ملخص
Abstract
Ad-hoc networking is a concept in computer communications, which means that users wanting to communicate with each other form a temporary network, without any form of centralized administration. Each node participating in the network acts both as host and a router and must therefore be willing to forward packets for other nodes.In a multi-hop wireless packet network, a packet traverses more than one wireless link in traveling from its source to its destination. Such wireless networks have received considerable attention recently in the context of ad hoc networks. Ad-Hoc networks are self-organizing multi-hop wireless networks where not all links along the path of a packet can be activated simultaneously. Hence the performance of a higher level mechanism, such as congestion window or Transport Layer protocol (TCP) will depend crucially on how the links are scheduled for transmission. In multiple access networks, such as the IEEE802.11 wireless LAN, the scheduling of transmissions is done by the medium access control (MAC) protocol. This transmission scheduling is completely unaware of the needs of the TCP congestion window protocol. This thesis is concerned with the poor interaction between the TCP adaptive window protocol, the IEEE 802.11 MAC protocol, and the IEEE 802.11 Ad-Hoc routing protocol. Our thesis follow on the work done by the researchers K. Nahm and C. Kuo in the paper entitled “TCP over 802.11 Multihop networks issues and Performance Enhancement” [1]. Our contributions are two-fold. First we have repeated and confirmed the simulation results reported by K.Nahm and C.Kuo using NS-2 network simulator. Second we have introduced a simple modification in the 802.11 DSR routing protocol which reduces the unnecessary VI reroutings triggered by MAC packet collisions and enforces the TCP to enter the fast retransmit / fast recovery phase to quickly recover the packet loses. This modification has been simulated using NS-2 network simulator and from the results we have found that it provides a substantial improvement in TCP performance in the situations that we have considered in the simulation models