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.
Original patent title: “Multipoint data communication system with collision detection”
What this patent covers
The actual claim
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 this patent does NOT cover
The boundaries
- 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
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.
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Ethernet networks
Wired local area networks (LANs)
Network interface cards (NICs)
Why it matters
The bigger picture
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.
Filed
March 31, 1975
Granted
December 13, 1977
Claim 1 — Plain English
What this patent 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.
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.
What it does not 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.
Patent Journey
From filing to expiry
Patent Filed
1975
Patent Granted
1977 · 3yr after filing
Highly Cited
301 patents cite this
Patent Expired
1995
PatentBrief Score
Impact Score
Moderate
Citation count
40/40
Highly cited
Claim breadth
17/20
Very broad protection
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assignee
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
The original legal language
Original claims
26 claims as filed with the patent office.
Citations
Patent lineage
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