CWNA Guide to Wireless LANs, Second Edition

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

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

4. IEEE Wireless LAN Configurations: Basic Service Set Figure 5-1: Basic Service Set (BSS)

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

6. IEEE Wireless LAN Configurations: Extended Service Set Figure 5-2: Extended Service Set (ESS)

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

8. IEEE Wireless LAN Configurations: Independent Basic Service Set Figure 5-3: Independent Basic Service Set (IBSS)

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

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

11. MAC Frame Formats • 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

12. MAC Frame Formats • 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

13. MAC Frame Formats • 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

14. MAC Frame Formats • Control frames: Provide assistance in delivering frames that contain data Figure 5-5: Control frame

15. MAC Frame Formats • Data frame: Carries information to be transmitted to destination device Figure 5-6: Data frame

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

17. Discovering the WLAN: Beaconing Figure 5-7: Beaconing

18. Discovering the WLAN: Beaconing • 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

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

20. Discovering the WLAN: Scanning Figure 5-8: Active scanning

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

22. Joining the WLAN: Authentication Figure 5-9: Open system authentication

23. Joining the WLAN: Authentication (continued) Figure 5-10: Shared key authentication

24. Joining the WLAN: Authentication • 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

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

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

27. Transmitting on the WLAN: Distributed Coordination Function • 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

28. Transmitting on the WLAN: Distributed Coordination Function Figure 5-11: Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

29. Transmitting on the WLAN: Distributed Coordination Function Figure 5-11 (continued): Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

30. Transmitting on the WLAN: Distributed Coordination Function Figure 5-12: Hidden node problem

31. Transmitting on the WLAN: Distributed Coordination Function • 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

32. Transmitting on the WLAN: Distributed Coordination Function • 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

33. Transmitting on the WLAN: Distributed Coordination Function Figure 5-13: CSMA/CA and ACK

34. Transmitting on the WLAN: Distributed Coordination Function • 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

35. Transmitting on the WLAN: Distributed Coordination Function Figure 5-14: Request to Send/Clear to Send (RTS/CTS)

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

37. Transmitting on the WLAN: Interframe Spacing (continued) Figure 5-15: CSMA/CA with one station transmitting

38. Transmitting on the WLAN: Interframe Spacing Figure 5-16: CSMA/CA with two stations transmitting

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

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

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)

42. Transmitting on the WLAN: Point Coordination Function Figure 5-18: DIFS and DCF frames

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

44. Transmitting on the WLAN: Quality of Service and 802.11e Table 5-1: Wi-Fi Multimedia (WMM)

45. Transmitting on the WLAN: Quality of Service and 802.11e • 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

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

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

48. Remaining Connected to the WLAN: Power Management Figure 5-19: Power management in infrastructure mode

49. Remaining Connected to the WLAN: Power Management • 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

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