Ethernet & LAN Technologies (IEEE 802.3)

A Small Problem in the 1970s

Imagine an office in the early 1970s.

Computer existed, but they were isolated machines.

Each computer was expensive and powerful, but:

• printers were shared manually
• files were moved using tapes
• collaboration was slow
• data sharing was painful

Companies needed something new:

A way for many computers in the same building to communicate easily.

This need gave birth to Local Area Networks (LANs).

Add one technology would dominate everything:

Ethernet

What Is Ethernet?

Ethernet is:

A set of rules that define how devices communicate inside a local network.

Technically:

• Works at Layer 2 and Layer 2
• Defines signalling + framing + access rules
• Standardized as IEEE 802.3

Ethernet answers:

• How bits travels on cables
• How devices identify each other
• When devices can transmit
• How collisions are handled

The First Ethernet Idea

Researcher at Xerox PARC asked:

“What if computers shared one communication cable like people sharing a radio channel?”

So early Ethernet looked like this:

A -----
        \
B ------- Shared Cable -------- Printer
        /
C -----

This was called a:

Bus Topology:

One wire. Everyone connected.

Simple and cheap.

How Early Ethernet Worked

The cable acted like a shared conversation space.

Rules were needed.

Ethernet introduced:

CSMA/CD (Carrier Sense Multiple Access with Collision Detection)

Step-by-Step Story

Step 1: Listen First (Carrier Sense)

Before talking, a computer listens:

“Is this cable free?”

If busy -> wait

Step 2: Speak (Transmit)

If silent -> send data.

Step 3: Collision Happens

Two devices may start at the same time.

Signals collide.

Signal A + Signal B = Corruption

Step 4: Detect Collision

Devices notice abnormal signals.

They immediately stop.

Step 5: Retry Later

Each device waits a random time and tries again.

This prevents repeated collisions.

Real-Life Analogy

Like people in a meeting:

• Listens people in a meeting
• If interruption happens -> both pause
• Try again later

Ethernet made shared communication practical.

Ethernet Frames – The Language of LANs

Layer 2 introduced frames.

Ethernet defines exactly how frames look.

Ethernet Frame Structure

| Preamble | Dest MAC | Source MAC | Type | Data | CRC |

1 Preamble

Synchronization pattern

Helps receiver prepare to read incoming bits.

2 Destination MAC Address

Who should receive the fram

3 Source MAC Address

Who sent it.

4 Type/Length Field

Indicates protocol inside data (IP, ARP, etc.).

5 Data (Payload)

Actual informatin being sent.

Typically:

• 46-1500 bytes.

6 CRC (Error Check)

Detects corrupted frames.

This structure made reliable local communication possible.

Ethernet Speed – Evolution Over Time

Ethernet continuously evolved.

Same idea.

Faster technology.

From Bus Networks to Star Networks

Early Ethernet had problems:

• cable break = entire network fails
• many collisions
• hard troubleshooting

Solution:

Switch-Based Ethernet

Modern Ethernet Topology

      PC A
        |
PC B — SWITCH — PC C
        |
      Server

Each device connects to a switch.

What Changed?

Instead of one shared cable:

• Each device gets its own connection.
• Switch forwards frames intelligently.

Result:

• fewer collisions
• higher speed
• scalable networks

Hubs vs Switches

Hub (Layer 1)

• repeats signals
• broadcasts everything
• collisions common

Switch (Layer 2)

• understand MAC addresses
• sends data only where needed
• eliminate collisions

Switches made Ethernet dominate networking.

Full Duplex Communication

Old Ethernet:

• Send OR receive at one time

Modern Ethernet:

• send AND receive simultaneously.

Called:

Full Duplex

Benefits:

• no collisions
• double throughput
• efficient communication

CSMA/CD is mostly unnecessary today.

Ethernet Cabling Types

Ethernet defines physical media too.

Twisted Pair (Most Common)

Examples:

• Cat5e
• Cat6
• Cat6a

Used in homes and offices.

Fiber Optic Ethernet

Uses light instead of electricity.

Advantages:

• long distance
• hight speed
• immune to interference

Used in:

• ISPs
• data centers
• backbone networks

Why Ethernet Won Over Other Technologies

Other LAN technologies existed:

• Token Ring
• FDDI
• ATM LAN

But Ethernet succeeded because it was:

• simple
• cheap scalable
• backward compatible
• easy to upgrade

Today:

Nearly every wired network on Earth uses Ethernet.

Ethernet in Your Daily Life

When you:

• plug laptop into router
• connect office computers
• use data centers servers
• access wired internet

You are using Ethernet.

Even Wi-Fi networks eventually connect to Ethernet infrastructure.

Ethernet's Role in Networking Layers

Ethernet acts as the foundation carrier for higher protocols.

The Bigger Picture

Physical Layer: signals move

Data Link Layer: devices communicate locally

Ethernet: practical implementation of both layers