802.11 MAC Architecture Module-7

1. 802.11 MAC ArchitectureModule-7 Jerry Bernardini Community College of Rhode Island Wireless Networking J. Bernardini

2. Presentation Reference Material • CWNA Certified Wireless Network Administration Official Study Guide (PWO-104), David Coleman, David Westcott, 2009, Chapter-9 • The purpose of 802.11 is to transfer data from computer to another --- not to be forgotten Wireless Networking J. Bernardini

3. Bits, Bytes, Octets, Frames, Packets • Bits =1 or 0 • Bytes = 8 bits • Octets = 8 bits = Byte • Octet is used by telecommunication people • Byte is used by IT people • Frames = grouping of bits at layer-2 • Packets = grouping of bits at layer-3 • Datagrams = another term for packets Wireless Networking J. Bernardini

4. OSI data flow CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

5. IEEE 802.11 Physical Layer Standards • IEEE wireless standards follow OSI model, with some modifications • Data Link layer divided into two sublayers: • Logical Link Control (LLC) sublayer: Provides common interface, reliability, and flow control • Media Access Control (MAC) sublayer: Appends physical addresses to frames • Physical layer divided into two sublayers: • Physical Medium Dependent (PMD) sublayer: Makes up standards for characteristics of wireless medium (such as DSSS or FHSS) and defines method for transmitting and receiving data • Physical Layer Convergence Procedure (PLCP) sublayer: Performs two basic functions • Reformats data received from MAC layer into frame that PMD sublayer can transmit • “Listens” to determine when data can be sent CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

6. Where the IEEE 802.11 Standard Fits

7. Data Link Layer - Physical Layer- Data Units MSDU (MAC Service Data Unit) (From upper layers 2304 bytes max) 802.2 Logical Link Control LLC Data Link Layer (Layer-2) MPDU (MAC Protocol Data Unit) 802.11 Media Access Control MAC PSDU (PLCP Service Data Unit) (MPDU = PSDU name change to indicated service needed) PLCP PHY Layer Convergence Protocol Physical Layer (Layer-1) PPDU (PLCP Protocol Data Unit) PMD Physical Medium Dependent Modulated Radio Signal PHY = Physical Layer

8. Ethernet and 802.11 Frames 1518 7 1 6 6 2 46 - 1500 4 • Ethernet Frame • Wireless Frame Preamble Source Destination Data FCS Type or Length Field Start Of Frame 10 or 18 2 4 or 6 Sync PLCP Header Start Of Frame 2 2 6 6 6 2 6 0 - 2304 4 FCS Source Destination Rec. Adr Trans. Adr Data Sequence Cntrl Duration ID Frame Cntrl MAC Packet DATA Unit, (MPDU)

9. Frame Categories / Types • Management Frames • Beacon Frame • Probe Frames • Association Frames… more • Control Frames • RTS and CTS Frames • ACK – Acknowledgement Frames… more • Data Frames • Data Payload Frames

10. Twelve Management Frame Types Wireless Networking J. Bernardini

11. Eight Control Frames • Used to assist with the delivery of data frames Wireless Networking J. Bernardini

12. Fifteen Data Frames • The frames that actually carry application data Wireless Networking J. Bernardini

13. IEEE 802.11 Frame Formats (Bytes per field) CWNA Guide to Wireless LANs, Second Edition

14. Frame Types and Sizes Wireless Networking J. Bernardini

15. Header Payload Trailer Beacon Frame Beacons • Beacon – information frame sent by an AP. Beacon frames are approximately 50-bytes with the following information: • Timestamp • Beacon Interval • Capability Info • Service Set Identifier • Support Rates • Parameter Sets • Traffic Indication Map

16. Beacon Management Frame • A special management frame that is used by a client stations seeking a wireless network to join. • Instead of beacon frames a station could use probe request and probe response frames • In an ad hoc (IBSS ) wireless network all stations take turns broadcasting the beacon frame Beacon Beacon S2 S1 AP Control Point Wireless Networking J. Bernardini

17. Passive Scanning (Beacons) • Client stations listens for a beacon from an access point (AP) • If multiple beacons are received the strongest one is selected • The listening station then requests authentication and association Beacons Beacons S2 S1 AP Control Point Wireless Networking J. Bernardini

18. Active Scanning (Probes) • A station could use probe request and probe response frames Instead of beacon frames • Station is configured with SSID and switched to a channel • Probe request sent by requesting station • All stations that have the same SSID and have normal configurations respond with a Probe Response frame • The process also involves waiting for ProbeDelay and MinChannel Timers Probe Response Probe Request S2 S1 AP Control Point Wireless Networking J. Bernardini

19. Authentication and Association • Using the IEEE 802.11 State Machine • Stations are in one of three states • Unauthenticated / Unassociated • Authenticated / Unassociated • Authenticated / Associated • You cannot transmit data frames for processing until you are associated • You cannot transmit associated frames for processing until you are Authenticated Wireless Networking J. Bernardini

20. Wireless Connection Process • 2-Step Connection Process • Authentication Phase • Association Phase Authentication Phase Association Phase

21. Wireless Connection Process • Authentication is the process used by a station to verify that another station is approved for communications. This is a station authentication and not a user authentication. • Authentication Step • Two IEEE 802.11 standards: • Open System Authentication • Shared Key Authentication

22. Authentication • Authentication - A means to establish or prove identity; verifying eligibility of users, devices, or applications. • Only authorized clients are allowed to gain access to the network. • For this level of authentication it applies to the stations and not the user.

23. Association • Association – The binding of a wireless network client to an Access Point for the purpose of data transfer. Again the stations not the user.

24. Connection State Machine Unauthenticated and Unassociated De-authentication Notification Successful Authentication Authenticated and Unassociated Disassociation Notification Successful Association Authenticated and Associated De-authentication Notification

25. Unauthenticated & Unassociated • In the first state: • Wireless Client/Node is not connected to the network. Wired LAN Access Points Wireless Client

26. Authenticated & Unassociated • In the Second State: • Wireless Client/Node has passed the authentication process but is not associated with the AP. Wired LAN Association Response Access Points Association Request Wireless Client

27. Authenticated & Associated • In the Third State: • Wireless Client/Node is now connected and associated with the AP. Data can now be transferred between the devices. Wired LAN Access Points Data Link Wireless Client

28. Open System Authentication • Open System / Null Authentication • IEEE 802.11 Default • Authentications based on empty string SSID • Client sends empty string SSID • Receiving station, (AP) sends acknowledgment

29. Open System Authentication Wired LAN Authentication Management Frame - SSID ACK Access Points Authentication Management Frame Wireless Client ACK

30. Closed System Authentication • Closed System • IEEE 802.11 • Authentications based only on SSID • Client sends SSID • Receiving station, (AP) sends acknowledgment

31. Shared Key Authentication • Shared Key • IEEE 802.11 Wired Equivalent Privacy, (WEP). • Authentications based on Text and WEP Keys. • Challenge – Response Scheme

32. Shared Key Authentication Wired LAN Authentication Management Frame - SSID Challenge Phrase Access Points Encrypted Phrase Wireless Client Authentication Secure Channel WEP Key WEP Key

33. Roaming • Roaming - The ability for a user to function when the serving network is different from their home/associated network. The process of a client moving seamlessly from one area or cell to another while maintaining a data link.

34. Roaming Wired LAN Access Points 1 2 1 1 Wireless Clients Usually a 20%-30% cell overlap

35. Unauthenticated & Unassociated • In the first state: • Wireless Client/Node is not connected to the network. Wired LAN Access Points Wireless Client

36. Authenticated & Unassociated • In the Second State: • Wireless Client/Node has passed the authentication process but is not associated with the AP. Wired LAN Association Response Access Points Association Request Wireless Client

37. Authenticated & Associated • In the Third State: • Wireless Client/Node is now connected and associated with the AP. Data can now be transferred between the devices. Wired LAN Access Points Data Link Wireless Client

38. IEEE 802.11 MAC Functions • Scanning- discover AP or BSS • Synchronization- all stations have the same clock • Frame Transmission- rules for frame transfer • Authentication-allow device in network • Association-after authentication associate with AP • Reassociation-roaming and association with new AP • Data Protection-data encryption protects data • Power Management-save power by sleeping transceiver • Fragmentation-breakup frame for efficiency and interfer. • RTS/CTS- solution to hidden node problem Wireless Networking J. Bernardini

39. 802.11 Open Association Process Wireless Networking J. Bernardini

40. Wireless Networking J. Bernardini

41. Wireless Networking J. Bernardini