1. BSS Transition Improvements
Month Year
doc.: IEEE yy/xxxxr0
Dec 2009
BSS Transition Improvements
Date:
Authors:
Allan Thomson, Cisco Systems
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
Dec 2009
Abstract
This proposal highlights improvements to BSS Transition that enable the AP to provide a “better” set of transition candidates to the non-AP STA
The goal is to provide an improved roaming behavior for both the non-AP STA and the AP managing the BSS
Allan Thomson, Cisco Systems
John Doe, Some Company

3. Month Year
doc.: IEEE yy/xxxxr0
Dec 2009
Problem
The AP determines the BSS Transition Candidate list based on its own knowledge of the environment
This results in potentially a BSS Transition Candidate List that does match the non-AP STA’s list or preferences
Non-AP STAs have no way of providing to the AP the list of preferred BSS Transition candidates from their perspective
Allan Thomson, Cisco Systems
John Doe, Some Company

4. Month Year
doc.: IEEE yy/xxxxr0
Dec 2009
Solution Overview
Extend BSS Transition Query and Request/Response frames to enable improved BSS Transition candidate lists
Define additional behavior on the protocol
Allan Thomson, Cisco Systems
John Doe, Some Company

5. Use Case 1 Dec 2009 Use case Possible reasons
Ensuing actions with a cooperating client
Ensuing actions with a non-cooperating client
A transition event is triggered on the client or
the client feels that the preferred AP list that it last supplied to the AP is now stale
Client feels performance over the link with current AP is not satisfactory
Client feels that it has roamed to a different location
Client sends BSS Transition Query to current AP with client’s list of preferred APs.
Current AP sends BSS Transition Req to client with current AP’s list of preferred APs. (The Disassociation Imminent bit here is assumed to be set to 0, since this reply from the AP was initiated by the client’s query.)
If client decides to transition, then it should attempt to associate with the APs on the AP’s list.
Client moves to an AP that it likes without consulting with current AP, or ignores the recommendations sent by the current AP.
(The implication in here and the following use cases is that the AP would have used the client’s list and preferences to make best recommendation in the request.)
Allan Thomson, Cisco Systems

6. Use Case 2 Dec 2009 Use case Possible reasons
Ensuing actions with a cooperating client
Ensuing actions with a non-cooperating client
Current AP wishes to move a client to another AP
Current AP performs load balancing
Current AP feels there’s another AP that can give client better performance, etc.
Current AP sends a BSS Transition Req with the Disassociation Imminent field set to 0 and without a list of preferred APs.
Client shall replies with a BSS Trans Response with a list  of his preferred APs.
Then AP sends a BSS Transition Req with the Disassociation Imminent field set to 1 and with a list of preferred AP.
Since the client just supplied his list to the AP, the client has no doubts with the staleness of it and the client does not feel need to delay or reject the transition request, so the client replies with a BSS Trans Response with Status Code of Accept.
Since it’s an Accept, then the client shall attempt to associate with the APs on the AP’s list.
Client responds with a bogus or empty list.
Client ignores the recommendations sent by the current AP and moves to an AP that he likes.
If Client does nothing, AP eventually disassociates this client
Allan Thomson, Cisco Systems

7. Use Case 3a Dec 2009 Use case Possible reasons
Ensuing actions with a cooperating client
Ensuing actions with a non-cooperating client
Current AP wishes to move a client to another AP .
AP has never asked the client for the client’s preferred list or it’s been a while since the AP asked.
The cooperating client feels it should supply its preferences to the AP.
Same as Case 2
Then AP sends a BSS Transition Request with the Disassociation Imminent field set to 1 and with a list of preferred AP.
Client sends a BSS Transition Query to the AP with his latest preferred list (probably after the client has performed some scanning)
Client will have at least 30 seconds to do this before being disassociated
Upon receiving of the Query frame, the AP replies with a BSS Transition Request to client with a (possibly new) list of preferred APs (and the Disassociation Imminent field set)
The client is more satisfied with the present candidate list and there’s no need to delay or reject the transition request, so the client replies with a BSS Transition Response with Status Code of Accept.
Since it’s an Accept, then the client shall attempt to associate with the APs on the AP’s list.
Client ignores the recommendations sent by the current AP and moves to an AP that he likes.
If Client does nothing, AP eventually disassociates this client
Allan Thomson, Cisco Systems

8. Use Case 3b Dec 2009 Use case Possible reasons
Ensuing actions with a cooperating client
Ensuing actions with a non-cooperating client
Current AP wishes to move a client to another AP .
AP has never asked the client for the client’s preferred list or it’s been a while since the AP asked.
But the cooperating client is happy with the AP’s list of target BSS.
Same as Case 2
Then AP sends a BSS Transition Req with the Disassociation Imminent field set to 1 and with a list of preferred AP.
The client is satisfied with the current candidate list and there’s no need to delay or reject the transition request, so the client replies with a BSS Transition Response with Status Code of Accept.
Since it’s an Accept, then the client shall attempt to associate with the APs on the AP’s list.
Client ignores the recommendations sent by the current AP and moves to an AP that he likes.
If Client does nothing, AP eventually disassociates this client
Allan Thomson, Cisco Systems

9. Use Case 4 Dec 2009 Use case Possible reasons
Ensuing actions with a cooperating client
Ensuing actions with a non-cooperating client
Current AP wishes to obtain a fresh preferred target AP list from the client
AP is just planning ahead
Current AP sends a BSS Transition Request with the Disassociation Imminent field set to 0 and without a list of preferred AP.
Client shall reply with a BSS Trans Response with a list of its preferred APs, list may be empty.
Client responds with a bogus or empty list.
Allan Thomson, Cisco Systems

10. Query Frame Changes Add BSS Transition Candidate List Entries field
Dec 2009
Query Frame Changes
Add BSS Transition Candidate List Entries field
Allows the non-AP STA to provide a list of target transition BSSs and relative preferences
Add BSS Transition Query Reason code
(ANA) Providing STA BSS Transition Candidate List
Enhance AP Behavior
Will consider them in addition with the AP’s own candidates when determining the best BSS transition candidates for the non-AP STA
Allan Thomson, Cisco Systems

11. Response Frame Changes
Month Year
doc.: IEEE yy/xxxxr0
Dec 2009
Response Frame Changes
Add BSS Transition Candidate List Entries field
Allows non-AP STA to provide the AP with a list of target transition BSSs and the relative preferences for transitioning there
Add a Status code
(5) Reject – STA BSS Transition Candidate List provided
Enhance AP Behavior
AP will consider them in addition with the AP’s own candidate list when determining the best BSS transition candidates for this non-AP STA
Allan Thomson, Cisco Systems
John Doe, Some Company

12. Dec 2009
Behavior Changes (1a): The AP wishes to obtain the non-AP STA’s preferred list of BSS candidates
The AP sends a BSS Transition Management Request frame with
Disassociation Imminent field set to 0
Preferred Candidate List Included field set to 0
The non-AP STA shall reply with a BSS Transition Management Response frame, wherein the Status Code is set to one of these:
Reject – STA BSS Transition Candidate List provided
The BSS Transition Candidate List shall be non-empty in this case
Reject – No suitable BSS transition candidates
Allan Thomson, Cisco Systems

13. Dec 2009
Behavior Changes (1b): The AP must include at least one BSS that the non-AP STA has provided
The non-AP STA has provided a non-empty BSS Transition Candidate List Entries in a Query or Response frame
Then in any BSS Transition Management Request frames that the AP sends to this non-AP STA, the frame shall include at least one BSS from the non-AP’s list entries
This entry shall not have preference value of 0
(0 = refrain from associating to this listed BSS)
Allan Thomson, Cisco Systems

14. Dec 2009
Behavior Changes (2): The non-AP STA is satisfied with the AP’s preferred list of BSS transition candidates
The AP sends a BSS Transition Management Request frame with:
Disassociation Imminent field set to 1
Preferred Candidate List Included field set to 1
An non-empty list of ranked BSS transition candidates
If the non-AP STA is satisfied with the AP’s list:
The non-AP shall reply with a BSS Transition Management Response frame with Status code set to 0 (Accept)
The non-AP shall disassociate with the current AP
The non-AP STA shall attempt to re-associate with the BSSs in listed in the Request frame’s BSS Transition Candidate List
Allan Thomson, Cisco Systems

15. Dec 2009
Behavior Changes (3): Time before disassociation is at least 30 seconds
After the first BSS Transition Management Request frame sent to a non-AP STA with Disassociation Imminent field set to 1:
The Disassociation Timer field is a number of TBTTs that is at least 30 seconds or “0” (disassociate time not decided yet)
AP keeps a countdown timer starting with the Disassociation Timer value (if it is not “0”) or a number of TBTT at least 30 seconds (if Timer is “0”)
Countdown timer is decremented by 1 with each beacon transmitted
In any subsequent BSS Transition Management Request frame sent to this non-AP STA, the Disassociation Timer shall be set to the value in the countdown timer
Allan Thomson, Cisco Systems

16. Dec 2009
Behavior Changes (4): The AP cannot disassociate a STA before time in countdown timer is up for this STA
After the first BSS Transition Management Request frame sent to a non-AP STA with Disassociation Imminent field set to 1
AP begins a countdown timer for this non-AP STA (see previous slide)
The AP shall not send a Disassociate frame to this non-AP STA before the countdown timer for this non-AP STA has attained value 0
If the AP changes its mind on disassociating this non-AP STA, then the AP simply sends a new Request frame with Disassociation Imminent set to 0
Allan Thomson, Cisco Systems

17. Dec 2009
Behavior Changes (5): The non-AP STA shall reply with a Response for unicast Request
If the received BSS Transition Management Request is an individually-addressed frame
Then non-AP STA shall reply with a BSS Transition Management Response frame
No change to behavior if the Request frame is group-addressed
i.e., that the non-AP STA shall not respond in this case
Allan Thomson, Cisco Systems

18. Dec 2009
Questions ?
Allan Thomson, Cisco Systems