IEEE 802.11 Mobility

Standard defines the following mobility types:

• No-transition: no movement or moving within a local BSS.

• BSS-transition: station moves from one BSS in one ESS to another BSS within the same ESS.

• ESS-transition: station moves from a BSS in one ESS to a BSS in a different ESS (continuos roaming not supported).

Roaming

Roaming: station changes network (BSS).

STA may go:

• Outside the coverage area of their AP.

• But still under the coverage area of another AP.

Reassociate the STA with the new AP allows the communication to continue.

STA decides that the signal with the current AP is bad.

STA does scanning (act/pas) to find new AP.

STA reassociates with the New AP (NAP).

• Includes authorization.

Without positive answer.

• STA does new scan.

With positive answer:

• STA changed network to the new NAP.

• AP informs the ESS of the new association.

• Information in the distributed system is always updated.

Attachment to a BSS

The STA finds a BSS/AP through Scanning/Probing.

Both Authentication as well as Association are necessary to enter a BSS.

Phase 1: Scanning

The STA searches for APs.

• Passive Scanning

  • STA analizes channels looking for Beacon packets, which are periodically sent by the AP, announcing its presence and SSID.

• Active Scanning

  • STA sends Probe Request packets to all channels in sequence.

  • AP’s listening in these diferente channels respond with a Probe Response.

Phase 2: Authentication

After finding and selecting an AP, the STA has to authenticate with it.

Two main methods:

• Method 1: Open System Authentication

  • Default procedure, executed in 2 steps:

    • 1 - STA sends an authentication frame including its identity.

    • 2 - AP responds with a frame as a Ack/NAck.

• Method 2: Shared Key Authentication

  • STA and AP have a shared secret, obtained in some other way.

  • 1 – STA sends an initial authentication request.

  • 2 – AP replies to the STA with a challenge.

  • 3 – STA decyphers the challenge with its own key and sends it to the AP.

  • 4 – AP uses its own key to decifer the challenge and compares results.

Phase 3: Association

After authenticated, the STA begins the association process, i.e.,

Exchange roaming and capacity information between STA and AP.

Procedure:

1. STA sends a Associate Request to AP, indicating supported transmission rates and intended association SSID.

2. AP allocates resources and decides if it accepts or rejects the STA.

3. P sends an Association Response, indicating the association identifying and supportted transmission rates, in case the association is accepted.

4. (optional) – In case of a handover (transition of the STA between two different APs), the new AP informs the old AP.

Only after associating to the AP, can the STA start to send and receive data.