1. doc.: IEEE 802.11-12/0370r0 Submission January 2012 Haiguang Wang et. al, I2R, SingaporeSlide 1 TIM Compression Date: 2012-03-11 Authors:

2. doc.: IEEE 802.11-12/0370r0 Submission January 2012 Jaya Shankar et. al, I2R, Singapore Slide 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.

3. doc.: IEEE 802.11-12/0370r0 Submission January 2012 Haiguang Wang et. al, I2R, SingaporeSlide 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. –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.

4. doc.: IEEE 802.11-12/0370r0 Submission TIM IE Format 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 March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 4

5. doc.: IEEE 802.11-12/0370r0 Submission 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. March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 5 00000000 00001000 00000000 00001000 18 bytes Partial Virtual Bitmap

6. doc.: IEEE 802.11-12/0370r0 Submission 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 March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 6

7. doc.: IEEE 802.11-12/0370r0 Submission 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. March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 7 Number of bits for each AID AID 1 AID 2 AID N...

8. doc.: IEEE 802.11-12/0370r0 Submission 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 AID_1, AID_2, …, AID_n. –Compress the AID_1, AID_2, …, AID_n with differential encoding as follows: ∆AID_1 = AID_1 ∆ AID_i = AID_i – AID_i-1, i = 2, …, n. –Transmit ∆ AID_1, ∆ AID_2, …, ∆ AID_n in the TIM IE. March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 8 Number of bits for each AID AID 1 ∆ AID 2 ∆ AID N...

9. doc.: IEEE 802.11-12/0370r0 Submission January 2012 Haiguang Wang et. al, I2R, SingaporeSlide 9 Simulation Parameters Simulator –Qualnet network simulator Simulation parameters DCF mode with 1 MHz bandwidth With implemented power saving protocols. PHY modulation: MCS0-REP2 defined by 802.11ah Transmission range: 1 km Basic parameters ParameterValueParameterValue Data Rate150 KbpsBackoff Win15 - 1023 Number of Nodes1000Maximum Short Retry7 DIFS250 usMaximum long Retry4 SIFS160 usTime slot45 us Beacon Interval200 millisecondsDTIM Period25 (5 seconds) PS Mode Listen Interval25 (5 seconds)

10. doc.: IEEE 802.11-12/0370r0 Submission January 2012 Haiguang Wang et. al, I2R, SingaporeSlide 10 Topology

11. doc.: IEEE 802.11-12/0370r0 Submission Traffic Model Data packets from AP to 1000 stations follows Exponential distribution. The destination follows a uniform distribution among nodes associated with the AP. 2 packets, 20 bytes each per station per hour March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 11

12. doc.: IEEE 802.11-12/0370r0 Submission Performance Results: 2 pkts/station/hour March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 12 ItemPartial Virtual BitmapAID DirectlyAID with Differential Encoding Number of beacon transmitted16084 Total number of bytes for TIM IE1379947230819212553 Average Size of TIM IE85.814.3513.21 Maximum Size of TIM IE1313128 Compression Rate**10.2370.214 * Statistics duration: from 300s – 3600s ** Compression Rate: using Partial Virtual Bitmap as a base

13. doc.: IEEE 802.11-12/0370r0 Submission TIM IE Size vs. Active AIDs March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 13 Number of Active AIDs

14. doc.: IEEE 802.11-12/0370r0 Submission January 2012 Slide 14 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. Haiguang Wang et. al, I2R, Singapore

15. doc.: IEEE 802.11-12/0370r0 Submission January 2012 Slide 15 References [1] Chao-Chun Wang, “Supporting Large Number of STAs in 802.11ah”, IEEE 802.11-11/1019, July 2011. Haiguang Wang et. al, I2R, Singapore

16. doc.: IEEE 802.11-12/0370r0 Submission 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: January 2012 Haiguang Wang et. al, I2R, SingaporeSlide 16

17. doc.: IEEE 802.11-12/0370r0 Submission 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: March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 17

18. doc.: IEEE 802.11-12/0370r0 Submission Straw Poll (3) Do you agree to use differential encoding to further compress the binary form AIDs? Y: N: A: March 2012 Haiguang Wang et. al, I2R, SingaporeSlide 18