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Transcript
802.11 MAC Architecture Module-7 Jerry Bernardini Community College of Rhode Island 5/25/2017 Wireless Networking J. Bernardini 1 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 5/25/2017 Wireless Networking J. Bernardini 2 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 5/25/2017 Wireless Networking J. Bernardini 3 OSI data flow CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini 4 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 5 Where the IEEE 802.11 Standard Fits 6 Data Link Layer - Physical Layer- Data Units MSDU (MAC Service Data Unit) (From upper layers 2304 bytes max) LLC MAC 802.2 Logical Link Control Data Link Layer (Layer-2) 802.11 Media Access Control MPDU (MAC Protocol Data Unit) PSDU (PLCP Service Data Unit) (MPDU = PSDU name change to PLCP PHY Layer Convergence Protocol indicated service needed) Physical Layer (Layer-1) PPDU (PLCP Protocol Data Unit) PMD Physical Medium Dependent PHY = Physical Layer Modulated Radio Signal Ethernet and 802.11 Frames • Ethernet Frame 7 1 Preamble 1518 6 6 2 Source Destination 46 - 1500 4 Data FCS Type or Length Field Start Of Frame • Wireless Frame 10 or 18 2 4 or 6 Sync PLCP Header Start Of Frame 2 2 6 6 6 Source Destination Rec. Adr Duration ID Frame Cntrl 2 6 0 - 2304 4 Trans. Adr Data FCS Sequence Cntrl MAC Packet DATA Unit, (MPDU) Frame Categories / Types • Management Frames o Beacon Frame o Probe Frames o Association Frames… more • Control Frames o RTS and CTS Frames o ACK – Acknowledgement Frames… more • Data Frames o Data Payload Frames Twelve Management Frame Types 5/25/2017 Wireless Networking J. Bernardini 10 Eight Control Frames • Used to assist with the delivery of data frames 5/25/2017 Wireless Networking J. Bernardini 11 Fifteen Data Frames • The frames that actually carry application data 5/25/2017 Wireless Networking J. Bernardini 12 IEEE 802.11 Frame Formats Management Duration (2) Frame Control (2) Control Data Frame Control (2) Frame Control (2) Duration (2) Des. Address (6) Duration (2) Address 1 (6) Source Address (6) Receiver Address (6) Address 2 (6) Address 3 (6) BSSID (6) Seq. Control (2) Transmit Address ( 6) Frame Check Seq. (6) Seq. Control (2) Address 4 (6) Frame Body ( 1 to 2311) Data ( 1 to 2311) Frame Check Seq. (6) Frame Check Seq. (6) (Bytes per field) CWNA Guide to Wireless LANs, Second Edition 13 Frame Types and Sizes Protocol Data Unit (PDU) Frame Type and Vendor Support MTU (Bytes) TCP Transport maximum segment size 1460 Layer-3 default size 1500 IEEE 802.3 Ethernet default 1500 MPDU IEEE 802.11 default 1534 MPDU IEEE 802.11 maximum 2304 MAC Jumbo Frame >1500 MAC Cisco Baby giant 1552 1600 MAC Cisco Catalyst 4000 9198 9216 MAC Cisco Catalyst 6000 9216 9234 IP MAC 5/25/2017 Wireless Networking J. Bernardini MTU + Overhead (Bytes) 1518 14 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 Header Payload Beacon Frame Support Rates Parameter Sets Traffic Indication Map Trailer 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 5/25/2017 Wireless Networking J. Bernardini 16 Passive Scanning (Beacons) 1. Client stations listens for a beacon from an access point (AP) 2. If multiple beacons are received the strongest one is selected 3. The listening station then requests authentication and association Beacons Beacons S1 5/25/2017 AP Control Point Wireless Networking J. Bernardini S2 17 Active Scanning (Probes) • A station could use probe request and probe response frames Instead of beacon frames 1. Station is configured with SSID and switched to a channel 2. Probe request sent by requesting station 3. 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 S1 5/25/2017 AP Control Point Wireless Networking J. Bernardini S2 18 Authentication and Association • • 1. 2. 3. • 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 5/25/2017 Wireless Networking J. Bernardini 19 Wireless Connection Process • 2-Step Connection Process – Authentication Phase – Association Phase Authentication Phase Association Phase 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 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. 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. Connection State Machine Unauthenticated and Unassociated Successful Authentication De-authentication Notification Authenticated and Unassociated Successful Association Disassociation Notification Authenticated and Associated De-authentication Notification Unauthenticated & Unassociated • In the first state: – Wireless Client/Node is not connected to the network. Wired LAN Access Points Wireless Client 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 Association Request Wireless Client Access Points 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 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 Open System Authentication Wired LAN Authentication Management Frame - SSID ACK Wireless Client Authentication Management Frame ACK Access Points Closed System Authentication • Closed System – – – – IEEE 802.11 Authentications based only on SSID Client sends SSID Receiving station, (AP) sends acknowledgment Shared Key Authentication • Shared Key – IEEE 802.11 Wired Equivalent Privacy, (WEP). – Authentications based on Text and WEP Keys. – Challenge – Response Scheme Shared Key Authentication Wired LAN Authentication Management Frame - SSID Challenge Phrase Encrypted Phrase Wireless Client WEP Key Access Points Authentication Secure Channel WEP Key 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. Roaming Wired LAN Access Points 1 1 2 1 Wireless Clients Usually a 20%-30% cell overlap Unauthenticated & Unassociated • In the first state: – Wireless Client/Node is not connected to the network. Wired LAN Access Points Wireless Client 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 Association Request Wireless Client Access Points 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 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 5/25/2017 Wireless Networking J. Bernardini 38 802.11 Open Association Process 5/25/2017 Wireless Networking J. Bernardini 39 5/25/2017 Wireless Networking J. Bernardini 40 5/25/2017 Wireless Networking J. Bernardini 41