Yuzo Taenaka

Terminal-centric ap selection algorithm based on frame retransmissions

In the near future, wireless local area networks (WLANs) will overlap to provide continuous coverage over a wide area. In such ubiquitous WLANs, a mobile node (MN) moving freely between multiple access points (APs) requires not only permanent access to the Internet, but also continuous communication quality during handover. In order to satisfy these requirements, an MN needs to (1) select an optimal AP and (2) execute a handover seamlessly. Requireing (2), we have proposed a seamless handover method in previous work. In this paper, in order to achieve (1), we propose and implement a terminal-centric AP selection algorithm. In our proposed AP selection method, an MN investigates the wireless link condition of an AP based on the number of frame retransmissions in addition to RSSI. In our demonstration, we show that the number of frame retransmissions has the potential to be an effective decision metric for AP selection.

An efficient handover decision method based on frame retransmission and data rate for multi-rate WLANs

To enhance the communication performance at handover between multi-rate WLANs, we propose a new handover decision method that can be applied to our previously reported handover management scheme, which handled a handover by utilizing two WLAN interfaces (IFs) through cross-layer collaboration between layer 2 and layer 4. It should be noted that we here propose a new handover decision scheme for traversing between multi-rate WLANs, while our previous decision scheme works only in fixed-rate WLANs. In this paper, to treat a handover between multi-rate WLANs, we employ two kinds of information: (1) the most frequently used data rate (MFDR) for assessing the stable communication performance of a multi-rate WLAN, and (2) the frame retransmission ratio (FRR) for assessing its exact communication performance. The MFDR enables us to estimate the area where we should start handover. If the MFDRs of two interfaces are same in the area, the FRR allows us to compare the wireless condition on the two interfaces precisely to give an optimal handover point. Through simulation experiments, we show that our proposed scheme certainly estimates an appropriate handover point as a result of multi-path transmission (s), thereby providing handover successfully. That is, the proposed method can determine handover at an optimal point depending on the various distances between access points, the mobile node (MN) velocity, and the MN moving pattern. Moreover, our proposed scheme prevents the redundant network load caused by multi-path transmission as much as possible, thereby providing the ideal TCP communication performance.

Design and Implementation of Cross-layer Architecture for Seamless VoIP Handover

In the near future, wireless local area networks (WLANs) will overlap to provide continuous coverage over a wide area. In such ubiquitous WLANs, a mobile node (MN) freely moves between WLANs with different IP subnets during VoIP communication. In such situations, since an MN experiences several handovers, the communication quality is degraded. In previous studies, in order to solve this problem, we proposed a seamless handover scheme based on frame retransmissions and demonstrated its effectiveness through simulation experiments. However, no existing scheme has demonstrated the effectiveness of cross-layer architecture exploiting the number of frame retransmissions on a real system. In the present paper, therefore, we design and implement a handover scheme based on the number of frame retransmissions. In our implementation, we propose a cross-layer architecture using a shared memory to pass the information, i.e., the number of frame retransmissions, from MAC layer to Transport layer. Finally, we show preliminary results in a real wireless environment and evaluate the performance of the proposed prototype system in a simple topology.

MS-initiated handover decision criteria for VoIP over IEEE 802.16e

The emerging mobile WiMAX (IEEE 802.16e) has gained serious attention as a means of providing wireless broadband access to mobile users. 802.16e supports high data rates and QoS for various applications, and is thus a powerful contender for next generation wireless access technology. On the other hand, IEEE 802.11 a/b/g/n have already been widely used to provide high data rates in a limited area. Therefore, in the near future, 802.16e and 802.11 will co-exist and a multi-homed mobile station (MS) will be likely to execute many handovers (HOs) due to free movement among 802.16es hotzones and 802.11s hotspots. Many studies of HO decision criteria for 802.11 have been conducted but, so far, HO decision criteria for 802.16e have not yet been sufficiently studied, in particular that for MS-initiated HO in 802.16e. In this paper, we propose MS-initiated HO decision criteria for VoIP over 802.16e considering wireless link condition and congestion state of the 802.16e network, i.e, CINR and MS queue length. Our simulation results show that the combined use of MS queue length and CINR can be a proper MS-initiated HO decision criterion for VoIP over 802.16e.

Network capacity expansion methods based on efficient channel utilization for multi-channel wireless backbone network

This study presents new channel utilization methods for a multi-channel wireless backbone network (WBN). To flexibly and efficiently use multiple channels, we exploit OpenFlow as a baseline function. In this paper, we propose a new framework to build an OpenFlow-based multi-channel WBN and then implement two new channel utilization methods: (i) a flow direction aware channel assignment method (DCA) and (ii) a flow balance aware channel assignment method (BCA). BCA is classified into two simple multi-hop transmission procedures: (ii-1) each flow is transmitted on a persistent channel (BCA-PC), while (ii-2) being transmitted on a different channel for each hop (BCA-DC). Finally, we evaluate the proposed channel utilization methods in a real testbed. As a result, our methods can obtain approximately three times as much network capacity as the conventional WBN. In addition, BCA-DC can avoid the radio interference with neighbor hops effectively, thereby not only achieving efficient channel utilization but also expanding network capacity significantly.

An implementation design of a cross-layer handover method with multi-path transmission for VoIP communication

This paper presents an implementation design of a handover method for VoIP communication, namely Frame-retry-based handover method with Multi-path Transmission (FMT). Because VoIP communication significantly suffers from packet losses occurring at handover, the FMT uses the frame-retry information obtained from the MAC layer through a cross-layer architecture as a handover trigger and then performs handover accordingly with multi-path transmission to reduce packet losses around the handover. In the previous paper, we proposed the FMT and examined its theoretical effectiveness in extensive simulations. In this work, we design the implementation and examine the effective performance in a real environment because typically there are some system constraints in an OS. Actually, depending on design, the implementation can often degrade the system performance even if the method shows good theoretical performance in mathematical analysis and simulation. In this paper, we first show the implementation design of the FMT on a Linux OS. This includes a novel asynchronous cross-layer design to achieve the collaboration between the two layers (i.e., the MAC and transport layer) while avoiding degradation of system performance. We then provide an implementation of all necessary functions for handover operations supporting VoIP communication by using the proposed cross-layer architecture. We finally examine the prototype system in a real environment in terms of effectiveness of the frame retry-based handover trigger, the multi-path transmission, and voice quality by comparing the FMT with comparative methods. The experimental results show that the FMT efficiently maintains voice quality with low additional network load in a real environment.

Pro-Reactive Route Recovery with Automatic Route Shortening in Wireless Ad Hoc Networks

In this paper, we propose a relay recovery route maintenance protocol for ad hoc networks to combine the benefits of both proactive and reactive route recovery strategies and to minimize their drawbacks. In our proposal, one or more substitute routes usually become ready for the recovery of every link in a route before its break, while the route recovery process actually starts only when the upstream node of a link confirms the link break. Since this scheme does not broadcast any control packet, it can effectively recover a broken link without heavy control overhead traffic. Also, it helps reduce the time delay due to the recovery, since substitute routes are already available when the upstream node initiates the route recovery process. We further propose two automatic route shortening schemes to optimize the route during successful packet forwarding without causing extra control overhead. We have implemented our proposed schemes based on AODV and compared their performance with competitive schemes including original AODV. Simulation results demonstrate that our proposal definitely reduces the time delay and control overhead traffic in route repairing process, and that the route shortening schemes further leads to shorter time delay and average route length.

A Radio Interference Aware Dynamic Channel Utilization Method on Software Defined WMN

This paper proposes a dynamic channel utilization method enabling to consider the effect of radio interference on software defined multi-channel wireless mesh network (SD-WMN). In our SD-WMN, a set of APs, each of which uses a single and different channel, is connected by Ethernet each other and thus constructs a VAP handling multiple channels. The OpenFlow technology, which enables us to programmably control traffic, is employed to effectively utilize multiple channels in SD-WMN for improvement of the network capacity. Although we already proposed several channel utilization methods, either channel may be saturated even if other channels still have residual capacity. This is because the radio interference inevitably occurs inside WMN due to multi-hop transmission nature. Thus, to avoid the saturation of either channel while utilizing all channels effectively, we propose a new method that can consider the effect of radio interference inside WMN. In the method, all interfered APs can be reliably detected and the network load is balanced in conjunction with the interfered APs by considering the radio interference. From the experimental results, the proposed method can avoid the saturation of either channel successfully, thereby increasing the network capacity drastically.

An Adaptive Channel Utilization Method with Traffic Balancing for Multi-hop Multi-channel Wireless Backbone Network

This chapter presents an effective channel utilization method for a multi-hop wireless backbone network (WBN) constructed with multiple channels. In the previous work, we proposed a novel OpenFlow based management framework for the WBN, which enables access points to handle the unlimited number of channels simultaneously. Since the framework also enables us to easily and programmably use channels for packet forwarding, the utilization of multiple channels can be potentially optimized, thereby maximizing the network capacity of the WBN. However, since the previous work focused on the framework, the effective use of multiple channels was not completely addressed. Therefore, in this chapter, we propose a channel utilization method that balances the amount of traffic among multiple channels to maximize the network capacity. Through the performance evaluation in a real testbed, the proposed method can effectively use all channels in a 3-hop WBN.

Seamless handover management scheme under multi-rate WLANs

In ubiquitous Wireless LANs (WLANs), Mobile Nodes (MNs) are likely to experience many and frequent handovers between WLANs managed by different organizations/ISPs during TCP communication. Although we proposed a WLAN handover management scheme based on the number of frame retransmissions, the proposed scheme cannot adapt to multirate WLAN used in a more realistic environment. Furthermore, the handover is controlled based on only a predetermined threshold of the number of frame retransmissions, thereby degrading the performance drastically when the inappropriate threshold is employed. Therefore, in this paper, to enhance the practicality of handover scheme in a realistic environment where WLAN supports multiple data rates and automatically changes the rate in response to wireless link condition, we propose a handover management scheme adaptable to multi-rate WLANs. Our scheme exploits two sorts of information to recognize the wireless condition appropriately: (1) data rate used most frequently (DRMF) by each interface and (2) frame retransmission ratio (FRR) on each interface for some duration. The former of two criteria first enables us to estimate an area where we should start handover, and if DRMFs of two interfaces are same in the area, the latter then allows us to compare wireless condition on two interfaces precisely, thereby giving an optimal handover point. The simulation results demonstrate the advantages of the proposed method, especially in the multi-rate WLAN environment.