As mentioned above, in the LHA protocol when a node detects a merger it has to send only one message that informs about the changes in its network. Basically, the merger is solved com- pletely if every node receives the merger message. To ensure the successful delivery of the mer- ger message, LHA uses a merger timeout in which every node analyses its neighbor’s messages (this is the eavesdropping mechanism). The study now reported is, therefore, of the efficiency in terms of latency, message redundancy and merger success rate when LHA follows its own mechanism (reliable merger) or does without it (pure broadcast). In other words, if a node fol- lows the pure broadcast a node depends only on the ongoing broadcast mechanism used in the network to deliver its message, such as the simple flooding broadcast mechanism. Because there are many reasons for message drops in wireless networks, it is important to study the merger of networks in different scenarios. Three merging scenarios have therefore been selected, represent- ing the merging between one large and one small network, a pattern shown in Figure 4-13. The small network consists of 5 nodes and it is used only to trigger the merging function of the bor- der nodes in the large network in which the merger message is observed. Depending on the node distribution in the larger network, the three scenarios can be described as a sparse, dense and special (clustered nodes). For example, Figure 4-13 shows a sparse scenario which indicates the case of messages dropped because of long distances between nodes. In this scenario to get full coverage from the border (node 0) a merge message has to be forwarded a minimum of 21 times. In contrast, the dense scenario reveals the problem of high collisions and contentions because of the high density of nodes, although the full coverage can be achieved with 8 forwarding episodes at a minimum.
4.3 Impact of Network Mergers
Figure 4-13: Merging networks, sparse scenario
For complex cases, a special scenario (clustered nodes) which includes all the issues from the last two scenarios as shown in Figure 4-14 is defined and studied. In this scenario, the minimum number of times the merger message is forwarded from node 0 to all nodes in the network is about 13. In addition to the “dense” and “sparse” issues, the clustered nodes scenario includes bottleneck nodes, such as nodes 2 and 3 in the figure. Because the bottleneck nodes are consid- ered a connection node between two portions of the nodes, the dropping of the packets in one of these nodes results in a low packet delivery success rate. To show the performance of LHA mer- ger function by using the reliable merger or pure broadcast mechanisms, we present each scenar- io measured by three metrics, called merger latency, message redundancy and delivery rate (i.e. full coverage success rate) as follows:
Range 230 m
4.3 Impact of Network Mergers
Figure 4-14: Merging networks, clustered nodes scenario
• Success rate of merger:
The delivery metric is the success rate of all attempts, wherein the success of an attempt indi- cates that the broadcast message sent during the attempt is delivered successfully to all nodes in the network which means that message has got full coverage. Figure 4-15 shows this metric for each scenario in both cases, that of using the reliable merger in LHA and that of the pure broad- cast mechanism. The delivery metric shows that LHA is able to overcome the problem resulting from the broadcast mechanism. In the figure, the statistics shows that use of a pure broadcast mechanism to deliver the message to all nodes results in some unsuccessful attempts. However, the eavesdropping function in the LHA obviates this shortcoming. As can be seen, in all scenari- os broadcast with the LHA eavesdropping function there is full and successful coverage. In the figure, the pure broadcast suffers mainly in sparse scenario because the forwarding requires sev- eral hops and the probability of receiving a copy of the merger message from neighboring nodes is lower than other scenarios. However, high node density in dense scenario may mitigate this problem but in turn it increases the issues of contentions and collisions between nodes. There- fore, as the figure show, the clustered nodes scenario which has moderate density of nodes shows a better success rate than the other scenarios.
4.3 Impact of Network Mergers
Figure 4-15: success rate of merger
• Merger latency:
Now follows a study of the delay in handling the merger as a function of reliable and pure broadcast mechanisms: in other words, the time required to finish the merger process successful- ly in both cases. Because pure broadcast is not able in all attempts to deliver a merger message successfully the latency value in Figure 4-16 indicates the latency only of successful attempts. It can be seen that the maximum average time needed to inform all nodes in the network by using pure broadcast mechanism in all scenarios is lower than that in the reliable mechanism and that the latency is less than 0.5 sec in all scenarios. Basically, the reliable mechanism used in LHA merging function results in higher latency due to the waiting state needed for reliable merging, i.e, for success in all attempts as presented in Figure 4-15 above. As explained in Section 3.6.2, during this state each node forwarding any merger message (S_Merge or H_Merge) must detect and resend the message if the first forwarding has failed to achieve full coverage. Of course the time needed in a dense scenario in some cases is higher because the number of nodes trying to forward or resend the dropped messages is higher and this leads to increased transmission time. This effect is less in the sparse scenario but, on other hand, there are more outliers indicating resend function due to dropped packets in this scenario because of the many hops and few neigh- boring nodes. In this scenario 90% of latencies are between 0.4 and 0.5 second because the min- imum number of forwarding episodes required to get full coverage is bigger than that in the other two scenarios. In clustered nodes scenario the outliers is due to the bottleneck nodes but the most latencies in this scenario is lower than that in sparse scenario because it needs less number of hops for the forwarding.
100 100 100 88,9 82,45 91,1 0 10 20 30 40 50 60 70 80 90 100
dense sparse special
Deli ver y r ate % Scenario
4.3 Impact of Network Mergers
Figure 4-16: Latency of successful networks merger in different scenarios
• Message redundancy:
Figure 4-17 illustrates the message redundancy for each scenario, measured by number of messages sent per node. It is clear that in successful pure broadcasting attempts the merger mes- sage will be sent once by every node. However, to get full success in all attempts, the reliable merger of LHA allows a node to resend a message if there is a message drop. Therefore, more messages are sent per node in a dense scenario than in the other two scenarios, where many neighboring nodes try to resend the dropped message to the same destination. In contrast, the sparse scenario does not suffer from this case because of the low number of neighboring nodes distributed over big number of hops. Unsurprisingly, the results for the clustered nodes scenario show an average value between the two first scenarios because it has less node density than the dense scenario and more than the sparse scenario.
Figure 4-17: Message redundancy of successful networks merger in different scenarios
0.5 1.0 1.5 2.0 2.5 M er ger Lat enc y( se c)
Rel. Pure Rel. Pure Rel. Pure
Dense Sparse Special
Scenarios 1.0 1.5 2.0 2.5 M es sages per nod e
Rel. Pure Rel. Pure Rel. Pure
Dense Sparse Special
4.3 Impact of Network Mergers