An Overview of Trunked Radio Systems by Black Axe In the past few years, many public service agencies have decided to move their operations from conventional FDMA (frequency division, multiple access) repeater-based land mobile systems to a new breed of trunked radio systems. Just what, exactly, is a trunked radio system? How does it work? What different types are there? As a monitoring enthusiast, what do I need to do to be able to efficiently monitor these systems? History One of the best and most well-known examples of a trunked radio system would be the analog cellular system (as in cellular telephones, AMPS). As all good phreakers know, a cellular system is based on a control channel, and a number of associated voice channels. Data flowing over the control channel instructs the mobile units to switch frequencies and unsquelch audio, amongst other things. In the cellular system, the control channel would usually address a specific mobile unit. In a trunked radio system, the control channel addresses different talkgroups. Talkgroups are programmable groups of radios; each talkgroup forms a logical "channel" within the trunked system. However, because of the nature of the system, talkgroups can use different frequencies within the system, as allocated by the control channel. In the past, police departments were limited to those frequencies that they were licensed on. So a local police department with 2 licensed frequencies has 2 channels, divided by frequency. In a trunked system, however, the operator can program hundreds of different talkgroups into a trunked system using only 5 or 6 frequencies. The benefits here are obvious: these agencies are no longer limited to only 2 channels. A communications officer can have a talkgroup for EMS calls, another for traffic units, another for detectives, another for the SWAT team. Or, they can divide up their coverage area, with different talkgroups for each section of the town. For these reasons, many agencies have decided to "go trunked". And who can blame them? The advantages are excellent. Another implementation of the trunked system would be a SMR (Specialized Mobile Radio) system. A SMR system is generally owned by a private business. These businesses can then provide communications, on their trunked system, to others for a fee. The basic concept here is that a small organization can rent or buy radios from the SMR business, and rent their own talkgroup within the system. Monitoring Systems/Setup: How it works Before any idiot could walk into Radio Shack, drop a few bucks, and walk away with a radio capable of following trunked systems; trunked monitoring was for the technically inclined only. The original setup consisted of 2 receivers (scanners, if you will) and a computer that controlled the "trunktracking". One receiver had a discriminator tap and fed the control channel data stream into the computer through the appropriate interface. The other radio was controlled by the computer, and this was the radio that actually skipped from frequency to frequency, following calls. Back then, the actual commands as to which frequencies to switch to, etc. were decoded from the control channel only. Nowadays, trunktracking scanners operate in a different fashion. At first, when there's no activity, these new radios listen to the control channel. When activity appears on a talkgroup that is programmed into the scanner, the scanner's only receiver jumps to the frequency in use. Now, you may ask, what happens when the conversation changes frequency? In addition to data over the control channel, there is data encoded into the voice channel (somewhat similar to DPL tones, if you're familiar) that instructs the radios as to where they should now look for activity. This method is used in both trunktracking scanners and the actual mobile units that you're monitoring. Which way is the better way to follow the system? Depends on the situation. If you want to run around town, drinking and acting like a bunch of hooligans, I'd recommend the commercially produced handheld. If you're sitting at home, the original method (using 2 scanners) provides much more information as to how the trunked system works, and as to exactly what's going on within the system. Even if you choose to listen to a commercial trunktracking scanner at home, I highly recommend decoding the control channel on some old 386, just to give you a clearer picture. ** Note: within trunked radio systems, there exists the capability to place telephone calls over the system, also known as an autopatch. On these autopatch calls, a "privacy bit" is set. The call is still on the system, and it's still in analog mode (usually). Uniden, when designing their radios, decided to have the radio skip over and not notice any call with the privacy bit set - yet another reason as to why one may want to use the original setup described above. Equipment When trunked radio systems became popular, Uniden figured that it had better cash in on this new trend in land mobile communications. To date, the only trunktracking radios (with one exception) have been made by Uniden. My opinions, and a few specs to boot: Uniden BC235XLT: The first radio to hit the market. Handheld, 300 channels, can decode Motorola Type I/II systems. Uses rechargeable battery pack. Price: around $200 or so. Uniden BC895XLT: An excellent base radio. It's fairly large, but has many features (computer control, S-meter, easy discriminator mod). 300 channels, follows Motorola Type I/II systems. Price seems to hover around $220-230. Uniden BC245XLT: This one is fairly new. A handheld by Uniden, it was the first handheld to track EDACS (Ericsson) in addition to Motorola Type I/II systems. Specs are much the same as the 235XLT, except that the 245 has a port for computer control. Price: around $230, you can find it cheaper in some places. Optoelectronics Optocom: This offering from Optoelectronics is a "black box" receiver; that is, it's entirely computer-controlled. Channels limited only by your hard drive space, decent control software, and a sensitive receiver. Capable of following Motorola, EDACS, and LTR systems. Reaction tune capability (with the Scout). Price: around $550 (ouch!). ** Note: although the following radios are sold by Radio Shack, they are actually made by Uniden.. if you doubt me, open any of the radios up and look - Uniden likes to mark their own work. RS PRO-90 An _exact copy_ of the 235XLT. Not really worth your money at $300 or so. RS PRO-91 A 150-channel, Motorola only trunktracker. Again, RS shifts their prices around, but it's probably overpriced. The only advantage to this radio is that it may be fairly cheap, and it is the only currently available trunktracking handheld that will take AA batteries. RS PRO-2050 A 300-channel trunktracking base. Nothing spectacular here, Motorola Type I/II only. Price: around $300. RS PRO-2066 A 150-channel trunktracking mobile unit - fits perfectly into a car stereo slot. Price is around $220, so the only reason that I'd be buying this is if I needed something in the car. ** Note: the following radios aren't available for sale yet; they should be out late 1999 or early 2000. RS PRO-92 I'm really drooling over this one. 4 line dot matrix LCD, you can alphatag everything, SAME weather alert, follows Motorola Type I/II, EDACS, and LTR systems. Since this radio is made by GRE, and not Uniden, they may or may not "block" the autopatch calls. 500 channels, divided into 10 banks of 50 channels each. This is the scanner nut's dream handheld. Runs on AA batteries. Price: around $360, but it's worth every penny. RS PRO-94 An interesting handheld. Same case styling as the PRO-91 (and the 67, and the 26). 1000 channels, Motorola/EDACS following capability. Appears to run on AA's. Doesn't appear to be a bad radio; price should be around $300. Try for the PRO-92 though, unless you really need all of those channels. RS PRO-2052 The base version of the 94. Same as the 94, except in a PRO-2050 case. Price: around $340-350. If you handed me a wad of cash, and asked me to buy you the best radios, what would I say? As far as base radios go, the 895XLT blows em all out of the water. Even though it only has 300 channels, and can't do EDACS, it's still a great radio. If you really need the EDACS or the extra storage, however, the PRO-2052 is your only choice. As far as handhelds go... I tend to prefer having AA batteries in my radios - easy to replace, and you can't get fully charged Ni-Cd packs at your local friendly 7-11. Conclusion When you originally heard that your local PD was going trunked, you may have freaked. Hopefully, after reading this, you will have realized that it isn't such a bad change (can even make monitoring more interesting!). Grab a wad of cash, and when they do switch over, trek on down to your local electronics establishment (e.g. Rat Shack) and pick up that oh-so-sweet PRO-92 that I know you want to buy. Before you do this, though, monitor the trunked system and make sure they're transmitting in the analog mode. If they have went "full digital", that is, using a form of digital modulation as opposed to regular FM communications, you're screwed. Almost. More on decoding digital voice, another day.