# A Multi-Technology Communication Platform for Urban Mobile Sensing
## 2 Related Works
### DTN Forwarding Strategies  
Traditional routing protocols for wired and wireless networks fail to work in challenging environments since they demand a stable end-to-end connection between sources and destinations [25]. Since DTNs suffer from frequent disconnections, in long-duration partitioning with no end-to-end path, routing protocols for this type of networks must adapt themselves to the challenging environment. Usually, the routing protocols implement a trade-off between controlled replication and network knowledge. A pure replication protocol, e.g. flooding, consumes high resources since the packets are transmitted to all vicinity nodes, hence leading to high network congestion. However, a pure knowledge protocol requires also high resources to process complex routing algorithms and maintain updated routing tables in each node. Thus, it is necessary to find a trade-off between both approaches. Several studies on DTN routing approaches have been performed over the years. [PERSON] and [PERSON] [26], [PERSON] et al. [27], [PERSON] et al. [25], and [PERSON] et al. [28] presented some of those studies about the overall evolution of the several DTN routing/forwarding strategy approaches. Some forwarding strategies with high relevance to our opportunistic and delay-tolerant network were studied, e.g, Epidemic, the Hybrid of Probability and message Redundancy algorithm, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**MAC Protocol**} & **Single-Gateway** & **Single-Channel** & \multirow{2}{*}{
\begin{tabular}{c} **Media** \\ **vs.** \\ \end{tabular} } & **Network** & **Communication** \\  & **Multi-Gateway** & **Multi-Channel** & & **Access** & **Type** & **Flow** \\ \hline LoRaWAN & single & multi & Aloha + LBT & single-hop & bi-directional \\ LoRaBlink & single & single & Slotted Aloha & multi-hop & bi-directional \\ Multi-gateway & multi & single & RTS/CTS/CCTS & single-hop & bi-directional \\ LoRa & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: LoRa medium access communication (MAC) protocol overview. RTS: Request To Send; CTS: Clear To Send; CCTS: Control CTS. GeoSpray, the Geographical Routing scheme with Angle-Based Decision, Velocity and Direction, and Delegation Query. The Epidemic [29] protocol is a multi-copy protocol that implements a replication scheme that simply floods the bundle through the network, i.e., it transmits all bundles to all encountered neighbors that have not those bundles already. Thus, it does not require any prior knowledge about the network. In a contact opportunity, the nodes exchange the bundles that they do not have in common. This can be considered the optimal solution in an environment with no memory and bandwidth constraints. Hence, the Epidemic routing protocol minimizes the delivery delay and maximizes the delivery ratio as messages may reach the destination through multiple paths. The Hybrid of Probability and message Redundancy (HPR) [30] routing algorithm is based on a combination of message delivery probability 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, in order to provide a more precise and reasonable estimation of delivery probability. GeoSpray [31] is a geographic routing protocol for delay-tolerant networks (DTN) over a vehicular environment. Geographic routing relies mainly on location information and other mobility parameters provided by positioning devices such as global positioning systems (GPS). The GeoSpray routing protocol employs the concept of \"spray phase\" from the Spray and Wait protocol, where a fixed number of bundle copies are distributed to distinct nodes in the network. However, instead of doing blind replication (as proposed in Spray and Wait), GeoSpray guarantees that bundle copies are spread to the network nodes closer (and/or arrive sooner) to the bundle's destination.