General Discussion

In this section, we compare the performance of ODMRP, ADMR and MRDC op-erating in different transmission modes (broadcast, unicast and adaptive). These protocols use different types of delivery structure. ODMRP constructs a group

Maximum speed (m/s)

Distance from members to core

Average interior node

0 30.8 11.3

1 23.8 9.4

5 22.8 9.4

10 22.6 9.3

15 21.6 8.9

20 21.4 9.0

Table 4.8: Distance and Multicast tree size under MRDC-broadcast as a function of Maximum speed

shared mesh. ADMR uses source-based tree. MRDC provides group-shared tree.

During packet delivery, the former two create redundant forwarding state within nodes in the network against routes broken caused by topology change or control message loss. Whereas MRDC does not use this technique. The simulation results show that the greatest difficulty for these protocols to use redundant forwarding state is how to find the compromise between robustness and efficiency. Rich re-dundant forwarding state permits routing protocols to maintain its performance in dynamic networks without the requirement of extra routing overhead. However, the forwarding overhead as a result of redundant forwarding state degrades protocol’s performance in high load cases. As for MRDC, it provides the best performance since this protocol uses the least connectivity and in most cases control messages are correctly and duly transmitted. However, the group-shared trees do not provide the same performance for non-core sources. When group members are badly dis-tributed in the network, MRDC provides a worse packet delivery compared to other two protocols under the condition that all of them broadcast multicast packets.

Unicast mode creates more forwarding overhead and depends more on the cor-rectness of the multicast tree than broadcast mode since multicast transmission strictly respects tree structure. Thus MRDC-unicast degrades more quickly than MRDC-broadcast as network load and mobility increase. MRDC-adaptive some-times outperforms both MRDC-unicast and MRDC-broadcast because it takes ad-vantage of unicast mode and broadcast mode at the same time. In fact, when MRDC broadcasts multicast packets, a tree member can receive multicast pack-ets from neighbors which are not listed in its multicast routing entry. That provides a certain redundancy to improve packet delivery. Unicast multicast packets offers certain degree of reliability for transmission but deprive transmission redundancy.

In ideal cases, MRDC-adaptive uses broadcast mode in hot spot to create sion redundancy and avoid congestion and uses unicast mode to assure transmis-sion in other regions so that this protocol can provide the highest packet delivery.

4.7 Conclusion

In this chapter, we studied the performance of MRDC in ns2. At first, we se-lected key parameters of MRDC: tree refresh period PERIOD REF and thresholds of mode selection. A longer period generates less routing overhead for multicast tree refresh but reduces the robustness and efficiency of MRDC since the packet delivery ratio decreases as nodes’ mobility changes. On the other hand, a shorter refresh interval makes MRDC robust against topology changes at the cost of high control overhead. The simulation results show a 5-second period yields a compro-mise between robustness and low routing overhead. As for mode selection thresh-olds, a small INTR and QLEN make nodes easily think that they are in a hotspot and switch to broadcast mode. This is appreciated in high load networks to avoid congestion created by unicast mode. However, it is not welcome in low load net-works because broadcast mode generates significant MAC layer packet collision.

(

) pair shows a good trade-off between delivery ratio and delivery delay, and is used in performance comparison simulations.

Then, we evaluated performance of MRDC under different movement and traf-fic scenarios. The evaluation contains two parts: the first, the characteristics of MRDC multicast tree and the second, the performance comparison. The simulation results of MRDC multicast tree such as the average number of interior nodes and the average number of non-group-member routers allow us to estimate the routing overhead and forwarding overhead of MRDC. The results also show that MRDC multicast tree scales well in terms of tree size as the group size increases, while the number of tree repair times is proportional to link changes during a simulation.

On the other hand, the correctness of MRDC greatly depends on the transmission of control message. As the network load increases, tree fragmentation becomes more frequent owing to the loss of control messages. In the performance com-parison, MRDC operating in different transmission modes (broadcast, unicast and adaptive) are compared with other two multicast routing protocols, ODMRP and ADMR using four metrics: packet delivery ratio, transmission delay and routing and forwarding overhead. Thanks to the tree structure, MRDC-broadcast gener-ates the least forwarding overhead. MRDC-adaptive provides optimal results and sometimes the best multicast packet delivery since it yields redundent transmission in high load network and offers reliable delivery in low load networks.

From the simulations we find that the packet loss is due to different effects. The first is physical condition, for example network partition makes some group mem-bers unreachable by other memmem-bers. Routing protocols can do nothing to solve this problem. The second is low layer transmission failures such as packet collision oc-curring in the MAC layer. Routing protocols can alleviate this problem through redundant transmission. However too much redundancy can aggravate packet col-lision and even create congestion. The third is due to routing protocol problems for example if delivery structures are not constructed in time or the routing protocol does not repair the fragmentation in time, multicast packets cannot be delivered to the involved receivers during that period. MRDC does suffer from this kind of

problem because it does not preserve the forwarding state during tree refresh. One solution is to introduce a forwarding state into the states of multicast entry. Mul-ticast routers firstly degrade from tree member to forwarding group member when receiving a CA message. Forwarding group members continue forwarding multi-cast packets during tree refresh. The forwarding group membership expires after a short while and the node either becomes multicast tree member or leaves multicast tree by setting entry state to non-forwarder after tree refresh. In this way, multicast delivery continues even during multicast tree reconfiguration.

Chapter 5

RELIABLE MULTICASTING