Trunked Radio System Monitoring

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Scanning of Smart Zone and Smartnet Trunking Systems

Written by: Wiz, City of Seattle, WA

Network Manager for King County Regional 800 MHz Trunked Radio System

Revised: 15 June 2001


Some History

Two-way radio, or Police radio, are very broad terms that encompass a wide range of different technologies. Lets look at some of the terms you have heard used both correctly and incorrectly. As a subject becomes more technical, it becomes very important that we all speak the same language.

Simplex - (Portable to Portable Operation)

Simplex is the simplest form of 2-way radio communication. It is when two or more radios transmit and receive on the same frequency. First one radio transmits and the rest of the radios listen. When the transmission is completed, that radio listens on the same frequency for someone else to transmit. Radios transmit directly to each other. The range is limited, because of the low wattage in a portable radio (up to 5 watts), and because these transmissions are line of sight (usually less than 1 mile). To overcome some of this range limitation, base stations were placed at high locations and wire controlled from a dispatch point. This allowed the dispatcher to talk to, and hear from, field units further away but did nothing to improve range between field units. The location of the radio transmitter is called the radio site or site and is usually given some geographical name that is meaningful to the locals. Because the frequency is normally shared with others, users must wait for a clear channel before transmitting much as the old party line telephones (If you are old enough to remember those!)

Scanning Issues: Program in one frequency to listen to the radio users. Can normally hear the base transmissions from a long distance but depending on distance, terrain, and frequency will have very mixed results when the field units transmit. Conversations are easy to follow.

Duplex - (Repeater Operation)

To overcome the short range of simplex, we started putting repeaters on hills, tall buildings and other high places. Because of the time it would take to receive, record, and then rebroadcast a signal on the same frequency, we went to a two-frequency concept. The field units transmit on one frequency, which the repeater receives. The repeater then instantaneously re-transmits the voice signal on a second frequency, which is received by the field units. Because it is using two frequencies (or a frequency pair), this is called duplex operation. The field units are said to be half duplex since they use two frequencies, one for transmit and one for receive, but cannot operate on both at the same time. The repeater is said to be full duplex since it transmits and receives at the same time. Range for duplex (or repeater) operation is generally 5-10 miles, while operating on 75-100 watts. This range can vary greatly depending on frequency, local terrain, and height of the repeater antenna.

Scanning Issues: Normally program in the repeater output frequency to listen to the radio users. Can normally hear both base and field transmissions from a long distance. If field units go to talk-around (to bypass the repeater) they will use the repeater output frequency and scanners will have very mixed results when field units transmit, as if units are simplex (because they are). If field units are in the talk-around mode, the dispatcher will normally not be able to hear them, but a scanner may still pick them up.


To further increase range, a technology called simulcast was developed. With this method, two or more transmitters (normally repeaters) on the same frequency are located at multiple high locations (called remote sites). The inputs of each repeater transmitter are tied together so they all transmit the same audio signal at the same time. In this way no matter which repeater the field unit is within range of, it will hear all transmissions. If a field unit (radio) is close to one repeater, it's receiver will be captured by the strong signal and the received audio will be clear. If, however, the field unit is approximately equidistant from two or more repeaters, it will receive multiple signals. If these multiple signals are not exact in frequency, timing, volume, and audio frequency response, the received audio can be garbled to the point it is unintelligible. This equidistant geographical area is called the overlap area. For this reason we have to use some rather expensive technical methods to control the transmitter frequency, audio characteristics, and delay of the audio so all transmitters transmit the exact same signal at exactly the same time. On the repeater receive (or talk-in) side, we normally feed all the receivers (normally one at each transmitter site and sometimes additional receivers at non-transmitter sites) into a device called a voter or comparator. This unit electronically checks (or compares) each receiver signal for its signal to noise ratio and then selects (or votes) the best signal to be sent to the dispatcher and to be re-broadcast. The location of the simulcast controller (computer) and other equipment (including the voters) that makes simulcast possible is called the prime site and may, or may not, be located at one of the remote site locations. Simulcast can be either single frequency or trunked (see next section).

Scanning Issues: Same as for duplex operation. Since coverage area is normally larger than for a single repeater, scanners will also be able to monitor over a larger area. If one of the simulcast transmitters goes out of service, coverage for its area will be reduced. If you are in that area, you may notice a weaker signal or you may stop receiving that system all together. Since the FCC allows only six sites on a license, you may find multiple licenses covering the simulcast system. Note that it is not possible (except under some very special circumstances) for a receiver user to tell which simulcast transmitter site (or combination of sites) is providing the signal being received. Sometimes this may be the farthest site, sometimes the closest, but many times will be a combination of two or more of the remote sites. Thus it is inaccurate and confusing to refer to a received simulcast signal by one remote site name instead of the name for the simulcast system or subsystem.

Trunked Repeater Operation

In the older radio systems, one or more groups of users were assigned to a particular pair of frequencies. If no one in any of those groups were talking, the frequency (or frequency pair) stood idle. At the same time, the groups of users on a different assigned frequency pair may be so busy that each of the groups is constantly waiting for someone to get through talking so they can get a message through. Thus there was a lot of available airtime wasted.

As the number of user groups increased in large metropolitan areas, all available frequencies were assigned to existing user groups (King County being a prime example). As new groups (such as new police departments or additional precincts in an existing police department) were created, no frequencies were available without sharing with an existing user.

A solution, known as trunking was borrowed from the telephone industry. With this technology, several frequency pairs are grouped together and put under the control of a computer (called a Trunking Controller). This computer then accepts each request to talk, assigns the next available frequency pair, directs all the radios in the talk group to switch to the assigned frequency pair, and sends a permit to talk tone to the requesting party. The whole process takes less than half a second. The computer also monitors the activity (not the conversation, but the fact the frequency pair is in use). When the frequency pair becomes idle, it is returned to the pool and is available for re-assignment to another user group. By pooling several frequencies, we also pool the unused airtime thus allowing more user-groups to work independently. In this manner we can get 3 to 10 times more use from a given set of frequencies than the old non-trunking method of assigning frequencies to users. A trunked repeater system can be either single site or simulcast.

A simple analogy to trunking is the system used to serve in-person customers at a bank. Formerly, customers got in line at individual teller windows to wait for the teller at that window to assist them. All of us have experienced getting in what appears to be the shortest line, when in fact, the transactions in front of us end up taking longer than anticipated, resulting in a longer wait for service than other lines might have offered. Also, you may have experience waiting in an assigned line for your type of transaction, even if another window has no waiting customers. Now, many banks have customers form a single queue, and the next available teller waits on the customer at the front of the line.

Rather than being stuck with a specific teller line (frequency), the trunking system will accept a request to talk and assign whatever frequency is available to the next request in line. When all frequencies are assigned, the system will put all requests in a waiting queue, and is able to prioritize customers in the line. The next available channel will be assigned to requests with the highest priority. Requests with equal priority will be served on a first-come, first-served basis. Typically, no users should experience any wait except in the event of a major disaster when radio traffic would be unusually high.

Thus, rather than scanning frequencies, you will be scanning talk groups.

Scanning Issues: The range that a scanner will work is influenced by the same factors as repeater and simulcast operation, depending on configuration. Frequency, however, is a very different issue. Since a computer is used to assign the next frequency, sometimes as often as every time a field unit keys up, the scanner needs to be able to follow these assignments just as the field units or it will not follow the conversation. To do this, a special scanner that can either be controlled by an external computer or has the internal circuitry to follow the frequency assignments is required. Most of the older trunking scanners also need to know all the frequencies in use by the trunking system. Thus, to properly scan, all frequencies used in a trunked system must be programmed into this type of scanner. If you are using one of the computer software products such as Trunker or one of the newest trunking scanners, this is not a requirement, but you must still program the control channel for the system you wish to scan. Since the FCC allows only six sites on a license and the system may use frequencies from more than one band, you may find multiple licenses covering the trunked or trunked simulcast system. You will need to determine which frequencies on these multiple licenses (probably all the frequencies on all the licenses) are in use for the trunked system you wish to monitor.

When the King County system was first installed (Seattle was the first to implement), Seattle recognized this as an issue and made arrangements with the vendor to sell radios to those wishing to scan at the discounted price Seattle paid. This was an expensive way to scan, but there were no alternatives on the market. Although this offer is still available, Seattle will not program any requested talkgroup, will only program Seattle talkgroups (other sub-regions would program their talkgroups), and the radios are several times more expensive than the trunking scanners now on the market.

Smartnet Operation

Smartnet is the Motorola implementation of trunking and is their registered trademark.

In a Smartnet trunked repeater system, generally 5 to 28 frequency pairs are grouped together. One of these frequency pairs is called the control channel and is used to convey information between the field units and the trunking system. When a mobile or portable unit (also called field, or subscriber, unit) is not listening to a talkgroup conversation, it is monitoring (listening to) the control channel. A Smartnet system can be either single site or simulcast. Note, also, that there is no requirement to have the frequencies in any particular order. They can be set up in ascending, descending, or some random sequence and the system will work just as well. It then follows that the frequency order in the scanner does not have a bearing on the scanner operation.

Each subscriber unit is assigned a unique ID number. Each talkgroup is also assigned a unique ID number and turning the channel selector knob now selects a talkgroup ID number instead of a frequency. The system uses these numbers to track and authorize subscriber use of the system. This tracking starts when the subscriber unit is first turned on and then each time the operator changes the talkgroup.

The Smartnet trunked system is controlled by a Trunked System Controller which is located (in the case of simulcast) at what we call the Prime Site along with any simulcast control equipment. The transmitters are located at what we call Remote Sites (if simulcast) along with the Remote Site Controller. Note that a Smartnet system can be one site only (not simulcast) with everything co-located.

With the introduction of Smartnet, Motorola also introduced a failure mode called Failsoft. If the trunking system loses its control channel or has certain other failures, it is no longer able to operate in the trunking mode. So instead of going into a condition that stops all communication, the system enters Failsoft. In this state all transmitters (channels) turn on and operate in a conventional repeater mode. The subscriber radios are able to recognize this state and switch to a predetermined frequency (one of the trunk system frequencies, but not the control channel frequency) depending on their selected talkgroup. In most systems several talkgroups will share a frequency. Some talkgroups may not be assigned a failsoft frequency and these talkgroups will cease to operate during the failsoft period. If a particular failsoft frequency has also failed, the talkgroups assigned to that frequency will also be off the air during failsoft.

Scanning Issues: Same as Trunked Repeater operation for single site or simulcast, depending on configuration. The location of the prime site has no bearing on the operation of the scanner. The King County system uses Type II trunking.

During Failsoft, all transmitters turn on, whether or not there is an on-going conversation. This means a scanner will stop on each frequency and stay there until intervention by the scanner operator.

SmartZone Operation

As much as trunking solved traffic handling ability and simulcast improved wide-area operation, some limitations prevented this from being the panacea we hoped for. Trunking has a limited number of frequencies it can handle (limiting the amount of traffic it can carry). Simulcast has a limited number of sites it can handle (limiting the amount of signal delivered to the coverage area- with larger areas getting less signal for a given number of sites). Simulcast requires a transmitter at every site for each frequency whether or not that site is required for a particular call (increases the system cost), and requires exclusive frequency usage in a very large area (limiting the ability to reuse frequencies- an FCC issue). So we once again turned to the computer to solve these problems and increase the geographical coverage. Motorola responded to our needs with a system called SmartZone (also a Motorola registered trademark) which takes two or more Smartnet trunking systems (any combination of simulcast and non-simulcast) and ties them together with a master computer called the Zone Controller. Smart Zone allows us to combine the good points of both single site and simulcast trunking to achieve a better overall system. The job of the Zone Controller is to keep track of every radio in use, what site it is using (from the Zone Controller perspective, each entire simulcast sub-system (no matter how many remote sites it has) is called a site and treated as one site), and what talk group that radio is switched to. When a radio is turned on it affiliates with the system, reporting its ID, site it is on, and talkgroup selected. When a user keys a radio, or changes the channel selector knob, the radio talks to the site it is affiliated with, which in turn passes the information (desire to talk or change of talk group) to the Zone Controller. The system thus knows which sites (remember an entire simulcast sub-system is just another site as far as the Zone Controller is concerned) require the audio for a particular talkgroup. In the case of PTT (Press To Talk or keying the radio) activation, the Zone Controller then tells all affected site controllers to make a channel available, and ties the audio of these sites together to complete the call. The individual site controllers then (via the control channel) tell all of the subscriber units with the talkgroup selected which frequency to listen to. All this happens in less than half a second.

Note that, in most cases, if no radio affiliated to a particular site has the talkgroup being talked on selected, the talkgroup audio will not be sent to that site, and anyone scanning that site will not hear the conversation.

Each SmartZone site (whether single site or simulcast sub-system) can be programmed for Failsoft operation. Those sites that are programmed for failsoft operation can enter the failsoft mode even while the rest of the SmartZone system is operating normally in the wide-area, trunked, mode. Conversations on a site in failsoft will not be carried on other SmartZone sites.

Scanning Issues: With todays scanners, you can only scan the talkgroups of one SmartZone site at a time. Remember that a simulcast subsystem, no matter how many sites or how may licenses (call signs), is one SmartZone site. This is because all of the remote sites have the same frequencies and any conversation is simultaneously transmitted (simulcast) from all sites. The King County Regional Trunked Radio System uses Type II trunking. Range for each Smart Zone site is same as Trunked Repeater and Simulcast operation, depending on the configuration of the Smart Zone site you are listening to. In addition, if no radio affiliated to the site you are scanning has the talkgroup you wish to listen to selected, you will not hear activity on that talkgroup. Note that programming the frequencies of more than one site into your scanner will not allow you to scan multiple sites simultaneously. This is because the scanner selects only one control channel to listen to and that control channel does not convey any information about frequency assignments for other sites. In fact, if you program too many frequencies and one or more of the extra frequencies is the control channel of another site, the trunking scanner may actually use the wrong control channel. If you have not programmed in all the frequencies for that second site (a very likely event from what I have seen with posted frequency lists) you will probably miss parts or all of many conversations. Again, this is not an issue with computer software products such as Trunker or one of the newest trunking scanners that only require knowledge of the control channel.

Some scanners may claim to be able to scan multiple trunking sites or conventional frequencies as well as trunking systems simultaneously. They do this by sequentially moving from one system or site to the next. While scanning a particular site or frequency, you may miss something on the others. This is because the scanner has only one receiver and in order to know anything about the talkgroup activity on a trunked system, you must listen to the control channel.

So, this begs the question what is the best site to scan for the talkgroups I want to hear? The best site to scan for a particular talkgroup is a site that will have at least one subscriber unit affiliated with your desired talkgroup selected most, or all, of the time. And here is the short answer for the King County Regional Trunked Radio System: to scan Seattle talkgroups, program in the Seattle simulcast subsystem; to scan EPSCA talkgroups, program in the EPSCA simulcast subsystem; and to scan King County or Valley Com talkgroups, program in the KC/VC simulcast subsystem. If you cant get a good signal from the best simulcast subsystem, you can try another, but be aware that you may not hear any or all of the talkgroup conversations that you are interested in scanning! The same holds true if you want to scan talkgroups that have different best sites. Whether or not you can hear a particular talkgroup on a selected SmartZone site fully depends on if an authorized subscriber unit, with that talkgroup selected, is affiliated with the site you are scanning.

OmniLink Operation

OmniLink is a method of tying two or more Smart Zone systems together. OmniLink is also a Motorola registered trademark.

Scanning Issues: Same issues as for Smart Zone.

Other Reading

This paper was meant to give a basic understanding of Motorola trunked two-way radio operation so you might understand some of the weird things that seem to happen as you try to scan. As you become more familiar with your equipment and more comfortable in your understanding of the basic operation of Smart Zone, you may want to read further.

Some topics you will probably want to learn more about are:

The format of talkgroup IDs and how to convert from one format to another.

Status bits and how they may affect your scanner.

The different software scanning products available.

Antennas and how they affect your scanner performance.

Other manufacturer's trunking formats.

There are several good papers covering these topics at various technical levels on this board as well as other scanning related sites.


Following are some common terms used in two-way radio and trunking.

Antenna- The device (normally similar to a wire) used to convert the electromagnetic energy in the air to an electrical signal that a receiver can process. The dimensions of an antenna are mathematically related to the frequency it will work best for. Normally, higher frequencies will use smaller antennas.

Base Station- A fixed, or stationary, transmitter-receiver.

Channel- the frequency or talkgroup a radio is operating on.

Comparator- An electronic device to automatically the best of two or more signals. Also called a voter.

Control Channel- One frequency in a trunking system that is used to convey system information such as frequency assigned for a talkgroup call and what talkgroup a radio has selected.

Controller- A computer that is used to control a simulcast or trunking system, or a remote simulcast site.

Dispatch- The act of sending instructions to field personnel in day to day operations. Also an abbreviation for Dispatch Center, the place where Dispatchers work.

Duplex- A method of transmitting and receiving via two different channels or paths. Full duplex means transmit and receive can occur simultaneously and half duplex means one function needs to wait for the other to be completed.

Failsoft- A trunking system failure mode. Under certain conditions, the trunking system may not be able to continue trunking. Each frequency then operates as a repeater with two or more talkgroups sharing the frequency.

FCC- Federal Communications Commission

Field Unit- A mobile or portable radio. Also called subscriber unit.

Key- To cause a radio to transmit

Mobile- A two-way radio that is designed for installation in a vehicle.

Modulation- The method of putting intelligence on a radio signal.

OmniLink- A wide area trunked radio system consisting of two or more Smart Zone systems.

Portable- A two-way radio that is designed to be carried by a person.

Prime Site- The grouping of electronics that controls a simulcast system. The Zone Controller and its associated equipment can also be called a Prime Site.

PTT- Press To Talk button on a portable or microphone used to cause the transmitter to transmit.

Remote Site- One of two or more simulcast transmitter sites.

Repeater- A radio receiver/transmitter that receives on one frequency and rebroadcasts the signal on a different frequency. Repeaters normally operate in full duplex mode.

RF- Radio Frequency- The 800 MHz signal

Satellite Receiver- One of two or more receivers connected to a voting comparator.

Signal- The radio frequency information received by a receiver that carries voice information.

Simplex- Transmitting and receiving on the same frequency.

Simulcast- Transmitting the same signal from two or more sites simultaneously.

Site- Normally refers to the location of transmitting and receiving equipment. For a Smart Zone system, site also refers to an entire simulcast subsystem.

Site Trunking- This is when a Smart Zone site is disconnected from the Zone Controller. The site will continue to process traffic and trunk, but it is acting in a stand-alone manner and cannot share traffic with other Smart Zone sites.

Smartnet- The brand name given to a Motorola trunking system. A Smartnet system can be either single-site or simulcast.

Smart Zone- The Motorola brand name for a wide-area trunking system made by tying two or more Smartnet systems together. The individual Smartnet systems can be any mix of single-site and simulcast.

Subscriber (Unit)- A mobile or portable radio used by personnel not at fixed locations.

Talkgroup- A specific set of radio users with a common ID that are allowed to talk to each other and are assigned a particular frequency.

Trunk (ed) (ing)- A method of grouping of frequencies so several users can use the grouped resources, thus increasing capacity to carry conversations.

Voter- Another name for comparator or voting comparator. A device that selects the best receive signal from two or more receivers.

Zone Controller- The central computer used to control a Smart Zone trunking system.

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