What is Trunking?
For computer networking, trunking describes using multiple network cables / ports in parallel to increase the link speed beyond the limits of any one single cable or port. In telecommunication, the term trunk has the following meanings [4]: In a communications network, a single transmission channel between two points that are switching centers or nodes, or both. A circuit between switchboards or other switching equipment, as distinguished from circuits which extend between telephone exchange switching equipment and information origination/termination equipment. Trunking permits a large number of users to share a relatively small number of communication paths - or trunks. This sharing of communication paths is managed automatically by a computer and channel assignment is automatic and completely transparent to the individual users. [6] Trunks may be used to interconnect switches, such as major, minor, public and private switches, to form networks. Trunking offers many benefits, including:
- Faster system access
- Better channel efficiency
- More user privacy
- Flexibility to expand
Trunking in an exchange
Trunking also refers to the connection of switches and circuits within a telephone exchange [1]. 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 [2][3].
Example:
In this type of system, all the radios on the system (except the computer controlled set of repeaters, of course) listen to a common control channel (CC) output frequency and transmit (initially) on a common control channel input frequency, unless they are listening to a conversation on a talkgroup. Let's say that Patrolman Bob (from Smalltown Police Department) wants to tell the dispatch office that he is now in service. The following actions take place in a very short time, much shorter than it takes you to read this. He picks up his microphone and keys the mike, his radio sends a signal on the CC input frequency, which the controlling computer understands as a request for a channel grant for the talkgroup assigned to Smalltown Police, his radio then instantly goes back into receive mode. The computer looks at the system for an empty channel pair and issues that channel grant on a specific channel pair and sends that channel grant information out on the CC output channel. This channel grant information tells all radios on the system, if you are listening (monitoring) for communications on the Smalltown Police talkgroup: change to channel pair XX on the system for a communication. All radios tuned to Smalltown Police's talkgroup, including Patrolman Bob's, then switch frequencies to that channel pair granted by the computer. Patrolman Bob's radio, after changing frequencies, goes into transmit mode and he can start to talk. As he talks, all the radios monitoring the Smalltown Police's talkgroup are now listening on the assigned repeater output channel and are ready to talk on the assigned repeater input channel. This continues until Patrolman Bob has finished his transmission. On some types of systems, further communication may be on the initially assigned channel pair or it may move to another, but the process stays the same. [5]
Derivation:
From an early stage in the development of telephony, the need was found for thick cables (up to c. 10cm diameter). These were usually covered in lead. Thus, both in colour and size they resembled an elephant's trunk. Also, since they connect branch offices they act much like the trunk of a tree.
References:
- Flood, J.E., Telecommunications Switching, Traffic and Networks. Chapter 4: Telecommunications Traffic. New York: Prentice-Hall, 1998.
- Motorola, Trunking Communications Overview [1], Last accessed 13 February 2005.
- The Genesis Group, Trunking Basics [2], Last accessed 13 February 2005.
- From the Federal Standard 1037C and from MIL-STD-188 and from the Code of Federal Regulations, Telecommunications Parts 0-199
- "http://www.radioreference.com/wiki/index.php/Trunking_Basics"
- CommUSA go wireless: "http://www.commusa.com/catalog.aspx/Trunking"
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Last edited by Mookho Tsilo on Feb 17, 2006 6:33 am US/Central.