Department of Computer Science Southern Illinois University Carbondale

1. Department of Computer Science Southern Illinois University Carbondale CS591 – Wireless & Network Security Lecture 7: IEEE 802.11 Summary Dr. Kemal Akkaya E-mail: kemal@cs.siu.edu Wireless & Network Security 1

2. IEEE 802.11 Wireless LAN Standard • First Standard of WLANs • IEEE Std 802.11 – 1999 • The 802.11 standard provides MAC and PHY functionality for wireless connectivity of fixed, portable and moving stations moving at pedestrian and vehicular speeds within a local area Usage: • Avoid the high installation and maintenance costs incurred by traditional additions, deletions, and changes in wired LANs • Physical and environmental necessities • Operational environment; temporary usage Challenges and Constraints: • Frequency allocation • All users operates on a common frequency band • Must be approved and licensed by the government • Inference and reliability • Collision: begin transmission at the same time; hidden terminal; multipath fading • Security • Power consumption • Human safety • Mobility Wireless & Network Security 2

3. 802.11 Architecture • Works in Two Operational Modes • Infrastructure Mode • Ad-Hoc Mode Infrastructure Mode Access Point Access Point Wireless Link Wireless Link Wireless Link Mobile Station Extended Service Set (ESS) Basic Service Set (BSS) All traffic has to go through access points Access point provides connectivity to the wired backbone Wireless & Network Security 3

4. 802.11 MAC Sublayer • Primary operations • Accessing the wireless medium • Joining the network • Providing authentication and privacy • Wireless medium access • Support two different MAC modes depending on the operational mode of the Wireless LAN • DCF: Distributed Coordination Function • Based on CSMA/CA • Carrier Sensing: Physical and Virtual. • PCF: Point Coordination Function • Connection oriented • Contention free service • Polling based DCF and PCF coexist and alternate; PCF logically sits on top of DCF Wireless & Network Security 4

5. Sense channel (CS) If busy Back-off to try again later Else Send RTS If CTS not received Back-off to try again later Else Send Data If ACK not receive: Back-off to try again later Next packet processing Maintain a value CW (Contention-Window) If Busy: Wait till channel is idle. Then choose a random number between 0 and CW and start a back-off timer for proportional amount of time If transmissions within back-off amount of time, freeze back-off timer and start it once channel becomes idle again If Collisions (Control or Data) Binary exponential increase (doubling) of CW CSMA/CA Algorithm Wireless & Network Security 5

6. CSMA/CA – Transmission of MPDU (Data) using RTS/CTS DIFS RTS DATA Source SIFS SIFS SIFS CTS ACK Destination Others DIFS Defer Access for NAV(RTS) Defer Access for NAV(CTS) Backoff afterDefer Defer Access for NAV(Data) Wireless & Network Security 6

7. Channel Assignment and Registration • In multi-access environment, the operator should try to allocate non-overlapping channels to the physically adjacent channels • If adjacent access points use overlapping channels, then interference can be high • A mobile station periodically tunes to all channels and evaluates the signal strength received over each channel • Depending on the signal strength received over the channels, a mobile selects an access point and registers with that provided that the access points accepts the mobile. This is also called association • Re-association with a new access point occurs when • The mobile moves away from the current access point • When the signal conditions changes between the mobile and current access point • When there are a lot of users associated with the current access point Wireless & Network Security 7

8. Re-association at the PHY layer Access Point (AP) A Access Point (AP) B Signal from A Signal from B Associated withAccess Point B Associated withAccess Point A Mobile tunes to the channel of AP B when it moves into its range. Wireless & Network Security 8

9. 802.11b Frame Format IEEE 802.11b MAC Frame Format FC(2 bytes) ID(2) Add1(6) Add2(6) Add3(6) SC(2) Add4(6) Data(0-2312 bytes) CRC(4) Frame Control Format (2 bytes) Protocol(2 bits) Type(2) Subtype(4) To DS(1) From DS(1) More Frag(1) Retry(1) Pw Mgt(1) More Data(1) WEP(1) Order(1) Protocol Version: version of 802.11 standardType: Management. Control, Data frameSubtype: RTS, CTS, ACK frameTo DS: 1 if frame is sent to Distribution System (DS)From DS: 1 if frame is received from Distribution SystemMore fragment: 1 if there are more fragments belonging to the same frame following the current frame. Retry: indicates that is fragment is retransmission of previously transmitted fragment. Power Management: the type of power management mode that the station will be after the transmission of the frame. More Data: indicates that there are more frames buffered at the sender for this station. WEP: indicates that frame body is encrypted according to WEP. Order: indicates that the frame is sent using the strictly-ordered service class. Frame Control (FC): protocol version and frame typeDuration/ID (ID): power-save poll message frame type and for NAV calculationAddress Fields: contains up-to 4 MAC addressesSequence Control: fragmentation and sequence number. Data: higher layer data that is maximum 2312 bytes. CRC: 32 bit cyclic redundancy check for detecting error on the frame. Wireless & Network Security 9