doc.: IEEE 802.11-12/0370r2

Submission

January 2012

Haiguang Wang et. al, I2R, SingaporeSlide 1

TIM Compression

Date: 2012-03-11Authors:

Name Affiliations Address Phone email Haiguang Wang I2R 1 Fusionopolis Way, #21-01,

Connexis South Tower, Singapore 138632

+65 6408 2256 hwang@i2r.a-star.edu.sg

Jaya Shankar I2R 1 Fusionopolis Way, #21-01, Connexis South Tower, Singapore 138632

+65 6408 2239 jshankar@i2r.a-star.edu.sg

Shoukang Zheng I2R

Yeow Wai Leong I2R

Zander Lei I2R

Oh Ser Wah I2R

Hoang Anh Tuan I2R

Joseph Teo Chee Ming

I2R

Rongshan Yu I2R

doc.: IEEE 802.11-12/0370r2

Submission

January 2012

Jaya Shankar et. al, I2R, SingaporeSlide 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.

doc.: IEEE 802.11-12/0370r2

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.

doc.: IEEE 802.11-12/0370r2

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

doc.: IEEE 802.11-12/0370r2

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

0000000000000000 00001000 0000000000000000 0000000000000000 0000000000000000 0000000000000000

0000000000000000 0000000000000000 0000000000000000 00000000000000000000000000000000 0000000000000000

0000000000000000 0000000000000000 0000000000000000 00000000000000000000000000000000 00001000

18 bytes

Partial Virtual Bitmap

doc.: IEEE 802.11-12/0370r2

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

doc.: IEEE 802.11-12/0370r2

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 1AID 1 AID 2AID 2 AID NAID N. . .

doc.: IEEE 802.11-12/0370r2

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 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.

March 2012

Haiguang Wang et. al, I2R, SingaporeSlide 8

Number of bits for each AID

∆ AID 1∆ AID 1 ∆ AID 2

∆ AID 2 ∆ AID n∆ AID n. . .

doc.: IEEE 802.11-12/0370r2

Submission

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.

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 9

doc.: IEEE 802.11-12/0370r2

Submission

Performance Results

• 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.

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 10

doc.: IEEE 802.11-12/0370r2

Submission

64 Stations

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 11

doc.: IEEE 802.11-12/0370r2

Submission

256 Stations

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 12

doc.: IEEE 802.11-12/0370r2

Submission

512 Stations

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 13

doc.: IEEE 802.11-12/0370r2

Submission

1204 Stations

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 14

doc.: IEEE 802.11-12/0370r2

Submission

2048 Stations

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 15

doc.: IEEE 802.11-12/0370r2

Submission

8192 Stations

March 2012

Jaya Shankar et. al, I2R, SingaporeSlide 16

doc.: IEEE 802.11-12/0370r2

Submission

January 2012

Slide 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

doc.: IEEE 802.11-12/0370r2

Submission

January 2012

Slide 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

doc.: IEEE 802.11-12/0370r2

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 19

doc.: IEEE 802.11-12/0370r2

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 20

doc.: IEEE 802.11-12/0370r2

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 21