TRUNKED REPEATER OPERATION:
In the older radio systems, one or more groups of users were assigned to a particular pair of frequencies. If no one in any of those groups were talking, the frequency (or frequency pair) stood idle. At the same time, the groups of users on a different assigned frequency pair may be so busy that each of the groups is constantly waiting for someone to get through talking so they can get a message through. Thus there was a lot of available airtime wasted.
As the number of user groups increased in large metropolitan areas, all available frequencies were assigned to existing user groups (King County being a prime example). As new groups (such as new police departments or additional precincts in an existing police department) were created, no frequencies were available without sharing with an existing user.
A solution, known as trunking was borrowed from the telephone industry. With this technology, several frequency pairs are grouped together and put under the control of a computer (called a Trunking Controller). This computer then accepts each request to talk, assigns the next available frequency pair, directs all the radios in the talk group to switch to the assigned frequency pair, and sends a permit to talk tone to the requesting party. The whole process takes less than half a second. The computer also monitors the activity (not the conversation, but the fact the frequency pair is in use). When the frequency pair becomes idle, it is returned to the pool and is available for re-assignment to another user group. By pooling several frequencies, we also pool the unused airtime thus allowing more user-groups to work independently. In this manner we can get 3 to 10 times more use from a given set of frequencies than the old non-trunking method of assigning frequencies to users. A trunked repeater system can be either single site or simulcast.
A simple analogy to trunking is the system used to serve in-person customers at a bank. Formerly, customers got in line at individual teller windows to wait for the teller at that window to assist them. All of us have experienced getting in what appears to be the shortest line, when in fact, the transactions in front of us end up taking longer than anticipated, resulting in a longer wait for service than other lines might have offered. Also, you may have experience waiting in an assigned line for your type of transaction, even if another window has no waiting customers. Now, many banks have customers form a single queue, and the next available teller waits on the customer at the front of the line.
Rather than being stuck with a specific teller line (frequency), the trunking system will accept a request to talk and assign whatever frequency is available to the next request in line. When all frequencies are assigned, the system will put all requests in a waiting queue, and is able to prioritize customers in the line. The next available channel will be assigned to requests with the highest priority. Requests with equal priority will be served on a first-come, first-served basis. Typically, no users should experience any wait except in the event of a major disaster when radio traffic would be unusually high.
Thus, rather than scanning frequencies, you will be scanning talk groups.
From:
http://www.northwestradio.com/interceptnw/101.htm
