soren_ragsdale (Soren Ragsdale) writes:
: What exactly is a "trunk" as pertaining to radio?  I know it in a sense
: of an elephant's nose and a thing to pack for a long trip, but this radio
: definition eludes me.  Is it just tones for automating a radio network,
: or can you make something of listening to "trunks" (whatever they are)?

Think of standing in line at a bank.  Instead of one line of people
standing behind EACH teller, there is just ONE line, and the next
person in line gets the next teller.  This distributes the work load
evenly between tellers, and makes most efficient use of their capability
(theoretically).  That's a form of trunking.  I really do not know
where the word "trunk" comes from in this context,  except that
my dictionary defines it as a "direct line between telephone
switchboards".  I suppose metaphorically you could picture a
"tree trunk" supporting many smaller "limbs and branches and leaves",
which is kinda what radio and telephone trunking does.  OK, OK, I'm
reaching on this one.  Anyway.

With telephones, when you dial long distance from New York to Miami,
there is not a direct line between your phone and the person in
Miami.  Locally in New York, you are connected to one of many "trunk"
lines that go to Miami.  In other words, when you call Miami from
New York, you are given the next available "trunk" line to Miami,
automatically.  This by the way explains why calling into LA during
a earthquake is impossible - all long distance trunk lines are in
use with people calling to see how friends/relatives are doing.

Now in radio, the same holds true.  Trunking in radio communications
simply assigns the radio to the next available frequency in the 
pool of frequencies allocated to the agency using the radios.  This
is done with cellular phones.  Rather than have 1 frequency for each
individual car phone, there are bunches of frequencies shared by all,
and when you make your car phone call, you are given the next 
available frequency in the pool, i.e., you have been trunked to the
next available frequency.  Just like the bank.  Increasingly, police
departments are using trunked systems where no single radio uses the
same frequency all the time, rather, they are shared.  This makes
more efficient use of frequencies when a group (fleet) of radios
is being used.  Needless to say, trunking is almost nightmarish to
follow with a scanner because you never know what frequency the
person you are listening to will be switched to next.  

The switching of frequencies is done by a central computer over
one of the frequencies, called a data channel.  This data channel
sounds growly on a scanner, but it tells all the radios (in a police
department, or a cell phone area) what freqency to shift to next. 
When in an idle state, cell phones, police trunked radios, etc, all
revert to listening to the data channel.  Kinda like standing in
line at the bank, waiting your turn until the teller "waves" you over
or yells "next in line".

This description leaves out considerable detail about how handoffs
and so forth are handled, but it gives you some idea.  

Check out the good description of trunked radios in Police Call.
You can get it at Radio Shack for about 11 bucks.  Worth it.

Hope it helps.

Mike
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