Trunking
Encyclopedia
In modern communication
s, trunking is a concept 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.
A trunk is a single transmission
channel
between two points, each point being either the switching center or the node.
, Canada's Grand Trunk Railway
) was based on the natural model of a tree trunk
and its branches. It is likely that the same analogy drove the communications usage.
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 exchange
s during the years when only open wire was used.
Trunked radio system
s (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.
connecting telephone switchboard
s (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.
When dealing with a private branch exchange (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
. The dial tone
lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks.
. See subscriber trunk dialling
and Trunk vs Toll
.
es 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.
or ports
in parallel to increase the link speed beyond the limits of any one single cable or port. This is called link aggregation
. These aggregated links may be used to interconnect switches.
and Cisco
use the term "trunking" to mean "VLAN multiplexing
" - 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
header, 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 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.
Communication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
s, trunking is a concept 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.
A trunk is a single transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
between two points, each point being either the switching center or the node.
Etymology
How the term came to apply to communications is unclear, but its previous use in railway track terminology (e.g., India's Grand Trunk RoadGrand Trunk Road
The Grand Trunk Road also formerly known as Uttarapatha, Shah Rah-e-Azam or Sadak-e-Azam or Badshahi Sadak is one of South Asia's oldest and longest major roads...
, Canada's Grand Trunk Railway
Grand Trunk Railway
The Grand Trunk Railway was a railway system which operated in the Canadian provinces of Quebec and Ontario, as well as the American states of Connecticut, Maine, Michigan, Massachusetts, New Hampshire, and Vermont. The railway was operated from headquarters in Montreal, Quebec; however, corporate...
) was based on the natural model of a tree trunk
Trunk (botany)
In botany, trunk refers to the main wooden axis of a tree that supports the branches and is supported by and directly attached to the roots. The trunk is covered by the bark, which is an important diagnostic feature in tree identification, and which often differs markedly from the bottom of the...
and its branches. It is likely that the same analogy drove the communications usage.
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
Elephant
Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct...
's trunk. This leaves open the question of what term was applied to connections among exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s during the years when only open wire was used.
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 system
Trunked radio system
A trunked radio system is a complex type of computer-controlled radio system. Trunked systems use a few channels , and can have virtually unlimited talkgroups. The control channel computer sends packets of data to enable one talkgroup to talk together, regardless of frequency...
s (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.
Trunk line
A trunk line is a circuitElectrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
connecting telephone switchboard
Telephone switchboard
A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...
s (or other switching equipment), as distinguished from local loop
Local loop
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 carrier or telecommunications service provider's network...
circuit which extends from telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
switching equipment to individual telephones or information origination/termination equipment.
When dealing with a private branch exchange (PBX), trunk lines are the phone lines coming into the PBX from the telephone provider. This differentiates these incoming lines from extension
Extension (telephone)
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,...
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
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
, E1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
, ISDN or PRI
Primary rate interface
The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....
. The dial tone
Dial tone
A dial tone is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed...
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 callLocal call
In telephony, the term local call has the following meanings:# Any call using a single switching center; that is, not traveling to another telephone network;# A call made within a local calling area as defined by the local exchange carrier;...
. See subscriber trunk dialling
Subscriber trunk dialling
Subscriber trunk dialling is a term for a telephone system allowing subscribers to dial trunk calls without operator assistance.- Terminology :...
and Trunk vs Toll
Trunk vs Toll
In the US, under the purview of the Bell System, local telephone calls were flat rate; i.e., included in the subscribers' fixed fees except in a few big cities, and the term toll was adoptedfor long distance calls because they were subject to tolls....
.
Telephone exchange
Trunking also refers to the connection of switchSwitch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
es and circuits
Telecommunication circuit
A telecommunication circuit is any line, conductor, or other conduit by which information is transmitted.A dedicated circuit, private circuit, or leased line is a line that is dedicated to only one use...
within a telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
. Trunking is closely related to the concept of grading
Grade of service
In telecommunication engineering, and in particular teletraffic engineering, the quality of voice service is specified by two measures: the grade of service and the quality of service ....
. Trunking allows a group of inlet
Inlet
An inlet is a narrow body of water between islands or leading inland from a larger body of water, often leading to an enclosed body of water, such as a sound, bay, lagoon or marsh. In sea coasts an inlet usually refers to the actual connection between a bay and the ocean and is often called an...
switches at the same time. Thus the service provider
Service provider
A service provider is an entity that provides services to other entities. Usually, this refers to a business that provides subscription or web service to other businesses or individuals. Examples of these services include Internet access, Mobile phone operators, and web application hosting...
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.
Link aggregation
In computer networking, trunking is a slang term referring to the use of multiple network cablesPhysical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
or ports
Computer port (hardware)
In computer hardware, a port serves as an interface between the computer and other computers or peripheral devices. Physically, a port is a specialized outlet on a piece of equipment to which a plug or cable connects...
in parallel to increase the link speed beyond the limits of any one single cable or port. This is called link aggregation
Link aggregation
Link aggregation or trunking or link bundling or Ethernet/network/NIC bonding or NIC teaming are computer networking umbrella terms to describe various methods of combining multiple network connections in parallel to increase throughput beyond what a single connection could sustain, and to provide...
. These aggregated links may be used to interconnect switches.
VLANs
In the context of VLANs, AvayaAvaya
Avaya Inc. is a privately held computer networking, information technology and telecommunications company that is a global provider of business communications systems. The international head quarters is in Basking Ridge, New Jersey, United States...
and Cisco
Cisco
Cisco may refer to:Companies:*Cisco Systems, a computer networking company* Certis CISCO, corporatised entity of the former Commercial and Industrial Security Corporation in Singapore...
use the term "trunking" to mean "VLAN multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
" - 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
IEEE 802.1Q
IEEE 802.1Q is the networking standard that supports Virtual LANs on an 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...
adds a tag to the Ethernet frame
Ethernet frame
A data packet on an Ethernet link is called an Ethernet frame. A frame begins with Preamble and Start Frame Delimiter. Following which, each Ethernet frame continues with an Ethernet header featuring destination and source MAC addresses. The middle section of the frame is payload data including any...
header, labeling it as belonging to a certain VLAN. Since 802.1Q is an open standard
Open standard
An open standard is a standard that is publicly available and has various rights to use associated with it, and may also have various properties of how it was designed . There is no single definition and interpretations vary with usage....
, it is the only option in an environment with multiple-vendor equipment.
Cisco also has a proprietary trunking protocol called Inter-Switch Link
Cisco Inter-Switch Link
Cisco Inter-Switch Link is a Cisco Systems proprietary protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches....
which encapsulates the Ethernet frame with its own container, which labels the frame as belonging to a specific VLAN.