In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of this include telephone systems and the VHF radios commonly used by police agencies. More recently port trunking has been applied in computer networking as well.
A trunk is a single transmission channel between two points, each point being either the switching center or the node.
A trunk line is a circuit connecting telephone switchboards (or other switching equipment), as distinguished from local loop circuit which extends from telephone exchange switching equipment to individual telephones or information origination/termination equipment.
When dealing with a private branch exchange (PBX), trunk lines are the phone lines coming into the PBX from the telephone provider. This differentiates these incoming lines from extension lines that connect the PBX to (usually) individual phone sets. Trunking saves cost, because there are usually fewer trunk lines than extension lines, since it is unusual in most offices to have all extension lines in use for external calls at once. Trunk lines transmit voice and data in formats such as analog, T1, E1, ISDN or PRI. The dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks.
Trunk call
In the UK and the Commonwealth countries, a trunk call was a long distance one as opposed to a local call. See Subscriber trunk dialling and Trunk vs Toll.
Telephone exchange
Trunking also refers to the connection of switches and circuits within a telephone exchange. Trunking is closely related to the concept of grading. Trunking allows a group of inlet switches at the same time. Thus the service provider can provide a lesser number of circuits than might otherwise be required, allowing many users to “share” a smaller number of connections and achieve capacity savings.
