802.11: Introduction Reference: “IEEE : moving closer to practical wireless LANs”; Stallings, W.; IT Professional, Volume: 3 Issue: 3, May- June 2001; Page(s): 17 –23 (802.11Intro-1.pdf) Chapter 4, sections 2-4, Wireless Communications and Networks, by William Stallings, Prentice Hall

Terminology

Architecture

Architecture (cont)

Services Implemented in every station Provided between BSS ( in an AP)

Services (cont) Association –Before a station can transmit or receive frames on a wireless LAN, it must make its identity and address known –To do so, it establishes an association with an access point –The access point can then communicate this information to other access points, which makes it easier to route and deliver addressed frames Disassociation –Makes it possible for either a station or an access point to notify other access points that an existing association is terminated

Services (cont) Authentication –Stations must use an authentication service to establish their identity with other stations –IEEE does not mandate any particular authentication scheme, which could be anything from relatively unsecure handshaking to public-key encryption –It does specify two authentication algorithms, which vendors can decide to include in their products  Open-system authentication  Shard-key authentication

Services (cont) Distribution –The primary service used by stations to exchange MAC frames when the frame must traverse the DS (Distribution System) to get from a station in one BSS to a station in another BSS Integration –Enables transfer of data between a station on an IEEE LAN an a station on an integrated IEEE 802.x LAN

Services (cont) Privacy –To ensure privacy, IEEE provides for the optional use of encryption by specifying a scheme based on the Wired Equivalent Privacy (WEP) algorithm –To provide both privacy and data integrity, the WEP algorithm uses an encryption scheme based on the RC4 encryption algorithm –The idea in RC4 is that two communicating parties must share a 40-bit key which encrypts and decrypts all frames –For much stronger protections, some vendors offer optional 128-bit encryption

Protocol Architecture

Protocol Architecture (cont)

12 IEEE MAC Logic

13 IEEE MAC Timing Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA)

14 SIFS (short inter-frame space) –The shortest IFS, used for all immediate response actions PIFS (point coordination function IFS) –A mid-length IFS, used by the centralized controller in the PCF scheme when issuing polls DIFS (distributed coordination function IFS) –The longest IFS, used as a minimum delay for asynchronous frames contending for access IEEE MAC Timing (cont)

15 SIFS is used for –Acknowledgment (ACK)  MAC-level ACK provides for efficient collision recovery –Clear to send (CTS)  Sender sends Request to Send (RTS) frame  If receiver is ready to receive, responds with a CTS frame  All other stations defer using the medium until they see a corresponding CTS, or timeout –Poll response  For PCF IEEE MAC Timing (cont)

16 IEEE MAC Timing (cont)

17 IEEE MAC Timing (cont)

18 IEEE MAC Timing (cont)

MAC Frame Format

Physical Layer Spec.

Physical Layer Spec. (cont)