WLAN

Networks: Wireless LANs2 Distribute Coordination Function (DCF) Distributed access protocol Contention-Based Uses CSMA/ CA – Uses both physical and virtual carrier sensing. Suited for ad hoc network and ordinary asynchronous traffic

Networks: Wireless LANs3 Physical Carrier Sensing Station senses the channel when it wants to send. If idle, station transmits. – Station does not sense channel while transmitting. If the channel is busy, station defers until idle and then transmits. Upon collision, wait a random time using binary exponential backoff.

Networks: Wireless LANs4 Virtual Channel Sensing in CSMA/CA Figure The use of virtual channel sensing using CSMA/CA. C (in range of A) receives the RTS and based on information in RTS creates a virtual channel busy NAV(Network Allocation Vector). D (in range of B) receives the CTS and creates a shorter NAV.

Networks: Wireless LANs5 Virtual Channel Sensing in CSMA/CA “virtual” implies source station sets duration field in data frame or in RTS and CTS frames. Stations then adjust their NAV accordingly!

– Alternative access method on top of DCF – Centralized access protocol – Contention-Free – Works like polling PCF uses a base station to poll other stations to see if they have frames to send. Base station sends beacon frame periodically. No collisions occur. – Suited for time bound services like voice or multimedia Point Coordinated Function (PCF)

Networks: Wireless LANs7 DCF and PCF Co-Existence Distributed and centralized control can co-exist using InterFrame Spacing. SIFS (Short IFS) :: is the time waited between packets in an ongoing dialog (RTS,CTS,data, ACK, next frame) PIFS (PCF IFS) :: when no SIFS response, base station can issue beacon or poll. DIFS (DCF IFS) :: when no PIFS, any station can attempt to acquire the channel. EIFS (Extended IFS) :: lowest priority interval used to report bad or unknown frame.

Networks: Wireless LANs8 Interframe Spacing in DIFS (MAX) > PIFS > SIFS (MIN) SIFS - Used for immediate response actions e.g ACK, CTS PIFS - Used by centralized controller in PCF scheme DIFS - Used for all ordinary asynchronous traffic

RTS-CTS-DATA-ACK

Security In theory, spread spectrum radio signals are inherently difficult to decipher without knowing the exact hopping sequences or direct sequence codes used The IEEE standard specifies optional security called "Wired Equivalent Privacy" whose goal is that a wireless LAN offer privacy equivalent to that offered by a wired LAN. The standard also specifies optional authentication measures.

Security WEP – A protocol to protect link-level data during wireless transmission between clients and access points. Services: – Authentication: provides access control to the network by denying access to client stations that fail to authenticate properly. – Confidentiality: intends to prevent information compromise from casual eavesdropping – Integrity: prevents messages from being modified while in transit between the wireless client and the access point.