End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks - PowerPoint PPT Presentation

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End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks

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End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks V. Gambiroza, B. Sadeghi, and E. Knightly Department of Electrical and Computer Engineering – PowerPoint PPT presentation

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Title: End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks


1
End-to-End Performance and Fairness in Multihop
Wireless Backhaul Networks
  • V. Gambiroza, B. Sadeghi, and E. Knightly
  • Department of Electrical and Computer Engineering
  • Rice University
  • MobiCom04, Sept. 26Oct.
  • Presented by Yeong-cheng Tzeng

2
Outline
  • Introduction
  • Performance and fairness objectives
  • Performance study
  • Capacity and fairness
  • Conclusions

3
Introduction
  • Wireless LANs remain slower due to slow wired
    backhaul
  • Develop wireless backhaul networks via wirelessly
    multi-hopping to a Internet entry point
  • Existing protocols result in
  • Unfairness
  • Starvation
  • Poor performance

4
Introduction (cont.)
  • TAP System Model
  • System wide performance

5
Introduction (cont.)
  • TAP System Model
  • System wide performance
  • If branches are on different frequencies or
    sufficiently spatially separated

6
Introduction (cont.)
  • Parking Lot Scenario
  • Similar to parking lot with one exit

7
Introduction (cont.)
  • Parking Lot Scenario
  • Fairness problem

8
Introduction (cont.)
  • Parking Lot Scenario
  • Fairness problem

9
Introduction (cont.)
  • Parking Lot Scenario
  • Fairness problem

10
Introduction (cont.)
  • Parking Lot Scenario
  • Fairness problem

11
Outline
  • Introduction
  • Performance and fairness objectives
  • Objectives
  • TAP fairness reference model
  • Performance study
  • Capacity and fairness
  • Conclusions

12
Objectives
  • Temporal fairness
  • vs. throughput fairness
  • Performance isolation
  • Ingress aggregate
  • Each TAP corresponds to a single residence, small
    business, or hot spot
  • Spatial bias
  • No penalty for flows further away from the wired
    TAP
  • Spatial reuse
  • Resources can be reclaimed when they are unused
  • Maximize

13
TAP fairness reference model
  • Present a formal definition that determines if a
    set of candidate allocated temporal shares
    (expressed as a matrix T) is TAP-fair
  • Contention neighborhood
  • A subset of the set of all links with the
    property that no two links from the subset can be
    active simultaneously
  • A matrix T satisfying following constraints is
    said to be feasible
  • , for all flows (i, j)
  • , for all k and all links n

14
TAP fairness reference model (cont.)
  • Definition 1. A matrix T is said to be TAP fair
    if it is feasible and if for each flow ,
    cannot be increased while maintaining
    feasibility without decreasing for some flow
    for which
  • , when
  • , when

15
TAP fairness reference model (cont.)
  • Considering temporal shares we ensure temporal
    fairness
  • We ensure the ingress aggregate objective by
    satisfying the below inequality
  • , when
  • We ensure the spatial bias objective by
    satisfying the below equality
  • By allowing no spare time capacity we ensure
    spatial reuse

16
Outline
  • Introduction
  • Performance and fairness objectives
  • Performance study
  • UDP Baseline Scenario
  • TCP fairness
  • Inter-TAP Fairness Algorithm (IFA)
  • Summary of Findings
  • Capacity and fairness
  • Conclusions

17
Performance Study
  • Goal
  • Study end-to-end performance and fairness
  • Factors investigated
  • Fairness algorithms
  • Uncontrolled UDP, TCP, IFA
  • Media access control
  • CSMA and CSMA/CA
  • Antenna technologies
  • Omni directional, sector
  • Carrier sense range, multiple topologies and flow
    scenarios

18
UDP Baseline Scenario
  • Parking lot scenario
  • MU-TAP and TAP-TAP transmissions on orthogonal
    channels
  • 5 Mobile Users (MU) per TAP
  • Constant rate UDP traffic with 1000 byte packets
  • Channel rate constant 2 Mb/sec
  • TAPs two or more hops away not in carrier sense
    range

19
UDP Baseline Scenario (cont.)
  • Traffic originating at TAP1 starved
  • Hidden terminal problem
  • An increased no. of hops leads to a corresponding
    throughput decrease
  • UDP/802.11 achieved 92 of the objective
  • Capacity and fairness need to be considered
    jointly
  • No multi-rate transmission
  • Temporal throughput

20
Fairness with TCP
  • TAP4s ACK traffic
  • Hidden terminal problem
  • RTS/CTS introduces information asymmetry problem
  • CSMA obtains slightly higher goodput than CSMA/CA
  • TCP traffic is higher than objective

21
Fairness with TCP (cont.)
  • Sector antennas eliminate the hidden terminal
    problem and information asymmetry problem
  • Total goodput is increased
  • TAP(1) and TAP(2) are not starved
  • Second air interface
  • Reduced collisions

22
Fairness with TCP (cont.)
  • A sensitive carrier sense range can mitigate the
    impact of hidden terminals and information
    asymmetry
  • Such a sensitive carrier sense range not always
    true
  • Realistic - due to hardware limitations
  • Desirable - reduces spatial reuse

23
Inter-TAP Fairness Algorithm (IFA)
  • A layer 2 multi-hop wireless fairness algorithm
  • Allocate resources according to reference model
  • By limiting flows at the first hop to mitigate
    unfairness and starvation
  • Key components
  • Measurement of Offered Load and Capacity
  • Measurement can be performed at TAPs or MUs
  • Message Distribution
  • Message distribution interval
  • Overhead
  • Control message require priority or spare
    capacity
  • Aggregate Fair Share Computation
  • Compute the aggregate time shares in each
    contention neighborhoods and chooses the minimum
    value
  • Convert the time share to rate via use of the
    available link capacity
  • Ingress Rage Limiting
  • The TAP can signal each MU of its fair share
  • The TAP can CTS or poll MUs at the desired rate

24
Inter-TAP Fairness Algorithm (IFA) (cont.)
25
Inter-TAP Fairness Algorithm (IFA) (cont.)
26
Inter-TAP Fairness Algorithm (IFA) (cont.)
27
Inter-TAP Fairness Algorithm (IFA) (cont.)
  • End-to-end performance considerably improved
  • Hidden terminal problem mitigated
  • Contention considerably decreased
  • Spatial bias
  • IFA cannot eliminate it
  • Spatial reuse
  • IFA is able to exploit spatial reuse
  • Rates lower than the objective

28
Summary of Findings
  • Parking Lot scenario results in hidden
    terminals and information asymmetry
  • Starvation of upstream flows (UDP, TCP, with or
    w/o RTS/CTS)
  • Sector antennas and carrier sense range mitigate
    the problem
  • Sector antennas Throughput as low as 26 of
    targeted values
  • Sensitive CSR not always realistic and desirable
  • Ingress rate limiting mitigates the problem

29
Outline
  • Introduction
  • Performance and fairness objectives
  • Performance study
  • Capacity and fairness
  • Maximum throughput without fairness
  • Fairness objectives and throughput
  • Throughput comparison
  • Conclusions

30
Maximum throughput without fairness
Assign time-shares to maximize network throughput
No spare time-capacity
31
Maximum throughput without fairness (cont.)
  • Solution
  • Assign time shares to only one flow

32
Fairness objectives and throughput
  • Temporal Fairness Constraint
  • Time is resource
  • Divided equally among flows

33
Fairness objectives and throughput (cont.)
  • Spatial Bias removal constraint
  • Equal share regardless of number of hops
  • for all ,


Fair share of flow f, defined by
system policies
Example policy

34
Fairness objectives and throughput (cont.)
Throughput is proportional to the throughput of
ingress link
35
Fairness objectives and throughput (cont.)
20 km
20 km
A
B
v120km/h 1hr
v210km/hr 2hr
36
Fairness objectives and throughput (cont.)
  • Ingress Aggregate Constraint
  • Treat same-ingress flows as one

Ingress-Egress flow granularity
Ingress Aggregate flow granularity
37
Fairness objectives and throughput (cont.)
38
Fairness objectives and throughput (cont.)
39
Throughput comparison
  • Example scenario
  • Summary
  • Total throughput
  • Temporal vs. throughput
  • Throughput highly probable depending on link
    capacity

40
Outline
  • Introduction
  • Performance and fairness objectives
  • Performance study
  • Capacity and fairness
  • Conclusions

41
Conclusions
  • Fairness
  • Fairness reference model formally defined
  • Designed for multi-hop wireless networks
  • Performance study
  • Starvation of upstream flows
  • Sector antennas, larger carrier sense range, IFA
    mitigate the problem
  • IFA approximates performance of reference model
  • Capacity and fairness
  • Need to be considered jointly

42
  • The End
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