Upgrading Security - The Drinking Water - page 300

and completing the electrical circuit. This activates any linked equipment.
These types of systems are often used for high-voltage applications, such as
in some automotive and other manufacturing processes.
Two-Way Radios
Two-way radios, as discussed here, are limited to direct unit-to-unit radio
communication, either via single unit-to-unit transmission and reception or
via multiple hand-held units to a base station radio contact and distribution
system. Radiofrequency spectrum limitations apply to all hand-held units
and are directed by the Federal Communications Commission (FCC). This
discussion also distinguishes between a hand-held unit and a base station
or base station unit, such as those used by amateur (ham) radio operators,
which operate under different wavelength parameters.
Two-way radios allow a user to contact another user or group of users
instantly on the same frequency and to transmit voice or data without the
need for wires. They use half-duplex communication, which means that they
cannot transmit and receive data simultaneously. In other words, only one
person may talk while other personnel with radios can only listen. To talk,
the user depresses the talk button and speaks into the radio. The audio then
transmits the voice wirelessly to the receiving radios. When the speaker has
finished speaking and the channel has cleared, users on any of the receiv-
ing radios can transmit, either to answer the first transmission or to begin a
new conversation. In addition to carrying voice data, many types of wireless
radios also allow the transmission of digital data, and these radios may be
interfaced with computer networks that can use or track these data. Some
two-way radios can send information such as global positioning system
(GPS) data or the ID of the radio, and others can send data through a SCADA
system.
Wireless radios broadcast these voice or data communications over the
airwaves from the transmitter to the receiver. This can be an advantage in
that the signal emanates in all directions and does not require a direct physi-
cal connection to be received at the receiver, but it can also make the com-
munications vulnerable to being blocked, intercepted, or otherwise altered.
Additional security features are available, however, to ensure that the com-
munications are not tampered with.
Wireless Data Communications
A wireless data communication system consists of two components—a wire-
less access point (WAP) and a wireless network interface card (sometimes also
referred to as a client )—which work together to complete the communica-
tions link. These wireless systems can link electronic devices, computers,
and computer systems together using radiowaves, thus eliminating the need