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Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks

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Title: Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks


1
Improving Loss Resilience with Multi-Radio
Diversity in Wireless Networks
  • Allen Miu, Hari Balakrishnan
  • MIT Computer Science and Artificial Intelligence
    Laboratory
  • C. Emre Koksal
  • Computer and Communication Sciences, EPFL

2
The problem
  • New wireless applications demand high performance
  • But wireless channels are loss-prone
  • Interference
  • Noise
  • Attenuation
  • Multi-path
  • Mobility, etc
  • ? Inconsistent and poor performance

3
Current solutions are inefficient for recovering
losses
  • Coding (e.g., FEC)
  • Hard on highly variable channels
  • Retransmission
  • Wasteful outage durations can be long (tens to
    hundreds of milliseconds)
  • Bit-rate adaptation
  • Hard on variable channels
  • Slows down other clients

4
Todays wireless LAN (e.g., 802.11)
May use only one path
  • Uses only one communication path

AP1
5
Multi-Radio Diversity (MRD) Uplink
Todays wireless LAN (e.g., 802.11)
May use only one path
  • Allow multiple APs to simultaneously receive
    transmissions from a single transmitter

10
AP1
20
Loss independence ? simultaneous loss 2
6
Multi-Radio Diversity (MRD) Downlink
  • Allow multiple client radios to simultaneously
    receive transmissions from a single transmitter

AP1
AP2
7
Are losses independent among receivers?
  • Broadcast 802.11 experiment at fixed bit-rate 6
    simultaneous receivers and 1 transmitter
  • Compute loss rates for the 15 receiver-pair (R1,
    R2) combinations
  • Frame loss rate FLR(R1), FLR(R2) vs. simultaneous
    frame loss rate FLR(R1 n R2)

8
Individual FLR gt Simultaneous FLR
y x
FLR
R1 R2 R1R2
FLR(R1 n R2)
9
Challenges in developing MRD
  • How to correct simultaneous frame errors?
  • Frame combining
  • How to handle retransmissions in MRD?
  • Request-for-acknowledgment protocol
  • How to adapt bit rates in MRD?
  • MRD-aware rate adaptation

10
Bit-by-bit frame combining
TX 1100 1010
Combine failure
Problem Exponential of CRC checks in
of unmatched bits.
11
Block-based frame combining
  • Observation bit errors occur in bursts
  • Divide frame into NB blocks (e.g., NB 6)
  • Attempt recombination with all possible block
    patterns until CRC passes
  • of checks upper bounded by 2NB
  • Failure rate increases with NB

12
Failure decreases with NB and burst size
1.0
Frame size 1500B
Probability of failure
0.8
0.6
NB 2
0.4
NB 4
0.2
NB 6

NB 16
0
10
20
30
40
50
0
Burst error length parameter
13
Flawed retransmission schemes
  • Conventional link-layer ACKs do not work
  • Final status known only to MRDC
  • Two levels of ACKs are redundant
  • Cannot disable link-layer ACKs

14
Request-for-acknowledgment (RFA) for efficient
feedback
IP
IP
MRD
MRD
link
link
link
MRDC
15
MRD-aware rate adaptation
  • Standard rate adaptation does not work
  • Reacts only to link-layer losses from 1 receiver
  • Uses sub-optimal bit-rates
  • MRD-aware rate adaptation
  • Reacts to losses at the MRD-layer

Implication First use multiple paths, then
adapt bit rates.
16
Experimental setup
20 m
R2
R1
L
  • 802.11a/b/g implementation in Linux (MADWiFi)
  • L transmits 100,000 1,500B UDP packets w/ 7
    retries
  • 802.11a _at_ auto bit rate (6, 9, 12, 18, 24, 36,
    48, 54)
  • L is in motion at walking speed, gt 1 minute per
    trial
  • Variants R1, R2, MRD (5 trials each)

17
MRD improves throughput
Throughput (Mbps)
R1
R2
MRD
Each color shows a different trial
18
MRD maintains high bit-rate
Fraction of transmitted frames
Frame recovery data ( of total losses at R1) via
R2 42.3 frame combining 7.3 Total
49.6
0
6
9
12
18
24
36
48
56

Selected bit rate (Mbps)
19
Delay Analysis
Fraction of delivered packets
User space implementation caused high delay
0
10-3
10-2
10-1
1
10-4
One way delay (10x s)
20
Related work
  • Physical layer spatial diversity techniques
  • Antenna diversity
  • 802.16 MIMO/802.11n
  • Macro-diversity in CDMA networks
  • Retransmission with memory Sindhu 77
  • Opportunistic forwarding Biswas 05Jain 05
  • Bit rate selection (AARF, RBAR, MiSer, OAR,
    Sample Rate)

21
Summary
  • Design of Multi-Radio Diversity WLAN
  • Block-based frame combining
  • Request-for-acknowledgment protocol
  • MRD-aware rate adaptation
  • Analysis of block-based frame combining
  • Experimental evaluation
  • MRD reduces losses by 50 and improves throughput
    by up to 2.3x
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