Presented by Fei Huang
Virginia Tech
April 4, 2007
Original Paper
 W. 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.
Outline
 Background
 Preliminaries
 Double-covered Broadcast Algorithm
 Probability Analysis
 Performance
 Conclusions
Background
 Mobile ad hoc networks (MANETs) enable wireless
communications between participating mobile nodes
without the assistance of any base station.
Communications
C
G
D
B
F
A
E
A -> B
H
Background
 Mobile ad hoc networks (MANETs) enable wireless
communications between participating mobile nodes
without the assistance of any base station.
Characteristics
C
G
D
B
F
A
E
ad hoc
H
Background
 Mobile ad hoc networks (MANETs) enable wireless
communications between participating mobile nodes
without the assistance of any base station.
Characteristics
C
G
D
B
F
A
E
ad hoc
H
overhearing
Background
 Exposed Terminal Problem:
an outgoing transmission collides with an incoming
transmission
Communications
C
G
D
B
F
A
E
A -> E & E -> F
H
Background
 Hidden Terminal Problem:
two incoming transmissions collide with each other
Communications
C
G
D
B
F
A
E
A -> D & F->D
H
Background
 Blind Flooding:
every node forwards the packet once and only once
C
G
D
B
F
A
E
H
Background
 Blind Flooding:
every node forwards the packet once and only once
Cons
C
G
D
B
F
A
E
H
broadcast storm
problem
Background
 MANETs suffers from a high transmission error rate
because of the high transmission contention and
congestion in broadcast
C
G
D
B
F
A
E
H
Background
 Solution: select a subset of nodes to forward the
broadcast message without sacrificing the broadcast
performance
 How? This paper proposed a double-covered broadcast
algorithm.
Preliminaries
 MANET can be described by graph G=(V,E)
V : nodes
C
G
D
F
A
E
B
Preliminaries
 MANET can be described by graph G=(V,E)
V : nodes, E: bidirectional links between nodes.
C
G
D
F
A
E
B
Preliminaries
 Two nodes are neighbors only when their distance is
less than transmission range.
C
A
Preliminaries
 Nk(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)
Algorithm
 Basic 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 neighbors
2) 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
C
1 –hop
neighbors
2 –hop
neighbors
Forwarding
nodes
C, D, E
F, G
C,E
D
B
F
A
E
H
G
Double-covered Broadcast (DCB)
Algorithm
C
1 –hop
neighbors
2 –hop
neighbors
Forwarding
nodes
C, D, E
F, G
C,E
1) C, E cover F,G
D
B
F
A
E
H
G
Double-covered Broadcast (DCB)
Algorithm
C
1 –hop
neighbors
2 –hop
neighbors
Forwarding
nodes
C, D, E
F, G
C,E
1) C, E covers F,G
2) D is covered twice
by A and E.
B
D
F
A
E
H
G
Double-covered Broadcast (DCB)
Algorithm
 After receiving a new broadcast packet, each forwarding
node records the packet, computes its forwarding nodes,
and rebroadcasts the packet as a new sender.
C
D
B
F
A
E
H
Double-covered Broadcast (DCB)
Algorithm
 The 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.
C
D
B
F
A
E
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.
A’s Timer
3
C
D
B
F
A
E
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.
A’s Timer
2
C
A
D
B
F
E
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.
A’s Timer
1
C
A
D
B
F
E
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.
A’s Timer
0
C
A
D
B
F
E
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.
A’s Timer
0
C
ACK
A
D
B
F
E
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.
A’s Timer
0
C
A
New
forwarding
nodes
C, D
D
B
F
E
H
Double-covered Broadcast (DCB)
Algorithm
Pros:
1) The scheme avoids the broadcast storm problem
2) The scheme avoids the ACK implosion problem
Basic Algorithm
Forwarding 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 Selection
Forwarding 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 Selection
Forwarding 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.
DCB
 Both 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-EDC
 N(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}.
DCB
Reliability Issues
1) Nonforwarding nodes: two chances to hear the
message
2) Forwarding nodes: transmission error may propagate
Two Reasons for Packet Loss
1) High transmission error
2) Out-of-range movement
DCB
Network with Asymmetric Links
v will send ACK to u via w.
C
D
A
Probability Analysis
 The 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.
u
f
Probability Analysis
 The 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.
u
v
f1
f2
：
fK-1
Probability Analysis
 Generally,
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
v
packet; otherwise Y=0.
u
f1
f2
：
fK-1
Probability Analysis
u
v
f1
f2
：
fK-1
Probability Analysis
 From the above, we can derive that
u
v
f1
f2
：
fK-1
Probability Analysis
 Also, we can have
u
v
f1
f2
：
fK-1
Probability Analysis
 Combing the above, we have
u
v
f1
f2
：
fK-1
Performance Analysis
 Performance 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
Conclusions
 This 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.