Trunked radio system

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A central-controlled trunked system uses a control channel (as shown). Another type, Scan based trunked systems, (not shown) do not have a control channel. Frequencies are for discussion purposes and do not correspond to any specific system.

A trunked radio system is a complex type of radio system that is used to maximize available capacity in a two-way radio system, usually UHF. Groups of users are given a logical talkgroup to share for their communications, rather than a dedicated radio frequency.^[1] Trunking is used by many government entities to provide two-way communication for fire departments, police and other municipal services, who all share spectrum allocated to a city, county, or other entity.

In 1997, radio scanners compatible with trunked systems appeared on the market. One of the first companies to bring these devices to market, Uniden, trademarked the term trunk tracking on December 5, 1997.^[2]

[edit] Principles of Operation

[edit] Control channels

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.

[edit] 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.

[edit] 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 pool of channels.

[edit] Advantages of trunking

Trunked radio takes advantage of the probability that in any given number of user units, not everyone will need channel access at the same time. Therefore with a given number of users, 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 who 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 channels at all: when changed, it refers to an internal software program which causes a talkgroup affiliation to be broadcast 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.

[edit] Notes

1. ^ 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.
2. ^ US Patent and Trademark Office Registration Number 2407576, Serial Number 75400608, registered to Uniden America Corporation.

[edit] Types of Trunked Radio Systems

1. Ericsson GE
   • EDACS Provoice
   • EDACS
   • GE Mark V
2. Logic Trunked Radio
   • LTR Standard
   • LTR Passport
   • LTR Standard and Passport
   • LTR MultiNet
   • LTR-Net
3. Motorola
   • Type I
   • Type II
   • Type IIi Hybrid
   • Type II SmartZone
   • Type II SmartZone OmniLink
   • iDEN (integrated Digital Enhanced Network)
   • Motorola Harmony (see iDEN)
4. MPT-1327
5. OpenSky System
6. APCO Project 16
7. APCO Project 25
8. SmarTrunk
9. TETRA
10. TETRAPOL