Impact of Block ACK Window sliding on IEEE 802.11n throughput performance

Impact of Block ACK Window sliding on IEEE 802.11n throughput performance

2014 YU-ANTL Lab Seminar

June 7, 2014

Shinnazar SeytnazarovAdvanced Networking Technology Lab. (YU-ANTL)

Dept. of Information & Comm. Eng, Graduate School, Yeungnam University, KOREA

(Tel : +82-53-810-3940; Fax : +82-53-810-4742http://antl.yu.ac.kr/; E-mail : [email protected])

http://antl.yu.ac.kr/

http://antl.yu.ac.kr/

Advanced Networking Tech. Lab.Yeungnam University (YU-ANTL)

YU-ANTL Lab. SeminarShinnazar Seytnazarov2

OUTLINE Introduction

Frame aggregations BAW sliding

The analytical model Expected A-MPDU length derivation Throughput derivation

Analytical results Conclusion References



Introduction (1) A-MPDU (Aggregation of MPDUs) - aggregation

scheme [1] Sender can aggregate up to 64 MPDUs in A-MPDU frame If receiver receives at least one of the MPDUs successfully, it

sends back Block ACK (Block acknowledgement) frame informing about transmission status MPDUs

PLCP header Tail/Pad

MPDU delimiter

MAC header MAC payload FCS Pad

MPDU1 MPDU2 ... MPDU64

A-MPDU

MPDU

Fig. 1. Aggregation of MPDUs

PLCP header

1

FrameControl

Receiver Address

BA Control FCSTransmitter

AddressBA

Info

Starting Sequence Number

2 3 64... 63

Bitmap

Fig. 2. Block ACK frame format



Introduction (2) Block ACK Window (BAW) sliding [1]

BAW size is equal to 64 that is the maximum allowed A-MPDU length

Sender can transmit the MPDUs that are within the BAW BAW continues sliding forward unless any of the MPDUs inside

the BAW fails

100 101 102 101 102 103 ... 163 164 165 166 167

BAW sliding direction

Previous position of BAW Current position of BAW



Introduction (3) Simple example for BAW = 4

100 101 102 103 104 105 ...

101102103104

Sender’s window

TX

Error

Receiver’s window

A-MPDU1

1011TX

BlockACK1

102 103 104 105 106 107 ...

103105106TX

A-MPDU2

0110TX

BlockACK2

103TX

ACKTX

102 103 104 105 106 107 ...Error

106 107 108 109 110 111 ...

107108109110TX

A-MPDU3

Receiver is anticipating SNs: 101~104Sender is sending SNs: 101~104

Receiver is anticipating SNs: 103, 105, 106

Sender is sending SNs: 103, 105, 106

Receiver is anticipating SN: 103

Sender is sending SN: 103

Receiver is anticipating SNs: 107~110

Sender is sending SNs: 107~110

...

...

...

...100 101 102 103 104 105 ...

102 103 104 105 106 107 ...

102 103 104 105 106 107 ...

106 107 108 109 110 111 ...

Transmitted and successfully received MPDU

Transmitted but failed MPDU

New MPDU in A-MPDU and BAW

MPDU outside of BAW



Expected A-MPDU length derivation (1) We introduce several random variables:

L – number of MPDUs in A-MPDU i.e. length of A-MPDU, L = 1, 2, . . , 64

N – number of new MPDUs in A-MPDU, N = 0, 1, 2, . . , L S – number of successful MPDUs in A-MPDU, S = 0, 1, 2, . . , L F – number of failed/erroneous MPDUs in A-MPDU, F = 0, 1, 2, . . , L X – number of successful MPDUs until the first failure in A-MPDU, X =

1, 2, . . , L We need to find:

Expected number of MPDUs in A-MPDU - E[L] Expected number of successful MPDUs in A-MPDU - E[S] Expected number of failed MPDUs in A-MPDU - E[F]

Assumptions: Sender’s buffer always has enough number of MPDUs to fill the BAW

window MPDU errors occur independently and identically over MPDUs of A-

MPDU



Expected A-MPDU length derivation (2) [2]

Considering assumption (2), the number of failed MPDUs has binomial distribution F ~ B(pe, L), where pe is MPDU error probability and L is the number of MPDUs in A-MPDU: (1)

So, the expected number of failed/erroneous MPDUs is: (2) Number of successfully transmitted MPDUs also has a binomial distribution S ~

B(1 - pe, L): (3)

So, the expected number of successful MPDUs per A-MPDU is: (4)

PMF for the number of first successful MPDUs in A-MPDU can be written as: (5) Using the above PMF we can calculate expected number of new MPDUs in A-

MPDU; gives the expected window shift, where W depicts the window size which is 64: (6)Here,



Expected A-MPDU length derivation (3) [2]

The length of A-MPDU – L is the composition of failed MPDUs of previous A-MPDU and newly included MPDUs. (7)

It is obvious that under certain channel conditions, the expected length of A-MPDU is the sum of the expectations of failed MPDUs and new MPDUs: (8)

Thus, we will use the expected A-MPDU length instead of A-MPDU length for Equations (1-6): (9)

Equation (9) has unique solution for E[L] under the given pe and can be solved numerically.



Performance of BAW sliding under different channel conditions (1)

Expected length of A-MPDU for different window sizes under different channel conditions

0

20

40

60

80

100

120

140

24.3014.57

35.34

20.65

E[L](W=64) E[L](W=128)

MPDU error probability



Performance of BAW sliding under different channel conditions (2)

Expected length of A-MPDU, expected number of successful and failed MPDUs under different channel conditions

00.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

9 0.1 0.11

0.12

0.13

0.140.1

50.1

60.1

70.1

80.1

9 0.2 0.210.2

20.2

30.2

40.2

50.2

60.2

70.2

80.2

9 0.30

10

20

30

40

50

60

70E[L](W=64) E[S](W=64) E[F](W=64)

MPDU error probability



Discrete time Markov chain [3]



Transmission probability Transmission probability that a station transmits in a randomly

chosen slot time.

(10)

p is backoff stage increment probability due to either collision or A-MPDU failure because of channel noise:

(11)

Equations (10) and (11) can be solved using numerical method and have unique solution for .



Slot durations Idle slot duration Ti: When all STAs are counting down, no station

transmits a frame and we have

(12)

Successful slot duration Ts: At least one MPDU in A-MPDU successfully received by receiver, the slot duration is the sum of a A-MPDU, a SIFS and an Block ACK duration

(13)

Collision and ‘A-MPDU failure due to noise’ slot durations Tc and Tf:

(14)



Probabilities of Time Slots Idle slot is observed if none of the stations transmits:

(15)

Successful slot is observed if only one station transmits and A-MPDU is not fully failed

(16)

Failure slot is observed if only one station transmits and A-MPDU is fully failed

(17)

Collision slot is observed if none of other slots is observed:

(18)



Network throughput Network throughput can be defined as:

(19)



Parameters for numerical analysis

MAC header 34BMPDU payload 1000BPHY header duration 44usData transmission rate 300/600MbpsBlock ACK transmission rate 24MbpsMaximum backoff stage (m) 5CWmin 15Slot duration (σ) 9usDIFS 28usSIFS 10us



Performance analysis of IEEE 802.11n considering BAW sliding (1)

Network throughput “with BAW” at R = 300Mbps

1 10 20 3075

125

175

225

275

Pe = 0.0 Pe = 0.1 Pe = 0.3

Number of stations




Network throughput “with BAW” at R = 600Mbps

1 10 20 30100

200

300

400

500

Pe = 0.0 Pe = 0.1 Pe = 0.3

Number of stations




Network throughput comparison “with and without BAW” at R = 300Mbps

1 10 20 3050

100

150

200

250w/oBAW_Pe = 0.1 withBAW_Pe = 0.1 w/oBAW_Pe = 0.3 withBAW_Pe = 0.3

Number of stations




Network throughput comparison “with and without BAW” at R = 600Mbps

1 10 20 30150

200

250

300

350

400

450

w/oBAW_Pe = 0.1 withBAW_Pe = 0.1 w/oBAW_Pe = 0.3 withBAW_Pe = 0.3

Number of stations




Difference (%) between 'with BAW' and 'without BAW' at different PHY rates

1 10 20 300

10

20

30

40

50

60600M_Pe = 0.3 300M_Pe = 0.3 600M_Pe = 0.1 300M_Pe = 0.1

Number of stations



Conclusion In this presentation

We analyzed the BAW sliding effect on A-MPDU length under different channel conditions When MPDU error probability increases from 0.0 to 0.3 BAW decreases the

A-MPDU length from – 64 to 14.57 for window size of 64– 128 to 20.65 for window size of 128

BAW model was applied in DTMC model for IEEE 802.11n Network throughput was analyzed for different number of nodes and

different channel conditions Existing DTMC models for IEEE 802.11n performance have huge

difference:– Over 20% when MPDU error probability 0.1 at 600Mbps PHY rate– Over 10% when MPDU error probability 0.1 at 300Mbps PHY rate

Conclusion BAW sliding has significant impact on A-MPDU size and network

performance under erroneous channel conditions It is essential to consider BAW effect in order to have an accurate network

performance estimations



References[1] IEEE 802.11n, Part 11: Standard for Wireless LAN Medium Access Control (MAC) and

Physical Layer (PHY) Specifications Amendment 5: Enhancements for Higher Throughput, Sept. 2009.

[2] Ginzburg, Boris, and Alex Kesselman. "Performance analysis of A-MPDU and A-MSDU aggregation in IEEE 802.11 n." In Sarnoff symposium, 2007 IEEE, pp. 1-5. IEEE, 2007.

[3] G. Bianchi, “Performance analysis of the IEEE 802.11 distributed coordination function,” IEEE JSAC, vol. 18, no. 3, pp. 535–547, Mar. 2000.

[4] T. Li, Q. Ni, D. Malone, D. Leith, Y. Xiao, and R. Turletti, “Aggregation with fragment retransmission for very high-speed WLANs,” IEEE/ACM Transactions on Networking, vol. 17, no. 2, pp. 591–604, Apr. 2009.

[5] Chatzimisios, P., A. C. Boucouvalas, and V. Vitsas. "Influence of channel BER on IEEE 802.11 DCF." Electronics letters 39.23 (2003): 1687-9.

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Impact of Block ACK Window sliding on IEEE 802.11n throughput performance