AML MT7570 Vehicle Mount Computer - Manual (Page 42)

7-1
USER’S GUIDE MT7570 Vehicle Mount Terminal
Internal 802.11 b/g Radio (Linux ® )
The M7570 vehicle mount terminal comes standard with an 802.11b/g radio and internal antenna. This
radio is specifically designed to communicate with any 802.11b or 802.11g 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 802.11 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.
802.11 Fallback
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 the 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. For 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
802.11 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 com-
mercially available radio devices other than wireless LAN devices are permitted to use the same
frequency band as 802.11. Consequently, these co-existing radio devices can interfere or "jam" the
wireless LAN (and vice versa). 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 802.11 (it should be
noted that 802.11b/g divides the available frequency band into 11 channels [US]. 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.
Chapter 7