Privately Owned Switches

In a corporate environment, where large numbers of staff need access to each other and the outside, individual telephone lines are not economically viable. A PBX is a smaller, privately owned version of the CO switches used by telephone companies, as illustrated in Figure 1-4.


Most businesses have a PBX telephone system, a key telephone system, or a Centrex service. Large offices with more than 50 telephones or handsets choose a PBX to connect users, both in-house and to the PSTN.

PBXs come in a variety of sizes, from 20 to 20,000 stations. The selection of a PBX is important to most companies, because a PBX has a typical life span of seven to ten years.

All PBXs offer a standard, basic set of calling features. Optional software provides additional capabilities.

A PBX connects to telephone handsets using line cards and to the local exchange using trunk cards.

A PBX has three major components:

• Terminal interface The terminal interface provides the connection between terminals and PBX features that reside in the control complex. Terminals can include telephone handsets, trunks, and lines. Common PBX features include dial tone and ringing.

• Switching network The switching network provides the transmission path between two or more terminals in a conversation. For example, two telephones within an office communicate over the switching network.

• Control complex The control complex provides the logic, memory, and processing for call setup, call supervision, and call disconnection.

PBX Installations

PBX switches are installed in large business campuses to relieve the public telephone company switches from having to switch local calls. When you call a coworker locally in your office campus, the PBX switches the call locally instead of having to rely on the public CO switch. The existence of PBX switches also limits the number of trunks needed to connect to the telephone company's CO switch. With a PBX installed, not every office desktop telephone needs its own trunk to the CO switch. Rather, the trunks are shared among all users.

Small organizations and branch offices often use a key telephone system, as shown in Figure 1-5, because a PBX offers functionality and extra features that they may not require. A key system offers small businesses distributed answering from any telephone, unlike the central answering position required for a PBX. Notice in Figure 1-5 that telephones interconnect to a key system via connector blocks, while trunks coming in from the local exchange interconnect to the key system via termination blocks.


Today, key telephone systems are either analog or digital and are microprocessor based. Key systems are typically used in offices with 30 to 40 users, but can be scaled to support over 100 users.

A key system has three major components:

• Key service unit A key service unit (KSU) holds the system switching components, power supply, intercom, line and station cards, and the system logic.

• System software System software provides the operating system and calling-feature software.

• Telephones (instruments or handsets) Telephones allow the user to choose a free line and dial out, usually by pressing a button on the telephone.

Larger companies use proprietary telephone networks with PBXs. In a key telephone system, each telephone has multiple lines that allow users to access outside lines to their CO. When a call comes into the company, a line or a key lights up on the telephone and indicates that a particular line is in use. Users can call another extension or let another person know where to pick up a call by using an intercom function, such as an overhead paging system or speakerphone.

Key telephone system functionality has evolved over time to include a class called hybrid telephone systems. The hybrid system adds many features that were previously available only in PBXs. There is no single definition of the functions and features that are classified as a hybrid system because all vendors provide a mix that they believe gives them a competitive advantage.

The main difference between a key telephone system and a hybrid telephone system is whether a single-line telephone can access a single CO local loop or trunk only (key telephone system) or whether the single-line telephone can access a pool of CO local loops or trunks (hybrid telephone system).