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2014 YU-ANTL Lab Seminar. 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
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2014 YU-ANTL Lab Seminar 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 : firstname.lastname@example.org)
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  • 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  • 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)  • 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)  • 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)
Network throughput • Network throughput can be defined as: (19)
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  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.  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.  G. Bianchi, “Performance analysis of the IEEE 802.11 distributed coordination function,” IEEE JSAC, vol. 18, no. 3, pp. 535–547, Mar. 2000.  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.  Chatzimisios, P., A. C. Boucouvalas, and V. Vitsas. "Influence of channel BER on IEEE 802.11 DCF." Electronics letters 39.23 (2003): 1687-9.