In Ad Hoc Tcp Protocol Analysis And Improvement

As the most widespread transport layer protocol, TCP was originally designed for fixed wired network and was only fit for it. Ad Hoc Network is a distributed network which has new features of the lower layers that could be wildly different with the traditional network. TCP can not deal with these new features so its performance drops dramatically. As a result, the key to improve Ad Hoc's performance depends on either TCP's evolution of refit and redesign, or on strenghthening the fragile low layers of Ad Hoc itself.This issue is focused on the Ad Hoc protocol stack research and retrofit. IEEE 802.11 DCF, AODV DSR protocol and TCP-Reno are selected as the protocol stack of link layer, network layer and transport layer. Researches are concentrated on the exploiting of network's undeveloped information. The more useful information there is, the more capable the nodes can be when facing perplex network environment.The bottleneck detect and eliminate method, frame error ratio based link status evaluate and predict mechanism, stability first routing method for AODV protocol and successfully recieved packet ratio based TCP congestion windows modify mechanism are proposed. They are all based on mining and using the underlying information of network.(1) The competitions of Ad Hoc MAC layer is very fierce and bottlenecks are easy to shape. To give the bottleneck nodes more advantages in channel competition, the bottleneck detect and eliminate method is used to find and help these nodes, letting them occupy the channel faster and ease the tension of the network bottleneck.(2) The frame error ratio based link status evaluate and predict mechanism evaluate and predict the channel quality without increasing any network cost. This mechanism can get an accurate depiction of these channels and then launch certain policies.(3) Based on this mechanism, in the network layer, stability first routing mechanism for AODV protocol changed the way that AODV used to pick best route, which was routing by minimum hops. The new mechanism chooses the most stable route when routing and launches a re-routing when the MAC layer has predicted a neighbor vanishing, thus enhances the stability of active network layer routes and decreases the possibility of route breakdown, finally raises the entire throughput of the whole network.(4) TCP-Reno protocol is a traditional TCP protocol which has congestion avoid and control mechanisms. That leads to the serrate advance of the sender's congestion window. This issue evaluates the end-to-end routes of transport layer by analyze the successfully recieved packet ratio of the receiver, then give a certain feedback to the sender so it can change its congestion window according to this evaluation. Through this way, the TCP streams on the network can become smooth, the congestion happening possibility of the network will be decreased and available resources of the network could be used better.In the last, related modules of opnet simulation software are modified in order to add the obstacle environment simulation function.