Rheinisch-Westfälische Technische Hochschule Aachen Lehrstuhl für Informatik IV Prof. Dr. rer. nat. Otto Spaniol

Transcription

1 Rheinisch-Westfälische Technische Hochschule Aachen Lehrstuhl für Informatik IV Prof. Dr. rer. nat. Otto Spaniol COMPARISON of BROADCASTING TECHNIQUES for MOBILE AD HOC NETWORKS Seminar: Data Communication and Distributed Systems Summer 2003 Şerife Burcu Dilbaz Matrikelnummer: Betreuung: Mesut Güneş Lehrstuhl für Informatik IV, RWTH Aachen

2 TABLE OF CONTENTS ABSTRACT 3 1. INTRODUCTION 3 2. MOTIVATION 4 3. BROADCASTING TECHNIQUES IN MOBILE AD HOC NETWORKS SIMPLE FLOODING PROBABILITY BASED METHODS Probabilistic Scheme Counter-Based Scheme AREA BASED METHODS Distance-Based Scheme Location-Based Scheme NEIGHBOR KNOWLEDGE METHODS Flooding with Self Pruning Collection of Static Information Collection of Dynamic Information Scalable Broadcast Algorithm (SBA) Dominant Pruning Multipoint Relaying Ad Hoc Broadcast Protocol CDS-Based Broadcast Algorithm LENWB COMPARISONS OF THE BROADCASTING TECHNIQUES ALGORITHM EFFICIENCY CONGESTED NETWORK MOBILE NETWORK COMBINED NETWORKS CONCLUSION REFERENCES 20 2

3 Abstract Broadcasting is essential in ad hoc routing algorithms. Broadcasting is used in mobile ad hoc networks for paging, alarming, location updates and route discoveries. Many unicast routing protocols use broadcasting to establish routes. The simplest approach to broadcasting is flooding. But flooding causes redundancy of broadcast packets, contention and collision. To overcome the redundancy of flooding more efficient broadcasting methods than flooding have been proposed. This paper describes the broadcasting techniques and compares the performance of these techniques under different network conditions. 1. INTRODUCTION Mobile wireless networks continue to attract attention with the improvements in wireless communication and the increasing demand and availability of mobile devices. Mobile wireless networks have two variations. Infrastructured mobile networks and infrastructureless mobile networks. Infrastructured mobile networks need an infrastructure support consisting of fixed and wired gateways (see Figure 1). A mobile node communicates with a fixed base station in the network. As the mobile node moves, it can go out of the range of the base station and enter a new base station s range. In this case handoff is performed and the mobile node starts communicating with the new base station. Figure 1 - Infrastructured mobile network Infrastructureless mobile networks are known as mobile ad hoc networks. Mobile ad hoc networks are wireless multi-hop networks that don t require a fixed infrastructure. All nodes are mobile and can be connected dynamically and arbitrarily. Mobile nodes cooperate with each other to enable communication by functioning as routers which discover and maintain routes to other nodes in the network (see Figure 2). Figure 2 Ad hoc mobile network 3

4 Ad hoc Networks are useful when infrastructure is not available, is impractical, or expensive. Ad hoc networks are easy to deploy, therefore it has many applications in personal area networking (cell phone, laptop, ear phone, wrist watch), in military environments (soldiers, tanks, planes), in civilian environments (taxi cab network, meeting rooms, sports stadiums, boats, small aircraft), in emergency operations (search-and-rescue, police and fire fighters). Typical limitations of ad hoc networks are high power consumption, low bandwidth, high error rates and high mobility of the nodes. 2. MOTIVATION In mobile ad hoc networks, each node has a transmission radius and is able to send a message to all of its neighbors that are located within the radius. Packets sent by the source node are relayed by several intermediate nodes before reaching the destination node if the destination node is outside of the source node s transmission radius. In a broadcasting task, a source node sends the same message to all the nodes in the network. It is an important task used for paging, alarming, location updates, route discoveries. Network wide broadcasting is more frequent in ad hoc networks than in wired networks because of the node mobility and scarce system resources. Actually, broadcasting is essential in ad hoc routing algorithms. Many unicast routing protocols such as Dynamic Source Routing (DSR), Ad Hoc on Demand Distance Vector (AODV), Zone Routing Protocol (ZRP) and Location Aided Routing (LAR) use broadcasting to establish routes. The usual approach for broadcasting is through flooding. Flooding is the process in which the same message is transmitted by all nodes that receive it. Flooding is appropriate for mobile ad hoc networks as it requires no topological knowledge. However it is not optimal because it generates a high number of redundant messages. The redundant messages waste limited resources such as bandwidth and energy supply. The wireless medium does not have the capacity to be used abundantly for the unnecessary traffic. Flooding is very costly, since it causes redundancy, contention and collision. To overcome the redundancy of flooding more efficient broadcasting methods than flooding have been proposed. In this paper, broadcasting techniques will be described. Some of these techniques will be selected to be compared. Section 3 describes the broadcasting techniques, which are categorized in four categories: Simple Flooding, Probability-Based methods, Area-Based methods, Neighbor Knowledge based methods. Simple flooding ensures the coverage of the network but it has the largest number of rebroadcast packets. Probability-Based and Area-Based methods estimate its contribution to the overall broadcasting before rebroadcasting a packet. If the estimated contribution is lower than a threshold value, they don t rebroadcast the packet. In neighbor knowledge methods exchange hello packets in order to maintain topology knowledge of their neighborhood. Using this knowledge a small subset of neighbors is selected as rebroadcasting nodes. In Section 4 comparison results of the broadcasting methods will be given. Sample protocols from each category will be selected and the performance of these protocols will be examined under different network conditions. The effects of increasing node density, increasing congestion and increasing network mobility on the selected protocols will be demonstrated on graphs. In section 5 conclusions of the paper is presented. 4

5 3. BROADCASTING TECHNIQUES IN MOBILE AD HOC NETWORKS 3.1. Simple Flooding In simple flooding nodes have no neighborhood information. A node simply broadcasts a packet to all its neighbors. Each neighbor rebroadcasts the packet upon receiving it and this continues until all the nodes of the network receive the packet. Each node rebroadcasts the packet only once (See Figure 3). Simple flooding provokes many redundant packet rebroadcasts, because of the overlaping transmission coverages of some nodes. Redundant rebroadcasts causes contention and packet collisions. Figure 3 Retransmissions in simple flooding 3.2. Probability Based Methods Probabilistic Scheme The probabilistic scheme [2] reduces the broadcast redundancy by randomly having some of the nodes broadcast and some not. A node in the network broadcasts a message with a predetermined probability p and takes no action with probability 1-p. In dense networks the probability p is small, since multiple nodes transmission coverages overlap. In sparse networks the probability p is chosen higher. This scheme is equivalent to simple flooding when the broadcast probability is equal to Counter-Based Scheme The counter-based scheme [2] reduces the broadcast redundancy by preventing a node from rebroadcasting if the additional coverage of that node is too low. There is an inverse relation between the number of times a packet is received at a node and the probability that the node reaches additional area. The expected additional coverage after hearing the message k times, is expected to decrease quickly as k increases (See Figure 4). The counter-based scheme uses this fact and counts the number of times the same packet is received at a node within a predefined period, Random Assessment Delay (RAD). When RAD expires, the packet is rebroadcasted if the count is less than a threshold value, otherwise the packet is prohibited from rebroadcasting. The threshold value is chosen such that all nodes rebroadcast in sparse 5

6 networks and some nodes don t rebroadcast in dense networks. To maintain a high delivery ratio in sparse networks, a higher threshold value is needed; in dense networks a lower threshold value can be used. Figure 4 The expected additional coverage vs. number of transmissions heard 3.3. Area Based Methods Distance-Based Scheme The distance-based scheme [2] reduces the broadcast redundancy by having a node rebroadcast depending on its distance from its neighbors. The relative distance between nodes determines the additional coverage area (See Figure 5). If the distance is small, than the additional coverage area provided by rebroadcasting is small. If the distance is large, the provided additional coverage is large. In this scheme, a node compares the distances between itself and its neighbors and finds the minimum of them. If the minimum distance is less than a threshold value, the packet is considered as redundant and not sent. Signal strengths are used to calculate the distances between the nodes. Additional Coverage Area Figure 5 Distance-Based Scheme 6

7 Location-Based Scheme The location-based scheme [2] reduces the broadcast redundancy by having a node rebroadcast depending on the additional coverage area it provides. In this scheme the additional coverage area is estimated from the location information of the nodes. Therefore each node must determine its own location, e.g. by using Global Positioning System (GPS). Each node adds its location to the header of the packet before broadcasting or rebroadcasting it. Upon receiving a packet a node learns the location of the sender and calculates the additional coverage area provided if it rebroadcasts the packet. If the additional coverage area is less than a threshold value, then the node doesn t rebroadcast the packet Neighbor Knowledge Methods Flooding with Self Pruning Neighbor knowledge based methods are divided into neighbor-designating methods and selfpruning methods. In neighbor-designating methods, the forwarding status of each node is determined by its neighbors. In self-pruning methods, each node determines its forwarding status; forwarding or non-forwarding. This decision is based on node s local information. In this protocol every node knows its 1-hop neighbors [3]. To provide this information periodic hello packets are sent. A node adds the list of its neighbors to the header of the packet before broadcasting it. The node which receives this packet, compares the neighbors of the source node with its neighbors. If they are the same, the node doesn t rebroadcast the packet since rebroadcasting is redundant. Self pruning requires additional overhead of exchanging neighborhood information. To reduce the overhead, the received neighbor lists of the adjacent nodes can be cached. A node doesn t send its neighbor list if it s not been changed. The node that receives a packet without a neighborlist uses the entry in its cache. If a neighborlist is piggybacked in the packet, the receiver node updates its cache. An ad hoc network is considered as a graph G = (V,E) [4]. V is the set of nodes and E is the set of bidirectional links between the nodes. Each node has a neighbor set N(V) = {u (u,v) E}. The set of forward nodes is denoted by F and F V. A broadcast protocol ensures the coverage if it guarantees successful broadcast, that is every node receives the broadcast packet. Local information collected at a node v is represented formally as L v. L v is a triple (G v, p, F v ) where G v = (V v, E v ) is a subgraph of G which represents the topology of the small vicinity of the node v, p is the priority function on V v and F v represents a list of forward nodes extracted from incoming broadcast packets. F v F V v. The nodes in F v are called the black nodes. The nodes which are not black and whose priority is higher than v are called the gray nodes. The other nodes are called the white nodes. If u is a black node and a neighbor of v, then another neighbor w of v is covered by u if there exists a replacement path with gray nodes as intermediate nodes between u and w. More formally, there exists a path (u, v 1, v 2,..., v k, w) in G v, where p(v i ) > p(v) for i=1, 2,..., k. For example, in Figure 5, the black node s covers nodes v and x. Figure 5 Local Information for node u 7

8 In flooding with self pruning, each node decides its own forwarding status based on its local information. Local information of each node is divided into two categories: static and dynamic information. Static information includes neighbor topology and an attribute which is used as priority value. Dynamic information includes a small set of nodes that have forwarded the broadcast packet. When a node receives a broadcast packet, it either rebroadcasts it or drops it. When a node drops a broadcast packet, it is pruned from the forward node set. A node can be pruned from the forward node set, if each of its neighbors is either a black node or covered by a black node. This is called the self-pruning rule. According to this rule, node u in Figure 5 can be pruned because one of its neighbors is a black node and the remaing neighbors are covered by the black neighbor. Broadcast algorithm of self-pruning for each node v is defined as follows: 1. Periodically exchange hello message with neighbors and update the static information G v and p(v v ). 2. On receiving a broadcast packet, build up the black node set F v. 3. Test the self-pruning rule with the local information Lv = (G v,p,f v ). If the rule applies do nothing; otherwise forward the received packet Collection of Static Information Static information includes neighborhood topology and priority values. Neighborhood topology Gv in each node v is collected by exchanging hello messages periodically among neighbors. k-hop information can be collected after k rounds of hello message exchanges. Mobility pattern of an ad hoc network effects the maintenance overhead of k-hop information. A highly mobile network needs a smaller interval for exchanging hello packets than a slightly mobile network. If k is chosen larger, the interval for exchanging hello packets must be small so that the topology changes are disseminated quickly. Considering this overhead k must be chosen small. k = 0 means that there is no hello message, and each node knows only the black neighbors. The self-pruning rule cannot apply in this case, as it requires a complete list of neighbors. When k = 1, each node advertises its id via hello messages, so every node knows the ids of its neighbors but not the ids of the neighbors of its neighbors. Therefore self-pruning is still difficult to apply except for the case that all neighbors of a node are black. If all neighbors of the node are black, that means all neighbors are in the forward node set. Then the node can be pruned, since a broadcast is redundant in this case. Most self-pruning methods require 2-hop information. When k = 2, each node advertises its id and its neighbor set by hello messages. In this case replacement paths can be constructed and self-pruning rule can be applied considering not only the black nodes but also the neighbors that are covered by the black nodes. Generally k-hop knowledge can be collected when each node relays (k-1)-hop information in its hello packet. Figure 6 Neighborhood topology of node u 8

9 Collection of Dynamic Information The set of black nodes F v carried by the incoming broadcast packet contains the dynamic information, which is also named as history information. A self-pruning protocol is a static protocol if it doesn t collect any dynamic information, otherwise, it s a dynamic protocol. In a static protocol the static self-pruning rule is applied in step 3 of the broadcast algorithm. Static self-pruning algorithm states that a node can be pruned from the forward node set if each neighbor is covered simultaneously by all other neighbors. The benefit of static protocols is that the step 3 of the broadcast algorithm can be applied prior to any broadcasting. Because the static self-pruning rule doesn t require dynamic information which is carried in the broadcast packet. Instead, static information which is collected via hello packets is used. Static protocols reduce the computation delay but they usually produce a larger CDS than the dynamic protocols. In dynamic protocols, if a node has a larger black node set F v, than its chance to be pruned increases. There are two methods to increase the number of black nodes in the set: backoff delay and piggybacked history. The backoff delay postpones the testing of the self-pruning rule for a period of backoff delay, so that new black nodes can be observed forwarding the same packet within this period. Backoff delay also increases the overall delay. In piggybacking broadcast history information for a self-pruning method that uses k-hop information, the last k-1 visited nodes are piggybacked into the broadcast packet. Using more than k-hop history is a waste, since only black nodes within k-hops are useful to the selfpruning rule Scalable Broadcast Algorithm (SBA) In this algorithm a node doesn t rebroadcast a packet if all its neighbors have received the packet in the previous transmission [6]. All nodes are required to have 2-hop neighbor knowledge. This information is retrieved by sending hello packets that are composed of the node s identifier and the list of its neighbors. The node who receives this hello packet gains the knowledge of its own neighbors and the neighbors of them. In this algorithm, when a node receives a packet, it knows the transmitter node; because it s one of its neighbors. It also knows the neighbors of the transmitter node. Therefore it has the knowledge of the nodes which has been covered by this transmission. The node rebroadcasts the packet if it has additional neighbors that were not reached by the previous transmission. The broadcast algorithm can be divided into two parts: local neighborhood discovery and data broadcasting. Local neighborhood discovery is achieved by hello message exchanges. When a node receives a broadcast message, it discovers which nodes have been covered by this transmission by checking the neighbor list of the transmitter. Then these nodes including the transmitter are added into the broadcast cover set of the message. When making rebroadcast decision, the node checks the broadcast cover set of the message. If all its neighbors are in the set, then the rebroadcast operation is unnecessary and can be canceled. The data broadcasting procedure is illustrated as follows. 1. Source s broadcasts messages to all its neighbors and ignores duplicate messages received later. 2. A node u receives a broadcast message m from node r. Let N(u) denote the neighbor set of u. Node u performs the following operations. a) If N(u) N(r) {r}, then it doesn t rebroadcast the message. It ignores the duplications received later. b) Else, if the message is received firstly, then let broadcast cover set for the messsage m be C(u,m) = N(r) {r}. A rebroadcast is scheduled to be performed after a period of Random Assessment Delay (RAD). In this period, if a dublicate message is 9

10 received, the nodes that are covered by the received transmission are recorded in the broadcast cover set. C(u, m) = C(u, m) N(r) {r}. c) After RAD is expired, node u checks whether all its neighbors are in the broadcast cover set. If N(u) C(u, m), then the rebroadcast is canceled. Otherwise, u rebroadcasts the message m and ignores the dublicate messages received later Dominant Pruning In this protocol 2-hop neighbor knowledge is used like in the scalable broadcast algorithm. The sender node selects some or all of its 1-hop neighbor nodes that should rebroadcast [3]. A list of identities of the nodes that should rebroadcast are added in the header of the packet. It s called a forward list. Only the selected nodes can rebroadcast the packet. Selected neighbors also determine a forward list and add it in the rebroadcast packet and this process continues until all the nodes are covered. When a node receives a broadcast packet, it checks whether its identity is listed in the forward list. If its identity is in the forward list, it determines which of its neighbors should rebroadcast the packet. A Greedy Set Algorithm is used to determine the forward list so that the number of transmissions is minimum. If node u receives a broadcast packet from node r, and if it is in the forward list of r, it must select its own forward list before rebroadcasting the packet so that all nodes within 2-hop distance from u receive the packet. The forward list should be minimized to decrease the number of transmissions. r, N(r) have already received the packet, and N(u) will receive the packet when u forwards the packet. Therefore node u determines its forward list from the set N(u) N(r) so that all nodes in N(N(u)) - N(r) - N(u) receive the packet (See Figure 7). N(r) N(u) N(u) N(r) r u N(N(u)) N(N(u)) - N(r) - N(u) Figure 7 Dominant pruning method Multipoint Relaying In this algorithm number of rebroadcasts is reduced by selecting a subset of the neighbors as the rebroadcasting nodes [7]. The selected neighbors are called multipoint relays (MPRs). Selection of the neighbors is done explicitly by the senders as in the dominant pruning protocol. Each node selects its own multipoint relays. Only these multipoint relays can rebroadcast a packet. A receiving node rebroadcasts a packet if it is a multipoint relay of the sender (See Figure 4). To chose multipoint relays a node needs the information of its 1-hop neighbors and 2-hop neighbors. 1-hop neighbor information is gained via periodic hello packets. Each node 10

11 declares its presence to its neighbor. For 2-hop neighbor information list of neighbors is attached to the hello packet. IIn this way, each node gains its 1-hop and 2-hop neighbor set. Once a node has this information, it can select the minimal number of 1-hop neighbors which covers all of its two-hop neighbors. A node chooses its MPRs using the following algorithm[7]: First it finds all 2-hop neighbors that can be reached by one 1-hop neighbor. It puts these 2-hop neighbors in the cover set and assigns these 1-hop neighbors as MPRs. From the left 1-hop neighbors it finds the one that would cover the most 2-hop neighbors which are not in the cover set. This process continiues until all the 2-hop neighbors are in the cover set. Each node calculates its MPRs and adds the list of its chosen MPRs in the hello packets. A node receiving a hello packet, checks if its in the MPR list of the sender node. If it s an MPR of the sender node, it rebroadcasts the packets received from it. The concept of multipoint relaying is to reduce the number retransmissions while forwarding a broadcast packet. This technique restricts the number of retransmitters as much as possible by efficiently selecting a small subset of neighbors which covers the same network region that would be covered by the complete set of neighbors. This small subset of neighbors is called multipoint relays of a given network node. The scheme of multipoint relays (or MPRs) provides an adequate solution to reduce the flooding of broadcast messages in the network, while attaining the same goal of transferring the message to every node in the network with a high probability. Figure 8 Retransmissions in multipoint relaying In multipoint relaying technique each node calculates its own set of multipoint relays, which is completely independent of other nodes' selection of their MPRs. Each node reacts when its neighborhood nodes change and accordingly modifes its MPR set to cover its two-hop neighbors. An important aspect for the utilization of the multipoint relays is the manner in which these multipoint relays are selected by each node. Obviously, the goal is to achieve the maximum performance by selecting an optimal set of these MPRs by each node. But this task is not a trivial one. If the mechanism of selecting the MPRs is too simple, it may not select efficiently the MPRs in the dynamic and complex situations, and the expected performance gain would 11

12 not be achieved. On the other hand, if the algorithm of MPR selection is very long and complicated to provide a near to optimal MPR set, it may become difficult to implement it or it may generate its own control traffic to gather information for its functioning. So, there must be a compromise in designing such an algorithm for the selection of multipoint relays: it should be easy to implement, and it should give near to optimal MPR set in the majority of cases. The information needed to calculate the multipoint relays is the set of one-hop neighbors and the two-hop neighbors. To get the information about the one-hop neighbors, most protocols use some form of hello messages, that are sent locally by each node to declare its presence. In a mobile environment, these messages are sent periodically as a keep alive signals to refresh the information. To obtain the information of two-hop neighbors, one solution is that each node attach the list of its own neighbors, while sending its hello messages. In this way, each node can independently calculate its one-hop and two-hop neighbor set. Once a node has this information, it can select the minimal number of one-hop neighbors which covers all of its two-hop neighbors. One heuristic for the selection of multipoint relays is as follows. To select the multipoint relays for the node x, lets call the the set of one-hop neighbors of node x as N(x), and the set of its two-hop neighbors as N 2 (x). Let the selected multipoint relay set of node x be MPR(x). 1. Start with an empty multipoint relay set MPR(x). 2. First select those one-hop neighbor nodes in N(x) as the multipoint relays which are the only neighbor of some node in N 2 (x), and add these one-hop neighbor nodes to the multipoint relay set MPR(x). 3. While there still exist some node in N 2 (x) which is not covered by the multi-point relay set MPR(x) : a) For each node in N(x) which is not in MPR(x), compute the number of nodes that it covers among the uncovered nodes in the set N 2 (x). b) Add that node of N(x) in MPR(x) for which this number is maximum Ad Hoc Broadcast Protocol The Ad Hoc Broadcast Protocol (AHBP) is an extend form of dominant pruning and multipoint relaying. In this protocol Broadcast Relay Gateways (BRGs) are utilized as MBRs. BRGs are the only nodes that are allowed to rebroadcast. Each node selects its broadcast relay gateways at the time when a broadcast packet is transmitted. List of BRGs of the sender is transmitted within the broadcast packet, not via hello packets. hello packets are used to gain 2-hop neighbor information as in the previous protocols. A similar algorithm as in multipoint relaying is used for the selection of the BRG set. But in this algorithm when a node u receives a broadcast packet and is listed as a BRG in the header of this packet, u determines which neighbors have also received this packet by using its 2-hop neighbor knowledge. These nodes are considered as already covered and therefore they are removed from the set of possible BRGs of the node. In multipoint relaying MBRs selection is not done considering the source route of the broadcast packet. For high mobility networks extended AHBP can be used. When a node u receives a broadcast packet from node r, there might be a case that, at that time node u doesn t have node r in its neighbors list. This can happen if u and r have not yet exchanged hello messages. In extended AHBP (AHBP-EX) node u behaves as a BRG in this case and rebroadcasts the packet CDS-Based Broadcast Algorithm The Connected Dominating Set (CDS)-Based Broadcast algorithm is similar to the algorithms used in multipoint relaying and AHBP, but CDS-Based Broadcast algorithm performs more 12

13 calculations in the selection of the BRGs. AHBP considers the 1-hop neighbors of the source of the broadcast packet to be in the cover set. CDS-Based Broadcast algorithm also considers the higher priority BRGs of the previous sender in determining the cover set. Priority of the BRGs are designated in the BRG list which is transmitted in the header of the broadcast packet. The order of the BRG in the list shows its priority. If node B is in a higher priority(order) than node C, node C adds neighbors which are common to node B in the cover set. Once a node determines the cover set considering the source node and the BRGs with higher priorities, the node chooses its BRGs from the set of its neighbors that are not in the cover set LENWB The Lightweight and Efficient Network-Wide Broadcast (LENWB) protocol chooses the nodes that shoud rebroadcast implicitly, instead of specifying them explicitly. It utilizes hello packets to gain 2-hop neighbor knowledge as in the previous protocols. The hello packets contain the identity of the node, number of neighbors of the node, list of neighbors of the node and the number of their neighbors. Each node has a priority. The priority of a node is proportional to the number of neighbors of that node. The decision of rebroadcasting is made using the knowledge of the priorities of the neighbor nodes. A node considers its 1-hop and 2- hop neighbors that have higher priority and that have also received the broadcast packet in determining which of its neighbors are expected to rebroadcast. A node expects its higher priority neighbors to rebroadcast. If all of its low priority neighbors have already received the rebroadcasts of its higher priority neighbors, the node won t rebroadcast; otherwise it rebroadcasts the packet. More formally, whenever node v receives a broadcast packet from a neighbor u, it computes the set C of nodes that are connected to u via nodes that have higher priority values than v. If v s neighbor set, N(v) is contained in C, node v doesn t rebroadcast; otherwise, it is rebroadcasts. 4. COMPARISONS OF THE BROADCASTING TECHNIQUES Comparisons of broadcasting methods are provided in the paper of Brad Williams and Tracy Camp[1]. Simple Flooding, the Counter-Based scheme, the Location-Based scheme, SBA and AHBP are the selected broadcasting techniques in this paper. Simple Flooding is the only protocol in its category. Therefore it is chosen to represent its category. From the probability based methods the Counter-Based scheme is chosen, because it performed better than the Probabilistic scheme in most of the comparisons [2]. Area Based methods are represented by the Location-Based scheme. From the neighbor knowledge category two methods are selected: SBA and AHBP. Neighbor knowledge based protocols can be classified in two parts depending on the way a node makes rebroadcasting decision. In Self Pruning, SBA and LENWB this decision is made by the node itself. In Dominant Pruning, Multipoint Relaying, AHBP and CDS-Based Broadcasting a node is told to rebroadcast or not. From the first class where rebroadcast decision is local, SBA is chosen and the second class is represented by AHBP. Comparisons are performed considering different network conditons, like node densities, node mobilities and traffic rates. Selected broadcasting techniques are examined under four aspects in the comparisons: algorithm efficiency, congested networks, mobile networks and combined networks. The following sections describe these aspects and and the results of the comparisons. 13

14 4.1. Algorithm Efficiency To compare the algorithm efficiencies of the selected broadcasting techniques, a static network and Null MAC are used in the studies. Static network avoids the effects of node mobility. Null MAC is used to remove the congestion effect. All algorithms depend on the density of the network. In sparse networks, behaviours of the protocols converge to Simple Flooding since most of the nodes need to rebroadcast in sparse networks. In dense networks the differences of the algorithm efficiencies appear. The worstcase bound is provided by Simple Flooding since it has the largest forward set. The bestcase bound is provided theorotically by the Minimum Connected Dominating Set (MCDS). MCDS is the smallest set of rebroadcasting nodes, such that all the nodes in the set are connected and all the nodes that are not in the set are 1-hop neighbors of a node in the set. Figure 9 Delivery Ratio vs. Number of Network Nodes Figure 9 shows the delivery ratio of each protocol as the number of nodes increases in the static network with Null MAC. Delivery ratio is the percent of the nodes that receive the broadcast packet. From the graph, it s seen that Simple Flooding and the protocols representing neighbor knowledge based methods, namely SBA and AHBP perform better in sparse networks. Figure 10 shows the number of retransmitting nodes in each protocol as the number of nodes in the network increases. Protocols that have more complicated algorithms have fewer retransmitting nodes than the others. The neighbor knowledge based methods have the fewest retranmitting nodes in the studies. AHBP approximates theoretical the best-case. The figure also shows that area based schemes have fewer retransmitting nodes than probability based schemes. The threshold values for the Counter-Based and Location-Based schemes are held constant in the graphs of Figure 9 and Figure 10. For the Counter-Based scheme the threshold is 3, and for the Location-Based scheme it is 45 meters. But the threshold values effects the results. In the Counter-Based scheme, a higher threshold value in sparse networks and a lower threshold value in dense networks increases the delivery ratio. In Location-Based scheme, a lower threshold value is used to maintain a high delivery ratio in sparse networks and a higher threshold value is used in dense networks. 14

15 Figure 10 - Number of Retransmitting Nodes vs. Number of Network Nodes 4.2. Congested Network To compare the broadcasting protocols under the effect of congestion a static network and the contention based MAC scheme are used in the studies. Packet origination rate is increased keeping the packet size fixed to obtain congestion. Number of nodes remains constant. Figure 11 shows graph of delivery ratio versus packet origination rate. Delivery ratio decreases in every protocol as the network becomes more congested. Delivery ratio of Simple Flooding suffers the most, since congested network causes more collisions and queue overflows. Protocols that minimizes the number of retransmissions perform better in congested networks. Therefore AHBP and SBA has higher delivery ratios. Figure 11 Delivery Ratio vs. Packet Origination Rate 15

16 Figure 12 Number of Retransmitting Nodes vs Packet Origination Rate Figure 12 shows the number of retransmitting nodes as the packet origination rate is increased and the network becomes more congested. In Simple Flooding number of retransmitting nodes decreases because of the collisions and the queue overflows. The protocols which utilize RAD have an increase in the retransmitting nodes as the network becomes more congested. SBA, the Counter-Based scheme and the Location-Based scheme uses RAD in their algorithms and the effect of more congestion is an increase in the number of retransmitting nodes. This is due to the transmission delays of the redundant packets. Congestion prohibits redundant packets to arrive before RAD expires. Therefore more nodes rebroadcast. More rebroadcasts make the network even more congested and this causes more delays. We see that RAD based methods suffer in congested situations. A solution is adapting RAD so that it increases as the network becomes more congested. In general, neighbor knowledge based methods perform better than area based methods, which perform better than the probability based methods in congested networks. That means better performance comes with higher algorithmic cost and for neighbor knowledge based methods also with the cost of exchanging hello packets Mobile Network Selected broadcasting protocols are compared focusing on their reaction to node mobility in the network. A null MAC is used to avoid the effect of congestion. The number of nodes in the network is fixed to 60 and the packet source rate is set to 10 packets per second. Random way point mobility model with zero pause time is used in the simulations. Figure 13 shows the results of the simulations. Delivery ratios of the broadcasting protocols are compared as the average node speed is increased. The graph shows that delivery ratio of AHPB decreases as the network mobility increases. In AHBP the broadcast packet is rebroadcasted by the Broadcast Relay Gateways (BRGs). As the average node speed increases, it becomes very likely that a chosen BRG leaves the transmission range of the choosing node between hello message exchange. In AHBP-EX when a node moves inside another node s transmission range between hello message exchange, and receives a broadcast packet, it rebroadcasts the packet even though it s not listed as a BRG. Because of this property, AHBP-EX has better performance than AHBP in increased mobility, but still it under performs the other broadcasting protocols. An adaptation to AHBP-EX is to use shorter intervals for hello 16

17 packets. This enables AHBP-EX to maintain a high delivery ratio with a changing topology. However, there is a tradeoff between the cost of hello packets and the delivery ratio. Figure 13 Delivery Ratio vs. Average Node Speed The other neighbor knowledge based method, SBA doesn t behave like AHBP as the average node speed changes. The reason for this is the fact that a node assesses its own topology and makes rebroadcast decision locally in SBA. More nodes rebroadcast as the network mobility increases to maintain high delivery ratio. Figure 14 shows the number of retransmissions for each protocol. AHBP and AHBP-EX, both have less nodes rebroadcasting as the mobility increases. From the graphics we see that delivery ratios of the other protocols; Simple Flooding, the Counter-Based and Location-Based schemes, are not effected by topology changes. Figure 14 - Number of Retransmitting Nodes vs. Average Node Speed 17

18 4.4. Combined Networks In the three studies above the comparisons are performed by changing a particular network condition. Node density, network congestion and mobility is increased to examine the effects of these conditions on the broadcasting protocols. In this study combinations of different network conditions are used in the simulations to see the combined effect of them on the protocols. These combinations are given in Figure 15. The severity of the network environment increases as the trial number increases. Figure 15 Parameter Combinations for Combined Networks Figure 16 shows the delivery ratios of each protocol in each trial. Each protocol has a so called breaking point, where it can no londer deliver packets because of the severity of the network conditions. Simple Flooding is the first protocol that breaks. It breaks after Trial 2. The Counter-Based and the Location-Based protocols break after Trial 3 and neighbor knowledge based protocols break after Trial 4. SBA has the highest performance in Trial 4. But it breaks because of the increasing congestion. AHBP-EX is the worst performer in the first three trials, because of the untolerance of this protocol to increasing mobility. However the delivery ratio of AHBP-EX decreases more gracefully than the other protocols, and it is the best value in the fifth trial. Figure 17 shows the number of retransmitting nodes for each protocol in each trial. The number of rebroadcasting nodes in all protocols increases, as the severity of the network environment increases. The Counter-Based scheme starts behaving like Simple Flooding after the third trial in terms of delivery ratio and number of retransmitting nodes. The neighbor knowledge based methods have the fewest number of retransmitting nodes in every trial. Figure 16 Delivery Ratio as Severity of Network Increases 18

19 Figure 17 Number of Rebroadcasting Nodes as Severity of Network Environment Increases 5. CONCLUSION Increasing density in static networks effects the Probability-Based and Location-Based methods and increases the number of retransmitting nodes. The threshold value can be adapted to neighbor size in these methods to decrease the number of retransmitting nodes. Neighbor knowledge based methods approximate the best case, MCDS, as the node density of the network increases. Random Assessment Delay (RAD) utilizing schemes suffer in congested networks. RAD can be adabted by each node according to its congestion level. Neighbor knowledge based methods suffer in high-mobility networks. hello packet intervals can be reduced. Adapting RAD to the current congestion level improves SBA s performance in congested networks. The simulations show that the delivery ratios of adaptive SBA is higher than AHBP, but to achieve this higher delivery ratio, adaptive SBA requires higher number of retransmitting nodes. AHBP-EX performs better than other protocols in most severe network environments. However its delivery ratio is effected by the network mobility and it decreases as the network mobility increases. There is a trade off between algorithm complexity and redundant transmissions. Power requirements for more complex calculations is generally less than the power requirements of transmitting redundant packets. Simple Flooding is the worst case in generating redundant packets. Probability-Based and Area-Based methods have simple algorithms. But they fail to operate efficiently in congested networks. They can be improved but the improvement requires addition of hello messages and an adaptive RAD. The improvements spoil the simplicity of these protocols. It s not logical to choose one of the improved extension of these protocols since there are better protocols utilizing hello packets. Considering these, we can conclude that neighbor knowledge methods are preferred over other types of broadcasting protocols. But there is no clear conclusion between the two neighbor knowledge methods. AHBP-EX is recommended for networks having low mobility or extremely congested networks. Adaptive SBA is recommended for all other conditions. 19

20 6. REFERENCES [1] B. Williams, T. Camp. Comparison of Broadcasting Techniques for Mobile Ad Hoc Networks. Mobihoc, [2] S. Ni, Y. Tseng, Y. Chen, and J. Sheu. The Broadcast Storm Problem in a Mobile Ad Hoc Network. Mobicom, [3] H. Lim and C. Kim. Multicast Tree Construction and Flooding in Wireless Ad Hoc Networks. MSWIM, [4] J. Wu, F. Dai. A Generic Broadcast Protocol in Ad Hoc Networks Based on Self-Pruning. [5] J. Wu, F. Dai. Broadcasting in Ad Hoc Networks Based on Self-Pruning. [6] W. Peng and X. Lu. On the Reduction of Broadcast Redundancy in Mobile Ad Hoc Networks. Mobihoc, [7] A. Qayyum, L. Viennot, and A. Laouiti. Multipoint Relaying: Networks. INRIA - Rapport de recherche, [8] Y. Sasson, D. Cavin, A. Schiper. Probabilistic Broadcast for Flooding in Wireless Mobile Ad hoc Networks. Swiss Federal Institute of Technology, 2002 [9] V. Shurbanov, J. Redi. Energy-Efficient Flooding in Mobile Ad Hoc Networks. U.S. Army Research Laboratory s Collaborative Technology Alliance on Communications and Networking. [10] J. Wu, F. Dai, M. Gao, and I. Stojmenovic. On Calculating Power-Aware Connected Dominating Sets for Efficient Routing in Ad Hoc Wireless Networks. [11] I. Stojmenovic, M. Seddigh and J. Zunic. Dominating Sets and Neighbor Elimination- Based Broadcasting Algorithm in Wireless Networks. IEEE Transactions on Parallel and Distributed Systems. Vol. 13 No.1, [12] J. Sucec, I. Marsic. An Efficient Distributed Network-Wide Broadcast Algorithm for Mobile Ad Hoc Networks. CAIP Center. 20