Wireless LANs (Wi-Fi -- IEEE 802.11)

The New Problem: Wires Everywhere

By the 1990s, Ethernet had become successful.

Offices looked like this:

PC ─ cable ─ Switch ─ cable ─ Server

It worked well.

But a new problem appeared.

People wanted:

• laptops instead of desktops
• mobility inside offices
• internet in homes without drilling walls
• connectivity in cafes, airports, and universities

Ethernet required cables.

Humans wanter freedom.

The question became:

“Can computers communicate without physical wires?”

The answer became: Wireless Lan – Wi-Fi

What Is Wi-Fi?

Wi-Fi is:

A wireless implementation of local networking using radio waves.

Technically:

• Standardized as IEEE 802.11
• Works at Physical + Data Link Layers
• Wireless version of Ethernet-like communication

Instead of electrical signals in cables:

Wi-Fi sends electromagnetic radio singals through air.

A Simple Story – Talking Through the Air

Imagine Computer A wants to send data to Computer B.

Instead of a cable:

A ))))   AIR   (((( B

The NIC converts bits into radio waves.

Air becomes the transmission medium.

But this creates new challenges never seen in wired networks.

Why Wireless Networking Is Harder Than Wired

A cable is predicatable.

Air is chaotic.

Wireless networks face problems like:

Intereference

Walls, microwaves, Bluetooth devices disturb signals.

Signal Loss

Distance weakens radio waves.

Shared Space

Everyone nearby shares the same air.

Hidden Devices

Two devices may not hear each other but collide at receiver.

Because of these challenges, Wi-Fi needed new rules.

Core Components of a Wi-Fi Network

1 Wireless NIC

Every device needs a wireless adapter.

It converts:

bits ↔ radio signals

2 Access Point (AP)

The central device in Wi-Fi networks.

Example:

• Home router
• Office wireless AP

Acts like a wireless switch.

Laptop
   \
Phone ---- Access Point ---- Internet
   /
Tablet

The Access Point connects wireless devices to wired Ethernet networks.

How Wi-Fi Communication Works

Let's follow one message

Step 1: Device Joins Network

Your laptop scans available networks:

Home_WiFi
OfficeNet
Cafe_Free

This process is called Scanning.

Step 2: Authentication

Device proves permission to join.

(password, encryption keys)

Step 3: Association

Device registers with Access Point.

Now it becomes part of the LAN.

Step 4: Data Transmission

Data is sent as wireless frames using radio signals.

Wi-Fi Frames (Similar but Different from Ethernet)

Wi-Fi also uses frames.

But wireless frames include extra fields because mobilty exists.

Example structure:

| Control | Source | Destination | BSSID | Data | Error Check |

Why extra information?

Because devices may move and signals travel differently than cables.

The Biggest Wireless Problem – Collisions

In Ethernet (wired):

Devices can detect collisions easily.

In wireless:

A device cannot transmit and listen simultaneously.

So collision detection (CSMA/CD) cannot work.

Wi-Fi invents a new method.

CSMA/CA – Collision Avoidance

(Carrier Sense Multiple Access with Collision Avoidance)

Step 1: Listen

Devie checks if channel is free.

Step 2: Wait Random Time

Even if free, device waits a small random delay.

Reduces chance of simultaneous transmission.

Step 3: Transmit

Device sends frame.

Step 4: Acknowledgement (ACK)

Receiver confirms successful reception.

If ACK not received -> resend.

Wi-Fi avoid collisions instead of detecting them.

Wi-Fi Frequency Bands

Wi-Fi uses radio frequencies.

2.4 GHz Band

• longer range
• slower speeds
• more interference

5 Ghz Band

• shorter range
• faster
• less crowded

6 Ghz (Wi-Fi 6E / Wi-Fi 7)

• extremely fast
• minimal interference
• modern devices

Evolution of Wi-Fi Standards

Each generation improves efficiency and capacity.

Wireless Security

Because signals travel through air:

Anyone nearby could listen.

Security became essential.

Evolution of Wi-Fi Security

Encryption protects data from eavesdropping.

Range and Coverage

Wi-Fi depends on:

• Obstacles (walls)
• interference
• frequency band
• antenna strength

Typical indoor range:

~20-40 meters.

Large networks use multiple access points.

Ethernet vs Wi-Fi

Real-Life Network Example

At home:

Phone ──┐
Laptop ─┼── Wi-Fi → Router → Ethernet → ISP
TV ─────┘

Wireless devices connects via Wi-Fi.

Router connects to internet using Ethernet or fiber.

Wi-Fi extends Ethernet into the air.