How does a station connect to an Access Point?

Control services at MAC

Synchronization, Roaming and Association.

• Functions to find a network.

• Change APs.

• Search APs.

Power Management.

• Sleep mode without losing packets.

• Power management functions.

MIB: Management information base.

Security: authentication and cypher.

SSID

Mechanism used to segment wireless networks.

• Multiple independent wireless networks can coexist in the same location.

Each AP is programmed with a SSID that corresponds to its network.

Client computer presents correct SSID to access AP.

Security Compromises.

• AP can be configured to “broadcast” its SSID.

• Broadcasting can be disabled to improve security.

• SSID may be shared among users of the wireless segment.

Association Management

Scanning

Scanning is needed to:

• Find and connect to a networks.

• Find a new AP during roaming.

Passive Scanning:

• Station simply listens for Beacon and get info of the BSS. Power is saved.

Active Scanning:

• Station transmits Probe Request; elicits Probe Response from AP. Saves time.

Scanning, and Joining

Station must associate with an AP before they can use the network.

• AP must know about them so it can forward packets.

Re-association (roaming): association is transferred.

• Supports mobility in the same ESS.

Disassociation: station or AP can terminate association.

Stations can detect AP based on scanning.

Joining a BSS.

• Synchronization in Timestamp Field and frequency (i.e., channel).

• Adopt PHY parameters.

• Other parameters: BSSID, WEP, Beacon Period, etc.