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 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. A B Communications A -> B C D E F G H

Background Mobile ad hoc networks (MANETs) enable wireless communications between participating mobile nodes without the assistance of any base station. A B Characteristics ad hoc C D E F G H

Background Mobile ad hoc networks (MANETs) enable wireless communications between participating mobile nodes without the assistance of any base station. A B Characteristics ad hoc overhearing C D E F G H

Background Exposed Terminal Problem: an outgoing transmission collides with an incoming transmission A B Communications A -> E & E -> F C D E F G H

Background Hidden Terminal Problem: two incoming transmissions collide with each other A B Communications A -> D & F->D C D E F G H

Background Blind Flooding: every node forwards the packet once and only once A B C D E F G H

Background Blind Flooding: every node forwards the packet once and only once A B Cons broadcast storm problem C D E F G H

Background MANETs suffers from a high transmission error rate because of the high transmission contention and congestion in broadcast A B C D E F G 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 A B C D E F G

Preliminaries MANET can be described by graph G=(V,E) V : nodes, E: bidirectional links between nodes. A B C D E F G

Preliminaries Two nodes are neighbors only when their distance is less than transmission range. A C

Preliminaries N k (v): the k-hop neighbor set of node v which includes all nodes within k hops from v (and also includes v itself). H k (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 A B C D E F G 1 –hop neighbors 2 –hop neighbors Forwarding nodes C, D, EF, GC,E H

Double-covered Broadcast (DCB) Algorithm A B C D E F G 1 –hop neighbors 2 –hop neighbors Forwarding nodes C, D, EF, GC,E H 1) C, E cover F,G

Double-covered Broadcast (DCB) Algorithm A B C D E F G 1 –hop neighbors 2 –hop neighbors Forwarding nodes C, D, EF, GC,E H 1)C, E covers F,G 2)D is covered twice by A and E.

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. A B C D E F 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. A B C D 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. A B C D 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. A B C D 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. A B C D 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. A B C D 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. A B C D 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. A B C D E F H A’s Timer 0 New forwarding nodes C, D

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 P f (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. f u

Probability Analysis The probability P v (R=1) of a successful reception for 1- hop nonforwarding node with a set of forwarding nodes {f1,f2,..f K -1} can be carried out by where P=1-Q is the probability of a successful transmission, K is the number of forwarding nodes. v u f1 f2 f K -1 ：

Probability Analysis Generally, the probability P v (R) of a successful reception for 1-hop nonforwarding node with a set of forwarding nodes {f1,f2,..f K -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. v u f1 f2 f K -1 ：

Probability Analysis v u f1 f2 f K -1 ：

Probability Analysis v u f1 f2 f K -1 ： From the above, we can derive that

Probability Analysis v u f1 f2 f K -1 ： Also, we can have

Probability Analysis v u f1 f2 f K -1 ： Combing the above, we have

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.