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



V. Gambiroza, B. Sadeghi, and E. Knightly
            Rice University
                           Backhaul Networks



                              Backhaul
                              network            Internet
       Residential user
       or small business




   •   Backhaul networks technologies
        – Wireline: coax-, copper-based, fiber
        – Wireless



Violeta Gambiroza
           Wireless Backhaul Networks
                                                      TAP Networks

        Residential user
        or small business

                                Ethernet




                                                                  Internet


                                                       Ethernet
        Wireless Backhaul
                      Network
                                           Ethernet
           Ethernet




   •   Multihop wireless infrastructure
       – High bandwidth, good economics, deployability
   •   Transit Access Point (TAP)

Violeta Gambiroza
                              Fundamental Scenario


                                    Ethernet




                                                                       Ethernet
                                                                                               One branch of the access tree
                                               Ethernet
        Ethernet




                                                                                                         •   Traffic matrix
                   Ethernet




                                                            Ethernet
                               Ethernet




                                                                                   Ethernet




                                                                                              Internet        – Traffic to and from
       TAP1                   TAP2                        TAP3                    TAP4
                                                                                                                Internet



Violeta Gambiroza
                    Parking Lot Scenario

 •   Similar to parking lot with one exit




                                       Ethernet




                                                              Ethernet
                                                  Ethernet




                                                                          Ethernet
                                                                                     Internet

                                       TAP1       TAP2       TAP3        TAP4


Violeta Gambiroza
                    Fairness Problem




Violeta Gambiroza
                    Fairness Problem




Violeta Gambiroza
                    Fairness Problem




                          Goal
     Ensure equal shares independent of spatial location



               We need multihop fairness


Violeta Gambiroza
                      Contributions


 •   Fairness reference model

 •   Performance study
     – TCP




                                                     Ethernet
     – Inter-TAP fairness algorithm   Wireless




                                                                           Ethernet
                                      Backhaul
                                      Network




                                                                Ethernet
 •   Capacity and fairness



                                          Ethernet




Violeta Gambiroza
                            Outline


 •   Fairness reference model
     – Limitations of existing models
     – Fairness objectives
     – Algorithm solution space




                                                       Ethernet
                                        Wireless




                                                                             Ethernet
 •   Performance study                  Backhaul
                                        Network




                                                                  Ethernet
                                            Ethernet
 •   Capacity and fairness



Violeta Gambiroza
   Limitations of Existing Fairness Models:
      Ingress-Egress Flow Granularity

  •   Fairness with Ingress-Egress (IE) flow granularity
       – Provide fair share to each ingress-egress pair
         Ingress-Egress
        flow granularity

  •   Node corresponds to TAP
      Ingress-Egress business/residence
       – TAP is small
      Ingress Aggregate
      flow granularity
             Provide fair shares to TAPs independent of number of
        flow granularity
           flows
              Treat TAP’s traffic as a single aggregate


  •   Ingress Aggregate (IA) flow granularity
       –Provide fairness on both IA and IE flow granularities -
       Fundamentally different
Violeta Gambiroza
                          Our Objectives
              (Our Objectives vs. Classical Objectives)

       Our Objectives                     Classical Objectives
 Flow granularity
     –    Ingress aggregate (IA) and      – Ingress-Egress (IE)
          Ingress-Egress
 Spatial properties
      – Provide fair shares independent
               Formal definition in paper model
      of spatial location       Depends on fairness
      – Maximize spatial reuse – flows
      sufficiently spatially separated
      can transmit simultaneously
 Resource
      –    Channel access time            – Bandwidth


 Medium
      –    Multirate shared wireless      – Wired link
           channel

Violeta Gambiroza
                      Problem Statement


 •   Fairness reference model defined

 •   Distributed algorithm
      – Targeted at achieving shares defined by reference model

 •   Solution space
       – Local solution – insufficient
              Example: Parking lot
       – Multihop solution
              Flow e2e – TCP
           Multihop wireless network e2e – Inter-TAP
           Fairness Algorithm (IFA)

Violeta Gambiroza
                           Outline


 •   Fairness reference model

 •   Performance study
     – Performance factors
     – TCP fairness




                                                           Ethernet
     – Inter-TAP Fairness Algorithm (IFA)   Wireless




                                                                                 Ethernet
                                            Backhaul
                                            Network




                                                                      Ethernet
 •   Capacity and fairness


                                                Ethernet




Violeta Gambiroza
            Performance Factors (1/2)
 Goal
 •   Study end-to-end performance and fairness
 Factors investigated
 • Fairness algorithms
     – Uncontrolled UDP, TCP, IFA
 •   Media access control
     – 802.11 with two-way and four-way handshake
 •   Antenna technologies
     – Omni directional, sector
 •   Carrier sense range, multiple topologies and flow scenarios…

 Other simulation specs
 • Channel rate constant 2 Mb/sec
 • 1000 byte packets

Violeta Gambiroza
            Performance Factors (2/2)

 Topology
 Well understood topologies




 Increased no. of hops from destination
                                          Increased no. of source-dest. pairs
          Reduced throughput
                                                 Reduced throughput




Violeta Gambiroza
            Performance Factors (2/2)

 Topology
 Parking lot




           MU-TAP and TAP-TAP transmissions on orthogonal channels

                     TA(1)
                                        TA(2)
                                                           TA(3)
                             Ethernet




                                                                   Ethernet
                                                Ethernet




                                                                               Ethernet




                                                                                          Internet
                         TAP1              TAP2               TAP3            TAP4



Violeta Gambiroza
                                              Fairness with TCP
                      MAC, Hidden Terminals and Information Asymmetry


        MUs generate long lived TCP-Sack flows
        Carrier sense range = transmission range




                                                                                       Ethernet




                                                                                                              Ethernet
                                                                                                  Ethernet




                                                                                                                         Ethernet
                                                                                      TAP1        TAP2       TAP3        TAP4



                                    Obj.      Basic      RTS/CTS
                   1600


                                                                                     Idealized objective
Goodput [kb/sec]




                   1200
                                                                      1000
                                                                                 •
                                                                                     – Assumes perfect collision-
                   800
                                                                                       free MAC
                   400    320.5     320.5     320.5

                                                        ACK Traffic
                                                          38.5
                     0
                            TA(1)     TA(2)     TA(3)       TA(4)        Total



  Violeta Gambiroza
                                                   Fairness with TCP
                   MAC, and Hidden Terminals and Information Asymmetry


        MUs generate long lived TCP-Sack flows
        Carrier sense range = transmission range




                                                                                                   Ethernet




                                                                                                                          Ethernet
                                                                                                              Ethernet




                                                                                                                                     Ethernet
                                                                                                   TAP1       TAP2       TAP3        TAP4



                                      Obj.         Basic          RTS/CTS
                   1600
                                                                                             •   TAP1 and TAP2 traffic
                                                                                                 starved
                                                                                      1247
Goodput [kb/sec]




                   1200                                    1177

                                                                               1000



                   800
                                                                                                 – Both are hidden terminals
                                                                                                 – Timeouts – significant
                   400    320.5       320.5        320.5                                           throughput penalty
                                                                  ACK Traffic
                                  2           20                   38.5   48                         TCP generates bursts
                     0                                                                                of packets
                            TA(1)       TA(2)        TA(3)           TA(4)        Total



  Violeta Gambiroza
                                                          Fairness with TCP
                   MAC, and Hidden Terminals and Information Asymmetry


        MUs generate long lived TCP-Sack flows
        Carrier sense range = transmission range




                                                                                                                        Ethernet




                                                                                                                                               Ethernet
                                                                                                                                   Ethernet




                                                                                                                                                          Ethernet
                                                                                                                       TAP1        TAP2       TAP3        TAP4



                                          Obj.            Basic                RTS/CTS
                   1600

                                                                                                                   •   Capacity and fairness
                                                                                                                       RTS/CTS exchange
                                                                                                       1247
Goodput [kb/sec]




                   1200                                           1177
                                                                                                                   •
                                                                                                          1058.7
                                                                                                                       need to be considered
                                                                                                                       introduces information
                                                                                                1000
                                                                         988


                   800                                                                                                 jointly
                                                                                                                       asymmetry [KSSK02]
                                                                                                                       – Total is up to 125% of
                   400    320.5           320.5           320.5
                                                                                                                         objective no information
                                                                                                                       – TAP1 has while two
                                                  20 27
                                                                           ACK Traffic
                                                                                      48 40.7
                                                                                                                         of TAP3-TAP4 trans.
                                                                                                                         flows are starved
                                  2   3                                        38.5
                     0
                            TA(1)           TA(2)           TA(3)                TA(4)             Total



  Violeta Gambiroza
                                        TCP and Sector Antennas

                     MUs generate long lived TCP-Sack flows
                     TAPs use sector antennas




                                                                                                                        Ethernet




                                                                                                                                               Ethernet
                                                                                                                                   Ethernet




                                                                                                                                                          Ethernet
                                                                                                                       TAP1        TAP2       TAP3        TAP4




                                                                                                               •   Impact of hidden terminals
                   2400                Obj.        Basic            RTS/CTS
                                                                                                                   and information asymmetry
                                                                                                                   mitigated
                                                                                            2000
                   2000
Goodput [kb/sec]




                   1600                                                                            1386.3


                   1200
                                                                                                        1219
                                                                                                               •   Severe spatial bias
                                                                                                                   – TAP1 traffic obtains 26% of
                    800                                             730 692
                          641               641               641
                                                                                                                     objective
                                                  356
                    400
                                                                                                                   Total goodput increased
                                                        313
                                                                              ACK Traffic
                                                                                                               •
                                247
                                      167
                                                                               77 53.3 47
                      0
                           TA(1)             TA(2)             TA(3)            TA(4)         Total
                                                                                                               •   Total goodput is 67% of the
                                                                                                                   objective
    Violeta Gambiroza
     Inter-TAP Fairness Algorithm (IFA)


 •   Idealized version of algorithm
      – Omniscient calculation of fair rates
          Practical algorithm needs messaging




 •   Limit traffic rate at ingress




Violeta Gambiroza
                                                                      TCP and IFA

            MUs generate long lived TCP-Sack flows




                                                                                                           Ethernet




                                                                                                                                  Ethernet
                                                                                                                      Ethernet




                                                                                                                                             Ethernet
            Carrier sense range = transmission range
                                                                                                          TAP1        TAP2       TAP3        TAP4



                   1200           Obj.           Basic 802.11/IFA                               •   End-to-end performance
                                                                                                    considerably improved
                                                                                   1000
Goodput [kb/sec]




                   800                                                                               – TAP-aggregated throughput is
                                                                                          679
                                                                                                •   Spatial bias
                                                                                                       59% to 75% of the objective

                   400                                                                          •
                                                                                                      – IFA alone cannot eliminate
                                                                                                     Hidden terminal problem it
                                                                                                     mitigated
                          320.5          320.5         320.5

                                  190
                                                 223           240
                                                                                                •   Rates lower than the objective
                                                                     ACK Traffic
                                                                       38.5   26                     – Contention considerably
                     0
                                                                                                       decreased
                          TA(1) TA(2) TA(3) TA(4) Total
                                                                                                     – TCP cannot inject bursts of
                                                                                                       packets

      Violeta Gambiroza
                      Inter-TAP Performance Isolation
                                        Provide inter-TAP performance isolation
                                              independent of traffic types
                   Each TAP has one MU
                   TAP1: MU transmits TCP traffic




                                                                                                            Ethernet




                                                                                                                                   Ethernet
                                                                                                                       Ethernet




                                                                                                                                              Ethernet
                   TAP2 and TAP3: MU transmits UDP traffic
                                                                                                            TAP1       TAP2       TAP3        TAP4


                                                                                  993.4
                   1000
                                    Obj.         Basic 802.11
                    800                                                                           •   TCP achieves 64% of idealized
Goodput [kb/sec]




                                                                                                      objective, while UDP obtains
                                                                                          709.4


                    600                                                                               75%
                    400   326.8         326.8         326.8                                       •   Even with balanced contention
                    200
                                  209
                                                246           246
                                                                                                      TCP reduces its rate
                                                                    ACK Traffic
                                                                                                      – Having more MUs per TAP TCP
                                                                                                        performance degraded
                                                                      13   8.4
                      0
                          TA(1) TA(2) TA(3) TA(4) Total


    Violeta Gambiroza
            Summary of Findings (1/2)

 •   Starvation of upstream flows (UDP, TCP, with or
     w/o RTS/CTS)
     – “Parking Lot” scenario results in hidden terminals and
       information asymmetry

 •   Sector antennas and carrier sense range mitigate
     the hidden terminal problem
     – Severe spatial bias
         SA: Throughput as low as 26% of targeted values

         CSR: Throughput as low as 34% of targeted values



 •   TCP able to exploit spatial reuse


Violeta Gambiroza
            Summary of Findings (2/2)

 •   IFA approximates reference model performance

 •   The impact of hidden terminal problem and
     information asymmetry mitigated
     – Without any modifications to CSMA/CA


 •   TCP over IFA achieves 59% to 75% of idealized
     objective
     – Without any modifications to TCP


 •   Inter-TAP performance isolation

Violeta Gambiroza
                          Outline


     Fairness reference model




                                                            Ethernet
 •
                                             Wireless




                                                                                  Ethernet
                                             Backhaul
                                             Network
     Performance study




                                                                       Ethernet
 •




                                                 Ethernet
 •   Capacity and fairness
     – Maximum throughput without fairness
     – Fairness objectives and throughput




Violeta Gambiroza
                    Problem Statement


 •   Compute maximum aggregate throughput
     – No fairness constraint
 •   System model
     – One transmission possible at time
     – Perfect collision-free MAC




                                  Single contention neighborhood
Violeta Gambiroza
            Aggregate Throughput
     with and without Fairness Constraints

 Assign time-shares to maximize network throughput
                                         F                   F
                         max  
                          f
                                                f
                                                    s.t.    tlf  1
                           {tl }         f 1               f 1 lr f


                    F
Solution:            t
                    f 1 lr f
                                 l
                                     f
                                         1                No spare time-capacity

                    tif Ci  t jf C j                      i, j  r f


 Fairness constraints
   Temporal fairness constraint
   Spatial bias removal constraint
   Ingress aggregate constraint


Violeta Gambiroza
                    Conclusions

 Fairness
 • Fairness reference model formally defined
 • Designed for multihop 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


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



V. Gambiroza, B. Sadeghi, and E. Knightly
            Rice University

				
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