Trunking

This is about the network-design strategies; for riding in auto cargo space, see Trunking (auto), and for the UK term for electrical wireways, see Electrical conduit#Trunking.

Trunking is a concept in modern communications 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.

Etymology

How the term came to apply to communications is unclear, but it probably derives from transport. In the middle 19th century the principal road of India was named Grand Trunk Road. The Grand Trunk Railway in Canada was named in 1852, long before any telephone cable. Since telephone trunks, trunk railways, and trunk roads connect branch offices or branch roads, they act much like the trunk of a tree.

An alternative explanation is that, from an early stage in the development of telephony, the need was found for thick cables (up to around 10 cm diameter) containing many pairs of wires. These were usually covered in lead. Thus, both in colour and size they resembled an elephant's trunk. This leaves open the question of what term was applied to connections among exchanges during the years when only open wire was used.

Radio communication

In radio communication (public safety, etc.), trunking refers to the ability of a signal to hop frequencies. Initially, all communication is received at known frequencies, but as silence is detected the site controller will broadcast new frequencies on which to communicate via a control channel, and the entire group of listening units will simultaneously migrate to that next frequency.

Telecommunications

In telecommunications, a trunk is one of:

In a communications network, a single transmission channel between two points, which are the switching centers or nodes or both. See Trunked radio system.
A circuit between telephone switchboards or other switching equipment, as distinguished from local loop circuits which extend from telephone exchange switching equipment to individual telephones or information origination/termination equipment.

Trunk line

When dealing with a 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. See illustration here 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.

Computer networks

Link aggregation

In computer networking, trunking refers to the use of multiple network cables or ports in parallel to increase the link speed beyond the limits of any one single cable or port. This is called port trunking or link aggregation. Trunks may be used to interconnect switches, such as major, minor, public and private switches, to form networks.

VLANs

In the context of VLANs, Cisco uses the term "trunking" to denote a network link carrying multiple VLANs between 2 switches or between a switch and a router, 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 header, labeling it as belonging to a certain VLAN; Since 802.1Q is a non-proprietary standard, it is the only option in an environment with multiple vendor equipment. Cisco also has a 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.