1. CWNA Guide to Wireless LANs, Second Edition
Chapter Five
IEEE 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 MAC frames different from 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 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 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 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 standard specifies two procedures for transmitting on the WLAN, distributed coordination function (DCF) and an optional point coordination function (PCF)
The standard provides for an optional polling function known as Point Coordination Function (PCF)
CWNA Guide to Wireless LANs, Second Edition