Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Future work. <s> A novel multipoint communication paradigm, in which each destination receives a subset of the source's signal that corresponds to that destination's terminal and access bandwidth constraints, is presented. The approach to realizing this paradigm is based on integration of layered coding of the source's signal, routing based on bandwidth demand, optimization of signal parameters, and layered error control. The author gives an overview of several hierarchical signal coding techniques; presents methods for finding maximum bandwidth available to destination, establishing maximum-bandwidth routes; and optimally assigns bandwidth to the signal layers to maximize overall reception quality. Error control procedures whereby the network, source, and destinations cooperate to maintain layered-based data integrity, using erasure recovery coding and prioritized packet detection are also presented. > <s> BIB001 </s> Multicast Routing Protocols in Mobile Ad Hoc Networks: A Comparative Survey and Taxonomy <s> Future work. <s> This article focuses on the compressed representations of pictures. The representation does not affect how many bits get from the Web server to the laptop, but it determines the usefulness of the bits that arrive. Many different representations are possible, and there is more involved in their choice than merely selecting a compression ratio. The techniques presented represent a single information source with several chunks of data ("descriptions") so that the source can be approximated from any subset of the chunks. By allowing image reconstruction to continue even after a packet is lost, this type of representation can prevent a Web browser from becoming dormant. <s> BIB002
We believe that research on the use of multicast in mobile ad hoc networks is still in its infancy. Open issues include QoS guarantee, reliable multicast, security provisioning, power efficiency, congestion control, scalability, and efficient membership updates. It is difficult to design a multicast routing protocol that takes all these issues into consideration, that is, a one-size-fits-all design. One possible solution would be to develop an adaptive approach to routing, and this may be the best way forward. Possible topics for future research on multicast routing protocols include the following. (i) Interoperability: most of the existing multicast routing protocols for mobile ad hoc networks were not designed to interoperate with other networks such as wired networks, wireless mesh networks, WiMAX, and so forth. However, it is difficult to design a multicast routing protocol that performs efficiently in a mobile ad hoc network while still being able to interoperate with other networks. In order to offer seamless interoperation, novel mechanisms must be developed to achieve the best performance. (ii) Interaction: mobile ad hoc networks would typically have to support different simultaneous network applications, such as unicast and broadcast. The performance of almost all the existing multicast routing protocols is evaluated in isolation of unicast and broadcast protocols. The interaction effects of unicast and/or broadcast on the performance of multicast routing protocols must be investigated, since these interactions may significantly alter the behavior of multicast protocols. (iii) Heterogeneity: the nodes in mobile ad hoc networks are expected to be heterogeneous with a set of multicast destinations that differ greatly in their QoS requirements and end devices. Existing multicast protocols are developed under the assumption that destinations wish to receive all the information sent by a source. Hierarchical encoding techniques BIB001 BIB002 are proposed for the efficient use of resources in heterogeneous networks. These techniques are a layered way of encoding information, such as audio and video, to obtain different quality levels. New multicast protocols must be developed for layered multicast. Not all destinations in layered multicast receive the same amount of data. Each destination has its preferred number of audio/video layers according to its QoS requirements. Open issues include multicast tree construction, audio/video layer assignment, and (re)joining/leaving a multicast group. (iv) Integration: in general, we believe that no single multicast routing protocol is optimal for all mobile ad hoc network scenarios, given the diverse nature and wide range of operating conditions. Therefore, it is desirable to design multicast protocols that adapt well to network conditions (mobility, traffic load, etc.) and optimize functional and performance requirements, such as reliability, control overhead, QoS, and security. (v) Mobility: the continuous and random mobility of nodes in mobile ad hoc networks can easily make the information derived from the network topology stale. As a result, group membership information, such as leaving or joining a multicast group, may induce frequent updates on the protocol states. Moreover, the transmission of data packets can be obstructed during the update process. Thus, group membership approaches should efficiently cope with membership changes in order to minimize their impact on the overall performance of the protocol. (vi) Congestion control: adjacent nodes in mobile ad hoc networks compete with each other to access the wireless medium and transmit their packet. Thus, the network can be easily congested. Congestion, especially in dense networks, introduces long endto-end delay and buffer overflows and decreases reliability. Instead of leaving the MAC layer to deal with congestion control, multicast protocols should deploy additional novel mechanisms to overcome this congestion. (vii) Power efficiency: the nodes in mobile ad hoc networks are battery operated. Thus, the multicast protocols should provide data delivery with the minimum level of power usage. Multicast protocols that optimize the use of battery power in order to maximize the lifetime of the network should be explored. (viii) Network Coding: the notion of performing coding operations on the contents of packets while they are in transit through the network, commonly called network coding, was originally developed for wired networks. However, it can also be applied with success to MANETs. The main advantage of network coding, substantial performance gains, can be seen in multicast scenarios: improvement in multicast capacity and better resource utilization (in terms of energy consumption, e.g.).