AML M7225 Handheld Computer - Manual (Page 32)

M7225 User’s Guide
32
Chapter 5 – Wireless Networking
General Overview
The M7225 can contain an optional 802.11b/g radio and internal antenna. This radio is specifically designed to
communicate with any 802.11b/g Access Point.
The range of the radio depends greatly on the
quality of the Access Point and the RF commu-
nications characteristic of the environment
where the device is used. The typical range for
an 802.11 radio is 300 feet through free air.
Additional Access Points must be added to
improve coverage in a larger area, or in electri-
cally 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 sig-
nal 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 fall-
backs 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 commercially avail-
able 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 suscepti-
ble to microwave oven interference).
In each instance, jamming occurs only when the cordless telephone or microwave oven is active.