Title: Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks
1Improving 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
2The problem
- New wireless applications demand high performance
- But wireless channels are loss-prone
- Interference
- Noise
- Attenuation
- Multi-path
- Mobility, etc
- ? Inconsistent and poor performance
3Current 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
4Todays wireless LAN (e.g., 802.11)
May use only one path
- Uses only one communication path
AP1
5Multi-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
6Multi-Radio Diversity (MRD) Downlink
- Allow multiple client radios to simultaneously
receive transmissions from a single transmitter
AP1
AP2
7Are 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)
8Individual FLR gt Simultaneous FLR
y x
FLR
R1 R2 R1R2
FLR(R1 n R2)
9Challenges 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
10Bit-by-bit frame combining
TX 1100 1010
Combine failure
Problem Exponential of CRC checks in
of unmatched bits.
11Block-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
12Failure 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
13Flawed 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
14Request-for-acknowledgment (RFA) for efficient
feedback
IP
IP
MRD
MRD
link
link
link
MRDC
15MRD-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.
16Experimental 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)
17MRD improves throughput
Throughput (Mbps)
R1
R2
MRD
Each color shows a different trial
18MRD 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)
19Delay 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)
20Related 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)
21Summary
- 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