1. Outline What is Wireless LAN Wireless Transmission Types
Difference Between Wired and Wireless LAN
IEEE
Two modes of (Steps to join Network in Infrastructure mode )
CSMA/CD cannot be used in WLAN
( MACA ) Multiple Access with Collision Avoidance

2. Wireless LAN
A wireless LAN or WLAN is a wireless local area network that uses radio waves or Infrared as its carrier

3. Wireless Transmission Types Infrared , Radio
No licenses needed
License is required
Many things absorb IR light (cant penetrate walls, furniture e.t.c ) , Requires line of sight (LOS)
Can penetrate walls, furniture e.t.c
Example : Home-entertainment remote-control boxes
Example : Bluetooth

4. Visibility Node to Node on same network
Characteristics
Installation
Visibility Node to Node on same network
Cost
Mobility
Speed and Bandwidth
Security
Signal Loss And Fading
Types
Standards

5. Characteristics
Wired
Wireless
Installation
Difficult
Easy
Visibility Node to Node on same network
All of the nodes on a wired network can hear all other nodes
Many nodes on a wireless network cannot hear all of the other wireless nodes on the same network
Cost
Less (such Ethernet, cables, switches are not expensive)
More (wireless adapters and access points are quite expensive )
Mobility
Limited
Outstanding
Speed and Bandwidth
High
Low
Security
Signal Loss And Fading
Less
More (due to more interference, absorption, refraction and reflection etc.)
Types
Local Area Network(LAN) Metropolitan Area network(MAN) Wide Area Network
Infra structure based network
Infra structure less (ad hoc) network
Standards
802.3
802.11

6. IEEE
IEEE   defines specifications for implementing wireless local area network (WLAN)
All the specifications use Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) for path sharing

7. Two Modes of IEEE 802.11 1. Infrastructure Mode
Terminals communicate to an access point.
Devices on the network all communicate through a single access point, which is generally the wireless router. For example, let’s say you have two laptops sitting next to each other, each connected to the same wireless network. Even when sitting right next to each other, they’re not communicating directly. Instead, they’re communicating indirectly through the wireless access point. They send packets to the access point — probably a wireless router — and it sends the packets back to the other laptop. Most Wi-Fi networks function in infrastructure mode.

8. Access Point (AP)
Base station that communicates with the wireless hosts
Basic Service Set (BSS)
Coverage of one AP
AP acts as the master AP
BSS

9. If nodes move out of range of the access point
If nodes move out of range of the access point . Then communication is not possible!!

10. 2. Ad Hoc Mode
Terminals communicate in a peer-to-peer basis without any access point.

11. If Nodes are not in direct range of each other (Ad Hoc Mode)
If Nodes are not in direct range of each other (Ad Hoc Mode). Then communication is not possible!! X X X

12. What if ?? Multi-hop Infrastructure Access Multi-hop Ad Hoc Network OR
NOT part of

13. Steps to Join a Network in Infrastructure Mode
Discover available network i.e. basic service set (BSS)
Select a BSS
Authentication
Association

14. 1. Discovering Available Network
Active Scanning
Station sends a Probe Request frame
Access Point responses with a Probe Response frame, which includes
Access Point MAC address, Network name , ID etc.
Passive Scanning
Each Access Point broadcasts periodically a Beacon frame, which includes:

15. 2. Choosing a Network The user selects from available networks
Common criteria:
User choice
Strongest signal
Most recently used

16. 3. Authentication (A station proves its identity to the Access Point.)
4. Association
The station needs to register to the AccessPoint.
Association Request
Association Response
Station
Access Point

17. Why CSMA/CD is not applicable in WLAN
Wireless Properties
Signal strength decreases proportional to the square of the distance due to which there is a lack of full connectivity and Hidden Terminal Problem occurs
Exposed Terminal Problem
Due to these reasons we cant apply CSMA/CD properly

18. Hidden Terminal Problem ( A is hidden from C)
A and B hear each other
B and C hear each other
But, A and C do not
A and C want to send to B
First A senses the channel got it free and send data to B
After that C senses the channel got it free as A is hidden from C and send data to B (CS fails)
Collision occurs at B. A B C A B C
A’s signal
strength
space
C’s signal

19. Exposed Terminal Problem
B wants to send to A, C wants to send to some other i.e not A not B
C has to wait, since he finds the medium in use due to the signals produced by B

20. ( MACA ) Multiple Access with Collision Avoidance
IEEE is based on the idea of MACA
MACA uses a three-way handshake protocol
RTS (request to send) : sender sends RTS before sending data
CTS (clear to send) : receiver sends CTS
The sender then sends the data.
Signaling (RTS/CTS) packets contain
sender address
receiver address
packet size (duration of data transmission)

21. MACA Solution to hidden & exposed terminal problem
MACA avoids the problem of hidden terminals
A and C want to send to B
A sends RTS first
Therefore B sends CTS to both A,C
C waits after receiving CTS from B as in CTS senders and receivers address is there
A sends data after getting CTS
MACA avoids the problem of exposed terminals
now C does not have to wait, as it cannot receive CTS from A