# Networking of Multi-Robot Systems: Architectures and Requirements
## 4 Communication Issues and Requirements for MRS Systems
### MANETs and MRS  
Since robots are mobile during a significant portion of their operational time, they constitute a specific type of mobile ad hoc networks (MANETs). Certain protocols that are appropriate for MANETs can be used at the various layers of the open system architecture (OSI) model. These layers include the physical, data link, network, transport, and application layers (The session and presentation layers are commonly merged with the application layer). At the physical and data link layers, which constitute the underlying network, MRS communication can use the IEEE 802.11 (Wi-Fi) protocol. It has a line-of-sight range of several hundred meters. The latest version is the IEEE 802.11 ac protocol, which has higher data rates (up to a theoretical rate of 3.4 Gbps) that can support a wider range of data traffic. It uses the carrier sense multiple access with collision avoidance (CSMA/CA) protocol at the medium access control (MAC) layer as well as orthogonal frequency division multiplexing (OFDM) with multiple-input and multiple-output (MIMO) technologies at the physical layer. MRS communication can involve both best effort (BE) as well as QoS guaranteed traffic. For the earlier type of traffic, which is delay-tolerant, the distributed coordination function (DCF) mode can be used, which involves a contention-based access strategy. For the latter type of traffic, which might be delay, jitter, and bandwidth sensitive, such as audio, video, multimedia, and real-time, the point coordination function (PCF) mode can be used, which involves a contention-less access strategy using guaranteed time slots (GTSs). The super frame in the IEEE 802.11 protocol allows both types of accesses in its corresponding two phases. Since the network layer is responsible for end-to-end routing from the source node to the destination, we can take advantage of the large amount of research that is done for MANET routing protocols, which can be used and adopted for MRS systems. Such protocols can be classified into the following categories:  
* Static routing: In this class of protocols, the route is pre-programmed in to the routing tables of the individual nodes before the mission. Although this can be useful for some missions with  
Figure 1: Robot-to-Robot (R2R) and Robot-to-Infrastructure (R2I) Communication. relatively fixed and predetermined topology, it is inflexible and very limiting for MRS systems, which are characterized by higher mobility and variable topology. * Proactive routing: In this class of protocols, a routing table is constructed and periodically maintained by the nodes even before transmission requests arrive from the upper layers to the network layer. This strategy increases the overhead of route discovery and maintenance messages which reduces efficiency in cases where the node-to-node traffic is limited and decreases the scalability of the corresponding MRS network. However, it can be suitable in cases where the application cannot tolerate large and unpredictable route discovery delays such as MRS networks with high real-time coordination involving most of the nodes. The destination-sequenced distance-vector (DSDV) protocol is an example of such protocols. * Reactive routing: In this case, an end-to-end path between the source and the destination is only discovered on-demand when it is needed. Once the route is discovered the routing table at the source node is updated with the corresponding entry. All subsequent transmissions from that node to the destination use the discovered path as long as the related topology and relative locations of the intermediate nodes are not changed in a way that leads to the breaking of one or more intermediate links. If the path is no longer valid, a new path discovery process is initiated. This routing strategy reduces the control message overhead, since the routes are only discovered when they are needed.