With mobile devices such as smart phones, PDAs and tablets, users communicate with each other and obtain social network services. There are multiple wireless communication connections by which users can flexibly access. The GSM network is cheap but the data rate for connection is low. 3G and 4G networks provide faster transmission rates than GSM networks but requiring a higher service cost. Wi-Fi and Bluetooth become popular as they can provide a fast and free access for users [9-11].
In MSNs, the content needs to be delivered from the source (e.g. content provider) to the destination (e.g. user), mainly via the following access methods. One is to use the centralized networks infrastructures (e.g. cellular network) which mainly belong to the network operators or the content providers. The other is opportunistic network infrastructures (e.g. ad-hoc networks or delay tolerant networks) which are used to deliver data between users.
• Cellular network: Cellular networks can support users to generate content and provide communication information to other users. These information can be used to determine the social relations to construct the MSN , and study users’ social activities. Because the social relations between users are dynamic in the real world, the communication records are mapped into a time sequence with the correlation model. The model has been used to quantize the strength of the social relation between users to analyze the social relation among users.
• Opportunistic network: Due to the mobility of users, the communication in MSNs between the source user and destination user is dynamic without a fixed topology. For example, when users use Wi-Fi or Bluetooth to connect with each other, the opportunistic networks can be formed for MSNs. Because of the characteristics such as intermittent connectivity and long time disconnect, the opportunistic networks can support a store-carry-forward way to spread data in MSNs. The store- carry-forward way allows relay users to store the data when there is no chance to transmit it. Then the relay users transmit the data to the users who are close to the destination user by opportunistic contacts. When the relay user encounters another user, the relay user decides whether to forward the data to the encountering node or not by estimating the probability that this user will encounter the destination user . Figure 1.3 shows the process that a source user transmits the data to the destination user in MSNs in opportunistic networks. Here, the source user A needs to transmit the data to user E which is the destination user. As the distance between user A and E is long to transmit the data from user A to user E directly, user A needs to find the relay users to transmit data to E. In this figure, user A selects user B and C as the relay users. Then user C transmits the data to user D who finally transmits the data to E.