Two-way radio is used to communicate analog or digital data. Systems can
be simplex or duplex and may employ selective calling features such as CTCSS.
In full-duplex systems, data can be sent real-time between two points. In
simplex or half-duplex, data can be sent with a time lag between many points.
Some two-way digital systems carry both audio and data over a single
data stream. Systems of this type include NXDN and APCO Project 25. Other more
advanced systems under the TETRA standard are capable of joining time slots
together to improve data bandwidth, allowing advanced data polling and
telemetry applications over radio. The method of encoding and decoding the
audio stream is called a codec, such as the AMBE or the ACELP family of codecs.
After market GPS tracking and mobile messaging devices can be interfaced
with popular two-way radio models providing a range of features.
Analog
Analog systems may communicate a single condition, such as water level
in a livestock tank. A transmitter at the tank site continually sends a signal
with a constant audio tone. The tone would change in pitch to indicate the
tank's water level. A meter at the remote end would vary, corresponding to the
tone pitch, to indicate the amount of water present in the livestock tank. Similar
methods can be used to telemeter any analog condition. This type of radio
system serves a purpose equivalent to a four-to-twenty milliampere loop.[9] In
the US, mid-band 72–76 MHz or UHF 450–470 MHz interstitial
channels are often used for these systems. Some systems multiplex telemetry of
several analog conditions by limiting each to a separate range of tone pitches,
for example.[10]
Digital
Digital systems may communicate text messages from computer-aided
dispatch (CAD). For example, a display in a tow truck may give a textual
location for a call and any related details. The tow truck driver may press an
acknowledge button, sending data in the opposite direction and flagging the
call as received by the driver. They can be used for analog telemetry systems,
such as the livestock tank levels, as described above. Another possibility is
the lubricating oil pressure in a transit bus engine, or the current speed of
the bus. Analog conditions are translated into data words. Some systems send
radio paging messages which can either 1) beep a paging receiver, 2) send a
numeric message, or 3) send a text message.[11]
Digital systems typically use data rates in the 1,200–19,200 kilobit-per-second rates and may employ
modulation schemes such as frequency shift keying, audio frequency shift
keying, or quadrature phase shift keying to encode characters. Modern equipment
have the same capabilities to carry data as are found in Internet Protocol.
Working within the system's protocol constraints, virtually anything can be
sent or received.
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