Sometimes the terminology utilized can be a bit arcane.
This glossary, though incapable of covering every single technological
definition available in the discipline, will at least give you an idea
of what's going on.
Aircraft Band
The portion of the VHF band extending from
108 to 136 MHz. Used for all civil aircraft communications.
Aircraft transmissions are in AM mode (in contrast to other VHF and UHF
two-way transmissions, which are in FM mode) and can be monitored only
with receivers equipped for this band.
Ambient Noise
Environmental or background electro-magnetic
noise.
Antenna Gain
An antenna characteristic that increases the
power density in a given direction by concentrating of radiated energy
Association of Public Safety Communications Officers or APCO
Oldest and largest association of public safety
personnel, they are the authors of Project 16, Project
25 and designated FCC frequency coordinator for the Public Radio Service,
the Local Government Radio Service and the Public Safety "pool" of frequencies
in the 800MHz band.
Attenuation
The decrease in amplitude of a signal during
its transmission from one point to another. This procedure allows
the receiver to decrease the amount of RF coming into the Front End, thus
lowering the distortion and intermixing of signals. Usually found
only on high-end receivers.
Automatic Vehicle Locator or AVL
Using Loran-C, the AVL continually pools vehicles
to identify the unit(s) closest to an incident. Other technologies
include "dead reckoning" systems are available.
Bandwidth
The range of frequencies assigned to a channel
or system. Also, the difference, expressed in Hertz, between the
highest and lowest frequencies of a given band.
Call Signs
Each station and mobile unit is assigned a
call sign by the Federal Communications Commission (FCC). Base station
call signs usually are in the form of three or four letters followed by
three numbers (e.g., KOP911 or KNBB445). Control stations, operating
as inputs to repeaters, formerly used three letter, three number call signs
beginning with the letters WAA - WXZ. Presently, the FCC is assigning
the same call sign to all stations and units within a system. All
stations are supposed to identify with their call signs every half-hour,
but this regulation is widely ignored.
CCS
Also known as Hundred Call Seconds, this is
a unit which communications traffic is measured. Traffic in CCS is
given by" Total number of calls X total duration of calls (in seconds)
/100.
Cellular Radio
An advanced mobile telephone system which
provides communications by dividing a metropolitan area into smaller geographic
units, or cells, each served by a transmitter and controller. Each
cell can reuse channels assigned to mobile communications, greatly increasing
capacity and service quality. When a portable or mobile telephone
user crosses from one cell to another while talking, a central computer
in the cellular system automatically reroutes the conversation from one
transmitter to another, eliminating interference between callers and allowing
each channel to be used several times simultaneously.
Channel
A communication path; a route for electrical
transmission between two or more points. A channel may be a wire,
a radio carrier or a light wave. A channel may also be called a circuit,
line, link or path.
Channel Loading Standards
FCC standards designating the number of mobile
units that must be assigned to a single channel before that channel is
considered "fully loaded," thereby justify the assignment of additional
channels. Peak loading refers to anticipated traffic level of a channel
during the busy hour.
City or County Services
Also known as "local government" channels, these
are catch-all listings that may be used for any city or county department,
including sheriff, police, fire, ambulance, road crews, transportation,
elderly services, etc.
Common Carrier
A company which provides telecommunications
facilities to the public. Be definition, they must offer service
to anyone who requests at a reasonable fee (in contrast to private services
which offer service internally to a business or a specified group such
as local governments). They are regulated by the FCC and the state
Public Utility Commissions.
Communications Act of 1934
An act of Congress, this establishes the U.S.
Federal Communications Commissions. It gives responsibility for regulating
national and international communications to the FCC, reserving the regulation
of intrastate communications and exchange services to the states.
The line of demarcation between state PUC and FCC areas of jurisdiction
is continuously being challenged in FCC dockets and in the courts.
Computer Aided Dispatch
Also known as CAD, a system intended to help dispatchers respond to emergencies faster and more accurately by accessing addresses, records, emergency vehicle status information and so fourth, by computer. Within this general concept, systems are designed with varying capabilities to meet a jurisdictions individual requirements.
Concentrator
A communications device that provides communications
capability between many low speeds, usually asynchronous channels and one
or more high speeds, usually synchronous channels.
Control Channel
A selected repeater in a trunked system through
which there is a continuous two-way data communication path between the
Central Controller and all mobiles and portables in a system.
Controller
Microprocessor based computer which is the
heart of the trunked system. All coordination of channel assignments
is made by the Central Controller through the control channel.
Conventional System
Unlike a trunked system, frequencies are assigned
to particular channels and a user is either signed onto a channel or not.
These are simpler technologies are are less expensive; they allow persons
to monitor desired channels in contrast with a trunked system (using conventional
receivers).
Dead Spots
Locations from which effective, understandable
communications cannot be established because the transmitted signal is
blocked by buildings, trees, distances or terrain.
Decibel
The standard unit for expressing transmission
gain or loss and relative power ratio. The decibel is a relative
measure based on a logarithmic scale.
Duplex
In duplex systems, base and mobiles operate
on separate frequencies. Base stations can receive all mobile units
and mobile units can receive the base station, but not he other mobile
units. Duplex systems frequently operate in conjunction with "car-to-car"
or other simplex frequencies to which mobile units can switch if they wish
to communicate with one another. Many state police systems operate
in duplex mode. Duplex systems may be recognized when different base
and mobile frequencies are shown in a directory. In other words,
the ability of communications circuits to transmit and receive simultaneously.
Digital Voice Protection or DVP
DVP is a scrambling system where base and
mobile units are programmed with code keys, chosen from a hugh number (ten
followed by eighteen zeros) of possibilities. Voice frequencies are
encoded by digital methods and sound like burst of static when intercepted
on ordinary receiving equipment. Up until recently, there were no
known means of unscrambling DVP or DES, another form of scrambling used
by many Federal agencies. However, by utilizing the Internet and
pooling all the resources of those computers together, the code was finally
broken.
Enhanced 911 or E-911
Basic 911 routes a call to the dispatchers.
Enhanced 911 has added capabilities, most notably, it provides the caller's
address and/or phone number on the screen automatically.
Earth Stations
An earth station is the physical equipment
used to send data communications to a satellite (called the up-link) and/or
receive data communications from a satellite (called a down-link).
The land-mobile radio frequency band with
the range of 806 - 960 MHz. Both conventional repeater and trunked
systems operate in the band. See Repeaters
and
Trunking.
Local Area Network
A computer and communications network without a central
node or processor covering a limited geographical area, allowing every
node to communicate with every other node and allowing users simultaneous
access to data files and software.
Loran-C
A database of geographic coordinates developed
and operated by the Coast Guard.
Low Band
The portion of the VHF band extending from
30 through 50 MHz. Used for broad coverage. See Skip.
MED Channels
Med channels are utilized by emergency medical
service technicians and paramedics for communication with hospitals.
Ambulances will generally work through C-MED (centralized or coordinated
medical dispatch) centers which patch the ambulance through (from radio
to telephone line) to a hospital where the patient is to be transported.
Med channels are UHF frequencies which may or may not be repeater-controlled.
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Microwave
A form of communication transmission constituting
a narrow beam (usually five degrees) which can be separated into multiple
channels. Transmission is limited in the distance signals can travel
before requiring retransmission through a repeater station. Thus
repeater stations are typically placed approximately thirty miles apart
on towers, the tops of tall building or hilltops to provide line of sight
paths.
Mobile Phone
Generally refers to radio telephones installed
in automobiles that can operate from any location within the system coverage
area.
Mobile Data Terminal or MDT
Small computer terminals operating in an automobile,
MDTs are usually linked with CAD systems and can be used for all non-voice
communications, from providing an officer's status to searching for information
on a driver's license and vehicle registration.
Multiplex
The process of combining a number of radio
signals on a single transmission wire to or from an antenna.
Path Loss
The attenuation of electro-magnetic energy
over the path between two points.
PL (CTCSS) Tone Controls
PL ("Private Line" or "Channel Guard") codes
are sub-audible tones which are utilized by many agencies to help limit
interference. PL, in effect, slices a single frequency into many
portions, allowing nearby departments to use the same frequency in systems
with more that one repeater site. In this way, a turnpike authority,
for instance, may use a single frequency for the length of the tool road,
but patrol cars on the west end of the state will not receive interference
from units on the east end as they will be using a distinct sub-audible
tone. DPL (Digital Private Line) is another format utilized.
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Portable Phone
Generally refers to a hand-held radio telephone
that can be operated from any location within the system coverage area.
POTS
Plain old telephone system.
Project 16
A study addressing the economic, technical
and managerial questions surrounding the 800MHz band designed for public
use. Provides recommendations and specifications for the implementations.
Propagation Delay
The delay between the time a signal enters
a channel and the time it is received.
PSK
Phase shift keying.
PSTN
Public switched telephone network.
RBOC
Regional Bell operating system.
Repeaters receive transmissions from mobile
or hand-held units and rebroadcasts these transmissions from higher-power
transmitters or locations offering wide coverage, thus extending the range
of mobile units. Repeaters are usually located on mountain tops or
high structures to achieve a greater service area. Typically, police
radio systems in larger cities employ repeaters to allow cars and officers
on foot to communicate with one another using portable or mobile radios.
Since repeaters receive mobile transmission on one frequency (the "input")
and retransmit them on another frequency (the "output"), it is only necessary
to monitor the output frequency to hear all the traffic. Repeaters
operate in all frequency bands, but follow certain rules in the UHF bands.
In system operating between 450 and 470 MHz, the input frequency is always
5 MHz higher than the output frequency. In the 470 - 512 MHz band,
input frequencies are always 3 MHz higher. In the 800 MHz band (see
800/900
MHz), mobile units transmit 45 MHz lower than base stations and
repeaters. Not all systems in the UHF bands (450 - 512 MHz) use repeaters.
Generally speaking, repeater inputs, whether BHF or UHF, have not been
included in this collection. In a few cases, bases and mobiles simply
operate on different frequencies without a repeater. In such instances,
though, base stations operate 5 MHz (450 - 470 MHz) or 3 MHz (470 - 512
MHz) lower than mobile stations, and both frequencies must be monitored
to hear both sides of the conversation. This is often the case with
C-MED channels, which are usually not repeater-controlled. Note that
most systems operating above 400 MHz are repeater-controlled.
Scramblers
Many public safety and federal government
agencies (and, recently, some business radio users) utilize scrambling
to maintain communications security. The simplest scramblers ("single-inversion"
units) operate by exchanging high and low audio frequencies for one another.
Scramblers such as this deter casual eavesdroppers, but are easily defeated
by commercially available descramblers. A variety of more complex
and costly scramblers are available, including Digital
Voice Protection.
Simplex
In simplex systems, all units (bases and mobiles)
operate on the same frequency. All frequencies listed in the collection
are simplex unless otherwise noted (although in some cases I may not have
firm information as to whether a repeater is or is not in operation).
VHF and UHF radio waves normally are "line-of-sight":
Their range is limited by the horizon and other obstructions. At
certain times VHF transmissions can "skip" - bouncing off energized layers
of the ionosphere - and return to the Earth to be received a long distance
away from their origin. Particularly on low bands, stations from
thousands of miles away may suddenly interfere with one another's transmissions.
Subfleet
A subset of a given fleet whose normal communications
do not require interfacing with other subsets of the fleet. Typically
the majority of an individual's communication will be within his own subfleet.
A subfleet conversation can be assigned to any available frequency.
10-Codes
Used for brevity and security by many agencies.
The APCO 10-Codes are a standardization set of codes designed to facilitate
communications between different agencies. Many agencies use some
or all of those codes. Other informal codes and abbreviations, unique
to each agency, are often heard.
Transceiver
The combination of radio transmitter and receiving
equipment in a common housing. Usually for portable or mobile use.
In trunked systems, operating in the 800 MHz band,
bases and mobiles are assigned from 3 to 30 (or more) channels. When
a microphone is keyed, the system automatically switches to an open channel.
Certain channels can be restricted to command, data, supervisory, intercity,
telephone interconnect or area use. (Data channels will often routinely
change to save wear on a repeater. Telephone interconnect and intercity
channels are used in the conventional mode.) Because of their efficient
use of limited frequencies, trunked systems have become the standard in
public safety communications. Many different users are assigned to
the set of frequencies and is therefore difficult to monitor (on a conventional
receiver) one user, or one unit, as any one of may channels may be utilized.
However, since the introduction of Uniden's TrankTrack receiver incorporating
software that will track Motorola systems, this listening restriction has
been greatly reduced. The typical trunking system operates between
853 and 870 MHz. On trunking systems, no agency uses a single frequency:
Instead, several frequencies are utilized and either multiple agencies,
or multiple divisions of a single large agency, operate in separate "fleets"
and "sub-fleets" where units only hear the radio traffic of their own division
by means of computer-controlled technology. Many trunking systems,
such as Ericsson's EDACS, incorporate "anti-scanner" technology that produces
beeps, tomes and other sounds whenever there is no voice on the air.
This anti-scan technology is designed to deter monitoring of voice communications.
However, over the years there has been other technology introduced that
combats the anti-scanner technology. Most have been very successful.
Trunking Formulas
Mathematical formulas taking into account various
assumptions and indicating the number of channels required for a given
application.
UHF Band
Extends from 450 to 512 MHz. Its upper portion
is known as the T-Band, from 470 - 512 MHz, usually found in larger metropolitan
areas.
Uninterruptable Power Supply
A device which provides a constant source of power
in the event of loss of commercial power. Many UPS' will also "clean-up"
street power, preventing computer errors and crashes.
VSAT
Very small aperture terminal (satellite).
WAN
Similar to a local area network, but incorporating
several LANs to create a wide area network. The Internet is an example
of a WAN.
