Lines, Switches, and Trunks

It is helpful to think of lines and trunks as a highway system, with “lines” as streets and “trunks” as highways.  The lines originate from individual phone numbers (or addresses), and are collected into larger and larger trunks that may ultimately be compared to Interstates, with special load limitations, restricted access, more commercial traffic and more sophisticated exchanges, with associated higher costs of construction and maintenance.

Lines

The term line refers generally  to a connection to support a voice call generated by an individual. In the case of a private branch exchange (PBX), it refers to the connection from the PBX to a desktop station. In the case of Centrex, it refers to the connection from the customers’ location to the Central Office (CO).   In today’s usage, the term line is often linked with adjectives that describes what the line can be used for:  “Voice Grade Line” or T1, T2, etc., which denote different grades of  telecommunications quality and capacity.

Trunks and Switches

It doesn’t take long to realize that every address is not connected to every other address by a separate line.  We regard that as too absurd to consider, but without switches it is what we would need if universal access were our goal.  Thus lines are collected into trunks, and redistributed to their targeted lines, through switches.

The ability of any switching system, such as a CO or a PBX, to establish connection is intentionally limited by design (otherwise we’d have all those wires).  The difference between switching capacity and the number of line signals coming into the switch at any one time (because of actual usage) defines the efficiency of the switch. Switching systems are configured by precise mathematical formulae that yield the ability of a given switch to handle X percent of the calls coming into it Y percent of the time. 

This technology is called Queuing Theory.  It was developed for this purpose by the Bell System very early in its history, but is used today by enterprises from McDonalds (How many registers do I buy for my store?), to airports (How many runways do we need?  How many gates?), to cell phones (how many antennae do we put up?  Where?)

Thanks to this technology, much higher utilization is achieved for a given number of connections.  The principal benefit of a good PBX design is its ability to require fewer connections than telephones. The CO must be configured so that it can provide connection services to such trunks at this higher utilization rate, thus using more of the CO’s overall switching and connection capacity.  Likewise, a telephone company is able to bill a PBX trunk at a higher rate than a single business line, even though the PBX trunk might be (through its own switches) physically identical to that of a single line.

The term trunk generally refers to a circuit configured to support the calling loads generated by a group of users. Such a group may contain just a few, or many thousands of users.  A general-use circuit from a PBX to a Co would usually be described and billed as a trunk. Connections between COs or offices higher in the network hierarchy would also be referred to as trunks. Note that trunks utilize technology and materials that are also common to lines.  The distinctions lie in origin, access and volume.

Comparisons

A line is an end from a central switching service, such as a CO or a private automated branch exchange (PABX). A line carries one single conversation at a time on the physical channel capacity. A trunk, on the other hand, connects two intelligent switching systems. The trunk might be a single circuit carrying a single call, or it might be a bundled service that is switched and multiplexed to carry multiple conversations. The difference is that a trunk is used for switching and routing decisions from the switching offices (CO or PABX).