With the widespread use of wireless Internet and wireless LANs, different wireless technologies, such as 3G cellular networks and WLAN, will cooperate to support more users and applications with higher data rates over wider areas. When a mobile node moves between hybrid networks, a need arises for seamless vertical handoffs between different wireless networks in order to provide high performance data transmission. When an application with a TCP connection in a mobile node performs a vertical handoff, TCP performance is degraded due to packet loss even while the previous TCP state information is maintained during handoff, as 3G and WLAN have different available bandwidths. In this paper, a new congestion control algorithm is proposed for a vertical handoff that improves TCP performance by measuring the rough end-to-end available bandwidth and calculating the slow-start threshold. By ns-2 simulation it is shown that, compared to previous algorithms, the proposed algorithm enhances TCP performance during vertical handoffs.
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