Lines Versus Trunks
The term line unfortunately refers to more than one type of circuit. In most cases, it includes a connection configured to support a normal voice calling load generated by one individual (typically about 10 minutes per hour for a business user). But in the case of a PBX , the term line usually corresponds to one connection from the PBX to a desktop. In the case of Centrex, a line is normally one physical connection from the customer site to the CO. With a key system, a line corresponds to one telephone number—but it might also be referred to as a trunk .
The term trunk normally refers to a circuit configured to support the calling loads generated by a group of users—possibly many thousands of users. Thus, a general-use circuit from a PBX to a CO would usually be described—and billed—as a trunk (but see DID lines). Connections between COs or higher in the network hierarchy would also be referred to as trunks. But note that these trunks are (or at least can be) physically identical to lines. Why then the different terminology? The ability of any given switching system such as a CO or a PBX to establish connections is limited. For example, although a PBX might be able to support 200 connections or ports, it might only be able to actually provide 80 paths at any one time. In such a case, if 80 people connected to 80 other people (some of them possibly off site), that would account for 160 of the ports; if any of the remaining 40 telephones or ports attempted to be serviced, it would fail. That is, a user could pick up the telephone and not receive a dial tone. Some systems are configured so that no such failures can happen. If only 160 physical connections were made to the PBX, then it could provide simultaneous service to all of them.
Normally, a PBX’s connections to the CO are configured so that a much higher utilization than 10 minutes per hour is achieved on those ports; a primary benefit of a PBX is the ability to buy fewer telco connections than one has 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 (COs are not normally configured as nonblocking switches). So, the telco will naturally bill a PBX trunk at a higher rate than a single business line—even though the PBX trunk might be physically identical to that single line.
As a final comparison of the definition of line versus trunk, then would be as follows:
A line is an end point from a central switching service, such as a CO or a PABX. The line is represented as the end point on the pair of wires regardless of where the intelligence resides. A line carries one single conversation at a time on the physical channel capacity. It is a billable location for the telephone companies.
A trunk on the other hand connects between two intelligent switching systems. The trunk might be a single circuit carrying a single call at a time, or it might be a bundled service that is multiplexed and carries multiple simultaneous conversations. The difference is that a trunk will be used for switching and routing decisions from the switching offices (CO or PABX). The trunk is continually used rather than occasionally used. It is a billable address that can have additional sub addressing capabilities behind it. In a telephone company world, it is the connection between and among other offices in the hierarchy. In the private user (customer) world, it might be a single connection to the intelligent PABX from the CO. These distinctions offer some variations in billing and utilization. With this in mind, here are some common configurations.
This introduction is meant to teach you about the functions and technology of a Central Office.
Analog to Digital Bandwidth
The Telephone Network
A Topology of Connection
Network Hierarchy (pre 1984)
Network Hierarchy (post 1984)
North American Numbering Plan
The Subscriber Extension
Local Access and Transport Areas
Wiring Connections: Hooking Things Up
Types of Communication
Lines Vs. Trunks
Foreign Exchange Signal