Presented by Fei Huang

Virginia TechApril 4, 2007

Original PaperW. Lou and J. Wu, “Toward broadcast reliability in

mobile ad hoc networks with double coverage,” IEEE Transactions on Mobile Computing, Vol. 6, No. 2, Feb. 2007, pp. 148-163.

It makes use of broadcast redundancy to improve the delivery ratio in an environment with high transmission error rate.

OutlineBackgroundPreliminariesDouble-covered Broadcast AlgorithmProbability AnalysisPerformanceConclusions

BackgroundMobile ad hoc networks (MANETs) enable

wireless communications between participating mobile nodes without the assistance of any base station.

AB

Communications

A -> B

CD

E

F

G

H

BackgroundMobile ad hoc networks (MANETs) enable

wireless communications between participating mobile nodes without the assistance of any base station.

AB

Characteristics

ad hoc

CD

E

F

G

H

BackgroundMobile ad hoc networks (MANETs) enable

wireless communications between participating mobile nodes without the assistance of any base station.

AB

Characteristics

ad hoc

overhearing

CD

E

F

G

H

BackgroundExposed Terminal Problem: an outgoing transmission collides with an

incoming transmission

AB

Communications

A -> E & E -> F

CD

E

F

G

H

BackgroundHidden Terminal Problem: two incoming transmissions collide with

each other

AB

Communications

A -> D & F->D

CD

E

F

G

H

BackgroundBlind Flooding: every node forwards the packet once and

only once

AB

CD

E

F

G

H

BackgroundBlind Flooding: every node forwards the packet once and

only once

AB

Cons

broadcast storm problem

CD

E

F

G

H

BackgroundMANETs suffers from a high transmission

error rate because of the high transmission contention and congestion in broadcast

AB

CD

E

F

G

H

BackgroundSolution: select a subset of nodes to forward

the broadcast message without sacrificing the broadcast performance

How? This paper proposed a double-covered broadcast algorithm.

PreliminariesMANET can be described by graph G=(V,E)

V : nodes

AB

CD

E

F

G

PreliminariesMANET can be described by graph G=(V,E)

V : nodes, E: bidirectional links between nodes.

AB

CD

E

F

G

PreliminariesTwo nodes are neighbors only when their

distance is less than transmission range.

A

C

PreliminariesNk(v): the k-hop neighbor set of node v which

includes all nodes within k hops from v (and also includes v itself).

Hk(v): the k-hop node set of v which includes all nodes that are exactly k hops away from v.

Double-covered Broadcast (DCB) AlgorithmBasic Idea: When a sender broadcasts a

packet, it selects a subset of 1-hop neighbors as its forwarding nodes to forward the packet based on a greedy approach.

Such selected neighbors should satisfy two requirements:1) they cover all the sender’s 2-hop neighbors2) the sender’s 1-hop neighbors are either forwarding nodes or non-forwarding nodes covered by at least two forwarding nodes

Double-covered Broadcast (DCB) Algorithm

AB

CD

E

F

G

1 –hop neighbors

2 –hop neighbors

Forwarding nodes

C, D, E F, G C,E

H

Double-covered Broadcast (DCB) Algorithm

AB

CD

E

F

G

1 –hop neighbors

2 –hop neighbors

Forwarding nodes

C, D, E F, G C,E

H

1) C, E cover F,G

Double-covered Broadcast (DCB) Algorithm

AB

CD

E

F

G

1 –hop neighbors

2 –hop neighbors

Forwarding nodes

C, D, E F, G C,E

H

1) C, E covers F,G2) D is covered twice

by A and E.

Double-covered Broadcast (DCB) AlgorithmAfter receiving a new broadcast packet, each

forwarding node records the packet, computes its forwarding nodes, and rebroadcasts the packet as a new sender.

AB

CD

E

F

H

Double-covered Broadcast (DCB) AlgorithmThe retransmissions of the forwarding nodes are

overheard by the sender as the acknowledgement of the reception of the packet. The nonforwarding 1-hop neighbors of the sender do not acknowledge the receipt of the broadcast.

AB

CD

E

F

H

Double-covered Broadcast (DCB) Algorithm If the sender fails to detect all its forwarding nodes

retransmitting during a predetermined time, it assumes that a transmission failure has occurred for this broadcast. The sender then resends the packet until all the forwarding nodes’ retransmissions are detected or the maximum number of retries is reached. When the forwarding node receives a duplicated broadcast packet, it sends an ACK to acknowledge the sender.

AB

CD

E

F

H

A’s Timer

3

Double-covered Broadcast (DCB) Algorithm If the sender fails to detect all its forwarding nodes

retransmitting during a predetermined time, it assumes that a transmission failure has occurred for this broadcast. The sender then resends the packet until all the forwarding nodes’ retransmissions are detected or the maximum number of retries is reached. When the forwarding node receives a duplicated broadcast packet, it sends an ACK to acknowledge the sender.

AB

CD

E

F

H

A’s Timer

2

Double-covered Broadcast (DCB) Algorithm If the sender fails to detect all its forwarding nodes

retransmitting during a predetermined time, it assumes that a transmission failure has occurred for this broadcast. The sender then resends the packet until all the forwarding nodes’ retransmissions are detected or the maximum number of retries is reached. When the forwarding node receives a duplicated broadcast packet, it sends an ACK to acknowledge the sender.

AB

CD

E

F

H

A’s Timer

1

Double-covered Broadcast (DCB) Algorithm If the sender fails to detect all its forwarding nodes

retransmitting during a predetermined time, it assumes that a transmission failure has occurred for this broadcast. The sender then resends the packet until all the forwarding nodes’ retransmissions are detected or the maximum number of retries is reached. When the forwarding node receives a duplicated broadcast packet, it sends an ACK to acknowledge the sender.

AB

CD

E

F

H

A’s Timer

0

Double-covered Broadcast (DCB) Algorithm If the sender fails to detect all its forwarding nodes

retransmitting during a predetermined time, it assumes that a transmission failure has occurred for this broadcast. The sender then resends the packet until all the forwarding nodes’ retransmissions are detected or the maximum number of retries is reached. When the forwarding node receives a duplicated broadcast packet, it sends an ACK to acknowledge the sender.

AB

CD

E

F

H

A’s Timer

0

ACK

Double-covered Broadcast (DCB) Algorithm If the sender fails to detect all its forwarding nodes

retransmitting during a predetermined time, it assumes that a transmission failure has occurred for this broadcast. The sender then resends the packet until all the forwarding nodes’ retransmissions are detected or the maximum number of retries is reached. When the forwarding node receives a duplicated broadcast packet, it sends an ACK to acknowledge the sender.

AB

CD

E

F

H

A’s Timer

0

New forwarding

nodes

C, D

Double-covered Broadcast (DCB) AlgorithmPros:1)The scheme avoids the broadcast storm

problem2)The scheme avoids the ACK implosion

problem

Basic AlgorithmForwarding Node Set Selection Process

(FNSSP)

where X is all the candidate neighbors, U represents the uncovered 2-hop neighbors, and F is the set of forwarding nodes.

DCB Forwarding Node SelectionForwarding Node Set Selection Process – Double Coverage (FNSSP-DC):1. Each node v computes X=H(v) and

U=N2(v)-{v}.2. Node v uses the FNSSP to find F(v) in X to

cover U.

DCB Forwarding Node SelectionForwarding Node Set Selection Process – Enhanced Double Coverage (FNSSP-EDC):1. Each node v computes X=H(v)-N(u) and

U=N2(v)-N(u)-N(F(u)-{v}).2. Node v uses the FNSSP to find F(v) in X to

cover U.

DCBBoth FNSSP-DC and FNSSP-EDC algorithms provide a

set of forwarding nodes that cover all the nodes within a 2-hop neighbor set and doubly cover the nonforwarding nodes within a 1-hop neighbor set.

FNSSP-EDCN(2)={1,2,3,5,6} and N2(2)={1,2,3,4,5,6,7}(a)node 2 selects forwarding nodes {1,3,5}(b)node 5 selects forwarding nodes {2,4,6}. So node 2’s uncovered 2-hop

neighbor set is N2(2)-N(5)-N({4,6})={3}. Node 2 then selects {3}.

DCBReliability Issues1)Nonforwarding nodes: two chances to hear

the message2)Forwarding nodes: transmission error may

propagate

Two Reasons for Packet Loss1)High transmission error2)Out-of-range movement

DCBNetwork with Asymmetric Linksv will send ACK to u via w.

C

D

A

Probability AnalysisThe probability Pf(R) of a successful reception

for a 1-hop forwarding node f can be carried out by

where Q is the probability of a failed transmission, R is the number of retransmission attempts. fu

Probability AnalysisThe probability Pv(R=1)of a successful

reception for 1-hop nonforwarding node with a set of forwarding nodes {f1,f2,..fK-1} can be carried out by

where P=1-Q is the probability of a successful transmission, K is the number of forwarding nodes.

vuf1

f2

fK-1：

Probability AnalysisGenerally, the probability Pv(R)of a successful

reception for 1-hop nonforwarding node with a set of forwarding nodes {f1,f2,..fK-1} can be carried out by

where X represents the number of retransmissions for u to successfully deliver a packet and Y=1 when v receives the packet; otherwise Y=0.

vuf1

f2

fK-1：

Probability Analysis

vuf1

f2

fK-1：

Probability Analysis

vuf1

f2

fK-1：

From the above, we can derive that

Probability Analysis

vuf1

f2

fK-1：

Also, we can have

Probability Analysis

vuf1

f2

fK-1：

Combing the above, we have

Performance AnalysisPerformance is simulated in ns-2.Metrics:1) Broadcast delivery ratio. 2)Broadcast forwarding ratio. 3)Broadcast overhead. 4)Broadcast end-to-end delay.

Broadcast delivery ratio

Broadcast forwarding ratio

Broadcast overhead

Broadcast end-to-end delay

ConclusionsThis is a simple broadcast algorithm with satisfactory

result.It is reported that the DCB is sensitive to node’s

mobility.The performances is good and have a high delivery ratio.KEY: (1)Only a subset of nodes are selected as forwarding

nodes.(2) Double coverage of forwarding selection process

increases the reliability.DCB mainly increases the reliability of forwarding nodes

but not for nonforwarding nodes.