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

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

June 7,2014

Shinnazar Seytnazarov

Advanced Networking Technology Lab. (YU-ANTL)

Dept. of Information & Comm. Eng, Graduate School,

Yeungnam University, KOREA

(Tel : +82-53-810-3940; Fax : +82-53-810-4742

http://antl.yu.ac.kr/; E-mail : [email protected])

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

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

Introduction (3)

- Simple example for BAW = 4

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 peand 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

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

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 Tcand 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)

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

- Network throughput “with BAW” at R = 300Mbps

Performance analysis of IEEE 802.11n considering BAW sliding (2)

- Network throughput “with BAW” at R = 600Mbps

Performance analysis of IEEE 802.11n considering BAW sliding (3)

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

Performance analysis of IEEE 802.11n considering BAW sliding (4)

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

Performance analysis of IEEE 802.11n considering BAW sliding (5)

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

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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