Guide to Wireless Communications - Chapter 6: Introduction to Wi-Fi Wireless Local Area Networks

WLAN Components

• Minimal hardware needed for a WLAN:

  • Mobile device or computer

  • Wireless Network Interface Card (WNIC)

  • Access Point (AP)

Wireless Network Interface Card (WNIC)

• A WNIC connects a computer to a network using an antenna.

• When transmitting, WNICs:

  • Modulate data onto RF waves

  • Determine when to send packets

  • Transmit packets

• Mini PCI is a compact version of a standard PCI card.

Access Points (AP)

• Function: Provides wireless LAN devices access to a wired network.

• Components of an AP:

  • Radio transmitter/receiver

  • Antenna

  • RJ-45 wired network interface port

• Characteristics:

  • Range: Approximately 375 feet (115 meters)

  • Supports over 100 users (practically 20-25 for heavy usage)

  • Uses Dynamic Rate Selection (DRS) to adjust transmission data rate based on signal quality

  • Power over Ethernet (PoE) allows power delivery through Ethernet cables.

WLAN Operating Modes

• Ad hoc mode (also known as Independent Basic Service Set - IBSS):

  • Devices connect directly without an AP.

  • Useful for sharing an Internet connection among devices.

• Infrastructure mode (Basic Service Set - BSS):

  • Involves wireless clients connecting through an AP.

  • If more than one AP is used on the same SSID, it's called an Extended Service Set (ESS).

  • Each AP has a unique MAC address called the Basic Service Set Identifier (BSSID).

IEEE 802.11 WLAN Standards

• Defines a local area network with cable-free data access at speeds of 1 or 2 Mbps (using RF or infrared).

• Transmission methods include:

  • Frequency Hopping Spread Spectrum (FHSS)

  • Direct Sequence Spread Spectrum (DSSS)

• Frame formatting is managed in the PHY and MAC layers.

IEEE 802.11b Standard

• Enhancements included support for speeds of 5.5 Mbps and 11 Mbps.

• Encoding and modulation methods specified:

  • Radio Frequency (RF), DSSS, and Complementary Code Keying (CCK).

• Frame structure includes:

  • Preamble

  • Header

  • Data

• Preamble and header are transmitted at 1 Mbps.

Power Management Features

• Functionality: Allows WNICs in WLANs to conserve power.

  • Clients can go into sleep mode to save energy.

  • AP buffers frames for clients in sleep mode, indicated by Traffic Indication Map (TIM).

Coordinating Communications

• Devices share the same frequency channel and must manage data transmission to prevent collisions:

  • Distributed Coordination Function (DCF): Prevents collisions by requiring devices to listen before sending.

  • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA): Involves random backoff periods to reduce the likelihood of collisions.

  • Request-to-Send/Clear-to-Send (RTS/CTS) helps manage medium reservations.

  • Fragmentation Breaks larger frames into smaller ones for transmission to improve reliability.

Association and Reassociation

• Association Process:

  1. Passive Scanning: Device listens for beacon frames.

  2. Active Scanning: Device sends probe frames and awaits responses.

  3. Device sends an associate request to join the WLAN (includes capabilities).

• Reassociation enables mobile clients to switch APs for better coverage. It requires:

  • Sending a reassociate request to a new AP

  • The new AP sending a disassociate message to the previous AP.

MAC Frame Formats

• WLAN communication involves three types of frame formats:

  • Management Frames: Establish connections.

  • Control Frames: Aid in delivering data frames.

  • Data Frames: Carry actual information to clients.

Interframe Spaces (IFS)

• IFS are timing gaps between transmissions to manage access to the medium:

  • Short IFS (SIFS): Time to allow signals to decode.

  • DCF IFS (DIFS): Time for devices to wait before transmitting after a data frame.
    -Following transmission, devices must listen for ACK and manage their transmission attempts based on the reception of ACKs.