802.11 Architecture
What is unique about wireless?
•	Difficult media
–	interference and noise
–	quality varies over space and time
–	shared with “unwanted” 802.11 devices
–	shared with non-802 devices (unlicensed spectrum, microwave ovens)
•	Full connectivity cannot be assumed
–	“hidden node” problem
•	Multiple international regulatory requirements
Medium Variations
Uniqueness of Wireless (continued)
•	Mobility
–	variation in link reliability
–	battery usage: requires power management
–	want “seamless” connections
•	Security
–	no physical boundaries
–	overlapping LANs
Requirements
•	Single MAC to support multiple PHYs.
–	Support single and multiple channel PHYs.
–	PHYs with different “Medium Sense” characteristics.
•	Should allow overlap of multiple networks in the same area and channel space.
•	Need to be “Robust for Interference”.
–	Microwave, other non-802.11 interferers.
–	Co-channel interference.
•	Need mechanisms to deal with “Hidden Nodes”.
•	Need provisions for Time Bounded Services.
Architecture Overview
•	One MAC supporting multiple PHYs
–	currently Frequency Hopping, Direct Sequence and Infrared PHYs
•	Two configurations 
–	“Independent” (ad hoc) and “Infrastructure”
•	CSMA/CA (collision avoidance) with optional “point coordination”
802.11 Protocol Entities
802.11 Protocol Architecture
•	MAC Entity
–	basic access mechanism
–	fragmentation
–	encryption
•	MAC Layer Management Entity
–	synchronization
–	power management
–	roaming
–	MAC MIB
•	Physical Layer Convergence Protocol (PLCP)
–	PHY-specific, supports common PHY SAP
–	provides Clear Channel Assessment signal (carrier sense)
802.11 Protocol Architecture (cont.)
•	Physical Medium Dependent Sublayer (PMD)
–	modulation and encoding
•	PHY Layer Management
–	channel tuning
–	PHY MIB 
•	Station Management
–	interacts with both MAC Management and PHY Management
802.11 Configurations - Independent
•	Independent
–	one “Basic Service Set”, BSS
–	“Ad Hoc” network
–	direct communication
–	limited coverage area
802.11 Configurations - Infrastructure
•	Infrastructure
–	Access Points and stations
•	Distribution System interconnects Multiple Cells via Access Points to form a single Network.
–	extends wireless coverage area
Distribution System
•	Used to interconnect wireless cells 
–	multiple BSS connected together form an ESS, Extended Service Set
–	Allows mobile stations to access fixed resources
•	Not part of 802.11 standard
–	could be bridged IEEE LANs, wireless, other networks …
–	Distribution System Services are defined
Access Points
•	Stations select an AP and “associate” with it
•	Support roaming 
•	Provide other functions
–	time synchronization (beaconing)
–	power management support
–	point coordination function
•	Traffic typically (but not always) flows through AP
–	direct communication possible
802.11 Defines the Airwaves IF
•	The airwaves interface between stations (including that between station and AP) is standardized
–	PHY and MAC
•	No exposed MAC/PHY interface specified
•	No exposed interface to Distribution System
–	required DS services are defined
•	Internals of Distribution System not defined
MAC Services
•	Asynchronous MSDU Data Delivery
–	provided to LLC (2304 octet maximum)
•	Time Bounded Services
–	optional point coordination function
•	Security Services
–	confidentiality, authentication, access control
•	Management Services
–	scanning, joining, power management
MAC Functionality
•	Independent and Infrastructure configuration support
–	Each BSS has a unique 48 bit address
–	Each ESS has a variable length address
•	CSMA with collision avoidance
–	MAC-level acknowledgment
–	allows for RTS/CTS exchanges
»	hidden node protection
–	MSDU fragmentation
–	“Point Coordination” option
»	AP polling
MAC Functionality (continued)
•	Roaming support within an ESS
–	station scans for APs, association handshakes
•	Power management support
–	stations may power themselves down
–	AP buffering, distributed approach for IBSS
•	Authentication and privacy
–	Optional support of “Wired Equivalent Privacy” (WEP)
–	Authentication handshakes defined
PHY Layer Services
•	PHY_DATA transfers
–	multiple rates

•	Clear Channel Assessment (CCA)
–	carrier sense

•	PHY Management
–	channel tuning
Three PHYs
•	Frequency Hop Spread Spectrum
–	2.4 GHz band, 1 and 2 Mbps transmission
–	2GFSK, 4GFSK 
–	hop over 79 channels (North America)
•	Direct Sequence Spread Spectrum
–	2.4 GHz band, 1 and 2 Mbps transmission 
–	DBPSK, DQPSK
–	11 chip Barker sequence
•	Baseband IR
–	Diffuse infrared
–	1 and 2 Mbps transmission, 16-PPM and 4-PPM