WLAN Overview and Design Goals

WLAN Overview

• WLANs utilize radio waves for wireless communication.
• The last user connection is wireless, enabling network access within a building, campus, or specific rooms.
• The core network typically uses cables for higher stability and reliability.

WLAN Goals

• Wireless access flexibility in offices, homes, and manufacturing settings.
• Coverage typically limited to buildings, campuses, or individual rooms.
• Aiming to replace wired networks in offices, enhance connection flexibility (especially for portable devices), and enable ad-hoc communications for meetings.

WLAN Design Goals

• Global functionality of WLAN products for seamless operation.
• Low power consumption for battery-powered devices (power-saving modes and management functions).
• No special permissions or licenses required (license-free band operation).
• Robust transmission technology to ensure reliable data transfer.
• User-friendly design for ease of use and simple management.
• Protection of wired network investments (support the same data types and services).
• Strong security features: data privacy, user profile protection, and low radiation safety.

WLAN Benefits

• Mobility: Increases working efficiency and productivity, extends online periods.
• Installation in challenging areas: Inside buildings, road crossings, and other difficult-to-wire locations.
• Enhanced reliability: More resilient against disaster scenarios (earthquakes, fires).
• Reduced installation time: Faster deployment than wired networks, convenient for users, particularly in challenging installations.
• Long-term cost savings: Generally cheaper than wired networks, easier maintenance, reduced cabling costs, improved efficiency, and accuracy.

WLAN Known Problems

• Wireless link characteristics: Error-prone communication media, higher bit error rate (BER) compared to wired networks.
• IEEE 802.11n standard: Up to 300 Mbit/s data rate (2 spatial streams), 600 Mbit/s (4 spatial streams).
• Carrier-sense multiple access with collision avoidance (CSMA/CA) used for multiple access.

WLAN Standards

• 2.4 GHz and 5 GHz frequency bands are used for wireless LANs.
• 802.11, 802.11b, 802.11g, HiSWANa, 802.11a, HiperLAN2 are common standards with varying data rates.
• HomeRF 2.0, Bluetooth, and HomeRF 1.0 are other technologies with different data rates.
• 802.11e (Quality of Service), 802.11f (Inter Access Point Protocol), 802.11h (Transmit Power Control & Dynamic Frequency Selection), and 802.11i (Security) are additional standards.

Wireless Network Technology Options

• Wireless Personal Area Network (WPAN): IEEE 802.15.1, known as Bluetooth.
• Low-rate WPAN (LR-WPAN): IEEE 802.15.4, known as ZigBee.
• Wireless Local Area Network (WLAN): IEEE 802.11, known as WiFi.
• Wireless Metropolitan Area Network (WMAN): IEEE 802.16, known as WiMAX.

WPAN

• Wireless personal area network (WPAN) utilizes Bluetooth technology for short-range radio communication.
• Short range, low power consumption, and low cost define Bluetooth.
• Bluetooth operates in the 2.4 GHz band, a license-free industrial, scientific, and medical (ISM) frequency range.
• Bluetooth uses frequency-hopping spread spectrum (FHSS) for data transmission, dividing data into packets transmitted on one of 79 distinct Bluetooth channels (each channel has a 1 MHz bandwidth).

Low-rate WPAN (LR-WPAN)

• Designed for low data rates, extended battery life (months or years), and low-complexity applications.
• Differences lie in the physical and link layers, with distinct media and access control mechanisms.

IEEE 802.11 Terminology

• Basic Service Set (BSS): Group of stations sharing the same radio frequency.
• Access Point (AP): Station integrated into the WLAN and the distribution system.
• Station (STA): Terminal with wireless medium access and radio contact to the access point.
• Portal: Bridge to other (wired) networks.
• Distribution System (DS): Interconnection network forming a logical network.
• Extended Service Set (EES): Based on multiple BSSs.

Distribution System (DS)

• Interconnects wireless cells (multiple BSSs to form an ESS).
• Enables multiple mobile stations to access fixed resources.
• Interconnects 802.11 technology.

Access Points (AP)

• Bridges wireless LAN traffic to the wired LAN.
• Acts as a repeater for wireless nodes, extending the maximum distance between nodes.
• Allows stations to associate with it.
• Supports Distributed Coordination Function (DCF) and Point Coordination Function (PCF).
• Provides management features: Joining/Associating with BSS, time synchronization (beaconing), and power management.
• All traffic flows through APs.
• Supports roaming.

BSS, ESS, and DS

• BSS (Basic Service Set): A group of stations connected to the same access point, sharing the same wireless channel.
• ESS (Extended Service Set): Multiple BSSs interconnected to form a larger network, allowing roaming between APs.
• DS (Distribution System): The infrastructure that connects and manages multiple BSSs in an ESS.

Physical Layer

• Wireless transmission via infrared (IR) or radio frequency (RF).
• Radio frequency: Typically using the license-free 2.4 GHz band.
• Advantages: Experience from wireless WAN (microwave links) and mobile phones, coverage of larger areas, higher transmission rates (~11-54 Mbit/s).
• Disadvantages: Limited license-free frequency bands, shielding challenges, interference from other senders or electrical devices.

Media Access Control (MAC)

• The MAC sublayer and logical link control (LLC) sublayer together form the data link layer.
• Provides flow control and multiplexing for the transmission medium, enabling multiple network nodes to communicate.
• Two basic access mechanisms defined for IEEE 802.11:
  • CSMA/CA (mandatory), known as distributed coordination function (DCF).
  • Point Coordination Function (PCF), a contention-free polling method for time-bounded service.

DCF and PCF

• DCF provides asynchronous service.
• PCF offers both asynchronous and time-bounded service, but requires the access point to control medium access and avoid contention.
• DCF is also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol:
  • An asynchronous access method based on contention for shared channels.
  • A contention-free access mechanism is provided through RTS/CTS (Request to Send/Clear to Send) exchange.
• PCF is used for time-bounded data transfer with the access point polling terminals based on a list.

Carrier Sense Multiple Access (CSMA)

• Carrier refers to a shared transmission medium.
• Sense implies nodes can listen and detect activity on the medium.
• Multiple Access indicates all nodes have the right to access the shared medium.
• CSMA means a node can detect activity on the transmission medium (regardless of the medium type).
• If a node detects ongoing activity, it waits (backoff) for a random time before checking again.

Hidden Station Problem

• When a node (A) is communicating with another node (B), a third node (C) might be hidden from A's perspective but within range of B.
• Node C might start transmitting to B without being aware of the ongoing communication between A and B, leading to collisions.

Collisions in WLAN

• Occur when two or more nodes transmit simultaneously.
• A key difference between WLAN and LAN protocol design is the difficulty in detecting all collisions in WLANs due to:
  • Weak received signals (fading) making it difficult to sense collisions while transmitting.
  • With receiver and transmitter antennas close together, a station cannot see any signal except its own.
  • As a result, the complete packet is sent before an incorrect checksum reveals a collision.
  • Receiver and transmitter are not typically active at the same time.
• Collisions cannot be sensed in all cases due to hidden terminals and fading.

CSMA/CA

• In LANs, protocols use collision detection (CD) mechanisms to detect collisions and take corrective actions.

RTS-CTS Exchange Scenario

• A node (A) sending data to another node (B) via an access point (AP) can use a Request to Send (RTS) and Clear to Send (CTS) exchange to avoid collisions.
• Node A sends an RTS message to the AP, reserving the channel for communication.
• Node B also sends an RTS message, potentially resulting in a collision if sent at the same time.
• Node A's RTS takes priority, and the AP sends a CTS to A, confirming the channel reservation.
• Node A sends the data to B, and B sends an ACK (acknowledgment) to A.

Addressing

• Frame control fields include:
  • MAC addresses of the source and destination devices.
  • Router interface address to which the wireless access point is attached.
  • Address used only in ad-hoc mode.
  • Sequence number.
  • Duration field for the frame.
  • Payload: Data to be transmitted.
  • Cyclic Redundancy Check (CRC): Error detection code.

Addressing (continued)

• Internet (Internet Protocol)
• H1 (Wireless host 1)
• R1 (Router 1)
• AP (Wireless Access Point)
• The destination MAC address indicates the receiving device.
• The frame includes the router's MAC address, the access point's MAC address, the source device's MAC address, and the destination device's MAC address.