Private Telephone Networks : Overview

Private Telephone Networks
Private telephone networks are communication systems that are owned, leased or operated by the companies that use these systems. These telephone systems include key systems, private branch exchange (PBX) systems, computer telephony, and local area network (LAN) telephones. Private telephone systems that are owned or operated by a company or private individual are called customer premises equipment (CPE).

Private telephone systems primarily allow the interconnection of multiple telephones within the private network with each other and provide for the sharing of telephone lines from a public telephone network. Private telephone systems can vary from simple multi-line telephones (key systems) to integrated voice and data service LAN telephone networks.

The first private telephone systems were key telephone systems. These systems used multi-line telephones to provide access to outside lines and used intercom features to allow inter-station connections.

As switching technology improved, small switching systems were installed to provide private branch exchange (PBX) systems. PBX systems provide switching between incoming trunk lines (multiple channel lines) and provide for advanced inter-system calling features.

To offer similar services as PBX systems, central exchange (Centrex) services were developed for end office (EO) switches. Centrex software allows local telephone companies to provide similar features as private telephone systems. These features include 3 or 4 digit abbreviated dialing, automated attendant (call transfer), least cost routing (LCR), and other local switching functions.

CTI systems integrate computer networks and telephony systems. CTI allows PBX technology to provide for voice mail, interactive voice response (IVR), and automatic call distribution (ACD) functions.

The combining of LAN systems with telephone systems is called LAN telephony. LAN telephony allows the sharing of equipment data network cost with telephone system cost.

Figure 1 shows the different types of private telephone systems. This diagram shows the first telephone systems were multiple line key telephone systems. This changed to private branch exchange (PBX) systems. Computer telephony (CT) systems are communication networks that merge computer intelligence with telecommunications devices and technologies. Local access network (LAN) telephony (sometimes called TeLANophy) use LAN systems to transport voice communications.

Figure 1: Private Telephone Systems


Private telephone systems are composed primarily of telephones (called “stations”), local wiring, and switching systems. Telephone stations are the interface between the user and the telephone network. Wiring connects telephone stations to switching systems or distribution points. Local wiring in private systems varies from shared lines (key systems) to individual lines (digital stations). Switching systems interconnect stations to each other or to outside telephone lines or interoffice trunks.

Companies purchase or lease private telephone system and have one or more of their personnel trained to handle day-to-day administrative functions of the system. Practically all PBX’s and key systems today are computer-based and thus allow for soft changes to be made through an administration terminal or PC. Unless the business has a need for technical telecommunications personnel on staff for other reasons, the business will normally contract with their vendor for routine adds, moves, and changes of telephone equipment.

PBX systems are often equipped with key assemblies and systems including voice mail, call accounting, a local maintenance terminal, and a dial-in modem. The voice mail system is controlled by the PBX only receiving calls when the PBX software determines a message can be left or retrieved. The call accounting system receives system message details on all call activities that occur within the PBX. The local terminal provides onsite access to the PBX for maintenance activities. The dial-in capability also provides access to the PBX for maintenance activities.

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