⭐ UNIT – 3 (Easy Notes + PDF References)

Wireless LAN • MAC Problems • Hidden/Exposed Terminal • Near/Far • Infrastructure vs Ad-hoc • IEEE 802.11 • Mobile IP • Ad-hoc Routing

I have rewritten everything in easy school-level English so you can understand quickly.

🔵 1. What is WLAN? (Wireless LAN)

(Page 1

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WLAN = Wireless Local Area Network.
It allows devices to connect without cables using radio, infrared, or microwave signals.

Why WLAN?
✔ No wires needed
✔ Quick installation
✔ Can attach to existing wired network
✔ Useful in disasters or old buildings where wiring is impossible

🔵 2. Design Goals of WLAN

(Page 2–3 – Table shown in PDF

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✔ Simple to use

Users should easily connect.

✔ Low power use

Mobile devices need long battery life.

✔ No license needed

Using license-free bands reduces cost.

✔ Handle interference

There is a lot of radio noise; WLAN must work even in interference.

✔ Work globally

Frequencies should be allowed in many countries.

✔ Security

Wireless signals can be stolen easily, so strong security needed.

✔ Safety

Should not disturb medical devices & stay within safe radiation limits.

✔ Quality of Service (QoS)

Should support voice/video smoothly.

🟢 3. Advantages of WLAN

(Page 2–3

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Very flexible
No wiring cost
High data rate in small area
Works even without line-of-sight
Useful in disasters

🔴 4. Disadvantages of WLAN

(Page 3

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Limited coverage
Interference from other devices
Slower than wired networks
Less security
Environment can weaken signals
Access points are costly

🔵 5. Transmission Mediums in WLAN

(Page 3–5 diagrams included

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✔ Radio Waves

Travel long distances
Pass through walls
Used in WiFi

✔ Microwaves

Straight-line travel
Require line-of-sight
Affected by weather

✔ Infrared

Short range (like TV remote)
Cannot cross walls

✔ Laser Light

Very narrow beam
Can’t cross obstacles
Very secure (hard to tap)

🔵 6. MAC Problems in Wireless

(Page 5–7 with diagrams

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CSMA/CD works in wired networks
BUT does not work well in wireless because:

Receiver detects collisions, not sender
Signal becomes weak over distance
Nodes may not hear each other

🔴 Hidden Terminal Problem

(Page 6 – Fig 3.7

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Node A → sends to B
Node C → cannot hear A
C also sends to B at same time → collision at B
BUT A and C never know about it.

Solution → RTS/CTS method (IEEE 802.11)

🔴 Exposed Terminal Problem

(Page 7 – Fig 3.8

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Node B talks to A
Node C wants to talk to D
C hears B and waits
But D is out of B’s range → C could have transmitted

Result: unnecessary delay.

🔴 Near and Far Terminal Problem

(Page 8 – Fig 3.9

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A and B send with same power
B is near station C → strong signal
A is far → weak signal
C only hears B → A’s message lost

Needs power control.

🟦 7. Infrastructure vs Ad-hoc Networks

(Table 3.1 on Page 8

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Infrastructure	Ad-hoc
Uses access point	No access point
Long range	Short range
More secure	Less secure
Faster	Slower
Suitable for large networks	Suitable for small devices only

🟦 8. IEEE 802.11 (Wi-Fi Standard)

(Page 8–10 diagrams

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Defines:
✔ Physical Layer
✔ MAC Layer
✔ Security
✔ Power management
✔ Roaming between access points

Architecture:

BSS (Basic Service Set):
One AP + stations (Fig 3.10)

ESS (Extended Service Set):
Multiple BSS connected together (Fig 3.11)

AP allows roaming between cells.

🟦 9. Protocol Architecture (IEEE 802.11)

(Page 10 – Fig 3.12

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Layers involved:

Application
TCP/UDP
IP
LLC
MAC
PHY (PLCP + PMD)

MAC handles:
✔ Access control
✔ Fragmentation
✔ Encryption
✔ Power saving

🟢 10. Mobile IP – Simple Meaning

(Page 11–12 – Fig 3.13

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Mobile IP allows a mobile phone/laptop to keep the same IP address while moving between networks.

Important Terms

MN: Mobile Node
HA: Home Agent
FA: Foreign Agent
COA: Care-of Address
CN: Correspondent Node
(Page 11 list clearly shows this)

How Mobile IP works?

CN sends packets to MN’s home IP
MN is in another network
FA sends COA to HA
HA tunnels packets to FA
FA delivers to MN

🔵 11. Ad-hoc Network (Very Easy)

(Page 12 –14 diagrams

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Devices connect directly to each other without AP.
Every device also acts as a router.

Types (Page 12):

MANET
VANET
Mesh Network
Smartphone Ad-hoc
Sensor Networks (WSN)

🔵 12. Ad-hoc Routing Protocols (Important for exam)

(Page 13–14 diagrams

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✔ AODV (On-demand) – Fig 3.17

Messages:

RREQ
RREP
RERR

Uses sequence numbers → loop-free.

✔ DSDV (Table-based) – Fig 3.18

Proactive
Maintains full routing table
Solves count-to-infinity
No loops

✔ DSR (Source routing)

Source writes full path inside packet
No periodic updates
Saves bandwidth

✔ ZRP (Hybrid) – Fig 3.19

Uses proactive + reactive
Fast delivery + low overhead

⭐ ONE-LINE SUMMARY (Super Short)

1️⃣ WLAN is wireless LAN using radio/microwave/infrared.
2️⃣ Wireless MAC faces hidden, exposed, and near–far problems.
3️⃣ IEEE 802.11 defines Wi-Fi architecture, MAC, PHY, security.
4️⃣ Mobile IP keeps same IP while moving.
5️⃣ Ad-hoc networks have no AP; devices route data themselves.
6️⃣ AODV, DSDV, DSR, ZRP are major ad-hoc routing protocols.