How did Ethernet solve the problem of multiple computers transmitting simultaneously?

Ethernet uses a distributed algorithm called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Computers listen before transmitting (carrier sense), only transmit if the medium is clear (multiple access), and if a collision occurs, they detect it and reschedule their transmission using a randomized process called exponential backoff.

What was the original physical setup for Ethernet?

Early Ethernet used a bus topology with a single, long coaxial cable running through a building. Computers were connected to this main cable using devices called 'vampire taps' or later, T-pieces, with terminators at each end of the bus.

Why is it called 'Ethernet' and not something like 'CableNet'?

The name 'Ethernet' was chosen by Bob Metcalfe and colleagues to refer to the 'ether,' a concept from late 19th-century physics referring to the medium through which electromagnetic signals travel. This generalized term allowed for future flexibility in transmission media beyond just coaxial cables.

How has Ethernet evolved over the past 50 years?

Ethernet has dramatically increased in speed, from the original 3 Mbps to today's multi-gigabit and terabit per second standards. While early Ethernet used shared bus media, modern networks typically use switches with point-to-point connections, eliminating collisions and enabling full-duplex communication.

What is exponential backoff in Ethernet?

Exponential backoff is a mechanism used when collisions occur on an Ethernet network. If two computers collide while transmitting, they both wait a random amount of time before trying again. If another collision happens, the maximum waiting time they consider is doubled, reducing the probability of repeated collisions.

Are modern networks still based on the original Ethernet principles?

Yes, even though modern wired networks often use switches and higher speeds, the fundamental packet format, addressing, and the core ideas behind shared access and reliable data transmission are descendants of the original Ethernet design from 1973.

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Ethernet (50th Birthday) - Computerphile

Computerphile

Education4 min read27 min video

May 31, 2023|141,739 views|4,867|239

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TL;DR

Ethernet turns 50! From bus networks with "vampire taps" to today's switches, CSMA/CD ensures data gets through.

Key Insights

Ethernet, celebrating 50 years, was invented by Bob Metcalfe and others at Xerox PARC in 1973.

Early Ethernet used a bus topology requiring "vampire taps" to connect devices physically to a shared cable.

The core innovation is Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to manage shared access.

CSMA/CD allows devices to listen before transmitting and detect/handle collisions using exponential backoff.

The name "Ethernet" reflects the initial idea of a broadcast medium (ether) rather than a specific cable type.

Modern networking, while faster and using switches, retains Ethernet's fundamental packet format and principles.

BIRTH OF ETHERNET AND THE NEED FOR NETWORKING

Ethernet marks its 50th anniversary, originating in 1973 from Xerox PARC, pioneered by Bob Metcalfe and colleagues. At a time of diverse network technologies, like Hawaii's ALOHA wireless system, Ethernet focused on Local Area Networks (LANs) designed for connecting computers within a kilometer. The initial concept considered the physical cable length needed to connect numerous machines within a single room or building, highlighting the potential for significant cable usage even over relatively short distances.

THE CHALLENGE OF CONNECTIVITY: EXPLOSION OF CABLES

Connecting multiple computers directly to each other, creating a fully meshed network where each machine can communicate with every other machine, leads to a rapid increase in cable requirements. For instance, connecting ten machines directly would require 45 cables. This exponential growth in cables and ports would make machines bulky and impractical, necessitating a more efficient approach to network connectivity.

THE ETHERNET SOLUTION: A SHARED BUS AND MEDIA ACCESS

Ethernet's solution was to use a single shared cable, a 'bus,' to which all devices connect. This drastically reduced the number of cables needed. However, it introduced the challenge of managing simultaneous transmissions. The core problem was preventing two or more machines from transmitting at the same time, which would lead to garbled data, akin to two people speaking simultaneously. This led to the development of Media Access Control (MAC) protocols.

THE NAME AND THE CONCEPT: THE ETHER

The name 'Ethernet' was chosen deliberately by its inventors. They envisioned a network that could transcend various transmission media, not just coaxial cable, drawing inspiration from the 19th-century concept of 'luminiferous ether' through which electromagnetic waves were thought to travel. This foresight allowed the standard to remain general, accommodating future technologies like radio, microwaves, or any other medium suitable for broadcast transmissions.

EARLY IMPLEMENTATION: 10BASE5 AND 'VAMPIRE TAPS'

Early Ethernet implementations, like the 10BASE5 standard, utilized a thick coaxial cable running across buildings. Devices were connected using 'transceivers' with sharp spikes, colloquially known as 'vampire taps.' These taps physically punctured the cable to establish a connection without needing to cut and splice it, maintaining signal integrity. A drop cable then connected the transceiver to the computer's network card.

TERMINATION AND SIGNAL INTEGRITY

A crucial aspect of bus networks is proper termination. If a signal traveling down the cable reaches an end without a terminator, it reflects back, corrupting data. Terminators absorb the signal's energy, preventing reflections. This ensures that data transmissions are clean and reliable across the network segment.

MEDIA ACCESS CONTROL: CSMA/CD EXPLAINED

The genius of Ethernet lies in its distributed algorithm for managing shared access: Carrier Sense Multiple Access with Collision Detection (CSMA/CD). 'Carrier Sense' means devices listen to the network before transmitting. 'Multiple Access' means multiple devices share the medium. 'Collision Detection' allows devices to detect if their transmission has interfered with another. If a collision occurs, devices stop transmitting and use 'exponential backoff'—randomly delaying their next attempt, doubling the delay range after repeated collisions—to re-establish access.

THE POWER OF THE ALGORITHM AND BEST EFFORT DELIVERY

Ethernet's CSMA/CD algorithm allows devices to acquire access to the 'ether' without central arbitration. While it doesn't guarantee delivery (a 'best effort' network), it effectively manages shared access and minimizes collisions through clever detection and backoff mechanisms. This design, originating from principles seen in the ALOHA network, proved highly effective for its time.

EVOLUTION TO MODERN NETWORKS: SWITCHES AND FULL DUPLEX

While original Ethernet used shared media, modern wired networks predominantly use switches. Switches create dedicated connections between devices, eliminating collisions on the local link and enabling full-duplex communication (simultaneous sending and receiving). Despite these advancements, the fundamental packet format and many core principles of the original Ethernet protocol are still in use today.

INCREASING SPEEDS AND CHANGING MEDIUMS

Over 50 years, Ethernet has seen dramatic speed increases, from the original 3 megabits per second to multi-gigabit and terabit speeds. The transmission media have also evolved, moving from thick coaxial cable (10BASE5) and thinner coaxial (10BASE2) to Unshielded Twisted Pair (UTP) cables (Cat5, Cat6, etc.) and fiber optics, adapting to the demands for higher bandwidth and longer distances.

THE LEGACY OF ETHERNET

Ethernet's enduring popularity stems from its robustness, scalability, and the innovative algorithms that governed its early operation. While technologies like Token Ring and ARCNET existed, Ethernet ultimately prevailed, becoming the de facto standard for wired networking. Its foundational concepts continue to influence network design, even as the physical infrastructure and speeds have transformed dramatically.

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Ethernet: The Bus Network Explained

Practical takeaways from this episode

Do This

Utilize a shared medium (bus network) for connecting multiple computers.

Terminate the cable at both ends.

Implement Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to manage transmissions.

Detect and react to collisions by deferring and using exponential backoff.

Ensure proper termination to prevent signal reflections.

Connect devices using 'vampire taps' (10BASE5) or T-pieces (10BASE2) on coaxial cable.

Avoid This

Don't allow two machines to transmit simultaneously on the shared medium.

Don't leave the cable unterminated, as it causes reflections.

Don't rely on a central point for arbitration in a bus network; use a distributed algorithm.

Avoid collisions by deferring transmission if the network is busy.

Common Questions

Ethernet is a fundamental networking technology that celebrated its 50th anniversary recently. It evolved from early research at Xerox Park and became the dominant standard for local area networks, enabling communication between computers within a building or campus.