Equipment: Private Branch Exchanges

Private Branch Exchange (PBX)

The term PBX, or Private Branch Exchange, refers to privately owned switching equipment, usually located on the property of the owner.  A PBX may be either digital or analog in basic format, and it may or may not use a live receptionist to answer and direct calls.  It is the typical telephone system for large to medium sized organizations, where it provides internal dial tone to individual workstations within the organization. A single-line telephone set with a dial tone may work but it won’t satisfy the needs of the organization, and the costs of renting a dial tone line for each user are prohibitive. In such an environment, dial tone service from the ILEC requires the high-volume calling and handling capacity provided by a PBX.

There are many PBX arrangements, but they share the common goal of processing voice calls for the organizations that own them. Originally, PBX handled only voice calls, but more recent technological advances have expanded their application to handling the full host of services offered by the digital revolution. Prices of new all-digital PBXs range from $500 to $1000 for each end user station, depending on the range of services the PBX will be required to handle. These prices typically include the costs of hardware and services needed for installation and interconnection.

PBX Components

• The PBX’s internal switching network.
• Central processor unit (CPU) or computer inside the system, including  memory.
• Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation.
• Stations or telephone sets, sometimes called lines.
• Outside Telco trunks that deliver signals to (and carry them from) the PBX.
• Console or switchboard allows the operator to control incoming calls.
• Uninterruptible Power Supply (UPS) consisting of sensors, power switches and batteries.
• Interconnecting wiring.
• Cabinets, closets, vaults and other housings.

Analog Systems

                Analog PBXs use electromagnetic analog components to interconnect inside lines with one another, and with outside trunks. A voice call is introduced in much the same way that a regular telephone call enters the telephone company network.  The user produces a call or line request by picking up the handset or activating his station.  This generates an input/output request signal to the PBX. The PBX returns a dial tone and the user activates the connection or completes the call.

                From there, the telephone produces a request to a trunk card. The trunk card is the interface between trunks and the central office (CO) to gain outside dial tone. On receiving dial tone at the trunk card, the PBX produces the electronic access request. The CO processes these signals in the same way that it does individual line requests from a residential user.

All lines converge at a central point in the Main Distribution Frame, where connection blocks terminate the wires.  Access to the PBX’s main switching frames is provided in a telephone closet, where patch panels are available for service changes, maintenance and reprogramming.  Patch panels can be used for voice, data, or LAN connections, and are usually separated by these categories.

Digital PBX

Today’s digital PBX is a stored-program, common-controlled unit. As a telephone installation, it is a resource sharing system that provides the capacity to access local dial tone and outside trunks to the end user. This is made possible by its all-digital architecture.

                All newer PBXs are digital, in the sense that they process information by digital encoding and switching.  In cases where the initial signal is analog (usually voice), a digital coder/decoder (codec) converts the analog information into digital format, so a consistent digital signal is presented to the PBX for processing.

If the call is directed outside the PBX mainframe, information must be reformatted from digital to analog even if the call is to traverse a digital link to the world. The PBX might have to go through a digital-to-digital changeover because the digital signal and the PBX interface is unipolar signal, whereas the signal to the telephone company is a bipolar signal.  As digital telecommunications technology continues its spread throughout the world, however, the need for such acrobatics is decreasing.