How Computers Share a Network Cable Without Crashing

This patent describes how multiple computers can share a single communication cable by listening for other transmissions and stopping if they detect a collision, then trying again later.

What it covers

This system allows several data processing stations to communicate over a shared cable, called a "communicating medium" (Claim 1). Each station has a "transceiver" (Claim 1) that can send and receive signals. Before transmitting, a station checks if the cable is busy using a "signal detecting means" (Claim 5). If the cable is clear, it starts sending data. Crucially, if a station transmits a signal and simultaneously receives a signal from another station, a "collision detecting means" (Claim 1) senses this interference. When a collision is detected, the system immediately stops the transmission using "means connected to each transceiver and responsive to the presence of said collision signal for interrupting the transmission" (Claim 1). After stopping, the station waits a random amount of time before attempting to transmit again, with the waiting time increasing if repeated collisions occur, as described in the abstract.

What it doesn't cover

• —Does not cover wireless communication systems, as it specifically describes a "communication cable" and a "communicating medium" that is physically tapped.
• —Does not cover network systems that prevent collisions entirely through strict scheduling, such as token passing or time-division multiplexing, as its core is about detecting and recovering from collisions.
• —Does not cover point-to-point communication links where only two devices are connected, as it is designed for "multipoint data communication."
• —Does not cover systems where devices transmit without first checking if the medium is busy, as Claim 5 includes "signal detecting means" for detecting a carrier signal before transmitting.

The clever bit

The innovation was combining two key ideas: first, listening to the cable before transmitting to see if it's already in use (carrier sense), and second, continuing to listen *while* transmitting to detect if another device started sending data at the same time (collision detection). If a collision happened, both devices would stop and wait a random amount of time before trying again, preventing a continuous jam.

Why it matters

This patent is foundational to Ethernet technology, which became the dominant standard for wired local area networks (LANs). Developed at Xerox PARC by Robert Metcalfe and his team, the principles outlined here enabled many computers to reliably share a single network cable, making networked computing practical and widespread. Its concepts are still central to how most wired networks function today.

Real-world examples

• 1.Ethernet networks
• 2.Wired local area networks (LANs)
• 3.Network interface cards (NICs)