IEEE 802.11 Wireless LANs Notes

IEEE 802.11 - BSS

• IEEE 802.11 working group developed most prominent WLAN spec.

• IEEE 802 physical layer divided into:

  • PLCP: Maps 802.11 MAC layer protocol data units (MPDUs) into a framing format for data and management info.

  • PMD: Defines characteristics for transmitting/receiving user data through wireless medium.

OSI vs. IEEE 802 Reference Model

• OSI Model: Application, Presentation, Session, Transport, Network, Data Link, Physical.

• IEEE 802 Model: Upper-layer protocols, LLC, MAC, PLCP, PMD.

• IEEE 802 focuses on Data Link and Physical layers.

802.11 Standards

• Define radio frequencies for wireless links.

• Most standards use one antenna; newer ones use MIMO for higher speeds.

• MIMO uses multiple antennas to improve performance; up to four antennas supported.

IEEE WLAN Standards

• 802.11: 2.4 GHz, up to 2 Mbps.

• 802.11a: 5 GHz, up to 54 Mbps, smaller coverage, not compatible with 802.11b/g.

• 802.11b: 2.4 GHz, up to 11 Mbps, longer range, better penetration.

• 802.11g: 2.4 GHz, up to 54 Mbps, backward compatible with 802.11b.

• 802.11n: 2.4 GHz/5 GHz, 150-600 Mbps, up to 70m range, MIMO, backward compatible with 802.11a/b/g.

• 802.11ac: 5 GHz, 450 Mbps-1.3 Gbps, MIMO, up to eight antennas, backward compatible with 802.11a/n.

• 802.11ax: 2.4 GHz/5 GHz, released in 2019, High-Efficiency Wireless (HEW), higher data rates, improved power efficiency (Wi-Fi 6).

IEEE 802.11 Terminology

• AP: Provides access to distribution system for associated stations.

• BSS: Set of stations controlled by coordination function.

• Coordination Function: Determines when station can transmit/receive.

• DS: Interconnects BSSs and integrated LANs to create an ESS.

• ESS: Interconnected BSSs and LANs appearing as single BSS to LLC layer.

• MPDU: Data unit exchanged between MAC entities.

• MSDU: Information delivered between MAC users.

• Station: Device with IEEE 802.11 MAC and physical layer.

Wi-Fi Alliance

• WECA (now Wi-Fi Alliance) formed in 1999 to certify interoperability for 802.11b products.

• Certified 802.11b products are called Wi-Fi.

• Certification extended to 802.11g and 802.11a (Wi-Fi5) products.

• Focus areas: enterprise, home, hot spots.

IEEE 802.11 Architecture

• BSS: Smallest WLAN building block with stations executing same MAC protocol and competing for shared medium.

  • Connects to DS via AP.

  • Stations don't communicate directly; frames go through AP.

IEEE 802.11 Architecture - Extended Service Set

• ESS with multiple BSSs connected via DS.

• Stations communicate through APs.

• DS connects to IEEE 802.x LAN via portal.

IEEE 802.11 Architecture - BSS Overlap

• Each station belongs to single BSS.

• BSSs may overlap geographically.

• Association between station and BSS is dynamic.

IEEE 802.11 Architecture - Independent BSS (IBSS)

• IBSS: All stations are mobile with no connection to other BSSs (ad hoc network).

  • Stations communicate directly, no AP involved.

IEEE 802.11 Architecture - Extended Service Set (ESS) Details

• ESS: Two or more BSSs interconnected by DS.

  • Appears as single logical LAN to LLC level.

  • AP provides access to DS.

• Portal: Integrates IEEE 802.11 with wired LAN, implemented in bridge/router attached to DS.

IEEE 802.11 Services

• Defines nine services for WLAN functionality equal to wired LANs.

• Categorized by service provider (station or DS) and function (access/security or MSDU delivery).

• MSDU: Data block passed from MAC user to MAC layer.

  • Fragmented if too large for single MAC frame.

IEEE 802.11 Services - Categories

Distribution of Messages Within a DS

• Distribution: Primary service for stations to exchange MAC frames across DS.

• Integration: Enables data transfer between 802.11 LAN and 802.x LAN.

  • IS handles address translation and media conversion.

Association Related Services

• MAC layer transfers MSDUs via distribution service (DS).

• DS requires info about stations within ESS, provided by association services.

• Stations must associate before communicating.

• Transition types: no transition, BSS transition, ESS transition.

Station Location

• DS needs to know AP to deliver messages to.

• Station must maintain association with AP.

• Services: Association, Reassociation, Disassociation.

Access and Privacy Services - Authentication

• Authentication establishes station identity.

• 802.11 supports multiple authentication schemes.

• Mutual authentication required before association.

Access and Privacy Services - Deauthentication and Privacy

• Deauthentication terminates existing authentication.

• Privacy uses encryption to prevent message reading.

IEEE 802.11 Medium Access Control

• MAC layer covers reliable data delivery, medium access control, security.

Reliable Data Delivery

• 802.11 physical and MAC layers are subject to unreliability.

• 802.11 includes frame exchange protocol with ACK frames.

Four Frame Exchange

• Basic data transfer involves 2 frames.

• Enhanced reliability uses 4-frame exchange (RTS, CTS, data, ACK).

Media Access Control

• Two proposal types: Distributed access protocols and Centralized access protocols

Media Access Control - DWFMAC

• MAC algorithm is Distributed Wireless Foundation MAC (DWFMAC).

  • DCF: Contention algorithm for all traffic.

  • PCF: Centralized MAC algorithm for contention-free service.

Distributed Coordination Function

• DCF uses CSMA.

  • No collision detection (CSMA/CD) due to signal dynamic range.

• DCF includes delays (IFS) for smooth functioning.

Interframe Space

• IFS values: SIFS, PIFS, DIFS.

• Rules: sense medium, wait for IFS, back off random time if needed.

Priority - IFS Values

• SIFS: Shortest, highest priority, immediate response.

• PIFS: Midlength, used by PCF for issuing polls.

• DIFS: Longest, minimum delay for asynchronous frames.

SIFS Use - ACK

• SIFS gives highest priority.

• Used for ACK, MAC-level recovery, and efficient delivery of multiple frame LLC PDUs.

Point Coordination Function (PCF)

• Alternative access method on top of DCF.

• Uses polling by centralized polling master (point coordinator) using PIFS.

802.11 Frame Structure

• Layer 2 frames: header, payload, FCS.

• Similar to Ethernet frame format with more fields.

802.11 Frame Structure - Fields

• Frame Control, Duration, Address1, Address2, Address3, Sequence Control, Address4, Payload, FCS.

Wireless Client and AP Association

• Three-stage process: Discover AP, Authenticate with AP, Associate with AP.

Wireless Client and AP Association - Specific Parameters

• SSID, Password, Network mode, Security mode, Channel settings.

Passive and Active Discover Mode

• Wireless clients connect using passive or active scanning.

Passive Mode

• AP broadcasts beacon frames with SSID, standards, security settings.

Active Mode

• Client broadcasts probe request with SSID; AP responds with probe response.

802.11 Physical Layer

• Four stages: 1997 (original), 1999 (802.11a/b), 2002 (802.11g).

Physical Layer Standards

• IR, FHSS, DSSS, OFDM.

Frequency Channel Saturation

• Occurs when demand for specific channel is too high.

Frequency Channel Saturation Techniques

• DSSS, FHSS, OFDM.

Direct-Sequence Spread Spectrum (DSSS)

• Spreads signal over larger frequency band.

• Used by 802.11b to avoid interference.

Frequency-Hopping Spread Spectrum (FHSS)

• Rapidly switches carrier signal among frequency channels.

• Used by original 802.11 standard.

Orthogonal Frequency-Division Multiplexing (OFDM)

• Uses multiple sub-channels on adjacent frequencies.

• Used by 802.11a/g/n/ac; 802.11ax uses OFDMA.