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

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

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

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

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

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

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

Data-Link Layer Divided into two sublayers Upper portion – 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 based networks standard defines operations at the MAC sublayer 8 Certified Wireless Network Administrator: CWNA – PW0-105

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 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 n amendment 9 Certified Wireless Network Administrator: CWNA – PW0-105

MAC Protocol Data Unit (MPDU) The 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

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

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

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 radios PMD sublayer takes the PPDU, modulates the data bits and begins transmitting 13 Certified Wireless Network Administrator: CWNA – PW0-105

and Interoperability max frame size is just over 1,500 bytes 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 frame has two MAC address fields frame has up to four MAC address fields (most frames only use three address fields) 14 Certified Wireless Network Administrator: CWNA – PW0-105

and Interoperability (continued) address fields –Source address (SA) –Destination address (DA) address fields –“Address1” through “Address4” –The contents of the 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

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

Three 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

Management Frames Majority of the frame types Management MAC Protocol Data Unit (MMPDU) Used by 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

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

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

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

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

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]

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

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

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

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

Authentication First step when connecting to a basic service set (BSS) Should be referred to as “ Authentication” Often misunderstood or misinterpreted Authenticates that both the station and the AP are capable of 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

Association Occurs after 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

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

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

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

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

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

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

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

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

Fragmentation Breaks and 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

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

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

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

Data Frames Most common data frame is the simple data frame AP and controller integration services take the MSDU payload of an data frame and transfers it into an 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

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

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

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

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

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

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

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

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

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