Resource and Performance Tradeoffs in Delay-Tol...

图1.系统中某时刻单包的副本数，每个状态i代表系统中包副本数目，R代表至少一个包到达收集站点的状态。γ为相遇速率。the rate of offloading is i*γ，the probability of offloading is i*γ*Δt。参数p用来管理系统资源使用，设为成功接受包的概率。

假设是无线传感器节点，各节点相遇情况相同，且呈泊松分布。Epidemic routing is based on replication and propagationof copies of a message to many mobile nodes.
This Markov model assumes that node-to-node encounters and node-to-station encounters occur at exponentially distributed intervals and that all nodes have the same prob- ability of reaching a relay or a destination. This is a reasonable assumption for a sensor network with mobile nodes whose motion is independent from the location of the data- collecting stations. For example, consider a network of tags on animals where the animals are foraging for food. Each node independently follows a pseudo-random pattern and is equally likely to meet another node or station in its area at any time.

Finding the probability of the system in the state R as a function of time provides us with the cumulative distribution function for the delays of the packets, F(T). (即：系统处于R状态概率是包累积时延分布时间函数F（T）)

The energy consumption of the system is calculated by recording the number of transitions between states in this model. （系统能耗通过记录模型中各状态的传输数量）。The system storage at timestep T, for a unique packet and its copies is found by adding up the probabilities of the different states multiplied by the number of packet copies in those states.包副本增加到达概率。

From the cumulative distribution of the delay, we are able to determine the threshold probabilityP_thresh that represents the confidence with which we desire each packet to be successfully offloaded to a collection station. 从时延累积分布中，能决定决定包成功到达收集站点的门限概率 Pthresh。

For example, if P_thresh = 0.5 and F⁻¹(0.5) = 200, then the packets need to remain in the system for 200 time-steps, after which time they are successfully offloaded with probability 0.5, and all the copies could (and should) then be removed from the system.

传输时延(包数量会增加)和能量、存储的tradeoff。

PS：(Science Citation Index, SCI)是由美国科学信息研究所(ISI)1961年创办出版的引文数据库。SCI(科学引文索引 )、EI(工程索引 )、ISTP(科技会议录索引 ) 是世界著名的三大科技文献检索系统，是国际公认的进行科学统计与科学评价的主要检索工具，其中以SCI最为重要