# A Distance-Vector-Based Multi-Path Routing Scheme for Static-Node-Assisted Vehicular Networks
## 2 Related Work
### Enhancing Vehicular Ad Hoc Networks with Unwired Static Nodes  
Several studies have been made to enhance vehicular ad hoc networks with unwired static nodes. As is already described, SADV [12] is based on link-state routing in which static nodes compute the shortest paths as a sequence of static nodes and build their own routing tables. Each node creates two duplicated copies of a packet and forwards them to the next-hops corresponding to the primary and secondary shortest paths. SADV has no other packet duplication strategy so that it is vulnerable to unexpected packet loss due to vehicles' movement such as going into narrow streets or parking lots. Note that this effect grows large when static nodes are sparsely located at intersections on the roadmap. Thus, in SADV, static nodes are expected to be installed at a high percentage of intersections.  
RDV [13] improved this point by duplicating the required number of packets to suffice the preconfigured expected packet delivery ratio. This not only improves reliability of packet forwarding, but also enables us to make sparse deployment of static nodes located at intersections, i.e., even if we place static nodes at only major intersections between which there is vehicle reach with relatively low probability, packets can be forwarded with a sufficiently high delivery ratio. Sparse deployment reduces deployment cost and makes the routing scheme practical.  
ETGR (Efficient Traffic Geographic Routing) [24] is a reactive routing protocol in which the path discovery process using RREQ (Route REQUest) and RREP (Route REPIy) packets is assisted by static nodes. ETGR in the original paper treats real-time communications in dense-vehicle scenarios, and compared the communication performance with geometric routing schemes such as GSR [6] and GyTAR [10]. In the sparse scenario, however, due to the dynamic nature of VANET, this approach suffers from large paths repairing cost as well as a large delay to search for available paths to the destinations.  
AQRV (Adaptive Quality-of-service (QoS)-based Routing for VANETs) [25] proposed QoS-based routing in static-node-assisted vehicular networks. AQRV assumes urban scenarios in which static nodes are installed at a high percentage of intersections, and computes the communication quality for each road segment between neighboring static nodes. Then, the static nodes behave as routers that dynamically select routes based on an ACO (Ant-Colony Optimization) algorithm. Although AQRV provides high-quality communications with the help of static nodes in urban scenarios with high vehicle density, it does not provide communication quality in sparse scenarios.  
EGSR (Efficient GSR) [26] does not explicitly assume stationary devices at intersections, and instead assumes that vehicle density is high enough to hold required information in cooperation with vehicles around intersections. EGSR measures the quality of links (here, a link means the road segment between two adjacent intersections) based on the ACO approach, and keeps sharing the information around intersections. Every vehicle computes the shortest paths using the [PERSON]'s shortest-paths algorithm with the measured link quality as link weight. The weakness of this method, however, appears when it faces sparse cases in which information sharing among vehicles is impossible.  
ROSTER (Routing based on SpatioTemporal Encounter Records) [27] utilizes daily trajectory data of vehicles to find good paths with low delay performance in sparse scenarios. By computing the intersection region of trajectories and placing RSUs there, they achieves low delay and high delivery ratio communications in sparse scenarios. Although ROSTER covers majority of communication requirements, they cannot provide full coverage of communication requests in the region because it covers only the major requirements that appear in the trajectory data. In addition, their method highly depends on traffic patterns so that performance could be degraded when traffic pattern changes.