Radio Scanners

Radio communications

In two-way radio communications, trunking refers to the ability of transmissions to be served by free channels whose availability is determined by algorithmic protocols. In conventional (i.e., not trunked) radio, users of a single service share a one or more exclusive radio channels and must wait their turn to use them. This can be likened to the operation of a group of cashiers in a grocery store, where each cashier serves their own line of customers. The cashier represent each radio channel, and each customer represents a radio user transmitting on their radio. When the grocery store gets busy, shoppers get backed up and congested as some cashiers have long lines and other cashiers sit idle.
Trunked radio systems (TRS) pool all of the cashiers (channels) into one group and use a store manager (site controller) that assigns incoming shoppers to free cashiers as determined by the store's policies (TRS protocols).
It should be noted that in a TRS, individual transmissions in any conversation may take place on several different channels, much as if a family of shoppers checked out all at once, they may be assigned different cashiers by the traffic manager. Similarly, if a single shopper checks out more than once, they may be assigned a different cashier each time.
Trunked radio systems provide greater efficiency at the cost of greater management overhead. The store manager's orders must be conveyed to all the shoppers. This is done by assigning one or more radio channels as the "control channel". The control channel transmits data from the site controller that run the TRS, and is continuously monitored by all of the field radios in the system so that they know how to follow the various conversations between members of their talkgroups (families) and other talkgroups as they hop from radio channel to radio channel.
TRS's have grown massively in their complexity since their introduction, and now include multi-site systems that can cover entire states or groups of states. This is similar to the idea of a chain of grocery stores. The shopper generally goes to the nearest grocery store, but if there are complications or congestion, the shopper may opt to go to a neighboring store. Each store in the chain can talk to each other and pass messages between shoppers at different stores if necessary, and they provide backup to each other: if a store has to be closed for repair, then other stores pick up the customers.
TRS's have greater risks to overcome than conventional radio systems in that a loss of the store manager (site controller) would cause the system's traffic to no longer be managed. In this case, most of the time the TRS automatically reverts to conventional operation. In spite of these risks, TRS's usually maintain reasonable uptime.
TRS's are more difficult to monitor via radio scanner than conventional systems. However, larger manufacturers of radio scanners have introduced models that, with a little extra programming, are able to follow TRS's quite efficiently.

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