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In mobile database systems, the communication between clients and servers is weak connectivity, which is disadvantageous to synchronization and prone to bring about synchronization rollback. In order to solve the weak connectivity problem, researchers have put forward many solutions and their focuses are on application layer of network, however, the application layer does not know the real network conditions and can not adjust itself accordingly. This paper resolves this problem at transport layer. The paper adopts the TCP Westwood protocol as transport protocol, which adapts to wireless network better. Further, we use cross-layer method, allowing synchronization program to make use of the bandwidth estimated by TCP Westwood, to reduce the waste of communication resources induced by rollback of synchronization transactions in wireless network. Then we use Sybase SQL Anywhere to do experiments. The experimental results show that the cross-layer method can improve the synchronization performance of mobile database remarkably.