# An Aquatic Mobile Sensing USV Swarm with a Link Quality-Based Delay Tolerant Network
## 2 Related Work
### DTN Forwarding Strategies  
Traditional routing protocols demand a stable end-to-end connection between source and destination, failing in challenging environments. Delay tolerant networks experience high heterogeneity and volatility in the network, not always having end-to-end connection. Routing protocols for this type of architecture have to adapt to challenging environments. The developedrouting protocols over the years present a trade-off between controlled replication and network knowledge. On the one hand, a pure replication protocol, such as flooding, uses high resources due to the broadcasting of the packets to all vicinity nodes, leading to high network congestion. On the other hand, a pure knowledge protocol requires high resources to process and gather the information, while maintaining an updated routing table [16].  
The Epidemic protocol implements a replication protocol flooding the bundle through the network. It does not require any prior knowledge [17]. This protocol increases the network overhead, since the packet can reach the destination by multiple paths. The Direct Contact protocol only delivers the packet when the source is directly connected to the destination; it has a small network overhead, but a high delay, and a low delivery ratio [18].  
[PERSON] et al. [19] proposed a DTN-based routing through LQE that uses a hybrid monitoring, through active probing and overhearing. This LQE uses RSSI, link stability and available bandwidth. [PERSON] et al. [20] proposed Q-PRoPHET, a quality-based routing protocol capable of performing multi-hop routing decisions based on the paths' qualities. This protocol is based on PRoPHET and uses a decay function and a transitive probability, avoiding sending replicas to poor link qualities. The LQE used is based on RSSI and link stability, making it an active monitoring estimator.  
QoN-BSW presented by [PERSON] et al. [21] is an improvement of the Binary Spray and Wait protocol, since it improves the delivery rate, reduces the average delay, and reduces the network overhead. This protocol presents the notion of QoN, that is, the number of nodes met in a period of time. In the first phase, when a node encounters another, they will update their QoNs and then exchange QoNs with each other. After this, the amount of message copies that are forwarded takes into account the QoN number of the destination node. In the wait phase, if a node only has one copy left, it switches to direct transmission and only forwards this message to the destination. The Hybrid of Probability and message Redundancy (HPR) [22] routing algorithm is based on a combination of packet delivery ratio (PDR) and message redundancy, with the aim of reducing the communication overhead while keeping the high message delivery ratio. This algorithm estimates the delivery probability of the node based on the history of encounter information and contact duration to provide a more precise and reasonable estimation of delivery probability.  
Our proposal is to develop a software-based LQE with only passive monitoring, combining multiple metrics. To have a better understanding of each protocol, and also our proposed LQE Passive Multihop Link Quality Estimator (PAmuLQE), Table 1 summarizes the differences and similarities between them.