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

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

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

4. 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. 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. 6 Introduction

7. 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. 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. 9 Components of a Scheduler Error-free service model Lead/Lag counter Compensation model A means of monitoring and predicting the channel state

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

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

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

13. 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. 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. 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. 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. 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. 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 α value (between 0 & 1) is used for compensation between leading and lagging sessions

19. 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. 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. 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. 22 Scheduling in Wireless CDMA Networks (2/2) Simultaneous transmission is possible as long as following inequality is satisfied:

23. 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. 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. 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. 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. 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. 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. 29 Scheduling in Multihop Networks Scheduler Properties Topology transparency Low connectivity information requirement Basic Principles Node Activation Link Activation