802.11 Tutorial
802.11 MAC Entity:
MAC Basic Access Mechanism
Privacy and Access Control
802.11 Protocol Entities
Main Requirements
•	Single MAC to support multiple PHYs.
–	Support single and multiple channel PHYs.
–	and PHYs with different  Medium Sense  characteristics
•	Should allow overlap of multiple networks in the same area and channel space.
–	Need to be able to share the medium.
–	Allow re-use of the same medium.
•	Need to be Robust for Interference.
–	Microwave interferers
–	Other un-licensed spectrum users
–	Co-channel interference
•	Need mechanisms to deal with Hidden Nodes.
•	Need provisions for Time Bounded Services.
•	Need provisions for Privacy and Access Control.
Basic Access Protocol Features
•	Use Distributed Coordination Function (DCF) for efficient medium sharing without overlap restrictions.
–	Use CSMA with Collision Avoidance derivative.
–	Based on Carrier Sense function in PHY called Clear Channel Assessment (CCA).
•	Robust for interference.
–	CSMA/CA + ACK for unicast frames,  with MAC level recovery.
–	CSMA/CA for Broadcast frames.
•	Parameterized use of RTS / CTS to provide a Virtual Carrier Sense function to protect against  Hidden Nodes.
–	Duration information is distributed by both transmitter and receiver through separate RTS and CTS Control Frames.
•	Includes fragmentation to cope with different PHY characteristics.
•	Frame formats to support the access scheme
–	For Infrastructure and Ad-Hoc Network support
–	and  Wireless Distribution System.
CSMA/CA Explained
•	Reduce collision probability where mostly needed.
–	Stations are waiting for medium to become free.
–	Select Random Backoff after a Defer, resolving contention to avoid collisions.
•	Efficient Backoff algorithm stable at high loads.
–	Exponential Backoff window increases for retransmissions.
–	Backoff timer elapses only when medium is idle.
•	Implement different fixed priority levels.
–	To allow immediate responses and PCF coexistence.
CSMA/CA + ACK protocol
Throughput Efficiency
“Hidden Node” Problem
“Hidden Node” Provisions
RTS/CTS Overhead Impact
Optional Point Coordination Function (PCF)
Contention Free operation
PCF Burst
Fragmentation
•	Burst of Fragments which are individually acknowledged.
–	For Unicast frames only.
•	Random backoff and retransmission of failing fragment when no ACK is returned.
•	Duration information in data fragments and Ack frames causes NAV to be set, for medium reservation mechanism.
Frame Formats
•	MAC Header format differs per Type:
–	Control Frames (several fields are omitted)
–	Management Frames
–	Data Frames
•	Includes Sequence Control Field for filtering of duplicate caused by ACK mechanism.
Address Field Description
•	Addr 1 = All stations filter on this address.
•	Addr 2 = Transmitter Address (TA)
–	Identifies transmitter to address the ACK frame to.
•	Addr 3 = Dependent on To and From DS bits.
•	Addr 4 = Only needed to identify the original source of  WDS (Wireless Distribution System) frames.
Privacy and Access Control	
•	Goal of 802.11 is to provide “Wired Equivalent Privacy” (WEP)
–	Usable worldwide
•	802.11 provides for an Authentication mechanism
–	To aid in access control.
–	Has provisions for “OPEN”, “Shared Key” or proprietary authentication extensions.
•	Optional (WEP) Privacy mechanism defined by 802.11. 
–	Limited for Station-to-Station traffic, so not “end to end”.
»	Embedded in the MAC entity.
–	Only implements “Confidentiality” function.
–	Uses RC4 PRNG algorithm based on:
»	a 40 bit secret key (No Key distribution standardized)
»	and a 24 bit IV that is send with the data.
»	includes an ICV to allow integrity check.
–	Only payload of Data frames are encrypted.
»	Encryption on per MPDU basis.
Privacy Mechanism
•	WEP bit in Frame Control Field indicates WEP used.
–	Each frame can have a new IV, or IV can be reused for a limited time.
–	If integrity check fails then frame is ACKed but discarded.
Support for Mobility
MAC Management Layer
•	Synchronization
–	finding and staying with a WLAN
–	Synchronization functions
»	TSF Timer, Beacon Generation
•	Power Management
–	sleeping without missing any messages
–	Power Management functions
»	periodic sleep, frame buffering, Traffic Indication Map
•	Association and Reassociation
–	Joining a network
–	Roaming, moving from one AP to another
–	Scanning
•	Management Information Base
Synchronization in 802.11
•	Timing Synchronization Function (TSF)
•	Used for Power Management
–	Beacons sent at well known intervals
–	All station timers in BSS are synchronized
•	Used for Point Coordination Timing
–	TSF Timer used to predict start of Contention Free burst
•	Used for Hop Timing for FH PHY
–	TSF Timer used to time Dwell Interval
–	All Stations are synchronized, so they hop at same time.
Synchronization Approach
•	All stations maintain a local timer.
•	Timing Synchronization Function
–	keeps timers from all stations in synch
–	AP controls timing in infrastructure networks
–	distributed function for Independent BSS 
•	Timing conveyed by periodic Beacon transmissions
–	Beacons contain Timestamp for the entire BSS
–	Timestamp from Beacons used to calibrate local clocks
–	not required to hear every Beacon to stay in synch
–	Beacons contain other management information
»	also used for Power Management, Roaming
Infrastructure Beacon Generation
Power Management
•	Mobile devices are battery powered.
–	Power Management is important for mobility.
•	Current LAN protocols assume stations are always ready to receive.
–	Idle receive state dominates LAN adapter power consumption over time.
•	How can we power off during idle periods, yet maintain an active session?
•	802.11 Power Management Protocol:
–	allows transceiver to be off as much as possible
–	is transparent to existing protocols
–	is flexible to support different applications
»	possible to trade off throughput for battery life
Power Management Approach
•	Allow idle stations to go to sleep
–	station’s power save mode stored in AP
•	APs buffer packets for sleeping stations.
–	AP announces which stations have frames buffered
–	Traffic Indication Map (TIM) sent with every Beacon
•	Power Saving stations wake up periodically
–	listen for Beacons
•	TSF assures AP and Power Save stations are synchronized
–	stations will wake up to hear a Beacon
–	TSF timer keeps running when stations are sleeping
–	synchronization allows extreme low power operation
•	Independent BSS also have Power Management
–	similar in concept, distributed approach
Infrastructure Power Management
•	Broadcast frames are also buffered in AP.
–	all broadcasts/multicasts are buffered
–	broadcasts/multicasts are only sent after DTIM
–	DTIM interval is a multiple of TIM interval
Infrastructure Power Management
•	Broadcast frames are also buffered in AP.
–	all broadcasts/multicasts are buffered
–	broadcasts/multicasts are only sent after DTIM
–	DTIM interval is a multiple of TIM interval
•	Stations wake up prior to an expected (D)TIM.
Infrastructure Power Management
•	Broadcast frames are also buffered in AP.
–	all broadcasts/multicasts are buffered
–	broadcasts/multicasts are only sent after DTIM
–	DTIM interval is a multiple of TIM interval
•	Stations wake up prior to an expected (D)TIM.
•	If TIM indicates frame buffered 
–	station sends PS-Poll and stays awake to receive data
–	else station sleeps again
Wireless LAN Infrastructure Network
Roaming
Roaming
Roaming
Roaming
Roaming Approach
•	Station decides that link to its current AP is poor
•	Station uses scanning function to find another AP
–	or uses information from previous scans
•	Station sends Reassociation Request to new AP
•	If Reassociation Response is successful 
–	then station has roamed to the new AP
–	else station scans for another AP
•	If AP accepts Reassociation Request
–	AP indicates Reassociation to the Distribution System
–	Distribution System information is updated
–	normally old AP is notified through Distribution System
Scanning
•	Scanning required for many functions.
–	finding and joining a network
–	finding a new AP while roaming
–	initializing an Independent BSS (ad hoc) network
•	802.11 MAC uses a common mechanism for all PHY.
–	single or multi channel
–	passive or active scanning
•	Passive Scanning
–	Find networks simply by listening for Beacons
•	Active Scanning
–	On each channel
»	Send a Probe, Wait for a Probe Response
•	Beacon or Probe Response contains information necessary to join new network.
Active Scanning Example
Active Scanning Example
Active Scanning Example
Active Scanning Example
Active Scanning Example
Active Scanning Example
MAC Management Frames
•	Beacon
–	Timestamp, Beacon Interval, Capabilities, ESSID, Supported Rates, parameters
–	Traffic Indication Map
•	Probe
–	ESSID, Capabilities, Supported Rates
•	Probe Response
–	Timestamp, Beacon Interval, Capabilities, ESSID, Supported Rates, parameters
–	same for Beacon except for TIM
•	Association Request
–	Capability, Listen Interval, ESSID, Supported Rates
•	Association Response
–	Capability, Status Code, Station ID, Supported Rates
More MAC Management Frames
•	Reassociation Request
–	Capability, Listen Interval, ESSID, Supported Rates, Current AP Address
•	Reassociation Response
–	Capability, Status Code, Station ID, Supported Rates
•	Disassociation
–	Reason code
•	Authentication
–	Algorithm, Sequence, Status, Challenge Text
•	Deauthentication
–	Reason
802.11 MAC
the end…