[2][3] Trunk lines are used for connecting a private branch exchange (PBX) to a telephone service provider.
Trunk lines transmit voice and data in formats such as analog, T1, E1, ISDN, PRI or SIP.
This is in contrast to making a local call which involves a single exchange and typically no trunk lines.
The most common and preferred method, IEEE 802.1Q, adds a tag to the Ethernet frame labeling it as belonging to a certain VLAN.
[8] 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 one or more exclusive radio channels and must wait their turn to use them, analogous to the operation of a group of cashiers in a grocery store, where each cashier serves his/her own line of customers.
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).
In this case, most of the time the TRS will automatically switch to an alternate control channel, or in more rare circumstances, conventional operation.