How Multiple 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 their own if a collision occurs, then trying again later.
Original patent title: “Multipoint data communication system with collision detection”
This patent describes how multiple computers can share a single communication cable by listening for other transmissions and stopping their own if a collision occurs, then trying again later. Granted to Xerox Corp in 1977 with 26 claims and 301 forward citations, and it is now in the public domain.
Coverage
What does this patent actually cover?
The patent describes a system for multiple data processing stations to communicate over a shared cable, like a bus. Each station has a "transceiver" (a device that transmits and receives) connected to the cable. When a station wants to send data, it first listens to the cable (ClaimclaimA numbered sentence at the end of a patent that legally defines what the inventor owns. The most important section.Read more → 5, "signal detecting means"). If the cable is busy, it waits. If the cable is clear, it starts transmitting. Crucially, if another station starts transmitting at the same time, causing a "collision," the system detects this (Claim 1, "collision detecting means"). Upon detecting a collision, the transmitting station immediately stops sending data (Claim 1, "interrupting the transmission"). It then waits a random amount of time before attempting to transmit again, with the wait time increasing if collisions keep happening (AbstractabstractA short summary at the front of the patent describing the invention. Not legally binding.Read more →, "random number generator"). For example, if two computers try to send data at the exact same moment on a shared cable, this system detects the garbled signal, tells both computers to stop, and makes them wait different random times before trying again, preventing a continuous jam.
The gap
What does this patent NOT cover?
- Does not cover communication systems where each device has its own dedicated connection (e.g., point-to-point links).
- Does not cover network protocols that use tokens or central controllers to manage access to a shared medium.
- Does not cover wireless communication systems where collisions are handled by different methods like listen-before-talk (LBT) without explicit collision detection on the medium.
- Does not cover systems where the transmitting means does not interrupt its transmission upon detecting a collision.
- Does not cover systems that do not use a random backoff mechanism to reattempt transmission after a collision.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
Key facts
What made this novel
The really smart part is the combination of listening before transmitting (carrier sense) with the ability to detect when two transmissions clash (collision detection) and then using a randomized delay (exponential backoff) to retry. This ensures that even if multiple devices try to send data at the same time, they don't permanently jam the network.
The Patent Drawing
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
Early Ethernet networks
10BASE-T Ethernet (though modern switched Ethernet largely avoids collisions)
Coaxial cable networks
Shared bus architectures for data communication
Why it matters
The bigger picture
This patent is foundational to Ethernet, the most widely used local area network (LAN) technology. Robert Metcalfe, one of the inventors, is credited with co-inventing Ethernet at Xerox PARC. The principles described here, particularly Carrier Sense Multiple Access with Collision Detection (CSMA/CD), enabled the reliable and efficient sharing of a common communication medium, paving the way for interconnected computers in offices and homes.
Filed
March 31, 1975
Granted
December 13, 1977
Market context
Who's building on this
Companies in this space
While the direct implementation of CSMA/CD is less common in modern switched Ethernet networks, the core principles of network access control and collision avoidance are built upon by companies like Cisco, Juniper Networks, and Broadcom in their networking hardware and chipsets. The fundamental ideas of shared media communication and robust error handling continue to influence network protocol design.
Market impact
This patent laid the groundwork for Ethernet, which revolutionized local area networking. It enabled the creation of a standardized, cost-effective way for computers to communicate within an office or campus. This led to the proliferation of networked personal computers and workstations, fostering the growth of the entire IT industry and becoming the dominant wired LAN technology globally.
Claim 1 — Plain English
What this patent covers
The patent describes a system for multiple data processing stations to communicate over a shared cable, like a bus. Each station has a "transceiver" (a device that transmits and receives) connected to the cable. When a station wants to send data, it first listens to the cable (Claim 5, "signal detecting means"). If the cable is busy, it waits. If the cable is clear, it starts transmitting. Crucially, if another station starts transmitting at the same time, causing a "collision," the system detects this (Claim 1, "collision detecting means"). Upon detecting a collision, the transmitting station immediately stops sending data (Claim 1, "interrupting the transmission"). It then waits a random amount of time before attempting to transmit again, with the wait time increasing if collisions keep happening (Abstract, "random number generator"). For example, if two computers try to send data at the exact same moment on a shared cable, this system detects the garbled signal, tells both computers to stop, and makes them wait different random times before trying again, preventing a continuous jam.
The clever bit
The really smart part is the combination of listening before transmitting (carrier sense) with the ability to detect when two transmissions clash (collision detection) and then using a randomized delay (exponential backoff) to retry. This ensures that even if multiple devices try to send data at the same time, they don't permanently jam the network.
What it does not cover
- Does not cover communication systems where each device has its own dedicated connection (e.g., point-to-point links).
- Does not cover network protocols that use tokens or central controllers to manage access to a shared medium.
- Does not cover wireless communication systems where collisions are handled by different methods like listen-before-talk (LBT) without explicit collision detection on the medium.
- Does not cover systems where the transmitting means does not interrupt its transmission upon detecting a collision.
- Does not cover systems that do not use a random backoff mechanism to reattempt transmission after a collision.
Patent timeline
Application submitted to the patent office
Application published, typically 18 months after filing
Patent officially issued
Patent enters public domain
This patent is in the public domain
See the Freedom to Build guide — what is free to use, what is not, and how to cite this patent.
PatentBrief Score
Impact Score
Strong
Citation count
40/40
Highly cited
Claim breadth
17/20
Very broad protection
Recency
0/20
Older than 20 years
Assignee scale
20/20
Major company or institution
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$82K – $262K
Midpoint $164K · expired or expiring · industry ×1.4
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
Patent Claims
0 independent claims · 1 dependent
Claims are the legal boundaries of the patent. An independent claim stands alone. A dependent claim adds limitations to its parent, narrowing — but not broadening — the scope.
The original legal language
Original claims
26 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Metcalfe, R. M., Lampson, B. W., Thacker, C. P., & Boggs, D. R. (1977). How Multiple Computers Share a Network Cable Without Crashing (U.S. Patent No. 4,063,220). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/4063220/ethernet-packet-network
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
Embed
Add this patent to your site
Drop this plain-English patent card into any blog post or article — free, no signup. It always links back to the full breakdown here.
<div data-patentlens-widget data-patent-number="US4063220"></div> <script src="https://patentbrief.org/embed.js" async></script>
Stay in the loop
Get a weekly digest of new patents.
One email per week. No spam. Unsubscribe anytime.
Keep exploring
Related patents you should know
US 4683195 · 1987
How to Make Billions of Copies of a DNA Segment
This patent describes the Polymerase Chain Reaction (PCR), a method to rapidly create many copies of a specific piece of DNA or RNA, enabling its detection and analysis.
Cetus Corp
US 8697359 · 2014
How to Edit Genes in Human Cells Using an Engineered CRISPR System
This patent describes an engineered CRISPR-Cas9 system for precisely cutting DNA in eukaryotic cells to change how genes work, opening the door for gene editing in complex organisms.
Massachusetts Institute of Technology
US 7657849 · 2010
How the iPhone's Slide-to-Unlock Gesture Works
Apple's 2010 patent describes unlocking a device by dragging a specific graphical image across the touchscreen along a predefined path, a gesture that became iconic with the original iPhone.
Apple Inc
US 4733665 · 1988
How Doctors Implant a Permanent Stent Using a Balloon
This patent describes the method for placing a permanent, expandable wire mesh tube inside a blood vessel or other body tube using a balloon-tipped catheter to widen it and keep it open.
Expandable Grafts Partnership
US 4965188 · 1990
How to Make Many Copies of a DNA Piece with Heat
This patent describes the Polymerase Chain Reaction (PCR) method, a technique to make millions of copies of a specific DNA segment using a heat-resistant enzyme and repeated temperature changes.
Cetus Corp
US 4235871 · 1980
How to Encapsulate Active Materials in Lipid Bubbles Efficiently
This patent describes a method for trapping biologically active substances inside tiny, multi-layered fat bubbles called liposomes, using a specific water-in-oil emulsion and gel-forming process to improve how much material gets captured.
Individual
Semantically similar
You might also find these interesting
US 4677614 · 1987 · Process Systems Inc
How Self-Healing Data Loops Automatically Elect a New Master Controller
US 6665308 · 2003 · Terayon Communication Systems Inc
How Cable Modems Fix Signal Distortions Before Sending Data
US 7904576 · 2011 · Hewlett Packard Development Co LP
How to Send Data Streams Independently Without Connection Overload
US 5487069 · 1996 · Commonwealth Scientific and Industrial Research Organization CSIRO
CSIRO's High-Frequency Wireless Network Technology
More to explore
More in Telecom & Wireless
US 5347632 · 1994 · Prodigy Services Co
Prodigy's System for Interactive Online Information and Shopping
US 3906166 · 1975 · Motorola Inc
How Early Cell Phones Handled Calls Across Different Towers
US 4200770 · 1980 · Leland Stanford Junior University
How to Create a Secret Code Key Without Meeting First
US 5487069 · 1996 · Commonwealth Scientific and Industrial Research Organization CSIRO
CSIRO's High-Frequency Wireless Network Technology
New to patents?
Common Questions
Frequently Asked Questions
What does How Multiple Computers Share a Network Cable Without Crashing cover?
This patent describes how multiple computers can share a single communication cable by listening for other transmissions and stopping their own if a collision occurs, then trying again later.
Who owns patent US 4063220?
Xerox Corp owns this patent, granted in 1977.
When does this patent expire?
This patent has expired and is now in the public domain — anyone can use the invention freely.
What is patent US 4063220 cited by?
This patent has been cited by 301 later patents that build on its ideas.
What problem does this patent solve?
This patent is foundational to Ethernet, the most widely used local area network (LAN) technology. Robert Metcalfe, one of the inventors, is credited with co-inventing Ethernet at Xerox PARC. The principles described here, particularly Carrier Sense Multiple Access with Collision Detection (CSMA/CD), enabled the reliable and efficient sharing of a common communication medium, paving the way for interconnected computers in offices and homes.
What does this patent NOT cover?
Does not cover communication systems where each device has its own dedicated connection (e.g., point-to-point links).
Same assignee
More from Xerox Corp
Patent monitoring