lecture 15b - wireless lan

8/3/2019 Lecture 15b - Wireless Lan

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CS65-Computer Networks Dept. of EEE

S.Muralidharan 1

A wireless local area network(WLAN) links two ormore devices using some wireless distribution

method (typically spread-spectrum or OFDM radio),

and usually providing a connection through an

access point(AP) to the wider internet. This gives

users the mobility to move around within a local

coverage area and still be connected to the network

IEEE has defined the specifications for a wireless

LAN, called IEEE 802.11, which covers the physicaland data link layers.

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Standard defines two kinds of services :Standard defines two kinds of services :

The basic service set (BSS)The basic service set (BSS)

The extended service set (ESS)The extended service set (ESS)

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S.Muralidharan 8

Use of handshaking to prevent hidden station problem

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RTS

RTS = Request-to-Send

IEEE 802.11

Pretending a circular range

C FA B ED

RTS

RTS = Request-to-Send

IEEE 802.11

NAV = 10

NAV = remaining duration to keep quiet

Network Allocation Vector(NAV)

C FA B ED

CTS

CTS = Clear-to-Send

IEEE 802.11

CS65 C t N t k D t f EEE


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S.Muralidharan 9

C FA B ED

CTS

CTS = Clear-to-Send

IEEE 802.11

NAV = 8

C FA B ED

DATA

DATA packet follows CTS. Successful data reception acknowledged

using ACK.

IEEE 802.11

IEEE 802.11

C FA B ED

ACK

C FA B ED

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IEEE 802.11

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CS65 Computer Networks Dept of EEE


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S.Muralidharan 10

IEEE 802.11

C F A B ED

DATA

Transmit range

Interference

rangeCarrier sense

range

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Frame Control field defines the type of the frame and some control information.

Subfields in Frame Control(FC) field

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CS65 Computer Networks Dept of EEE


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CS65-Computer Networks Dept of EEE


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CS65 Computer Networks Dept. of EEE

S.Muralidharan 13

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WEP WPA 802.11iCipher Algorithm RC4 RC4 (TKIP) AES-CCMP

Encryption Key 40-bit 128-bit 128-bit

Initialization Vector 24-bit 48-bit 48-bit

Authentication Key None 64-bit 128-bit

Integrity Check CRC-32 Michael CCM

Key Distribution Manual 802.1X (EAP) 802.1X (EAP)

Key Unique To: Network Packet, Session, UserPacket, Session, User

Key Hierarchy No Derived from 802.1X Derived from 802.1X

Ad-hoc Security (P2P No No Yes (IBSS)

Pre-authentication No No Yes (EAPOL)

lecture 15b - wireless lan

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