TIM Compression

1. TIM Compression Authors: Date: 2012-03-11 Haiguang Wang et. al, I2R, Singapore

2. Abstract Considering that the data transmission rate of 802.11ah 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

3. Motivation • IEEE 802.11ah 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 802.11ah PHY can be as low as 150 kbps. • Itmay 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 (minimum 6 bytes) • 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 TIM IE Format Haiguang Wang et. al, I2R, Singapore

5. 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. Problem with Current Partial Virtual Bitmap Partial Virtual Bitmap 00000000 00001000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 18 bytes 00000000 00000000 00000000 00000000 00000000 00001000 Haiguang Wang et. al, I2R, Singapore

6. 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 TIM Compression Haiguang Wang et. al, I2R, Singapore

7. 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. Using AID Directly in TIM Bitmap Number of bits for each AID AID 1 AID 2 . . . AID N Haiguang Wang et. al, I2R, Singapore

8. 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. AID with Differential Encoding Number of bits for each AID ∆ AID 1 ∆ AID 2 . . . ∆ AID n Haiguang Wang et. al, I2R, Singapore

9. 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. Encoding TIM Bitmap with AID Differential Encoding + Partial Virtual Bitmap (PVTB) Jaya Shankar et. al, I2R, Singapore

10. Comparison with other TIM Bitmap encoding schemes • Partial Virtual Bitmap (PVTB): defined in the IEEE 802.11-2007 • Page-Block TIM encoding scheme: proposed in IEEE 802.11-12/0388r1. Performance Results Jaya Shankar et. al, I2R, Singapore

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

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

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

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

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

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

17. 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. References • [1] Chao-Chun Wang, “Supporting Large Number of STAs in 802.11ah”, IEEE 802.11-11/1019, July 2011. • [2] Minyoung Park, “TGah Efficient TIM Encoding”, IEEE 802.11-12/388, March 2012. Haiguang Wang et. al, I2R, Singapore

19. 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: Straw Poll (1) Haiguang Wang et. al, I2R, Singapore

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

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