*note: The following information relates to Uniden scanners only.  I have no experience with any of the Radio Shack Pro-xx scanners.

Trunking Basics for Newbies


If you are new to trunking radio scanners, have no fear, we've all been there.  Luckily, the public safety systems in SD county are 800 mHz Motorola type II systems, and are the simplest to program, IMHO.

In its simplest form, a trunked system consists of a group of frequencies, one of which at any time acts as the controller.  This controller channel, called the DATA channel coordinates all the activities for all the users on the trunked system.  Each radio on the system essentially listens in on two frequencies at a time. One that is tuned to the data channel and the other that transmits and receives the voice (or digital data in the case of digital modulation).  When a radio is tuned to a specific talkgroup, this information is transmitted to the computer via the data channel.  When a user on a specific talkgroup transmits, the computer assigns a frequency for that transmission to transmit on.  All radios in the system affliliated (or tuned) to that talkgroup are switched to the assigned frequency for the duration of the transmission.  A user on a different talkgroup that transmits at the same time will have its traffic routed to a different frequency.  When a transmission ceases, the frequency is freed up to be used by another talkgroup user.  Using this method, many talkgroups (or channels) can be carried on fewer frequencies.  When listening to trunked frequencies in conventional mode, a conversation will jump from frequency to frequency every time the carrier is dropped and you will hear bits and pieces of many users if listening to a single frequency.

 

Programming a scanner for Trunked Systems

 

There are some things you need to know when programming and listening to a trunked system which are different than programming/listening to conventional frequencies.

Most scanners usually have 10 banks of channels. 

The following must be kept in mind when programming a trunked system.

bulletEach trunked system you want to track must reside in its OWN bank.  In other words, you must put each system into a separate bank if you want to actively listen to it.  (You can, however, program the frequencies of more than one system into the same bank, but you will only be able to track ONE system per bank at a time.)
bulletYou need to enter ALL the frequencies for the particular system you want to monitor into the bank of choice. (unless you are trunking in Motorola CC (control channel mode) only.)
bulletALL frequencies must be entered in the scanner in TRUNKING MODE.  (Different scanners have different methods of programming in TRUNKING mode.  I am only familiar with the BC785D/796D, BC780xlt, BC245 and BC235 and indirectly, the BC250D/296D.)
bulletTo track the system you must scan in TRUNK mode.

 

Tracking the Trunked System

 

When trunk tracking, individual radio channels are now called TALKGROUP IDs.

When tracking a trunked system in SEARCH mode, the scanner will stop on any talkgroup ID that is not locked out.  You may find everything from the police department to the dogcatcher to the trash collector on a particular system.

Talkgroups in Motorola Type II systems (in 3600 baud control channel systems) extend from 16 (the lowest) to 65536 (the highest) in multiples of 16.  (i.e. 16, 32, 48, 64, 80 and so on up to 65536)

Each talkgroup ID has 16 unique modes which allow specialized communications.  With your scanners "STATUS BIT" ON (not available in the BC235) you will receive ALL of the 16 modes of each Talkgroup ID by listening only to the primary mode.  In other words, if the talkgroup ID in question is 32, with the STATUS BIT ON, you will hear any traffic on ID 33, 34, 35 through 47 on talkgroup 32.  If you choose to keep the scanners STATUS BIT OFF, then any traffic on IDs 33-47 can only be listened to on their respective talkgroup alone.  By keeping the STATUS BIT ON you are listening to the 16 modes on one ID.  You cannot however differentiate which mode the radio is in if you are listening STATUS BIT ON, but you usually don't need to.  Click here For a more detailed explanation of how and why there are 16 modes of a talkgroup ID.

Once you have determined what you want to listen to, you can program interesting Talkgroup IDs into the scanners memory known as LISTS.   LISTS typically have space for 10 IDs per List.  The BC245, BC780xlt, BC250D/BC296D and BC785D/BC296D allow 10 LISTS per bank for a total of 100 memory positions.  The BC235 allows 5 lists per bank for a total of 50.

 

Once programmed, LISTS and talkgroup IDs can be locked out, just like individual channels and banks in conventional scanning.

 

 

FAQs



Q: What is trunking?

A: Trunking is a system that allows a limited number of radio
frequencies to be used by many users.  Since radio communications are
not continuous, the frequencies in a trunked system are assigned by
the system as the need arises.  There are several different types of
trunked systems.



Q: How does trunking work?

A: A computer controls the operation of a trunked system by way of a
data channel (control channel). When the push-to-talk button is
activated on a radio, the computer at the receiving end assigns a
radio frequency from the pool to carry the transmission.  In order to
monitor a trunked system, your scanner must be tuned to receive the
data channel and, unless your scanner supports control channel only
trunking, must contain all of the systems frequencies.


Q: How do I program my scanner to track a trunked system?

A: First, you must specify what type of trunking system will reside
in the bank you are programming. Second, you must program the system
freqs.  There are 2 ways to program a trunked system.  1. All the
trunk system frequencies must be programmed into the scanner.  or 2.
The Data Channel only is programmed. This is sometimes known as
control channel trunking.  If your scanner supports control channel
only trunking, it will say so.  Only Motorola trunk systems support
control channel only trunking.  Once your scanner is receiving the
data channel and the bank is programmed with the trunk system type,
you can monitor the different TALKGROUPS on the system. 

Note: If your scanner is scanning the trunked frequencies, you are
NOT trunktracking.


Q: What is a Talkgroup?

A: A talkgroup in trunking is the equivalent to a frequency in
conventional scanning.  In order to listen to a user or agency in
trunked mode, you must scan or search their talkgroups. 



Q: Can I put a trunked system and conventional frequencies in the
same bank and have it work?

A: Yes, all but the earliest trunktrackers support this.  If you do
though, the radio will alternate monitoring the trunked system (by
way of the control channel) and the conventional freqs.  This may not
be the best way to monitor a trunked system (or any frequency) since
the scanner will cycle through all of the conventional freqs before
retracking the trunked system again.  A large trunked system can have
hundreds of talkgroups so potentially it can be a long time elapsed
before the conventional frequencies are resampled again.  Remember,
the trunked system is only ONE frequency (the control channel) in the
scanning sequence, but may have as many simultaneous conversations as
there are frequencies in the system.  Similarly, if you have many
busy conventional frequencies in the bank, it may be a while before
the trunk system is tracked the next sequence.




Q: Can I track multiple trunked systems with my scanner?

A: All newer trunktracking scanners can monitor ONE trunked system
per bank.  Therefore if your scanner has 10 banks, you can monitor 10
trunked systems.




Q: Can I put more than one trunked system in the same bank?

A: Yes, however because the scanner LOCKS onto the first control
channel it finds, you can only monitor ONE system at a time. 
Additionally, they must all be of the same trunk type.  ie. you can't
mix a Motorola system with an EDACS system.  If you do, you must
reprogram the bank with the proper trunk type, before you attempt to
trunktrack it.


Q: Speaking of EDACS, whats that?

A: EDACS is a type of trunk system that requires the frequencies be
programmed in the proper order and often in specific channel
locations in the scanner.




There is much more to trunking than you want to know but once you
understand the concept, the whole thing becomes simpler.

I've probably overlooked something important, but if you have any
questions, just ask.