Winter 2004 UCSC CMPE252B1 CMPE 257: Wireless and Mobile Networking SET 3f: Medium Access Control Protocols

Spring 2005CMPE257 UCSC2 MAC Protocol Topics n Fair medium access and fair scheduling (queueing) p MACAW p Topology independent fair queuing

Spring 2005CMPE257 UCSC3 n Unequal opportunity to access the channel n Severe throughput degradation n Causes p Binary exponential backoff (BEB) p Location dependent contention The Fairness Problem Node A Node B Node CNode D Example: Flow CD will capture the channel.

Spring 2005CMPE257 UCSC4 Prior Work on Fairness n Max-min fairness p Reduce the ratio between max throughput and min throughput of flows p Backoff or dynamic adjustment of channel access. n Fair queuing (FQ) approach p Adapt wireline FQ disciplines to ad hoc networks. p Flow contention graph as a useful abstraction p Flows are tagged as either leading or lagging, and then backoff window is adjusted accordingly.

Spring 2005CMPE257 UCSC5 MACAW ([BDSZ94]) n One of the earliest work on throughput and fairness enhancement n Note: Packet sensing (not carrier sensing) is used! n Per-node and per-stream fairness p Maintain backoff windows for different streams (also appears in IEEE e) n Backoff copy and MILD backoff p Multiplicative increase, linear decrease to address the fairness problem

Spring 2005CMPE257 UCSC6 Topology Independent Fair Queuing Design Goals [LL00, LL05] n 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

Spring 2005CMPE257 UCSC7 Outline n Issues in Ad Hoc Wireless Fair Queueing n Fair Queueing Model for Packet Scheduling n An Idealized Centralized Algorithm n A Distributed Implementation n Performance Evaluation n Conclusion and Future Work

Spring 2005CMPE257 UCSC8 Network Model n A Single Shared Physical Channel n Collision – Receiver in Transmission Range of More than One Transmitting Node n Flow – Stream of Packets from Source to Destination n CSMA/CA MAC Framework

Spring 2005CMPE257 UCSC9 Design Issues n Location Dependent Contention & Channel Reuse Spatial Reuse F1 F2 F3 X Spatial Collision

Spring 2005CMPE257 UCSC10 Design Issues F1 F2 F3 F4 No Spatial Contention No Spatial Reuse

Spring 2005CMPE257 UCSC11 Flow Contending Graph F1 F2 F4 F3 F1 F4 F2 F3 Flow Graph Node Graph

Spring 2005CMPE257 UCSC12 Design Issues (cont’d) n To Maximize Channel Utilization F1 F2 F4 F3F5 F1 F2 F4 Node Graph F3 Flow Graph F5 Inherent Conflict between Achieving Fairness & Maximizing Channel Utilization F3 F5 Schedule F3 & F5 always (MIS) F3F5 F1 F2 F4 Starve F1, F2 & F4 F1 F2 F4 Schedule the Maximal Number of Flows in Flow Graph

Spring 2005CMPE257 UCSC13 Design Issues (cont’d) n Distributed Nature of Packet Scheduling in Ad Hoc Wireless Networks p Unlike Wireline or Packet Cellular Networks. NO Single Logical Entity for Scheduling p NO Direct Access to All Contending Flow Info n Provide QoS at Finest Time Scale (Packet Level)

Spring 2005CMPE257 UCSC14 Solution Space n Maximize Channel Utilization Always: p Schedule Largest Number of Non-conflicting Flows p Starvation of Certain Flows n Ensure Fairness: p Maximize Spatial Channel Reuse Subject to Fairness Constraint

Spring 2005CMPE257 UCSC15 Enable Spatial Channel Reuse F1F2 F4F3 Wireline FQ: F1.1F2.2F2.1F3.1F4.1F1.2F3.2F4.2 F1.1F2.2F2.1F3.1F4.1F1.2F3.2F4.2 FQ Tagging: F1 F3

Spring 2005CMPE257 UCSC16 Look Ahead Window F1F2 F4F3 Wireline FQ: F1.1F2.2F2.1F3.1F4.1F1.2F3.2F4.2 F1.1F2.2F2.1F3.1F4.1F1.2F3.2F4.2 FQ Tagging: FQ with Lookahead: F2.1 F4.1 F1.2 F3.2 F2.2 F4.2 Lookahead window = 4 bits F1.1 F3.1 F1.1F3.1

Spring 2005CMPE257 UCSC17 Minimum Graph Coloring n Maximizing Spatial Reuse within a Look Ahead Window is a Minimum Coloring Problem F1 F2 F3 F5 F0 F4 Flow Contending Graph F1 F2 F3 F5 F0 F4

Spring 2005CMPE257 UCSC18 Dynamic Graph Coloring n Look Ahead Window Moves Forward n Balance Two Design Goals: p Transmit Current Window of Bits ASAP p Move Window Ahead AFAP

Spring 2005CMPE257 UCSC19 An Adaptive Algorithm n Two Key Components: p A Basic Scheduling Loop p Adaptive Dynamic Coloring

Spring 2005CMPE257 UCSC20 Basic Scheduling Loop n WFQ to Assign Flow Tags n V(t): Smallest Finish Tag n A Lookahead Window of Bits: [V(t),V(t)+ ] p Partition Flows into Disjoint Sets p Schedule the Set with Least Tagged Flow p Move Window Forward to Next Least Tagged Flow

Spring 2005CMPE257 UCSC21 Adaptive Dynamic Coloring n As Lookahead Window Moves from [V(t- 1),V(t-1)+ ] to [V(t),V(t)+ ] p Retain Disjoint Sets of Unserved Packets in [V(t-1),V(t-1)+ ] p Merge Newly Joined Packets, Create New Set if No Merge Possible p Retain All Disjoint Sets until t+1

Spring 2005CMPE257 UCSC22 Properties of Central Algorithms n Number of Disjoint Sets Non-increase n Adaptively Reduce Total Number of Disjoint Sets n Move the Window Forward n Fairness Guarantee n Spatial Channel Reuse n Throughput & Delay Bounds

Spring 2005CMPE257 UCSC23 Distributed Implementation n Approximate Central Algorithm: A Back-off Based Approach n Within CSMA/CA MAC Framework n Approximate WFQ with Modified WRR n Backoff-based Implementation of Largest- degree First (LF) Coloring Algorithm n Backoff-based Implementation of Adaptive Algorithms

Spring 2005CMPE257 UCSC24 An Example F0 F1 F2 F3 F4 F5 Partition Flows into Disjoint Sets (5) (2) (3) (4) n Adaptive Dynamic Coloring

Spring 2005CMPE257 UCSC25 An Example n Largest Degree First: Set Backoff = – Flow_Degree F1 F2 F3 F4 F5 F0 (5) (2) (3) (4) Time F2 F3 F4 F5 F0 F

Spring 2005CMPE257 UCSC26 An Example F1 F2 F3 F4 F5 F0 Time F2 F3 F4 F5 F0 F Wireline FQ: F1.1F0.1F4.1F3.1F2.1F5.1F1.2F4.2 FQ Tagging: F1.1 F5.1 F1.1F5.1

Spring 2005CMPE257 UCSC27 An Example F1 F2 F3 F4 F5 F0 Time F2 F3 F4 F5 F0 F Wireline FQ: F0.1F4.1F3.1F2.1F1.2F4.2 FQ Tagging: F1.1 F5.1 F4.1 F0.1 F4.1F0.1

Spring 2005CMPE257 UCSC28 An Example F1 F2 F3 F4 F5 F0 Time F2 F3 F4 F5 F0 F Wireline FQ: F3.1F2.1F1.2F4.2 FQ Tagging: F1.1 F5.1 F4.1 F0.1 F2.1 F3.1 F2.1 ……

Spring 2005CMPE257 UCSC29 Other Issues n Detailed MAC Layer Design p CSMA/CA Paradigm n Global Flow Information (i.e. Flow Weights) Propagation p Conflict-free Multicast Tree

Spring 2005CMPE257 UCSC30 Simulation Example n 17 flows n 15 source nodes n Simulation slots: 100,000 n Poisson & MMPP Traffic

Spring 2005CMPE257 UCSC31 Simulation Example n Normalized Throughput Flow ID Throughput Normalized Throughput

Spring 2005CMPE257 UCSC32 Average Delay Flow ID Delay Average Delay n Average Delay

Spring 2005CMPE257 UCSC33 Convergence of Adaptive Coloring n Numbers of Disjoint Sets

Spring 2005CMPE257 UCSC34 Related Work n Distributed Fair Queuing p Adapt Fair Queuing Algorithm to Wireless LAN p Distributed Fair Scheduling p Backoff Based on Virtual Time p No Explicit Consideration of Spatial Channel Reuse p “Distributed Fair Scheduling in a Wireless LAN,” by N. Vaidya, P. Bahl & S. Gupta, MOBICOM 2000 n Multihop Packet Scheduling p Focus on Resolving Conflict between Fairness & Maximal Channel Utilization p Per-flow Service is not Fair p “A New Model for Packet Scheduling in Multihop Wireless Networks” by H. Luo & S. Lu, MOBICOM 2000

Spring 2005CMPE257 UCSC35 References  [BDSZ94] Bharghavan et al., MACAW: Media Access Protocol for Wireless LANs, in ACM SIGCOMM  [LL00] H. Luo and S. Lu, A Topology- Independent Fair Queueing Model in Ad Hoc Wireless Networks, in IEEE ICNP  [LL05] H. Luo and S. Lu, A Topology- Independent Wireless Fair Queueing Model in Ad Hoc Networks 2005, in IEEE JSAC (Extended version of [LL00]).

Spring 2005CMPE257 UCSC36 Acknowledgments n Parts of the presentation are adapted from the following sources: p Prof. Luo’s ICNP 2000 presentation