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QoS in IEEE 802.11 Networks

QoS in IEEE 802.11 Networks. Resources. Introduction. IEEE 802.11. Simple, Effective Designed for Best Effort Service Real Time Services: Throughput and Delay Sensitive End-to-End QoS Guarantees, IEEE 802.11e. IEEE 802.11 Architecture. ESS. Existing Wired LAN. AP. AP. STA. STA. STA.

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QoS in IEEE 802.11 Networks

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  1. QoS in IEEE 802.11 Networks Resources

  2. Introduction

  3. IEEE 802.11 Simple, Effective Designed for Best Effort Service Real Time Services: Throughput and Delay Sensitive End-to-End QoS Guarantees, IEEE 802.11e

  4. IEEE 802.11 Architecture ESS Existing Wired LAN AP AP STA STA STA STA BSS BSS Infrastructure Network STA STA Ad Hoc Network BSS BSS STA STA

  5. IEEE 802.11e • QAP = QoS AP • QSTA = QoS Station

  6. Layers server Station infrastructure network AP TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

  7. PHY Layer • HR/DSSS: High Rate Direct Sequence Spread Spectrum • FHSS : Frequency Hopping Spread Spectrum

  8. MAC Sublayer PCF (Point Coordination Function) DCF (Distributed Coordination Function)

  9. PCF -Point Coordinator (PC) -Only in infrastructure networks -Designed for delay-bounded services PIFS EIFS: Extended IFS

  10. PCF - Centralized: location-dependent errors • Stations must wait for polling: Delay at low load • AP needs to contend for the channel using DCF to begin a CFP: variable CFP • Managing large number of stations using polling affects the applications that use DCF • No admission control

  11. DCF • Distributed, Contention-based • CSMA/CA • Binary Exponential Back-Off (CW: Contention Window) • Physical channel sensing • Virtual channel sensing (NAV: Network Allocation Vector) • Hidden Terminal problem: RTS/CTS • Timers • Retry limits • Fragmentation

  12. Fragmentation

  13. NAV: Network Allocation Vector DIFS: DCF Inter Frame Space SIFS: Short IFS PIFS: PCF IFS EIFS: Extended IFS Priority DIFS DIFS PIFS SIFS medium busy contention next frame time

  14. Tunable Parameters

  15. QoS Mechanisms

  16. Service Differentiation Priority: classification of traffic Fair Scheduling: partitioning the bandwidth fairly by regulating the wait times of traffic classes according to given weights

  17. EDCF (Enhanced DCF) Virtual Collision Handler ~ Internal Collisions AC: Access Category AIFS: Arbitrary IFS

  18. Persistent Factor DCF (P-DCF) • A persistent factor P is selected; small P means higher priority traffic class • A uniform random number r is generated every slot in back-off stages. • A flow stops the back-off and starts transmission only if r > p in the current slot given no transmission occurs in previous slots  The back-off interval is a geometric distributed random variable with P Geometric random variable is the number of trials required to obtain the first failure

  19. Distributed Weighted Fair Queue 2 schemes are proposed: • CW for a flow = Difference between actual and expected throughput A station decreases the CW to get higher priority  Lower CW when the actual throughput is lower than the expected one • Li’ = Ri/Wi Ri = the actual throughput Wi = the weight Each station adjust its CW by comparing others Li’ • Selfishness • More stations will have small value of CW

  20. Distributed Fair Scheduling (DFS) The back-off interval is based on the packet length and traffic class For flow i, BIi proportional to: 1. The weight (higher for higher throughput classes) 2. The packet length 3. A scaling factor (to min the probability of collisions in case different stations have same back off interval)

  21. Distributed Deficit Round Robin (DDRR) 1. Each throughput class i at station j is given a service quantum rate (Qij) equal to its required throughput 2. A deficit counter (DCij) is advanced at the rate Qij in a round robin fashion 3. Once a DCij becomes positive, the ith queue is allowed to send one packet 4. After transmission, DCij will be decreased by packet length each time a packet is transmitted  DCij is used to calculate IFSij (time before transmit or back-off): larger DCij, smaller IFSij

  22. DDRR Queues of different throughput classes Polling in a round robin way

  23. Admission Control and Bandwidth Reservation • Service differentiation does not perform well under high traffic loads • There is a need to protect existing streams • A wireless node has no knowledge of exact condition of the network • With CSMA/CA, bandwidth provision is quite difficult

  24. Measurement-Based Admission Control • The decision is made on measurement of existing network status (delay, throughput, …) • Different methods used: • Virtual MAC: the use of virtual MAC frames, and using a virtual source algorithm to tune the virtual MAC. • Probe packet: the use of probe packet for ad hoc • Data probe: the use of data packets

  25. Calculation-Based Admission Control • Performance metrics or criteria for evaluating the network status • Permissible throughput propagation • Saturation-based

  26. Scheduling and Reservation-Based Schemes • ARME (ASSURED RATE MAC EXTENSION • ): - Extension of DCF - Uses token bucket-based algorithm to detect overloading condition - improvements mad by adjusting CW

  27. Scheduling/Reservation • AACA: - RTS/CTS used for reservation - Mainly was for solving hidden terminal problem

  28. Link Adaptation Dynamically change the transmit rate , specified in the PLCP header of the PHY layer, that depend on channel conditions

  29. Link Adaptation • Received Signal Strength (RSS) • Each station maintains 12 RSS thresholds and corresponding transmission rate • Measure RSS and adjust the transmission rate • PER-Prediction • Decisions are based on Packet Error Rate-Prediction • MPDU-Based • Success/Fail Thresholds • Code Adapts To Enhance Reliability

  30. Direct Link Protocol (DLP) • QSTA transmits directly to another QSTA • Set up with the QAP is needed • STAs cannot go into power saving mode for active duration of the direct stream. • DLP is not applied in Ad Hoc networks • DLP messages can include security information

  31. Group ACK • Send a group of frames before any ACK to reduce overhead • GroupAckReq • GroupAck frame with an ACK bitmap • Sender retry unacknowledged frames with a retry limit • Receiver should keep the state of burst data received (sender address, bit map, sequence numbers)

  32. Challenges • IEEE 802.11e and DiffServ • IEEE 802.11e and IntServ • Integration of WLAN and MANET • Integration of WLAN and Bluetooth • Integration of WLAN and 3G wireless networks

  33. Resources 1. “A SURVEY OF QUALITY OF SERVICE IN IEEE 802.11 NETWORKS” By: HUA ZHU, MING LI, IMRICH CHLAMTAC, AND B. PRABHAKARAN THE UNIVERSITY OF TEXAS AT DALLAS 2. www.eecs.berkeley.edu/~ergen/docs/IEEE-802.11overview.ppt 3. www.cs.ucla.edu/classes/ winter04/cs117/chap7wlanRvsd.ppt 4. http://www.it.iitb.ac.in/~kirang/academic/MTP/Firststage/slides.pdf Up

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