Wireless LAN• Advantages

1. Flexibility

2. Planning

3. Design

4. Robustness

5. Cost

WHERE ?? HOTELS,AIRPORT,LECTUREHALLS

WLAN

• Disadvantages

1. QoS (Lower Quality)

2. Restrictions –limited to low power senders

and certain frequency bands

3. Safety and Security

(Radio Transmission Interference Hospital Equipment)

DESIGN ISSUES FOR COMMERICAL

SUCCESS

• Global Operations

• Low Power

• License Free Operations

• Robust Transmission Technology

• Easy to use ( Plug –and-Play)

Infrared VS Radio Transmission

Infrastructure Vs Adhoc Networks

IEEE 802.11

• Famous Family Of WLAN in which many products available

• Time Bounded and Asynchronous Services

Architecture of an Infrastructure based on IEEE 802.11

Architecture of IEEE 802.11 Adhoc Wireless Lan

IEEE 802.11 Protocol Architecture and Bridging

Layers and Functions

Physical Layer• 3 Different Variants

• All Phy Variants have CCA(Clear Channel Assessment) Signal

• FHSS

• DSSS

• PHY Layer, based on Infrared transmission uses near light at 850-950nm.

IEEE 802.11 Medium Access Control

• MAC layer covers three functional areas:

–Reliable data delivery

–Access control

–Security

MAC LAYER• 3 BASIC ACESS MECHANISM

(1) CSMA/CD by avoiding Hidden Terminal

Problem

(2) DCF (Distributed coordination Function)

(3) PCF (Point Coordination Function)

MAC MECHANISMS ARE ALSO CALLED

AS DISTRIBUTED FOUNDATION

WIRELESS MAC (DFWMAC)

Inter Frame Spacing

• All access methods; controlling waiting time is important.

Reliable Data Delivery

• More efficient to deal with errors at the MAC level than higher layer (such as TCP)

• Frame exchange protocol–Source station transmits data–Destination responds with acknowledgment (ACK)–If source doesn’t receive ACK, it retransmits frame• Four frame exchange–Source issues request to send (RTS)–Destination responds with clear to send (CTS)–Source transmits data–Destination responds with ACK

802.11 DFWMAC

802.11 FRAME FORMAT

802.11 FRAME FORMAT Contd..• Duration Id : Time in which medium is occupied • Checksum : To protect the frames • To DS & From DS• More Frag-> 1 (if another frames follows)• Retry ->1 (Earlier Frame retransmit)• Power Management -> 1-PS mode ,0 –Active• Wired Equivalent Privacy(WEP): Standard

security mechanism applied • Order : Received Frames to be processed in the

strict order

Interpretation of the MAC Address in an 802.11 MAC Frame

MAC Management• Synchronization :

Finding WLAN, Synchronization of Internal Clocks, Generation of Beacon Signals Generation

• Power Management :

Periodic Sleep,Buffering,without missing a Frame

• Roaming :

Joining Network; Changing AP;Scanning for AP

• Management Information Base :

Current state of Wireless station is stored for internal and external access

Synchronization

• Node of 802.11 network maintains INTERNAL CLOCK

• Timing Synchronization Function

• Within BSS,Periodic transmission of a BEACON FRAME

• BEACON –Timestamp and MF(PM,Roaming)

• From Time to Time Internal Clock to be adjusted.

Beacon Transmission in a Busy 802.11 Infrastructure Network

• AP Performs Synchronization

Beacon Transmission in a Busy 802.11 Adhoc Network

• NO AP;Each Node has its own timer & starts the transmission of a beacon signal after the beacon interval

• Random Back off Algorithm –Beacon Frames & only one BEACON Wins.

POWER MANAGEMENT

• Switch off the transceiver whenever it is not needed –Should support all applications

• Sending device –Simple

• Receiving device –Not aware-WakeUp Periodically

• Longer off periods save battery life but reduces average throughput.

Power SAVING

• Two States for a Station –SLEEP & AWAKE & Buffering of data in senders

• Receivers awake periodically and stay awake for a certain time

• Senders Announce the destinations of the buffered data

• Destination wakes until the transmission takes place

Power Management –Adhoc Networks•No AP to buffer data

•Nodes ability to buffer data frames

•Synchronization –All stations wake up at the same time

•All Stations will announce the list of Buffered Frames-AdhocTIM

•Announcement -ATIM Window

•ATIM Interval –wakes & goto sleep if no frame is buffered

•Others transmit and get the ACK

ROAMING

• 10-20m transmission quality

• User move from ap-ap

• Station scans for NEW AP

• Scanning defined in PHY Layer

• PASSIVE & ACTIVE

Passive -Simply Listens to Medium to find the other network.

ACTIVE SCANNING • PROBE on each channel & wait for a response

• Based on the response ;the station decides to join BSS

• Station selects AP;Send the Association Request

• Get the Association Response

• After joining new AP ; DS will update and inform the old AP