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IEEE 802.11 WLAN Configurations & MAC Procedures

Explore IEEE 802.11 WLAN configurations - Basic Service Set, Extended Service Set, Independent Basic Service Set, and MAC frame formats and procedures. Learn about WLAN signal discovery, joining, transmitting, and staying connected.

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IEEE 802.11 WLAN Configurations & MAC Procedures

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  1. CWNA Guide to Wireless LANs, Second Edition Chapter Five IEEE 802.11 Media Access Control and Network Layer Standards

  2. Objectives • List and define the three types of WLAN configurations • Tell the function of the MAC frame formats • Explain the MAC procedures for joining, transmitting, and remaining connected to a WLAN CWNA Guide to Wireless LANs, Second Edition

  3. IEEE Wireless LAN Configurations: • Basic Service Set • Extended Service Set • Independent Service Set CWNA Guide to Wireless LANs, Second Edition

  4. IEEE Wireless LAN Configurations: Basic Service Set • Basic Service Set (BSS): Group of wireless devices served by single AP • infrastructure mode • BSS must be assigned unique identifier • Service Set Identifier (SSID) • Serves as “network name” for BSS • Basic Service Area (BSA): Geographical area of a BSS • Max BSA for a WLAN depends on many factors (technology, obstructions, interference, battery power,…etc.) CWNA Guide to Wireless LANs, Second Edition

  5. IEEE Wireless LAN Configurations: Basic Service Set (continued) Figure 5-1: Basic Service Set (BSS) CWNA Guide to Wireless LANs, Second Edition

  6. IEEE Wireless LAN Configurations: Extended Service Set • Extended Service Set (ESS): Comprised of two or more BSS networks connected via a common distribution system • APs can be positioned so that cells overlap to facilitate roaming • Wireless devices choose AP based on signal strength • Handoff CWNA Guide to Wireless LANs, Second Edition

  7. IEEE Wireless LAN Configurations: Extended Service Set (continued) Figure 5-2: Extended Service Set (ESS) CWNA Guide to Wireless LANs, Second Edition

  8. IEEE Wireless LAN Configurations: Independent Basic Service Set • Independent Basic Service Set (IBSS): Wireless network that does not use an AP • Wireless devices communicate between themselves • Peer-to-peer or ad hoc mode • IBSS useful for quickly and easily setting up wireless network • When no connection to Internet or external network needed CWNA Guide to Wireless LANs, Second Edition

  9. IEEE Wireless LAN Configurations: Independent Basic Service Set (continued) Figure 5-3: Independent Basic Service Set (IBSS) CWNA Guide to Wireless LANs, Second Edition

  10. IEEE 802.11 Media Access Control (MAC) Layer Standards • Media Access Control (MAC) layer performs several vital functions in a WLAN • Discovering WLAN signal • Joining WLAN • Transmitting on WLAN • Remaining connected to WLAN • Mechanics of how functions performed center around frames sent and received in WLANs CWNA Guide to Wireless LANs, Second Edition

  11. MAC Frame Formats • Frames: Packet at MAC layer • Or Data Link layer in OSI model • IEEE 802.11 MAC frames different from 802.3 Ethernet frames in format and function • Used by wireless NICs and APs for communications and managing/controlling wireless network CWNA Guide to Wireless LANs, Second Edition

  12. MAC Frame Formats (continued) • Frame control field identifies: • Specific 802.11 protocol version • Frame type • Indicators that show WLAN configuration • All frames contain • MAC address of the source and destination device • Frame sequence number • Frame check sequence for error detection CWNA Guide to Wireless LANs, Second Edition

  13. MAC Frame Formats (continued) IEEE 802.11 standard specifies three categories of MAC frames: • Management Frames • Control Frames • Data Frames CWNA Guide to Wireless LANs, Second Edition

  14. MAC Frame Formats (continued) • Management Frames: Initialize communications between device and AP (infrastructure mode) or between devices (ad hoc mode) • Maintain connection Figure 5-4: Structure of a management frame CWNA Guide to Wireless LANs, Second Edition

  15. MAC Frame Formats (continued) • Types of management frames: • Authentication frame • Association request frame • Association response frame • Beacon frame • Deauthentication frame • Disassociation frame • Probe request frame • Probe response frame • Reassociation request frame • Reassociation response frame CWNA Guide to Wireless LANs, Second Edition

  16. MAC Frame Formats (continued) • Control frames: Provide assistance in delivering frames that contain data Figure 5-5: Control frame CWNA Guide to Wireless LANs, Second Edition

  17. MAC Frame Formats (continued) • Data frame: Carries information to be transmitted to destination device Figure 5-6: Data frame CWNA Guide to Wireless LANs, Second Edition

  18. Discovering the WLAN: Beaconing • At regular intervals, AP (infrastructure network) or wireless device (ad hoc network) sends beacon frame • Announce presence • Provide info for other devices to join network • Beacon frame format follows standard structure of a management frame • Destination address always set to all ones (broadcasting) CWNA Guide to Wireless LANs, Second Edition

  19. Discovering the WLAN: Beaconing (continued) Figure 5-7: Beaconing CWNA Guide to Wireless LANs, Second Edition

  20. Discovering the WLAN: Beaconing (continued) • Beacon frame body contains following fields: • Beacon interval • Timestamp • Service Set Identifier (SSID) • Supported rates • Parameter sets • Capability information • In ad hoc networks, each wireless device assumes responsibility for beaconing • In infrastructure networks beacon interval normally 100 ms, but can be modified CWNA Guide to Wireless LANs, Second Edition

  21. Discovering the WLAN: Scanning • Passive scanning: Wireless device simply listens for beacon frame • Typically, on each available channel for set period • Active scanning: Wireless device first sends out a management probe request frame on each available channel • Then waits for probe response frame from all available APs CWNA Guide to Wireless LANs, Second Edition

  22. Discovering the WLAN: Scanning (continued) Figure 5-8: Active scanning CWNA Guide to Wireless LANs, Second Edition

  23. Joining the WLAN: Authentication • Unlike standard wired LANS, authentication performed before user connected to network • Authentication of the wireless device, not the user • IEEE 802.11 authentication: Process in which AP accepts or rejects a wireless device • Open system authentication: Most basic, and default, authentication method • Shared key authentication: Optional authentication method • Utilizes challenge text CWNA Guide to Wireless LANs, Second Edition

  24. Joining the WLAN: Authentication (continued) Figure 5-9: Open system authentication CWNA Guide to Wireless LANs, Second Edition

  25. Joining the WLAN: Authentication (continued) Figure 5-10: Shared key authentication CWNA Guide to Wireless LANs, Second Edition

  26. Joining the WLAN: Authentication (continued) • Open system and Shared key authentication techniques are weak • Open System: Only need SSID to connect • Shared Key: Key installed manually on devices • Can be discovered by examining the devices • Digital certificates: Digital documents that associate an individual with key value • Digitally “signed” by trusted third party • Cannot change any part of digital certificate without being detected CWNA Guide to Wireless LANs, Second Edition

  27. Joining the WLAN: Association • Association: Accepting a wireless device into a wireless network • Final step to join WLAN • After authentication, AP responds with association response frame • Contains acceptance or rejection notice • If AP accepts wireless device, reserves memory space in AP and establishes association ID • Association response frame includes association ID and supported data rates CWNA Guide to Wireless LANs, Second Edition

  28. Transmitting on the WLAN: Distributed Coordination Function (DCF) • MAC layer responsible for controlling access to wireless medium • Channel access methods: Rules for cooperation among wireless devices • Contention: Computers compete to use medium • If two devices send frames simultaneously, collision results and frames become unintelligible • Must take steps to avoid collisions CWNA Guide to Wireless LANs, Second Edition

  29. Transmitting on the WLAN: Distributed Coordination Function (continued) • Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Before networked device sends a frame, listens to see if another device currently transmitting • If traffic exists, wait; otherwise send • Devices continue listening while sending frame • If collision occurs, stops and broadcasts a “jam” signal • CSMA/CD cannot be used on wireless networks: • Difficult to detect collisions • Hidden node problem CWNA Guide to Wireless LANs, Second Edition

  30. Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) CWNA Guide to Wireless LANs, Second Edition

  31. Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-11 (continued): Carrier Sense Multiple Access with Collision Detection (CSMA/CD) CWNA Guide to Wireless LANs, Second Edition

  32. Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-12: Hidden node problem CWNA Guide to Wireless LANs, Second Edition

  33. Transmitting on the WLAN: Distributed Coordination Function (continued) • Distributed Coordination Function (DCF): Specifies modified version of CSMA/CD • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Attempts to avoid collisions altogether • Time when most collisions occur is immediately after a station completes transmission • All stations must wait random amount of time after medium clear • Slot time CWNA Guide to Wireless LANs, Second Edition

  34. Transmitting on the WLAN: Distributed Coordination Function (continued) • CSMA/CA also reduces collisions via explicit frame acknowledgment • Acknowledgment frame (ACK):Sent by receiving device to sending device to confirm data frame arrived intact • If ACK not returned, transmission error assumed • CSMA/CA does not eliminate collisions • Does not solve hidden node problem CWNA Guide to Wireless LANs, Second Edition

  35. Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-13: CSMA/CA and ACK CWNA Guide to Wireless LANs, Second Edition

  36. Transmitting on the WLAN: Distributed Coordination Function (continued) • Request to Send/Clear to Send (RTS/CTS) protocol: Option used to solve hidden node problem • Significant overhead upon the WLAN with transmission of RTS and CTS frames • Especially with short data packets • RTS threshold: Only packets that are longer than RTS threshold are transmitted using RTS/CTS CWNA Guide to Wireless LANs, Second Edition

  37. Transmitting on the WLAN: Distributed Coordination Function (continued) Figure 5-14: Request to Send/Clear to Send (RTS/CTS) CWNA Guide to Wireless LANs, Second Edition

  38. Transmitting on the WLAN: Fragmentation • Fragmentation: Divide data to be transmitted from one large frame into several smaller ones • Reduces probability of collisions • Reduces amount of time medium is in use • If data frame length exceeds specific value, MAC layer fragments it • Receiving station reassembles fragments • Alternative to RTS/CTS • High overhead • ACKs and additional SIFS time gaps CWNA Guide to Wireless LANs, Second Edition

  39. Transmitting on the WLAN: Point Coordination Function (PCF) • Polling:Channel access method in which each device asked in sequence if it wants to transmit • Effectively prevents collisions • Point Coordination Function (PCF): AP serves as polling device or “point coordinator” CWNA Guide to Wireless LANs, Second Edition

  40. Summary • A Basic Service Set (BSS) is defined as a group of wireless devices that is served by a single access point (AP) • An Extended Service Set (ESS) is comprised of two or more BSS networks that are connected through a common distribution system • An Independent Basic Service Set (IBSS) is a wireless network that does not use an access point • Frames are used by both wireless NICs and access points for communication and for managing and controlling the wireless network CWNA Guide to Wireless LANs, Second Edition

  41. Summary (continued) • The MAC layer provides four major functions in WLANs: discovering the WLAN signal, joining the WLAN, transmitting on the WLAN, and remaining connected to the WLAN • Discovery is a twofold process: the AP or other wireless devices must transmit an appropriate frame (beaconing), and the wireless device must be looking for those frames (scanning) • Once a wireless device has discovered the WLAN, it requests to join the network; This is a twofold process known as authentication and association CWNA Guide to Wireless LANs, Second Edition

  42. Summary (continued) • The IEEE 802.11 standard specifies two procedures for transmitting on the WLAN, distributed coordination function (DCF) and an optional point coordination function (PCF) • The 802.11 standard provides for an optional polling function known as Point Coordination Function (PCF) CWNA Guide to Wireless LANs, Second Edition

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