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What is a Trunk Line?

Malcolm Tatum
Updated: May 17, 2024

In the world of telecommunications, a trunk line is a term that is used to refer to a high speed connection via a phone line that is routed through a central office on a telephone network. In common usage, a trunk line or a trunk is often used to refer to telephone lines that are routed through a telephone carrier network and provide voice and data transfer between two parties. It is important to note that a trunk line today is a fiber optic line that is capable of carrying both voice and data, rather than the older copper lines that were capable of carrying sound or voice only.

Use of the fiber optic trunk line has steadily increased around the world since the 1980s, and is distinguished by its ability to carry far more data than the older copper lines. Thanks to this type of technology, it is possible to use a single line for a residential voice telephone, Internet access, and in some cases, television programming. While the actual transmission speed of voice communications is no faster than with copper lines, this approach does have the benefit of carrying a more sensitive transmission that makes it possible to hear and be heard more clearly, and helps to eliminate some of the sound issues that were common with the older copper wire systems.

While voice communications are carried at the same speed, the benefit of a phone trunk line is that it is possible to simultaneously transmit all types of data without experiencing interruptions or breaks in the transmissions. This is particularly important for businesses that rely on the use of the Internet to stay in contact with customers, conduct presentations and workshops, and hold investor meetings that include voice and data conferencing as part of the platform. Even manufacturing companies benefit from the trunk line, as it is possible to utilize a video conference feed to allow a remote technician to diagnose a problem with a piece of equipment on the production floor and instruct plant personnel on how to correct the situation. In times past, this type of activity would have meant flying in the technician, wasting valuable time while the trip was made.

While a trunk line usually does refer to actual fiber optic lines that are routed through a public switched telephone network or PSTN, the term is widely used today to refer to any type of telephone line that is used for home or business purposes. In the world of teleconferencing, it is not unusual to refer to each of the telephone lines that terminate on a teleconference bridge as trunks or trunk lines. In like manner, many businesses refer to the lines that terminate onto a server or an office PBX as trunks, indicating the number of lines that are available to make inbound calls or receive inbound calls simultaneously.

About Mechanics is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Malcolm Tatum
By Malcolm Tatum
Malcolm Tatum, a former teleconferencing industry professional, followed his passion for trivia, research, and writing to become a full-time freelance writer. He has contributed articles to a variety of print and online publications, including About Mechanics, and his work has also been featured in poetry collections, devotional anthologies, and newspapers. When not writing, Malcolm enjoys collecting vinyl records, following minor league baseball, and cycling.
Discussion Comments
By AnswerMan — On Mar 05, 2014

@Phaedrus, my cousin works for a telephone repair company, and he says the problems with fiber optic trunk lines are still preferable to the problems with the old copper trunk lines. He said that all of those heavy copper lines can be replaced with a bundle of fiber optics only two inches thick. He spends most of his time pulling out the old copper trunk lines and preparing the conduits for new fiber optic lines. He said the phone company uses specially trained crews to handle those lines because they can be damaged easily if mishandled.

By Phaedrus — On Mar 04, 2014

Several years ago, our city had an almost 100% telephone and Internet blackout. It lasted almost 24 hours in some places. The phone company traced the problem to a fiber optic trunk line which ran through a pipe under a bridge. A construction crew accidentally severed the line while digging around the foot of the bridge.

The phone company sent out a specialized crew, and it took them two days to finally restore everything. Fiber optic trunk lines can carry an entire city's worth of data, but they are not easy to repair.

Malcolm Tatum
Malcolm Tatum
Malcolm Tatum, a former teleconferencing industry professional, followed his passion for trivia, research, and writing...
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