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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.

In modern communications, 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.



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.[citation needed] This leaves open the question of what term was applied to connections among exchanges 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 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, 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 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 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.



Trunk line

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]

When dealing with a private branch exchange (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.

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.

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]

Computer networks

Link aggregation

In computer networking, trunking is a slang term referring 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 link aggregation. These aggregated links may be used to interconnect switches.


In the context of VLANs, Cisco uses 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.


  1. ^  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 encyclopedia, last accessed 18 Apr 2007
  4. ^ Flood, J.E., Telecommunications Switching, Traffic and Networks Chapter 4: Telecommunications Traffic. New York: Prentice-Hall, 1998.
  5. ^ Motorola, Trunking Communications Overview, last accessed 13 February 2005.
  6. ^ The Genesis Group, Trunking Basics, last accessed 13 February 2005.

Study guide

Up to date as of January 14, 2010

From Wikiversity

Author: Michel Le Vieux

Module 22 of the Teletraffic Hyperlinked Textbook


What is Trunking?


In telecommunications systems, trunking is the aggregation of multiple user circuits into a single channel. The aggregation is achieved using some form of multiplexing. Trunking theory was developed by Agner Krarup Erlang, Erlang based his studies of the statistical nature of the arrival and the length of calls. The Erlang B formula allows for the calculation of the number of circuits required in a trunk based on the Grade of Service and the amount of traffic in Erlangs the trunk needs cater for.


In order to provide connectivity between all users on the network one solution is to build a full mesh network between all endpoints. A full mesh solution is however impractical, a far better approach is to provide a pool of resources that end points can make use of in order to connect to foreign exchanges. The diagram below illustrates the where in a telecommunication network trunks are used.

Trunking.jpg Figure 1: - A Modern Telephone Network Indicating where trunks are used. SLC - Subscriber line concentrator (HANRAHAN, 2001)

Erlang and Trunking Theory

The Danish mathematician Agner Krarup Erlang [2] is the founder of teletraffic engineering. Erlang developed the fundamentals of trunking theory while investigating how a large population can be serviced by a limited number of servers [3]. Trunking theory leverages off the statistical behaviour of users accessing the network, these characteristics discussed in the assumptions of the Erlang B equation.

Grade of Service is a measure of the probability that a user may not be able to access an available circuit because of congestion. The busy hour is the time when the network is the most busy and is dependent on the users. The highest traffic may not occur at the same time every day so the concept of time consistant busy hour is defined, TCBH, as those 60 minutes (within 15 minute accuracy) that has the highest traffic [4]. Business users may have there busy hour between 8:30am and 9:30am while residential users may have their busy hour between 6:00pm and 7:00pm.

Erlang B formula

GoS = \frac{\frac{A^C}{C!}}{\sum_{i=0}^{C}{\frac{A^i}{i!}}} [5]


  • GoS Grade of Service is the probability of blocking during the busy hour
  • C is the number of resources such as servers or circuits in a group
  • A = λh is the total amount of traffic offered in Erlangs

and based on the following assumptions, taken from Kennedy 2007 [5].

1. The assumption of pure-chance traffic means that call arrivals and terminations are independent, identically distributed random events. The number of call arrivals in a given time also has a Poisson distribution.

2. Statistical equilibrium assumes that the probabilities do not change with time.

3. Full availability means an arriving call can be connected to any free outgoing circuit. If switches make the connection from incoming calls to outgoing, each switch must have sufficient outlets to provide connection to every outgoing circuit.

4. Any attempted call that encounters congestion is lost because the derivation assumed lost-calls. If this congestion did occur, the customer is likely to make another attempt in a short while, thus increasing the traffic offered when there is congestion.


In order to have multiple communication channels use the same medium some form of multiplexing needs to be used. The two main types of multiplexing are;

The two main types of multiplexing are: -

  • Time Division Multiplexing

Multiple channels are combined onto a single medium for transmission. The channels are separated in the medium by their time slot.

  • Frequency Division Multiplexing

Multiple channels are combined onto a single medium for transmission. The channels are separated in the medium by their frequency.


Assume a fictitious residential telephone network with 10 users connected to Local Exchange A and 10 users connected to Local Exchange B. If we would like 10 users on LE A to connect to 10 users on LE B a proposed architecture could be as follows we would need a connection between the two exchanges with 10 circuits as in the diagram below.


Figure 2: - Diagram showing proposed connections between two exchanges

If the circuits in the above diagram were to be investigated it would be seen that there utilisations would in fact be very low.

The low usage of the circuits in the above Scenario leads us to look at Trunking theory. Looking at the assumptions made by the Erlang B equations we have.

1. Pure-chance traffic - A user may make a call at any time of the day

2. Statistical equilibrium - A user may or may not make a call directly after a previous call

3. Full availability - If there is a circuit on the trunk availiable an incoming call may make use of it

4. Calls which encounter congestion are lost - If there were no available circuits on the trunk the call will be lost and the user will recieve a busy tone

Investigations have also indicated that a residential user generates 0.02 erlangs of traffic and if a Grade or Service of 0.001 (1 in a thousand calls will be lost) is selected.

Applying this information into the the Erlang B equation above

A = 0.2 (10 x 0.02 Erlangs)

GoS = 0.001

We can calculate that the actual number of circuits required between LE A and LE B is 4. Trunking theory has been a major driver in making communication networks economically viable and affordable to service providers and users.


Figure 3: - An illustration of the use of trunking theory)


The management of a fixed line voice operator would like to try a decrease costs, they have suggested reducing the Grade of Service on their E1, E3, T1, T3 and STM-1 trunk circuits.

(a) Calculate and graph the efficiency per circuit of each of the carrier’s trunks with the following GoS 0.001, 0.002 and 0.005. You may make use of the following Erlang-B calculator

(b) Does reducing the grade of service significantly increase circuit efficiency? Could you suggest a better way of reducing costs?



  1. Hanrahan, H. E., Telecommunications Access Networks – ELEN 5010, Department of Electrical Engineering, University of the Witwatersrand, 2001.
  2. Wikipedia: Agner Krarup Erlang
  3. Rappaport, T. S., Wireless Communications – Principles and Practice, Second Edition, 2002.
  4. Iverson, V.B., Teletraffic Engineering and Network Planning, COM Course 34340, Technical University of Denmark, May 2006.
  5. Kennedy, I.G., ELEN7015 lecture notes, Department of Electrical Engineering, University of the Witwatersrand, 2007.


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