## 2 Mechanisms Specific to QoS and Multicast in Ad Hoc Networks
### Mechanisms for Multicast Routing
#### 2.1.2 Routing Scheme  
The routing scheme reflects the basic ad hoc routing schemes, namely proactive, reactive, geographic routing, and hybrid approach. Among these three types, reactive and hybrid approaches form the majority. Only two protocols in [31, 33] use a proactive approach. In these schemes, the routes to the destinations are maintained in the multicast table of each node. Since the source is aware of the topology (or part of topology) of the network, the multicast routes can be computed immediately. The routing table at each node is updated periodically. MCEDAR [31] is an extension of the CEDAR unicast protocol, where a backbone is established and maintained as the basis for all forwardings. The protocol HQMGA [33] divides the network into clusters and computes a tree for each cluster as well as a tree connecting all clusters. The trees are calculated by a genetic algorithm. In the reactive approach, the nodes do not collect topology information in advance. Whenever a source or receiver joins a multicast group, join requests are flooded and the possible alternatives are unicast to the initiator from existing member of the multicast group. Reactive routing reduces the overhead at the expense of delay and broadcasting traffic to identify possible paths. There is a close resemblance between the dynamic signaling of joining and leaving groups and the reactive methods used for multicast routing. The two processes can therefore easily be combined. This might be the reason for a fair number of proposals based on reactive routing such as schemes in [11, 15, 16, 18, 32]. In geographic routing, packets are routed based on the location of nodes. With only local information, packets can be forwarded to the node that is closest to the destination. Route discovery and maintenance are then not needed. For multicast, geographic routing is used whenever applications require geographic coverage like requesting a target for a specific location. None of the protocols in the survey is solely based on geographic routing. However, two hybrid protocols in [14, 28] combine proactive and geographic routing. The hybrid approach combines the reactive and the proactive approach or combines the geographical and proactive approach. In [12, 13, 21], the network is divided into clusters. Each cluster has a cluster head which proactively maintains the local cluster topology. Reactive protocols are used to build the multicast tree/mesh between clusters. The clusters can be formed based on the geographical location of nodes, connectivity, signal range, mobility, or relative location between nodes. CQMRP [12] uses a self-terminating algorithm to build clusters. In LTM [23], the local link-state information is collected periodically for each node to identify the lantern which has one or more sub-paths existing between two nodes. The lantern-tree or multicast tree is then constructed reactively. The protocol QMOST [26] relies on a QoS unicast link state protocol (like QOLSR [56]). The overlay multicast tree is built actively after the source has all the necessary topology information. Two hybrid protocols in [14, 28] combine proactive and geographic routing. LACMQR [28] is based on a distributed cluster-based QoS multicast routing algorithm. It requires maintaining the local state at each node. The location information provided by the positioning device is used to discover and maintain routes. HVDB [14] also divides the network into clusters and uses the mobility prediction and location-based clustering technique to form stable clusters. The forwarding between logical nodes is done by a location-based unicast routing algorithm. If the number of receivers is high and the receivers are well distributed, it is not given that separate multicast trees/mesh is needed. It might be more efficient to broadcast the information to all nodes. The overhead could be lower if the broadcast is done efficiently. One example is Simplified Multicast Forwarding (SMF) [34]. SMF may utilize the Multi Point Relay nodes (MPR) of OLSR [49].