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CMPE 257: Wireless and Mobile Networking SET 3f:

CMPE 257: Wireless and Mobile Networking SET 3f:. Medium Access Control Protocols. MAC Protocol Topics. Fair medium access and fair scheduling (queueing) MACAW Topology independent fair queuing. The Fairness Problem. Unequal opportunity to access the channel Severe throughput degradation

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CMPE 257: Wireless and Mobile Networking SET 3f:

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  1. CMPE 257: Wireless and Mobile NetworkingSET 3f: Medium Access Control Protocols UCSC CMPE252B

  2. MAC Protocol Topics • Fair medium access and fair scheduling (queueing) • MACAW • Topology independent fair queuing CMPE257 UCSC

  3. The Fairness Problem • Unequal opportunity to access the channel • Severe throughput degradation • Causes • Binary exponential backoff (BEB) • Location dependent contention Node B Node C Node D Node A Example: Flow CD will capture the channel. CMPE257 UCSC

  4. Prior Work on Fairness • Max-min fairness • Reduce the ratio between max throughput and min throughput of flows • Backoff or dynamic adjustment of channel access. • Fair queuing (FQ) approach • Adapt wireline FQ disciplines to ad hoc networks. • Flow contention graph as a useful abstraction • Flows are tagged as either leading or lagging, and then backoff window is adjusted accordingly. CMPE257 UCSC

  5. MACAW ([BDSZ94]) • One of the earliest work on throughput and fairness enhancement • Note: Packet sensing (not carrier sensing) is used! • Per-node and per-stream fairness • Maintain backoff windows for different streams (also appears in IEEE 802.11e) • Backoff copy and MILD backoff • Multiplicative increase, linear decrease to address the fairness problem CMPE257 UCSC

  6. Topology Independent Fair Queuing Design Goals [LL00, LL05] • QoS Support for Advanced Applications in Ad Hoc Wireless Networks • Fair Allocation of Channel Bandwidth • Maximal Channel Utilization under Fairness Constraint • Communication Intensive Applications • Limited-bandwidth Wireless Channel • Distributed Packet Scheduling Design CMPE257 UCSC

  7. Outline • Issues in Ad Hoc Wireless Fair Queueing • Fair Queueing Model for Packet Scheduling • An Idealized Centralized Algorithm • A Distributed Implementation • Performance Evaluation • Conclusion and Future Work CMPE257 UCSC

  8. Network Model • A Single Shared Physical Channel • Collision – Receiver in Transmission Range of More than One Transmitting Node • Flow – Stream of Packets from Source to Destination <Sender, Receiver, Flow_ID> • CSMA/CA MAC Framework CMPE257 UCSC

  9. F3 F1 X F2 Spatial Collision Design Issues Spatial Reuse • Location Dependent Contention & Channel Reuse CMPE257 UCSC

  10. F4 Design Issues F3 F1 F2 No Spatial Reuse No Spatial Contention CMPE257 UCSC

  11. F3 F4 F1 F2 Node Graph Flow Graph Flow Contending Graph F3 F4 F1 F2 CMPE257 UCSC

  12. F1 F1 F1 F1 F3 F3 F4 F4 F4 F4 F5 F5 F3 F3 F5 F5 F2 F2 F2 F2 Node Graph Flow Graph • Schedule F3 & F5 always (MIS) Schedule the Maximal Number of Flows in Flow Graph • Starve F1, F2 & F4 Design Issues (cont’d) • Inherent Conflict between Achieving Fairness & Maximizing Channel Utilization • To Maximize Channel Utilization CMPE257 UCSC

  13. Design Issues (cont’d) • Distributed Nature of Packet Scheduling in Ad Hoc Wireless Networks • Unlike Wireline or Packet Cellular Networks. NO Single Logical Entity for Scheduling • NO Direct Access to All Contending Flow Info • Provide QoS at Finest Time Scale (Packet Level) CMPE257 UCSC

  14. Solution Space • Maximize Channel Utilization Always: • Schedule Largest Number of Non-conflicting Flows • Starvation of Certain Flows • Ensure Fairness: • Maximize Spatial Channel Reuse Subject to Fairness Constraint CMPE257 UCSC

  15. FQ Tagging: F1.1 F2.1 F3.1 F4.1 F1.2 F2.2 F3.2 F4.2 F1 F2 1 2 3 4 5 6 7 0 Wireline FQ: F4 F3 F1 F3 Enable Spatial Channel Reuse F1.1 F2.1 F3.1 F4.1 F1.2 F2.2 F3.2 F4.2 CMPE257 UCSC

  16. F1.1 F3.1 Lookahead window = 4 bits F1.1 F3.1 Look Ahead Window F1 F2 FQ Tagging: F1.1 F2.1 F3.1 F4.1 F1.2 F2.2 F3.2 F4.2 1 2 3 4 5 6 7 0 Wireline FQ: F4 F3 F1.1 F2.1 F3.1 F4.1 F1.2 F2.2 F3.2 F4.2 FQ with Lookahead: F2.1 F4.1 F1.2 F3.2 F2.2 F4.2 CMPE257 UCSC

  17. F3 F3 F4 F4 F1 F1 F5 F5 F0 F0 F2 F2 Flow Contending Graph Minimum Graph Coloring • Maximizing Spatial Reuse within a Look Ahead Window is a Minimum Coloring Problem CMPE257 UCSC

  18. Dynamic Graph Coloring • Look Ahead Window Moves Forward • Balance Two Design Goals: • Transmit Current Window of Bits ASAP • Move Window Ahead AFAP CMPE257 UCSC

  19. An Adaptive Algorithm • Two Key Components: • A Basic Scheduling Loop • Adaptive Dynamic Coloring CMPE257 UCSC

  20. Basic Scheduling Loop • WFQ to Assign Flow Tags • V(t): Smallest Finish Tag • A Lookahead Window of Bits: [V(t),V(t)+ ] • Partition Flows into Disjoint Sets • Schedule the Set with Least Tagged Flow • Move Window Forward to Next Least Tagged Flow CMPE257 UCSC

  21. Adaptive Dynamic Coloring • As Lookahead Window Moves from [V(t-1),V(t-1)+ ] to [V(t),V(t)+ ] • Retain Disjoint Sets of Unserved Packets in [V(t-1),V(t-1)+ ] • Merge Newly Joined Packets, Create New Set if No Merge Possible • Retain All Disjoint Sets until t+1 CMPE257 UCSC

  22. Properties of Central Algorithms • Number of Disjoint Sets Non-increase • Adaptively Reduce Total Number of Disjoint Sets • Move the Window Forward • Fairness Guarantee • Spatial Channel Reuse • Throughput & Delay Bounds CMPE257 UCSC

  23. Distributed Implementation • Approximate Central Algorithm: A Back-off Based Approach • Within CSMA/CA MAC Framework • Approximate WFQ with Modified WRR • Backoff-based Implementation of Largest-degree First (LF) Coloring Algorithm • Backoff-based Implementation of Adaptive Algorithms CMPE257 UCSC

  24. (3) F3 (4) F4 F1 (5) (2) (2) F5 F0 (2) F2 An Example • Adaptive Dynamic Coloring • Partition Flows into Disjoint Sets CMPE257 UCSC

  25. 4 1 4 3 2 4 An Example • Largest Degree First: Set Backoff = –Flow_Degree F0 (3) F3 F1 (4) F4 F1 F2 (5) F3 F5 F0 (2) (2) F4 F2 (2) F5 Time CMPE257 UCSC

  26. FQ Tagging: F1.1 F5.1 F1.1 F4.1 F3.1 F2.1 F5.1 F0.1 F1.2 F4.2 1 2 3 4 5 6 7 0 Wireline FQ: F1.1 F5.1 An Example F0 4 F3 1 F1 F4 F1 F2 4 3 F3 F5 F0 2 F4 F2 4 F5 Time CMPE257 UCSC

  27. F4.1 F0.1 F4.1 F0.1 An Example FQ Tagging: F4.1 F3.1 F2.1 F0.1 F1.2 F4.2 1 2 3 4 5 6 7 0 Wireline FQ: F1.1 F5.1 F0 4 F3 1 F1 F4 F1 F2 4 3 F3 F5 F0 2 F4 F2 4 F5 Time CMPE257 UCSC

  28. …… F3.1 F2.1 F2.1 F3.1 …… An Example FQ Tagging: F3.1 F2.1 F1.2 F4.2 1 2 3 4 5 6 7 0 Wireline FQ: F1.1 F5.1 F4.1 F0.1 F0 4 F3 1 F1 F4 F1 F2 4 3 F3 F5 F0 2 F4 F2 4 F5 Time CMPE257 UCSC

  29. Other Issues • Detailed MAC Layer Design • CSMA/CA Paradigm • Global Flow Information (i.e. Flow Weights) Propagation • Conflict-free Multicast Tree CMPE257 UCSC

  30. Simulation Example • 17 flows • 15 source nodes • Simulation slots: 100,000 • Poisson & MMPP Traffic CMPE257 UCSC

  31. Simulation Example • Normalized Throughput Normalized Throughput 1.05 1 0.95 Throughput 0.9 0.85 0.8 0 2 4 6 8 10 12 14 16 CMPE257 UCSC Flow ID

  32. Average Delay • Average Delay Average Delay 4 3.5 3 2.5 Delay 2 1.5 1 0.5 0 0 2 4 6 8 10 12 14 16 Flow ID CMPE257 UCSC

  33. Convergence of Adaptive Coloring • Numbers of Disjoint Sets CMPE257 UCSC

  34. Related Work • Distributed Fair Queuing • Adapt Fair Queuing Algorithm to Wireless LAN • Distributed Fair Scheduling • Backoff Based on Virtual Time • No Explicit Consideration of Spatial Channel Reuse • “Distributed Fair Scheduling in a Wireless LAN,” by N. Vaidya, P. Bahl & S. Gupta, MOBICOM 2000 • Multihop Packet Scheduling • Focus on Resolving Conflict between Fairness & Maximal Channel Utilization • Per-flow Service is not Fair • “A New Model for Packet Scheduling in Multihop Wireless Networks” by H. Luo & S. Lu, MOBICOM 2000 CMPE257 UCSC

  35. References • [BDSZ94] Bharghavan et al., MACAW: Media Access Protocol for Wireless LANs, in ACM SIGCOMM 1994. • [LL00] H. Luo and S. Lu, A Topology-Independent Fair Queueing Model in Ad Hoc Wireless Networks, in IEEE ICNP 2000. • [LL05] H. Luo and S. Lu, A Topology-Independent Wireless Fair Queueing Model in Ad Hoc Networks 2005, in IEEE JSAC 2005. (Extended version of [LL00]). CMPE257 UCSC

  36. Acknowledgments • Parts of the presentation are adapted from the following sources: • Prof. Luo’s ICNP 2000 presentation CMPE257 UCSC

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