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Private Branch Exchanges (PBXs)

Clearly, a better approach would involve effectively moving a partition of the central office exchange to the customer premise, as portrayed in Figure 4.2. In this fashion, a Private Branch of the Exchange (PBX) would yield significant benefit to Telco and the user organization.


Figure 4.2  PBX vs. individual telephone set connections to a telephone company Central Office (CO) exchange.

The first PBX reportedly was placed into service in the Old Soldiers’ Home in Dayton, Ohio in 1879. While the first systems were little more than nonstandard modifications of telephone company central office switches, AT&T offered a standard PBX (No. 1 PBX) in 1902 [4-4]. PBXs evolved according to the same order as did other circuit switches, such as central office exchanges. As noted in Table 4.1, the following generally accepted designations evolved, as well.

Table 4.1 PBX Generations

Designation Nature of Technology

Cordboard Manual Switchboard
PBX Electro-Mechanical
(Private Branch eXchange) Step-by-Step (SxS)
PABX Electro-Magnetic
(Private Automatic Branch eXchange Crossbar (XBar)/Crossreed
EPABX Electronic Common Control (ECC)
(Electronic Private Automatic Analog/Digital
Branch eXchange) Stored Program Control (SPC)

The terms PBX, PABX and EPABX often are used interchangeably. Contemporary PABXs generally can be characterized as being Electronic Common Control (ECC), Stored Program Control (SPC), and digital. Additionally, they accommodate data switching, as well as voice. PABXs vary from 10 stations to 10,000+ stations, or extensions, with the average being in the range of 200 stations. As noted in Table 4.2, U.S. market share leaders include Lucent Technologies (previously AT&T), Nortel, and Siemens Rolm [4-3]. The market continues to grow in terms of both lines shipped and revenues, with market leadership remaining fairly stable [4-5].

Table 4.2 1995 U.S. PBX market share (line shipments)

Vendor Market Share (%)

AT&T (now Lucent) 25.0
Nortel 24.7
Siemens Rolm 13.3
Mitel 9.7
NEC 8.5
Fujitsu 6.0
Ericsson 2.8
Intecom 2.7
Hitachi 2.5

Source: MultiMedia Telecommunications Association

PBX Components

Modern PBXs are specialized computer systems that include a circuit-switching matrix for the primary purpose of connecting voice calls, although many also handle a limited amount of data exceptionally well. PBXs serve large numbers of station sets, which may include fully-featured electronic terminals. They primarily serve voice terminals on a wired basis, although wireless appliqués often are available as an option. Otherwise, they are much like any other computer consisting of cabinets, shelves, printed circuit boards, power supplies. The primary physical and logical components (Figure 4.3) of a PBX include common control, switching matrix, trunk interfaces, line interfaces, and terminal equipment


Figure 4.3  PBX components, including common control, switching matrix, trunks and trunk interfaces, stations and line interfaces.

Common Control

Common Control is a common set of stored program logic that controls the activities of the system and all of its various elements. In reality, it consists of multiple microprocessors operating under a stored program, that is secured so as to be bulletproof, according to the manufacturers. As in any computer, the control processor consists of memory, input/output equipment, bulk memory equipment, and software.

The Central Processing Units (CPUs) control the operation of the system. They may be truly centralized, but generally are distributed among cabinets, shelves, or even cards (printed circuit boards), for purposes of effectiveness, efficiency, and survivability. The CPUs control functions such as call setup, call maintenance, call release, performance monitoring, system diagnostics, and storage of operational data for analysis and report presentation. Processor Redundancy is common in contemporary PBXs. Not only are the processing functions distributed amongst multiple microprocessors at the cabinet, shelf, and card levels, but many PBXs feature hot standby processors.


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