TRUNKING RADIO COMMUNICATIONS SYSTEMS
Increasing population. Growing demands for service. Expanding services provided by public safety agencies. Shrinking budgets. All these factors, plus the rapid consumption of the radio frequency spectrum, contribute to an impending radio communications crisis. The advent of many highpowered FM entertainment radio stations has a great, negative impact on the public safety channels in the 140MHz to 450-MHz range. Allocating to the private sector radio channels formerly assigned to the public sector represents an erosion of radio frequencies once available to public safety.
The existing methods for using radio frequencies, or the radio-frequency spectrum, must be changed, since the available radio spectrum is relatively insignificant compared with the escalating demand for this resource. Otherwise, the fire service soon may find itself within a “communications Tower of Babel.” Technological and philosophical intervention are needed if a serious crisis is to be averted. The trunking radio system appears to be one solution.
THE CURRENT SYSTEM
Administration and “assignment” of radio frequency channels. In the United States, the Federal Communications Commission (FCC) is the ultimate authority having jurisdiction for assignment. The FCC derives its authority from federal law and U.S./foreign nation treaty agreements relating to the worldwide allocation of the frequency spectrum to certain uses. This is a reasonable policy, since the propagation, or travel, of radio signals does not respect political jurisdiction boundaries; the use of radio frequencies is becoming a grave international problem.
Current radio communications operations/technology. Historically, emergency communications frequency, or channel, assignments have been made so that, if possible, a fire department has its own unshared frequency for primary operations communications. A second frequency for administrative or “overload” communications also may be assigned. Admittedly, some communications arrangements in counties served by volunteer firedepartments may have many volunteer departments share a single channel, but that does not significantly change the direction of this discussion, since channel use “assigned” is for fire service communications exclusively.
The same radio operating environment occurs in the police, public works, utilities, and street departments. It is easy, therefore, to see that historically each of the many departments in a municipality or county has been assigned a radio channel for its almost exclusive use. Admittedly, it often is true that public works, utilities, animal control, and other local government agencies share frequencies. This practice often results in five to seven radio channels being assigned to a smallto medium-sized municipality. If you listen to the radio traffic on any one of these channels, it immediately becomes obvious that the percentage of “quiet time” on each frequency is greater than the percentage of “traffic time.” Put another way, the resource, the radio spectrum, is not being utilized effectively. For the sake of this discussion only, effective utilization means that the percentage of time that radio traffic occurs exceeds 70 percent. It is not unusual to have utilization factors of five to 10 percent or even lower in smallto medium-sized communities. Utilization often approaches 35 to 50 percent in large communities. Nevertheless, even in larger communities, much available radio transmission time is left untapped on certain channels.
The traditional approach was to assign radio channels to government users based on the specific types of services they provide, such as fire, police, medical, or general local government. Order in the communications “world” was the principal reason for this approach. Also, it was very important that public safety communications paths, or channels, enjoy open communications lines whenever the need to communicate arose— not very good use of the natural resource. But that was then.
RADIO COMMUNICATIONS TRUNKING SYSTEMS
Decades ago, the telephone industry. realizing that it needed advanced technology to meet the escalating demands for telephone services, switched from its conventional wireline system to a trunking system (see sidebar). This same technology now is being applied to radio communications. The reason this telephone industry technology was not applied to radio communication lies principally in the failure to recognize that the radio frequency spectrum was a natural resource that must be conserved.
The goal of a radio trunking system is to increase the percentage of time a radio channel is used for communications by allowing that channel to broadcast any type of service communications instead of being dedicated to a single service.
In a conventional radio system, each fleet (the total of mobile and portable radio units in a group of subscribers—such as the public services sector of a municipality: public works, public utilities, animal control, jurisdiction management, and emergency medical services—sharing a radio channel) would have its own operating channel. Not so in a trunking system. The combined trunking radio system proposed in this discussion has only four frequencies allocated. One of the frequencies, F4, is the Trunk Management Control Frequency (TMCF). The FI, F2, and F3 frequencies are the actual communications channels. This proposed system provides communications to seven agencies while using only three frequencies. The other interesting part of this system is that the arrangement is “transparent” (the subscriber does not know it is a trunking system) to the radio communications user. It appears to operate in the same way as a conventional system. The operating procedures of each agency may remain the same—a very important consideration.
In a trunking radio system, each field radio unit is equipped with all of the trunk system radio frequency transmit and receive channels as well as the TMCF channel. One of the frequencies, however, is not used for base-to-mobile communications in any case. Here, channel F4, the TMCF, constantly transmits operating instructions to all field radios. The TMCF addresses and updates each fleet field radio with digitally encoded instructions from time to time. These instructions are received and executed by the field radio whenever the radio is turned on. The radio user knows nothing of the contents of the instructions. When the user wishes to communicate, all that is necessary is to select the desired user and push the transmit button. The TMCF tells the field radios the frequency to which they should go to receive and transmit. Typically, the trunking system frequency used for communications differs from one fleet transmission to another.
As do all systems, the trunking system has some drawbacks. The major shortcoming obviously is that theoretically one could conceive a situation in which all of the field fleets or base agencies sharing the system might transmit communications simultaneously. Basic arithmetic demonstrates three channels cannot handle the peak demand. In such a situation, the concept of the“channel busy signal” is introduced. Each base agency and each fleet is assigned a transmission priority. For the sake of argument, assume that transmissions in various agencies are assigned a “priority number.” The highest priority, number 1, is assigned to public service agencies such as fire and police. The priority levels then proceed through numbers 2 to 8, which are assigned to other public agencies.
Field priorities, for instance, may range from number 1 for police and fire field units to number 8 for public works. This simply means that if channels FI, F2, and F3 are in use, the user gains system access based on assigned priority number. The low-priority user is “bumped” off the system to make way for the higher-priority user. A busy signal similar to that used in the telephone industry tells the lowpriority user that immediate access is not available. When a channel becomes available, an audible tone notifies tlie waiting user that a channel is available.
The computer-operated trunk manager performs “traffic cop” functions—-determining which frequency should handle the transmission, telling the field units to set themselves up to receive and transmit on appropriate channels, and finally determining which user has priority if all the communications channels are busy. Simultaneously, it tells the lower-priority user to stand by and be patient.
Switching from a system of less frequently used channels assigned to specific services to a system wherein computers determine how to “squeeze” many users onto fewer frequencies that are used more often and are not assigned to specific services may strike terror into the hearts of some managers, particularly those in the public safety area. The reality, however, is that the question is not whether this should be done but rather how it will be done.
Municipalities and counties in highpopulation-density areas and areas of high growth almost are certain candidates for this type of system. They, therefore, should begin to think in this direction now. Communities and departments that have no growth on the horizon may possibly retain their present communications systems. On the other hand, if the jurisdiction is planning to expand or consolidate field services, the total community public sector communications system should be reviewed for direction and design. No one is exempt from the review and planning function regarding public sector radio communications systems.
Major manufacturers of communications systems can help with the review of present and future designs based on needs. A few qualified independent professional consultants also can assist. A qualified and experienced professional should determine equipment specifications and develop performance specifications.
The communications manager must understand the difference between technical data used for licensing purposes and data relating to “real-life” systems performance. A number of computer software programs, which can be used on personal computers, offered by the private sector can help determine antenna locations and heights, transmitter power levels, and receiver sensitivity. These programs must be used within the context for which they were intended. In almost all cases, the programs are for reference and guideline purposes only and are more for determining licensing procedures than for designing trunking systems. A municipality or county should not proceed on its own to “design” a trunking system based on these programs. Concerns voiced by radio system manufacturers who respond to specifications for equipment to be purchased on the basis of these programs alone should be heeded.
This article discusses the trunking radio communications systems in a veny basic way. Fire service and emergency service managers would be wise to study the approach and to make sure that they understand the advantages and disadvantages. While there are some trunking systems presently in use, they are few. For the rest of us. trunking systems are the future, and we can’t afford to ignore the obvious by not paying attention to this technology.