Efficient TIM Element for Multiple BSSIDs in IEEE 802.11

1. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 1 Efficient TIM element supporting multiple BSSIDs Date: 2008-05-13 Authors:

2. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 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.

3. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 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 (2 n -1) are used for buffered broadcast/multicast frame indications AID 2 n 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

4. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 4 Current TIM Element for Multiple BSSIDs (General Case) Octet index Bit index 0 0 (2 n -1) 0 1 0 0 Values of all bits are 0s N2 (N2+1) Bits values depend on whether the corresponding STAs have unicast frames buffered at the AP or not Bits 0 to (2 n -1) indicate buffered broadcast/multicast frames at AP or not 2007 Bits values depend on whether the corresponding BSSIDs have broadcast/multicast frames buffered at the AP or not Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 Traffic Indication Virtual Bitmap Bits 2 n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA

5. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 5 Current TIM Element for Multiple BSSIDs (Special Case) Octet index Bit index 0 0 Values of all bits are 0s (2 n -1) 0 1 0 0 Bits 0 to (2 n -1) indicate buffered broadcast/multicast frames at AP or not 2007 Bits 2 n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 (Encoded as a single all 0s octet) Traffic Indication Virtual Bitmap Values of all bits are 0s

6. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 6 Review of TIM Element for Single BSSID in Base Spec (General Case) Octet index Bit index Values of all bits are 0s 0 (N1-1) N1 1 0 1 1 0 0 Values of all bits are 0s N2 (N2+1) Bits values depend on whether the corresponding STAs have unicast frames buffered at the AP or not 2007 Contents of the Partial Virtual Bitmap field, and BitmapOffset = N1/2 Traffic Indication Virtual Bitmap Bits 1 to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA 0 0

7. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 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

8. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 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 Method A Method B General Case (buffered uc and bc/mc frames) Octets numbered 0 to N2 (currently defined in P802.11v_D2.0) A concatenation of Octets numbered 0 to N0-1 and Octets numbered N1 to N2 Special Case 1 (no buffered frames) A single zero octet (currently defined in P802.11v_D2.0) A single zero octet Special Case 2 (buffered bc/mc but no buffered uc frames) Octets numbered 0 to N0-1 Octets numbered 0 to N0-1

9. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 9 Proposed Modification (General Case) Octet index Bit index 0 0 Values of all bits are 0s (2 n -1) (N0-1) N0 (N1-1) N1 0 1 0 1 1 0 0 Values of all bits are 0s N2 (N2+1) Bits values depend on whether the corresponding STAs have unicast frames buffered at the AP or not Bits 0 to (2 n -1) indicate buffered broadcast/multicast frames at AP or not 2007 Bits values depend on whether the corresponding BSSIDs have broadcast/multicast frames buffered at the AP or not Contents of the Partial Virtual Bitmap field, and BitmapOffset = (N1-N0)/2 Traffic Indication Virtual Bitmap Bits 2 n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA 0

10. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 10 Proposed TIM Modification (Special Case 1) Octet index Bit index 0 0 Values of all bits are 0s (2 n -1) 0 1 0 0 Bits 0 to (2 n -1) indicate buffered broadcast/multicast frames at AP or not 2007 Bits 2 n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 (Encoded as a single all 0s octet) Traffic Indication Virtual Bitmap Values of all bits are 0s

11. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 11 Proposed Modification (Special Case 2) Octet index Bit index 0 0 Values of all bits are 0s (2 n -1) (N0-1) N0 0 0 0 Bits 0 to (2 n -1) indicate buffered broadcast/multicast frames at AP or not 2007 Bits 2 n to 2007 indicate buffered unicast frames at AP or not; Each bit corresponds to one STA Bits values depend on whether the corresponding BSSIDs have broadcast/multicast frames buffered at the AP or not Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 Traffic Indication Virtual Bitmap

12. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 12 Comparison General Case Existing Method in P802.11v_D2.0 Proposed Method Contents of the Partial Virtual Bitmap field Octets numbered 0 to N2 (methods neither specify nor use N0 and N1) A concatenation of octets numbered 0 to N0-1 and octets numbered N1 to N2 Values of Bitmap Offset in the Bitmap Control field 0 (N1-N0)/2 Size of the Partial Virtual Bitmap field N2+1 octets N2+1-(N1-N0) octets Size of TIM element N2+4 octets N2+ 4 -(N1-N0) octets

13. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 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.

14. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 14 Motion Move to adopt the text proposal in document IEEE 802.11- 07/2898r2 into the TGv draft Mover: Seconder: Result:

15. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 15 References IEEE P802.11v TM /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

16. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 16 Appendix

17. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 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 (2 n -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 2 n bits of this Modified Partial Virtual Bitmap.

18. doc.: IEEE 802.11-07/2897r3 Submission May 2008 Qi Wang, Broadcom Corporation Slide 18 Interpretation at Stations (PVB a single all-zero octet) BitmapOffset = 0 BitmapOffset 0 Single BSSID Multiple BSSIDs Single BSSID Multiple BSSIDs 11v STAs (understanding of multiple BSSIDs) No need to prepend the received PVB No need to prepend the received PVB Prepend (2 BitmapOffs et) octets of the received PVB Infer N0 from 2 n , and insert (2 BitmapOffset) octets between bit (N0 8-1) and bit N0 8 of the received PVB Legacy STAs (no understanding of multiple BSSIDs) No need to prepend the received PVB No need to prepend the received PVB Prepend (2 BitmapOffs et) octets of the received PVB Prepend (2 BitmapOffset) octets of the received PVB (Note: method B can only be used with legacy STAs in the BSS if such a prepending does not incur PVB misinterpretation by legacy STAs) 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