1. TIM Compression Date: 2012-03-11 Authors: January 2012 Month Year
doc.: IEEE yy/xxxxr0
January 2012
TIM Compression
Date:
Authors:
Haiguang Wang et. al, I2R, Singapore
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
January 2012
Abstract
Considering that the data transmission rate of ah can be as low as 150 kbps, it is necessary to improve the media transmission efficiency. In this contribution, we propose a few methods to reduce the size of TIM bitmap and simulation results shows that the size of TIM IE can be reduced significantly.
Jaya Shankar et. al, I2R, Singapore
John Doe, Some Company

3. January 2012
Motivation
IEEE ah is required to support up to 6000 stations by a single AP [1]. The TIM bitmap can be as large as a few thousand bit in size.
The transmission speed of the ah PHY can be as low as 150 kbps.
It may take more than 6.7 milliseconds to transmit a TIM bitmap with a size of 1000 bits.
The overhead may consume a significant portion of channel time.
Compression can help in reducing the time required for TIM bitmap transmission.
Haiguang Wang et. al, I2R, Singapore

4. TIM IE Format TIM IE Format (minimum 6 bytes) March 2012
Element ID and Length
DTIM count
DTIM Period
Bitmap control
Bit 0: for multicast/broadcast
Bit 1 – 7: partial bitmap offset
Partial Virtual Bitmap
consists of octets numbered N1 through N2 of the traffic indication virtual bitmap
N1: the largest even number such that bits numbered 1 through (N1 × 8) – 1 in the bitmap are all 0.
N2 is the smallest number such that bits numbered (N2 + 1) × 8 through 2007 in the bitmap are all 0
Haiguang Wang et. al, I2R, Singapore

5. Problem with Current Partial Virtual Bitmap
March 2012
Problem with Current Partial Virtual Bitmap
Encoding of TIM Bitmap with the partial virtual bitmap as specified in the current standard is not efficient.
Many zero octets may be included in the partial bitmap, which are unnecessary and can be removed with simple encoding method.
Partial Virtual Bitmap
18 bytes
Haiguang Wang et. al, I2R, Singapore

6. March 2012
TIM Compression
TIM IE can be further compressed to reduce beacon transmission time.
Reduce the TIM IE from 6 bytes to 4 bytes when there is no packet in the buffer by excluding.
Bitmap control field (one byte)
One byte with all zeros for Partial Virtual Bitmap
Haiguang Wang et. al, I2R, Singapore

7. Using AID Directly in TIM Bitmap
March 2012
Using AID Directly in TIM Bitmap
When the number of bits being enabled (set to’1’) in the Partial Virtual Bitmap is sparse, representing AID directly in a binary form can reduce the size of TIM IE significantly. The scheme should include
A way to specify the number of bits required to encode AID.
All AIDs are encoded with the same number of bits.
Number of bits for each AID
AID 1
AID 2
. . .
AID N
Haiguang Wang et. al, I2R, Singapore

8. AID with Differential Encoding
March 2012
AID with Differential Encoding
Using differential encoding can further reduce the size of TIM IE when AIDs are encoded in binary.
E.g. Sort AIDs that need to be transmitted in the TIM IE in ascending order, denoted as AID1, AID2, …, AIDn.
Compress the AID1, AID2, …, AIDn with differential encoding as follows:
∆AID1 = AID1-1
∆ AIDi = AIDi – AIDi-1-1, i = 2, …, n.
Transmit ∆ AID1, ∆ AID2, …, ∆ AIDn in the TIM IE.
Number of bits for each AID
∆ AID 1
∆ AID 2
. . .
∆ AID n
Haiguang Wang et. al, I2R, Singapore

9. March 2012
Encoding TIM Bitmap with AID Differential Encoding + Partial Virtual Bitmap (PVTB)
AID differential encoding can encode TIM Bitmap size efficiently when the 1’s in the bitmap is sparse and the PVTB encoding scheme is more efficient when the 1’s in the bitmap is dense.
The two encoding scheme can be used alternatively based on the density of 1’s in the bitmap.
AP choose one of the schemes for TIM encoding by comparing the size of bitmap after encoding.
AP can derive the size of encoded bitmap based on calculation instead of create a bitmap.
Jaya Shankar et. al, I2R, Singapore

10. Performance Results Comparison with other TIM Bitmap encoding schemes
March 2012
Performance Results
Comparison with other TIM Bitmap encoding schemes
Partial Virtual Bitmap (PVTB): defined in the IEEE
Page-Block TIM encoding scheme: proposed in IEEE /0388r1.
Jaya Shankar et. al, I2R, Singapore

11. March 2012
64 Stations
Jaya Shankar et. al, I2R, Singapore

12. March 2012
256 Stations
Jaya Shankar et. al, I2R, Singapore

13. March 2012
512 Stations
Jaya Shankar et. al, I2R, Singapore

14. March 2012
1204 Stations
Jaya Shankar et. al, I2R, Singapore

15. March 2012
2048 Stations
Jaya Shankar et. al, I2R, Singapore

16. March 2012
8192 Stations
Jaya Shankar et. al, I2R, Singapore

17. January 2012
Conclusions
In this contribution, we propose a few methods to reduce the size of TIM IE. Firstly, we propose that the TIM IE may be transmitted with the DTIM count and DTIM period only when there is no data packet in the buffer of AP; Secondly, we propose to allow AIDs in TIM IE to be directly encoded in its binary form and using differential encoding on the list of binary form AIDs to further reduce the size of TIM IE. We have compared the AID with Differential Encoding with the Encoding algorithm proposed in [2] and the improvement can be as high as 40%.
Haiguang Wang et. al, I2R, Singapore

18. January 2012
References
[1] Chao-Chun Wang, “Supporting Large Number of STAs in ah”, IEEE /1019, July 2011.
[2] Minyoung Park, “TGah Efficient TIM Encoding”, IEEE /388, March 2012.
Haiguang Wang et. al, I2R, Singapore

19. January 2012
Straw Poll (1)
Do you agree that TIM IE should exclude the Bitmap Control field and Partial Virtual Bitmap when there is no data packet in the buffer of AP? Y: N: A:
Haiguang Wang et. al, I2R, Singapore

20. March 2012
Straw Poll (2)
Do you agree to use the binary form representation of AIDs to help reduce the size of TIM IE? Y: N: A:
Haiguang Wang et. al, I2R, Singapore

21. March 2012
Straw Poll (3)
Do you agree to use differential encoding to further compress the binary form AIDs? Y: N: A:
Haiguang Wang et. al, I2R, Singapore