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. 

