Trunking
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- 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.
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[edit] 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 Road, 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 exchanges during the years when only open wire was used.
[edit] 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 a one or more exclusive radio channels and must wait their turn to use them. This can be likened to the operation of a group of cashiers in a grocery store, where each cashier serves their own line of customers. The cashier represent each radio channel, and each customer represents a radio user transmitting on their radio. When the grocery store gets busy, shoppers get backed up and congested as some cashiers have long lines and other cashiers sit idle.
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).
It should be noted that in a TRS, individual transmissions in any conversation may take place on several different channels, much as if a family of shoppers checked out all at once, they may be 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 run 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 automatically reverts to 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.
[edit] 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.[1][2]
[edit] Trunk line
When dealing with a PBX, trunk lines are the phone lines coming into the PBX from the telephone provider [3]. 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.
[edit] 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.
[edit] Telephone exchange
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][6]
[edit] Computer networks
[edit] 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.
[edit] VLANs
In the context of VLANs, Cisco uses the term "trunking" to denote a network link carrying multiple VLANs between two 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 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.
[edit] See also
- Multi-Link Trunking (MLT): Nortel's Port Aggregation Protocol
- Split multi-link trunking (SMLT): Nortel's proprietary enhancement to LACP
[edit] References
- ^
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188). - ^ Title 47 of the Code of Federal Regulations, Parts 0-199
- ^ Versadial, Call recording encyclopedia, last accessed 18 Apr 2007
- ^ Flood, J.E., Telecommunications Switching, Traffic and Networks Chapter 4: Telecommunications Traffic. New York: Prentice-Hall, 1998.
- ^ Motorola, Trunking Communications Overview, last accessed 13 February 2005.
- ^ The Genesis Group, Trunking Basics, last accessed 13 February 2005.
