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27
Understanding Scanning and the BCT15
scanner. Now, t h e Fort Worth systems are lo cked out a nd System Quick K eys [1],
[2], and [3] allo w sele ct ion between the P ho enix-are a systems.
Multi-Site Trunking
Some systems covering a very large geogra phic area use multiple anten na sit es
that each operate on different frequencies, but that use the same Talk Gr oup ID’s
for traff ic. When progra mming su ch a syste m, yo u prog ra m ea ch sit e’s fr equencies
a nd othe r sett in gs into the syste m, th en pro gram t he chann el s in to cha nn el gro up s
within that system. Ea ch site can have its own quick key, so you can turn e ach
indivi dual site on or off while you scan. Since all sites share all the channels within
the syst em, mult i-site trunking is much more e f ficien t than progra mming each site
as a separa t e syst em.
Not only does your BCT15 scan channels l ike a conventional scanner, it actuall y
follows the users o f a t runked radio syste m. Once you kn ow a nd progra m a t alk
group’ s ID (TGID ), you won’t miss any of the action.
If you are a new scanner enthusiast, you mi ght want to r ead the first part of this
manual and use your scanner in conventional mode before you begin trunk
tracki ng. U nder standing scanning fundamentals and terminology wi ll make trunk
tr acking much ea si er . If y ou are a l re ady a n e xpe ri e nc e d scann er oper ator , you can
review the programming worksheets and their associated pages in the Refer ence
Section of this manual .
Types of Trunking Systems
Trunking syst ems divid e a few f re qu en cie s a mo ng ma ny d ifferent users, but the
way that e ach system does t his is slight ly diff erent. Th is sect ion describ es some of
the t echnical da ta behind Mot orola, EDA CS, and LTR trun ked radio syst ems.
Motorola Trunking
All Moto rola t ru nking systems use the same basic t ru nking method . Th e syst em
consists of o ne con t rol ch ann el (o r as man y a s 4 pe r system but on ly one is act ive
at any one ti me), pl us one or more voic e channel s (ty pical ly 10, 20, or 28 total
channe ls). When a user pre sses Push To Talk (PTT) to tr an smit, th e ir ra dio first
sends the person’s tal k group inform ation to the control channel. The computer
then assigns that talk group to a specific voice channel and tr ansmits that data
over the control channel. All radios in that talk group switch over to the assi gned
voice channel and the user can begi n speaking. T his al l typical ly takes plac e i n
about a sec ond...the pers on transm i tti ng hears a beep fr om their r adio w hen the
channel is assigned and then it is OK to star t talking.