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Enhancing bss transition management
Enhancing BSS Transition Management



Kenan Xu, Nortel Networks


  • Amendment proposals (on IEEE802.11v) are made to enable a streamlined network-initiated BSS transition process when the current AP cannot honor more resource (TSPEC) requests, while neighboring APs have abundant unused radio resource.

  • The resulting network-initiated load shifting process is more effective, efficient and deterministic than an STA-initiated BSS transition process.

Kenan Xu, Nortel Networks


  • BSS transition management in IEEE 802.11v

  • Issues with current BSS transition management process

  • Proposed solution to enhance the BSS transition process

  • Standard amendment proposal

  • Straw polls

Kenan Xu, Nortel Networks

802 11v bss transition management procedures
802.11v BSS Transition Management Procedures

Current AP


1.TSPEC rejection

2. Load balancing

3. Excessive frame loss

4. Excessive delay, etc.

BSS Transition Query (Reason Code )

BSS Transition Request (Neighbor Report element)

BSS Transition Response (status code, target BSSID)

Kenan Xu, Nortel Networks

Sta initiated load shifting due to tspec rejection
STA-initiated Load Shifting due to TSPEC Rejection

Associated AP







5. Tclas is not supported

6. Have no sufficient Tclas resource

There is no existing status code to indicate the opportunity of BSS transition. Hence, individual vendors must chose which status code to send and how to interpret the code.

ADDTS Response

(?status code)

BSS Transition Query (TSPEC rejection)

BSS Transition Request (Neighbor Report element)

AP is better aware of the available network resources than the STA. AP could inform the STA of the opportunity to transition and initiate the process.

BSS Transition Response

Kenan Xu, Nortel Networks

Consequences of sta initiated load shifting
Consequences of STA-initiated Load Shifting

  • If the ADDTS Request is rejected, the mobile STA doesn't know its full range of possible next steps.

  • Currently, the mobile STA can:

    • retry with new TSPEC settings,

    • transition to another AP based on its own logic,

    • issue a BSS transition query to the current AP,

    • reject the session, or

    • wait.

  • Each of those alternatives involves a multistep sequence which could fail, require successive attempts or result in delays in obtaining the desired traffic service.

  • The best alternative is missing – this where the AP proactively orchestrates a BSS transition by the mobile STA to another nearby AP that can accommodate the ADDTS Request.

Kenan Xu, Nortel Networks

Benefits of network initiated load shifting alternative
Benefits of Network Initiated Load Shifting Alternative

  • Driving the BSS transition from the AP at the precise time it is needed would create an efficient network initiated load shifting mechanism

  • Improves the efficiency of the transition process

    • This is especially important for voice handsets at call initiation, when response time is critical.

  • Coupling the initiation of the BSS transition to the ADDTS Response reduces the multiple alternative courses of action into a definitive directive to the mobile STA.

    • a single atomic operation, which makes the transition process much more deterministic.

Kenan Xu, Nortel Networks

Network initiated load shifting solution
Network-Initiated Load Shifting Solution

  • A status code indicates the BSS Transition opportunity.

  • Neighbor report element piggybacked on the ADDTS Response frame leads to a streamlined BSS transition process.


Associated AP


ADDTS Response (status, BSS Transition Candidate List)

Kenan Xu, Nortel Networks

Addts response frame
ADDTS Response Frame

n+3 BSS Transition Candidate List Entries

Kenan Xu, Nortel Networks

Including all the transition information
Including All the Transition Information

  • For an effective network initiated BSS Transition, more information than just the BSS Transition Candidate List is needed as part of the ADDTS Response frame.

  • That information is already defined as the BSS Transition Request frame, see 802.11v D4.01 clause

  • Hence, all the needed BSS transition information can be included by appending the entire contents of the BSS Transition Request frame to the ADDTS Response frame under the cited conditions, i.e. when the new status code is used.

Kenan Xu, Nortel Networks

Addts response frame1
ADDTS Response Frame

n+3 BSS Transition Request frame

Kenan Xu, Nortel Networks

Standard amendment proposal
Standard Amendment Proposal

  • Add the new status code REJECTED_WITH_SUGGESTED_BSS_TRANSITION, to be optionally used by an AP within an ADDTS Response frame.

  • If an AP rejects a TSPEC request and is aware of alternative candidate APs that can meet the request and the non-AP STA supports the BSS Transition capability, the AP can set the status code to REJECTED_WITH_SUGGESTED_BSS_TRANSITION and append the BSS Transition Request frame to the ADDTS Response frame.

  • Within the appended BSS Transition Request frame the following must be provided:

    • Preferred Candidate List Included = 1

    • Abridged = 0

    • BSS Transition Candidate List Entries

  • A non-AP STA receiving such a message immediately starts the BSS transition process.

  • Affected clauses:

    • Status Code field

    • ADDTS Response frame format

    • MLME-ADDTS.confirm

    • MLME-ADDTS.response

    • 11.4.4 TS Setup

    • 11.20.6 BSS Transition Management for Network Load Balancing

Kenan Xu, Nortel Networks


  • Proposal to streamline network-initiated BSS transition (load shifting) by coupling the TS setup process to the BSS transition process using a designated status code.

  • A STA becomes aware of all opportunities when a TSPEC request is denied. The BSS transition process is expedited by adopting one of the entries in the BSS Transition Candidate List in the ADDTS Response frame.

  • The solutions are simple (limited amendments) and effective (compared to STA-initiated process).

Kenan Xu, Nortel Networks


  • IEEE Std. 802.11-2007

  • IEEE 802.11v D4.01, January 2009

  • 11-08-1295-02-000v-Enhancing-BSS-Transition-Management.doc, this is the corresponding .doc submission to accompany this presentation.

Kenan Xu, Nortel Networks

Straw poll questions
Straw Poll Questions

  • Do you agree that there is a need to streamline the BSS transition process as described in this presentation?Yes: No: Abstain:

  • Do you agree that the solution presented in this presentation should be included in the new revision of the 802.11v draft?Yes: No: Abstain:

Kenan Xu, Nortel Networks