u8.3 802.11 Data Link Layer

u8.3 802.11 Data Link Layer

1. Hidden Nodes

   • When 2 devices are in range of an AP but not each other
     • AP must mediate between the 2 devices that are hidden from each other
       • Sending device transmits a request to send to AP
         • AP responds with a clear to send to all devices
           • Delays transmission until the sending device has successfully sent its data

2. 2 Sub Layers

   • Logic Link Control
     • Uses same 48 bit addressing as 802.3 ethernet
   • Media Access Control (MAC)
     • Cant use CSMA/CD (carrier sense multiple access / collision detection
       • Radio in wireless adapter cant receive and transmit a signal at the same time
         • So collisions go unnoticed
           • Instead it uses CSMA / CA = Collision avoidance
             • Sender of a wireless frame waits until the air is clear before sending
               • receiver has to send an acknowledgement to let the sender know the data arrived in tact
                 • If none is received then the sender knows a collision happened and resends
                   • Downside to this is overhead, acknowledgements must be sent for every piece of data

3. Joining a Basic Service Set

   • AP broadcasts a beacon, an advertisement for that AP containing info about the service set it represents
     • Station can receive beacons from a number of AP's within range
       • Station chooses the AP that represents the BSS it wants to join
         • Station must then authenticate to the AP
           • After authentication the station and AP are associated and transfer can begin
             • Reassociation
               • 2 events that may cause a station to reassociate with another AP
                 • If it finds a stronger signal or more reliable AP for the same BSS
                 • if the signal strength or signal-to-noise ratio for the current access point drops below a useable level