The document provides an overview of the IEEE 802.11 MAC protocol through a presentation. It discusses topics such as IEEE 802.11 layers, channels, infrastructure networks, ad hoc networks, joining a network, synchronization, communication approaches, MAC functionality including PCF and DCF, encryption, fragmentation, management functions, and MAC frame formats. The presentation was given on May 9th, 2001 by Mahdi Ahmed Jama to provide an introduction to the IEEE 802.11a MAC protocol.

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Welcome
Presentation of the IEEE 802.11a MAC
May 9th
2001 Mahdi Ahmed Jama

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IEEE 802.11 MAC Overview
 Introduction
Communication Approach
MAC Functionality
MAC Frame Formats
 Conclusion

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IEEE 802.11 MAC Overview
 Introduction
Layers description
Sublayers description
IEEE 802.11a Channels
Infrastructure
Ad hoc Network
 Communication Approach
 MAC Functionality
 MAC Frame Formats
 Conclusion

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IEEE 802.11 Layers Description
802.2 Logical Link ControlData
Link
Layer
MAC 802.11
802.11
e
Prcl 802.11 std 802.11b 802.11a
Tech IR FHSS DSSS DSSS OFDM
Band ~350 103
GHz
2.4GHz Band 5GHz
Band
PHY
Layer
Data
Rate 1 and 2 Mbps
1, 2,
5.5, 11
Mbps
6*,9,12*,
18, 24*,
36, 48 or
54 Mbps
Same
PHY
as
802.11
MAC
*Mandatory

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Sublayers Description
MAC
Sublayer
MAC layer
management
PLCP Sublayer
PMD Sublayer
PHY layer
management
LLC
MAC
PHY
PLCP: Physical Layer Convergence Protocol
PMD: Physical Medium Dependent
SAP (Service Access Point)

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IEEE 802.11a Channels
 Channel Center Frequency (CCF) at every 5 MHz
between 5GHz and 6GHz
 CCF = 5000 + 5*NCH (MHz)
 NCH = 1, 2 …. 200
 Only some channels are available according to
the Country regulation

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Example: US (FCC regulation)
Band (GHz) Ch. Number Ch. Center
freq. (MHz)
Lower band
5.15- 5.25
36
40
44
48
5180
5200
5220
5240
Middel band
5.25- 5.35
52
56
60
64
5260
5280
5300
5320
Upper Band
5.725- 5.825
149
153
157
161
5745
5765
5785
5805

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Infrastructure Network
BSS 1 BSS 2
wired or wireless DS
ESS
DS= Distributed System
ESS= Extended Service Set
BSS= Basic Service Set

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Ad hoc Network
IBSS
IBSS= Independent Basic Service Set (= ad hoc)

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IEEE 802.11 MAC Overview
 Introduction
 Communication Approach
Joining a BSS
Synchronization
Communication scheme
Hidden Node Problem
 MAC Functionality
 MAC Frame Formats
 Conclusion

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Joining a BSS
 STA (Station) performs scanning function:
 Passive scanning: listen for AP’s Beacons
 Active scanning: send a Probe-Request on each
channel en wait for a Probe-Response
 If AP found
 STA and AP perform Authentication function
 If authentication succeeds
 STA and AP perform Association function

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Synchronization
 A TSF (Timing Synchronization Function) timer
with modulus 264
is used
 TSF timer increments in μsec.
 AP is the timing master
 AP sends periodically a copy of its TSF in a
Beacon Frame
 All STA’ s synchronize their local TSF by
listening to Beacons

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Communication scheme
Medium
Busy
RTS
CTS
RTS
time
CTS
Medium
Busy
Data
ACK

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Hidden Node Problem
RTS-
Range
CTS-
Range
STA3 can’t “hear”
the RTS of STA1 and
sense the medium
free
STA1 STA2
STA3

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IEEE 802.11 MAC Overview
 Introduction
 Communication Approach
MAC Functionality
Point Coordination Function (PCF) -Optional
Distributed Coordination Function (DCF)
Encryption/ Decryption (WEP)
Fragmentation
Management
 MAC Frame Formats
Conclusion

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IEEE 802.11 MAC Functionality
 Point Coordination Function (PCF) -Optional
 Distributed Coordination Function (DCF)
 CSMACA +ACK
 Combination of DCF and PCF
 Encryption/ Decryption (WEP)
 Fragmentation/ Defragmentation
 Management
 Roaming (Mobility)
 Power management

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Point Coordination Function (PCF)
Optional
 QOS Support (not efficiently)
 Based on centralised Control (AP polling, no IBSS
configuration)
 Overlap restrictions
 PCF provides contention-free transfer
 The PC resides in AP
 AP informs STA’ s the beginning of a CFP via a
Beacon

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PCF access mechanism
AP
STA
Beacon
Data+
ACK
Data2+ CF-
ACK CF-Poll
SIFS SIFS= 16μs
SIFS
PIFS=25μsDIFS= 34 μs
NO ACK!
PIFS= 25μs
Data1+
CF-Poll
Data3+ CF-
ACK CF-Poll
Data+
ACK
SIFS
SIFS
CF-
end
SIFS

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Distributed Coordination Function
 Efficient medium sharing without overlap
restrictions.
 Use CSMA/CA +ACK (Unicast frames) based on
Carrier Sense function in PHY called Clear
Channel Assessment (CCA)
 CSMA/(CA) (Broadcast frames and small packets)
 No QOS Support

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CSMA/CA +ACK 1
 Sense medium:
 check state of NAV (Network Allocation Vector),
a Virtual Carrier Sense
 use CCA (Clear Channel Assessment) function
of the PHY
 If medium is free and remains idle for a DIFS
period time, send a RTS
 If CTS send data, expect ACK after SIFS time

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CSMA/CA +ACK 2
Src.
Dest.
Others
RTS
CTS
Data
ACK
SIFS SIFS= 16μs
SIFS
DIFS
NAV (RTS)
NAV (CTS)
Next MPDU
Random value*1 slot time
Backoff
1 slot time= 9μsDefer Access
DIFS= 34 μs
Collision!

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RTS/CTS with fragmented MSDU
RTS
CTS
Frag. 0 Frag. 1 Frag. 2
ACK0 ACK1 ACK2
SRC
Dest
Others NAV (RTS)
NAV (CTS)
NAV (frag.0)
NAV (ACK0)
NAV (frag.1)
NAV (ACK1)
SIFS
SIFS SIFS
SIFS SIFS SIFS SIFS DIFS
NAV update

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Backoff Procedure
 Backoff procedure resolves the medium
contention conflicts
 It is invoked if the medium is busy or a
transmission failed
 Procedure is base on a Backoff algorithm: all
Sta’s defer the access to the medium according
to this algorithm

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Backoff Algorithm
 Backoff Time= Random() * aSlotTime
 aSlotTime= 9µs
 Random()= Pseudorandom integer drawn from a
uniform distribution over the interval [0,CW]
(CW=Contention Window)
 CWmin <= CW <= Cwmax (CWmin = 15 and Cwmax= 1023)
 CW= 2(4+ n)
-1 {thus CW[15, 31, 63, 128, 255, 512, 1023]}
 n= number of retransmission given by the Retry
Counter (see next slide)

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Retry Counters
 All Sta’s maintain a Short Retry Count and a Long
Retry Count for the transmission of an MPDU
 A Short Retry Counter (SRC) counts the number of
retransmission of a short MAC frame (i.e. MAC frame
sent without RTS/CTS)
 A Long Retry Counter (LRC) counts the number of
retransmission of a long MAC frame (i.e. MAC frame
sent with RTS/CTS)
 SRC (resp. LRC) shall be incremented every time
transmission of a short (resp. long) MAC frame fails

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Example
CWmin
CWmax
15
1023
Initial attempt 1st
retransmission
2d 3th
31
63
Default values:
• n=3 max retry for Long frames
• n=6 max retry for short frames

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Encryption (WEP)
 Based on a WEP (wired Equivalent Privacy)
algorithm - optional
 Implementation in Hardware OR Software
 Uses RC4 PRN algorithm based on:
 a 40-bit key
 and a 24bit Initialization Vector (IV, send with data)
 includes an Integrity Check Value (ICV) to allow integrity
check
 Only Data Frame Body (Payload) is encrypted

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Encryption/ Decryption Process
Plain
text
encryption decryption
Shared Secret Key (SSK)
Ciphertext Original
Plaintext
Shared Secret Key (SSK)

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Encryption Block
IV
SSK
RC4
PRNG
Key Sequence
Plain Text
Integrity algorithm
ICV
IV
+
Cipher
text
Message

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Encrypted Payload
IV
4 Bytes
Data (MPDU)
>=1 Byte
ICV
4 Bytes
Encrypted
IV (3bytes)
Pad
6bits
Key ID
2bits
1 byte
Encrypted Frame Body:

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Decryption Block
IV
SSK
RC4
PRNG
Key
Sq
Plain Text
Integrity
algorithm
ICV
IV
+
Cipher
text
Message
ICV’
ICV= ICV’?

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Fragmentation
MSDU
MAC
HDR
Frame
Body
CRC
MAC
HDR
Frame
Body
CRC
MAC
HDR
Frame
Body
CRC

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MAC Management
 Roaming
 Power management
 Authentication/ De-authentication
 Association/ Reassociation/
Disassociation

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Roaming1
BSS 1
BSS 2
Distributed System (DS)
BSS 3

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Roaming2
 Quality of the link with current AP is bad
 Use scanning function to find an AP with better
link quality (or information from previous scans):
 Passive scanning: listen for AP’s Beacons
 Active scanning: send a Probe Request on each
channel en wait for a Probe Response
 Send Reassociation Request to new AP

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Roaming3
 New AP sends Reassociation Response and
indicates the Reassociation to Distributed
System
 Update of Distributed System information
 Disassociation old AP

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Power management
 In Power Save mode idle STA’s go to sleep and
wake up periodically to listen for Beacons
 TSF (Timing Synch. Funct.) keeps running and
assures synchronization with AP
 AP buffers packets for STA’s in PS mode
 AP sends with Beacon the Traffic Indication Map
(TIM) and announces which STA’s have frames
buffered

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IEEE 802.11 MAC Overview
 Introduction
 Communication Approach
MAC Functionality
MAC Frame Formats
General and Data MAC frame Format
Control Frame Formats
Management Frame Format
 Conclusion

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General and Data MAC Frame
Formats
 Length of Frame body (=Payload) is variable
 FCS= Frame Check Sequence containing a 32-bit CRC
Frame
Control
Durati
on/ ID
Frame Body
Sequence
Control
Addr
ess 1
Addr
ess 2
Addr
ess 4
Addr
ess 3
FCS
Bytes:
2 2 6 6 6 2 6 0- 2312 4
M A C H e a d e r

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Frame Control field
Prcl
vers.
Typ
e
More
data
More
Frag
Sub
type
To
DS
Ret
ry
From
DS
Ord
er
Bits: 2 2 4 1 1 1 1 1 1
pwr
mgt
W
EP
1 1
LSB MSB

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Types and Subtypes
 Data frames: Data, Data+ACK
 Control frames: RTS, CTS, ACK, PowerSave-Poll
 Management frames: Beacons, Association
(Request and Response), Disassociation,
Reassociation (Request and Response),
Authentication, De-authentication, Probe (Request
and Response), Announcement Traffic Indication
Message (ATIM)

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Address Field Contents
To DS
From
DS
Addr. 1 Addr. 2 Addr. 3 Addr. 4
0 0 DA SA BSSID N/A
0 1 DA BSSID SA N/A
1 0 BSSID SA DA N/A
1 1 RA TA DA SA
BSSID= Basic Service Set Identifier DA= Destination Address
DS= Distributed System SA= Source Address
RA= Receiver Address TA= Transmitter Address

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Address Field Contents in detail
To DS=0 From DS=0
ad1=DA Ad2=SA Ad3=BSSID ad4=N/A BSS
BSS
To DS=0 From DS=1
ad1=DA Ad2=BSSID Ad3=SA Ad4=N/A
To DS=1 From DS=0
ad1= BSSID Ad2=SA Ad3=DA Ad4=N/A
BSSTo DS=1 From DS=1
ad1=RA Ad2=TA Ad3=DA Ad4=SA

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Sequence Control field
Fragment number (4 bits) Sequence number (12 bits)
LSB MSB

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Fragment number
 A fragment number for each fragment of an MPDU
or MMPDU.
 0 for single or 1st
fragment and increase by 1 for next
fragment
 The number remains constant for retransmission.

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Sequence number
 A Sequence number for every MPDU or MMPDU
 Numbers are assigned from a single modulo 4096
counter, starting at 0 and increase by 1 for next
MPDU
 The number remains constant for retransmission.

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Control Frame Formats
Bytes:
Frame Control Duration RA FCS
2 2 6 4
RTS Frame
Bytes
: Frame Control Duration RA FCS
2 2 6 4
Frame Cntrl Duration RA TA FCS
Bytes: 2 2 6 6 4
MAC Header
CTS Frame
ACK Frame
PowerSave- Poll
Frame
Frame Cntrl AID BSS ID TA FCS
2 2 6 6 4

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Management Frame Format
Frame
Control
Durati
on
Frame Body
Sequence
Control
DA SA
BSS
ID
FCS
Bytes:
2 2 6 6 6 2 0- 2312 4
M A C H e a d e r
The frame body contains the management information
that has to be sent, like Challenge text (e.g.
Authentication frame) or Time Indication Message and
PHY parameters (Beacon frame).

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IEEE 802.11 MAC Overview
 Introduction
 Communication Approach
 MAC Functionality
MAC Frame Formats
 Conclusion

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Conclusion
 IEEE 802.11a uses the same MAC as IEEE 802.11
and the same PHY as Hiperlan2
 Main MAC functions are: DCF, PCF, WEP
encryption, fragmentation, roaming and power
management
 MAC frame has a variable length and consists of a
MAC header(max 30 bytes), a frame body (max
2312 bytes) and a 32-bit CRC

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Conclusion 2
 Advantage:
 High speed wirelessLAN
 Good access scheme for medium sharing (DCF)
 Disadvantage:
 No QOS support (DCF)
 WEP protocol is “easy to crack”
 Solution: IEEE 802.11e (MAC and security
enhancement)

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Thank you for your attention