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802.11: Introduction

This article serves as an introduction to the IEEE 802.11 wireless LAN standard, detailing its architecture and the critical services it offers to facilitate communication between devices. Key concepts covered include association and disassociation processes, authentication methods, data distribution across different Basic Service Sets (BSS), and privacy measures like WEP encryption. The paper also discusses MAC layer operations, including the timing mechanisms and frame formats vital for managing wireless communications. Understanding these elements is essential for optimizing wireless network performance and security.

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802.11: Introduction

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  1. 802.11: Introduction Reference:“IEEE 802.11: 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

  2. 802.11 Terminology

  3. 802.11 Architecture

  4. 802.11 Architecture (cont)

  5. 802.11 Services Implemented in every 802.11 station Provided between BSS ( in an AP)

  6. 802.11 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

  7. 802.11 Services (cont) • Authentication • Stations must use an authentication service to establish their identity with other stations • IEEE 802.11 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

  8. 802.11 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 802.11 LAN an a station on an integrated IEEE 802.x LAN

  9. 802.11 Services (cont) • Privacy • To ensure privacy, IEEE 802.11 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 802.11 vendors offer optional 128-bit encryption

  10. 802.11 Protocol Architecture

  11. 802.11 Protocol Architecture (cont)

  12. IEEE 802.11 MAC Logic

  13. IEEE 802.11 MAC Timing Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA)

  14. IEEE 802.11 MAC Timing (cont) • 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

  15. IEEE 802.11 MAC Timing (cont) • 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

  16. IEEE 802.11 MAC Timing (cont)

  17. IEEE 802.11 MAC Timing (cont)

  18. IEEE 802.11 MAC Timing (cont)

  19. 802.11 MAC Frame Format

  20. 802.11 Physical Layer Spec.

  21. 802.11 Physical Layer Spec. (cont)

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