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802.11 Wireless LANs
Abhishek Karnik,
Dr. Ratan Guha
University Of Central Florida
OVERVIEW
• Introduction
• 802.11 Basics
• 802.11e for QoS
• WEP
• In 1997 the IEEE adopted IEEE Std. 802.11-1997
• Defines MAC and PHY layers for LAN and wireless connectivity.
• Facilitate ubiquitous communication and location independent computing
• 802.11b operates at 11Mbps in the 2.4 GHz ISM Band (‘99)
• 802.11a operates at 54Mbps in the 5 GHz Band (’99)
• 802.11g operates at 54Mbps in the 2.4 GHz Band (’02)
• Increased deployment and popularity lead to introduction of QoS
• 802.11e for QoS – Draft Supplement – Nov 2002
INTRODUCTION
• Wireless LAN StationThe station (STA) is any device that contains the functionality of the 802.11
protocol, that being MAC, PHY, and a connection to the wireless media. Typically the 802.11 functions are implemented in the hardware and software of a network interface card (NIC).
Ex : PC , Handheld , AP (Access Point)
• Basic Service Set (BSS)802.11 defines the Basic Service Set (BSS) as the basic building block of an
802.11 wireless LAN. The BSS consists of a group of any number of stations.
802.11 BASICS
STA
STA
STA
STA
IBSS (Independent Basic Service Set – Ad-hoc Mode)
peer-peer connections
AP
Wired Backbone
Infrastructure Basic Service Set
AP
Wired Backbone
AP
ESS (Extended Service Set)
BSS1 BSS2
PCF DCF
Super Frame
DCF - Distributed Coordinated Function (Contention Period - Ad-hoc Mode)
PCF - Point Coordinated Function (Contention Free Period – Infrastructure BSS)
Beacon - Management Frame
Synchronization of Local timers
Delivers protocol related parameters
TBTT - Target Beacon Transition Time
Beacon TBTT
Distributed Coordinated Function (DCF)
• Also known as the Contention Period
• STAs form peer-peer connections. No central authority
• First listen and then speak
• Uses CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance)
• ACK indicates successful delivery
• Each node has one output buffer
Inter-Frame Spacing :
DIFS - 34 µsec
PIFS - 25 µsec ( Used in PCF )
SIFS - 16 µsec
Slot Time - 9 µsec
DIFS = SIFS + (2 * Slot Time)
SIFS required for turn around of Tx to Rx and vice versa
DATAA ACKBACK
DIFS SIFS
DIFSCWA
Data Transmission from Node A to B
• CW – Contention Window. Starts only after DIFS.
• Random number ‘r’ picked form range ( 0-CW )
• CWmin minimum value of CW
• CWmax maximum value the CW can grow to after collisions
• ‘r’ can be decremented only in CW
• CW doubles after every collision
DATAA ACKBACK
DIFS SIFS
DIFSCWA
• What if some node C wanted to send data while A was transmitting
data to B ?
• What about during SIFS ?
• What if after ACK, more than one say B,C,D,E nodes are waiting
to transmit data ?
Example :
rA = 4 and rC = 6
DATAA ACKBACK
DIFS SIFS
DIFS
DATAC
• What if rA and rC had both been picked as 4 ?
• What if rA and rC has collided and DATAA length was 10 while
DATAC length were 15 ?
DATAAACK
DIFS
DATAC
SIFSDIFS
A Collision between nodes A and C
• Length (DATAA) = 10 Slot times
• Length (DATAC) = 15 Slot times
• CW after Collision 1 0 – 7
• CW after Collision 2 0 – 15
• CW after Collision 3 0 – 31
• CW after Collision 4 0 – 63
NAV – Network Allocation Vector
DATA
ACK
STAA
STAB
STAC ACK
DIFS SIFS
DIFS
NAVB and C
STAA
STAB
STAC
Hidden Node Problem and Exposed Node Problem
RTS/CTS :
• RTS (Request To Send) - (Approx 20 bytes)
• CTS (Clear To Send) - (Approx 16 bytes)
• Use of RTS/CTS is optional
• Solves two problems :
1. Hidden Node Problem
2. Wastage of time due to collisions
• Maximum MSDU is 2304 bytes
A C
D
B
RTS
CTS
CTS
CTS
Preventing a collision at STAB
RTSSTAA
STAB
STAC
STAD
CTS
DATA
ACK
ACK NAV
NAV
NAVNew Node
DIFS SIFS SIFS SIFS DIFSCW
Point Coordinated Function (PCF)
• Also known as the CFP (Contention Free Period)
• Operation in an Infrastructure BSS
• STAs communicate using central authority known as PC
(Point Coordinator) or AP (Access Point)
• No Collisions take place
• AP takes over medium after waiting a period of PIFS
• Starts with issue of a Beacon
PCF DCF
Super Frame
Beacon TBTT
Beacon • Management Frame • Synchronization of Local timers• Delivers protocol related parameters• TBTT - Target Beacon Transition Time
DATA A
DIFS SIFS DIFS
PIFSB
DIFS - 34 µsecPIFS - 25 µsec SIFS - 16 µsecSlot Time - 9 µsecB - Beacon
AP taking over the Wireless medium using PIFS
B D1 + Poll
U1 + ACK
D2 + ACK + Poll
U1 + ACK
CF_End
Operation in CFP
CPCFP
SIFS
• Admission Control
• Purpose of having separate DCF and PCF
• Different 802.11 Working groups
• 802.11a (54Mpbs in 5GHz Band)
• 802.11b (11 Mbps in 2.4 GHz Band)
• 802.11c Wireless AP Bridge Operations
• 802.11d Internationalization
• 802.11e (QoS)
• 802.11f Inter-vendor AP hand-offs
• 802.11h Power control for 5Ghz region
• 802.11g (54Mbps in 2.4 GHz Band)
• 802.11i (Security)
802.11e for QoS
• QoS (Quality of Service)
• 802.11e for QoS – Draft Supplement – Nov 2002
• Introduction of new QoS mechanism for WLANs
PC
BSS
(Basic Service Set)
QBSS
(Basic Service Set for QoS)
HC
( Enhanced Station )
HCCA EDCAPCF DCF
QoS Support Mechanisms of 802.11e :
EDCA :
• Introduction of 4 Access Categories ( AC ) with 8 Traffic
Classes ( TC )
• MSDU are delivered through multiple back offs
within one station using AC specific parameters.
• Each AC independently starts a back off after
detecting the channel being idle for AIFS
• After waiting AIFS , each back off sets counter from
number drawn from interval [1,CW+1]
• newCW [AC] >= ((oldCW[TC] + 1 ) * PF ) - 1
Prioritized Channel Access is realized with the QoS parameters per TC, which include :
• AIFS[AC]
• CWmin[AC]
• PF[AC]
AC_VO [0] AC_VI [1] AC_BE [2] AC_BK [3]
AIFSN 2 2 3 7
CWmin 3 7 15 15
CWmax 7 15 1023 1023
EDCA
Virtual Collision
AC1 AC2 AC3 AC4TC
ACK BackOff[AC0] + Frame BackOff[AC1] + Frame
BackOff[AC2] + Frame
AIFS[AC0]
AIFS[AC1]
AIFS[AC2]
BackOff[AC3] + Frame
AIFS[AC3]
Access Category based Back-offs
Element IDCWmin[AC]
CWmin[0]….CWmin[3]CWmax[AC]
CWmax[0]….CWmax[3]
AIFSN[AC]AIFSN[0]….AIFSN[3]
TxOPLimit[AC]TxOP[0]….TxOP[3]
QoS Parameter Set Element Format
AIFS [AC] = AIFSN [AC] * aSlotTime + SIFS
HCCA ( Hybrid Coordination Function Controlled Channel Access )
Extends the EDCA access rules.
CP : TxOP
• After AIFS + Back off
• QoS Poll ; After PIFS
CFP : TxOP
• Starting and duration specified by HC using
QoS Poll .
HCCA EDCA
HC
PIFS
DATA A
AIFS SIFS AIFS
PIFS
DATA
Hybrid Coordinator
802.11e Operation in the CFP
• Guaranteed channel access on successful registration
• Each node will receive a TxOP by means of polls granted
to them by the HC
• TxOP based on negotiated Traffic specification (TSPEC) and
observed node activity
• TxOP is at least the size of one Maximum sized MSDU at the
PHY rate.
• Access Point advertises polling list
Traffic Specification (TSPEC)
Element ID (1)
Length (1)
Maximum MSDU size
(2)
TS info (2)
Nominal size MSDU (2)
Minimum Service
Interval (4)
Maximum Service
Interval (4)
Mean DataRate (4)
Inactivity Interval
(4)
Minimum Data Rate (4)
Maximum Burst Size
(4)
Minimum PHY Rate
(4)
Surplus Bandwidth Allowed (2)
Peak DataRate (2)
Delay Bound(2)
AC[0] AC[1] AC[2]
AIFSN 2 4 7
CWmin 7 10 15
CWmax 7 31 255
PF 1 2 2
Example :
AIFS[AC] = AIFSN[AC] * aSlotTime + SIFS
PIFS - 25 µsec ( Used in HCCA)SIFS - 16 µsecSlot Time - 9 µsec
AIFS[0] = (2 * 9) + 16 = 34 µsec = DIFS
AIFS[1] = (4 * 9) + 16 = 52 µsec (52 – 34) / 9 = 18/9 = 2 Slots
AIFS[2] = (7 * 9) + 16 = 79 µsec (79 – 34) / 9 = 45/9 = 5 Slots
Back-off Algorithm :
802.11 : CWRANGE = [ 0 , 2 2+i – 1 ]
802.11e : newCW[AC] = [(oldCW[AC] + 1) * PF] - 1
Collision1 Collision2 Collision3
AC[0] [(7+1)*1]-1 = 7
( 0 - 7 )
( 0-7 ) ( 0-7 )
AC[1] [(10+1)*2]-1 = 21
( 0 - 21 )
[(21+1)*2]-1 = 43
( 0 – 31 )
( 0 – 31 )
AC[2] [(15+1)*2]-1 = 31
( 0 – 31 )
[(31+1)*2]-1 = 63
( 0 – 63 )
[(63+1)*2]-1 = 127
( 0 – 127 )
WEP (Wired Equivalent Privacy)
• Optional in WLANS• Uses the RC4 (Rivest Cipher 4) Stream Cipher generated with a 64bit/128 bit Key• Key composed of 24 bit IV (Initialization Vector)• Key = (24 Bit IV, 40 Bit WEP Key) = 64 Bits• Key = (24 Bit IV, 104 Bit WEP Key) = 128 Bits• Goal to provide authentication, confidentiality and data integrity• Secret Key is shared between communicators• The encrypted packet is generated with a bitwise exclusive OR
(XOR) of the original packet and the RC4 stream.• 4-byte Integrity Check Value (ICV) is computed on the original
packet and appended to the end which is also encrypted with the RC4 cipher stream.
• Encryption done only between 802.11 stations.
Encrypted WEP Frame
http://www-106.ibm.com/developerworks/security/library/s-wep/
Encryption / Decryption :
• M – Original Data Frame
• CRC-32 (c) applied to M to obtain c (M)
• c (M) and M are concatenated to get Plain Text P = (M, c (M))
• WEP produces a Key-stream as a function 24 bit IV and 40-bit WEP Key
using RC4; equal to the length of P.
• Key Stream and the Plaintext are XORed to produce the Cipher Text
• The IV is transmitted in the clear (unencrypted)
• The receiver uses the IV and the shared key to decrypt the message
Draw Backs of WEP:
• A number of attacks can be used against WEP
• Passive Attacks based on statistical analysis
• Active Attacks based on known plain text
• WEP relies on a Shared Key to ensure that packets are not
modified in transit.
• There is no discussion on how these keys are distributed and
hence usually a single key is used which is shared amongst
all STA’s and the AP
• Shared Key is long lived – May last a week, month,
even a year or more
• Consider a busy AP which constantly sends packets
of length 1500 bytes at 11Mbps
• Since IV on 24 bits in length and Shared key is
unchanged, IV gets exhausted after
2^24 * (1500 * 8) / (11 * 10^6)
= 18000 secs = 5 hours
• Lucent wireless cards
All in a days work :
PT Key CT CT Key PT
XOR :
0 0 0
0 1 1
1 0 1
1 1 0
• XORing a Bit with itself gives 0
Sender
PT K CT
0 0 0
0 1 1
1 0 1
1 1 0
Receiver
CT K PT
0 0 0
1 1 0
1 0 1
0 1 1
PASSIVE ATTACK
MSG1 K C ( MSG1 )
MSG2 K C ( MSG2 )
• IV repeats generating K
• Identical K used to encrypt MSG1 and MSG2
• Obtain C( MSG1) and C( MSG2) and XOR them
• XORing causes Key Stream to cancel which yields
the XOR of MSG1 and MSG2 i.e. XOR of Plain Text packets
• This XOR can now be used to apply Statistical Analysis
Example :
MSG1 0 0 1 1
MSG2 1 0 1 1
MSG1
PT1 K CT1
0 0 0
0 1 1
1 0 1
1 1 0
MSG2
PT2 K CT2
1 0 1
0 1 1
1 0 1
1 1 0
CT1 XOR CT2
CT1 CT2
0 1 1
1 1 0
1 1 0
0 0 0
MSG1 XOR MSG2
MSG1 MSG2
0 1 1
0 0 0
1 1 0
1 1 0
Apply Statistical analysis on last three bits and educated guess on the rest
Attacker
AP Wired Network
Hi
xx
Active Attack :
• Attacker knows exact plain text for one encrypted packet
• Use this knowledge to construct correct encrypted packet
• Construct a new message , calculate CRC-32 and perform
bit flips on original encrypted packet to change the plaintext
to the new message.
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