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)
Association Request frame is having following frame format (CWAP Study Guide- Page 136)
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)
In 802.11-2013 (802.11ac) amendment below two additional fields added to above list.Here 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.
AP send an ACK for the Association Request frame sent by STA
After 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)
Here is the Association Response capture in the given frame exchange. Note that status code 0 (successful) & AID of 4 allocated for this client.
This Association Response will ACK by the STA.Here is the information contain in Association Response frame (source IEEE 802.11-2012).802.11ac amendment added following field onto Association Response frame.
1. CWAP Official Study Guide – Chapter 4
2. IEEE 802.11-2012
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
One more good post..
Thanks for your post, Here is one question if you give some advice?
For associate response frame, what’s the channel bandwidth value AP will reply to STA side, the max channel bandwidth it support? Or the intersection value between it and STA? Thank you again.
And what’s the difference with beacon/probe request frame?
1. Beacon is broadcast frame and probe response is unicast in nature .
2. Beacon can carry TIM information for Dozing clients and QoS Control info in BSS .
3. Probe response is acked from received client and beacons are not acked (bcast)
4. Probe response contains only the request information elements by clients
Hi Sandy, Thank you very much for answering these queries time to time, greatly appreciated
I’m curious about duration field. is it fixed to 44us?
frame body length or symbol number does not influence duartion?
Can you please tell me the Mac header size for 802.11ac
Thank you for your post.
Ravi Kumar said:
Thanks for the post.
I have a question like for security >WPA, association response & Re-association response contains RSN IE?