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CWNA Guide to Wireless LANs, Second Edition

CWNA Guide to Wireless LANs, Second Edition. Chapter Five IEEE 802.11 Media Access Control and Network Layer Standards. Objectives. List and define the three types of WLAN configurations Tell the function of the MAC frame formats

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CWNA Guide to Wireless LANs, Second Edition

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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 • Describe the functions of mobile IP CWNA Guide to Wireless LANs, Second Edition

  3. 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 • Dynamic rate shifting: As mobile devices move away from AP, transmission speed decreases CWNA Guide to Wireless LANs, Second Edition

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

  5. 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

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

  7. 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 • BSS more flexible than IBSS in being able to connect to other wired or wireless networks • 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

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

  9. 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

  10. MAC Frame Formats • Packet: Smaller segments of a digital data transmission • Strictly speaking, other terms used to describe these smaller segments • Frames: Packet at MAC layer • Or Data Link layer in OSI model • IEEE 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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 CWNA Guide to Wireless LANs, Second Edition

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

  18. 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

  19. Discovering the WLAN: Scanning • Receiving wireless device must be looking for beacon frames • 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

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

  21. 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

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

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

  24. 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

  25. 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

  26. 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

  27. 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

  28. 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

  29. 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

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

  31. 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

  32. 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

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

  34. 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 longer than RTS threshold transmitted using RTS/CTS CWNA Guide to Wireless LANs, Second Edition

  35. 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

  36. Transmitting on the WLAN: Interframe Spacing • Interframe spaces (IFS): Intervals between transmissions of data frames • Short IFS (SIFS): For immediate response actions such as ACK • Point Coordination Function IFS (PIFS): Time used by a device to access medium after it has been asked and then given approval to transmit • Distributed Coordination Function IFS (DIFS): Standard interval between transmission of data frames CWNA Guide to Wireless LANs, Second Edition

  37. Transmitting on the WLAN: Interframe Spacing (continued) Figure 5-15: CSMA/CA with one station transmitting CWNA Guide to Wireless LANs, Second Edition

  38. Transmitting on the WLAN: Interframe Spacing (continued) Figure 5-16: CSMA/CA with two stations transmitting CWNA Guide to Wireless LANs, Second Edition

  39. 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

  40. 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” • Point coordinator has to wait only through point coordination function IFS (PIFS) time gap • Shorter than DFIS time gap CWNA Guide to Wireless LANs, Second Edition

  41. Transmitting on the WLAN: Point Coordination Function (continued) • If point coordinator hears no traffic after PIFS time gap, sends out beacon frame • Field to indicate length of time that PCF (polling) will be used instead of DCF (contention) • Receiving stations must stop transmission for that amount of time • Point coordinator then sends frame to specific station, granting permission to transmit one frame • 802.11 standard allows WLAN to alternate between PCF (polling) and DCF (contention) CWNA Guide to Wireless LANs, Second Edition

  42. Transmitting on the WLAN: Point Coordination Function (continued) Figure 5-18: DIFS and DCF frames CWNA Guide to Wireless LANs, Second Edition

  43. Transmitting on the WLAN: Quality of Service (QoS) and 802.11e • DCF does not work well for real-time, time-dependent traffic • Quality of Service (QoS): Capability to prioritize different types of frames • Wi-Fi Multimedia (WMM): Modeled after wired network QoS prioritization scheme • 802.11e draft: defines superset of features intended to provide QoS over WLANs • Proposes two new mode of operation for 802.11 MAC Layer CWNA Guide to Wireless LANs, Second Edition

  44. Transmitting on the WLAN: Quality of Service and 802.11e (continued) Table 5-1: Wi-Fi Multimedia (WMM) CWNA Guide to Wireless LANs, Second Edition

  45. Transmitting on the WLAN: Quality of Service and 802.11e (continued) • 802.11e draft (continued): • Enhanced Distributed Channel Access (EDCA): Contention-based but supports different types of traffic • Four access categories (AC) • Provides “relative” QoS but cannot guarantee service • Hybrid Coordination Function Controlled Channel Access (HCCA): New form of PCF based upon polling • Serves as a centralized scheduling mechanism CWNA Guide to Wireless LANs, Second Edition

  46. Remaining Connected to the WLAN: Reassociation • Reassociation: Device drops connection with one AP and establish connection with another • Several reason why reassociation may occur: • Roaming • Weakened signal • When device determines link to current AP is poor, begins scanning to find another AP • Can use information from previous scans CWNA Guide to Wireless LANs, Second Edition

  47. Remaining Connected to the WLAN: Power Management • When laptop is part of a WLAN, must remain “awake” in order to receive network transmissions • Original IEEE 802 standard assumes stations always ready to receive network messages • Power management: Allows mobile devices to conserve battery life without missing transmissions • Transparent to all protocols • Differs based on WLAN configuration • AP records which stations awake and sleeping • Buffering: If sleeping, AP temporarily stores frames CWNA Guide to Wireless LANs, Second Edition

  48. Remaining Connected to the WLAN: Power Management (continued) Figure 5-19: Power management in infrastructure mode CWNA Guide to Wireless LANs, Second Edition

  49. Remaining Connected to the WLAN: Power Management (continued) • At set times AP send out beacon to all stations • Contains traffic indication map (TIM) • At same time, all sleeping stations switch into active listening mode • Power management in ad hoc mode: • Ad hoc traffic indication message (ATIM) window: Time at which all stations must be awake • Wireless device sends beacon to all other devices • Devices that previously attempted to send a frame to a sleeping device will send ATIM frame indicating that receiving device has data to receive and must remain awake CWNA Guide to Wireless LANs, Second Edition

  50. WLAN Network Layer Standards: WLAN IP Addressing • In standard networking, IP protocol responsible for moving frames between computers • Network layer protocol • TCP/IP works on principle that each network host has unique IP address • Used to locate path to specific host • Routers use IP address to forward packets • Prohibits mobile users from switching to another network and using same IP number • Users who want to roam need new IP address on every network CWNA Guide to Wireless LANs, Second Edition

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