It is a technique which allows a large number of users to share the relatively small number of channels in a cell by providing access to each user,on demand, from a pool of available channels.
In trunked radio system, each user is allocated a channel on a per call basis and upon termination of the call, the previously occupied channel is immediately returned to the pool of available channels.
Using trunking theory,it is possible to accommodate a large random user community.
In trunk-ed mobile system, When a particular user requests service and all the radio channels are already in use, the user is blocked or denied access to the system.
In some systems, a queue may be use to hold the requesting users for a predetermined time and if a channel is not assigned to it in this time, only then the user is blocked.
Erlang B system: Blocked call cleared trunk-ed system
Erlang C system: Blocked calls delayed trunk-ed system.