Introduction to Trunking Systems

From the Auxiliary Communications Service Newsletter,
developed by the ACS Training Unit of the State of California
Governor's Office of Emergency Services:

EMCOMM Bulletins #188-190

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AN INTRODUCTION TO TRUNKING RADIO SYSTEMS
By Mike Krueger, N6MIK
Assistant Radio Officer, Training Officer, County of Orange RACES

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The concept of a trunking radio system was first developed in
the mid-1980's for the commercial radio business. More and more
business customers required radio communications, while fewer
and fewer frequencies remained to issue. Motorola designed the
first trunking system to help alleviate the congestion in the
LMR spectrum, and allow more users to share the same frequencies
by way of dynamic re-use. Trunking is a method of using
relatively few communication paths for large number potential
users. This article will concentrate on the Motorola systems,
due to their popularity among California public safety agencies.
These radio systems are similar to the telephone trunking
concept that has been in use for years.

There are currently three types of Motorola trunking systems in
use by public-safety agencies: Type I, Type II and Type IIi. A
Type I system is the oldest and least robust of the Motorola
trunking formats and has a limited user capacity. The newer and
much more complex Type II, or SmartNet, system abolishes
Subfleets and introduces the Talkgroup (TG). This type of
system is very popular among public safety because of increased
security, emergency signaling, and remote radio monitoring and
control. Type II systems are capable of up to 4,000 talkgroups,
65,535 individual radio identifiers, and up to 28 system
repeaters.

A typical Type I trunked radio system, which is also called
Privacy Plus, consists of five to seven radio repeaters
connected by a central controller. A Type I system defines its
"channels" by using Fleet and Subfleets. For example, a city
fire department may be on one fleet and the public works
department may be on another fleet, with up to 16 subfleets per
fleet. Subfleets are referred to by letter, A~O, and are the
"channels" of a Type I system.

The Central Controller is the computerized brain of the trunked
system and performs many functions, the most obvious of which is
letting users talk by managing the talkgroups and system
repeaters. The controller also allows for administrative
functions, such as adding talkgroups, radios, or repeaters,
changing the trunking parameters, billing, and other functions.
The central controller uses one of the system's repeaters to
transmit a constant data stream (3600-baud FSK) on the control
channel. This stream is encoded with a 4-digit hexadecimal
system identifier and sends instructions to radios on the
system. The control channel usually rotates daily among a set
group of channels to avoid making one repeater do all the work.

(Begin Part 2 of 3)

When a trunking radio is powered on, it searches a preprogrammed
list of frequencies for the control channel. When the control
channel is found, the radio confirms that the System ID matches
its programming and starts quietly monitoring the data stream
for instructions from the central controller. This is where
trunking gets interesting!

Each radio has a unique identifier or Unit ID, which allows the
controller to address it individually, and one or more subfleet
or talkgroup programmed. How does this all work? It's actually
quite simple!

When the PTT is keyed, several things happen before the user can
speak. First, the radio sends a quick (21.67 milliseconds) burst
of data on the input frequency of the control channel repeater.
This is called the Inbound Status Word, or ISW. The ISW contains
the unit ID and the fleet/subfleet or talkgroup of the radio,
depending on system type. The controller then sends instructions
out over the control channel, steering all radios tuned to the
same subfleet/TG to an open repeater in the system and activates
it. This is the Outbound Status Word or OSW. At this point, the
user that pressed the PTT gets a "talk-permit" tone (usually
three short beeps) and can begin speaking. This entire process
takes about 58 milliseconds. There are several other low and
high speed data handshakes that occur on the system repeater
frequencies.

When the user releases the PTT, the radio transmits a 200-ms
disconnect tone (163.64 Hz) that tells the central controller
that the transmission has ended. During the repeater hang time,
users on the subfleet can transmit without requiring the central
controller's intervention. When the hang time expires, another
low speed disconnect tone is transmitted by the central
controller on the voice channel, releasing all radios to return
to the control channel and await further instruction.

In the event that all system repeaters are in use when the
central controller receives a talk request, the requesting radio
will sound low beeping tones similar to the telephone's
"busy-signal". Simultaneously, the controller adds the request
to a queue, and will signal the radio with a talk permit tone
when a repeater is free. Many public-safety systems (including
the current Fire radio system in Orange County) have an
EMERGENCY button that, when pressed, makes a repeater instantly
available, even if all system repeaters are busy. This is called
"ruthless preemption". The controllers can be configured to alert
the dispatcher by sounding an alarm at the console when the
EMERGENCY button is pressed.

(Begin Part 3 of 3)

Large users, such as public-safety agencies, may need more than
16 subfleets. Multiple independent systems can be installed and
programmed into user radios. In this case, the system number
appended to the subfleet (1-A, 4-B) is used to indicate the
group.

SmartNet, or Type II, systems operate similar to the Privacy
Plus systems with regards to ISW/OSW and high and low speed
handshaking. In addition, a SmartNet radio affiliates with the
system on power on and before processing data from the control
channel. This is similar to cellular/PCS phones that require an
automated "logon". Affiliation also occurs whenever a user
changes talkgroups. De-affiliation occurs when the radio is
powered off. System administrators can disable individual radios
as needed by setting the controller to refuse affiliation. A
non-affiliated radio can not process information from the
controller, and will not transmit or receive.

A Type II system can have up to 4,000 talkgroups, and the
controller must manage all of the talkgroups to avoid
overloading the system. Because each discrete talkgroup will
occupy a system repeater, talkgroups only become active when two
or more radios have affiliated with it. A single radio on a
talkgroup will be unable to transmit. This keeps the system load
down, especially during multi-select (a transmission over several
talkgroups, such as a general broadcast by a dispatcher). The
controller transmits to those groups that have radios
affiliated, leaving several system repeaters open.

A Type IIi system is a hybrid of a SmartNet with some Privacy
Plus components. This type of system is commonly used while a
department is migrating from Privacy Plus to a SmartNet system
and must maintain compatibility with older radios. In the
unlikely event of central controller failure, the system will go
into Fail Soft mode. Fail Soft conditions are similar to
conventional repeaters in that all trunking functions are lost.
A faint single beep is heard every 10 seconds to alert users of
the condition. Radios are preprogrammed using the system
repeaters in a conventional format, and all users share the
system's repeaters until the system is back on-line. The
controller sends the Fail Soft message in its data stream.

As communicators, it is important that we become familiar with
the equipment used by the agencies we support. Knowing the basic
operating principles of trunking may prove valuable in the future
when you may be asked to operate on a trunking system.

Motorola and all references to their trunking formats are
registered trademarks of Motorola.

(Series end: AN INTRODUCTION TO TRUNKING RADIO SYSTEMS By Mike
Krueger, N6MIK, Assistant Radio Officer, Training Officer,
County of Orange RACES.)
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