Osamu Takahashi

Impact of layer two ARQ on TCP performance in W-CDMA networks

We address the interaction between TCP and RLC (Radio Link Control), the link layer retransmission protocol in W-CDMA, a 3rd generation cellular wireless network technology, in evaluating TCP performance in following points; l) Supress delay-jitter in link layer to avoid excess retransmissions. 2) Clarify the trade-off between jitter-suppression and link utilization to improve TCP throughput. 3) Optimize the link layer and TCP parameters. We show how to find the TCP and link ARQ parameters that yield optimum overall performance; simulations and emulation confirm their effectiveness. The coexistence of ARQ and TCP can lead to inefficient interaction; the delay-jitter on the link layer may trigger spurious TCP retransmission. The solution is to suppress jitter on the link layer. To manage the trade-off between this suppression and link utilization, we optimized with link parameter.

Flow/congestion control for bulk reliable multicast

This paper proposes Monitor-based flow control (MBFC) to realize flow/congestion control needed for one-to-many bulk reliable multicast (RM) protocols. Bulk RM on top of UP multicast requires flow/congestion control because 1) it needs to adjust to the effective bandwidth to minimize packet losses and retransmission, and 2) to share the link bandwidth with other legacy traffic such as TCP so that RM does not override them aggressively. MBFC is a generic mechanism to implement such flow/congestion control. Thus we think it is very important to provide an effective flow and congestion control mechanism that enable RM traffic to coexist with legacy TCP traffic in Internet. MBFC is based on rate flow control and involves a monitor mechanism to adjust its sending rate. In order to realize existence with TCP traffic it mimics TCPs flow/congestion control algorithm, that is, additive increase/multiplicative decrease. To investigate its effectiveness MBFC was implemented in our RM protocol, and a series of simulations were performed in which simultaneous bulk RM and TCP flow were poured. The simulation result showed that our rate control policy effectively achieved bandwidth sharing between bulk RM and bulk TCP transfer traffic. We also conclude that 1) setting the transmission rate to the worst receiver leads to an extremely unfair bandwidth sharing occupied by TCP flow, 2) If Rm only experiences a few loses it should not give up its bandwidth, otherwise TCP monopolizes it. 3) It is more effective for MBFC to introduce RED gateways to prevent RM from an aggressive TCP traffic.

Automating detection of faults in TCP implementations

We propose a new environment for testing the behavior of TCP. We analyze existing test methodologies and show that there is still a need to reduce the loads imposed on both the worker and the expert. The Auto Detector is extensible and so well supports both the automation of predefined tasks and the communication tools needed to ensure gradual refinement by the participants. To show the effectiveness of our proposal, actual software flaws found in HP/UX and the Windows operating system are taken as examples of how to realize the automation of reproduction and detection tasks; it is flexible enough to resolve real-world issues.

Performance Evaluation on WAP and Internet Protocol over 3G Wireless Networks

This research analyses the performance of WAP 1.x in a comparison to the Internet protocol. We implement a WAP client and a WAP gateway based on WAP version 1.1 and assess the response time by comparing to that of HTTP and TCP. We use a W-CDMA simulator to evaluate its performance in high-speed wireless networks such as 2.5G and 3G. The results shows that both protocols have comparable performance (i.e. response time) except when transmitting large content sets (e.g. multimedia data files), in which case the performance of HTTP/TCP is better than that of WAP 1.x. We also evaluate WAP specific functions such as the binary encoding of WAP headers and contents. While binary encoding is effective for small content sets, its effectiveness and performance are questionable for large content sets. Finally, we propose a mobile Internet architecture that is suitable for 2.5G and 3G wireless networks based on the evaluation and our experience with the i-mode service. Our architecture consists of wireless optimized TCP, TLS, HTTP and XHTML.