Network Coding vs. Erasure Coding:Reliable Multicast in MANETs

Atsushi Fujimura*, Soon Y. Oh, and Mario Gerla

*NEC Corporation University of California, Los Angeles

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Motivation

Tactical networks require high reliability of multicast communications for effective mission accomplishment

Both Network coding (NC) and erasure coding (EC) can increase reliability in lossy networks

Which coding scheme is more reliable and efficient for MANETs?

“The jury is still out”

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NC and ECImplementation Both randomly and linearly encode:

– Block coding: stream of packets is split into blocks and encoded

– Coefficients are randomly drawn from a finite field– Receivers reconstruct original data

Erasure Coding Example

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Network Coding Example

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NC and EC Implementation Probabilistic Forwarding:

– In both NC and EC each intermediate nodes forwards with probability f

Forwarder generates random number x

Compare x and drop rate d

If (x < d) forwarding received packet

Otherwise drop packet

Packet Drop

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NC and EC Implementation

Network Coding Erasure Coding

Coding at Source

( Rate c = k/n )

Coding rate c = 1.A source does not generate redundant packets

k original packets are encoded into n > k source encoded packets, c < 1

Encoding at Intermediate

NodesYes No

Buffering and Forwarding

Intermediate nodes enqueue innovative packets for re-encoding

Intermediate nodes forward only non-duplicated packets

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Simulation Settings

Qualnet implementation– Random linear coding– 2Mbps channel bandwidth, 376m radio range– 802.11b MAC and PHY– 1KB/s traffic

Two topologies– Grid topology– Random topology

Performance Metrics– Packet Delivery Ratio (PDR)– Normalized Packet Overhead

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Grid Topology Setting

Grid Topology– One source and three

receivers– Each node has r

redundant paths (except the 1st hop)

– h: Number of hops from a source to receivers

Receivers

R

S

R R

h

Source

r

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Simulation (Grid Topology)

EC coding rate ranges between c=1 and c=1/6 EC requires twice as much line overhead to

achieve the same delivery ratio as NC

Packet delivery ratio when h =5 Overhead in term of h when f =1

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Simulation (Grid Topology)

EC Delivery ratio is very sensitive to hop number (while NC holds its performance variation smaller)

Packet drop probability on a link (d) has more impact on NC achievable delivery ratio

Packet delivery ratio for varying hop # Packet delivery ratio for variable packet drop rate, h=5

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Analysis (Grid Topology)

Single-hop models for NC (left) and EC (right) Different packets (NC) and duplicated packets (EC)

f f f

S ２

R

1-d

f f f

S

R

1-d

S １ S ３

R

S

R R

Network coding case Erasure coding case

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Random Topology

50 nodes including one source and 10 multicast members

Nodes are randomly distributed in a square field

Node density = average number of nodes within the transmission range (376m)

0

200

400

600

800

1000

1200

1400

0 200 400 600 800 1000 1200 1400

(m)

(m)

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Simulation (Random Topology)

EC suffers much more overhead (similar to the results in Grid Topology)

As for grid topology, EC delivery ratio equals NC delivery ratio between c =1/3 and c=1/2

Packet delivery ratio and overhead when the node density is 12

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Simulation (Random Topology)

Packet Drop decreases delivery ratio, but more significant in NC than EC

Node mobility helps both NC and EC recover from high packet drop rate

Packet drop probability (d) = 0.4

Packet delivery ratio with drip rate and mobility when node density is 12

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Summary

Compared NC and EC in MANETs NC can achieve high delivery ratio with much less overhead

Future Work Implementation of joint EC and NC scheme Extension to vehicular applications

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Thanks