14.8.4 How Does Ethernet Actually Work?
- What Is Ethernet?
- How Did Ethernet Begin?
- What Is CSMA/CD?
- Why Did Ethernet Change?
- What Is an Ethernet Frame?
- What Is a MAC Address?
- How Does an Ethernet Switch Work?
- Why Is Modern Ethernet Full-Duplex?
- How Fast Is Ethernet?
- What Transmission Media Does Ethernet Use?
- Why Has Ethernet Been So Successful?
- Will Ethernet Continue to Evolve?
- What Should You Remember?
Short Answer
Ethernet is the world's most widely used local area networking technology. It allows computers and other devices to exchange information by transmitting small units of data called frames across a shared communication medium. Over the past fifty years Ethernet has evolved from a simple shared coaxial cable system into a family of high-speed switched networks operating over copper cables, fiber-optic links, and wireless access networks. Its simplicity, reliability, low cost, and continual evolution have made Ethernet the foundation of modern computer networking.
What Is Ethernet?
Ethernet is a family of networking standards that defines how devices communicate within a local area network (LAN).
It specifies:
- the format of transmitted frames;
- how devices are identified;
- how access to the communication medium is managed;
- the physical transmission media; and
- the data rates that can be supported.
Today, Ethernet is used in homes, offices, factories, data centres, universities, and industrial facilities throughout the world.
How Did Ethernet Begin?
The original Ethernet systems, developed during the 1970s, connected many computers to a single coaxial cable.
Only one device could transmit at any instant. If two devices transmitted simultaneously, their signals collided and both transmissions were lost. To minimise collisions, Ethernet employed a technique called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Before transmitting, each station listened to determine whether another device was already using the cable.
If the medium was busy, the station simply waited.
What Is CSMA/CD?
CSMA/CD was designed for networks in which many devices shared the same transmission medium.
The procedure is straightforward:
- Listen to determine whether the channel is idle.
- Transmit only if no other station is transmitting.
- Monitor the channel while transmitting.
- If a collision is detected, stop transmitting immediately.
- Wait for a random period before trying again.
This approach worked well for relatively small, shared networks but became less efficient as the number of connected devices increased.
Why Did Ethernet Change?
As networks grew larger and faster, collisions became increasingly undesirable.
Engineers therefore replaced shared Ethernet with switched Ethernet. Instead of connecting every device to one shared cable, each device is connected to an Ethernet switch using its own dedicated link. This simple architectural change almost completely eliminated collisions while dramatically increasing network performance.
Today, virtually all Ethernet LANs operate in this way.
What Is an Ethernet Frame?
Ethernet does not transmit continuous streams of data.
Instead, information is organised into discrete units called frames. Each frame contains:
- destination MAC address;
- source MAC address;
- length or protocol information;
- user data; and
- an error-detection field known as the Frame Check Sequence (FCS).
The receiving device examines the frame and determines whether it is intended for that particular destination.
If transmission errors have occurred, the frame is discarded.
What Is a MAC Address?
Every Ethernet network interface is assigned a unique Media Access Control (MAC) address.
The MAC address identifies the device within the local network. When transmitting a frame, the sender places the destination MAC address into the frame header.
Ethernet switches examine this address and forward the frame only toward the appropriate destination whenever possible.
This selective forwarding greatly improves network efficiency.
How Does an Ethernet Switch Work?
An Ethernet switch learns the location of devices automatically.
Whenever a frame arrives, the switch records:
- the source MAC address; and
- the port on which the frame was received.
Over time, the switch builds a forwarding table containing the locations of active devices. When future frames arrive, the switch forwards them only through the port leading toward the destination device rather than broadcasting them throughout the entire network.
This intelligent forwarding reduces unnecessary traffic and allows many simultaneous conversations to occur.
Why Is Modern Ethernet Full-Duplex?
Because every device has its own dedicated connection to the switch, modern Ethernet normally operates in full-duplex mode.
This means that transmission and reception occur simultaneously. As a result:
- collisions are eliminated;
- CSMA/CD is no longer required;
- throughput is greatly increased; and
- communication delays are reduced.
Although CSMA/CD remains an important historical concept, it is rarely used on modern switched Ethernet networks.
How Fast Is Ethernet?
Ethernet has evolved continuously since its introduction.
Common Ethernet speeds include:
- 10 Mbps;
- 100 Mbps (Fast Ethernet);
- 1 Gbps (Gigabit Ethernet);
- 10 Gbps;
- 40 Gbps;
- 100 Gbps;
- 200 Gbps; and
- 400 Gbps.
Even higher-speed standards continue to be developed for data centres and backbone networks.
Remarkably, newer Ethernet standards remain compatible with the same fundamental frame format introduced decades ago.
What Transmission Media Does Ethernet Use?
Modern Ethernet operates over several different media.
These include:
- twisted-pair copper cable;
- fiber-optic cable;
- short-distance direct-attach cables; and
- backplane connections within network equipment.
The choice depends upon:
- transmission distance;
- required data rate;
- installation cost;
- electromagnetic environment; and
- power requirements.
This flexibility has helped Ethernet become suitable for almost every networking application.
Why Has Ethernet Been So Successful?
Few communication technologies have enjoyed the longevity of Ethernet.
Its success results from several important characteristics:
- open international standards;
- low equipment costs;
- continual performance improvements;
- excellent interoperability;
- backward compatibility;
- simple installation; and
- widespread industry support.
As networking requirements evolved, Ethernet evolved with them rather than being replaced.
Will Ethernet Continue to Evolve?
Almost certainly.
Although wireless networking continues to expand, wired Ethernet remains essential wherever:
- maximum performance;
- predictable latency;
- high reliability;
- large data volumes; or
- secure communication
are required.
Future Ethernet standards are expected to provide even higher data rates while continuing to support the same fundamental networking principles established in the original Ethernet systems.
What Should You Remember?
- Ethernet is the world's dominant local area networking technology.
- Early Ethernet used a shared transmission medium and CSMA/CD to manage collisions.
- Modern Ethernet uses switches and dedicated full-duplex links, eliminating collisions.
- Information is transmitted in frames containing MAC addresses and error-detection information.
- Ethernet switches learn device locations automatically and forward frames intelligently.
- Ethernet operates over copper cables, fiber-optic links, and other transmission media at speeds ranging from megabits to hundreds of gigabits per second.
- Ethernet's simplicity, interoperability, and ability to evolve have made it the foundation of modern wired networking.