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Many networks are arranged on a trunk and branch basis, analogous to the structure of a tree.

Contents 1 Computer networks 1.1 VLANs 2 Radio Communication 3 Telecommunications 3.1 Trunk line 3.2 Trunk call 3.3 Telephone exchange 3.4 Derivation 4 References

Main article: Link aggregation

In computer networking, trunking defines 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.

In the context of VLANs, the term "trunking" denotes 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 connections on one link, frames from individual VLANs must be identified by one of 2 common protocols. IEEE 802.1Q adds a tag to the Ethernet frame header, labeling it as belonging to a certain VLAN; this is the preferred method as of 2007 and the only option in an environment with multiple vendor equipment. There is also a Cisco proprietary trunking protocol called Inter Switch Link which encapsulates the Ethernet frame with its own container, each of which labels the frame as belonging to a specific VLAN.

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.^[1][2]

In telecommunications, a trunk is one of^[3]:

• 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.

When dealing with a PBX, trunk lines are the phone lines coming into the PBX from the telephone provider ^[4]. This differentiates these incoming lines from extension lines that leave the PBX and usually lead to 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

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.

Trunking also refers to the connection of switches and circuits within a telephone exchange.^[5] 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.^[6][7]

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.

1. ^ http://www.tech-faq.com/trunking.shtml (aeolianmeson)
2. ^ http://www.police-scanner.info/scanning/trunked-radio-systems.htm (aeolianmeson)
3. ^ From the Federal Standard 1037C and from MIL-STD-188 and from the Code of Federal Regulations, Telecommunications Parts 0-199
4. ^ Versadial, Call recording encyclopedia, last accessed 18 Apr 2007
5. ^ Flood, J.E., Telecommunications Switching, Traffic and Networks Chapter 4: Telecommunications Traffic. New York: Prentice-Hall, 1998.
6. ^ Motorola, Trunking Communications Overview, last accessed 13 February 2005.
7. ^ The Genesis Group, Trunking Basics, last accessed 13 February 2005.