# 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. - **Patent:** US 4063220 - **Original title:** Multipoint data communication system with collision detection - **Owner:** Xerox Corp - **Granted:** 1977 - **Status:** Public domain (expired) - **Times cited:** 301 - **Field:** telecommunications, software, consumer_electronics, semiconductors ## What it does 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. ## 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. ## 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. ## Real-world examples 1. Early Ethernet networks 2. 10BASE-T Ethernet (though modern switched Ethernet largely avoids collisions) 3. Coaxial cable networks 4. Shared bus architectures for data communication ## Why it matters 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. ## 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). **Full plain-English explainer:** https://patentbrief.org/patent/us/4063220/ethernet-packet-network **Original patent:** https://patents.google.com/patent/US4063220 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Self-Healing Data Loops Automatically Elect a New Master Controller](https://patentbrief.org/patent/us/4677614/data-communication-system-and-method-and-communication-controller-and-method-therefor-having-a-dataclock-synchronizer-and-method) — A communication system where multiple nodes in a loop can automatically take over as the master controller if the current one fails, ensuring the network stays synchronized. - [How Cable Modems Fix Signal Distortions Before Sending Data](https://patentbrief.org/patent/us/6665308/apparatus-and-method-for-equalization-in-distributed-digital-data-transmission-systems) — A method for cable modems to pre-filter data so it arrives clearly at the central office, preventing signal errors caused by the messy physical wires between them. - [How to Send Data Streams Independently Without Connection Overload](https://patentbrief.org/patent/us/7904576/reliable-datagram-via-independent-source-destination-resources) — A method for sending multiple streams of data between devices using independent, reliable paths that keep data in the correct order without needing a constant connection. - [CSIRO's High-Frequency Wireless Network Technology](https://patentbrief.org/patent/us/5487069/wifi-csiro-wireless-lan) — This 1996 patent from CSIRO describes a wireless local area network system that can send data reliably using radio waves above 10 GHz, even when signals bounce off walls. - [How Modems Use Guard Time to Switch Between Data and Commands](https://patentbrief.org/patent/us/4549302/hayes-modem-escape-sequence) — This patent describes a method for modems to safely switch from sending data to accepting commands without accidentally triggering that switch while transmitting normal files.