lecture 15b - wireless lan
TRANSCRIPT
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CS65-Computer Networks Dept. of EEE
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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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A Super Frame
time
Contention PeriodContention-free Period Contention-free Period Contention Period
A Super Frame
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CSMA/CA and NAV
NAV- Network Allocation Vector
RTS- Request To Send
CTS-Clear To Send
DIFS Distributed InterframeSpace
SIFS-Short Interframe Space
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CS65 C t N t k D t f EEE
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Use of handshaking to prevent hidden station problem
C FA B ED
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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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
ACK
IEEE 802.11
Reserved area
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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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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)