CITS3002 Computer Networks  
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Access-Point Association

Although two mobile nodes can communicate directly, the more usual approach is for all communication to be through fixed access-points.

We say that a mobile node associates with a single access-point if all of its communications are via that point, and all such related nodes form a cell.

Communication between nodes in different cells requires two access-points connected via a distribution system.

The mechanism employed by a mobile node to select an access-point is termed active scanning:

  1. the mobile client node sends Probe frames,
  2. all access-points within range reply (if they have capacity) with a Probe Response frame,
  3. the mobile node selects an access-point and sends an Association Request frame, and
  4. the access-point responds with an Association Response frame.
An alternative is for an access-point to use passive scanning:

  1. the access-point periodically sends Beacon frames advertising its existence and abilities (e.g. supported bandwidths),
  2. the mobile node may choose to switch to this new access-point using Disassociation and Reassociation frames.

When a node selects a new access-point, the new access-point is expected to inform the old access-point using the distribution system.

CITS3002 Computer Networks, Lecture 4, Local Area Networks (LANs and WLANs), p22, 20th March 2024.