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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.

Telecommunication Transmission of information between locations using electromagnetics

Telecommunication is the transmission of signs, signals, messages, words, writings, images and sounds or information of any nature by wire, radio, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology. It is transmitted through a transmission media, such as over physical media, for example, over electrical cable, or via electromagnetic radiation through space such as radio or light. Such transmission paths are often divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is often used in its plural form because it involves many different technologies.

Two-way radio A radio that can do both transmit and receive a signal

A two-way radio is a radio that can both transmit and receive a signal, unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, designed for bidirectional person-to-person voice communication with other users with similar radios. Two-way radios are available in mobile, stationary base and hand-held portable configurations. Hand-held two-way radios are often called walkie-talkies, handie-talkies or hand-helds.

Link aggregation computer networking technology to increase throughput by using multiple connections in parallel

In computer networking, the term link aggregation applies to various methods of combining (aggregating) multiple network connections in parallel in order to increase throughput beyond what a single connection could sustain, and to provide redundancy in case one of the links should fail. A link aggregation group (LAG) combines a number of physical ports together to make a single high-bandwidth data path, so as to implement the traffic load sharing among the member ports in the group and to enhance the connection reliability.


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. [1] [2]

Electrical network interconnection of electrical components or a model of such an interconnection, consisting of electrical elements

An electrical network is an interconnection of electrical components or a model of such an interconnection, consisting of electrical elements. An electrical circuit is a network consisting of a closed loop, giving a return path for the current. Linear electrical networks, a special type consisting only of sources, linear lumped elements, and linear distributed elements, have the property that signals are linearly superimposable. They are thus more easily analyzed, using powerful frequency domain methods such as Laplace transforms, to determine DC response, AC response, and transient response.

Telephone switchboard telecommunications system

A telephone switchboard is a telecommunications system used in the public switched telephone network or in enterprises to interconnect circuits of telephones to establish telephone calls between the subscribers or users, or between other exchanges. The switchboard was an essential component of a manual telephone exchange, and was operated by switchboard operators who used electrical cords or switches to establish the connections.

In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the common carrier or telecommunications service provider's network.

Trunk lines are used for connecting a private branch exchange (PBX) to a telephone service provider. [3] 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 residential telephony, an extension telephone is an additional telephone wired to the same telephone line as another. In middle 20th century telephone jargon, the first telephone on a line was a "Main Station" and subsequent ones "Extensions". Such extension phones allow making or receiving calls in different rooms, for example in a home, but any incoming call would ring all extensions and any one extension being in use would cause the line to be busy for all users. Some telephones intended for use as extensions have built in intercom features; a key telephone system for a small business may offer two to five lines, lamps indicating lines already in use, the ability to place calls on 'hold' and an intercom on each of the multiple extensions.

A dial tone is a telephony signal sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is working and is ready to initiate a telephone call. The tone stops when the first dialed digit is recognized. If no digits are forthcoming, the permanent signal procedure is invoked, often eliciting a special information tone and an intercept message, followed by the off-hook tone, requiring the caller to hang up and redial.

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.

In telephony, the term local call has the following meanings:

  1. Any call using a single switching center; that is, not traveling to another telephone network;
  2. A call made within a local calling area as defined by the local exchange carrier;
  3. Any call for which an additional charge, i.e., toll charge, is not billed to the calling or called party, or for which this charge is reduced because it is a short-distance call.

Trunking also refers to the connection of switches and circuits within a telephone exchange. [4] 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. [5]

In electrical engineering, a switch is an electrical component that can "make" or "break" an electrical circuit, interrupting the current or diverting it from one conductor to another. The mechanism of a switch removes or restores the conducting path in a circuit when it is operated. It may be operated manually, for example, a light switch or a keyboard button, may be operated by a moving object such as a door, or may be operated by some sensing element for pressure, temperature or flow. A switch will have one or more sets of contacts, which may operate simultaneously, sequentially, or alternately. Switches in high-powered circuits must operate rapidly to prevent destructive arcing, and may include special features to assist in rapidly interrupting a heavy current. Multiple forms of actuators are used for operation by hand or to sense position, level, temperature or flow. Special types are used, for example, for control of machinery, to reverse electric motors, or to sense liquid level. Many specialized forms exist. A common use is control of lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures.

A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted. Originally, this was analog, and was often used by radio stations as a studio/transmitter link (STL) or remote pickup unit (RPU) for their audio, sometimes as a backup to other means. Later lines were digital, and used for private corporate data networks.

Telephone exchange telecommunications system used in public switched telephone networks or in large enterprises

A telephone exchange is a telecommunications system used in the public switched telephone network or in large enterprises. An exchange consists of electronic components and in older systems also human operators that interconnect (switch) telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.

Computer networks

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 [ citation needed ] and Cisco [6] 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. [7]

Radio communications

In two-way radio communications, trunking refers to the ability of transmissions to be served by free channels whose availability is determined by algorithmic protocols. In conventional (i.e., not trunked) radio, users of a single service share one or more exclusive radio channels and must wait their turn to use them, analogous to the operation of a group of cashiers in a grocery store, where each cashier serves his/her own line of customers. The cashier represents each radio channel, and each customer represents a radio user transmitting on their radio.

Trunked radio systems (TRS) pool all of the cashiers (channels) into one group and use a store manager (site controller) that assigns incoming shoppers to free cashiers as determined by the store's policies (TRS protocols).

In a TRS, individual transmissions in any conversation may take place on several different channels. In the shopping analogy, this is as if a family of shoppers checks out all at once and are assigned different cashiers by the traffic manager. Similarly, if a single shopper checks out more than once, they may be assigned a different cashier each time.

Trunked radio systems provide greater efficiency at the cost of greater management overhead. The store manager's orders must be conveyed to all the shoppers. This is done by assigning one or more radio channels as the "control channel". The control channel transmits data from the site controller that runs the TRS, and is continuously monitored by all of the field radios in the system so that they know how to follow the various conversations between members of their talkgroups (families) and other talkgroups as they hop from radio channel to radio channel.

TRS's have grown massively in their complexity since their introduction, and now include multi-site systems that can cover entire states or groups of states. This is similar to the idea of a chain of grocery stores. The shopper generally goes to the nearest grocery store, but if there are complications or congestion, the shopper may opt to go to a neighboring store. Each store in the chain can talk to each other and pass messages between shoppers at different stores if necessary, and they provide backup to each other: if a store has to be closed for repair, then other stores pick up the customers.

TRS's have greater risks to overcome than conventional radio systems in that a loss of the store manager (site controller) would cause the system's traffic to no longer be managed. In this case, most of the time the TRS will automatically switch to an alternate control channel, or in more rare circumstances, conventional operation. In spite of these risks, TRS's usually maintain reasonable uptime.

TRS's are more difficult to monitor via radio scanner than conventional systems; however, larger manufacturers of radio scanners have introduced models that, with a little extra programming, are able to follow TRS's quite efficiently.

Related Research Articles

A network switch is a computer networking device that connects devices on a computer network by using packet switching to receive, process, and forward data to the destination device.

The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include backup links to provide fault tolerance if an active link fails.

A virtual LAN (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer. LAN is the abbreviation for local area network and in this context virtual refers to a physical object recreated and altered by additional logic. VLANs work by applying tags to network frames and handling these tags in networking systems – creating the appearance and functionality of network traffic that is physically on a single network but acts as if it is split between separate networks. In this way, VLANs can keep network applications separate despite being connected to the same physical network, and without requiring multiple sets of cabling and networking devices to be deployed.

Virtual private network Allows a private network to go through a public network

A virtual private network (VPN) extends a private network across a public network, and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. Applications running on a computing device, e.g. a laptop, desktop, smartphone, across a VPN may therefore benefit from the functionality, security, and management of the private network. Encryption is a common though not an inherent part of a VPN connection.

Cisco Discovery Protocol (CDP) is a proprietary Data Link Layer protocol developed by Cisco Systems. It is used to share information about other directly connected Cisco equipment, such as the operating system version and IP address. CDP can also be used for On-Demand Routing, which is a method of including routing information in CDP announcements so that dynamic routing protocols do not need to be used in simple networks.

IEEE 802.10 is a former standard for security functions that could be used in both local area networks and metropolitan area networks based on IEEE 802 protocols.

IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual LANs (VLANs) on an IEEE 802.3 Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames. The standard also contains provisions for a quality-of-service prioritization scheme commonly known as IEEE 802.1p and defines the Generic Attribute Registration Protocol.

VLAN Trunking Protocol (VTP) is a Cisco proprietary protocol that propagates the definition of Virtual Local Area Networks (VLAN) on the whole local area network. To do this, VTP carries VLAN information to all the switches in a VTP domain. VTP advertisements can be sent over 802.1Q, and ISL trunks. VTP is available on most of the Cisco Catalyst Family products. Using VTP, each Catalyst Family Switch advertises the following on its trunk ports:

Trunked radio system class of a radio system

A trunked radio system is a digital two-way radio system that uses a digital control channel to automatically assign frequency channels to groups of users. In a half-duplex land mobile radio system a group of users with portable two-way radios communicate over a single shared radio channel, with one user at a time talking. These systems typically have access to multiple channels, up to 40 - 60, so multiple groups in the same area can communicate. Trunked radio systems are an advanced alternative to conventional systems in which the channel selection is done manually. In a conventional system, before use the group must decide on which channel to use, and manually switch all the radios to that channel. There is nothing to prevent multiple groups in the same area from choosing the same channel, causing conflicts. In a trunked system, the channel selection process is performed automatically.


EtherChannel is a port link aggregation technology or port-channel architecture used primarily on Cisco switches. It allows grouping of several physical Ethernet links to create one logical Ethernet link for the purpose of providing fault-tolerance and high-speed links between switches, routers and servers. An EtherChannel can be created from between two and eight active Fast, Gigabit or 10-Gigabit Ethernet ports, with an additional one to eight inactive (failover) ports which become active as the other active ports fail. EtherChannel is primarily used in the backbone network, but can also be used to connect end user machines.

Cisco Inter-Switch Link (ISL) is a Cisco Systems proprietary protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches. ISL is Cisco's VLAN encapsulation protocol and is supported only on some Cisco equipment over the Fast and Gigabit Ethernet links. It is offered as an alternative to the IEEE 802.1Q standard, a widely used VLAN tagging protocol, although the use of ISL for new sites is deprecated by Cisco.

Bridging (networking) Device that creates a larger computer network from two smaller networks

A network bridge is a computer networking device that creates a single aggregate network from multiple communication networks or network segments. This function is called network bridging. Bridging is distinct from routing. Routing allows multiple networks to communicate independently and yet remain separate, whereas bridging connects two separate networks as if they were a single network. In the OSI model, bridging is performed in the data link layer. If one or more segments of the bridged network are wireless, the device is known as a wireless bridge.

Multiple Registration Protocol (MRP), which replaced Generic Attribute Registration Protocol (GARP), is a generic registration framework defined by the IEEE 802.1ak amendment to the IEEE 802.1Q standard. MRP allows bridges, switches or other similar devices to be able to register and de-register attribute values, such as VLAN identifiers and multicast group membership across a large LAN. MRP operates at the Data Link Layer.

The Link Layer Discovery Protocol (LLDP) is a vendor-neutral link layer protocol used by network devices for advertising their identity, capabilities, and neighbors on a local area network based on IEEE 802 technology, principally wired Ethernet. The protocol is formally referred to by the IEEE as Station and Media Access Control Connectivity Discovery specified in IEEE 802.1AB and IEEE 802.3 section 6 clause 79.

Computer network collection of autonomous computers interconnected by a single technology

A computer network is a digital telecommunications network which allows nodes to share resources. In computer networks, computing devices exchange data with each other using connections between nodes. These data links are established over cable media such as wires or optic cables, or wireless media such as Wi-Fi.

The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be used. It works on Layer 2 of the OSI model. VLAN trunks formed using DTP may utilize either IEEE 802.1Q or Cisco ISL trunking protocols.

Data center bridging (DCB) is a set of enhancements to the Ethernet local area network communication protocol for use in data center environments, in particular for use with clustering and storage area networks.

IEEE 802.1ad was an Ethernet networking standard informally known as QinQ as an amendment to IEEE standard IEEE 802.1Q-1998. 802.1ad was incorporated into the base 802.1Q standard in 2011. The technique is also known as provider bridging, Stacked VLANs, or simply QinQ or Q-in-Q. "Q-in-Q" can for supported devices apply to C-tag stacking on C-tag but this has limited application in the modern methodology of network routing.

TRILL is an IETF Standard implemented by devices called RBridges or TRILL Switches. TRILL combines techniques from bridging and routing and is the application of link state routing to the VLAN-aware customer-bridging problem. RBridges are compatible with and can incrementally replace previous IEEE 802.1 customer bridges. They are also compatible with IPv4 and IPv6 routers and end nodes. They are invisible to current IP routers and, like routers, RBridges terminate the bridge spanning tree protocol.


  1. PD-icon.svg This article incorporates  public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188 ).
  2. Title 47 of the Code of Federal Regulations, Parts 0-199
  3. Versadial, Call Recording Terms/Definitions, last accessed 8 June 2015
  4. Flood, J. E. (1998). "Telecommunications Traffic". Telecommunications Switching, Traffic and Networks. New York: Prentice-Hall. ISBN   0130333093.
  5. Motorola, Trunking Communications Overview, last accessed 13 February 2005.
  6. "VLANs and Trunking". Cisco Press. 2002-10-25. Retrieved 2012-03-15.
  7. "Connecting Common VLANs Between Switch Units". SuperStack II Switch 3000 TX 8 Port User Guide. June 1997. Document No. DUA1694-1AAA04.