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.

What it 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.

What it doesn't 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.

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.

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