IEEE 802.11 Wi-Fi Standards and Architecture

Study Notes

IEEE 802.11 is a standard for Wireless Local Area Networks (WLANs), commonly known as Wi-Fi.
Data transmission uses the electromagnetic spectrum, including radio waves and infrared.
It operates on a license-free ISM band of 2.4 GHz.
It's a supporting technology for wired networks, and not a replacement
802.11 supports two main architectures: infrastructure mode and ad-hoc mode.

Basic Service Set (BSS): a fundamental unit of a Wi-Fi network.
- Infrastructure: Stations communicate through an access point (AP).
- Ad-hoc: Stations communicate directly with each other without an AP.
Extended Service Set (ESS): Multiple BSSs connected through a distribution system form an ESS
- Distribution system is used to connect multiple BSS, to form an ESS.
Station Types: Classified based on mobility.
- No transition station: doesn't move between BSS or ESS
- BSS transition station: moves between BSSs but stays within a single ESS
- ESS transition station: moves between different ESSs.
Connection not guaranteed during movement: Communication may be disrupted or delayed during a station's movement

Active Scanning: A station actively probes to discover available networks. Sent probe request/response frames.
Passive Scanning: A station passively listens for beacon frames from networks. Sent association request/response frames after it finds a network it likes.

Different amendments are available, based on bandwidth.
Encoding schemes include FHSS (Frequency Hopping Spread Spectrum) and DSSS (Direct Sequence Spread Spectrum), and OFDM (Orthogonal Frequency Division Multiplexing)
Speeds: 1, 2, 5.5, 11, and 54 Mbps.
Range: ~250 meters.

Distributed Coordination Function (DCF): A contention-based access method using CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). It's mandatory.
Point Coordination Function (PCF): A contention-free access method used in infrastructure mode. It's optional

Multiple stations compete to access the carrier using CSMA/CA.
Collision Avoidance is used to prevent collisions on the network.
Reasons for not using Collision Detection: Stations are half-duplex, signal strength is very low, and hidden station problem.
CSMA/CA Process: Sender checks for carrier, waits for DIFS, sends RTS, waits for SIFS, receives CTS, waits for SIFS, sends data, waits for SIFS, receives acknowledgment.

Hidden Terminal Problem: Two stations are hidden from each other, and can transmit simultaneously without detecting each other. CSMA/CA solves by using RTS and CTS frames.
Exposed Terminal Problem: A station is exposed to the transmission of two other stations. If the exposed station waits for the transmission between the other two stations to complete, it can transmit data immediately without colliding with either. CSMA/CA solves this problem by using RTS and CTS frames.

Piconet: A master device and up to 7 slave devices communicate directly. Slaves can either be active or parked
Scatternet: Multiple piconets interconnected to allow communication between devices in different piconets.