Despite the steady migration of public-safety radio organizations to 800 MHz trunking systems over the past few years, many public-safety radio systems still operate in the VHF and UHF bands. Fortunately, the national frequency plan developed by the National Public Safety Planning Advisory Committee and adopted by the Federal Communications Commission in 1987 calls for certain 800 MHz frequencies to be set aside for the purpose of interoperability — or “mutual aid” — between public-safety entities operating in various frequency bands.

To review the requirements and objectives, I have excerpted a portion of Appendix C of NPSPAC's Channel Planning Analysis, as follows in the next section. (Visit www.pswn.gov/admin/librarydocs/800appendixc.pdf for the full document.)

The national plan requires that regional plans contain several sections concerning the implementation of the five nationwide mutual-aid channels. These sections typically define the use of each of these channels and provide detailed standards and operating procedures to govern the use of these channels.

Common Channel Implementation — The national plan sets forth the guidelines for using and implementing the five National Common Channels (see Figure 1). Four of these channels are dedicated as National Tactical Channels and one channel is dedicated as the National Calling Channel. The National Common Channels are to be available for use throughout the region.

The National Calling Channel, channel 601, is to be implemented as a full mobile relay, with wide area coverage transmitters to maximize coverage. Large system users (five or more channels) are required to monitor this channel and could be required to provide satellite receiver feeds into the wide coverage area.

The four National Tactical Channels are to be assigned throughout the region for use by all eligible entities. Large system users could be required to sponsor one or two localized mobile relays to cover specific geographic areas. The users of these channels must be eligible for licensing on other 800 MHz public-safety channels, but no special licensing is required.

Operation on Common Channels — The five common channels are only to be used for activities requiring intersystem communications between entities not already sharing communications systems and are not to be used for any daily operations. In emergencies, the channels may be assigned by the primary public-safety agency in that area. On all common channels, plain English and familiar words and phrases should be used. The calling channel is used to establish contact with other users and determine which tactical channel to use. It is not to be used as an ongoing working channel. Tactical channels are reserved for interagency communications and are used as directed by the primary public-safety agency in the area. Tactical channels can be assigned by the various public-safety services, or they can be assigned by county or area.

Network Operating Method — A wide area network will be established on the National Calling Channel. The tactical channel communications systems will be implemented by volunteer entities, and each primary geographic section of the region is covered by at least one tactical channel.

Coded Squelch on Mutual-Aid Channels — The National Common Tone Squelch of 156.7 Hz will be used on all equipment operating on the five common channels.

If these frequencies are used nationally, as the plan requires, then interoperability comes down to providing a link or bridge from public-safety radio systems operating in bands other than 800 MHz. In Figure 2, a conventional VHF radio is interconnected to a conventional 800 MHz radio to form a translator. The function of the translator is to receive radio transmissions on the 800 MHz system and retransmit (translate) them over the VHF radio system. The translator works bidirectionally — that is, it also picks up radio transmissions on the VHF radio system and retransmits them through the 800 MHz radio.

As shown in Figure 2, the audio output of the VHF transceiver in the translator is fed to the exciter input of the 800 MHz transceiver. The control circuitry keys the 800 MHz transceiver, thus transmitting the signal to the 800 MHz mobile relay station. The 800 MHz mobile relay station then retransmits the 800 MHz signal to the 800 MHz mobile (or portable) radio transceiver. The signal follows the path of the blue arrows. When the 800 MHz mobile transceiver is transmitting, the reverse operation takes place, and the signal follows the path of the red arrows.

Figure 3 shows another setup in which the translator is the only bridge between the 800 MHz radio and the VHF radio. The 800 MHz mobile relay station is omitted from the loop. It is important to note that the translator setups described above will only work on conventional 800 MHz frequencies.

More elaborate methods must be employed to link a conventional system at other bands to an 800 MHz trunking system. One solution can be found in the SmartBridge trunked radio interconnect systems from Melbourne, Australia-based Radio Systems Technologies. For example, the SB100 Talk Group extender — designed for the company's SmartNet/SmartZone trunked radio networks — can be installed at any conventional site within radio range of a trunked site.

When the bridge is operating in the connect state, any radio operating within the repeater's radio range becomes an extension of the TalkGroup. Thus, the conventional radio effectively behaves as though it were a trunked radio itself. The connect on/off state is controlled by using “over-the-air” dual-tone multi-frequency signaling. A unique “trunked system busy” protocol ensures that users of conventional radios are immediately informed of unsuccessful attempts by audible alerts. (For more information visit www.rstradio.com.)

Another device that has gained popularity is the TRP-1000 transportable interconnect system from JPS Communications, a division of Raytheon. The TRP-1000 provides interoperable communications between organizations using different radios on disparate frequencies. It is a self-contained, portable unit consisting of a power source and multiple radios pre-wired to the company's ACU-1000 solution. The ACU-1000 is designed to provide on-scene interoperability by linking radios, SATCOM, cellular, Wi-Fi and landline phones directly and over IP networks.

The TRP-1000, integrated with the ACU-1000, can serve as a bridge between conventional systems as well as between conventional systems and trunked systems. (More information on the TRP-1000 and ACU-1000 can be found at www.jps.com.)

Although trunked radio systems (800 MHz and other bands) currently serve a large number of public-safety agencies, many agencies continue to operate conventional radio systems in the VHF and UHF bands. Of these, many — if not most — are pleased with their radio systems and don't want to move to 800 MHz trunking systems. Consequently, until the time comes when all public-safety agencies are on the same radio system, bridging devices such as those described in this space will be required to achieve interoperability.

Until next time — stay tuned!