| Delay tolerant network is a multi-hop wireless network which has intermittent connection due to the mobility of nodes,short of wireless communication range,sleeping mode of nodes.As a result,delay tolerant network usually gets higher average network delay and overhead ratio,and the delivery rate of it is very low.However,delay tolerant network could meet the application requirements of some extreme environment,such as emergency communication,vehicle ad hoc network and so on.In recent years,many researchers work on how to improve transmission efficiency of DTNs,thoes research works mainly push on two diretions: fisrt,how to design a efficiency routing protocol in order to make nodes choose the proper relay node during the routing procedure or to control the copies of the messages to make the overhead ratio lower;second,how to manage the cache data of the node efficiently in order to minimize the impact of buffer overflow on routing performance.In recent years,network coding technology is very popular and it has a natural advantage of improving the transmission reliability,reducing the delay,improving the throughput,especially suitable for high packet loss rate of the wireless network.Based on the characteristics of network coding,this paper introduces how to apply inter-session network coding to delay tolerant network routing protocol,and then proposes the corresponding buffer management policy which could process the coded packet.The first research part of this paper,which is a routing scheme based on inter-session network coding,is different from the existing research of intra-session network coding based routing scheme : inter-session network coding coded messages from different flow together and uses the broadcast natrue of the wireless network to improve the transmission efficiency while intra-session network coding coded messages from same flow together and uses the redundancy of the coding vector to improve the transmission reliability.Inter-session network coding is the topology based routing scheme,it is proper to be used in wireless network.However,in wireless mesh network,COPE,a inter-session network coding scheme,can not directly be applied to the delay tolerant network due to the dynamic topology structure.In this paper,the inter-session network coding based delay tolerant network routing is based on the clustering technology of Wireless Sensor Networks,thus the node of coding structure in COPE becomes the cluster of network.In DTNs,we design extension coding structure of chain encoding structure,X encoding structure which are mentioned in COPE,and analyze the average delivery delay by using ordinary differential equation.From the numerical result of matlab and simulation result of the ONE simulator,we know that this inter-session network coding base routing scheme has lower average delivery delay.Then,we propose a cache management algorithm for network coding packets to make up for the shortcomings of the existing cache management algorithm in dealing with the special network coding packets.The delay of coded packet in the network is divided into transmission delay and coding delay,transmission delay is decided by the overall characteristics of the network while the coding delay is decided by the number of native packet in the coded package,which denoted by ?.Usually,under certain condition,the greater value of ? is,the higher decoding probability is,but we will get higher coding delay due to we need collect more coded packet before we can decoded one coded packet.Therefore,this paper presents a prior ity discarding policy which drop the packet which has the higher value of ? and the lower survival time when the buffer is overflowed,this is realized by dropping the packet which has the minimum ratio of ? and the survival time.Finally,the experiment result indicates that the cache management algorithm can effectively reduce the delivery delay while keep overhead ratio of the nework no increase.Thus,this algorithm is suitable for network coding based delay tolerant network. |
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