Crossbar switch

Crossbar switch. A crossbar switch is an electromechanical switching matrix used in telephone exchanges and other telecommunications systems to establish temporary connections between input and output lines. It employs a grid of horizontal and vertical bars with electromagnetically operated contacts at each intersection, allowing any input to be connected to any output. This architecture represented a major advancement over earlier Strowger switch systems, offering greater speed, reliability, and capacity for complex routing. The technology became the backbone of telephone networks worldwide for much of the mid-20th century.

Overview

The fundamental design of a crossbar switch consists of a rectangular array of contacts, with multiple horizontal and vertical paths forming a grid. Key components include the horizontal and vertical bars, select magnets, and hold magnets, which work in concert to physically close a connection at a specific crosspoint. This matrix structure is fundamentally different from the step-by-step motion of the Strowger switch. Major manufacturers of these systems included Western Electric, ITT Corporation, and L.M. Ericsson, each developing proprietary versions like the Number Five Crossbar and AXE telephone exchange systems. The architecture enabled more efficient use of switching resources and facilitated the introduction of new subscriber services.

Operation

Operation begins when a call setup request is received; a common control system, such as a marker, identifies an available path through the matrix. A select magnet energizes to choose a specific horizontal bar, and a hold magnet then activates to engage a vertical bar, with the mechanical contact at the intersection closing to complete the circuit. This process is supervised by centralized logic, a departure from the direct control of the Strowger switch. The establishment of a trunking path through the switch is temporary and is held until the call is terminated by the parties, at which point the magnets release. This method allowed for features like call forwarding and conference calls to be implemented more readily.

Applications

The primary application was in public switched telephone network central offices, forming the core of local and toll exchanges like the 4A switching system and 5ESS Switch. Crossbar technology was also adapted for use in military communications networks, including the Autovon system, and in private branch exchange installations for large organizations. Furthermore, the conceptual framework influenced early computer networking and the design of multistage interconnection networks for parallel computing. Its principles can be seen in the routing fabric of some modern Clos network architectures used in data centers.

Advantages and disadvantages

Key advantages included higher call handling capacity, reduced maintenance due to fewer moving parts compared to Strowger switch systems, and greater flexibility for introducing touch-tone dialing and long-distance calling features. The technology was also more reliable and quieter in operation. However, significant disadvantages were its substantial physical size, high power consumption from numerous electromagnets, and the inherent complexity of the common control systems. Ultimately, the crossbar switch was rendered obsolete by fully electronic systems like the Northern Telecom SP-1 and later digital switches such as the Ericsson AXE telephone exchange and AT&T 4ESS Switch, which offered vastly greater density and software control.

Historical development

Early electromechanical crossbar designs were pioneered in Sweden by engineers at L.M. Ericsson and in the United States by Bell Labs, with significant patents filed in the 1910s and 1920s. Widespread commercial deployment in telephone networks began in the 1930s, with systems like the AT&T Number One Crossbar entering service. The technology saw its peak adoption from the 1950s through the 1970s, dominating the infrastructure of the Bell System and many other national postal, telegraph and telephone service providers. Its decline commenced in the 1970s with the advent of stored program control exchanges, culminating in the rapid transition to digital switching by the 1990s, as seen with the Global System for Mobile Communications core networks.

Category:Telecommunications equipment Category:Telephony Category:Switches