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802.11 MAC Architecture
Module-7
Jerry Bernardini
Community College of Rhode Island
5/25/2017
Wireless Networking
J. Bernardini
1
Presentation Reference Material
• CWNA Certified Wireless Network
Administration Official Study Guide
(PWO-104), David Coleman, David Westcott,
2009, Chapter-9
• The purpose of 802.11 is to transfer
data from computer to another --not to be forgotten
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Bits, Bytes, Octets, Frames, Packets
• Bits =1 or 0
• Bytes = 8 bits
• Octets = 8 bits = Byte
– Octet is used by telecommunication people
– Byte is used by IT people
• Frames = grouping of bits at layer-2
• Packets = grouping of bits at layer-3
• Datagrams = another term for packets
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OSI data flow
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
4
IEEE 802.11 Physical Layer Standards
• IEEE wireless standards follow OSI model, with some modifications
• Data Link layer divided into two sublayers:
– Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow
control
– Media Access Control (MAC) sublayer: Appends physical addresses to frames
• Physical layer divided into two sublayers:
– Physical Medium Dependent (PMD) sublayer: Makes up standards for
characteristics of wireless medium (such as DSSS or FHSS) and defines method for
transmitting and receiving data
– Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic
functions
• Reformats data received from MAC layer into frame that PMD sublayer can transmit
• “Listens” to determine when data can be sent
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
5
Where the IEEE 802.11 Standard Fits
6
Data Link Layer - Physical Layer- Data Units
MSDU (MAC Service Data Unit)
(From upper layers 2304 bytes max)
LLC
MAC
802.2 Logical Link Control
Data Link Layer (Layer-2)
802.11 Media Access Control
MPDU (MAC Protocol Data Unit)
PSDU (PLCP Service Data Unit)
(MPDU = PSDU name change to
PLCP
PHY Layer Convergence Protocol
indicated service needed)
Physical Layer (Layer-1)
PPDU (PLCP Protocol Data Unit)
PMD
Physical Medium Dependent
PHY = Physical Layer
Modulated Radio Signal
Ethernet and 802.11 Frames
• Ethernet Frame
7
1
Preamble
1518
6
6
2
Source
Destination
46 - 1500
4
Data
FCS
Type or Length
Field
Start Of Frame
• Wireless Frame
10 or 18
2
4 or 6
Sync
PLCP Header
Start Of Frame
2
2
6
6
6
Source
Destination
Rec. Adr
Duration ID
Frame Cntrl
2
6
0 - 2304
4
Trans. Adr
Data
FCS
Sequence Cntrl
MAC Packet DATA Unit, (MPDU)
Frame Categories / Types
•
Management Frames
o Beacon Frame
o Probe Frames
o Association Frames… more
•
Control Frames
o RTS and CTS Frames
o ACK – Acknowledgement Frames…
more
•
Data Frames
o Data Payload Frames
Twelve Management Frame Types
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Eight Control Frames
• Used to assist with the delivery of data frames
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Fifteen Data Frames
• The frames that actually carry application data
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IEEE 802.11 Frame Formats
Management
Duration
(2)
Frame
Control
(2)
Control
Data
Frame
Control
(2)
Frame
Control
(2)
Duration
(2)
Des.
Address
(6)
Duration
(2)
Address
1
(6)
Source
Address
(6)
Receiver
Address
(6)
Address
2
(6)
Address
3
(6)
BSSID
(6)
Seq.
Control
(2)
Transmit
Address
( 6)
Frame
Check
Seq.
(6)
Seq.
Control
(2)
Address
4
(6)
Frame
Body
( 1 to
2311)
Data
( 1 to
2311)
Frame
Check
Seq.
(6)
Frame
Check
Seq.
(6)
(Bytes per field)
CWNA Guide to Wireless LANs, Second Edition
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Frame Types and Sizes
Protocol
Data Unit
(PDU)
Frame Type and Vendor Support
MTU
(Bytes)
TCP
Transport maximum segment size
1460
Layer-3 default size
1500
IEEE 802.3 Ethernet default
1500
MPDU
IEEE 802.11 default
1534
MPDU
IEEE 802.11 maximum
2304
MAC
Jumbo Frame
>1500
MAC
Cisco Baby giant
1552
1600
MAC
Cisco Catalyst 4000
9198
9216
MAC
Cisco Catalyst 6000
9216
9234
IP
MAC
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MTU + Overhead
(Bytes)
1518
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Beacons
• Beacon – information frame sent by an AP.
Beacon frames are approximately 50-bytes
with the following information:
–
–
–
–
Timestamp
Beacon Interval
Capability Info
Service Set Identifier
Header
Payload
Beacon Frame
Support Rates
Parameter Sets
Traffic Indication
Map
Trailer
Beacon Management Frame
• A special management frame that is used by a client stations
seeking a wireless network to join.
• Instead of beacon frames a station could use probe request
and probe response frames
• In an ad hoc (IBSS ) wireless network all stations take turns
broadcasting the beacon frame
Beacon
Beacon
S2
S1
AP Control Point
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Passive Scanning (Beacons)
1. Client stations listens for a beacon from an access point (AP)
2. If multiple beacons are received the strongest one is selected
3. The listening station then requests authentication and
association
Beacons
Beacons
S1
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AP Control Point
Wireless Networking
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S2
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Active Scanning (Probes)
• A station could use probe request and probe response frames
Instead of beacon frames
1. Station is configured with SSID and switched to a channel
2. Probe request sent by requesting station
3. All stations that have the same SSID and have normal
configurations respond with a Probe Response frame
• The process also involves waiting for ProbeDelay and
MinChannel Timers
Probe Response
Probe Request
S1
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AP Control Point
Wireless Networking
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S2
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Authentication and Association
•
•
1.
2.
3.
•
Using the IEEE 802.11 State Machine
Stations are in one of three states
Unauthenticated / Unassociated
Authenticated / Unassociated
Authenticated / Associated
You cannot transmit data frames for processing until you are
associated
• You cannot transmit associated frames for processing until
you are Authenticated
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Wireless Connection Process
• 2-Step Connection Process
– Authentication Phase
– Association Phase
Authentication Phase
Association Phase
Wireless Connection Process
• Authentication is the process used by a
station to verify that another station is
approved for communications. This is a
station authentication and not a user
authentication.
• Authentication Step
– Two IEEE 802.11 standards:
• Open System Authentication
• Shared Key Authentication
Authentication
• Authentication - A means to establish or
prove identity; verifying eligibility of users,
devices, or applications.
– Only authorized clients are allowed to gain access
to the network.
• For this level of authentication it applies to
the stations and not the user.
Association
• Association – The binding of a wireless
network client to an Access Point for the
purpose of data transfer. Again the stations
not the user.
Connection State Machine
Unauthenticated and Unassociated
Successful
Authentication
De-authentication
Notification
Authenticated and Unassociated
Successful
Association
Disassociation
Notification
Authenticated and Associated
De-authentication
Notification
Unauthenticated & Unassociated
• In the first state:
– Wireless Client/Node is not connected to the
network.
Wired LAN
Access
Points
Wireless Client
Authenticated & Unassociated
• In the Second State:
– Wireless Client/Node has passed the
authentication process but is not associated with
the AP.
Wired LAN
Association
Response
Association
Request
Wireless Client
Access
Points
Authenticated & Associated
• In the Third State:
– Wireless Client/Node is now connected and
associated with the AP. Data can now be
transferred between the devices.
Wired LAN
Access
Points
Data
Link
Wireless Client
Open System Authentication
• Open System / Null Authentication
–
–
–
–
IEEE 802.11 Default
Authentications based on empty string SSID
Client sends empty string SSID
Receiving station, (AP) sends
acknowledgment
Open System Authentication
Wired LAN
Authentication Management
Frame - SSID
ACK
Wireless Client
Authentication Management
Frame
ACK
Access
Points
Closed System Authentication
• Closed System
–
–
–
–
IEEE 802.11
Authentications based only on SSID
Client sends SSID
Receiving station, (AP) sends
acknowledgment
Shared Key Authentication
• Shared Key
– IEEE 802.11 Wired Equivalent Privacy, (WEP).
– Authentications based on Text and WEP Keys.
– Challenge – Response Scheme
Shared Key Authentication
Wired LAN
Authentication Management
Frame - SSID
Challenge Phrase
Encrypted Phrase
Wireless Client
WEP
Key
Access
Points
Authentication
Secure Channel
WEP
Key
Roaming
• Roaming - The ability for a user to function
when the serving network is different from
their home/associated network. The process
of a client moving seamlessly from one area or
cell to another while maintaining a data link.
Roaming
Wired LAN
Access Points
1
1
2
1
Wireless Clients
Usually a 20%-30% cell overlap
Unauthenticated & Unassociated
• In the first state:
– Wireless Client/Node is not connected to the
network.
Wired LAN
Access
Points
Wireless Client
Authenticated & Unassociated
• In the Second State:
– Wireless Client/Node has passed the
authentication process but is not associated with
the AP.
Wired LAN
Association
Response
Association
Request
Wireless Client
Access
Points
Authenticated & Associated
• In the Third State:
– Wireless Client/Node is now connected and
associated with the AP. Data can now be
transferred between the devices.
Wired LAN
Access
Points
Data
Link
Wireless Client
IEEE 802.11 MAC Functions
•
•
•
•
•
•
•
•
•
•
Scanning- discover AP or BSS
Synchronization- all stations have the same clock
Frame Transmission- rules for frame transfer
Authentication-allow device in network
Association-after authentication associate with AP
Reassociation-roaming and association with new AP
Data Protection-data encryption protects data
Power Management-save power by sleeping transceiver
Fragmentation-breakup frame for efficiency and interfer.
RTS/CTS- solution to hidden node problem
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802.11 Open Association Process
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