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channels (typically 10, 20, or 28 total channels). When a user presses Push To
Talk (PTT) to transmit, their radio first sends the person’s 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. 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 in
the Reference section in this manual. You can also find fleet map resources on the
web such as at http://www.radioreference.com.
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.
Motorola ASTRO 25 (APCO 25) — these radios are similar to Type II systems,
but send only the radio ID and radio channel code to the control channel,
transmitting voice in digital form.
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. Talk groups are assigned in an
Agency-Fleet-Subfleet (AFS) hierarchy.There is one variation of EDACS called
SCAT that your BCD996T 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