Practical and Efficient Broadcast in Mobile Ad hoc Networks

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Practical and Efficient Broadcast in Mobile Ad
hoc Networks
Infocom 2007 Paper

• L. (Erran) Li, R. Ramjee, M. Buddhikot, S. Miller
• Bell Labs, Lucent

Slides by Tan Apaydin
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• Problem Broadcasting packets to be received by
  all nodes.
• Wired vs. Wireless
• Observation Due to the broadcast nature of
  wireless media, not all nodes need to transmit.

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PREVIOUS WORK On BROADCAST SUPPORT

• Probabilistic Each node re-broadcast packet to
  its neighbors with a given forwarding probability
  (fp).
• Challenge tune the fp.
• Deterministic Predetermine neighbor nodes to
  forward the broadcast packet.
• In this paper deterministic approach.

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DETERMINISTIC APPROACH

• Two popular schemes
• Complete topology
• 1 or 2 hop topology
• First one above is not realistic for ad hoc
  mobile networks.
• In this paper 2-hop topology info is kept.

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What is presented in the paper?

• 1-) Select forwarding nodes to prevent each node
  broadcasting each packet PDP algorithm which is
  already published by other authors.
• 2-) NETWORK CODING Two algorithms for encoding
  packets.
• - XOR Based
• - Reed-Solomon Based (FEC forward error
  correction)

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Partial Dominant Pruning (PDP)

• N(u) Neighbors, N(N(u)) 2-hop neighbors
• Node u sends packet to node v
• v selects a forward list (min. nodes to
  rebroadcast to cover all nodes in N(N(v))
• Nodes in N(u) already received
• Nodes in N(v) will have when v broadcasts
• Nodes in N(N(u) n N(v)) will also receive
• Select forward node list from
• N(N(v)) N(u) N(v) N(N(u) n N(v))
• (A greedy set cover algorithm applied)

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Neighbor Reception Table at node v
Source nodes r, s, t, u
- (Using PDP algo) Node v is chosen as the
forwarder by all the source nodes. - v will
cover their respective 2-hop neighborhood.

• At least 1 node missing a pkt v makes 5
  transmissions.
• Send (p1 XOR p2) 4 trans.
• Reed Solomon 3 trans.

By sending nk bits, rcv can recover k bit
erasures. Send 3 pkts, rcvs recover up to 3
missing pkts.
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CODEB Overview

• Coding-based broadcast protocol
• Like COPE, new layer btw IP MAC
• Opportunistic listening exchange neighbor
  information, construct reception report tables.
• Forwarder selection and pruning Forwarder set is
  stamped in the pkt header.
• Opportunistic coding

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- CODEB Algorithm - Line 5?
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• Problem is NP-hard
• They apply a greedy algorithm.

Coding algorithm with XOR
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• The largest of pkts received by a neighbor is X
• k P - X
• At least k pkts need to be encoded

Coding algorithm with Reed-Solomon
As long as a node receives at least n-k distinct
native pkts (doesnt matter which set), node can
decode the rest of the missing pkts.
Vandermonde Matrix
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• How is packet decoding done?
• - Similar to COPE
• - Each node maintains a Packet Pool.
• Theorem For a reliable network, all nodes
  receives the set of pkts injected into the
  network.

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EXPERIMENTS

• XOR up to 45 gain.
• Reed-Solomon up to 60 gain.
• Coding-based deterministic approach vs.
  Coding-based probabilistic approach
• Deterministic performs better.