Network Coding vs Erasure Coding Reliable Multicast in

Network Coding vs. Erasure Coding: Reliable Multicast in MANETs Atsushi Fujimura*, Soon Y. Oh, and Mario Gerla *NEC Corporation University of California, Los Angeles

Motivation n n 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” Page 2

NC and EC Implementation n 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 Page 4

Network Coding Example Page 5

NC and EC Implementation n 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 Page 6

NC and EC Implementation Network Coding at Coding rate c = 1. Source A source does not ( Rate c = k/n ) generate redundant packets Encoding at Intermediate Nodes Buffering and Forwarding Yes Intermediate nodes enqueue innovative packets for re-encoding Erasure Coding k original packets are encoded into n > k source encoded packets, c < 1 No Intermediate nodes forward only nonduplicated packets Page 7

Simulation Settings n n n Qualnet implementation – Random linear coding – 2 Mbps channel bandwidth, 376 m radio range – 802. 11 b MAC and PHY – 1 KB/s traffic Two topologies – Grid topology – Random topology Performance Metrics – Packet Delivery Ratio (PDR) – Normalized Packet Overhead Page 8

Grid Topology Setting Source S n r h R R R Grid Topology –One source and three receivers –Each node has r redundant paths (except the 1 st hop) –h: Number of hops from a source to receivers Receivers Page 9

Simulation (Grid Topology) EC coding rate ranges between c=1 and c=1/6 n EC requires twice as much line overhead to achieve the same delivery ratio as NC n Packet delivery ratio when h =5 Overhead in term of h when f =1 Page 10

Simulation n n (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 Page 11

Analysis (Grid Topology) Single-hop models for NC (left) and EC (right) n Different packets (NC) and duplicated packets (EC) n S R R R S１ S２ S３ f f f S f f 1 -d R R Network coding case f Erasure coding case Page 12

Random Topology n n n 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 (376 m) Page 15

Simulation (Random Topology) n EC suffers much more overhead (similar to the results in Grid Topology) n 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 Page 16

Simulation (Random Topology) Packet Drop decreases delivery ratio, but more significant in NC than EC n Node mobility helps both NC and EC recover from high packet drop rate n Packet drop probability (d) = 0. 4 Packet delivery ratio with drip rate and mobility when node density is 12 Page 18

Summary Compared NC and EC in MANETs n NC can achieve high delivery ratio with much less overhead n Future Work Implementation of joint EC and NC scheme n Extension to vehicular applications n Page 19

Thanks Page 20