16-05-2012, 05:10 PM
Routing is an essential component of communication networks
Routing is an essential component of communication networks.docx (Size: 160.8 KB / Downloads: 29)
Routing is an essential component of communication networks, which defines act of moving information from source node(s) to destination node(s) in an inter network [3]. Multicast is a type of routing scheme, which transmit datagram’s to a set of desirable destinations identified by a single address [4]. If we compare with unicast routing multicast is a hard in wired networks, and is even more challenging in ad hoc networks due to the dynamic topology, limited wireless network bandwidth and energy resources. In ad hoc networks two nodes can communicate directly with each other only within the transmission range. Otherwise, the communication between them has to relay on other nodes. In Fig.1 nodes A and B are within each others transmission range so A can send that packet directly to B. But, A and C are not within each other’s range. If A wants to send a packet to C, it has to first send the packet to B and then B forward the packet to C.
When a link breakage is detected, the node that is furthest from the multicast group leader is responsible for repairing the broken link. If the tree cannot be reconnected, a new leader for the disconnected downstream node is chosen as follows. If the node that initiated the route rebuilding is a multicast group member, it becomes the new multicast group leader. On the other hand, if it was not a group member and has only one next hop for the tree, it prunes itself from the tree by sending its next hop a prune message. This process is continues until a group member is reached. Once these two partitions reconnect, a node eventually receives a group hello for the multicast group that contains group leader information that differs from the information it already has. If this node is a member of the multicast group, and if it is a member of the partition in which the group leader has the lower IP address, it can initiate reconnection of the multicast tree.
Multicast technique requires some technological constraints, where more capacity should be added into the network as multicast addressing, capacity multiplication and specific signalling protocols. Until now, only a few multicast routing protocols have been recently proposed for ad hoc networks. Actually, multicast protocols used for static networks such as DVMRP, CBT, PIM, and MOSPF do not perform well in ad hoc networks. This fact is due to the fragile multicast tree structure that should be reconstructed each time the connectivity changes. Furthermore, multicast trees usually require a global routing substructure such as link state or distance vector. This will require frequent exchange of routing vectors or link state tables due to continuous topology change causing excessive channel and processing overhead. In addition, storage capacity and power are severely limited requiring much less multicast state exchange. Traditional multicast protocols based on upstream and downstream links are not suitable because creating and maintaining upstream and downstream link status is not efficient in a wireless network [Chiang et al., 2]. To provide efficient multicast routing in MANETs, a different kind of protocols should be designed. These protocols should modify the conventional tree structure, or deploy a different topology between group members [Lee, 8]. Designing multicast routing protocols is a complex problem. Group membership can change, and network topology can also evolve (links can fail and nodes can disappear). Some technical challenges of multicast routing are as follows [Obraczka and Tsudik, 12]: