Hiroyasu Obata

A study on primary path switching strategy of SCTP

As a variety of communication devices emerging, we can easily install two or more communication interfaces on a PC or a PDA and make a multi-home environment. To utilize this multi-home environment effectively, SCTP (stream control transmission protocol) has been proposed and standardized. Extending functions of the original SCTP, a strategy which can adapt SCTP to real-time communications has been proposed. However, the typical existing strategy has not enough considered the switching conditions from the viewpoint of transmission delay, so there is room to improve them. In this paper, we clarify the drawbacks of the existing strategy and their influences on the performance. Then, we propose a new strategy of path switching to reduce the delay and to increase the amount of data successfully delivered. The effect of the proposed strategy is discussed and confirmed through experimental evaluations.

TCP-STAR: TCP Congestion Control Method for Satellite Internet

Satellite Internet is one of the most important networks for emergency communications because of its tolerant of disasters such as earthquake. Therefore, satellite Internet has received considerable attention over recent years. However, most standard implementations of TCP congestion control method perform poorly in satellite Internet due to its high bit error rate and long propagation delay. This paper proposes a new TCP congestion control method called TCP-STAR to improve the throughput over satellite Internet. TCP-STAR has three new mechanisms, namely Congestion Window Setting (CWS) based on available bandwidth, Lift Window Control (LWC), and Acknowledgment Error Notification (AEN). CWS can resist the reduction of the transmission rate when data losses are caused by bit error. LWC is able to increase the congestion window quickly based on the estimated available bandwidth. AEN can avoid the reduction of the throughput by mis-retransmission of data. The mis-retransmission is caused by ack losses or delay. Simulations show that TCP-STAR can obtain the best throughput comparing with other TCP variants (TCP-J and TCP-WestwoodBR). Furthermore, we found that the fairness of TCP-STAR is a little lower than that of TCP-WestwoodBR. However, the fairness of TCP-STAR is equal to TCP-J.

A new TCP congestion control method considering adaptability over satellite Internet

Most standard implementations of congestion control method perform poorly in the satellite Internet due to both a high bit error rate and a long propagation delay. This paper proposes a new TCP congestion control method called TCP-STAR to improve both TCP performance and adaptability of network conditions over the satellite Internet. The performance and the adaptability of network conditions are most important metrics for assurance. TCP-STAR has three new mechanisms, namely congestion window setting based on available bandwidth (CWS), lift window control (LWC), and acknowledgement error notification (AEN). CWS can avoid the reduction of the transmission rate when data losses are caused by bit error. LWC is able to increase the congestion window quickly based on the estimated available bandwidth. AEN can avoid the reduction of the throughput by mis-retransmission of data. The mis-retransmission is caused by ack losses or delay. Simulation experiments show that TCP-STAR improves the throughput comparing with other TCP variants and exhibits the adaptability of network conditions. Furthermore, the fairness of TCP-STAR is better than that of other TCP variants.

TCP performance analysis on asymmetric networks composed of satellite and terrestrial links

As the Internet users increase, asymmetric networks which provide asymmetric bandwidth or delay for the uplink and downlink have become a great attraction. However asymmetric networks which use both terrestrial and satellite links at the same time have not been adequately investigated. Therefore, this paper proposes a new formula for TCP performance evaluation for the asymmetric networks. Using this evaluation formula, we calculate the throughput of TCP Reno over the asymmetric networks taking slow start into account. The calculation results are compared with the following: (1) the value based on an existing theoretical formula, (2) the outputs of simulation by NS (Network Simulator), and (3) the experimental results using VSAT (Very Small Aperture Terminal) satellite communication system for satellite links and the Internet for terrestrial links. As a result, it is shown that the new formula is more precise than the one already proposed.

Experimental Evaluation of TCP-STAR for Satellite Internet over WINDS

Satellite link speeds are now increasing in unison with those of terrestrial links. For example, the maximum link speed of WINDS (Wideband Internetworking engineering test and Demonstration Satellite) which was launched on 2008 is 1.2Gbit/s. WINDS is expected to support applications in education, medicine including telemedicine, and disaster countermeasures. Therefore, such satellite communication service is one of the key technologies for network assurance. TCP-STAR was proposed to improve the throughput over satellite Internet. However, no experimental evaluation of TCP-STAR over satellite links such as WINDS has been published. Therefore, this paper evaluates the performance of TCP-STAR by comparing with TCP-New Reno and TCP-Hybla over a network consisting of WINDS, HDR-VSAT, and portable VSAT. As a result, it is found that TCP-STAR and TCP-Hybla achieve better throughput when segment losses do not occur, while TCP-STAR achieves the best throughput when segment losses occur.

TCP congestion control method of improving friendliness over satellite Internet

The number of wireless Internet service users that use wireless LAN or satellite links has increased. The broadband satellite Internet services have especially attracted attention because of its characteristics such as global coverage and robustness. TCP (Transmission Control Protocol) is used by many typical applications over satellite Internet. However, a typical TCP (such as TCP-NewReno) which has been developed for wired networks performs poorly in wireless networks. Furthermore, the long propagation delay of satellite links decreases performance of the typical TCP. TCP-STAR has been proposed to solve these problems by modifying TCP congestion control method. TCP-STAR achieves high-speed communication by using the estimated bandwidth. However, if TCP-STAR coexists with the typical TCP, TCP-STAR tends to reduce throughput of the typical TCP. On the other hand, TCP-Fusion has been proposed for wired high-speed networks. TCP-Fusion which uses delay-based and loss-based congestion control method achieves scalability and friendliness to the typical TCP. However, TCP-Fusion cannot obtain high performance over satellite Internet, since TCP-Fusion is developed for wired high-speed links. In this paper, we propose a TCP congestion control method for improving friendliness over satellite links. The proposed method combines TCP-Fusions congestion control method and TCP-STARs congestion control method. We evaluate the performance of the proposed method over satellite Internet using ns2 (Network Simulator version 2). As a result, the proposed method achieves good friendliness when the proposed method coexists with the typical TCP. Furthermore, it is found that the proposed method utilizes bandwidth of satellite links well.

A TCP congestion control method for securing stable throughput

The important communications or real time applications over IP networks such as the Internet have received considerable attention over recent years. However, it is very difficult for the standard TCP to maintain QoS (i.e. bandwidth) which is demanded by important communications or real time applications, when multiple TCP connections share the bandwidth of a bottleneck link. Thus, it is useful for stable communication to secure bandwidth for such applications. Several TCP congestion control methods which maintain a target bandwidth at end hosts have been proposed. However, these methods become unstable when there are many background TCP connections or multiple TCP connections which try to secure bandwidth. Therefore, this paper proposes a TCP congestion control method for achieving predictable stable throughput by decreasing a target bandwidth dynamically, when there are many back ground TCP connections or multiple TCP connections which try to secure bandwidth. Simulation results show that the proposed method can maintain throughput more stably than existing methods in heavy network congestion.

Throughput Characteristics Evaluation of Media Access Control Method Based on Synchronization Phenomena of Coupled Oscillators over WLAN Coexisting of CSMA/CA Terminals

Wireless local area networks, based on the distributed coordination function of the IEEE802.11 standard, usually use carrier sense multiple access with collision avoidance (CSMA/CA) for media access control. In the CSMA/CA method, if the number of wireless terminals increases, data frame collision often occurs among wireless terminals, which results in a decrease in the total throughput. Therefore, to improve the total throughput, a media access control method based on the synchronization phenomena of coupled oscillators, which we refer to as SP-MAC, is proposed. However, the performance of terminals based on SP-MAC, which coexist with CSMA/CA terminals, has not been sufficiently evaluated. Therefore, in this study, we evaluate the throughput performance in detail when terminals based on SP-MAC coexist with those based on CSMA/CA.

An access point selection mechanism based on cooperation of access points and users movement

Public areas, such as train stations and airports, providing wireless Local Area Network (WLAN) services are increasing and expanding because of the rapid development of WLANs based on IEEE 802.11 standard. Moreover, because of the advances in smartphone tethering technology, portable access points (APs) such as mobile Wi-Fi routers are being utilized more frequently. Consequently, there are increasing circumstances where a user needs to select and connect to one of the many APs. AP selection significantly determines the quality of service of the subsequent communication session. Existing AP selection algorithms consider user movement but not AP movement. We propose an AP selection method that handles both types of movement effectively. Moreover, we show that the proposed method improves the throughput significantly compared to the existing method.

Setting Radio Transmission Range Using Target Problem to Improve Communication Reachability and Power Saving

Ad hoc networks can be composed entirely of mobile wireless terminals, and do not require permanent network infrastructure such as access points. They are considered a useful network configuration technology for various situations. For example, they are used to construct sensor networks in which distributed, inexpensive sensors monitor environmental conditions such as temperature and humidity. Further, ad hoc networks can be implemented after severe disasters that have disabled other network infrastructures. In general, ad hoc network terminals are battery powered. Therefore, extending network lifetime by reducing terminal power consumption is an important issue in ad hoc network management. One method for reducing power consumption involves reducing the radio transmission range of each terminal. However, reducing the radio transmission range causes degradation in the reachability of each terminal. In this paper, we propose a method to set ad hoc network radio transmission ranges using a Target problem, to reduce power consumption and increase each terminal’s reachability. Next, we evaluate our method using various routing protocols, and define the applicability of our proposed method for each protocol. Simulation results show that the proposal improves communication reachability and power savings in ad hoc networks with normally distributed terminals, when the Destination-Sequenced Distance-Vector (DSDV) routing protocol is used.