Wi-Fi

IEEE 802.11

ARCHITECTURE

Network Components

Wireless Stations Access Point

Types of Architectures

Basic Service Set

Stationary or Mobile wireless stations

Optional Central Base Station

Extended Service Set

Distributed System

Uses LAN to connect Central Stations form Basic Service Sets

BASIC SERVICE SET [BSS]

AD HOC NETWORK INFRASTRUCTURE NETWORK

EXTENDED SERVICE SET [ESS]

DISTRIBUTION SYSTEM

BSS BSS BSS

TYPES OF STATIONS

NO Transition

BSS Transition

ESS Transition

802.11 Protocol Stack

802.11 FRAME FORMAT

802.11 define 3 classes of frames :

MANAGEMENT

CONTROL

DATA

802.11 FRAME FORMAT

FRAME CONTROL

0 : current version

PROTOCOL VERISION

00 : Management frame01 : Control frame10 : Data frame11 : Unspecified

Control (01)- RTS (1011) CTS (1100)

ACK (1101)

ToDS- 1: frame is addressed to AP for forwarding it to DS

0: other frames

FromDS- 1: frame coming from DS 0 : other frames

1 : When there are more fragments

0 : Sent fragment is last

1 : Frame is retransmitted

0 : Frame is transmitted for the first time

1 : Station in in sleep mode

0 : active mode

1 : indicates that the sender has additional frames

1 : frame body is encrypted

0 : not encrypted

1 : frames transmitted in order

0 : not transmitted in order

Data, Management frame : defines duration of transmission

Control frame : defines ID of frame

Address1 : next device

Address2 : Previous device

Address3 : final destination if not defined by addr1

Address4: original source if not defined by addr2

CASE 1:

ToDS : 0FromDS :0

CASE 2:

ToDS : 0FromDS :1

CASE 3:

ToDS : 1FromDS :0

CASE 4:

ToDS : 1FromDS : 1

Sequence field (16 bits) has 2 subfields:

1. Fragment no.(4 bits)

2. Sequence no.(12 bits)

It can transmit frame with max payload of 2312 bytes

Its is 32bits field containing 32 bits CRC

MANAGEMENT FRAME

RTS FRAME

CTS FRAME

ACK FRAME

CONTOL FRAMES

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