Hiroyuki Koga

Path selection using active measurement in multi-homed wireless networks

The mobile Internet is built upon a number of different wireless access networks with widely varying features in terms of coverage area, bandwidth, packet loss, and delay. To move across these different networks smoothly, issues associated with the changing features need to be addressed. In this paper, a path selection method for the coverage overlap area is proposed in which the mobile host actively measures the round trip time (RTT) and bottleneck bandwidth for each path and a path is selected based on four rules.

Performance evaluations of DCCP for bursty traffic in real-time applications

The Datagram Congestion Control Protocol (DCCP) has been proposed as a transport protocol which supports real-time traffic using window-based flow control. We investigate the DCCP performance for various traffic flows, focusing on how DCCP flows affect TCP flows and vice versa. Through those simulations, we examine an unfair bandwidth distribution problem caused by the incompatibility of DCCP with the fast recovery algorithm of TCP.

A framework for network media optimization in multihomed QoS networks

Future pervasive networks will be constructed by various access networks, which are managed by different operators. Wireless LAN hotspot services will be available in numerous locations, and cellular services, which provide a wide coverage area, will also be available. In order to realize efficient mobile computing in such an environment, mobile hosts must have multiple network media as well as functions to select the optimum network among them based upon the current situation without the disconnecting communications. In order to provide mobile hosts with such a function, we propose an association layer, which is inserted between the transport and network layers. The association layer selects the appropriate network medium for each flow according to the network conditions and flow characteristics, providing end-to-end QoS in multihomed networks. Furthermore, we show the effectiveness of the proposed scheme through simulation results and demonstrate proof of concept based on empirical evaluations of a prototype implementation.

TCP flow control using link layer information in mobile networks

Mobile Networks have been expanding and IMT-2000 further increases their available bandwidth over wireless links. However, TCP, which is a reliable end-to-end transport protocol, is tuned to perform well in wired networks where bit error rates are very low and packet loss occurs mostly because of congestion. Although a TCP sender can execute flow control to utilize as much available bandwidth as possible in wired networks, it cannot work well in wireless networks characterized by high bit error rates. In the next generation mobile systems, sophisticated error recovery technologies of FEC and ARQ are indeed employed over wireless links, i.e., over Layer 2, to avoid performance degradation of upper layers. However, multiple retransmissions by Layer 2 ARQ can adversely increase transmission delay of TCP segments, which will further make TCP unnecessarily increase RTO (Retransmission TimeOut). Furthermore, a link bandwidth assigned to TCP flows can change in response to changing air conditions to use wireless links efficiently. TCP thus has to adapt its transmission rate according to the changing available bandwidth. The major goal of this study is to develop a receiver-based effective TCP flow control without any modification on TCP senders, which are probably connected with wired networks. For this end, we propose a TCP flow control employing some Layer 2 information on a wireless link at the mobile station. Our performance evaluation of the proposed TCP shows that the receiver-based TCP flow control can moderate the performance degradation very well even if FER on Layer 2 is high.

Out-of-sequence packet arrivals due to layer 2 ARQ and its impact on TCP performance in W-CDMA networks

Data transmission over wireless networks has attracted more attention with the growth of the Internet and wireless networks. However, TCP is tuned to perform well in wired networks where the error rates are very low and packet losses occur mostly because of congestion. Therefore, the performance of TCP can be severely affected by high bit error rate characterizing wireless links. We clarify the performance of TCP over wireless links of IMT-2000 in which selective-repeat ARQ is performed for recovering lost PDUs of Layer 2 on the links. ARQ can cause out-of-order delivery of SDUs, which will result in TCP performance degradation. In particular we examine the impact of the timeout timer employed to maintain sequence integrity.

Multi-path Transmission Algorithm for End-to-End Seamless Handover across Heterogeneous Wireless Access Networks

In mobile networks, new technologies are needed to enable mobile hosts to move across various kinds of wireless access networks. In the past, many researchers have studied handover in IP networks. In almost all cases, special network devices are needed to maintain the host’s mobility. However, in these technologies, mobile hosts cannot move across different wireless access networks without closing the connection and degrading the goodput. To overcome these, we propose here a multi-path transmission algorithm for end-to-end seamless handover. The main purpose of this algorithm is to improve the goodput during handover by sending the same packets along multiple paths, reducing unnecessary consumption of network resources. We evaluate our algorithm through simulations and show that mobile hosts gain a better goodput.

Receiver-based flow control mechanism with interlayer collaboration for real-time communication quality in W-CDMA networks

Mobile networks are becoming increasingly prevalent, and this has led to an increase in the bandwidth available over wireless links in IMT-2000. Both non-real-time forms of communication, such as e-mail and web browsing, and real-time forms of communication, such as audio and video applications, are well suited to wireless networks. However, wireless networks are subject to relatively long transmission delays because of the need to recover lost packets caused by high bit error rates. This degrades the quality of real-time communications. Therefore, in the present paper, we propose a receiver-based flow control mechanism employing an interlayer collaboration concept to improve the quality of real-time communications without adversely affecting the performance of non-real-time communications on IMT-2000 networks. In addition, simulations are performed in order to evaluate the performance of the proposed mechanism and demonstrate its effectiveness.

Performance evaluation of TCP under dynamic allocation scheme for down‐link transmission rate in W‐CDMA systems

IMT-2000 has attracted much attention as the Next Generation Mobile Communication System. IMT-2000 can provide high-bit-rate data communication service, so that a great number of packets conveyed by TCP (transmission control protocol) can be transmitted over a wireless link. W-CDMA, standardized in 3GPP (which standardizes IMT-2000), allows dynamically allocating transmission rates to flow over a wireless link in response to a changing FER for each flow, which is thought to be essential for a next generation mobile communication system. Therefore, in this paper, we study the characteristics of the dynamic allocation scheme when TCP flows share a wireless link, and, in particular, we focus on the throughput performance of these TCP flows. First, we use simulations to examine the effectiveness of the dynamically allocating down-link transmission rate for TCP flows in response to changing the frame error rate (FER). Through the simulation results, we will show how it can improve the total throughput performance of TCP flows. Furthermore, we can obtain an effective way to allocate transmission rates to flows with different FERs in order to achieve high total throughput. Finally, we will deal with a case of multiple flows from a fixed host to a mobile host. In actual networks, this often happens. In this case, we will show that the total throughput of TCP flows degrades less than in the single-flow case, even when the FER is high. Copyright

Performance evaluation of TCP under dynamic allocation scheme for down link transmission rate in W-CDMA systems

IMT-2000 has attracted strong attention as the Next Generation Mobile Communication System. That can provide high-bit-rate data communication service, so that a great number of packets conveyed by TCP will be transmitted over a wireless link. W-CDMA standardized in 3GPP (standardizing IMT-2000) allows dynamic allocation of transmission rates to flows over a wireless link in response to changing link condition for each flow, which is thought to be essential for next generation mobile communication system. Therefore, in this paper, we study the characteristics of the dynamic allocation scheme when TCP flows share a wireless link, in particular by focusing on the throughput performance of those TCP flows. First, we examine the effectiveness of the dynamic allocation of down link transmission rate for TCP flows in response to changing FER by means of simulations. Through simulation results, we will show how it can improve the total throughput performance of TCP flows. Furthermore, we can obtain an effective way to allocate transmission rates to flows with different FERs in order to achieve high total throughput. Finally, we will deal with a case of multiple flows from a fixed host to a mobile host. In actual networks, it will very often happen. In this case, we will show that the total throughput of TCP flows less degrades than in the above single flow case even when FER becomes high.