| Wireless Ad hoc Networks are temporary autonomous systems which are made up of a group of terminal nodes. Especially with the nodes in mobility, it is called MANET(Mobile Wireless Ad hoc Networks). These nodes could not only work as terminal hosts but also play a roll of routers. Two nodes that are out of the transmission range of one another could communicate through intermediate multi-hop nodes. MANET could be configured easily and established quickly, and it could work not only independently but also as end subnet along with existed networks. These advantages make it widely applied in civil and military area.However in ad hoc networks, link contention is extensive, node mobility is unpredictable, route change is frequent, and transmission medium is unstable. These are problems not existing in traditional wired networks. As a classical transport protocol to support upper applications, TCP meets with great problems in Ad hoc networks. Improving TCP transport performance, to make high layer applications that basing on TCP work steadily under ad hoc environment, is with great realistic significance.The main work of this article could be divided in two parts:Firstly, in MANET, the mobility of nodes results in frequent route change, but original TCP could not recognize non-congestion packets losses. During route re-establishment, TCP will shrink the congestion window blindly and induce the unnecessary decline of throughput. Preemptive routing could improve the reliability of routes. It tries to warn the source node when a path is regarded to fail soon, and make it switch to a new stable route before the real path breakdown in so as to reduce packet losses. We fine that the time of warning initiation is a key factor. In order to improve the warning precision, we adopt a link status evaluation method basing on neighbor nodes mobility information which is more precise, and analyz the reaction time which is required during warning and route change. With the help of these two points, we modify the preemptive routing, and additionally we reserve the route cache mechanism which is unenabled in original preemptive routing.Secondly, in static ad hoc scenarios, the main factor of link unreliability is contention instead of node mobility, and this is determined by 802.11 and the character of wireless ad hoc network. The Link contention make BDP (Bandwidth-Delay Product) go far beyond the real network capacity. BDP is the guideline of original TCP congestion window adaptation, and it should be the reflection of network capacity. However in wireless environment, TCP will boost the congestion window aggressively above real capacity. This overshooting of congestion window results in more extensive contention, more packet losses, more route failure and reset of window, bad for throughput. We study this topic thoroughly and obtain some research conclusion. Basing on these inferences, we adopt a new link contention evaluation method basing on statistic, and it could depict the path status reasonably. On the other hand, TCP congestion window adaptation method is improved, so congestion window not only is alleviated from overshooting, but also could work steadily during a long period. |
