Ethernet Local Area Network (CSMA/CD)
US 4,063,220Carrier Sense Multiple Access, Coaxial Tap Transceivers, Analog Collision Detection, and Binary Exponential Backoff
How It Works: Step-by-Step Mechanical & Physical Breakdown
Ethernet operates as a distributed statistical arbitration mechanism across five coupled physical and logical stages: 1. Carrier Sensing (Listen Before Talk): When a host has a packet to send, its interface checks the cable. If the analog voltage on the 50-ohm coaxial line indicates an active transmission (carrier sensed), the station defers and waits for the channel to fall silent. 2. Transmission & Manchester Encoding: Once silent, the station transmits its packet bit-serially at 2.94 to 10.0 Mbps, encoding bits using Manchester phase modulation (where every bit has a mid-bit transition for reliable clock recovery). 3. Listen While Talk (Collision Detection): Because electricity travels through coaxial cable at roughly 200,000 km/s (~0.66c), there is a 5-nanosecond-per-meter propagation delay window where two distant stations might both find the cable quiet and transmit simultaneously. To catch this, the transmitting station continuously compares its outgoing signal against the voltage on the cable. When two open-collector drivers transmit together, their currents add constructively into the 25-ohm parallel bus impedance, pulling the cable voltage down from -1.0V to -2.0V. An analog comparator and delay-matched XOR gate detect this discrepancy instantly. 4. Collision Jam & Immediate Abort: The moment a collision is detected, the station aborts the transmission immediately (preventing the waste of a full multi-kilobyte packet) and broadcasts a 32-bit jam pattern so all other stations reliably detect the collision. 5. Binary Exponential Backoff (BEB): Each station increments its collision counter n and picks a random integer delay r from [0, 2^min(n, 10) - 1] slot times. By doubling the random delay window after every collision (1 slot, 2 slots, 4, 8, 16 ... up to 1024), contending stations rapidly un-synchronize and find clear transmission windows, maintaining over 95% channel efficiency even under intense traffic.
Interactive Real-Time Physical Simulation
Drag to rotate · Pinch to zoom · Shared controls update the displayed modelDetailed Component Architecture
1Coaxial Cable Medium & Termination
The transmission bus consists of a continuous coaxial line with characteristic impedance . Both physical ends are terminated with non-inductive resistors to ground, presenting an effective parallel AC impedance of to any tap. Electromagnetic signals travel along the polyethylene dielectric at ().
2Vampire Tap & Active Transceiver
The transceiver clamps onto the cable using needle-like contact pins (vampire tap). An open-collector NPN transistor driver pulls through the bus, creating a nominal logic pulse. High-input-impedance differential receivers sense line voltages with minimal capacitive loading (), and isolation transformers prevent ground loops between stations.
3Delay-Matched XOR Collision Detector
The outgoing transmit pulse is passed through an internal delay line (delaying the signal by to match receiver propagation latency) and fed into an XOR gate alongside the received line signal. When two stations transmit concurrently, the additive line voltage drops below the collision threshold, producing a logic mismatch at the XOR gate that asserts the collision line within nanoseconds.
4Binary Exponential Backoff Generator
A high-frequency asynchronous clock (fast clock) continuously runs a binary counter. When a collision occurs, a shift register tracks collision attempt . An AND-gate weighting matrix masks the fast counter output to select a random integer , loading an up-down counter that decrements once per slot time ().
5Manchester Phase Encoder / Decoder
Data bits are XORed with the transmitter bit clock. A binary '0' is represented by a high-to-low voltage transition at the center of the bit cell; a binary '1' is represented by a low-to-high transition. This ensures that every bit contains at least one signal transition, allowing receiving stations to extract clock synchronization directly from the data stream without a separate clock wire.
Governing Equations & Engineering Principles
Coaxial Cable Electromagnetic Wave Propagation & Delay
Electrodynamics & Transmission LinesClaim 1Electromagnetic Propagation Velocity
The speed at which electrical signals propagate down the coaxial transmission line, reduced by the dielectric material to approximately 200,000 km/s (5 ns per meter).
Because electricity does not propagate instantaneously, a station at one end of a 500-meter cable cannot know another station began transmitting until 2.5 microseconds later. This propagation latency creates the vulnerability window in which collisions can occur.
Historical Context: Understanding propagation delay led Metcalfe and Boggs to define the slot time as twice the maximum round-trip propagation time, guaranteeing that any collision anywhere on the cable is detected by all transmitting nodes.
Truncated Binary Exponential Backoff Retransmission Algorithm
Distributed Algorithms & Network Protocol DynamicsClaim 12Retransmission Backoff Delay
The duration a station must remain silent before attempting to retransmit a packet that previously suffered a collision.
If two stations pick the same delay after a collision, they will collide again forever. By doubling the delay pool after each collision (1 slot -> 2 -> 4 -> 8 -> ... -> 1024), the probability of repeated collisions rapidly approaches zero, automatically adapting to varying network loads.
Historical Context: Binary Exponential Backoff is one of the most celebrated algorithms in computer science history. It enabled decentralized local area networks to achieve over 95% channel throughput under heavy load without needing any central traffic arbiter.
Interactive Schematic Sheet (Figure 1)
Overall system architecture illustrating a plurality of communication stations (110, 120, 125, 126) connected to a shared passive coaxial cable medium (100) through individual non-invasive transceivers (111, 121) and interface units (115).
Select Any Numbered Pin
Click pins on the schematic or select from the list below to inspect historical specifications.
Why It Still Matters
US 4,063,220 is arguably the most consequential patent in the history of computer communications. It transformed local area networking from an expensive, fragile, centralized telecommunications paradigm into a ubiquitous, robust, and cost-effective standard. Ethernet became the universal protocol for local networks worldwide, underpinning Xerox Alto workstations, 3Com commercial adapters, the IEEE 802.3 standard, modern data centers, and the physical infrastructure of the global Internet.
Legal Claims Decoder (22 Numbered Claims)
The Historical Bottleneck
Why Prior Art Failed
- •Centralized polling switches created severe single points of failure and massive scheduling latency.
- •Token-passing rings suffered from token loss recovery delays and complicated node insertion/removal protocols.
- •ALOHA packet radio lacked instantaneous listen-while-talk collision detection, forcing corrupted frames to transmit to completion and wasting channel airtime.