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When 802.11 authentication (not the RSN-WPA/WPA2 authentication) completes, a STA move to Association phase to the BSS. The purpose of this exchange is to join the cell & obtain an Association Identifier (AID). Below shows a typical client association where you can see Association Request & Association Response followed by 802.11 Authentication frames (request & response)

CWAP-Association-01Association Request frame is having following frame format (CWAP Study Guide- Page 136)CWAP-Association-10

Listen Interval:
The Listen Interval field is used to indicate to the AP how often a STA in power save mode wakes to listen to Beacon management frames. It is expressed in units of beacon interval. The value 0 might be used by a STA that never enters power save mode. An AP may use the Listen Interval information in determining the lifetime of frames that it buffers for a STA.

Below table summarize the information contain in a Association Request frame. (source IEEE 802.11-2012)

CWAP-Association-02In 802.11-2013 (802.11ac) amendment below two additional fields added to above list.CWAP-Association-03Here is the Association Request frame capture. Note that its type is “management” or value 0 with subtype value of 0 (Association Request). Listen Interval listed as 10. As you can see it listed the SSID trying to associate, supported data rate, Power capability, Supported Channels, HT & VHT capabilities.

CWAP-Association-04AP send an ACK for the Association Request frame sent by STA

CWAP-Association-05After acknowledging reception of the Association Request frame, the AP examine each field of the request & verify they all match its own 802.11 parameters. If there is a mismatch, AP decides whether this difference is a blocking factor (to the association).If the differences is not blocking, the AP take note of those differences & grant access to the cell, indicating own parameters in the Association Response frame. If the difference is blocking AP reject the association (status code =1). Here is the  frame format of Association Response frame. (page 139- CWAP Study Guide)

CWAP-Association-11Here is the Association Response capture in the given frame exchange. Note that status code 0 (successful) & AID of 4 allocated for this client.

CWAP-Association-06This Association Response will ACK by the STA.CWAP-Association-07Here is the information contain in Association Response frame (source IEEE 802.11-2012).CWAP-Association-08802.11ac amendment added following field onto Association Response frame.CWAP-Association-09

1. CWAP Official Study Guide – Chapter 4
2. IEEE 802.11-2012

Related Posts

1. 802.11 Mgmt Frame Types
2. 802.11 Mgmt : Beacon
3. 802.11 Mgmt : Probe Req/Res
4. 802.11 Mgmt : Authentication Frame
5. 802.11 Deauthentication & Disassociation
6. 802.11 Mgmt : Information Elements
7. 802.11 Mgmt : Action Frames
8. 802.11 Mgmt : Spectrum & TPC
9. 802.11 Mgmt : Admission Control