Title: End-to-End Performance and Fairness in Multihop Wireless Backhaul Networks
1End-to-End Performance and Fairness in Multihop
Wireless Backhaul Networks
- V. Gambiroza, B. Sadeghi, and E. Knightly
- Rice University
2Backhaul Networks
Residential user or small business
- Backhaul networks technologies
- Wireline coax-, copper-based, fiber
- Wireless
3Wireless Backhaul Networks TAP Networks
Residential user or small business
- Multihop wireless infrastructure
- High bandwidth, good economics, deployability
- Transit Access Point (TAP)
4Fundamental Scenario
- One branch of the access tree
5Parking Lot Scenario
- Similar to parking lot with one exit
Internet
TAP3
TAP1
TAP2
TAP4
6Fairness Problem
7Fairness Problem
8Fairness Problem
We need multihop fairness
9Contributions
- Fairness reference model
- Performance study
- TCP
- Inter-TAP fairness algorithm
- Capacity and fairness
10Outline
- Fairness reference model
- Limitations of existing models
- Fairness objectives
- Algorithm solution space
- Performance study
- Capacity and fairness
11Limitations of Existing Fairness Models
Ingress-Egress Flow Granularity
- Fairness with Ingress-Egress (IE) flow
granularity - Provide fair share to each ingress-egress pair
- Node corresponds to TAP
- TAP is small business/residence
- Provide fair shares to TAPs independent of
number of flows - Treat TAPs traffic as a single aggregate
- Ingress Aggregate (IA) flow granularity
- Provide fairness on both IA and IE flow
granularities -Fundamentally different
12Our Objectives (Our Objectives vs. Classical
Objectives)
Our Objectives
Classical Objectives
- Flow granularity
- Ingress aggregate (IA) and Ingress-Egress
- Spatial properties
- Provide fair shares independent of spatial
location - Maximize spatial reuse flows sufficiently
spatially separated can transmit simultaneously
Depends on fairness model
- Resource
- Channel access time
- Medium
- Multirate shared wireless channel
13Problem 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)
14Outline
- Fairness reference model
- Performance study
- Performance factors
- TCP fairness
- Inter-TAP Fairness Algorithm (IFA)
- Capacity and fairness
15Performance 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
16Performance Factors (2/2)
Topology
Well understood topologies
17Performance Factors (2/2)
Topology
Parking lot
MU-TAP and TAP-TAP transmissions on orthogonal
channels
18Fairness with TCP MAC, Hidden Terminals and
Information Asymmetry
MUs generate long lived TCP-Sack flows Carrier
sense range transmission range
19Fairness with TCP MAC, and Hidden Terminals and
Information Asymmetry
MUs generate long lived TCP-Sack flows Carrier
sense range transmission range
- TAP1 and TAP2 traffic starved
- Both are hidden terminals
- Timeouts significant throughput penalty
- TCP generates bursts of packets
ACK Traffic
20Fairness with TCP MAC, and Hidden Terminals and
Information Asymmetry
MUs generate long lived TCP-Sack flows Carrier
sense range transmission range
- Capacity and fairness need to be considered
jointly - Total is up to 125 of objective while two flows
are starved
- RTS/CTS exchange introduces information asymmetry
KSSK02 - TAP1 has no information of TAP3-TAP4 trans.
ACK Traffic
21TCP and Sector Antennas
MUs generate long lived TCP-Sack flows TAPs use
sector antennas
- Severe spatial bias
- TAP1 traffic obtains 26 of objective
- Total goodput increased
- Total goodput is 67 of the objective
22Inter-TAP Fairness Algorithm (IFA)
- Idealized version of algorithm
- Omniscient calculation of fair rates
- Practical algorithm needs messaging
- Limit traffic rate at ingress
23TCP and IFA
MUs generate long lived TCP-Sack flows Carrier
sense range transmission range
- End-to-end performance considerably improved
- TAP-aggregated throughput is 59 to 75 of the
objective - Hidden terminal problem
mitigated - Contention considerably decreased
- TCP cannot inject bursts of packets
- Spatial bias
- IFA alone cannot eliminate it
- Rates lower than the objective
24Inter-TAP Performance Isolation
- Provide inter-TAP performance isolation
- independent of traffic types
25Summary 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
26Summary 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
27Outline
- Fairness reference model
- Performance study
- Capacity and fairness
- Maximum throughput without fairness
- Fairness objectives and throughput
28Problem Statement
- Compute maximum aggregate throughput
- No fairness constraint
- System model
- One transmission possible at time
- Perfect collision-free MAC
Single contention neighborhood
29Aggregate Throughputwith and without Fairness
Constraints
Assign time-shares to maximize network throughput
Solution
No spare time-capacity
- Fairness constraints
- Temporal fairness constraint
- Spatial bias removal constraint
- Ingress aggregate constraint
30Conclusions
- 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
31End-to-End Performance and Fairness in Multihop
Wireless Backhaul Networks
- V. Gambiroza, B. Sadeghi, and E. Knightly
- Rice University