Trunking

In telecommunications, trunking is a method for a system to 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 two-way radios commonly used by police agencies. Trunking, in the form of link aggregation and VLAN tagging, has been applied in computer networking as well. In telecommunications, trunking is a method for a system to 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 two-way radios commonly used by police agencies. Trunking, in the form of link aggregation and VLAN tagging, has been applied in computer networking as well. A trunk is a single communications 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. 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 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 or PRI. 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. 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. In computer networking, port trunking is the use of multiple concurrent network connections to aggregate the link speed of each participating port and cable, also called link aggregation. Such high-bandwidth link groups may be used to interconnect switches or to connect high-performance servers to a network. In the context of Ethernet VLANs, Avaya and Cisco use the term Ethernet trunking to mean carrying multiple VLANs through a single network link through the use of a trunking protocol. To allow for multiple VLANs on one link, frames from individual VLANs must be identified. The most common and preferred method, IEEE 802.1Q adds a tag to the Ethernet frame, labeling it as belonging to a certain VLAN. Since 802.1Q is an open standard, it is the only option in an environment with multiple-vendor equipment. Cisco also has a (now deprecated) proprietary trunking protocol called Inter-Switch Link which encapsulates the Ethernet frame with its own container, which labels the frame as belonging to a specific VLAN. 3Com used proprietary Virtual LAN Trunking (VLT) before 802.1Q was defined.