Resource and Performance Tradeoffs in Delay-Tol...

圖1.系統中某時刻單包的副本數，每一個狀態i表明系統中包副本數目，R表明至少一個包到達收集站點的狀態。γ爲相遇速率。the rate of offloading is i*γ，the probability of offloading is i*γ*Δt。參數p用來管理系統資源使用，設爲成功接受包的機率。

假設是無線傳感器節點，各節點相遇狀況相同，且呈泊松分佈。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：系統處於R狀態機率是包累積時延分佈時間函數F（T）)

The energy consumption of the system is calculated by recording the number of transitions between states in this model. （系統能耗經過記錄模型中各狀態的傳輸數量）。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.包副本增長到達機率。

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. 從時延累積分佈中，能決定決定包成功到達收集站點的門限機率 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.

傳輸時延(包數量會增長)和能量、存儲的tradeoff。

PS：(Science Citation Index, SCI)是由美國科學信息研究所(ISI)1961年創辦出版的引文數據庫。SCI(科學引文索引 )、EI(工程索引 )、ISTP(科技會議錄索引 ) 是世界著名的三大科技文獻檢索系統，是國際公認的進行科學統計與科學評價的主要檢索工具，其中以SCI最爲重要