## 1 Introduction  
Delay Tolerance Mobile Sensor Networks (DTMSNs) [1-5] have several unique characteristics such as sparse network density, short range radio and sensor node mobility, energy limits and so on, which result in intermittent connectivity and frequently changing network topology. Obviously, it is difficult to form well connected end-to-end paths for mobile sensor nodes to transmit data messages to the sink nodes in a DTMSN. Mobile nodes usually use probabilistic delivery schemes, that is, in the process of data delivery, a node and its neighbors form the temporary route for delivering the data in a carry-and-forward scheme. Since the probabilistic delivery cannot guarantee the data delivery performance, it is reasonable to employ multi-replica transmission schemes, which means generating multiple replicas for every data message and spreading these copies to different nodes, in order to enhance the successful delivery ratio and reduce delivery delays. Though multi-replica transmission schemes are generally efficient for improving data gathering performance, these schemes have to expend much more network resources, such as bandwidth, node energy and buffer space _etc._ compared to single-replica transmission schemes. For a DTMSN whose resources are usually limited, too many duplicate messages will dramatically increase traffic overhead, cause a large amount of wireless collisions, increase overall delays, and rapidly drain mobile nodes' limited battery energy. Therefore, an efficient data gathering scheme tailored for DTMSNs needs to maximize data delivery ratios while minimizing communication overhead (_i.e._, the number of data message replicas). In recent years, a number of multi-replica routing protocols [6-10] have been proposed to enable data delivery in such challenging environments as DTMSNs. These multi-replica protocols can be classified into two categories based on the number of replicas generated and injected to the network: flooding-based and quota-based. Flooding-based protocols send a replica of each message to as many nodes as possible, whereas quota-based protocols intentionally limit the number of replicas. A majority of existing multi-replica protocols, such as Epidemic [6], MaxProp [7], PREP [8], RAPID [9] and _etc._ are flooding-based. Though these protocols can effectively reduce the number of duplicate messages compared with the basic flooding algorithm, they usually fail to work well in DTMSNs. The main reason is that their high demands on network resources that are typically unavailable in such challenging environments as DTMSNs. Different from flooding-based routings, quota-based protocols such as Spray-and-Wait [10] and Spray-and-Focus [11] use fixed numbers of duplicate messages, which effectively limits network resource expansion and thus make them attain better performance than flooding-based routing schemes. However, quota-based protocols still have performance problem due to the fact that each message is delivered and transferred in the same and fixed amount of replicas, which would cause unwanted results as follows: (a) when a node is very close to the sink node, the number of duplicates exceeds the need, thereby causing redundancy and when the node is far enough from the sink node, the number of duplicates may not meet the performance needs; (b) the quota-based protocols do not take the difference of node delivery capability into account, and simply consider that every node has the same capability to deliver data to the sink nodes. However, in practice there are obvious differences between the delivery capabilities of nodes. This paper proposes a novel data gathering scheme, called Distance-aware Replica Adaptive Data Gathering protocol (DRADG), which is tailored for DTMSNs. By elaborately adapting the number of duplicates for every data message and computing the delivery probability so that data messages are forwarded to nodes with higher delivery capability, DRADG can achieve high network performance with low transmission overhead cost.