# An Adaptive Delay-Tolerant Routing Algorithm for Data Transmission in Opportunistic Social Networks
## 1 Introduction  
Opportunistic social networks [1] are a special combination of opportunistic networks and wireless multi-hop networks, in which clients carrying wireless mobile devices communicate with their peers in a communication area [1]. Without a stable and relay communication link in remote areas, data transmission in opportunistic social networks undergoes intermittent connection and delay, therefore the \"store-carry-forward\" strategy has also been used in this type of wireless networks [2]. As a special type of wireless multi-hop network, opportunistic networks (OPPNET) are derived from wireless Ad-Hoc networking and delay-tolerant networking [2]. At the dawn of 5G and big data, opportunistic networks are not only a special topological structure, but also a specific form of data routing and forwarding. The hot research areas on opportunistic networks include mobile model, routing algorithm, security mechanism, and application scenario [3]. Due to the intermittent connection and variable network structure of opportunistic social networks, the source node in unpredictable location attempts to communicate with the destination without an end-to-end path [4]. Consequently, the end-to-end communication connection between sources and destinations can be implemented by the opportunistic communication created by node mobility [5]. Recently, the primary application areason opportunistic networks consist of wildlife monitoring and rescue [6], vehicle networks [7], network in underdeveloped areas [8], networks in disaster relief [9], and social networks [10]. Routing algorithm is the key issue in the research area of opportunistic networks because of its utility and specificity [11]. To face the challenge of the intermittent communication connectivity, the \"store-carry-forward\" mechanism has been utilized to develop an end-to-end data transmission path [12]. Recently, cache-aware routing protocols have received the increasing attention of researchers [13]. This is because the improvement of cache spaces is helpful to alleviate the problem of data congestion [14]. Relay node is a short-time station for data packets in the \"store-carry-forward\" mechanism [15]. Since the cache management mechanism is improved, the capacity of information processing of relay node becomes more efficiently and prompt, which enhances the delivery ratio and declines delay [16]. In addition, energy consumption is another important limitation in opportunistic networks [17]. As the energy of the mobile nodes is going to be exhausted, they must delay the data transmission [18]. To this end, this implies that energy and cache are two significant cornerstones for the establishment of an opportunistic network routing algorithm [19]. In other words, routing algorithm should consider not only whether there is enough cache to store data information, but also whether there is sufficient energy to carry and transmit information. More importantly, an adaptive approach is needed for routing algorithm to dynamically evaluate the combined influence of the two factors [20]. Limited cache and energy are two major challenges for opportunistic network routing protocols. However, most of those algorithms tackle the issue of limited network resources from the perspective of the deterministic method, which could be formalized as a linear or nonlinear mathematical expression. Most existing routing algorithms, such as Prophet [21], RubbleRap [22], SCORP [23], DABBER [24], and ICMD [25] consider the issue of limited network resources as a deterministic problem and lack of an adaptive transmission mechanism. To demonstrate a more explicit comparison, we establish Table 1 to illustrate the implement details of these routing protocols. On the contrary, the relationship between the two-factor (energy and cache) and data transmission is actually a special type of fuzzy uncertain expression, while should be defined as the degree of membership in fuzzy inference system from the theoretical perspective.