#5 - Radio Systems

RADIO SYSTEMS

The majority of radio systems that are in use today are conventional systems. These include: Unit-To-Unit Central Dispatch Repeater Systems For many users, these systems provide the best combination of functionality and cost effectiveness.

UNIT-TO-UNIT SYSTEMS

The most basic type of conventional system provides unit-to-unit communications. This type of system allows field personnel to communicate directly with each other. In unit-to-unit systems, users are equipped with two-way radios, either mobile or portable, so that each person has the ability to both talk and listen. However, because these radios transmit and receive on the same frequency, a user cannot talk and listen simultaneously. Regardless of the number of radios in the system, only one radio user can talk at a time.

CENTRAL DISPATCH

In this type of system, a communications link is established between a central dispatcher and radio users in the field. When the dispatcher wants to call one or more field units, he or she transmits the message over the radio system. Everyone with radios tuned to the same frequency hears the message from the dispatcher. Likewise, field personnel can initiate a conversation or respond to the dispatcher.

REPEATERS

Recall that a repeater is a special type of base station. When the repeater receives a radio signal, it immediately retransmits that signal. Since the repeater is a base station which has a much higher transmitter output power than a mobile or portable radio, a repeater is an excellent way to extend the coverage range of a radio system. Many people who need and use radio communications cannot afford, or justify, the cost of their own repeater system. One way to get the benefits of a repeater system without the expense is by using a community repeater. A community repeater is a repeater system that can be shared by many users.

TRUNKING SYSTEMS

Trunking improves calling efficiency, with the most obvious benefit of a trunked radio system being its ability to minimize user waiting times. A Motorola trunking system consists of trunking repeaters, a central controller, and field units.

TRUNKING REPEATERS

A trunked radio system begins with a series of repeaters, it is possible for a Motorola trunked system to have up to 28 repeaters. One of the repeaters is assigned the duty of transmitting and receiving data information. This repeater is called the control channel. All other repeaters are designated as voice channels.

CENTRAL CONTROLLER

The central controller is a computer that acts much like a traffic cop. It processes all inbound and outbound data. When a request for a voice channel is received, it assigns an available repeater.

FIELD UNITS

In a trunked system, users are divided into groups. Each user group may consists of a control station at the dispatch point and a mixture of mobile and portable radios. All trunked radios are capable of switching to the frequencies of any of the repeaters in the system. They also contain a unique code word, which identifies each unit and indicates to which user group the radio belongs.

HOW A TRUNKED SYSTEM WORKS

Assume a trunking system is in the idle mode and repeater number one is the designated control channel. The idle mode is when no users are talking and all their radios are monitoring the data being sent out by the control channel. During the idle mode, the central controller consistently sends out data signals over the control channel. All user radios receive these data signals so they know which channel to monitor as the control channel. The central controller is also monitoring the control channel to see if anyone wants to make a call. All of this is done automatically. If a radio unit in user group B initiates a call, when the microphone is keyed, a burst of data identifying that individual radio and it's user group is automatically sent to the control channel indicating a unit in user group B is making a call request. First, the call request is sent through the control channel to the central controller. Next, the central controller processes the call request and assigns one of the idle voice channels to the entire user group. This channel assignment is transmitted in the form of another burst of data, back to the radios over the control channel. Next, each radio in the user group automatically switches to the assigned voice channel frequency and listens to the message. This is all done in less than half a second and in time to receive the caller's first words. All other radios not designated as members of user group B ignore the data and continue to monitor the control channel. Finally, when the call is completed, the radios in user group B switch back to the control channel frequency and once again continue to receive the data signals from the central controller. In a similar manner, if someone in user group A initiates a call to transmit a message, the control channel assigns one of the idle repeaters to the radios in user group A. Because each user group is assigned a different repeater with a different frequency, user group A will not hear user group B's transmissions, and vice-versa. Trunked radio systems are often used by government agencies where different departments, such as police and fire can use the same infrastructure, but operate as different user groups. Trunking can also be used for shared or public use. In these applications, different companies —like taxi cabs, construction companies, and delivery trucks—can be assigned as different user groups that use the same radio system, and are charged for their use of the service.

PAGING SYSTEMS

A radio paging system provides one-way communications to selective individuals. Pagers are FM receivers that alert a person that someone wants to communicate with him or her. The person wearing the pager is alerted when someone needs to get in touch with them. Unlike a two-way radio system, the people who do receive the paging message must respond by phone, or in person, since the pagers cannot talk back.

PAGING SYSTEM COMPONENTS

A paging system consists of a paging encoder, a radio transmitter, and individual pagers with a built-in radio receiver and decoder. Each pager has a specific address code that is unique to that pager—much like a mailing address that has a number, a street name, and a zip code.

HOW PAGING SYSTEMS WORK

To send a page, the caller enters the user's address code into the paging encoder. This code is transmitted over the air, much like a voice message, except it is digital information rather than voice. Each pager has a receiver and a digital decoder with its specific address code programmed into it. The pager compares the received code to its address code, if they match, the page is received and the user is alerted. If the codes do not match, the page is not allowed to pass through the decoder and nothing happens. Pagers can be programmed with individual, group or multiple code assignments. For example, a doctor may have a pager with an individual code so his/her office can contact him/her, and a group code that allows him/her and other members of an emergency team to be alerted via a single page during emergency response situations.

Types Of Pagers

There are a variety of pagers available which include: Basic pagers which are tone-only pagers that simply sound an alert. Alphanumeric pagers that alert the user and display a message, such as the telephone number to call, or an action to take. Tone and voice pagers which alert the user and then deliver a short voice message. Paging systems are for people who are on the move and they can be a cost effective and efficient means of communications in the right circumstances.