Efficient TIM element supporting multiple BSSIDs doc.: IEEE 802.11-07/2897r2 January 2008 Efficient TIM element supporting multiple BSSIDs Date: 2008-01-15 Authors: Qi Wang, Broadcom Corporation Qi Wang, Broadcom Corporation

doc.: IEEE 802.11-07/2897r2 January 2008 Abstract This presentation proposes an improvement to the TIM element supporting multiple BSSIDs. Compared to the existing method defined in IEEE P802.11v-D1.0, the proposed modification reduces the beacon frame size while incurring minimal additional complexity at an AP and no additional complexity at STAs. Qi Wang, Broadcom Corporation Qi Wang, Broadcom Corporation

January 2008 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

Current TIM Element for Multiple BSSIDs (General Case) doc.: IEEE 802.11-07/2897r2 January 2008 Current TIM Element for Multiple BSSIDs (General Case) Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 Octet index N2 (N2+1) Bit index (2n-1) 2007 1 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 Qi Wang, Broadcom Corporation

Current TIM Element for Multiple BSSIDs (Special Case) January 2008 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 Bit index (2n-1) 2007 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

Inefficiency of Current TIM Element January 2008 Inefficiency of Current TIM Element The current PVB encoding method of the TIM element is designed to be backwards compatible with legacy STAs at the cost of potentially always sending a long string of zero bits However, there are situations where a more efficient PVB encoding method is possible without incurring misinterpretation by the 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

PVB encoding efficiency can be improved January 2008 PVB encoding efficiency can be improved AP has full knowledge of the content of traffic indication virtual bitmap to improve the PVB efficiency PVB efficiency can reduce the beacon frame size, and subsequently reduce the transmission time and medium occupancy time and reduce collisions in the BSS increase the amount of the saved power at a non-AP STAs in power save (PS) mode Qi Wang, Broadcom Corporation

Proposed Modifications January 2008 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_D1.0) 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 Special Case 2 (buffered bc/mc but no buffered uc frames) Octets numbered 0 to N0-1 Qi Wang, Broadcom Corporation

Proposed Modification (General Case) January 2008 Proposed Modification (General Case) Contents of the Partial Virtual Bitmap field, and BitmapOffset = (N1-N0)/2 Octet index (N0-1) N0 (N1-1) N1 N2 (N2+1) Bit index (2n-1) 2007 1 1 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

Proposed TIM Modification (Special Case 1) January 2008 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 Bit index (2n-1) 2007 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

Proposed Modification (Special Case 2) January 2008 Proposed Modification (Special Case 2) Contents of the Partial Virtual Bitmap field, and BitmapOffset = 0 Octet index (N0-1) N0 Bit index (2n-1) 2007 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

Interpretation at Stations (PVB ≠ a single all-zero octet) January 2008 Interpretation at Stations (PVB ≠ a single all-zero octet) BitmapOffset = 0 BitmapOffset ≠ 0 Single BSSID Multiple BSSIDs New STAs (i.e., STAs understanding multiple BSSIDs) No need to prepend the received PVB Prepend (2×BitmapOffset) octets of the received PVB Infer N0 from 2n, and insert (2×BitmapOffset) octets between bit (N0×8-1) and bit N0×8 of the received PVB Legacy STAs (i.e., STAs not understanding multiple BSSIDs) (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: if the received PVB is a single zero octet, both new and legacy STAs interpret all bits in the Traffic Indication Virtual Bitmap as zeros. Qi Wang, Broadcom Corporation

Comparison – General Case January 2008 Comparison – General Case Existing Method in P802.11v_D1.03 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 (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 Qi Wang, Broadcom Corporation

January 2008 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 saving The proposed changes incur minimal additional complexity at an AP but no additional complexity at non-AP STAs. Qi Wang, Broadcom Corporation

January 2008 Motion Move to adopt the text proposal in document IEEE 802.11-07/2898r1 into the TGv draft Mover: Seconder: Result: Qi Wang, Broadcom Corporation

January 2008 References IEEE P802.11vTM/D1.03, 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