The technologies of Wireless local networks and cellular network are very popular and more and more service under the integrated heterogeneous environment. Stream Control Transmission Protocol (SCTP) is considered as an ideal to support the communication between them by enabling a mobile client to freely switch the IP address in different networks. In this paper, we propose an extended analytical model for SCTP which consider the congestion window and scalable of congestion control mechanism. A great advantage of our model is that establishing a relationship between the throughput and congestion control mechanism. Furthermore, the analytical model provides a useful tool to improve congestion control mechanism of SCTP.