A trunk is a communications line or link designed to carry multiple signals simultaneously to provide network access between two points. Trunks typically connect switching centers in a communications system. The signals can convey any type of communications data. A trunk can consist of multiple wires, cables or fiber optic strands bundled together to maximize the available bandwidth in a single physical cable, or it can consist of a single high-capacity link over which many signals are multiplexed.
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A trunk can also consist of a cluster of broadcast frequencies, as in a trunked radio system that enables the sharing of a few radio frequency channels among a large group of users.
In telephony, trunks interconnect switching nodes, such as private branch exchanges (PBX) and central offices. In enterprise telephony, the transition from traditional time-division multiplexing (TDM) trunks to SIP trunks began around 2009 to use Voice over IP (VoIP) to connect a PBX to the internet.
Data networks use two types of trunks. First, trunks can carry data from multiple local area networks (LANs) or virtual LANs (VLANs) across a single interconnect between switches or routers, called a trunk port. Second, trunks can bond or aggregate multiple physical links to create a single, higher-capacity, more reliable logical link, which is called port trunking.
A trunk port marks frames with special identifying tags -- defined by IEEE standard 802.1Q for VLAN tags for Ethernet frames -- as they pass between switches, so each frame can be routed to its intended VLAN at the other end of the trunked link. Using port trunking to aggregate links is defined by IEEE standard 802.1aq and by the 802.1AX standard for LANs and metropolitan area networks, as well by various vendor-proprietary methods.