Jouni Korhonen

Effect of vertical handovers on performance of TCP-friendly rate control

An intersystem or vertical handover is a key enabling mechanism for next generations of mobile communication systems. A vertical handover can cause an abrupt change of up to two orders of magnitude in link bandwidth and latency. It is hard for end-to-end congestion control to adapt promptly to such changes. This is especially a concern for slowly responsive congestion control algorithms, such as TCP-Friendly Rate Control (TFRC). TFRC is designed to provide a smooth transmission rate for real-time applications and, therefore, is less responsive to changes in network conditions than TCP. Using measurements and simulation, we show that TFRC has significant difficulties adapting after a vertical handover. TFRC receives only a fraction of TCP throughput over a fast link, but can be grossly unfair to concurrent TCP flows after handover to a slow link. We show that two proposals based on overbuffering and an explicit handover notification are effective solutions to these problems. Using them, TFRC can quickly adapt to new link characteristics after a handover, while otherwise maintaining a smooth transmission rate.

Using Quick-Start to Improve TCP Performance with Vertical Hand-offs

Vertical hand-offs between different wireless access technologies have become more relevant after the introduction of multi-access mobile terminals with wireless LAN (WLAN) and wireless WAN (WWAN) technologies. While the IP mobility mechanisms are rather well known, the performance of TCP still has problems when moving between WLAN and WWAN accesses. First, with a high-latency WWAN link technology such as GPRS it takes several seconds before the TCP congestion window has reached the path capacity. Second, when the notification of the first packet loss arrives at the TCP sender, several packets have already been lost due to the slow-start overshoot and the TCP sender needs to retransmit a large number of the packets from the last transmission window. Third, after a vertical hand-off the path characteristics might have changed dramatically in which case the TCP congestion control state is not valid any more. In this paper we investigate Quick-Start, a mechanism for avoiding the initial slow-start delay, in the context of wireless multi-access terminals. We also propose an enhancement to Quick-Start to alleviate the effects of slow-start overshoot and apply Quick-Start after a vertical hand-off to quickly learn the available capacity on the new end-to-end path. An explicit cross-layer hand-off notification is employed to trigger Quick-Start when the hand-off completes. We conduct simulations with different hand-off models, and our simulations yield promising results with Quick-Start