Don't let wireless LAN transmission impairments ruin your day!


The implementation of a wireless LAN may seem easy at first. All you have to do is install a few access points, connect them to an Ethernet switch, and you're ready to go. Right?

Well, it's not always that easy. Unlike wired networks, seen and unseen obstacles can often impede wireless network transmissions. Common wireless LAN transmission impairments include path loss, multipath distortion, and RF interference. If you don't carefully assess the environment where the wireless LAN will operate, then those impairments will cause problems that are difficult to diagnosis and correct after installation.

Path loss, which is the attenuation that a signal undergoes because of the propagation distance between the radio and the access point, is present in all wireless LAN transmissions. In general, path loss varies directly with transmission distance and frequency. As a radio moves farther away from the access point, path loss will increase and cause the signal-to-noise ratio at the receiver to decrease to the point at which the radio is unable to distinguish the data signal from the noise. When that occurs, the radio is operating in a fringe area, the maximum distance from the access point. Of course, that limited range is why you'll often need multiple access points to cover a particular facility.

A problem with path loss is that it will become much more significant as you begin to deploy higher frequency, 5 GHz wireless LANs as specified by the IEEE 802.11a standard. That higher operating frequency will require you to implement a relatively large number of access points to cover the same area of a contemporary 2.4 GHz system.

RF interference is a bit more disruptive than path loss. Because of carrier-sensing medium access protocols, a wireless LAN station will not transmit when it senses other stations transmitting. If the interfering signal falls within the same frequency of the wireless LAN, then the interfering signal will appear legitimate and block the wireless LAN from transmitting. The interfering signal can also strike a packet in transit, resulting in errors, retransmissions, and corresponding delays -- not something that you want to happen.

Microwave ovens and nearby radio stations can interfere with wireless LAN operation, but people are mostly worrying about potential interference between Bluetooth and IEEE 802.11 wireless LANs. Bluetooth is a relatively new wireless technology that offers data rates of 1 Mbps over short ranges (typically 30 feet), using frequency hopping spread spectrum in the 2.4 GHz band. Some Bluetooth-enabled products, including laptops and cell phones, are on the market today and many others will appear through 2001. Bluetooth devices offer the greatest potential for interference with 802.11-based wireless LANs when both are transmitting simultaneously within close proximity (30 to 50 feet).

As a result, it's important to manage the concurrent operation of Bluetooth and wireless LAN products. Task Group 2 (TG2) of the IEEE 802.15, however, is developing mechanisms that will facilitate coexistence of Bluetooth and 802.11 devices in the same area. IEEE will publish the results of that effort as an IEEE Recommended Practice by the end of 2001. Until then, you'd better carefully plan the integration of Bluetooth into a wireless LAN environment.

Multipath distortion, another culprit, occurs as portions of the radio signal bounce off physical obstacles, such as desks and metal bins, in route to the destination. Multipath tends to delay parts of the signal, causing it to be spread in time at the receiver. As a result, the receiver may not be able to detect the data correctly, especially with higher data rates. RF hostile areas, such as manufacturing plants, can offer significant multipath distortion, requiring 11 Mbps 802.11b wireless LANs to operate at the lower, 1 Mbps or 2 Mbps. As a result, be sure to understand the effects of multipath in your environment before getting too far with the wireless LAN implementation.

How do I ensure that path loss, RF interference, and multipath distortion don't disrupt my wireless LAN? One of the best preventative measures is to perform an RF site survey before installing the wireless LAN. An RF site survey will determine the number and location of access points and whether multipath distortion or RF interference will cause any problems. The results of the survey will help you plan an effective and supportable system.

Stay tuned. Next time we'll take a look at some test equipment that comes in handy when analyzing wireless LANs impairments.

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