revisiting the contract between layers
Post on 09-Feb-2016
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Revisiting the Contract Between Layers
Sachin KattiDina Katabi, Hari Balakrishnan, Muriel Medard
Mesh Networks Borrowed the Internet Contract
Conflicts with wireless mesh characteristics
PHY + LL Deliver correct packetsNetwork Forward correct packets to destination
Current contract builds reliability on a link by link basis
Spatial diversity more naturally provides reliability across multiple links
S
R1
R2
D99% (10-3
BER)
99% (10 -3 BER)
Wireless Naturally Provides Reliability Across Links
0%
0%
Even 1 bit in 1000 incorrect Packet loss of 99%
S
R1
R2
D99% (10-3
BER)
99% (10 -3 BER)
Wireless Naturally Provides Reliability Across Links
0%
0%
Current contract Link by link reliability 50 transmissions
Loss
Loss
S
R1
R2
D99% (10-3
BER)
99% (10 -3 BER)
Wireless Naturally Provides Reliability Across Links
0%
0%
Spatial diversity: Even if no correct packets, every bit is likely received correctly at some node
Exploit wireless characteristics 3 transmissions
Current contract 50 tx Low throughputExploit wireless characteristics 3 tx High throughput
Useful with High Quality Links?R1
R2
R3
R4
Sa
Pb
Db
Da
Sb
Pb
Pa
Pa
Pa
Pb
1%
2%
1%
3%
0%
0%
0%
0%
Loss
Loss
Loss
Loss
Useful with High Quality Links?R1
R2
R3
R4
Sa
Pb
Db
Da
Sb
Pb
Pa
Pa
Pa
Pb
1%
2%
1%
3%
0%
0%
0%
0%Current contract Inhibits concurrencyExploit wireless characteristics Enables high concurrency
Current Contract
Limits throughput, inhibits concurrency
PHY + LL Deliver correct symbols to higher layerNetwork Forward correct symbols to destination
PHY + LL Deliver correct packetsNetwork Forward correct packets to destination
High throughput, high concurrency
New Contract Exploiting Wireless Characteristics
MIXIT• New contract between layers to harness
wireless characteristics• Novel symbol-level network code that
scalably routes correct symbols• High concurrency MAC• Implementation and evaluation• 3-4x gain over shortest path routing• 2-3x gain over packet-level opp. routing
How does a Router Identify Correct Symbols?• PHY already estimates a confidence for every
decoded symbol [JB07]• PHY + LL delivers high confidence symbols to
network layer
PHY Confidence
Packet
PHY + LL Deliver correct symbols to higher layerNetwork Forward correct symbols to destination
What Should Each Router Forward?
R1
R2
DSP1P2
P1P2
P1P2
What Should Each Router Forward?
R1
R2
DSP1P2
But overlap in correctly received symbols Potential solutions1)Forward everything Inefficient2)Coordinate Unscalable
P1P2
P1P2
P1P2
P1P2
Forward random combinations of correct symbols
R1
R2
DSP1P2
MIXIT Prevents Duplicates using Symbol Level Network Coding
P1P2
P1P2
1s
…
…R1
R2
D
2s2
1
7s2s
2
7…
1s
…
…
2s
Routers create random combinations of correct symbols
2
1
9s5s
5
9…
MIXIT Prevents Duplicates using Symbol Level Network Coding
R1
R2
D2
1
7s2s
…
2
1
9s5s
…
21 s,sSolve 2
equations
Destination decodes by solving linear equationsRandomness prevents duplicates without co-ordination
MIXIT Prevents Duplicates using Symbol Level Network Coding
1s
…
…R1
R2
D
2s2
1
7s2s
2
7…
1s
…
…
2s
Routers create random combinations of correct symbols
15s5
0…
MIXIT Prevents Duplicates using Symbol Level Network Coding
R1
R2
D2
1
7s2s
…
15s …
21 s,sSolve 2
equations
Destination decodes by solving linear equations
Symbol Level Network Coding • No duplicates Efficient • No coordination Scalable
MIXIT Prevents Duplicates using Symbol Level Network Coding
Destination needs to know which combinations it received
21 9s5s
21 0s5s
21 9s0s
(if both symbols were correct)
(if only s1 was correct)
(if only s2 was correct)
Nothing (if neither symbol was correct)
Destination needs to know which combinations it received Use run length encoding
5
9
Original Packets Coded Packet
0
9
Original Packets Coded Packet
Use run length encoding
Destination needs to know which combinations it received
9
5
Original Packets Coded Packet
Destination needs to know which combinations it received Use run length encoding
0
5
Original Packets Coded Packet
Destination needs to know which combinations it received Use run length encoding
Run length encoding efficiently expresses combinations
Destination needs to know which combinations it received Use run length encoding
Routers May Forward Erroneous Bits Despite High Confidence
MIXIT has E2E error correction capability!
Symbol-LevelNetwork CodingECC Data
MIXIT’s Error Correcting Code (ECC)1. Routers are oblivious to ECC2. Optimal error correction capability3. Rateless
Decode ECCData
PHY + LL Deliver correct symbols to higher layerNetwork Forward correct symbols to destination
Source Destination
High Concurrency MAC
• Each node maintains a map of conflicting transmissions
• Map is based on empirical measurements and built in distributed, online manner
w & x NO!w & u YES!
xu w
Evaluation
• Implementation on GNURadio SDR and USRP• Zigbee (IEEE 802.15.4) link layer• 25 node indoor testbed, random flows• Compared to:
1. Shortest path routing based on ETX2. MORE: Packet-level opportunistic routing
Throughput (Kbps)
CDF
Throughput increase: 3x over SPR, 2x over MORE
Throughput Comparison
2.1x3x
Shortest PathMOREMIXIT
Throughput (Kbps)
CDF
Where do the gains come from?
Shortest PathMOREMIXIT
Take concurrency away from MIXIT
Where do the gains come from?
1.5x
Without concurrency, 1.5x gain over MOREThroughput (Kbps)
CDF
Shortest PathMORE
MIXIT withoutconcurrency
Take concurrency away from MIXIT
Where do the gains come from?
Throughput (Kbps)
CDF
MIXIT
Gains come from both moving to the symbol level and high concurrency
Shortest PathMORE
MIXIT withoutconcurrency
Multiple Flows
Shortest PathMOREMIXIT
No. of concurrent flows
Avg.
Net
wor
k Th
roug
hput
(Kbp
s)
MORE/SPR: Higher congestion Lower concurrencyMIXIT: Higher congestion High concurrency
Related Work• Opportunistic Routing• EXOR [BM05], MORE [CJKK07], Coop-diversity[LWT04]
• Soft information • SoftPHY [JB07], SOFT [WKK07], SOVA[HH89]
• Increasing wireless concurrency• CMAPS[VJB08], Conflict maps[JPPQ03], Interference
modeling [RMRWZ06]
• Network Coding• Linear codes[ACLY00], MRD [G:85, SKK:08, KK:07]
ConclusionMIXIT• New contract harnesses wireless characteristics• Symbol-level network coding to scalably route
correct symbols• High concurrency• Implementation and evaluation demonstrating 3-4x over shortest path, 2-3x gains over MORE
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