Trunking

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In telecommunications, trunking is a technology for providing network access to multiple clients simultaneously by sharing a set of circuits, carriers, channels, or frequencies, instead of providing individual circuits or channels for each client. This is reminiscent to the structure of a tree with one trunk and many branches. Trunking in telecommunication originated in telegraphy, and later in telephone systems where a trunk line is a communications channel between telephone exchanges.

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Other applications include the trunked radio systems commonly used by police agencies. [1]

In the form of link aggregation and VLAN tagging, trunking has been applied in computer networking.

Telecommunications

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. [2] [3]

Trunk lines are used for connecting a private branch exchange (PBX) to a telephone service provider. [4] When needed they can be used by any telephone connected to the PBX, while the station lines to the extensions serve only one station’s 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, PRI or SIP. The dial tone lines for outgoing calls are called DDCO (Direct Dial Central Office) trunks.

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.

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]

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.

VLAN

In the context of Ethernet VLANs, Cisco uses the term Ethernet trunking to mean carrying multiple VLANs through a single network link through the use of a trunking protocol. [7] 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. [8]

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

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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 providing fault tolerance if an active link fails.

A virtual local area network (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer. In this context, virtual, refers to a physical object recreated and altered by additional logic, within the local area network. 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.

A virtual private network (VPN) is a mechanism for creating a secure connection between a computing device and a computer network, or between two networks, using an insecure communication medium such as the public Internet.

Class of service is a parameter used in data and voice protocols to differentiate the types of payloads contained in the packet being transmitted. The objective of such differentiation is generally associated with assigning priorities to the data payload or access levels to the telephone call.

IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual local area networking (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.

<span class="mw-page-title-main">Trunked radio system</span> Class of a radio system

A trunked radio system is a two-way radio system that uses a control channel to automatically assign frequency channels to groups of user radios. In a traditional half-duplex land mobile radio system a group of users with mobile and 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 simultaneously. In a conventional (non-trunked) system, channel selection is done manually; before use the group must decide which channel to use, and manually switch all the radios to that channel. This is an inefficient use of scarce radio channel resources because the user group must have exclusive use of their channel regardless of how much or how little they are transmitting. There is also nothing to prevent multiple groups in the same area from choosing the same channel, causing conflicts and 'cross-talk'. A trunked radio system is an advanced alternative in which the channel selection process is done automatically by a central controller (computer).

<span class="mw-page-title-main">Link aggregation</span> Using multiple network connections in parallel to increase capacity and reliability

In computer networking, link aggregation is the combining of multiple network connections in parallel by any of several methods. Link aggregation increases total throughput beyond what a single connection could sustain, and provides redundancy where all but one of the physical links may fail without losing connectivity. A link aggregation group (LAG) is the combined collection of physical ports.

<span class="mw-page-title-main">Metro Ethernet</span> Metropolitan area network based on Ethernet standards

A metropolitan-area Ethernet, Ethernet MAN, or metro Ethernet network is a metropolitan area network (MAN) that is based on Ethernet standards. It is commonly used to connect subscribers to a larger service network or for internet access. Businesses can also use metropolitan-area Ethernet to connect their own offices to each other.

<span class="mw-page-title-main">EtherChannel</span> Computer networking link aggregation technology

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.

The Multiple Spanning Tree Protocol (MSTP) and algorithm, provides both simple and full connectivity assigned to any given Virtual LAN (VLAN) throughout a Bridged Local Area Network. MSTP uses BPDUs to exchange information between spanning-tree compatible devices, to prevent loops in each MSTI and in the CIST, by selecting active and blocked paths. This is done as well as in STP without the need of manually enabling backup links and getting rid of switching loop danger.

<span class="mw-page-title-main">Network bridge</span> 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.

<span class="mw-page-title-main">Router on a stick</span> Router that has a single connection to a network

In networking, a router on a stick, also known as a one-armed router, is a router that has a single physical or logical connection to a network. It is a method of inter-VLAN routing where one router is connected to a switch via a single cable. The router has physical connections to the broadcast domains where one or more VLANs require the need for routing between them.

<span class="mw-page-title-main">Computer network</span> Network that allows computers to share resources and communicate with each other

A computer network is a set of computers sharing resources located on or provided by network nodes. Computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are made up of telecommunication network technologies based on physically wired, optical, and wireless radio-frequency methods that may be arranged in a variety of network topologies.

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 is an amendment to the IEEE 802.1Q-1998 networking standard which adds support for Provider Bridges. It was incorporated into the base 802.1Q standard in 2011. The technique specified by the standard is known informally as stacked VLANs or QinQ.

TRILL is an Internet Standard implemented by devices called TRILL switches. TRILL combines techniques from bridging and routing, and is the application of link-state routing to the VLAN-aware customer-bridging problem. Routing bridges (RBridges) are compatible with and can incrementally replace previous IEEE 802.1 customer bridges. TRILL Switches are also compatible with IPv4 and IPv6, routers and end systems. They are invisible to current IP routers, and like conventional routers, RBridges terminate the broadcast, unknown-unicast and multicast traffic of DIX Ethernet and the frames of IEEE 802.2 LLC including the bridge protocol data units of the Spanning Tree Protocol.

References

  1. ↑ Sharp, D.S.; Cackov, N.; Laskovic, N.; Shao, Qing; Trajkovic, L. (2004). "Analysis of public safety traffic on trunked land mobile radio systems". IEEE Journal on Selected Areas in Communications. 22 (7): 1197–1205. doi:10.1109/JSAC.2004.829339. S2CID   4912845.
  2. ↑ PD-icon.svg This article incorporates public domain material from Federal Standard 1037C. General Services Administration. (in support of MIL-STD-188).
  3. ↑ Title 47 of the Code of Federal Regulations, Parts 0-199
  4. ↑ "Call Recording Terms/Definitions". Versadial.com. Retrieved 8 June 2015.
  5. ↑ Flood, J. E. (1998). "Telecommunications Traffic". Telecommunications Switching, Traffic and Networks. New York: Prentice-Hall. ISBN   0130333093.
  6. ↑ Motorola, Trunking Communications Overview, last accessed 13 February 2005.
  7. ↑ "VLANs and Trunking". Cisco Press. 2002-10-25. Retrieved 2012-03-15.
  8. ↑ "Connecting Common VLANs Between Switch Units". SuperStack II Switch 3000 TX 8 Port User Guide. June 1997. Document No. DUA1694-1AAA04.
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