1. Certified Wireless Network Administrator (CWNA) PW0-105 Chapter 9 802.11 MAC Architecture

2. Chapter 9 Overview Packets, Frames, and Bits Data-Link Layer Physical Layer 802.11 and 802.3 Interoperability Three 802.11 Frame Types Beacon Management Frame (Beacon) Passive Scanning Active Scanning 2 Certified Wireless Network Administrator: CWNA – PW0-105

3. Chapter 9 Overview (continued) Authentication Association Authentication and Association States Basic and Supported Rates Roaming Reassocation Disassociation ACK Frame Fragmentation 3 Certified Wireless Network Administrator: CWNA – PW0-105

4. Chapter 9 Overview (continued) Protection Mechanism RTS/CTS CTS-to-Self Data Frames Power Management Band Steering 4 Certified Wireless Network Administrator: CWNA – PW0-105

5. Packets, Frames, and Bits Main objective of a network – Transfer user data from one computing device to another Each layer of the OSI model adds header information as the frame is passed down to the next layer Headers tell the receiving computer how to process the data Data transfer process –Data starts at the application layer of the OSI model –Data moves down the OSI model until it reaches the Physical layer –Data is transferred across the network medium to the receiving device –Data moves back up the OSI model of the receiving device –Each layer processes the data based upon header information received from its peer layer on the sending device 5 Certified Wireless Network Administrator: CWNA – PW0-105

6. Packets, Frames, and Bits (continued) Network Layer - Packets –IP address and header is added –Network header contains layer 4-7 data Data-Link Layer - Frames –MAC address and header is added –IP packet is encapsulated inside the frame Physical Layer – Bits –PHY header is added –Data is encoded into a carrier signal and transmitted 6 Certified Wireless Network Administrator: CWNA – PW0-105

7. Data-Link and Physical Layers The following slide will review the components that make up the Data-Link and Physical layers 7 Certified Wireless Network Administrator: CWNA – PW0-105

8. Data-Link Layer Divided into two sublayers Upper portion – 802.2 Logical Link Control (LLC) sublayer –Identical for all 802 networks (although not used by all 802 networks) Lower portion - Media Access Control (MAC) sublayer –Identical for all 802.11-based networks 802.11 standard defines operations at the MAC sublayer 8 Certified Wireless Network Administrator: CWNA – PW0-105

9. MAC Service Data Unit (MSDU) The Network layer data (IP Packet) passed down to the Data-Link layer plus LLC data Contains data from the LLC and layers 3-7 802.11 standard states maximum size of the MSDU is 2,304 bytes Maximum frame body size is actually 2,304 octets plus any overhead from encryption Aggregate MSDU (A-MSDU) was introduced with 802.11n amendment 9 Certified Wireless Network Administrator: CWNA – PW0-105

10. MAC Protocol Data Unit (MPDU) The 802.11 frame Contains –Layer 2 header –Frame body –Trailer – 32 bit CRC known as the frame check sequence (FCS) 10 Certified Wireless Network Administrator: CWNA – PW0-105

11. Physical Layer Divided into two sublayers Upper portion – Physical Layer Convergence Procedure (PLCP) sublayer –Prepares the frame for transmission –Takes the frame from the MAC sublayer and creates the PLCP Protocol Data Unit (PPDU) Lower portion – Physical Medium Depenent (PMD) sublayer –Modulates and transmits the data as bits 11 Certified Wireless Network Administrator: CWNA – PW0-105

12. PLCP Service Data Unit (PSDU) Physical layer name for MPDU Think of a door –One side is the entrance –Other side is the exit –Two names – same door 12 Certified Wireless Network Administrator: CWNA – PW0-105

13. PLCP Protocol Data Unit (PPDU) When PLCP receives the PSDU, it prepares it to be transmitted and creates the PPDU PSDU + preamble + PHY header Preamble – used for synchronization between transmitting and receiving 802.11 radios PMD sublayer takes the PPDU, modulates the data bits and begins transmitting 13 Certified Wireless Network Administrator: CWNA – PW0-105

14. 802.11 and 802.3 Interoperability 802.3 max frame size is just over 1,500 bytes 802.11 max frame size is just over 2,300 bytes IP maximum transmission unit (MTU) is commonly 1,500 bytes, so frame size difference is usually not a problem 802.3 frame has two MAC address fields 802.11 frame has up to four MAC address fields (most frames only use three address fields) 14 Certified Wireless Network Administrator: CWNA – PW0-105

15. 802.11 and 802.3 Interoperability (continued) 802.3 address fields –Source address (SA) –Destination address (DA) 802.11 address fields –“Address1” through “Address4” –The contents of the 802.11 address fields represent Receiver address (RA) Transmitter address (TA) Basic Service Set Identifier (BSSID) Destination address (DA) Source address (SA) 15 Certified Wireless Network Administrator: CWNA – PW0-105

16. 802.11 and 802.3 Interoperability (continued) 802.11 MAC Header 16 Certified Wireless Network Administrator: CWNA – PW0-105

17. Three 802.11 Frame Types Three major frame types –Management –Control –Data Frame types are further subdivided into multiple subtypes Many frame types provide support for PCF media access method, and thus have never been implemented 17 Certified Wireless Network Administrator: CWNA – PW0-105

18. Management Frames Majority of the frame types Management MAC Protocol Data Unit (MMPDU) Used by 802.11 to join and leave the BSS Not necessary on wired networks due to physical connection Wireless network must establish logical network connection before data can be transmitted Do not carry upper-layer information 18 Certified Wireless Network Administrator: CWNA – PW0-105

19. Management Frames 14 Management Frame subtypes –Association request –Association response –Reassociation request –Reassociation response –Probe request –Probe response –Beacon –Announcement traffic indication message (ATIM) 19 Certified Wireless Network Administrator: CWNA – PW0-105 –Disassociation –Authentication –Deauthentication –Action –Action No ACK –Timing advertisement

20. Control Frames Assist with the delivery of the data frames Transmitted at one of the basic rates Provide 3 primary tasks –Clear the channel –Acquire the channel –Provide unicast frame acknowledgements Contain only header information 20 Certified Wireless Network Administrator: CWNA – PW0-105

21. Control Frames 9 Control Frame subtypes –Power Save Poll (PS-Poll0 –Request to send (RTS) –Clear to send (CTS) –Acknowledgement (ACK) –Contention Free-End (CF-End) –CF-End + CF-ACK –Block ACK Request (BlockAckReq) –Block ACK (BlockAck) –Controll wrapper 21 Certified Wireless Network Administrator: CWNA – PW0-105

22. Data Frames Carry the actual data Layer 3-7 MSDU payload is normally encrypted Some data frames carry no MSDU – provide a specific function Data subtype is referred to as the “simple data frame” 22 Certified Wireless Network Administrator: CWNA – PW0-105

23. Data Frames 15 Data Frame subtypes 23 Certified Wireless Network Administrator: CWNA – PW0-105 QoS Data [HCF] QoS Null (no data) [HCF] QoS Data + CF-ACK [HCF] QoS Data + CF-Poll [HCF] QoS Data + CF-ACK + CF-Poll [HCF] QoS CF-Poll (no data) [HCF] QoS CF-ACK + CF-Poll (no data) [HCF] Data (simple data frame) Null function (no data) Data + CF-ACK [PCF only] Data + CF-Poll [PCF only] Data + CF-ACK + CF-Poll [PCF only] CF-ACK (no data) [PCF only] CF-Poll (no data) [PCF only] CF-ACK + CF-Poll (no data) [PCF only]

24. Beacon Management Frame (Beacon) One of the most important frame types Heartbeat of the wireless network Contains timestamp used to synchronize station transmissions Transmitted by each AP Separate beacons per SSID Separate beacons per radio Approximately 10 times per second (per AP, per SSID, per radio) 24 Certified Wireless Network Administrator: CWNA – PW0-105

25. Beacon Management Frame (Beacon) Contains –Time Stamp (synchronization information) –Spread Spectrum Parameter Sets (DSSS-, OFDM-, HT-specific information) –Channel Information (channel used by AP or IBSS) –Data Rates (Basic and supported rates) –Service Set Capabilities (Extra BSS or IBSS parameters) –SSID –Traffic Indication Map (TIM) (Used during Power Save) –QoS Capabilities (QoS and EDCA information) –Robust Security Network (RSN) Capabilities (TKIP or CCMP info and authentication method) –Vendor Proprietary Information 25 Certified Wireless Network Administrator: CWNA – PW0-105

26. Passive Scanning Used by station to discover an AP before connecting Station listens for beacon frames Used to identify APs that are advertising SSIDs If multiple APs advertise the same SSID, station will connect to AP with strongest signal 26 Certified Wireless Network Administrator: CWNA – PW0-105

27. Active Scanning Used by station to discover an AP before connecting Station transmits probe request management frame Probe request could contain specific SSID (directed probe request) Probe request could contain null SSID (null probe request) APs reply with probe response management frame Probe response is almost identical to a beacon Commonly occurs even after a station is connected to an AP (allows station to identify other available APs) 27 Certified Wireless Network Administrator: CWNA – PW0-105

28. Authentication First step when connecting to a basic service set (BSS) Should be referred to as “802.11 Authentication” Often misunderstood or misinterpreted Authenticates that both the station and the AP are capable of 802.11 communications Analogous to plugging in an Ethernet cable and establishing a link beat Two legacy methods of authentication defined –Open System authentication (commonly used with new methods of security) –Shared Key authentication (Do not use) 28 Certified Wireless Network Administrator: CWNA – PW0-105

29. Association Occurs after 802.11 authentication process Station becomes a member of a BSS Station can send data to the AP and onto the distribution system medium Association identifier (AID) identifies each station 29 Certified Wireless Network Administrator: CWNA – PW0-105

30. Authentication and Association States Authentication states –Unauthenticated –Authenticated Association states –Unassociated –Associated Three possible states for the station –State 1: initial start state, unauthenticated and unassociated –State 2: authenticated and unassociated –State 3: authenticated and associated 30 Certified Wireless Network Administrator: CWNA – PW0-105

31. Authentication and Association States 31 Certified Wireless Network Administrator: CWNA – PW0-105

32. Basic and Supported Rates Basic Rates = required rates Client must be capable of the basic rates in order to join the BSS Supported Rates = the group data rates that the AP will use when communicating with the station Both are advertised as part of the beacon 32 Certified Wireless Network Administrator: CWNA – PW0-105

33. Roaming The ability for a station to transition from one AP to another, while maintaining upper-layer communications When to roam is decided by the station only There is no standard for when to roam When to roam is typically determined by –Signal level –Noise level –Bit-error rate 33 Certified Wireless Network Administrator: CWNA – PW0-105

34. Reassociation The process of roaming to another AP 34 Certified Wireless Network Administrator: CWNA – PW0-105

35. Disassociation A notification not a request Cannot be refused (except when management frame protection is enabled and MIC fails) Polite way to terminate the association Client or AP will disassociate when shutting down 35 Certified Wireless Network Administrator: CWNA – PW0-105

36. Deauthentication A notification not a request Cannot be refused (except when management frame protection is enabled and MIC fails) Deauthentication will automatically cause a disassociation 36 Certified Wireless Network Administrator: CWNA – PW0-105

37. ACK Frame Key component of CSMA/CA Used to confirm receipt of a transmitted frame Consists of 14 octets 37 Certified Wireless Network Administrator: CWNA – PW0-105

38. Fragmentation Breaks and 802.11 frame into smaller pieces Transmits each piece as an individual frame Inherently increases transmission time If network is experiencing data corruption, retransmission of smaller fragments can increase overall data throughput 38 Certified Wireless Network Administrator: CWNA – PW0-105

39. Protection Mechanism Provides compatibility for higher data rate devices to operate along side lower data rate devices Also known as mixed mode or protected mode Contrary to what some people think, faster devices do not simply switch to the lower data rate 39 Certified Wireless Network Administrator: CWNA – PW0-105

40. Request to Send/Clear to Send (RTS/CTS) Mechanism that performs NAV distribution Reserves the medium prior to the transmission of the data frame Primarily used as a protection mechanism or when hidden nodes exist 40 Certified Wireless Network Administrator: CWNA – PW0-105

41. CTS-to-Self Mechanism that performs NAV distribution Reserves the medium prior to the transmission of the data frame Used strictly as a protection mechanism 41 Certified Wireless Network Administrator: CWNA – PW0-105

42. Data Frames Most common data frame is the simple data frame AP and controller integration services take the MSDU payload of an 802.11 data frame and transfers it into an 802.3 Ethernet frame MSDU data payload is often encrypted Null function frame is often used to signal Power Save status 42 Certified Wireless Network Administrator: CWNA – PW0-105

43. Power Management Helps increase station battery life Active mode –Wireless station is always ready to transmit or receive –Sometimes referred to as Continuous Aware mode –Provides no battery conservation Power Save Mode –Station will shut down the transceiver for short periods of time to conserve power Traffic Indication Map (TIM) –Field in the beacon that lists all stations that have undelivered data, waiting for station to wake up to receive the data 43 Certified Wireless Network Administrator: CWNA – PW0-105

44. Power Management (continued) Delivery Traffic Indication Message (DTIM) –Ensures that stations using power management are awake when multicast or broadcast traffic is sent Announcement Traffic Indication Message (ATIM) –A frame used in an IBSS that notifies a station that another station has buffered data for it WMM Power Save and U-APSD –Wireless multimedia power save and unscheduled automatic power save delivery –Enhanced power management method –Uses a trigger mechanism to receive buffered unicast traffic 44 Certified Wireless Network Administrator: CWNA – PW0-105

45. Power Management (continued) 45 Certified Wireless Network Administrator: CWNA – PW0-105

46. Power Management (continued) 46 Certified Wireless Network Administrator: CWNA – PW0-105

47. 802.11n Power Management Spatial multiplexing power save (SM power save) –Enables MIMO device to power down all but one radio chain Power same multi-poll (PSMP) –An extension of APSD 47 Certified Wireless Network Administrator: CWNA – PW0-105

48. Band Steering Proprietary, non-standard Implemented by many vendors, often using similar but different techniques AP encourages multi-band station to connect using 5 GHz radio instead of 2.4 GHz radio Station will most likely experience higher throughput on 5 GHz network 48 Certified Wireless Network Administrator: CWNA – PW0-105

49. Chapter 9 Summary Packets, Frames, and Bits Data-Link Layer Physical Layer 802.11 and 802.3 Interoperability Three 802.11 Frame Types Beacon Management Frame (Beacon) Passive Scanning Active Scanning 49 Certified Wireless Network Administrator: CWNA – PW0-105

50. Chapter 9 Summary (continued) Authentication Association Authentication and Association States Basic and Supported Rates Roaming Reassocation Disassociation ACK Frame 50 Certified Wireless Network Administrator: CWNA – PW0-105

51. Chapter 9 Summary (continued) Fragmentation Protection Mechanism RTS/CTS CTS-to-Self Data Frames Power Management Band Steering 51 Certified Wireless Network Administrator: CWNA – PW0-105