A trunked radio system is a complex type of computer-controlled two-way radio
system that allows sharing of relatively few radio frequency channels among a
large group of users. Instead of assigning, for example, a radio channel
to one particular organization at a time, users are instead assigned to a
logical grouping, a "talkgroup". When any user in that group wishes to
converse with another user in the talkgroup, a vacant radio channel is
found automatically by the system and the conversation takes place on that
channel. Many unrelated conversations can occur on a channel, making use of
the otherwise idle time between conversations. Each radio transceiver
contains a microcomputer to control it. A control channel coordinates all the
activity of the radios in the system. The control channel computer sends
packets of data to enable one talkgroup to talk together, regardless of
frequency. The primary purpose of this type of
system is efficiency; many people can carry many conversations over only a few
distinct frequencies. 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. Principles of operation
= Control channels= 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 other's conversations. Trunked systems
primarily conserve limited radio frequencies and also provide other
advanced features to users. = Talkgroups=
A talkgroup is an assigned group on a trunked radio system. Unlike a
conventional radio which assigns users a certain frequency, a trunk system takes
a number of frequencies allocated to the system. Then the control channel
coordinates the system so talkgroups can share these frequencies seamlessly. The
purpose is to dramatically increase bandwidth. Many radios today treat
talkgroups as if they were frequencies, since they behave like such. For
example, on a radio scanner it is very common to be able to assign talkgroups
into banks or lock them out, exactly like that of conventional frequencies.
= Fleet maps and IDs= In some agencies, groups are assigned in
a fleet map. This was implemented to make it easier to assign group ID
numbers. For example, EMS would be on a separate 'fleet' than police. In those
fleets are subfleets or the actual talkgroups. This system is not as common
as the typical trunking method, but they do exist. In some communities the ID
corresponds to a location or agency. For example, it is common to see a '1' or
'2' in front of a police group and another number in front of a fire group.
= Scanning= Most scanners that can listen to trunked
radio systems are able to scan and store individual talkgroups just as if they
were frequencies. The difference in this case is that the groups are assigned to
a certain bank in which the trunked system is programmed. In other words,
the talkgroups are stored on the trunked bank.
= Comparison with telephone trunking= The concept of trunking 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.
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 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, called the
Erlang-B formula. This concept has simply been applied to
radio user groups, to determine the optimal number of channels needed, under
normal conditions, to accommodate 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 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 simply by changing the
software programming in the radio itself.
Since the talkgroups are constantly transmitting on different frequencies,
trunked radio systems makes it more difficult for a scanner listener without
a programmed trunk tracking scanner to keep up with the conversation.
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.
Types of trunked radio systems TETRA
OpenSky System APCO Project 16
APCO Project 25 SmarTrunk
TETRAPOL dPMR Mode 3
DMR Tier III NXDN
Kenwood NEXEDGE Digital trunked radio Icom IDAS Digital trunked Land Mobile
Radio Ericsson GE
EDACS Provoice EDACS
GE Mark V Hytera
Hytera DMR Tier 3 Trunking Pro Hytera DMR Tier 3 Trunking Lite
ACCESSNET-T IP TETRA trunking system Logic Trunked Radio
LTR Standard LTR Passport
LTR Standard and Passport LTR MultiNet
LTR-Net Motorola
Type I Type II
Type IIi Hybrid Type II SmartZone
Type II SmartZone OmniLink iDEN
Motorola Capacity Plus Motorola Connect Plus
Motorola Harmony Tait
TN9300 DMR Tier 3 system TN9400 P25 system
TN13/5100 MPT1327 System MPT-1327
Notes ^ 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. ^ The document "Conventional and Trunked
Radio Systems Comparison Report" from the Department of Homeland Security
analyzes and compares conventional, trunked, and hybrid LMR systems and
provides high-level definitions of the architecture alternatives associated
with each option. ^ US Patent and Trademark Office
Registration Number 2407576, Serial Number 75400608, registered to Uniden
America Corporation.