# DTN and Non-DTN Routing Protocols for Inter-CubeSat Communications: A comprehensive survey
## 3 Routing Protocols in Connected CubeSat network
### Multi-Layer Routing Protocols in Connected CubeSat Network  
Multi-layer routing was proposed to resolve some of the single-layer routing drawbacks, such as the low persistence and efficiency of the network, and the concept of Multi-Layer Satellite Networks (MLSNs) are introduced to enhance the performance of satellite networks. In the multilayered architecture, satellites are disseminated over two or three standard orbits, such as LEO, MEO, and GEO orbits. Figure 4 illustrates standard satellite orbits. In other words, MLSN architecture allows for the satellites in the higher layer to coordinate with those in the lower layer which means that all of the computational and storage complexity will be more relaxed on the lower layer satellites without the need for ground segment intervention. Figure 5 depicts the multi-layer architecture with three layers. In multi-layered architecture, the management of any lower layer is the responsibility of the higher layer, for instance, GEO satellites will manage the MEO satellites, whereas the LEO satellites are accountable to the ground access. While a satellite can communicate with another satellite on the  
Figure 4: Standard satellite orbits. Figure 5: Multi-layer satellite network hierarchy. same orbit using inter-satellite or intra-satellite links, the inter-layer links are used for establishing communication between the satellites on different orbits. Satellites in the higher orbit will have a wider communication window with satellites in the lower orbit, which increases the efficiency of the network management and provides flexible routing. In general, the routing algorithms proposed for multi-layer satellite networks depend on the master-slave concept. Table 3 summarizes the multi-layer routing algorithms that are proposed for a variety of multi-layered satellite networks. Satellite-Over-Satellite (SOS) architecture was introduced to enhance the routing efficiency over ISLs of broadband and real-time satellite networks [27]. The authors proposed a Hierarchical Satellite Routing Protocol (HSRP) that is a multilayered, dynamic, and QoS-based routing algorithm for high-density SOS networks to provide multimedia and real-time services. In SOS, satellites are assumed to be distributed over multiple orbits and communication between the satellites and the ground stations involves three types of links: inter-orbit links; inter-satellite links; and, ground links. Inter-orbit links are incorporated for communication between different layers and inter-satellite links to connect satellites on the same layer, while the ground links allow for satellites in the lower layer of the network hierarchy to communicate with the ground stations. The SOS network is modelled as a connected grid with logical locations around the Earth to provide full coverage, while assuming that each satellite from the higher layer will have a cluster of lower layer satellite covered by its footprint. Although SOS architecture can provide a better performance, the HSRP flooding mechanism that is used to collect the topology information leads to high overheads and complexity. A Multi-Layered Satellite Routing (MLSR) algorithm was developed for LEO, MEO, and GEO three-layered satellite IP architecture based on the same concept of SOS [28]. With a large number of multilayered satellites, MLSR decentralizes routing table calculations among the high-level management GEO satellites to reduce the computational complexity. MLSR also reduces the communication costs by grouping LEO satellites to update GEO satellites with outlined link delays. MLSR considers propagation delay as well as computation and queueing delays. The link recovery technique in the case of satellite failure is also considered in the routing table calculations. In the same way, a three-dimensional hierarchy satellite network is evaluated while using the OPNET network simulator [29].