1. Efficient TIM element supporting multiple BSSIDs
doc.: IEEE /2897r2
January 2008
Efficient TIM element supporting multiple BSSIDs
Date:
Authors:
Qi Wang, Broadcom Corporation
Qi Wang, Broadcom Corporation

2. doc.: IEEE /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

3. 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

4. Current TIM Element for Multiple BSSIDs (General Case)
doc.: IEEE /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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

16. 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