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Efficient TIM element supporting multiple BSSIDs

Efficient TIM element supporting multiple BSSIDs. Date: 2008-05-13. Authors:. Abstract.

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Efficient TIM element supporting multiple BSSIDs

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  1. Efficient TIM element supporting multiple BSSIDs Date: 2008-05-13 Authors: Qi Wang, Broadcom Corporation

  2. Abstract This presentation proposes an improvement to the TIM element supporting multiple BSSIDs. Compared to the existing method defined in IEEE P802.11v-D2.0, the proposed modification reduces the beacon frame size while incurring minimal additional complexity at an AP and no additional complexity at STAs. This presentation relates to LB123 CID 171, 842, 914. Qi Wang, Broadcom Corporation

  3. Introduction • A modified TIM element supporting multiple BSSIDs is defined in the draft 11v spec • A single AID space is shared by all stations associated with an AP which supports multiple BSSIDs • AID 0 to AID (2n-1) are used for buffered broadcast/multicast frame indications • AID 2n to AID 2007 are assigned to individual stations • A single beacon frame with a single TIM element is used to announce the buffered frames for all stations associated with one of the multiple BSSIDs supported by an AP Qi Wang, Broadcom Corporation

  4. Current TIM Element for Multiple BSSIDs (General Case) Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 Octet index 0 0 N2 (N2+1) Bit index 0 (2n-1) 2007 1 0 0 Bits values depend on whether the corresponding BSSIDs have broadcast/multicast frames buffered at the AP or not Bits values depend on whether the corresponding STAs have unicast frames buffered at the AP or not Values of all bits are 0s Bits 0 to (2n-1) indicate buffered broadcast/multicast frames at AP or not Bits 2n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Traffic Indication Virtual Bitmap Qi Wang, Broadcom Corporation

  5. Current TIM Element for Multiple BSSIDs (Special Case) Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 (Encoded as a single all 0s octet) Octet index 0 0 Bit index 0 (2n-1) 2007 0 0 Values of all bits are 0s Values of all bits are 0s 1 Bits 0 to (2n-1) indicate buffered broadcast/multicast frames at AP or not Bits 2n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Traffic Indication Virtual Bitmap Qi Wang, Broadcom Corporation

  6. Review of TIM Element for Single BSSID in Base Spec – (General Case) Contents of the Partial Virtual Bitmap field, and BitmapOffset = N1/2 Octet index 0 (N1-1) N1 N2 (N2+1) Bit index 0 2007 0 0 1 1 0 0 Values of all bits are 0s Values of all bits are 0s Bits values depend on whether the corresponding STAs have unicast frames buffered at the AP or not 1 Bits 1 to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Traffic Indication Virtual Bitmap Qi Wang, Broadcom Corporation

  7. Inefficiency of TIM Element with Multiple BSSIDs in P802.11v_D2.0 • Offset in TIM element with single BSSID provides efficiency • Offset = N1/2 and PVB enables the reconstruction of Traffic Indication Virtual Bitmap • Mandating Offset=0 in call scenarios with multi-BSSID is inefficient • Allowing Offset ≠ 0 with multi-BSSID restores efficiency • AP has full knowledge of the content of traffic indication virtual bitmap to improve the PVB efficiency • Reduced TIM element size reduces beacon size and improves power save Qi Wang, Broadcom Corporation

  8. Proposed Modifications • Specify both Method A and Method B for PVB encoding and BitmapOffset field setting • Based upon its knowledge of STAs’ capabilities and the traffic bitmap content, an AP shall use either Method A or Method B • Use Method B for its efficiency wherever not incurring misinterpretation of received PVB by STAs; Use Method A, otherwise • Multi-BSSID & Non-zero BitmapOffset signals the use of Method B Qi Wang, Broadcom Corporation

  9. Proposed Modification (General Case) Contents of the Partial Virtual Bitmap field, and BitmapOffset = (N1-N0)/2 Octet index 0 0 (N0-1) N0 (N1-1) N1 N2 (N2+1) Bit index 0 (2n-1) 2007 0 0 1 1 0 0 Bits values depend on whether the corresponding BSSIDs have broadcast/multicast frames buffered at the AP or not Values of all bits are 0s Values of all bits are 0s Bits values depend on whether the corresponding STAs have unicast frames buffered at the AP or not 1 Bits 0 to (2n-1) indicate buffered broadcast/multicast frames at AP or not Bits 2n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Traffic Indication Virtual Bitmap Qi Wang, Broadcom Corporation

  10. Proposed TIM Modification (Special Case 1) Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 (Encoded as a single all 0s octet) Octet index 0 0 Bit index 0 (2n-1) 2007 0 0 Values of all bits are 0s Values of all bits are 0s 1 Bits 0 to (2n-1) indicate buffered broadcast/multicast frames at AP or not Bits 2n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Traffic Indication Virtual Bitmap Qi Wang, Broadcom Corporation

  11. Proposed Modification (Special Case 2) Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 Octet index 0 0 (N0-1) N0 Bit index 0 (2n-1) 2007 0 0 Bits values depend on whether the corresponding BSSIDs have broadcast/multicast frames buffered at the AP or not Values of all bits are 0s Bits 0 to (2n-1) indicate buffered broadcast/multicast frames at AP or not Bits 2n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Traffic Indication Virtual Bitmap Qi Wang, Broadcom Corporation

  12. Comparison – General Case Qi Wang, Broadcom Corporation

  13. Summary • An AP decides whether to use method A or method B for PVB encoding and BitmapOffset setting of the TIM element • Whenever used in appropriate situations, Method B reduces the size of the TIM element and improves performance in terms of: • Reduction of beacon size and medium occupancy time as well as BSS collision rate • Improvement of power save • The proposed changes incur minimal additional complexity at an AP but no additional complexity at non-AP STAs. Qi Wang, Broadcom Corporation

  14. Motion • Move to adopt the text proposal in document IEEE 802.11-07/2898r2 into the TGv draft Mover: Seconder: Result: Qi Wang, Broadcom Corporation

  15. References • IEEE P802.11vTM/D2.0, Draft standard for information technology- telecommunications and information exchange between systems – Local and metropolitan area networks – specific requirements, Part 11: Wireless LAN Medium Access controal (MAC) and Physical Layer (PHY) specifications, Amendment 9: Wireless Network Management Qi Wang, Broadcom Corporation

  16. Appendix Qi Wang, Broadcom Corporation

  17. Example Scenarios of Using Method B • Example scenarios where a more efficient PVB encoding method is possible without incurring misinterpretation by the legacy and 11v STAs • Example: no legacy STAs in the BSS • Example: bits 0 to (2n-1) of the Traffic Indication Virtual Bitmap are all 0s • Example: The AP knows all legacy STAs cannot wrongly interpret the Partial Virtual Bitmap. • That is, when legacy STAs prepend 2xBitmap_Offset octets all 0 bits before octet 0 of the Partial Virtual Bitmap generated using a new method (i.e., Method B) to form a “Modified Partial Virtual Bitmap”, none of the legacy STAs’ AIDs corresponds to the last 2n bits of this “Modified Partial Virtual Bitmap”. Qi Wang, Broadcom Corporation

  18. Interpretation at Stations (PVB ≠ a single all-zero octet) Note 1: if PVB = single zero octet, both new and legacy STAs interpret all bits in the Traffic Indication Virtual Bitmap as zeros. Note 2: the combination of multi-BSSID and Offset ≠ 0 signals the use of Method B Qi Wang, Broadcom Corporation

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