Bell System in the context of Packet-switched

A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits, enabling telephone calls between subscribers. The term "central office" can also refer to a central location for fiber optic equipment for a fiber internet provider.

In historical perspective, telecommunication terminology has evolved with time. The term telephone exchange is often used synonymously with central office, a Bell System term. A central office is defined as the telephone switch controlling connections for one or more central office prefixes. However, it also often denotes the building used to house the inside plant equipment for multiple telephone exchange areas. In North America, the term wire center may be used to denote a central office location, indicating a facility that provides a telephone with a dial tone. Telecommunication carriers also define rate centers for business and billing purposes, which in large cities, might encompass clusters of central offices to specify geographic locations for distance measurement calculations.

In telecommunications, packet switching is a method of grouping data into short messages in fixed format, i.e., packets, that are transmitted over a telecommunications network. Packets consist of a header and a payload. Data in the header is used by networking hardware to direct the packet to its destination, where the payload is extracted and used by an operating system, application software, or higher layer protocols. Packet switching is the primary basis for data communications in computer networks worldwide.

During the early 1960s, American engineer Paul Baran developed a concept he called distributed adaptive message block switching as part of a research program at the RAND Corporation, funded by the United States Department of Defense. His proposal was to provide a fault-tolerant, efficient method for communication of voice messages using low-cost hardware to route the message blocks across a distributed network. His ideas contradicted then-established principles of pre-allocation of network bandwidth, exemplified by the development of telecommunications in the Bell System. The new concept found little resonance among network implementers until the independent work of Welsh computer scientist Donald Davies at the National Physical Laboratory beginning in 1965. Davies developed the concept for data communication using software switches in a high-speed computer network and coined the term packet switching. His work inspired numerous packet switching networks in the decade following, including the incorporation of the concept into the design of the ARPANET in the United States and the CYCLADES network in France. The ARPANET and CYCLADES were the primary precursor networks of the modern Internet.

The Number One Crossbar Switching System (1XB), was the primary technology for urban telephone exchanges served by the Bell System in the mid-20th century. Its switch fabric used the electromechanical crossbar switch to implement the topology of the panel switching system of the 1920s. The first No. 1 Crossbar was installed in the PResident-2 central office at Troy Avenue in Brooklyn, New York which became operational in February 1938.

Wire wrap is an electronic component assembly technique that was invented to wire telephone crossbar switches, and later adapted to construct electronic circuit boards. Electronic components mounted on an insulating board are interconnected by lengths of insulated wire run between their terminals, with the connections made by wrapping several turns of uninsulated sections of the wire around a component lead or a socket pin.

Wires can be wrapped by hand or by machine, and can be hand-modified afterwards. It was popular for large-scale manufacturing in the 1960s and early 1970s, and continues today to be used for short runs and prototypes. The method eliminates the design and fabrication of a printed circuit board. Wire wrapping is unusual among other prototyping technologies since it allows for complex assemblies to be produced by automated equipment, but then easily repaired or modified by hand.

AT&T Inc., an abbreviation of its predecessor's former name, the American Telephone and Telegraph Company, is an American multinational telecommunications holding company headquartered at Whitacre Tower in Downtown Dallas, Texas. It is the world's third largest telecommunications company by revenue and the third largest wireless carrier in the United States behind T-Mobile and Verizon. As of 2023, AT&T was ranked 32nd on the Fortune 500 rankings of the largest United States corporations, with revenues of $122.4 billion.

The modern company claims the history of the original AT&T founded in 1885 and all relevant history is found on the company's website. The company to bear the AT&T name began as a merger of the SBC Corporation (an original Baby Bell) and AT&T Corporation (Ma Bell). SBC began its history as the American District Telegraph Company, formed in St. Louis in 1878. After expanding services to Arkansas, Kansas, Oklahoma and Texas through a series of mergers, it became the Southwestern Bell Telephone Company in 1920. Southwestern Bell was a subsidiary of the original American Telephone & Telegraph Company, itself founded in 1885 as a subsidiary of the original Bell Telephone Company founded by Alexander Graham Bell in 1877. In 1899, AT&T became the parent company after the American Bell Telephone Company sold its assets to its subsidiary. During most of the 20th century, AT&T had a near monopoly on phone service in the United States through its Bell System of local operating companies. This led to AT&T's common nickname of "Ma Bell". The company was formally rebranded as AT&T Corporation in 1994.

Western Electric Co., Inc. was an American electrical engineering and manufacturing company that operated from 1869 to 1996. A subsidiary of the AT&T Corporation for most of its lifespan, Western Electric was the primary manufacturer, supplier, and purchasing agent for all telephone equipment for the Bell System from 1881 until 1984, when the Bell System was dismantled.

For much of the 20th century, Western Electric's equipment, especially their telephones, was ubiquitous throughout most of the United States and Canada. This was a result of the Bell System's near-total monopoly on phone service, combined with the legal ban on connecting third-party premises equipment to the telephone network.

The Bell System held a virtual monopoly over telephony infrastructure in the United States from around the early 20th century until January 8, 1982. It consisted of parent the American Telephone & Telegraph Company (AT&T), which directly provided long-distance service, while local service was provided by 24 local Bell Operating Companies, which owned whole or in part by AT&T, while its manufacturing subsidiary Western Electric produced almost all of its equipment, which was largely designed at the research and development subsidiary Bell Labs. As a result, AT&T had substantial control over the United States' communications infrastructure.

The breakup of the system was initiated in 1974 when the United States Department of Justice filed United States v. AT&T, an antitrust lawsuit against AT&T. Relinquishing ownership of Western Electric was one of the Justice Department’s primary demands.

AT&T Corporation, an abbreviation of its former name, the American Telephone and Telegraph Company, was an American telecommunications company that provided voice, video, data, and Internet telecommunications and professional services to businesses, consumers, and government agencies.

During the Bell System's long history, AT&T was at times the world's largest telecommunications company, the world's largest cable television operator, and a regulated monopoly. At its peak in the 1950s and 1960s, it employed one million people and its revenue ranged between US$3 billion in 1950 ($42.6 billion in present-day terms) and $12 billion in 1966 ($120 billion in present-day terms).

Unix (/ˈjuːnɪks/ , YOO-niks; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley (BSD), Microsoft (Xenix), Sun Microsystems (SunOS/Solaris), HP/HPE (HP-UX), and IBM (AIX).

The early versions of Unix—which are retrospectively referred to as "Research Unix"—ran on computers such as the PDP-11 and VAX; Unix was commonly used on minicomputers and mainframes from the 1970s onwards. It distinguished itself from its predecessors as the first portable operating system: almost the entire operating system is written in the C programming language (in 1973), which allows Unix to operate on numerous platforms. Unix systems are characterized by a modular design that is sometimes called the "Unix philosophy". According to this philosophy, the operating system should provide a set of simple tools, each of which performs a limited, well-defined function. A unified and inode-based filesystem and an inter-process communication mechanism known as "pipes" serve as the main means of communication, and a shell scripting and command language (the Unix shell) is used to combine the tools to perform complex workflows.

Graybar Electric Company, Inc. is an American wholesale electrical, communications and data networking products distribution business, which also supplies related supply-chain management and logistics services. The company is based in Clayton, Missouri and is an employee-owned corporation.

Graybar was incorporated on December 11, 1925, as the successor company of the general electric supply business of the Western Electric Company, which was founded in 1869 in Cleveland, Ohio, by Elisha Gray and Enos M. Barton. The separation of product lines was intended to provide a separate identity from the telephone supply function of Western Electric to the Bell System, given its importance as the largest merchandiser of electrical apparatus and related equipment in the world in the 1920s.

The T-carrier system is a member of the series of data-multiplexing carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls.

The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell System, and could transmit up to 24 telephone calls simultaneously over a single transmission line of twisted pair copper wire. Subsequent specifications carried multiples of the basic T1 (1.544 Mbit/s) data rates, such as T2 (6.312 Mbit/s) with 96 channels, T3 (44.736 Mbit/s) with 672 channels, and others.

In telecommunications, packet switching is a method of grouping data into short messages in fixed format, i.e., packets, that are transmitted over a telecommunications network. Packets consist of a header and a payload. Data in the header is used by networking hardware to direct the packet to its destination, where the payload is extracted and used by an operating system, application software, or higher layer protocols. Packet switching is the primary basis for data communications in computer networks worldwide.

During the early 1960s, American engineer Paul Baran developed a concept he called distributed adaptive message block switching as part of a research program at the RAND Corporation, funded by the United States Department of Defense. His proposal was to provide a fault-tolerant, efficient method for communication of voice messages using low-cost hardware to route the message blocks across a distributed network. His ideas contradicted then-established principles of pre-allocation of network bandwidth, exemplified by the development of telecommunications in the Bell System. The new concept found little resonance among network implementers until the independent work of British computer scientist Donald Davies at the National Physical Laboratory beginning in 1965. Davies developed the concept for data communication using software switches in a high-speed computer network and coined the term packet switching. His work inspired numerous packet switching networks in the decade following, including the incorporation of the concept into the design of the ARPANET in the United States and the CYCLADES network in France. The ARPANET and CYCLADES were the primary precursor networks of the modern Internet.

A Registered Jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. After their invention at Bell Labs by 1973, the standard was first defined in the Universal Service Ordering Code (USOC) system of the Bell System in the United States for complying with the registration program for customer-supplied telephone equipment mandated by the Federal Communications Commission (FCC). In 1980, they were codified in title 47 of the Code of Federal Regulations Part 68.

The specification includes physical construction, wiring, and signal semantics for numerous connection technologies. Accordingly, registered jacks are primarily named by the letters RJ, followed by two digits that express the type. Additional letter suffixes indicate minor variations. For example, RJ11, RJ14, and RJ25 are the most commonly used interfaces for telephone connections for one-, two-, and three-line service, respectively. Although these standards are legal definitions in the United States, some interfaces are used worldwide.

Stored program control (SPC) is a telecommunications technology for telephone exchanges. Its characteristic is that the switching system is controlled by a computer program stored in a memory in the switching system. SPC was the enabling technology of electronic switching systems (ESS) developed in the Bell System in the 1950s, and may be considered the third generation of switching technology. Stored program control was invented in 1954 by Bell Labs scientist Erna Schneider Hoover, who reasoned that computer software could control the connection of telephone calls.