Crossbar Switch

Part of Telephony

An electromechanical telephone switching matrix that replaced step-by-step switches with faster, more reliable crosspoint technology.

Why This Matters

The crossbar switch was the dominant telephone exchange technology from the 1940s through the 1970s, handling billions of calls before digital switching displaced it. Understanding crossbar is important because it represents a fundamental advance in switching architecture: instead of a call sequentially hunting for a path through a series of rotary mechanisms, crossbar establishes a direct two-dimensional connection in a matrix with a single operation.

Crossbar exchanges were quieter, faster, and more reliable than their step-by-step predecessors. They could hold more connections simultaneously, could be controlled by common logic rather than individual per-connection mechanics, and provided better supervision and billing information.

For anyone designing a telephone exchange with more than a handful of lines, crossbar principles guide the fundamental architecture even if the physical implementation uses relays, solid-state switches, or software-defined connections. The matrix model of switching is timeless.

The Crossbar Matrix Concept

Imagine a grid of horizontal and vertical conductors, with a switch at every intersection. Each row connects to one telephone; each column connects to another telephone or to a trunk. To connect telephone A (row 3) to telephone B (column 7), close the switch at the intersection of row 3 and column 7.

This crosspoint matrix allows any input to connect to any output without interfering with other established connections, as long as each output is used by at most one input at a time. In a 10x10 matrix, 10 simultaneous connections are possible (each using a different row and different column).

The crossbar switch mechanism implements this matrix electromechanically. Horizontal and vertical bars cross over each other. Electromagnets select one horizontal bar and one vertical bar; the intersection of the selected bars contains a mechanism that latches closed when both bars are selected simultaneously. This two-key interlock (horizontal AND vertical must both be selected) prevents accidental crosspoints from forming due to a single stray signal.

Mechanical Structure

A typical small crossbar switch has 10 horizontal selecting magnets and 10-20 vertical holding magnets, providing a 10x10 or 10x20 matrix. Each matrix position contains a set of contact springs — typically 4 to 8 contacts per crosspoint for connecting multiple conductors (tip, ring, sleeve) simultaneously.

The horizontal bar (select bar) rotates briefly when its magnet is energized, latching the corresponding contacts at each column against the horizontal bar. While the horizontal magnet is held, a vertical magnet fires and latches its column contacts into a held position that no longer requires the horizontal magnet to remain energized. After the horizontal magnet releases, the connection remains held by the vertical (hold) magnet.

This two-motion mechanism (rotate, then latch) uses very brief high-current pulses for the select operation and modest continuous current for holding. The horizontal magnets cycle rapidly; the vertical magnets hold the connection for its entire duration — potentially hours. Power consumption is low because vertical holding magnets are efficiently designed for continuous operation.

Common Control

The most significant architectural advance of crossbar over step-by-step was common control. In step-by-step systems, each switch in the path receives dial pulses directly and moves to find its own connection — the intelligence is distributed across every switch in the chain.

In a crossbar exchange, the path-finding intelligence is centralized in a common control unit. The call progress digits are received and buffered centrally. Common control then examines the entire exchange to find an available path, selects the appropriate crosspoints in each crossbar matrix in the path, and fires them simultaneously or in rapid sequence to establish the connection. Once established, common control releases and handles the next call.

Common control makes the exchange more flexible. Route modifications, class-of-service restrictions, and billing changes require only modifications to common control — not to the physical switch fabric. This logical separation of switching from control is the conceptual ancestor of modern software-defined networking.

Path Hunting and Blocking

A crossbar exchange faces a fundamental problem: when all column paths in a matrix are busy, a new call cannot connect through that matrix even if the destination line is free. This is call blocking, and the probability of blocking depends on the matrix dimensions and traffic load.

Traffic engineers use Erlang calculations to determine what matrix sizes and what interconnection between stages provides an acceptable blocking probability. For a community telephone system, a blocking probability below 1% (meaning no more than 1 call in 100 is blocked at peak hours) is typically the design target.

Small systems (under 50 lines) can often use a single-stage crossbar matrix. Larger systems use two or three interconnected stages, where the middle stages provide enough paths that blocking becomes vanishingly rare — this is the Clos network architecture that underlies modern data center switching fabrics.

Relay-Based Implementation

For a post-collapse community telephone system, a crossbar switch can be built from telephone relays rather than the specialized crossbar mechanism. The principle is identical: a matrix of relay contact sets provides crosspoints, selection relays activate a row, and holding relays maintain connections.

A 10-line relay crossbar requires approximately 100 crosspoint relay contact groups (each providing the 4-6 contacts needed per circuit), 10 select relays, and 10 hold relays. The wiring is complex but systematic — follow the matrix pattern consistently and the logic is straightforward. Relay-based crossbar switches with 10-20 lines are within the capability of a competent technician with basic relay experience and several days of wiring work.

Test each crosspoint individually after wiring by connecting a resistance to the terminal pair and verifying that activating the correct row and column select relays connects the resistance to the test point, while activating any other combination does not.