[This story originally ran in the mid-1990s, but still accurately describes the trunked systems of today.--Editor]
The words "800 MHz" have become the buzzword of public safety agencies lately, but behind the words lies a jungle of high technology, federal regulation and politics that few agencies have the experience to conquer.
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.
But for law enforcement officers, firefighters or EMS personnel working the front lines against crime, the arrival of this new radio technology had better mean just one thing--better performance. Yet a complicated process of specifying, purchasing, installing and maintaining such systems may mean that public safety agencies will have to use existing systems for several more years.
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 public safety agencies, 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 public safety 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 or fire departments can quickly "create" an additional channel to handle a hostage situation or major fire, or change channel assignments to allow public works or other city agencies to communicate on the police/fire 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.
The Questions
But with this increasingly complex technology comes some pretty stiff questions about the ability of 800 MHz to adequately serve a police or fire department. Trunked systems are always more expensive, more complex, take more training to understand and operate, and must be engineered carefully to operate successfully. The companies that sell these systems, who have a natural stake in seeing 800 MHz succeed, have tweaked the technology to answer most of these questions, but many pitfalls remain.
The first question police officers or firefighters usually raise about 800 MHz trunked relates to safety. "How can we guarantee access to a radio channel?" they ask. The radio marketers have answered this concern with several schemes that designate certain agencies as "priority", which insures they always have an available channel with they press the XMIT button.
Lock-out can occur when several radios attempt to transmit at the same time. For example, most trunked systems are configured in groups of four voice channels. If five units transmit at once, one unit won't have access to the radio system. In a well-configured system with sufficient channels, this outcome can be minimized, but the possibility always exists.
But establishing priorities can also create its own problem. If certain agencies are designated as "priority", others must be declared as "non-priority" units, capable of being bumped off the air. That concept, and the thought of being denied access, may be procedurally or politically unacceptable to the other agencies sharing the trunked system with the police department. The only solution, purchasing additional channels, is costly.
Money is always an issue with police and fire departments--there's never enough of it. And 800 MHz radio systems are expensive to purchase and maintain. Existing systems are never compatible with new trunked systems, so old radios and equipment can't be salvaged. The line-of-sight characteristics of 800 MHz radio demand more receiver sites, linked to the communications center by expensive telephone lines or microwave links.
Loaded with extra features, trunked systems are inherently more expensive, simply because they represent the "latest technology." When confronted by glitzy features, and an array of convenience and technical options, an officer suddenly designated as the department's "expert" might approve thousands of additional dollars for features that will never be used.
Trunked radio raises another set of questions that don't relate to the technology at all, but to another issue critical to the success of any system: inter-agency cooperation.
It's common for individual agencies to meet, draw up specifications and to implement technical systems. It's even fairly common for police and fire agencies to cooperate on operations and support systems. But it's quite another matter to bring police, fire, public works, electrical and other city agencies together into one room and decide on a single radio system.
The first difficult step is gathering all of the parties down at a single table. For some cities or counties, this may be the first such meeting they've held. For others who have shared microwave or other communications systems, or who have a jurisdiction-wide communications manager, such a meeting may be routine.
But either way, the difficulties of dealing with more than one agency are concrete. As the numbers of agency heads goes up, the likelihood of a consensus goes down--this is true for any organization. Second, the number of agendas increases as you add agencies. And third, the differences in perceived priorities among the agencies may create friction that may submarine the radio project before it's implemented.
Each public safety agency is likely to already have has its own radio system. The systems may have been installed by different companies and most likely on a different radio band. Each system manager may have his/her own preference for how the system should operate and what company should provide it.
Since each agency has different duties and a different set of "customers", the tasks that the radio system performs in vastly different for each agency. The police obviously have a need for clear, reliable communications for the protection of life and property, while the public works department requires a system capable of reaching into sewers, tunnels and near power lines.
As talks progress, the participants are bound to discover there is a perceived difference in which agency is leading the project. They will sense which agency is considered to have priority in deciding system questions and which is at the bottom of the totem pole. Naturally, these differences often spark resentments and political turmoil.
Compatibility is a big issue with 800 MHz, both across model lines and with other agencies. Eventually third-parties will supply mobile radios that work with one or more company's system. But for now, the big guys--Motorola and M/A-COMM (GE)--make distinctly different systems that force an agency to purchase subsequent hardware from the same company. And if you're trying to configure your trunked system to talk with another agency, just hope they purchased their system from the same company or you're locked out.
[Since this article was first written, the Project 25 standard has established a common technical format for digital trunked radio systems. Despite several companies providing Project 25-compliant gear, the choice of major vendor remains the same: Motorola or M/A-COMM.--Editor]
And lastly on the list of trunked questions, engineering is a critical issue for all 800 MHz systems because of the line-of-sight characteristics of high frequency mobile radio systems. An experienced radio technician might be able to best-guess the specifications for a lower frequency system, but not for 800 MHz.
The radio transmission paths must be plotted on maps using either theoretical data or live tests. Satellite receiver sites must be located accurately by computer and the use of multiple base transmitters must be considered. The effects of trees, foliage, rain, and even high humidity must be considered on reception. After all this, system managers should anticipate changes in system configuration after the radio system is operating based upon actual working conditions.
The National Plan
Besides money, the biggest hurdle facing police departments has been obtaining the required FCC approval for 800 MHz radio plans. The paperwork is copious and demanding, even for an experienced radio technician. The approval process must pass through several layers of local, regional and federal coordinators before it reaches the FCC.
As background, the concept of 800 MHz trunked radio was born from the need to provide more channels for public safety users. Over the years, public safety allocations have shot up the spectrum like a thermometer--from VHF to UHF, then displacing some unused UHF television frequencies and finally into the 800 MHz band. Of course, major advances in radio technology allowed this movement to continue to higher and higher frequencies.
But when the requests finally hit 800 MHz and the FCC decided to allot a section of the band for public safety, they also dropped a very heavy requirement on the requestors--you must develop regional plans for usage to insure maximum use of the frequencies. In effect, the FCC told public safety, "Here you are, but make the most of it because there's no more."
Since that initial plan, each area of the country has been grappling with how to best allocate the band among its many public safety agencies. Following the strict guidelines set out by the FCC for submitting plans, the process has been slow. At least three plans have been approved, but a FCC backlog is keeping many others from being finalized.
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.
So far, submission of regional plans is going slow and the FCC approval process even slower. A few regions have had plans stamped "OK" and individual agencies are now beginning to install trunked systems. But most police and fire trunked systems are still in the early planning stages.
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.
But what happens if you turn on your radio during a transmission directed to your talk group? Or what if you drive out of radio range when your group receives a transmission? The radio companies have come up with various schemes to handle these situations, but there's always the chance that your mobile radio will loose track of which channel it's supposed to be listening to.
Typical System
Most trunked systems are designed to incorporate several jurisdictions or agencies. The most common example is a city's police, fire, ambulance agencies and public works department.
Most medium-sized cities could easily justify obtaining separate frequencies for each of its departments. Some of the frequency assignments might be in different bands -- police in UHF, fire, EMS in VHF-high and public works in VHF-low. But in a trunked system, all four agencies would be equipped with 800 MHz radios, and each would be assigned a different talk group ID.
Normally all four agencies would operate on different talk groups and essentially have separate radio systems. But during planning for a Fourth of July parade, for example, you decide that all three agencies should be on a single channel to coordinate their activities. You simply create a new talk group, program the mobiles and portables from all four agencies with the new group ID and establish a command post using the same ID. Presto! You have a new July 4th-only channel. With a conventional system, you'd have to go out and find a new frequency--slim chance.
Of course, you can apply this same type of coordination to normal situations, too. For example, you might program the public work department's street sweeper radio to allow access to the police talk group, so the operator can report problems when he/she is working in the early morning hours. The possibilities for cooperation are endless once you have the technology and it's easy to use.
Some user of trunked radio say this reorganization process is complex and hard to perform on a routine basis. Others counter that it simply takes thorough training, strict system administration, and that the system's flexibility overcomes any increased complexity.
Fail Safe
As a cautious police officer or firefighter, 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
So far, trunked technology has outpaced the ability of police agencies to get approval for its use, to specify systems and to appropriate money to build the systems. But large trunked systems for both single and multiple jurisdictions are now beginning to go on the air. Shortly, we may realize the promise that promoters have long made --to improve communications and service.
copyright ©1990-1994 911 Dispatch Services, Inc.
Also check the Trunked Radio Web site, this explanation of trunked radio technology, and the GenesisWorld Web site on trunking.