Trunking

In telecommunications, trunking is a technology for providing network access to multiple clients simultaneously by sharing a set of circuits, carriers, channels, or frequencies, instead of providing individual circuits or channels for each client. This is reminiscent to the structure of a tree with one trunk and many branches. Trunking in telecommunication originated in telegraphy, and later in telephone systems where a trunk line is a communications channel between telephone exchanges. Other applications include the trunked radio systems commonly used by police agencies. In the form of link aggregation and VLAN tagging, trunking has been applied in computer networking. 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. Trunk lines are used for connecting a private branch exchange (PBX) to a telephone service provider. When needed they can be used by any telephone connected to the PBX, while the station lines to the extensions serve only one station’s telephones. 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, PRI or SIP. The dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks. In the UK and the Commonwealth countries, a trunk call was the term for long-distance calling which traverses one or more trunk lines and involving more than one telephone exchange. This is in contrast to making a local call which involves a single exchange and typically no trunk lines. 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.