From: (Curtis Wheeler)
Newsgroups: rec.radio.scanner
Subject: Re: Q: What is "trunking"?
Date: 31 Oct 94 18:50:46 GMT

In article <38rmbm$4ob@huron.eel.ufl.edu> browere@einstein.cs.ucdavis.edu (Eric B.) writes:
>Would anyone care to explain exactly what "trunked" means, and how this 
>relates to scanning a given "trunked" frequency?
>Thanks in advance!
>Eric


Eric the term "trunk" more or less comes from the telephone industry.  A trunk 
is a circuit between swicthing equipment.  Your home phone is on a subscriber 
loop.  When you call a phone on a different exchange the switching equipment 
uses an avialable trunk to route the call to the other exchange where it is 
connected to the called party's loop.

Trunked radio does sort of the same thing without dedicated paths to users 
(no loops) but instead, uses a control channel.  When a trunked radio 
transmits it does so on a control channel.  When the central controller sees 
who is transmitting it assigns a different channel and instructs all radios in 
the user's group to go to that channel.  When the transmissions are finished 
the radios go back to the control channel and wait for the next round of 
traffic.

This is how the handshaking works on a Motorola type I system.

Basics.

1. Control Channel will transmit outbound signalling words (OSW) to the radio 
and receives inbound signalling words (ISW) from it.

2. Control channel contiuously transmits background OSWs in the absence of 
activity on the system.  These OSWs are used for synchronization of the radio 
and contain no user information.

3. Radios that are turned on will search the four highest frequencies in the 
trunked channel group until they find the control channel.

Operation.

1. PTT key up.  Radio will transmit a request for service (ISW) which is 78 
bits long at 3600 BPS (total time = 21.67 ms).  This ISW is sent on a precise 
time slot in relation to the controller OSW and contains:
   a.  Radio's ID.
   b.  Fleet and subfleet of which he is a member.
Only 21 bits are information, the rest being parity bits for error detection 
and correction (FEC).

2. The central controller puts the request for service in a queing condition, 
and dependent on it's priority, will assign a channel.  The quickest a grant 
for service can be is about 58 ms after reception of the ISW.

3. The controller send a channel grant 2 times in rapid succession.  This 84 
bit OSW is transmitted at 3600 BPS and contains:
   a. Individual ID of the radio.
   b. Fleet and subfleet which is sent to the new channel.
   c. Type of call (group/individual) 
   d. Number of the voice channel to be used.

4. The radio decodes the OSW grant and moves to the assigned voice channel.  
For a group call, all other radios also move to the assigned channel.
   a. At the same time the controller is sending the OSW grant on the assigned 
channel, it is also transmitting data on the assigned channel called the "high 
speed handshake".
   b. This data is a 21 bit frame, transmitted at 3600 BPS.  Data within the 
frame consists of sync data, ID information and two parity bits.
   c. The radio looks for the HSHS on the voice channel to confirm that it has 
switch to the proper channel.

5. The radio will now key up on the assigned channel and transmit 80 ms of 
high speed acknowledge tone (1800 Hz).

6. The controller recognozes this tone, removes the HSHS from the outbound 
voice channel and replaces it with an identical subaudible version sent at 150 
BPS and low deviation.  Called the low speed hand shake (LSHS).

7. At this time, three optional "talk permit" tones are received by the user, 
the mic unmutes and voice transmission commences..  The radio will transmit 
continuous subaudible connect tone (105.88) to the controller that keeps the 
channel connected.

8. In addition, radios that are sent to the voice channel will also look for 
the HSHS.  If they detect it, they will unmute after detecting the LSHS 
signal.  The best case time from requesting PTT to receiving radios = 280 ms.

9. During all of the voice communcation, the controller duplicates grants on 
the control channel (OSWs) as time permits.  If a receiving radio drops out 
for more than 1 second, it will return to the control channel and be 
redirected to the assigned channel.

Termination of Call.

1. When the PTT is released, the radio sends 200 ms of disconnect tone (163.64 
Hz at low deviation).

2. At this point the system can be either message or transmission trunked.  

   a. Message trunking:  The central controller detects the disconnect tone 
and begins a message timeout timer (2-6 seconds).  If another radio keys up 
during this time, the controller resets the timer.  If no other radio keys up, 
the timer will timeout and the controller will disconnect the radios from the 
channel by sending 125 ms of of low speed disconnect sequence (sync patterns 
at 300 BPS with low deviation) on the voice channel.  The radios will return 
to the control channel and becomes available for channel reassignment.

   b. Transmission trunking:  In this mode, the central controller will issue 
the low speed disconnect sequence immediatly after detecting disconnect tone 
from the transmitting unit.  The radio will remain muted and return to the 
control channel.

Curtis Wheeler