Scanning FAQ'S

WHAT'S TRUNKING?

Trunked 800 MHz radio systems are slowly shaping the way all public safety agencies are organized, dispatched and operated. The Federal Communications Commission (FCC) is determined that trunking will be the radio standard into the 21st century because it uses the radio spectrum more efficiently and allows more users onto the airwaves.

The Advantages

Besides making greater use of an increasingly limited radio spectrum, trunked radio systems generally permit a larger number of users than a conventional system. For some police departments, this alone is justification enough to spend the money on trunked radio.

The key to this capability lies in the ability of a trunked radio system to use a pool of frequencies for any of the system's users. When an officer wants to transmit, a trunked system simply looks at which frequency is unused and quickly assigns it to the officer for the duration of the transmission. Since field units infrequently transmit at the same time and most police transmissions are relatively short, frequency sharing can create more on-air time.

Another advantage of trunked systems is their flexibility. They're controlled entirely by computer, so it's easy to set up complex channel assignments, and to reconfigure the system as needed without the services of a technician. Police departments can quickly "create" an additional channel to handle a hostage situation or change channel assignments to allow public works or other city agencies to communicate on the police channel during natural disasters.

In contrast, a conventional system limits each user to just their assigned frequency. If the police channel is busy, officers just have to wait their turn, even though the fire or public works channel is going unused at the moment. And in most systems, channel assignments can't be easily reconfigured or shared with other city agencies.

HOW DOES IT WORK?

Despite its sophistication, trunked 800 MHz is designed to shield the user from the all the technology. A radio operator needs to know nothing about how the system works or why, but simply presses the button to receive the benefits of years of research.

Basically, trunked radio is method of making a group of radio channels available to several different groups through an electronic switching system that is controlled by a computer.

For the end user, trunked radio systems don't have traditional channels.

Instead, each mobile radio in the system is assigned a "talk group," which is simply a programmable, internal identification that is transmitted each time the radio begins a broadcast. The ID is not heard by the dispatcher, but it's recognized by the computer controlling the radio system.

For the radio techs, trunked systems really do have channels, which are typically provided in groups of four or five. One channel is arbitrarily designated as the control channel and the other four are available for voice communications.

In a conventional, one-channel radio system all mobile radios listen for a transmission the single channel. Everyone checks for a clear channel before broadcasting. When someone pushes the XMIT button, all other radios on the channel hear the transmission. Another user pushes the XMIT button to respond and everyone hears that transmission, too.

In a trunked radio system, every radio is tuned to a control channel. When someone pushes the XMIT button, the mobile radio transmits its individual ID and assigned talk group ID on the control channel to the base station. It's heard only by the trunked radio system computer and the data is decoded. The computer does some "thinking", and instantly transmits a digital message back out on the control channel to all the system's radios.

The digital message says, "Every radio on talk group XX, switch to channel 1 (or 2 or 3 or 4). Again instantly, every mobile with the same talk group code as the transmitting mobile switches to the assigned channel and they hear the calling mobile unit's voice transmission. Radios assigned to other talk groups continue to monitor the control channel and do not hear the voice transmission.

When the transmitting mobile releases the XMIT button, all mobiles resume listening to the control channel -- and the process starts all over again if another unit transmits. A transmission can occur on any of the assigned voice channels. A user never knows exactly what frequency is being assigned--and doesn't have to know, either.

This complex channel-switching process happens in just milliseconds--so short a delay that users don't notice that it's happening under most circumstances.

WHERE DO I START SCANNING?

29.000 - 50.000 MHz - Business, Gov't, old style cordless phones baby monitors 50.000 - 54.000 MHz - 6 Meter Ham Radio Band 108.000 - 136.000 MHz - Aircraft, this is a very busy frequency range for listening to aircraft.

138.000 - 144.000 MHz - Gov't

144.000 - 148.000 MHz - 2-meter Amateur Radio local ham radio repeaters, simplex 150.000 - 174.000 MHz - Shared by Business, Trains, Taxi, Maritime, Weather National Weather Broadcasts!, public safety and alot more.

406.000 - 420.000 MHz - Gov't agencies

420.000 - 450.000 MHz - 70cm ham radio band 450.000 - 470.000 MHz - Business Band, Large corps, and industry, Gov't, &

State(s) Dept of public safety

470.000 - 512.000 MHz - Public Safety

806.000 - 940.000 MHz - 800 mhz Public Safety 851.000 - 866.000 MHz - Public Safety and businesses 866.000 - 869.000 MHz - Public Safety

WHAT IS APCO 25?
 

APCO 25 brings together representatives from many local, state and federal government agencies who evaluate basic technologies in advanced land mobile radio. The objective is to find solutions that best serve the needs of the public safety marketplace. In addition, the committee has encouraged the participation of numerous international public safety organizations, making this a truly worldwide recommended standard-setting initiative. Basically stated many state agencies using one system, verses one supplier.

What are the benefits of APCO 25?
 

Every aspect of APCO 25 is designed to benefit public safety professionals who seek a new level of performance, efficiency, capabilities, and quality in two-way radio communications. Four key objectives guided the steering committee through this open process:

Provide enhanced functionality with equipment and capabilities focused on public safety needs.

Improve spectrum efficiency. Ensure competition among multiple vendors through Open Systems Architecture.

allow effective, efficient, and reliable intra-agency and inter-agency communications.

By adhering to these objectives, APCO 25 makes it easier for users to make the most informed decision possible when planning to convert existing system to digital. Each vendor's system will begin on a level playing field determined by an agreed upon base line set of specifications. This allows users to more accurately compare the direct features and benefits of both entire systems and individual radio products. This will make bidding processes more competitive among prospective vendors. Plus, users have the opportunity to mix and match equipment among APCO 25-compliant suppliers since their equipment will follow all basic standards.