Dec 2009
doc.: IEEE 802.11-09/1299r0
BSS Transition Improvements
Date: 2009-12-xx
Authors:
Name
Affiliations
Address
Phone
email
Allan Thomson
Cisco Systems
+1-408-853-5570
[email protected]
Douglas Chan
Cisco Systems
+1-408 577 9344
[email protected]
Hari Rangarajan
Cisco Systems
+1 408 527 6869
[email protected]
Sandeep Shetty
Cisco Systems
170 W. Tasman Drive, San
Jose, CA95134
170 W. Tasman Drive, San
Jose, CA95134
170 W. Tasman Drive, San
Jose, CA95134
170 W. Tasman Drive, San
Jose, CA95134
+1 408 527 4952
[email protected]
Submission
Slide 1
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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
Submission
Slide 2
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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
Submission
Slide 3
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
Solution Overview
Extend BSS Transition Query and Request/Response
frames to enable improved BSS Transition candidate
lists
Define additional behavior on the protocol
Submission
Slide 4
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
Use Case 1
Use case
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
Possible reasons Ensuing actions with a cooperating
Ensuing actions with a
client
non-cooperating client
- Client feels - Client sends BSS Transition Query - Client moves to an
performance
to current AP with client’s list of
AP that it likes
over the link
preferred APs.
without consulting
with current - Current AP sends BSS Transition
with current AP, or
AP is not
Req to client with current AP’s list
ignores the
satisfactory
of preferred APs. (The
recommendations
- Client feels
Disassociation Imminent bit here is
sent by the current
that it has
assumed to be set to 0, since this
AP.
roamed to a
reply from the AP was initiated by
different
the client’s query.)
location
- If client decides to transition, then
it should attempt to associate with
the APs on the AP’s list.
(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.)
Submission
Slide 5
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
Use Case 2
Use case
Possible reasons Ensuing actions with a 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.
-
-
-
-
-
Submission
Ensuing actions with a
non-cooperating client
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.
Slide 6
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
Dec 2009
doc.: IEEE 802.11-09/1299r0
Use Case 3a
Use case
Possible
reasons
Ensuing actions with a cooperating
client
Current AP
wishes to move
a client to
another AP .
Same as Case 2
-
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.
-
-
-
Submission
Ensuing actions with a
non-cooperating client
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.
Slide 7
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
Dec 2009
doc.: IEEE 802.11-09/1299r0
Use Case 3b
Use case
Possible
reasons
Ensuing actions with a cooperating
client
Current AP
wishes to move
a client to
another AP .
Same as Case 2
-
AP has never
asked the client
for the client’s
preferred list or
it’s been a while
since the AP
asked.
-
-
Ensuing actions with a
non-cooperating client
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
But the
cooperating
client is happy
with the AP’s
list of target
BSS.
Submission
Slide 8
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
Use Case 4
Use case
Possible
reasons
Current AP
AP is just
wishes to obtain planning ahead
a fresh preferred
target AP list
from the client
Submission
Ensuing actions with a cooperating
client
-
-
Ensuing actions with a
non-cooperating client
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.
Slide 9
Client responds with a
bogus or empty list.
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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
Submission
Slide 10
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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
Submission
Slide 11
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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 5 (Reject - STA BSS Transition Candidate List
provided)
– If the non-AP STA has information on BSS transition candidates
or wishes to share such information, then the non-AP can include
them in the BSS Transition Candidate List of the response frame;
otherwise the list is empty.
Submission
Slide 12
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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)
Submission
Slide 13
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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
Submission
Slide 14
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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
Submission
Slide 15
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
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 nonAP 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
Submission
Slide 16
Allan Thomson, Cisco Systems
Dec 2009
doc.: IEEE 802.11-09/1299r0
Questions ?
Submission
Slide 17
Allan Thomson, Cisco Systems