800MHz Trunked Radio System

he words "800 MHz" have become the buzzword of police departments lately. 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.

Technically Speaking
Trunked radio is synonymous with the 800 MHz band, although the technology can be applied to frequencies in any radio band. Only FCC rules and channel availability currently make the 800 MHz UHF band the most efficient for trunking. In the future, trunking may become the standard in all radio bands.

Like most other UHF systems, 800 MHz trunked systems are designed to be repeated and to use satellite receivers to provide complete coverage. Each trunked channel consists of a pair of frequencies--one base station frequency and one mobile frequency spaced 45 MHz apart.

After the FCC allocated a portion of the 800 MHz band to public safety, they mandated a complex planning process for each region of the country which wanted to use the band. They established 240 channel pairs for police, fire and related agencies with 12.5 KHz spacing in the 821-823 band. They also established five specific channels that all agencies must have for mutual aid and coordination.

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.

Fail Safe
As a cautious police officer, you might feel that putting an entire city's public safety agencies on one system would create a tremendous opportunity for disaster if the system fails. But trunked radio systems are designed so that a failure of one or more components doesn't affect communications.

Back-up power supplies, such as batteries or a generator, insure that the system is always powered. Redundant radio components for each radio channel insure that a failed circuit board or electronic part doesn't cause that channel to go off the air.

And since a trunked system is controlled by computer software, when failures do occur they can be handled automatically in an "intelligent" manner. For example, if the control channel fails, its duties can be shifted in seconds to one of the remaining communications channels. Channel availability is decreased somewhat, but no one suffers from an outage.

Conclusion
The Philadelphia 800MHz system is now well into it's development period. Once installed, it is expected that it will result in more efficient radio communications. That translates directly into an increase to officer safety and their ability to quickly respond to your needs.

Source: Adapted from article that appeared in "Dispatch Monthly Magazine."

Information listed here is believed to be current at the time of publication. However, some of the material presented here may have expired since it was posted. Persons should contact a Philadelphia Police representative whenever relying on dated material or information that is subject to change.

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