Based on reader mail that I receive, questions about trunked radio are at the top of list for scanner subjects that cause confusion. This month I'll provide a basic introduction to the concepts underlying trunking and describe how the newest scanners are able to keep up with the digital networks that we all hear so much about.

Hi Dan,

My name is Mike and I live in northern Michigan and I have 1000-channel digital Uniden Bearcat BC-796 scanner and it is very hard to understand trunking or talk groups. I enjoy my scanner very much. I also have one mobile and three handhelds. I live in Petoskey, about 25 miles north of Gaylord. I also have a 30-foot tower. I have a discone antenna and i can pick up signals in an 80-mile radius from my house.

Mike in Petoskey

Petoskey is located on the shore of Little Traverse Bay in Emmet County, which is home to more than 30,000 residents. That area of northwest Lower Michigan is mostly suburban and rural, with a good number of hills and plenty of forests.

Mike, I hope things are defrosting up there -- for Petoskey, April is the first month with an average temperature above freezing! Despite the cold, your location is a good one for monitoring trunked radio and the Bearcat BC-796 scanner is capable of following the systems in your area. With that outside tower and antenna you should have little trouble picking up a lot of activity.

Simply put, trunking is about sharing. More specifically, trunking is a method of sharing a limited number of radio frequencies among a large number of users.

But let's start with the basics. Mobile and portable radios communicate with repeater sites installed at fixed locations. These locations are usually on hilltops or other high points, in order to provide good coverage. They're called repeaters because they receive signals from mobile and portable radios and repeat them on another frequency. These repeater sites are usually linked to dispatch centers, where trained personnel monitor and participate in the activity on the system. For police, fire and ambulance operations these centers are referred to as Public Safety Answering Points (PSAPs) because they are the places where those 9-1-1 calls end up.

Communication between a radio and a repeater site occurs in two directions. The repeater transmits on one frequency, called the output frequency. Mobile and portable radios receive on this frequency and are thus able to monitor the forward link. Radios transmit back to the repeater on the input frequency. Most scanner listeners tune to the repeater output frequency because it is typically much stronger, and therefore easier to receive, than the input frequency.

Conventional Operation

Agencies or departments that don't have too many users can get by with a very limited number of frequencies. For instance, many small town police departments may use just one frequency. The repeater site and each officer's radio are programmed with the same frequency and all activity occurs on it. When someone is using the system, everyone can hear it. Anyone who wants to talk on the system has to listen first to be sure no one else is already talking. If someone is talking, the other person has to wait until the first person is done. This is about as simple as sharing can get. Dedicated channel usage like this is called conventional.

The Petoskey Police Department operates a conventional repeater on 154.740 MHz from an antenna located on Highway 131 just south of town, close to the border with Charlevoix County. Entering 154.740 into your scanner will allow you to hear all of the Petoskey Police radio traffic, since all activity occurs on that frequency.

The Petoskey Fire Department repeater transmits on 155.220 MHz. Petoskey Public Schools can be heard on 151.865 MHz and 152.420 MHz. The Northern Michigan Hospital on Connable Avenue in Petoskey uses 155.385 MHz. All of these frequencies are conventional in operation.

Agencies or departments with a larger number of users may still be able to operate conventionally, but they need more frequency pairs. Each pair might be dedicated to a particular function. For instance, a medium-sized police department may use one pair for dispatch on the north side of town and another pair for dispatch on the south side. Officers would select one pair or the other, depending upon their location.

As an example, I have a listing for the Emmet County Sheriff's Department that shows two frequencies: 155.820 MHz for operations in the north end of the county and 155.685 for operations on the south end. You would need to program both frequencies into your scanner in order to hear all the activity, assuming you're in a location that can receive both frequencies.

For more county activity in northwest lower Michigan, there are a number of conventional frequencies to monitor. Three local counties in the Petoskey area have combined their resources to form the Charlevoix-Cheboygan-Emmet (CCE) Central Dispatch Authority, which operates an E-911 PSAP covering more than 1,600 square miles of land and 200 miles of shoreline, including the southern end of the Mackinac Bridge.

The Authority uses a set of common frequencies that allow the state police, three sheriff's departments and eight local police departments to talk with each other. This is the easiest type of interoperability - let everyone use the same radio frequencies, as if they're all part of one big agency. The Authority also provides radio services for 28 fire departments representing nearly 1,000 full-time and volunteer firefighters. Fire frequencies are divided by function and include dispatch, fireground operations, mutual aid and emergency medical services.

The CCE Central Dispatch Authority is licensed for operation from a number of repeater sites, including locations in or near the towns of Boyne City, Cheboygan, East Jordan, Harbor Springs, Mackinaw City, Petoskey and Wolverine. This is typical for a geographically dispersed system, where repeater sites are centrally controlled but physically located dozens of miles apart. CCE ties these repeater sites back to the dispatch center by way of dedicated microwave links.

Although the FCC has licensed CCE to use a dozen or so frequencies between 150 MHz and 155 MHz, not every repeater site transmits on every frequency. Some sites transmit on only one while others transmit on as many as seven. Because CCE is operating a conventional system and frequencies are dedicated to particular geographic areas, repeater sites transmit only on the frequencies that are relevant to the area they cover.

In Emmet County you should be able to hear Fire Dispatch on 154.400 MHz, which is transmitted simultaneously from more than one repeater site. By using more than one repeater, adequate reception is ensured from nearly any part of the county.

Trunking

Adding frequencies and splitting activities geographically works up to a point, but for agencies or departments that have a significant number of users, conventional operation isn't a viable option. No matter how they might try to divide up the activity, there are just too many users who want to use the system.

This is where trunking comes in. Instead of using each frequency pair for a specific purpose, the pairs are combined in a "pool" that can be shared among all users. When someone wants to use the system, he or she makes a request to some type of central controller, which looks at the pool of frequency pairs to see if there is one that is not currently in use. If so, that pair is temporarily assigned to the radio making the request and the person can talk on that frequency. When the person is done talking the channel is released and put back in the pool, available for someone else to use. So, if you were monitoring only one radio frequency, you would hear "snippets" of conversation whenever the controller happened to choose that frequency from the pool. If it selected a different frequency, you would miss that transmission.

Since these radio frequencies are shared, radios need a way to separate the transmissions they want from the transmissions they don't want. This is done through identifiers called talkgroups. Groups of users who share a common purpose are assigned a unique identifier that is programmed into the radio of each group member. A radio may have several talkgroups programmed into it, and the user selects the one he or she wants to use at any particular time.

So where a conventional system would dedicate a radio frequency to each group, a trunked system uses a talkgroup instead.

Control Channels

Radio frequency channels in a trunked system can be divided into two types: traffic and control.

Traffic channels are what the controller assigns to a user when he or she wishes to speak, and they carry the sound from the talking user out to all of the listening users. The sound may be carried on the channel in different formats. The oldest format is referred to as analog, where the sound is represented by a continuously varying signal. Every consumer scanner on the market works with analog traffic channels. Newer formats carry the sound as a stream of digital data -- binary digits ("bits") of 1's and 0's. Some scanners on the market are capable of correctly interpreting one particular digital voice format used in APCO Project 25 systems, which we'll discuss later on. Other digital voice formats cannot be decoded by consumer-grade scanners and thus are not able to be monitored.

Control channels carry instruction and status messages between radios and the controller. These channels are painful for a human to listen to because the messages are in digital form, so all you hear is a rough hissing sound. However, in a properly programmed radio (and in a trunk-tracking scanner), these digital messages are received and interpreted by a microprocessor, which then performs the appropriate action.

A site typically has one radio frequency set aside as a control channel while the rest are used to carry traffic. Because control channels are transmitted continuously from repeater sites, many systems change the control channel frequency from day to day in order to spread out the wear and tear on the repeater equipment.

Trunking Process

When a group member wishes to speak with the other members of his or her talkgroup, the following steps take place:

1. All radios are tuned to the repeater output frequency that carries the control channel. This is called the idle state.
2. The user starts the process by pressing the push-to-talk button on his or her radio.
3. The radio transmits a request to the repeater, along with the radio's current talkgroup identifier.
4. The repeater receives the request and forwards it to the controller.
5. The controller checks if there is a traffic channel not currently in use.
6. If there is a traffic channel available, the controller assigns it to the talkgroup and marks it as "in use."

(If all of the traffic channels are in use, the controller sends a "busy" message back to the user's radio, which in turn emits a busy tone to inform the user to try again later.)

7. The controller sends a message out to all radios, telling them that the talkgroup is active on the assigned traffic channel.
8. Radios that receive the message and are programmed with that talkgroup tune to the assigned traffic channel.
9. The requesting user's radio receives the message and emits a "go ahead" beep to the user.

Steps 1 through 9 happen very quickly, usually in less than one second.

10. The user begins speaking.
11. Eventually the user stops talking and releases the push-to-talk button.
12. The user's radio transmits a "finished" message to the repeater.
13. The repeater receives the message and forwards it to the controller.
14. The controller receives the message and in turn sends a message out to all radios indicating that the talkgroup is no longer active on the assigned traffic channel.
15. Radios that were tuned to the assigned traffic channel retune to the control channel.
16. The controller releases the active channel and marks it as "not in use."

These sixteen steps are repeated each that time a user wants to say something to a talkgroup. From the user's point of view the system is available whenever he or she wants to talk and doesn't really care which radio frequency is being used. From the controller's point of view the radio frequencies are loaned out on a temporary basis to talkgroups for only as long as they're needed.

Another way to envision this process is to imagine getting a table at a busy restaurant. When you arrive at the restaurant you give the hostess your name. The hostess then checks to see if there is a table that is not currently occupied. If all of the tables are occupied, you have to wait until some other dinner party finishes and leaves their table. If there is a table available, the hostess assigns your party to it and then announces your name over the public address system informing you that your table is ready. You and the rest of your party then move to the table selected by the hostess and occupy it for as long as you need it. When you leave, the hostess notices that you've departed and is now free to seat someone else at that table.

This is how trunking works. The hostess in our example is the controller, managing the radio frequency channels (tables) and assigning them to talkgroups (parties of dinner guests) as the requests come in. During busy times talkgroups may have to wait until a channel is available, but if the system was designed correctly (the restaurant has enough tables) in most cases the channel is available immediately. Radios (dinner guests) need to monitor the control channel (public address system) to determine which frequency (table) to use when one becomes available.

So trunking is a way of efficiently sharing a limited resource of radio frequencies and talkgroups are a way to identify groups of users who want to make use of those frequencies. A trunking system controller manages those frequencies by dynamically assigning active talkgroups to idle frequencies and informing talkgroup members which frequency they should be using.

There are a number of different kinds of trunked radio systems in operation, many of which you can monitor with the trunk-tracking scanners on the market today. A chart comparing the capabilities of trunk-tracking scanners, including which types of systems can be monitored, is available at http://www.signalharbor.com/trunking.html

It is important to match the scanner to the type of system you want to monitor, since the control channel formats are different among the various types of trunking systems.

Following is a brief overview of the most common trunking systems in use. More information is available on my web site, and I'm happy to answer reader questions.

Motorola Type I and Type II systems are the most common trunked radio systems used by public safety agencies today. Type I systems are older and make use of a "Fleet Map" to organize talkgroups. Type II systems are newer and have more capability than Type I. Every trunk-tracking scanner on the market is able to follow analog voice traffic on both types. Some Type II systems have a mixture of analog and digital voice traffic.

Programming a scanner for these systems requires only entering the control channel frequencies, since control channel messages include the traffic channel frequency. Each repeater site may have a maximum of 28 radio frequencies, with at least one and as many as four of those frequencies used as a control channel.

Enhanced Digital Access Communication System (EDACS) is another popular public safety radio system. Each repeater site will have at least one dedicated control channel and as many as 23 traffic channels. In addition to analog voice, EDACS can carry different proprietary digital voice formats, which system operators may also encrypt. Traffic on EDACS control channels may also be encrypted through an optional product known as ESK (EDACS Security Key). Scanners on the market today cannot decode the digital voice formats and will not work properly if the control channel is encrypted.

EDACS requires that the system frequencies be entered into the scanner in a specific order. Each radio frequency is assigned a Logical Channel Number (LCN), and the LCN should correspond to the scanner's channel number.

LTR systems are not as common in public safety but are often used for industrial and business applications. LTR systems do not have a separate control channel but use a technique called subaudible signaling to carry talkgroup and frequency information on the voice channel.

APCO Project 25

The Association of Public-Safety Communications Officials (APCO) created a set of standards for digital public safety radio. These standards are collectively referred to as Project 25 and were intended to inject competition into the public safety radio market by allowing agencies to purchase compatible equipment from different manufacturers.

Because APCO Project 25 (P25) is a set of standards, there are systems in operation that use some standards but not others. P25 has a Common Air Interface (CAI) and a specific format for digital voice, as well as a standard for trunking.

There are conventional P25 systems that do not use any trunking but do use P25 digital voice.

There are hybrid systems that mix analog and P25 digital voice traffic on a Motorola Type II control channel. You may find this on systems that are transitioning from older analog technology to fully digital but during the interim want to save money by continuing to use their old radios.

There are also "pure" P25 networks that use all digital voice and the P25 control channel standard for trunking.

For Mike in Petoskey, there is a statewide P25 network that can be monitored by the new APCO-25-capable scanners, including the BC-796. The Michigan Public Safety Communications System (MPSCS) is one of the largest and earliest P25 systems put into operation with more than 180 repeater sites. One of those sites, located in Petoskey, transmits on 866.4625, 867.4625, 868.4625 and 868.9625 MHz. Numerous federal, state and local agencies make use of MPSCS with hundreds of active talkgroups.

That's all for this month. Check my web site at www.signalharbor.com for trunk-tracking scanner details, and as always I welcome your e-mail at dan.veeneman@monitoringtimes.com. Until next month, enjoy the April showers and look forward to the May flowers!