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Types of Trunking Systems
Trunking systems divide a few frequencies among many different users, but the way that
each system does this is slightly different. This section describes some of the technical
data behind Motorola, EDACS, and LTR trunked radio systems.
Motorola Trunking
While there are different types of Motorola trunking systems, they all use the same basic
trunking method. The system consists of one control channel plus one or more voice
channels (typically 10, 20, or 30 total channels). When a user presses Push To Talk (PTT)
to transmit, their radio first sends their talk group information to the control channel. The
computer then assigns that talk group to a specific voice channel and transmits that data
over the control channel. All radios in that talk group switch over to the assigned voice
channel and the user can begin speaking. This all typically takes place in about a
second...the person transmitting hears a beep from their radio when the channel is
assigned and then it is OK to start talking.
The systems in use are:
Motorola Type I – the radios send the radio ID, the fleet and subfleet talk group ID to the
control channel each time they transmit. To program a Type I system, you need to know
the system’s fleet map. The most common fleet maps are included at the back of this
manual. You can also find fleet map resources on the web.
Motorola Type II – the radios only send the radio ID and radio channel code to the control
channel. The central computer keeps a database of radio ID’s and which talk group is
assigned to which channel code for each radio, so with this system the user’s radio sends
only about
1
/3 the data as a Type I system with each transmission. Type II systems do not
use Fleet-subfleet talk groups; instead they use a 5-digit ID for each talk group.
Type IIi Hybrid — these systems support a mix of both Type I and Type II users. Like Type
I systems, you must know the system’s fleet map to ensure proper tracking.
EDACS Trunking
EDACS trunking works in much the same way as Motorola trunking with a couple of major
differences. In an EDACS system, each frequency used by the system is assigned a
Logical Channel Number (LCN) so that less data needs to be transmitted by the control
channel. Also, talk groups are assigned in an Agency-Fleet-Subfleet (AFS) hierarchy. Also,
there is one variation of EDACS called SCAT that your BC898T can monitor.
Logical Channel Numbers – each frequency used by the system is assigned an LCN. This
information is programmed into each user radio. When a user presses PTT, their radio
sends their AFS information to the control channel. The computer then assigns that talk
group to a channel and sends the LCN so that all other radios in that talk group will switch
to the correct channel. To program an EDACS system in your scanner, you will need to