# Networking of Multi-Robot Systems: Architectures and Requirements
## 4 Communication Issues and Requirements for MRS Systems
### MANETs and MRS
It is appropriate for MRS networks with a relatively larger number of nodes and lower traffic which is limited to the occurrence of certain events with a selected number of nodes involved such as search and rescue, and environmental monitoring applications. Dynamic source routing (DSR), ad hoc on-demand distance vector (AODV), and temporally ordered routing algorithm (TORA) are examples of such protocols. * Hybrid routing: In this case, both proactive and reactive routing strategies are used in the same MRS network. This type of routing aims at benefiting from the advantages of both strategies by applying them under different conditions. For example, the network can be divided into multiple geographic clusters with selected cluster head (CH) nodes in each cluster. Consequently, intra-cluster traffic, which consists of data exchanges among nodes in the same cluster uses a proactive routing strategy since the nodes are likely to have more frequent exchanges due to closer cooperation and tighter coordination. The inter-cluster traffic, which consists of data exchanges among nodes in different clusters uses a reactive routing strategy, which routes the data among the CHs in different clusters. Traffic that is exchanged among nodes in different clusters is routed to the CH of the source cluster, which transmits the data to the CH in the destination cluster. The latter subsequently routes the data to the destination node in that cluster. This approach provides a scalability advantage and is appropriate for larger MRS networks with clusters of robots responsible for closely coordinated tasks in the same geographic area (or cluster) but need to communicate information to robots in other geographic areas (or clusters) less frequently. * Location/Geographic routing: In this type of routing, each node must be equipped with GPS circuitry or must be provided with the position information of other nodes in the network from a location service. This comes at the expense of increased node cost. Data is routed from the source to the destination using various routing strategies which are intended to keep moving the data closer to the geographic location of the destination until the latter is reached. This type of routing has the advantage that each node does not need to keep track of the topology of the network. It only needs to determine the next hop that gets the data closer to the destination. The routing overhead is reduced since the routing decision is based on local information and the routing process typically incurs lower delays. This type of routing can be useful for MRS networks stretching over larger geographic areas and the nodes are highly mobile leading to constant topology changes. Protocols in this category include Location-Aided Routing in mobile ad hoc networks (LAR) [32], Energy Efficient Location-Aided Routing (EELAR) [33], greedy location-aided routing protocol (GLAR) [34], Location-Aided Energy Efficient Routing (LAEER) [35], and Location-Based Efficient Routing Protocol (ALERT) [36]. * Hierarchical protocols: In this case, a multi-level hierarchy is adopted. Consequently, communication between the lower level nodes is done by passing the data to the upper levels. For example, a three-level hierarchy can be used where the lower level robots play a sensing/acting role. They are identified as basic robots (BR). The robots at the second level play a data relay role, where the data between various clusters of lower level robots is relayed among the data relay robots (DRR). The robots at the third level are identified as data dissemination robots (DDR), and are responsible for delivering the data collected by the DRRs to the control center (CC). This type of strategy provides higher scalability at the cost increased complexity, and cost. However, for MRS networks with a large number of small robots (hundreds or thousands) covering a wide geographic area, this hierarchical model would be appropriate.