Understanding Radio Scanning
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Conventional Radio Scanning
Conventional radio scanning is a relatively simple concept. Each group
of users in a conventional system is assigned a single frequency (for
simplex systems) or two frequencies (for repeater systems) to use. Any
time one of them transmits, their transmission always goes out on the
same frequency. Up until the late 1980’s this was the primary
way that radio systems operated. Even today, there are a lot of 2-way
radio users who operate using a conventional system:
When you are scanning a conventional system, the scanner
stops very briefly on each channel to see if there is activity. If there
isn’t, the scanner quickly moves to the next channel. If there
is, then the scanner pauses on the transmission until it is over.
Simplex systems use a single frequency for both transmit and receive.
Most radios using this type of operation are limited to line-of-sight
operation. This type of radio is frequently used at construction job
sites, and with inexpensive consumer radios such as GMRS/FRS radios.
The range is typically 1-5 miles, depending upon the terrain and many
Repeater systems use two frequencies: one transmits from the radio to
a central repeater; the other transmits from the repeater to other radios
in the system. With a repeater based system, the repeater is located
on top of a tall building or on a radio tower that provides great visibility
to the area of operation. When a user transmits (on an input frequency),
the signal is picked up by the repeater and retransmitted (on an output
frequency). The user’s radios always listen for activity on the
output frequency and transmit on the input frequency. Since the repeater
is located very high, there is a very large line of sight. Typical repeater
systems provide coverage out to about a 25 mile radius from the repeater
While conventional scanning worked great while there were only a few
groups wanting to use the frequencies, with the advent of smaller, lower-cost
radios more and more agencies and businesses wanted to take advantage
of the utility of 2-way radio. As a result, the bands that were used
most became full, so new users were not able to take advantage of the
technology as quickly as they wanted.
Trunking solved this frequency shortage by allowing multiple groups
to use the same set of frequencies in a very efficient way. While each
type of trunking system operates a little differently (see the next
few sections), they all work on the same basic premise: even in a system
with a lot of users, only a few users are ever transmitting at any one
Instead of being assigned a frequency, as with conventional systems,
each group is assigned a Talkgroup ID. A central computer controls the
frequency each group operates on...and this frequency selection is made
each time a user transmits. So, while on a conventional system queries,
replies, and follow-ups are all on a single frequency, they could each
be on completely different frequencies on a trunked system. This semi-random
frequency assignment made monitoring such a system impossible prior
to Uniden’s invention of the Trunktracking scanner.
While there are 4 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 talkgroup information to the control channel.
The computer then assigns that talkgroup to a specific voice channel
and transmits that data over the control channel. All radios in that
talkgroup 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 it is OK to start talking.
The four systems in use are:
Motorola Type I –
the radios send the radio ID, the fleet and subfleet talkgroup
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 talkgroup 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 talkgroups; instead
they use a 5-digit ID for each talkgroup.
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 fleetmap
to ensure proper tracking.
Motorola Astro Digital
- for channel control purposes, this type of system operates just
like a Type II system — although the control channel can
be a 3600 bps data rate (for mixed analog/digital systems) or
a 9600 bps (for digital only systems). Pure digital systems can
be implemented under APCO 25 Phase 1 or Phase 2 standards. The
are able to decode all unencrypted digitized voice traffic on
either mixed mode or digital-only APCO 25 Phase 1 systems.
One big difference you will notice with digital
versus analog transmissions, is that with analog systems, you might
be able to hear weak signals interspersed with hissing. As you move
further away from the system, the interference gradually increases
until you are unable to make out the transmission. With digital
systems, the cutoff point is much more abrupt. You might have a
small area where partial decoding occurs...in which case you will
hear partial and garbled audio. However, once the scanner is unable
to receive the data well enough to decode it, the audio stops entirely.
For the best range, antenna selection and placement is critical.
See “Attaching an Antenna” for more information.
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,
talkgroups are assigned in an Agency- Fleet-Subfleet (AFS) hierarchy.
Also, there is one variation of EDACS called SCAT that the BCD396T
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 talkgroup to a channel and sends the LCN so that
all other radios in that talkgroup will switch to the correct channel.
To program a EDACS system in BCD396T
scanners, you will need to know both the frequencies used by the system
and the LCN for each frequency so that you can program the frequencies
in LCN order.
talkgroup ID’s for EDACS systems are assigned in a way that
makes it easy to see at a glance the affiliation of the user. Each
radio is assigned a 2-digit agency identifier from 00 – 15.
For example, 01 might be used by the police, 02 by ambulance service,
03 by the fire department, and so on. Each agency is then subdivided
up to 16 times to provide fleet identification, and then 8 more times
to identify subfleets. For example, the complete AFS for the Police
Department West District’s dispatch channel might be 01-062.
01 identifies the agency as the police department, 06 identifies the
fleet as the West district, and 02 identifies the subfleet as the
dispatch channel. While these assignments are somewhat arbitrary and
vary from system to system, there are many resources on the web for
finding the assignments for most systems. Because of the logical hierarchy
of the AFS system, the BCD396T
let you assign wildcard ID’s that let you, for example, use
only one ID memory to identify all units in either an agency or a
EDACS SCAT –
EDACS SCAT (Single Channel
Autonomous Trunking) systems operate on a single channel and alternate
control data with analog voice traffic. While BCD396T
cannot track ID’s in this system, it can eliminate the control
data so that all you hear is the voice transmissions when you monitor
this type of system.
LTR® (Logic Trunked Radio) systems are trunking systems used primarily
by business or private communications service providers, such as taxicabs,
delivery trucks, and repair services. These systems encode all control
information as digital subaudible data that accompanies each transmission,
so there is no separate control channel. Users on an LTR system are
assigned to specific talkgroups, which are identified by the radio as
six digit numbers. These numbers are in the form AHHUUU, where:
A= Area code (0 or 1)
H= Home repeater (01 through 20)
U= User ID (000 through 254)
When the scanner receives a transmission on a channel
set to the LTR mode, it first decodes the LTR data included with the
transmission. In the ID Search mode, the scanner stops on the transmission
and displays the talkgroup ID on the display. In the ID Scan mode, the
scanner only stops on the transmission if the LTR data matches a talkgroup
ID that you have stored in the bank’s talkgroup ID list and have
not locked out.
LTR systems are frequently programmed so that each radio has a unique
LTR systems also need to be programmed into BCD396T
and BCD996T scanners
Since many LTR systems use only odd-numbered channel slots, you would
program these systems using only the corresponding odd-numbered channels
in a bank (for example, you would program a system with channels at
1, 3, 5, and 9 into Trunk 2 channels 101, 103, 105, and 109).
Banks and Channels
The memory in BCD396T
and BCD996T scanners
is organized into 10 banks of 100 channels each. Each bank can contain
conventional channels as well as 1 trunking system. For each trunking
system, each bank can also store 10 groups of 10 talkgroup ID’s
(100 per bank).