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Chapter 9 802.11 MAC Architecture

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

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Chapter 9 802.11 MAC Architecture

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  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) • Disassociation • Authentication • Deauthentication • Action • Action No ACK • Timing advertisement 19 Certified Wireless Network Administrator: CWNA – PW0-105

  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 • 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] • 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] 23 Certified Wireless Network Administrator: CWNA – PW0-105

  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

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