# Vortex: Network-Driven Opportunistic Routing for Ad Hoc Networks
## 3 VORTEX: Variable Opportunistic Routing Transferring Endpoint Expense  
### Overview  
As we have explained the strategies and principles of forwarding in ORs, ORs can achieve better routing performance than conventional unicast routing protocols, especially in lossy environments. Although the conventional ORs have achieved superior reliability and efficiency, most of the protocols take end pair-based routing procedures and the overall network resource usage has not been considered. In other words, a union of routing information between end pairs is not effectively reused for other end pairs even though some protocols maintain forwarding histories for further communication. Moreover, the difficulty of gathering routing information from the entire network prevents the protocols from adapting to changes in the communication environment. Therefore, VORTEX, our proposed OR, employs the following routing strategies for improved adaptability to communication environments, load-balanced forwarding, and efficient use of network resources.  
* Tier assignment based on a degree of terminal;
* Tier-based prompt forwarding without route discovery;
* Treat network as a union of forwarding information.  
The conventional OR protocols generally adopt a \"flat\" routing procedure in which all the terminals in a network play the same role. However, the flat routing principle might be an obstacle when we consider the overall network management due to the scalability issues derived from the computational and information-sharing complexity. Therefore, VORTEX takes hierarchy into account as we had already proposed the hierarchy-based clustered OR, CHOR, in [14]. The main difference between CHOR and VORTEX can be summarized as only CHOR requires the clustering procedure to establish forwarding guides as explained in the following paragraph.  
Another characteristic feature of VORTEX is its prompt forwarding procedure, whereas conventional ORs including CHOR generally require route discovery-like forwarding guide establishment. One of the advantages of omitting the discovery is its improved adaptability to changes in the communication environments because the conventional ORs require the guide for forwarding eligibility confirmation and they should alter the guide according to the changes. On the other hand, the hierarchical structure and opportunistic forwarding strategy of VORTEX enable adaptive forwarding without the use of such guides if the hierarchical structure is properly maintained. Thus, the hierarchical structure becomes a comprehensive guide for multiple end pairs and VORTEX does not require an individual guide for each end pair. Eventually, the hierarchical guide establishment by VORTEX creates backbone-like network structures that could make opportunistic forwarding efficient. This characteristic is achieved by enforcing reasonable restrictions on forwarding direction and the backbone-like structure guides packets in the appropriate direction toward destination terminals. In other words, the hierarchical structures of VORTEX become help packets to travel appropriate routes in a similar way to reactive P2P overlay routing.  
In addition to the above, VORTEX adopts two different control phases, namely network initialization, and network management. In general situations, the network initialization phase is invoked only when the first communication occurs or terminals intentionally form a network for further communication demands. Thus, VORTEX usually operates in the network management phase once the initialization is completed. In the network management phase, VORTEX performs three procedures according to the required process,namely participation, leave, and update procedures. These procedures are independently invoked as needed to deal with topological changes in the network.