Last Lecture: Data Link Layer



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23- WLAN Introduction (1) (1)

Channels and Association

  • 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies
    • Network administrator chooses frequency for AP
    • Interference if channel is same as neighboring AP
  • Access points send periodic beacon frames
    • Containing AP’s name (SSID) and MAC address
    • Host scans channels, listening for beacon frames
    • Host selects an access point to associate with

oBeacon frames from APs oAssociate request from host oAssociation response from AP

802.11: Passive/Active scanning


AP 2

AP 1


H1

BBS 2


BBS 1

1

2

2

3

4

Active Scanning:
  • Probe Request frame broadcast from H1
  • Probes response frame sent from

  • APs
  • Association Request frame sent:

  • H1 to selected AP
  • Association Response frame sent: H1 to selected AP

AP 2

AP 1


H1

BBS 2


BBS 1

1

2

3

1

Passive Scanning:
  • beacon frames sent from APs
  • association Request frame sent: H1 to selected AP
  • association Response frame sent: H1 to selected AP

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