AML M7140 Stationary Terminal - Manual (Page 10)

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The Optional M7140 Radio
The M7140 desktop terminal can be purchased with an optional 802.11b radio and antenna. This
radio is specifically designed to communicate with any 802.11b Access Point. The range of the
radio depends greatly on the quality of the Access Point and the RF communications
characteristic of the environment where the device is used. The typical range for an 802.11b
radio is 500 feet through free air. Additional Access Points must be added to improve coverage
in a larger area, or in electrically noisy RF environments.
802.11b Fallback Mode
Wireless LAN technology is designed to make maintaining a connection between two devices
as reliable and consistent as possible. Since the speed of the connection between wireless
devices will vary as range and signal quality varies, the wireless devices will intentionally
sacrifice throughput (data rate or connection speed as measured in bits per second) in exchange
for maintaining a reliable connection. In other words, a reliable connection at a lower speed is
preferred over an unreliable connection at a higher speed (i.e., it is easier to maintain the
connection if data rate is deliberately reduced, or put another way, lower data rates will tolerate a
higher range and/or worse signal quality). This characteristic is known as fallback. As example,
an 802.11b system will fallback from 11 Mbps to 5.5 Mbps as range increases or signal quality
decreases. Subsequent fallbacks from 5.5 Mbps to 2 Mbps and 1 Mbps are also supported
Interference and Coexistence
802.11b operates in a range of radio frequencies known as an "unlicensed" band (i.e. the FCC
does NOT require the use of a license in order to operate a radio transmitter in this range). This
means that commercially available radio devices other than wireless LAN devices are permitted
to use the same frequency band as 802.11b. Consequently, these co-existing radio devices can
interfere or "jam" the wireless LAN (and vice versa). Ironically, the most troublesome devices
are cordless telephones and microwave ovens.
Fortunately, higher quality cordless phones tend to "listen" for a clear channel before becoming
active and will thus avoid interfering with a wireless LAN (i.e., the cordless phone seeks a clear
channel for itself so naturally avoids being interfered with or being a source of interference).
Jamming from microwave ovens is more severe but is usually restricted to the upper frequency
range for 802.11b (it should be noted that 802.11b divides the available frequency band into 11
channels. The higher numbered channels are most susceptible to microwave oven interference).
In each instance, jamming occurs only when the cordless telephone or microwave oven is
active.
Security Issues
Much has been publicized in the mass media recently about security problems with wireless
LANs. Although it cannot be denied that the encryption algorithms currently used in 802.11b