1. © Kemal AkkayaWireless & Network Security 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

2. © Kemal AkkayaWireless & Network Security 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

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

4. © Kemal AkkayaWireless & Network Security 4 802.11 MAC Sublayer DCF and PCF coexist and alternate; PCF logically sits on top of DCF  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

5. © Kemal AkkayaWireless & Network Security 5 CSMA/CA Algorithm  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

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

7. © Kemal AkkayaWireless & Network Security 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

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

9. © Kemal AkkayaWireless & Network Security 9 802.11b Frame Format FC (2 bytes) ID (2) Add1 (6) Add2 (6) Add3 (6) SC (2) Add4 (6) Data (0-2312 bytes) CRC (4) 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) IEEE 802.11b MAC Frame Format Frame Control Format (2 bytes) Frame Control (FC): protocol version and frame type Duration/ID (ID): power-save poll message frame type and for NAV calculation Address Fields: contains up-to 4 MAC addresses Sequence 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. Protocol Version: version of 802.11 standard Type: Management. Control, Data frame Subtype: RTS, CTS, ACK frame To DS: 1 if frame is sent to Distribution System (DS) From DS: 1 if frame is received from Distribution System More 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.