Neighbors As Its Forward Nodes example essay topic

Reliable Broadcasting in Mobile Ad-Hoc NetworksPhaneesh KuppahalliDepartment of Computer Science The University of Texas at Dallas, ABSTRACT In a mobile ad-how network, providing a reliable broadcast is one of the most important requirements. In broadcasting, a source node sends a message to all the other nodes in the network. Broadcasting operation is expected to be executed more frequently in mobile ad-how networks MANETs. So the number of retransmissions in the broadcast has to be minimized. The reliable broadcast service ensures that all the hosts in the network deliver the same set of messages to the upper layer. The protocols that are used in wired networks are unsuitable for deployment on MANETs, as these do not take into account the node mobility, network load and congestion.

There have been a lot of protocols which are proposed for reliable broadcasting in MANETs. A straight forward way is by Simple Flooding [1, 2] which is very costly and very inefficient. The other protocols are Probability based methods [3], Area Based Methods [3] and Neighbor Knowledge Methods [4, 5, 6 and 7]. Also, efficiency and reliability conflict with each other. Hence it is hard to achieve both at a time with just one scheme. This paper will aim at proposing improvements for reliable broadcasting in MANETs.

1. INTRODUCTION The drastic improvements in the wireless communications and portable wireless devices have made mobile computing a reality. Recently, Mobile Ad Hoc Networks (MANETs) has attracted a lot of attention and research. MANETs are made of a group of independent mobile hosts which communicate with each other. A mobile host may not be able to communicate directly with all the other hosts. So, the packets traverse various intermediate nodes before reaching the destination.

All the nodes in the network assist in routing. The ad-how networks are created dynamically on the fly. The hosts are allowed to move around in the network. Routing protocols in ad-how networks should provide means to deliver packets to destination nodes given these dynamic topologies. Applications of MANETs occur in battle-fields, major disaster and some business environments where networks need to be deployed immediately without any base stations or fixed networks. Broadcasting is process by which a source node sends a message to all the other nodes in the entire network.

Ad how On-Demand Distance Vector (AODV) [10] and Dynamic Source Routing (DSR) [9] are the two most widely studied unica st on-demand ad-how routing protocols which use broadcasting. These broadcasts use flooding which is inefficient and very costly. The other protocols that use broadcasting are Probability based methods [3], Area Based Methods [3] and Neighbor Knowledge Methods [4, 5, 6 and 7]. Since broadcasting is the most basic operation in ad-how networks, the reliability of the broadcast is one of the most important requirements. This paper discusses the some of the existing reliable broadcast protocols and also proposes improvements for reliable broadcasting in MANETs. The rest of the paper is organized as follows.

In section 2, the broadcast storm [12] and implosion [13] problems are discussed. In section 3, the related work is discussed. In section 4, the comparison of some of the existing protocols and improvements for the reliable broadcast in mobile ad-how networks are described. In section 5, the conclusion is given.

2. PRELIMINARIES A MANET consists of a set of mobile hosts that may communicate with one another from time to time. No base stations are supported. Each host is equipped with a CSMA / CA (carrier sense multiple access with collision avoidance) [11] transceiver. In such environment, a host may communicate with another directly or indirectly.

In indirect communication, the packet is transmitted between sender and receiver through intermediate nodes. The broadcast problem refers to the sending of a message to all other hosts in the network. In such environments, any of the nodes can issue a broadcast message at any time so it can experience repetitive collisions. Acknowledgements may cause serious medium contention (and thus, another "storm") surrounding the sender. Broadcast storm caused by flooding straight-forward approach to achieve reliable broadcast is by simple flooding. A host, on receiving a broadcast message for the first time, has to rebroadcast the message.

Clearly, this costs n transmissions in a network of n hosts. In a CSMA / CA network, drawbacks of flooding include: o Redundant rebroadcasts. When a mobile host decides to rebroadcast a broadcast message to its neighbors, all its neighbors already have the message. o Contention. After a mobile host broadcasts a message, if many of its neighbors decide to rebroadcast the message, these transmissions (which are all from nearby hosts) may severely contend with each other. o Collision. Because of the deficiency of back-off mechanism, the lack of RTS / CTS dialogue, and the absence of CD, collisions are more likely to occur and cause more damage. Collectively, the above phenomenon is referred to as the broadcast storm problem [12].

Acknowledgement implosion problem The high transmission contention and congestion in MANETs will result in high transmission error rate. So providing a reliable broadcast under dynamic MANETs is a major challenge. As pointed out by other researchers, achieving total reliability for broadcasting in MANETs is impractical. Acknowledgements are the usual way to ensure broadcast delivery. The requirement for all the receivers to send ACKs in response to a packet reception may become a bottleneck.

This is called the ACK implosion problem [13]. 2 RELATED WORK The broadcast protocols can be divided into various categories like Simple Flooding; Probability based methods, Area Based Methods and Neighbor Knowledge Methods. In the following section the basic broadcast protocols are described. Simple Flooding The algorithm for Simple Flooding [1, 2] starts with a source node broadcasting a packet to all neighbors. Now each of the neighbors that receive this packet will rebroadcast the packet exactly once and this continues until all nodes in the network have received the packet.

Probability Based Methods In the Probability based methods [3] is similar to Flooding. Here only some nodes rebroadcast with a predetermined probability. By randomly having some nodes not rebroadcast saves node and network resources without harming delivery effectiveness, as more than one node in a dense network will cover the same area. In sparse networks, there is much less shared coverage. Thus all nodes might not receive all the broadcast packets with the Probabilistic scheme unless the probability parameter is high. When the probability is 100%, this scheme is identical to Flooding.

So this method is well suited for dense networks. Counter-Based Scheme [3] is an example for probability based method. Area Based Methods Suppose a node receives a packet from a sender that is located very close to the sender. If the receiving node rebroadcasts the packet then the additional area covered by the retransmission is very low. If a node is located on the boundary of the sender node then a rebroadcast would reach significant amount of additional area. A node using an Area Based Method can evaluate coverage area based on all received redundant transmissions.

Note that area based methods only consider the coverage area of a transmission. Neighbor Knowledge Methods Flooding with Self Pruning: The simplest Neighbor Knowledge Method protocol for broadcasting was Flooding with Self Pruning [4] proposed by Lim and Kim. This protocol requires that each node have knowledge of its 1-hop neighbors, which is obtained via periodic "Hello" packets. A node includes its list of known neighbors in the header of each broadcast packet. A node receiving a broadcast packet compares its neighbor list to the sender's neighbor list. If the receiving node would not reach any additional nodes then it will not re-broadcast.

Otherwise the node rebroadcasts the packet. Scalable Broadcast Algorithm (SBA): The Scalable Broadcast Algorithm (SBA) [14] requires that all nodes have knowledge of their two hop neighbors. This neighbor knowledge allows a receiving node to determine if it would reach additional nodes by re-broadcasting. The 2-hop neighbor knowledge is achieved via periodic "Hello" packets. Each "Hello" packet contains the node's identifier and the list of known 1-hop neighbors.

After a node receives a "Hello" packet from all its neighbors, it has two-hop topology information. Multipoint Relaying: In Multipoint Relaying [5], re-broadcasting nodes are explicitly chosen by upstream senders. For example, say Node A is originating a broadcast packet. It has previously selected some, or in certain cases all, of its one hop neighbors to rebroadcast all packets they receive from Node A. The chosen nodes are called Multipoint Relays (MPRs) and they are the only nodes that will rebroadcast a packet received from Node A. Each MPR is required to choose a subset of its one hop neighbors to act as MPRs as well. Since a node knows the network topology within a 2-hop radius, it can select 1-hop neighbors as MPRs that most efficiently reach all nodes within the two hop neighborhood.

The Multipoint Relaying proposes the following algorithm for a node to choose its MPRs: 1. Find all 2-hop neighbors that can only be reached by one 1-hop neighbor. Assign those 1-hop neighbors as MPRs. 2. Determine the resultant cover set (i. e., the set of 2-hop neighbors that will receive the packet from the current MPR set). 3.

From the remaining 1-hop neighbors not yet in the MPR set, find the one that would cover the most 2-hop neighbors not in the cover set. 4. Rep eat from step 2 until all 2-hop neighbors are covered. Ad Hoc Broadcast Protocol: The Ad Hoc Broadcast Protocol (AHBP) [6] is similar to Multipoint Relaying. In AHBP, only nodes that are designated as a Broadcast Relay Gateway (BRG) within a broadcast packet header will rebroadcast the packet. BRGs are chosen by each upstream sender which is a BRG itself.

The algorithm for a BRG to choose its BRG set is identical to that used in Multipoint Relaying, as described above. AHBP differs from Multipoint Relaying in three ways: 1. A node using AHBP informs 1-hop neighbors of the BRG designation within the header of each broadcast packet. This allows a node to calculate the most effective BRG set at the time a broadcast packet is transmitted. In contrast, Multipoint Relaying informs 1-hop neighbors of the MPR designation via "Hello" packets. 2.

In AHBP, when a node receives a broadcast packet and is listed as a BRG, the node uses 2-hop neighbor knowledge to determine which neighbors also received the broadcast packet in the same transmission. These neighbors are considered already "covered" and are removed from the neighbor graph used to choose next hop BRGs. In contrast, MPRs are not chosen considering the source route of the broadcast packet. 3. AHBP is extended to account for high mobility networks. Suppose Node A receives a broadcast packet from Node B, and Node A does not list Node B as a neighbor (i. e., Node A and Node B have not yet exchanged "Hello" packets).

In AHBP-EX (extended AHBP), Node A will assume BRG status and rebroadcast the node. Multipoint relaying could be similarly extended. CDS-Based Broadcast Algorithm: This is Connected Dominating Set (CDS) -Based Broadcast Algorithm given by Peng and Lu [15]. AHBP only considers the source of the broadcast packet to determine a receiving node's initial cover set, CDS-Based Broadcast Algorithm also considers the set of higher priority BRGs selected by the previous sender. For example, suppose Node A has selected Nodes B, C and D (in this order) to be BRGs. When Node C receives a broadcast packet from Node A, AHBP requires Node C to add neighbors common to Node A to the initial cover set.

CDS-Based Broadcast Algorithm also requires Node C to add all the neighbors common to Node B, because Node B is a higher priority BRG. Similarly, Node D is required to consider common neighbors with nodes A, B and C. Once the initial cover set is determined, a node then chooses which neighbors should function as BRGs. The algorithm for determining this is the same as that for AHBP and Multipoint Relaying. Reliable Broadcast ProtocolsRelayer Broadcast Sequence Number Method (RBS): Relayed Broadcast Sequence Number protocol (RBS) [16] focuses on the problem of sending broadcast data packet reliably and efficiently in a mobile ad how network.

Especially, when more than one node try to broadcast a packet at the same time, the reliability of the broadcast is degraded seriously since packets originated from different sources can collide, resulting in the loss of packets. To prevent reliability dropping under multi sources broadcasting, a new mechanism is required to support reliability without deteriorating efficiency. Broadcasting from a source in mobile ad how network proceeds by intermediate node's relaying. Hence overall broadcast reliability can be improved if a sender or a re layer detects a packet loss and retransmits it in one hop.

Each sender uses a re layer broadcast sequence number (RBS). Every node in mobile ad how network manages its own RBS and increases its RBS whenever it sends or relays the broadcast packet. While broadcast packet is being distributed, RBS in broadcast packet is constantly changed to re layer's RBS. In this mechanism, the receiver detects the lost packet identified by RBS. When receiver gets the packet, it compares the broadcast packet' current RBS with previous RBS in the previous packet from the same sender. If the difference between the previous packet's RBS and current packet's RBS is greater than one then the receiver sends a NACK message by piggybacking lost packet's RBS in Hello message.

Because Hello message is exchanged periodically, NACK can be delivered with small additional bandwidth. When a source or a re layer receives the packet loss notification piggybacked in Hello, it will retransmit the packet. Double-Covered Broadcast (DCB) [17]: A broadcast operation requires the packet be disseminated to all nodes in the network. But the interference of the transmission of neighbors and the movement of the nodes may cause the failure of some nodes to receive the broadcast packet. The broadcast redundancy can provide more chance for a node to successfully receive the packet. If the sender can retransmit the missed packet, the broadcast delivery ratio can also be improved.

The proposed double-covered broadcast algorithm works as follows: When a sender broadcasts a packet, it selects a subset of 1-hop neighbors as its forward nodes to forward the broadcast based on a greedy approach. The selected forward nodes satisfy two requirements: (1) They cover all the nodes within 2 hops of the sender. (2) The sender's 1-hop neighbors are either forward nodes or non forward nodes but covered by at least two neighbors, once by the sender itself and once by one of the selected forward nodes. After receiving the broadcast packet, each forward node records the packet, computes its forward nodes and re-broadcasts the packet as a new sender.

The retransmissions of the forward nodes are received by the sender as the acknowledgement of receiving the packet. The non-forward 1-hop neighbors of the sender do not acknowledge receipt of the broadcast. The sender waits for a predefined duration to overhear the rebroadcasting from its forward nodes. If the sender fails to detect all its forward nodes retransmitting during this duration, it assumes that a transmission failure has occurred for this broadcast because of the transmission error or because the missed forward nodes are out of its transmission range. The sender then re-sends the packet until all forward nodes are retransmitted or the maximum number of retries is reached.

The proposed algorithm utilizes the method that the sender overhears the retransmission of the forward nodes to avoid the ACK implosion problem. Also, the algorithm guarantees that each node is covered by at least two transmissions so that it can avoid a single error due to the transmission collision. Moreover, the algorithm does not suffer the disadvantage of the receiver-initiated approach that needs a much longer delay to detect a missed packet. IMPROVEMENT: In the Double-Covered Broadcast (DCB) [17] protocol, it could so happen that the following situation could occur, as shown in Figure 1.

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