In ubiquitous networks, Mobile Nodes (MNs) often suffer from performance degradation due to the following two reasons: (1) reduction of signal strength by the movement of an MN and intervening objects, and (2) radio interference with other WLANs. Therefore, handover initiation based on quick and reliable detection of the deterioration in a wireless link condition arising from the above two reasons is essential for achieving seamless handover. In previous studies, we focused on a handover decision criterion and described the problems related to the two existing decision criteria. Furthermore, we showed the effectiveness of the number of frame retransmissions through simulation experiments. However, a comparison of the signal strength and the number of frame retransmissions could not be examined due to the unreliability of the signal strength in simulations. Therefore, in the present paper, by employing FTP and VoIP applications, we compare the signal strength and the number of frame retransmissions as a handover decision criterion with experiments in terms of (1) and (2) in a real environment. Finally, we clarify the problem of the signal strength in contrast to the effectiveness of the number of frame retransmissions as a handover decision criterion.

In ubiquitous networks based on Wireless Local Area Networks (WLANs) with limited individual coverage, mobile nodes will be likely to traverse different WLANs during TCP communication. An effective handover management scheme for achieving seamless and efficient communication throughout the handover operation is therefore crucial. To achieve this, the following three requirements are essential: (i) early initiation of handover, (ii) elimination of communication interruption upon handover, (iii) selection of an optimal WLAN. The handover scheme proposed in this study employs frame retransmission over WLAN as an indicator of link degradation, and a handover manager (HM) on the transport layer obtains the number of frame retransmissions on the MAC layer using a cross-layer architecture in order to achieve (i) and (iii). Then, it also employs multi-homing in order to achieve (ii). Simulations demonstrate that the proposed scheme can satisfy all of the three requirements and is capable of maintaining TCP performance throughout the handover operation.

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 AP with better performance and (2) execute a handover seamlessly. To satisfy requirement (2), we proposed a seamless handover method in a previous study. Moreover, in order to achieve (1), the Received Signal Strength Indicator (RSSI) is usually employed to measure wireless link quality in a WLAN system. However, in a real environment, especially if APs are densely situated, it is difficult to always select an AP with better performance based on only the RSSI. This is because the RSSI alone cannot detect the degradation of communication quality due to radio interference. Moreover, it is important that AP selection is completed only on an MN, because we can assume that, in ubiquitous WLANs, various organizations or operators will manage APs. Hence, we cannot modify the APs for AP selection. To overcome these difficulties, in the present paper, we propose and implement a proactive AP selection method considering wireless link condition based on the number of frame retransmissions in addition to the RSSI. In the evaluation, we show that the proposed AP selection method can appropriately select an AP with good wireless link quality, i.e., high RSSI and low radio interference.

Switching a communication path from one Access Point (AP) to another in inter-domain WLANs is a critical challenge for delay-sensitive applications such as Voice over IP (VoIP) because communication quality during handover (HO) is more likely to be deteriorated. To maintain VoIP quality during HO, we need to solve many problems. In particular, in bi-directional communication such as VoIP, an AP becomes a bottleneck with the increase of VoIP calls. As a result, packets queued in the AP buffer may experience a large queuing delay or packet losses due to increase in queue length or buffer overflow, thereby causing the degradation of VoIP quality for the Mobile Nodes (MNs) side. To avoid this degradation, MNs need to appropriately and autonomously execute HO in response to the change in wireless network condition, i.e., the deterioration of wireless link quality and the congestion state at the AP. In this paper, we propose an HO decision strategy considering frame retries, AP queue length, and transmission rate at an MN for maintaining VoIP quality during HO. Through simulation experiments, we then show that our proposed method can maintain VoIP quality during HO by properly detecting the wireless network condition.

In order to prevent the degradation of TCP performance while traversing two WLANs, we present an implementation design of an inter-domain TCP handover method based on cross-layer and multi-homing. The proposed handover manager (HM) in the transport layer uses two TCP connections previously established via two WLANs (multi-homing) and switches the communication path between the two connections according to the handover trigger and the comparison of new/old APs. The handover trigger and comparison are conducted by assessing the wireless link quality using the frame-retry information obtained from the MAC layer (cross-layer). In a previous study, we proposed a preliminary concept for this method and evaluated its functional effectiveness through simulations. In the present study, we design an implementation considering a real system and then examine the effective performance in a real environment because a real system has several system constraints and suffers from fluctuations in an actual wireless environment. Indeed, depending on the cross-layer design, the implementation often degrades the system performance even if the method exhibits good functional performance. Moreover, the simple assessments of wireless link quality in the previous study indicated unnecessary handovers and inappropriate AP selection in a real environment. Therefore, we herein propose a new architecture that performs cross-layer collaboration between the MAC layer and the transport layer while avoiding degradation of system performance. In addition, we use a new assessment scheme of wireless link quality, i.e., double thresholds of frame retry and comparison of frame retry ratio, in order to prevent handover oscillation caused by fluctuations in the wireless environment. The experimental results demonstrate that the prototype system works well by controlling two TCP connections based on assessments of wireless link quality thereby achieving efficient inter-domain TCP handover in a real WLAN environment.

In future mobile networks, new technologies will be needed to enable a mobile host to move across heterogeneous wireless access networks without disruption of the connection. In the past, many researchers have studied handover in such IP networks. In almost all cases, special network devices are needed to maintain the host's mobility. Moreover, a host cannot move across heterogeneous wireless access networks without degradation of the goodput for real-time communication, although a mobile host with multiple network interfaces can connect to multiple wireless access networks. For these reasons, we consider that a mobile host needs to manage seamless handover on an end-to-end basis. In this paper, we propose 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, minimizing unnecessary consumption of network resources. We evaluate our algorithm through simulations and show that a mobile host gains a better goodput.

A prompt rescue is a serious operation when a catastrophic disaster occurs. In an uncommunicable area where the existing communication systems are damaged, it is, however, difficult to collect SOS messages of victims. So far, we have proposed a smartphone application named SOSCast to collect SOS messages via device-to-device transmission in such an area. However, with the limitation of coverage area and battery consumption, it decreases the possibility of finding the victim due to the risk of losing the SOS messages. In this paper, thus, we propose an information-sharing cluster to virtually extend the communication coverage area and to secure the SOS messages by reducing the battery consumption. In the performance evaluation, compared with the original SOSCast, we showed that the proposed method can reduce battery consumption to secure the message through a prototype system and simulation experiments.