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An Overview of Scheduling Algorithms in Wireless Multimedia Networks

An Overview of Scheduling Algorithms in Wireless Multimedia Networks. Hossam Fattah, Cyril Leung (The University of British Columbia). presented by Metin Tekkalmaz. Outline. Introduction Challenges Scheduler Components Scheduler Properties Classification Scheduling in TDMA Networks

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An Overview of Scheduling Algorithms in Wireless Multimedia Networks

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  1. An Overview ofScheduling Algorithms inWireless Multimedia Networks Hossam Fattah, Cyril Leung (The University of British Columbia) presented by Metin Tekkalmaz

  2. Outline • Introduction • Challenges • Scheduler Components • Scheduler Properties • Classification • Scheduling in TDMA Networks • Scheduling in CDMA Networks • Scheduling in Multihop Networks

  3. Introduction (1/3) • Function of scheduler is to select the session whose head-of-line (HOL) packet is to be transmitted next

  4. Introduction (2/3) • Important component for guaranteed QoS parameters • delay • delay jitter • packet loss rate • throughput • Scheduling is more difficult for wireless networks • Currently available wireline algorithms cannot be applied directly

  5. Introduction (3/3) • Different QoS service parameters for different applications as opposed to traditional best effort services • Service classes: • nrt-VBR: non-real-time variable bit rate • ABR: available bit rate • UBR: unsepecified bit rate • CBR: constant bit rate • rt-VBR: real-time variable bit rate

  6. Introduction

  7. Challenges in Wireless Network Scheduler (1/2) • Time- and location- dependent • signal attenuation • fading • interference • noise • Result in • bursty error • time-varying channel capacity

  8. Challenges in Wireless Network Scheduler (2/2) • For scheduling decisions • Number of sessions • Reserved rates • Statuses of session queues are necessary • Battery consumption • Handoffs • In CDMA, SIR requirements should be met • Rapidly changing topology • Communication range

  9. Components of a Scheduler • Error-free service model • Lead/Lag counter • Compensation model • A means of monitoring and predicting the channel state

  10. Properties of a Scheduler (1/2) • Efficient link utilization • Delay bound • Fairness • Throughput • Implementation complexity

  11. Properties of a Scheduler (2/2) • Graceful service degradation • Isolation • Energy consumption • Delay/bandwidth decoupling • Scalability

  12. Classification of Schedulers • work-conserving vs. non-work-conserving • timestamped • round-robin • sorted priority • frame-based

  13. Generalized Processor Sharing-Based Scheduling (1/2) • GPS is efficient, flexible and fair • Simulated by some timestamp-based algorithms • Work-conserving • Provides end-to-end delay bound • Provides equal normalized service • Fairness index is zero -> optimal

  14. Generalized Processor Sharing-Based Scheduling (2/2) • GPS is simulted in a TDMA packet network using a virtual time function: v(t) [total work performed in GPS] • B: Set of backlogged sessions • S: Start time • F: Finish time • L: Length • R: Reserved Transmit rate • a: arrival time

  15. Scheduling in Wireless TDMA Networks – NW Model • One BS, multiple MSs • Scheduling is at BS side • BS can communicate with all MSs • Direct MS-MS comm. is impossible • Channel errors may be experienced • Channel state and packet queue info for each session is available at BS

  16. Scheduling in Wireless TDMA Networks – CSDPS • Channel State Dependent Packet Scheduling • RR, LQF, ETF can be used • Errors are avoided at link level rather than recovering at transport or application level • Channel state of each link is monitored • No lead/lag concept

  17. Scheduling in Wireless TDMA Networks – IWFQ • Idalized Wireless Fair Queuing • Realization of PGPS for error prone sessions • Each service is assigned a service tag: virtual finish time of its HOL packet • Sessions with good channel are services according to their tags • Bounds are set for lead/lag counters

  18. Scheduling in Wireless TDMA Networks – CIF-Q • Channel-Condition-Independent Fair Queuing • Start Time Fair Queuing is used as error-free service model • Each session has lead and lag counter • an αvalue (between 0 & 1) is used for compensation between leading and lagging sessions

  19. Scheduling in Wireless TDMA Networks – SBFA • Server-Based Fairness Approach • Any wireline scheduler can be used as error-free service model • A portion of outgoing bandwidth is reserved for a hypothetical session • Packets that cannot be sent are scheduled for this session

  20. Scheduling in Wireless TDMA Networks – WFS • Wireless Fair Service • Delay and bandwidth are decoupled • Packet with lowest finish time with v(t) + x, which determines schedulable interval, is sent

  21. Scheduling in Wireless CDMA Networks (1/2) • CDMA provides higher (soft) system capacity • Accurate power control is required • A new session can be established as long as SIRs for all transmitting sessions can be mainteined above their target levels a certain percent of time

  22. Scheduling in Wireless CDMA Networks (2/2) • Simultaneous transmission is possible as long as following inequality is satisfied:

  23. Scheduling in Wireless CDMA Networks– packet-by-packet GPS • PGPS model where multiple services can be served simultaneously • Each packet is timestamped according to equation 2 • Packet with lowest timestamp is chosen to sent • Opposed to TDMA version, multiple packets can be sent at a time

  24. Scheduling in Wireless CDMA Networks– Scheduled CDMA • Hybrid CDMA/TDMA scheduler • Data is exchanged between BS and MS in capsules • CTR (capsule transmission request) is used • BS sorts requests according to the priorities or delay tolerances • BS returns with permission capsules containing transmission slot and power

  25. Scheduling in Wireless CDMA Networks– Dynamic Resource Scheduling • Modified version of SCDMA w/o TDMA aspect • BS classifies requests according to traffic characteristics • Two seperate queues are used: • Guaranteed • Best effort • Predefined rate is provided but there is no delay guarantee

  26. Scheduling in Wireless CDMA Networks– WISPER • Wireless Multimedia Access Control Protocol with BER Scheduling • Packets arrive in batches • All packets in a given batch have the same expiry time • Priority is directly proportional to the remaining number of packets in the batch and inversly proportional to remaining time before expiration and max MS transmission rate

  27. Scheduling in Multihop Networks • Ad-hoc networks with little infrastructure support • No Base Stations • In singlehop networks direct MS-MS communication is possible • Relays are needed as the number of MSs increase • Topology changes rapidly

  28. Scheduling in Multihop Networks – NW Model • Time is divided into slots • MSs have omnidirectional antennas • Channel is noise-free • Half-duplex communication is used • Conflicts occurs: • Primary conflict • Secondary conflict

  29. Scheduling in Multihop Networks • Scheduler Properties • Topology transparency • Low connectivity information requirement • Basic Principles • Node Activation • Link Activation

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