In simplest languages, trunking enables sharing of channels amidst a large number of users. Popularly employed in cellular wireless systems, the fundamentals of trunking theory was first developed by A K Erlang. Depending upon the working principle, wireless systems can be classified into two types of trunked systems, namely, Block Call Cleared Systems and Block Call Queued Systems. Before understanding the principle of both systems, some popular terms linked with trunked theory are explained as follows.

Average call request rate (l): This measures the average number of call request made by users in a trunked system per unit of time.

Average call holding time (H): The average duration of a call for users of a trunked system is known as the average call holding time.

Set-up time: When any user makes a call request, the time required to allocate a trunked channel to the user is known as set-up time.

Blocked call: Call request of any user that cannot be completed by the system at that instant due to unavailability of free resource is known as blocked or lost call.

Busy hour: A continuous time span of 60 minutes having the highest traffic intensity over the day is referred to as the busy hour for the system.

Traffic intensity and traffic load: Traffic intensity measures the channel occupancy, when measured for all users of a trunked system it is referred to as traffic load of the system.