Glossary...

    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.


To the Beginning


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).
 

800/900 MHz

    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.
 
 
Base
Mobile
Channel
Usage
463.0000
468.0000
MED 1
Ambulance to Hospital
463.0250
468.0250
MED 2
Ambulance to Hospital
463.0500
468.0500
MED 3
Ambulance to Hospital
463.0750
468.0750
MED 4
Ambulance to Hospital
463.1000
468.1000
MED 5
Ambulance to Hospital
463.1250
468.1250
MED 6
Hospital to Ambulance
463.1500
468.1500
MED 7
Hospital to Ambulance
463.1750
468.1750
MED 8
Hospital to Ambulance
462.9500
467.9500
MED 9
Coordinating Channel
462.9750
467.9750
MED 10
Coordinating Channel

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.
 
 
Hz
Code
Hz
Code
Hz
Code
67.0
XZ
107.2
1B
173.8
6A
69.3
WZ
110.9
2Z
179.9
6B
71.9
XA
114.8
2A
186.2
7Z
74.4
WA
118.8
2B
192.8
7A
77.0
XB
123.0
3Z
203.5
M1
79.7
SP
127.3
3A
206.5
8Z
82.5
YZ
131.8
3B
210.7
M2
85.4
YA
136.5
4Z
218.1
M3
88.5
YB
141.3
4A
225.7
M4
91.5
ZZ
146.2
4B
229.1
9Z
94.8
ZA
151.4
5Z
233.6
M5
97.4
ZB
156.7
5A
241.8
M6
100.0
1Z
162.2
5B
250.3
M7
103.5
1A
167.9
6Z
254.1
0Z


To the Beginning


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

    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).
 

Skip

    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.
 

Trunking

    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.
 


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