IEEE 802.11 Wireless LAN Standard Chapter 14

IEEE 802 Protocol Layers

Protocol Architecture Functions of physical layer: Encoding/decoding of signals Preamble generation/removal (for synchronization) Bit transmission/reception Includes specification of the transmission medium

Protocol Architecture Functions of medium access control (MAC) layer: On transmission, assemble data into a frame with address and error detection fields On reception, disassemble frame and perform address recognition and error detection Govern access to the LAN transmission medium Functions of logical link control (LLC) Layer: Provide an interface to higher layers and perform flow and error control

Separation of LLC and MAC The logic required to manage access to a shared-access medium not found in traditional layer 2 data link control For the same LLC, several MAC options may be provided

IEEE 802.11 Services

MAC Frame Format MAC control Destination MAC address Contains Mac protocol information, e.g. priority level Destination MAC address Destination physical attachment point Source MAC address Source physical attachment point Data The body of the MAC frame (LLC PDU) CRC Cyclic redundancy check

LLC Services Unacknowledged connectionless service No flow- and error-control mechanisms Data delivery not guaranteed Connection-mode service Logical connection set up between two users Flow- and error-control provided Acknowledged connectionless service Cross between previous two Datagrams acknowledged No prior logical setup

IEEE 802.11 IEEE 802.11 is a set of standards for implementing WLAN computer communications in 2.4, 3.7, and 5 GHz frequency band. The first standard was IEEE 802.11b The Wi-Fi Alliance Industry consortium formed in 1999. Wi-Fi is concerned whether products from different venders will successfully interoperate (Certification process).

Data rate per stream (Mbit/s) IEEE 802.11 802.11 protocol Release Freq. (GHz) Bandwidth (MHz) Data rate per stream (Mbit/s) Modulation Range (m) Jun 1997 2.4 20 1, 2 DSSS, FHSS 100 a Sep 1999 5 6, 9, 12, 18, 24, 36, 48, 54 OFDM 35 120 3.7 — 5,000 b 5.5, 11 DSSS 140 g Jun 2003 OFDM, DSSS 38 n Oct 2009 2.4/5 7.2, 14.4, 21.7, 28.9, 43.3, 57.8, 65, 72.2 70 250 40 15, 30, 45, 60, 90, 120, 135, 150 ac (DRAFT) Nov. 2011 80 433, 867 160 867, 1.73 Gbit/s, 3.47 Gbit/s, 6.93 Gbit/s

IEEE 802.11 Architecture Distribution system (DS) Access point (AP) Basic service set (BSS) Stations competing for access to shared wireless medium Isolated or connected to backbone DS through AP Extended service set (ESS) Two or more basic service sets interconnected by DS

IEEE 802.11 WLAN topologies Infrastructure mode Wireless hosts communicate with an Access Point Basic Service Set (BSS) (cell) contains: 1 access point (AP): base station Several wireless hosts Access point realizes the control functionality Medium access, mobility management, authentication Wireless stations only need a minimum of functionality

IEEE 802.11 WLAN topologies Ad-Hoc mode No AP Wireless hosts communicate with each other Transferring packet from wireless host A to B may need to route through other wireless hosts. Independent Basic Service Set (IBSS) When all stations in the BSS are mobile stations with no connection to other BSSs. Applications: “Laptop” meeting in conference room / interconnection of “personal” devices / battlefield

IEEE 802.11 WLAN Terminologies Access Point – perform the wireless to wired bridging function between networks Wireless medium – means of moving frames from station to station Station – computing devices with wireless network interfaces (phones, laptops, desktops, PDAs, play-stations) Distribution System – backbone network used to relay frames between access points e.g. Wired Ethernet 14

Distribution of Messages Within a DS Distribution service Used to exchange MAC frames from station in one BSS to station in another BSS Integration service Transfer of data between station on IEEE 802.11 LAN and station on integrated IEEE 802.x LAN

Transition Types Based On Mobility No transition Stationary or moves only within BSS BSS transition Station moving from one BSS to another BSS in same ESS ESS transition Station moving from BSS in one ESS to BSS within another ESS

Access and Privacy Services Authentication Establishes identity of stations to each other Deauthentication Invoked when existing authentication is terminated Privacy Prevents message contents from being read by unintended recipient

Association-Related Services Establishes initial association between station and AP Reassociation Enables transfer of association from one AP to another, allowing station to move from one BSS to another Disassociation Association termination notice from station or AP

Station Connecting to a WLAN When a station moves into the coverage area of a WLAN, the following procedures take place: 1- Scanning: the station searches for a suitable channel over which subsequent communication takes place. 2 - Association: the station associates with an AP 3 - IP address allocation: the station gets an IP address, for instance from a DHCP server 4 - Authentication: only if this security option is required

Handover to another AP When a station has noticed that the radio connection to another AP is a better than the existing connection: 1- Reassociation: the station associates with another AP. 2 - No new IP address is needed; however, the WLAN must be able to route downlink traffic via the new AP. 3 - Authentication: this security option, if required, will result in a substantially increased handover delay (complete procedure sequence: deauthentication, disassociation, reassociation, authentication).