First introduced to public safety/local government operations in the early to mid 1980s, trunked radio systems were initially looked upon with some justified skepticism, since the earliest systems suffered some major bugs that needed to be worked out. Once the bugs had been cured, though, it became obvious that trunked systems were ideal for public safety and local government operations, since they allowed improved interagency communication and greater flexibility. This is especially so for large metropolitan areas, where multiple agencies can share a single system and communicate with virtually any other agency with a single radio instead of two or three radios.
Trunked radio systems have a number of advantages and disadvantages:
Some Advantages:
1. All city and/or county agencies can share a single radio system.
2. Improved communication between agencies.
3. Reduced costs for all agencies over the long run.
Some Disadvantages:
1. Greater initial cost.
2. Increased complexity for users initially, especially when advanced features are used.
3. Risk of total loss of communications in the event of a catastrophicsystem failure.
Trunked radio systems are all identical in that their operation is controlled by a computer, called the Central Controller. All also allow nifty special features such as emergency banners, private/individual (radio to radio) calling, dynamic regrouping (where specified radios are assigned by the dispatcher to a new dedicated private talkgroup), telephone interconnect, and so forth. All also offer "failsoft" mode, in which communications can continue in the event of a controller failure.
To a greater or lesser extent, however, the similarities end there. Motorola and Ericsson systems both dedicate one system channel (the Control Channel) for a data signal that tells the radios in the field what to do; E.F. Johnson systems, on the other hand, use all system fequencies for voice communications, controlling the radios via subaudible data on each voice channel. Each talkgroup in an EFJ system is assigned a primary channel in the system; if that channel is busy, the subaudible data stream tells the user radios which channel is now in use for that particular talkgroup.
All of these systems have certain advantages over the others. E.F. Johnson LTR systems have the advantage of using all channels for voice communications, while Motorola, Ericsson and E.F. Johnson Multi-Net systems dedicate one channel for control data. Motorola Type II systems are far easier to set up, while Ericsson systems continue trunking in the event of a controller failure.
All systems make use of control channels, and most make use of separate data channels as well. Control channels are those which provide trunking control information, while data channels are those which handle data to and from mobile data terminals. Mobile data can be sent on conventional channels or trunked talkgroups; many trunked systems, if not most of them, are set up to run mobile data on separate (usually conventional) frequencies, so as to have a redundant communications backup in case one system fails. Information sent on the control channel includes unit ID, talkgroup and channel assignment. Up to eight simple status messages can also be sent via the control channel from mobile radios. GPS and mobile data, on the other hand, are sent on data channels or talkgroups. Most systems have both control channels and data channels.
The control channels are those at each site which all radios monitor when there are no transmissions on the talkgroup they are set to. When a trunked system is set up, a certain number of control channels are designated; Motorola systems are limited to four per system or site, while EDACS systems can use all of the frequencies at a site. These frequencies are programmed into the radio so it knows where to monitor for control. In a single-site or simulcast system, there is a single control channel. Networked (or "wide-area") systems, on the other hand, have a different control channel at each site. Many networked systems, however, are set up to reuse control channel frequencies, but they must be geographically separated by a certain distance in order to avoid interference.
Radios monitor the control channel for the site they are currently affiliated with; if the signal quality in a networked system drops below a certain level, the radio will start searching it's list of control channels for one with better signal quality and will then affiliate with the new site.
When a user radio keys up the PTT, it sends a signal on the control channel; the system controller then assigns a voice channel (a channel grant). All user radios set to that particular talkgroup receive that channel grant and go to the appropriate channel. Then while on the voice channel, there is a subaudible data stream that identifies the talkgroup; when the conversation or transmission ends, the system sends a disconnect tone and all radios return to the control channel.
There are other system types out there such as Tetra and MPT1327, but there are so few of these in the United States I'm not going to cover them here.
Motorola systems are found in four general types: I, II, IIi and Astro. Also known as Privacy Plus, Type I is the earliest common form of Motorola trunking. In a Type I system, the system is divided up into 8 equal-sized blocks, each of which can be configured various ways, with each user or group of users being a "fleet", operating on various "subfleets", with a set number of radios per fleet.
Some users may need several fleets, with few subfleets, and many individual radios, while other users may require only a few fleets with many subfleets and fewer individual radios. One of the limitations of a Type I system is that once the system map is programmed into each radio, it cannot be changed unless every radio is reprogrammed.
Type II systems are newer, and much more flexible than Type I systems. Rather than the fixed fleet/subfleet arrangement of a Type I system, Type II systems use a flexible arrangement of "talkgroups", each designated by a hexadecimal code which can be programmed for any user in any radio without having to reprogram every radio in the system.
Motorola Hybrid/Type II-i Systems
Hybrid systems are newer Type I systems that allow Type IIprogramming, while Type II-i systems are newer Type II systems that allow Type I programming. However, as far as trunking scanners are concerned, they are essentially the same. One thing to keep in mind, though, is that even if a system is listed somewhere as a Type II-i system, the chances are pretty good that is is programmed strictly as Type II. Thus, you should always start out using the default "E2" data type. If you get lots of odd numbers and can't follow both sides of the conversation, then start trying the Type I fleet maps.
Finally, Astro systems are the newest incarnation of Motorola trunking. Astro systems use digital modulation and are similar in operation to normal Type II systems. There are two types of Motorola Astro digital: VSELP and IMBE. VSELP (Vector Sum Excited Linear Prediction) is the orignal Motorola offering; it has been replaced in the product line by IMBE (Improved Multi-Band Excitation). IMBE is the CAI (Common Air Interface) digital modulation developed as part of APCO's Project 25 initiative, which set the public safety standard for digital modulation. Digital modulation is excellent from a radio spectrum efficiency standpoint, and it also allows more advanced signalling features. The debate rages among system suppliers as to whose digital format is best, but Project 25 is the clear winner numerically as it is the standard, and far more systems meet this standard than do not.
Most Astro systems are fully digital. Some Astro systems, though, use digital for police and/or fire only, while city services operate solely in analog mode. Others may have a mix of analog and digital talkgroups, with digital used for day-to-day operations and analog for mutual aid/interoperability with other agencies. Only a handful of scanners are capable of monitoring Project 25 digital: the Radio Shack Pro-96/Pro-2096, and the Uniden BC250D/785D/296D/796D.
One thing to watch out for when selecting a digital-capable scanner is the control-channel capability. Mixed-mode Astro systems use the standard 3600 baud Motorola control channel singalling format; all of the scanners mentioned above are capable of monitoring this. Fully digital systems, however, use the Project 25 compliant 9600 baud control channel format; only the Pro-96/2096, BC296D/796D scanners are capable of monitoring this. Select your scanner carefully according to the format used by your local system.
There are several types of Motorola systems available today, each of which offers various feature levels:
If you're trying to figure out what kind of trunked system you've got in your area, the easiest way to tell them apart is by the sound of the control channel. Check the Digital Modes Page for .WAV file examples of various digital signals that you can hear on the air.
One excellent capability of trunked systems is that they can be networked to cover a much wider geographic area, such as a region, an entire state, or even multiple states. Some good examples are Ohio's MARCS (Multi-Agency Radio Communications System) which covers the entire state; or the American Elecric Power EDACS system, which covers most of Ohio, Indiana and West Virginia, as well as parts of Michigan, Kentucky, Tennessee and Virginia. Networked systems allow users to travel over a wider area and still remain in communication with their dispatchers and other users. Communication can be carried out on regular talkgroups or using Private Call/Individual Call.
Monitoring networked systems is different from monitoring a regular single-site or simulcast system. Communications are not repeated at all sites; you'll only hear those communcations at the site(s) you're monitoring.
To monitor multiple sites, you'll need to program each site in a separate bank. With the Radio Shack Pro-96 and Pro-2096 scanners, however, monitoring wide-area Motorola systems is easier; the control channels for each site in your area can be entered in a single bank, making sure that the frequencies for each site are separated by a conventional frequency.
The ID codes of Type I and II systems differ dramatically. An example of a Motorola Type I fleet/subfleet display ID is 401-B, which indicates Block 4, Fleet 1, Subfleet B. Because the LCD of some trunking scanners do not allow alpha characters, they display the above as 401-2.
The advantage to knowing Motorola Type I display codes, especially in conjunction with the list of subfleet names in order as programmed in the radios, is that the Type I ID and name list can aid in selecting a fleet map if one knows approxmately how many fleets and subfleets are in a given block of the system. A Type I system is divided into 8 blocks, each of which is assigned a size code that determines the number of fleets, subfleets and radios each block can handle. Thus, knowing how many subfleets are allocated to a given user can provide useful clues to the size code for a particular memory block on a Type I system. More clues can be provided by knowing approximately how many radios a given user has.
If you are fortunate enough to know the Motorola size code assignments for a given system, here is a conversion chart with Motorola and Uniden size codes as well as number of users, fleets and subfleet to help you set up your trunking scanner:
| Motorola: | A | B | C | D | E | F | G | H | I | J | K | M | O | Q | X |
| Trunk Scanner: | S1 | S2 | S3 | S4 | S5 | S6 | S7 | S8 | S9 | S10 | S11 | S12 | S13 | S14 | S0 |
| Fleets: | 128 * | 16 | 8 | 1 | 64 | 32 | 32 | 16 | 8 | 4 | 2 | 1 | 1 | 1 | n/a |
| Subfleets: | 3 | 7 | 7 | 15 | 3 | 7 | 3 | 3 | 3 | 7 | 15 | 15 | 15 | 15 | 256 |
| Unit IDs: | 16 | 64 | 128 | 512 | 32 | 32 | 64 | 128 | 256 | 256 | 256 | 1024 | 2048 | 4096 | 8192 |
| Blocks Used: | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 4 | 8 | 1 |
* Note 1: Block 0 and Block 7 are only capable of 127 fleets (or 8191 unit IDs for a Type II block).
* Note 2: Each Type I fleet is capable of the total number of subfleets listed plus a "fleetwide" subfleet, for a total of 4, 8 or 16 subfleets. Most "fleetwide" subfleets are only used occasionally, especially in business systems. "Fleetwide" subfleets are displayed on scanners as xxx-0.
Type II Systems
An example of a Type II hex code is 18F, which displays as 6384 on scanners. Type II systems allow 2048 talkgroups per system (4096 talkgroups when priority monitor is not used), arranged in the following pattern (only the first group is shown):
| 001 | 003 | 005 | 007 | 009 | 00B | 00D | 00F |
| 011 | 013 | 015 | 017 | 019 | 01B | 01D | 01F |
| 021 | 023 | 025 | 027 | 029 | 02B | 02D | 02F |
| 031 | 033 | 035 | 039 | 039 | 03B | 03D | 03F |
| 041 | 043 | 045 | 047 | 049 | 04B | 04D | 04F |
| 051 | 053 | 055 | 057 | 059 | 05B | 05D | 05F |
| 061 | 063 | 065 | 067 | 069 | 06B | 06D | 06F |
| 071 | 073 | 075 | 077 | 079 | 07B | 07D | 07F |
| 081 | 083 | 085 | 087 | 089 | 08B | 08D | 08F |
| 091 | 093 | 095 | 097 | 099 | 09B | 09D | 09F |
| 0A1 | 0A3 | 0A5 | 0A7 | 0A9 | 0AB | 0AD | 0AF |
| 0B1 | 0B3 | 0B5 | 0B7 | 0B9 | 0BB | 0BD | 0BF |
| 0C1 | 0C3 | 0C5 | 0C7 | 0C9 | 0CB | 0CD | 0CF |
| 0D1 | 0D3 | 0D5 | 0D7 | 0D9 | 0DB | 0DD | 0DF |
| 0E1 | 0E3 | 0E5 | 0E7 | 0E9 | 0EB | 0ED | 0EF |
| 0F1 | 0F3 | 0F5 | 0F7 | 0F9 | 0FB | 0FD | 0FF |
Each succeeding group is identical, beginning with 1-9, A, B, C, D, E or F. The corresponding trunking scanner display codes for the above group are:
| 16 | 48 | 80 | 112 | 144 | 176 | 208 | 240 |
| 272 | 304 | 336 | 368 | 400 | 432 | 464 | 496 |
| 528 | 560 | 592 | 624 | 656 | 688 | 720 | 752 |
| 784 | 816 | 848 | 880 | 912 | 944 | 976 | 1008 |
| 1040 | 1072 | 1104 | 1136 | 1168 | 1200 | 1232 | 1264 |
| 1296 | 1328 | 1360 | 1392 | 1424 | 1456 | 1488 | 1520 |
| 1552 | 1584 | 1616 | 1648 | 1680 | 1712 | 1744 | 1776 |
| 1808 | 1840 | 1872 | 1904 | 1936 | 1968 | 2000 | 2032 |
| 2064 | 2096 | 2128 | 2160 | 2192 | 2224 | 2256 | 2288 |
| 2320 | 2352 | 2384 | 2416 | 2448 | 2480 | 2512 | 2544 |
| 2576 | 2608 | 2640 | 2672 | 2704 | 2736 | 2768 | 2800 |
| 2832 | 2864 | 2896 | 2928 | 2960 | 2992 | 3024 | 3056 |
| 3088 | 3120 | 3152 | 3184 | 3216 | 3248 | 3280 | 3312 |
| 3344 | 3376 | 3408 | 3440 | 3472 | 3504 | 3536 | 3568 |
| 3600 | 3632 | 3664 | 3696 | 3728 | 3760 | 3792 | 3824 |
| 3856 | 3888 | 3920 | 3952 | 3984 | 4016 | 4048 | 4080 |
As you can see from the table above, trunking scanners display talkgroups in a "start at 16, add 32" progression. An exception to this is systems that do not use priority monitor; in these systems trunking scanners display talkgroups in a "start at 16, add 16" progression. Another exception is the newer APCO-25 compatible Motorola systems, which also use talkgroups spaced at 16. Still newer Astro version 6 systems allow nearly 65000 talkgroups, which is a "start at 1, add 1" progression; there are several systems now in use using this method, including Hamilton County, Ohio.
Knowing the hex code for a Type II talkgroup means that one can readily find the trunking scanner ID code using the scientific calculator found in Microsoft Windows 3.1, 3.11 or 95. Simply enter the hex code in hexadecimal mode (don't forget to add a trailing zero; for example, 18F should be entered as 18F0), and switch to decimal mode. The number displayed is the code that will be displayed on a trunking scanner. You can also enter the hex code, convert to decimal, and multiply by 16 to achieve the same result.
Another nifty trick for figuring out talkgroup codes is knowing that many systems are laid out in a logical progression. If Fire Dispatch is 16, and the next talkgroups in the list are Fire Tac 1, Fire Tac 2 and Fire Tac 3, then those talkgroups are likely to be codes 48, 80 and 112 in that order. This trick does not always work, however, as some systems were laid out following the hex progression originally but then new talkgroups were added to the system, and programmed into the radios in between the original talkgroups. The newly added talkgroups will not follow the normal hex progression.
Another way talkgroup IDs are displayed in a Motorola Type II system is in the form of a 6-digit 800 number. To convert these to Uniden display, use the following formula:
M6 - 800000 * 16 = Uniden
Using the previous hex code as an example:
800399 - 800000 * 16 = 6384
Finally, if you come across a talkgroup that doesn't quite match the "start at 16, add 32" or "start at 16, add 16" progressions, you may have found a talkgroup using a special function. The added numbers that define these are:
And now we move on to EDACS systems. Known in the hobby community mainly for their obnoxious and aggravating "scanner-buster" beeps (which can be defeated with a couple of after-market gadgets for your scanner), EDACS systems have some advantages over Motorola systems from an operational standpoint. For example, during "failsoft" (failure of the central controller), Motorola systems go into conventional mode, with each radio channel used as a conventional repeater for several users. All trunking features are lost until the controller is restored. EDACS systems, however, remain in basic trunked mode during failsoft; only special features such as private call and others are lost. This is a huge advantage.
A newer variant known as EDACS System Key is identical to normal systems, but uses bit inversion in the data stream to make the system untrackable; it is intended to prevent unauthorized two-way radios being programmed for systems. Few of these systems are in use.
EDACS systems are similar to Motorola Type I systems, in that they use a fleet/subfleet arrangement rather than discrete talkgroups like a Motorola Type II system. Fleets are all arranged in groups of either 4, 8 or 16, but there is no limitation on the total number of radios per fleet.
A major difference between Motorola and EDACS trunked systems is in the manner of channel assignment; this has a definite bearing on the way trunking scanners are programmed. Motorola control channels send out channel assignments using the FCC channel number to refer to the desired frequency; frequencies can thus be entered in any order in the TrunkTracker, and the scanner will know where the necessary frequency is. EDACS systems, on the other hand, send out channel assignments using a Logical Channel Number assigned to each frequency in a system. The system ID and LCN-to-frequency assignments are programmed into each radio, and the radio uses the Logical Channel Number in it's memory to know where to go when the control channel makes an assignment. This means that channels must be programmed into scanners in the proper Logical Channel order, or transmissions will be missed.
How do you determine the correct order? Well, many EDACS systems have the channels assigned in ascending order, lowest frequency to highest, so start with that. You'll probably find that it will work fine. If it doesn't work, you'll have to do a little research. Scan the system frequencies in conventional mode, and make notes of the direction the conversations are assigned frequencies. This works best at night when the system isn't very busy, and it will take a bit of doing, but eventually you'll notice a pattern. Write down the order you figure out and give it a try. Eventually, by dint of trial and error, you'll get it figured out. Of course, if you use the ETrunk program and monitor the system visually it's even easier!
EDACS is also available with digital modulation, known as ProVoice. This is similar in capability to Astro, but does not meet the APCO-25 standard. ProVoice cannot be monitored by any currently available scanners.
What about EDACS talkgroups? EDACS systems are usually set up divided into agencies, fleets and subfleets; an agency is an entire group of users (for example, fire and EMS), while a fleet is assigned to a specific group of users (a single fire department), and subfleets are the individual communication paths (dispatch, fireground, etc). Uniden EDACS-capable scanners default to what is called AFS (Agency-Fleet-Subfleet) display; while decimal display can be selected and used if you so choose, AFS provides some really fascinating functionality, allowing one to monitor an entire agency, an entire fleet, or a single subfleet, simply by altering the way the codes are entered.
For example, Agency 3 can be monitored by entering "03.", and all fleets and subfleets will be monitored (one at a time). Likewise, a fleet can be monitored by entering "03.05." and all subfleets in fleet 03-05x will be monitored. Finally, a single subfleet can be monitored by entering 03-051.
But what to do if you have a list of decimal talkgroup codes and would prefer to use AFS? Here's the conversion. Using talkgroup code 586 as an example, enter 586 into your Windows scientific calculator in decimal mode, and switch to binary mode. You'll see an 11-bit binary number, all ones and zeroes. If there aren't 11 bits, add leading zeroes until you get to 11. Binary is counted, right to left, as 1, 2, 4 and 8, and so:
(1) 586 decimal = 01001001010 binary (talkgroups are 11-bit binary)
| 8421 | 8241 | 421 | ||
| (2) Split the binary as follows | 0100 | 1001 | 010 | |
| (3) Convert each group to decimal | 04 | 09 | 2 |
(4) Format as 04-092
Not really easy at first if you're not too familiar with binary notation, but once you try it a few times you'll get the hang of it.
Finally we come to the last major supplier of trunked systems in the United States, E.F. Johnson.
Johnson's most common offering is the LTR (Logic Trunked Radio) system. This is a system which can have up to 20 channels, but does not use a dedicated control channel; all system frequencies are available for voice communications, controlling the radios via subaudible data on each voice channel. Each talkgroup in an EFJ system is assigned a primary channel in the system; if that channel is busy, the subaudible data stream tells the user radios which channel is now in use for that particular talkgroup.
LTR systems can be identified by the data burst sent out by each channel, about every 10 seconds. This sounds like a brief "key and release" of a microphone. LTR is very popular for business radio use; radios are inexpensive and commonly available. However, very few of these systems are used by public safety; the systems do not offer typical public safety features defined by the APCO-16 standard.
A newer type of LTR is known as Multi-Net; this is a different variation of LTR which uses a dedicated control channel, and offers all APCO-16 public safety features. Multi-Net cannot be tracked by any currently available scanners, but there are very few of these systems in use.
Next, what about those VHF and UHF trunked systems? Well, EDACS and LTR aren't a problem since you're simply entering the channels, but what about those pesky Motorola systems and their base and offset frequencies? First, select trunked programming mode and the system type (E2-Hi or E2-UHF), then the bank. Next, enter all of the system frequencies, from lowest to highest. Then hit "Data", enter the lowest frequency in the system as your base frequency. Hit enter, and select the offset. If all of the system frequencies end in 50 (ie, .950, .450, .150), use 50 kHz; if they are separated by 12.5 or 25 kHz steps, select appropriately.
Figuring these trunked systems out isn't terribly difficult; put a little thought and research into the process, and you'll soon have lots of information at your fingertips.
| I-CALL - 866.0125/821.0125 - Calling (PL 156.7) |
| I-TAC 1 - 866.5125/821.5125 - Tactical (PL 156.7) |
| I-TAC 2 - 867.0125/822.0125 - Tactical (PL 156.7) |
| I-TAC 3 - 867.5125/822.5125 - Tactical (PL 156.7) |
| I-TAC 4 - 868.0125/823.0125 - Tactical (PL 156.7) |