Technical Terms and Definitions

Learn about commonly used radio communications terms.


Whenever a person speaks, sound is projected in the form of a sound wave. These waves move at a certain frequency that determines the pitch of the sound. An analog radio network transmits the actual wave of a person’s voice over the air by modulating it onto a radio frequency carrier. An analog network operates differently from a digital network, which converts the vocal sound wave into a digital bit stream of ones and zeros. This information is then sent over the air, and eventually converted back into an analog wave to be heard.


This stands for the Association of Public Safety Communications Officials. This agency is composed of many different public safety employees and representatives from around the world. APCO was one of the lead agencies in the development of the Project 25 (P25) Standard on which the CREST system is based.


This is a type of radio communication system developed by Motorola that uses narrowband digital technology. This system makes good use of available frequencies, and allows for greater security than older systems. ASTRO can stand alone as a digital network or incorporate analog technology as well.

Backward Compatibility

This term refers to the capability of a piece of equipment (i.e. software program, hardware component) to be compatible with its predecessor in all forms. For instance, Microsoft Word 360 is backward compatible since it can create documents that will run in Microsoft Word ’97.

Base Station

This refers to a stationary radio connected to an antenna. The antenna is located where it can transmit into and receive from a geographic area where mobile and portable radios are being operated. On the CREST system, the mobile and portable radios can communicate at extended distances from each other by communicating through the base stations (see repeaters). The mobile and portable radios can also communicate with dispatchers at radio consoles who are remotely operating the base station radios via the CREST system.

Common Air Interface (CAI)

This refers to the protocol by which handheld and mobile radios communicate with the radio system infrastructure. In modern radio systems this is typically a proprietary format, however the CAI, defined by the Project 25 Standard, makes this an open protocol. This allows different manufacturers’ portable and mobile radios to work together on a single radio system.


A console is used by a dispatch operator to communicate with users in the field, to track radio activity, and to coordinate the efforts of various public safety agencies. A typical dispatch position consists of various types of equipment, which along with the radio console includes several different tracking and communication systems, usually running on anywhere from one to five computers. Dispatchers usually operate in a public safety facility, with consoles set up in their individual work areas.


A conventional radio network permanently allocates specific frequencies to specific groups of radio users. If nobody in a particular group is transmitting on their assigned frequency, then that channel remains open. This is in contrast to a trunking network that assigns frequencies to users only when they are needed, which can be more efficient.

Coverage/Coverage Area

A radio network’s coverage area refers to the entire area that gets a strong enough signal from the network for a radio in the field to transmit and receive. Once a signal from a network degrades so badly that it is essentially useless, and all transmissions are bad or impossible, then that area is considered to be out of the coverage area. The coverage area is often called the “footprint” of a network.


The term “digital” refers to the method of expressing information in one of two different electronic states, which are usually designated as ones or zeros. These ones and zeros form a pattern that can be translated into all kinds of information. Digital information is relayed through an electrical system by transmitting electronic pulses with one of two distinct electrical charges. These pulses are usually referred to as either “1” or “0”, with the “1” pulse usually having a higher voltage, or charge, than the “0” pulse. Electronic equipment such as computers can interpret the information by: a) receiving a set of electronic pulses, b) sensing the different voltages of the pulses, therefore determining whether each pulse is a “1” or a “0”, and c) combining many of these ones and zeros to form instructions that tell the computer what to do.


Digital transmissions can use encryption to secure information that is being transmitted. The reason this security technique is so effective is because the encrypted transmissions can only be deciphered by a radio with the proper decryption key. This key consists of a software application that is programmed into the authorized radios.


All radio networks broadcast their transmissions through antennas on a certain frequency. The number of the frequency refers to the number of times that an electromagnetic wave repeats in the span of one second. For example, a transmission being sent at 150 MHz means that its wavelength repeats 150,000,000 times per second. With sophisticated electronic equipment, these waves can be engineered to carry large amounts of information over great distances.

Industry Canada

This federal government department decides how frequencies are to be used, as well as who can transmit on them. It sets aside specific amounts of frequencies for public safety transmissions, commercial wireless carriers, television broadcasts, etc. Traditionally, it grants blocks of frequencies to public safety agencies, while commercial carriers must buy a license to transmit.

Intelligent Site Repeater

An Intelligent Site Repeater is a radio site that uses a device called a site controller. This controller can perform all call processing and channel assignment tasks that are required to operate the site’s base stations.


This term refers to the ability of people from different organizations to talk to one another directly vs. through their dispatch operators. CREST provides the technology to support interoperability between all participating agencies. However, procedures need to be in place and tested to ensure that service providers are able to use the system’s interoperability when needed.


In radio systems, the term mobile is usually used when referring to a vehicle-mounted radio unit. This is different from a portable radio, which refers to a handheld radio.


In radio systems, the term narrowband refers to the size of a channel with regards to frequency. In the CREST digital trunked radio system, all voice channels have a bandwidth of 12.5 kHz, which is considered to be narrowband. This is an upgrade from the previous conventional analog network, which used 25 kHz of bandwidth per voice channel.


In radio systems, the term portable usually refers to a handheld radio. This is different from a mobile radio, which would refer to a radio mounted inside a vehicle.

PTT (Push To Talk)

This term refers to the button on a radio that a user pushes to transmit. When somebody wants to talk over the air, they depress the PTT on their portable radio, mobile radio, or dispatch console, and if there is an available frequency, they will be able to speak over the network. When a user presses the PTT, it is often referred to as “keying” the radio.


When a radio user tries to make a call, and there are no available frequencies to transmit on, that user’s call gets placed in a queue. For the most part, the first user that gets placed into a queue will get to transmit as soon as a frequency becomes available, and any subsequent users in the queue will transmit when their turn arrives.


This term takes on multiple meanings when applied to a communications system. When the term radio is used, it can refer to any of the following: a portable device used to transmit audio, a base station at a transmit site that contains electronic equipment, electromagnetic waves in the air which carry a network’s information, or any device used to receive and/or transmit information across a medium.


A repeater is a piece of equipment that acts as a transmitter and a receiver. In a radio communications system, repeaters are used to extend the coverage of a wireless transmission. The repeater accomplishes this by first receiving a signal that has been transmitted from some other location, then amplifying and re-transmitting that signal from an antenna, thus giving the original transmission a boost.


A radio network that is simulcast transmits information from each of its transmission sites simultaneously. This means that when a radio user transmits from his/her radio, that transmission is rebroadcast from every tower or antenna that is part of the simulcast system. Because of this technique, any radio can pick up any transmission, regardless of its location.


Also called transmit site, cell site, radio site, or antenna site. Any radio network transmits and receives its signals through antennas that are placed strategically in different locations throughout their desired coverage area. These places are called sites. Usually the antennas at these sites are mounted high above ground on towers or on the sides of buildings.


A talkgroup is a group of radio users that are linked to each other through the radio system. For instance, if any member of a talkgroup initiates a call, any member of that group will hear that transmission. The CREST network incorporates many different talkgroups, and the users in these groups interact with the members of their own group as well as monitor other talkgroups throughout the network.


This term refers to the number of transmissions being made on the network at any given moment. Although most networks are designed to function even when very busy, an excess of traffic on a network may cause some radios to be placed in a queue when trying to transmit. Comprehensive traffic projections have been taken into account while designing the CREST network, and since this network provides radio coverage to public safety agencies, the standards have been raised much higher than that of a commercial wireless provider.


This term refers to a type of communications system that draws from a pool of available frequencies, and assigns them only when they are needed. For example, when a radio user wishes to talk over the air, they push their transmit button and the system dedicates a frequency to broadcast that user’s transmission. After the user lets go of the transmit button, the system can reassign that same frequency to a completely different radio. Trunking is different from a conventional radio network, which assigns one dedicated frequency to a group of radios indefinitely. In a conventional system, if nobody in a particular group is transmitting, their assigned frequency sits unused and is essentially wasted. Trunking can be more efficient, since any available frequency can be used whenever it is needed.


This piece of equipment transforms the sound of a person’s voice into a stream of digital information. It also reverses the process converting digital information back to voice. The vocoder is vital to the operation of a digital network, since without it, no audio transmissions could be sent or understood.