructure, will be a major throw backward as well for 802.11a's acceptance in the corporate environment. Although there are vendors touting hybrid chipsets combining both 802.11b and 802.11a in the same base stations, these are merely stop-gap measures and the real technology and migration challenges still lie ahead.

Until the dust settles with 802.11g (a backward compatible high data rate specification with 802.11b), and vendors can bring cheaper and more compatible 802.11a products to the market, there will still be resistance to adopt 802.11a technology to transcend the interference gripes with existing 2.4 GHz technologies.

Seamus Phan is research director at KnowledgeLabs News Center (www.knowledgelabs.net), an independent technology news bureau and writes for Network Computing-The Asian Edition. He can be reached at seamus@knowledgelabs.net

 
     
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