Principles of operation
See also: Erlang (unit)
In essence, a trunked radio system is a packet switching computer network. Users' radios send data packets to a computer, operating on a dedicated frequency called a Control Channel to request communication on a specific talk-group. The controller sends a digital signal to all radios monitoring that talkgroup, instructing the radios to automatically switch to the frequency indicated by the system to monitor the transmission. After the user is done speaking, the users' radios return to monitoring the control channel for additional transmissions.
This arrangement allows multiple groups of users to share a small set of actual radio frequencies without hearing each others' conversations. Trunked systems primarily conserve limited radio frequencies and also provide other advanced features to users.
Comparison with telephone trunking
The concept of trunking (resource sharing) is actually quite old, and is taken from telephone company technology and practice. Consider two telco central office exchanges, one in town "A" and the other in adjacent town "B". Each of these central offices has the theoretical capacity to handle ten thousand individual telephone numbers. (Central office "A", with prefix "123", has available 10,000 numbers from 123-0000 to 123-9999; central office "B", with prefix "124", the same.)
How many telephone lines are required to interconnect towns A & B? If all 10,000 subscribers in "A" were to simultaneously call 10,000 subscribers in "B", then 10,000 lines, (in telco parlance "trunk lines", or simply "trunks") would be required between the two towns. However, the odds of that happening are remote. Telephone companies have well-proven formulas which predict the optimal number of trunk lines actually needed, under normal conditions, to interconnect two telephone exchanges.
This concept has simply been applied to radio user groups, to determine the optimal number of channels needed, under normal conditions, to accommodate a given number of users. In the event of a widespread emergency such as a major earthquake, many more users than normal will attempt to access both the telephone and radio systems. In both cases once the trunking capacity of the systems is fully used, all subsequent users will receive a busy signal.
In our example of police dispatch, different talk-groups are assigned different system priority levels, sometimes with "preempt" capability, attempting to ensure that communication between critical units is maintained.
Differences from conventional two-way radio
"Trunked" radio systems differ from "conventional" radio systems in that a conventional radio system uses a dedicated channel (frequency) for each individual group of users, while "trunking" radio systems use a pool of channels which are available for a great many different groups of users.
For example, if police communications are configured in such a way that twelve conventional channels are required to permit citywide dispatch based upon geographical patrol areas, during periods of slow dispatch activity much of that channel capacity is idle. In a trunked system, the police units in a given geographical area are not assigned a dedicated channel, but instead are members of a talk-group entitled to draw upon the common resources of a smaller pool of channels.
Advantages of trunking
Trunked radio takes advantage of the probability that with any given number of user units, not everyone will need channel access at the same time, therefore fewer discrete radio channels are required. From another perspective, with a given number of radio channels, a much greater number of user groups can be accommodated. In the example of the police department, this additional capacity could then be used to assign individual talk groups to specialized investigative, traffic control, or special-events groups which might otherwise not have the benefit of individual private communications.
To the user, a trunking radio looks just like an "ordinary" radio: there is a "channel switch" for the user to select the "channel" that they want to use. In reality though, the "Channel switch" is NOT switching frequencies as in a conventional radio but when changed, it refers to an internal software program which causes a talkgroup affiliation to be transmitted on the control channel. This identifies the specific radio to the system controller as a member of a specific talkgroup, and that radio will then be included in any conversations involving that talkgroup.
This also allows great flexibility in radio usage - the same radio model can be used for many different types of system users (IE. Police, Public Works, Animal Control, etc.) simply by changing the software programming in the radio itself.
Trunked radio systems also provide a small level of extra privacy since the talkgroups are constantly transmitting on different frequencies. This makes it difficult for a scanner listener without a programmed trunk tracking scanner to keep up with the conversation.
^ Talk groups, scanning, and group calls are defined in, "Section 2: Needs Summary," Arizona Phase II Final Report: Statewide Radio Interoperability Needs Assessment, Macro Corporation and The State of Arizona, 2004, pp. 16.
^ US Patent and Trademark Office Registration Number 2407576, Serial Number 75400608, registered to Uniden America Corporation.
Types of trunked radio systems
GE Mark V
Logic Trunked Radio
LTR Standard and Passport
Type IIi Hybrid
Type II SmartZone
Type II SmartZone OmniLink
iDEN (integrated Digital Enhanced Network)
Motorola Harmony (see iDEN)
APCO Project 16
APCO Project 25
Trunked radio systems
Motorola systems: Type I Type II Type IIi Hybrid Type II SmartZone Type II SmartZone OmniLink iDEN
Other: APCO Project 16 APCO Project 25 EDACS EDACS Provoice MPT-1327 OpenSky TETRA TETRAPOL
Scan-based / distributed control
General Electric Mobile Radio: GE Marc V
Logic Trunked Radio: LTR Standard LTR Passport LTR Standard and Passport LTR MultiNet LTR-Net
Amateur and hobbyist
Amateur radio Amateur radio repeater Citizens' band radio Family Radio Service General Mobile Radio Service Mobile rig Multi-Use Radio Service PMR446 LPD433 UHF CB (Australia)
Aviation (aeronautical mobile)
Air traffic control Aircraft emergency frequency Airband Mandatory frequency airport Single Frequency Approach UNICOM
Land-based commercial and government mobile
Business band Base station Mobile radio Professional Mobile Radio Radio repeater Specialized Mobile Radio Trunked radio system Walkie talkie
2182 kHz 500 kHz Coast radio station Marine VHF radio Maritime mobile amateur radio
Signaling / Selective calling
CTCSS Dual-tone multi-frequency D-STAR MDC-1200 Push to talk Quik Call I Quik Call II Selcall
System elements and principles
Antenna Audio level compression Automatic vehicle location APRS Call sign CAD DC remote Dispatch Fade margin Link budget Rayleigh fading Tone remote Voice procedure Voting (diversity combining)
Categories: Trunked radio systems | Telecommunications terms | Radio resource management | Radio networksHidden categories: All articles with unsourced statements | Articles with unsourced statements from October 2008