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.

Recently, SCTP is attracting attention to support mobility in the Internet because it does not require additional equipment such as the Home Agent of Mobile IP. This paper focuses on an SCTP fast handover mechanism using a single interface because it is assumed that small mobile devices have a single interface per communication medium such as IEEE802.11b due to hardware limitations. The proposed mechanism called SCTPmx employs a cross layer control information exchange system called LIES to predict handover. LIES was originally designed to achieve network layer fast handover and then it was extended by adding the network layer primitives for efficient interaction among the link layer, the network layer, and the transport layer. Prior to handover, SCTPmx can generate a new address that will be used after handover and can execute duplicate address detection of IPv6. SCTPmx can suppress the delay caused by channel scanning at the link layer by employing selective background scanning mechanism which allows to continue data communication during channel scanning. In addition, SCTPmx can notify the correspondent node of the new address before handover. SCTPmx was implemented on FreeBSD. SCTPmx achieved better than 25 times lower handover latency (100 msec) and 2 times higher throughput than previous proposals.

For integrated WLAN/cellular networks, we propose an energy-efficient vertical handover mechanism that both improves the energy efficiency of the mobile nodes and reduces the WLAN frame overhead.

When handover events occur during the transmission of multimedia traffic, efficient handover control procedures and radio resource allocation are necessary to maintain the same QoS of transmitted multimedia traffic because the QoS may be degraded by additional delay and information loss. In this paper we propose a new handover control method for the next generation mobile multimedia networks, in which the handover setup process is done in advance of a handover request by predicting the handover cell from mobile terminal's current position. The handover procedures for real-time sessions are performed based on the handover cell information and the resource reservation condition. The radio resources in the estimated adjacent cells should be reserved and allocated to guarantee the continuity of the real-time sessions. We conduct a simulation model that is focused on the handover failure rate and packet loss rate. The simulation results show that our proposed method provides better performance than the previous methods.

Channel scanning is an important aspect of seamless handovers since it is required in order to find a target point of attachment (PoA). However, channel scanning in single radio devices may cause severe service disruptions with the current PoA so that the provided QoS will be further degraded during a handover. In this letter, we propose a dynamic channel scanning algorithm that supports QoS. Simulation results show that the proposed mechanism reduces the service disruptions and provides the desired QoS to users during the scanning period.

This paper proposes a handover method that uses a combination of bicast and forwarding (BIFO) of IP packets to achieve a short handover delay. BIFO achieves a lower amount of IP packet traffic in the backhaul for future IP-based radio access networks (RANs) than the bicast only method. To validate the effect of the proposed BIFO, we implement prototype experimental equipment comprising a RAN_access router (RAN_AR), Node Bs associated with a radio control server (RCS), and user equipment (UE) including a controller. The experimental results show that BIFO achieves a shorter handover delay than the conventional bicast method or forwarding method by taking advantage of the respective merits of bicast and forwarding. The results also confirm that BIFO achieves the handover delay time in the control plane of approximately 10-20 msec.

The authors propose a user-centric seamless handover (HO) scheme, which is a kind of a vertical HO from a new perspective, toward a next generation network where heterogeneous access networks converge. The users' experience-oriented scheme allows users to enjoy the optimal service quality for real-time applications in respective access networks. In addition, the scheme sustains on-going sessions during the vertical HO. The proposed scheme consists of two methods -- the bicasting of Different Quality-level Streams (DiffQS) and the Delay Difference Absorption (DDA) method. Initially, the authors propose two plausible methods for the SIP-based bicasting of DiffQS. This document introduces a SIP-capable network element named the HO Assistive Server (HOAS). HOAS controls bicasting of DiffQS and provides users with the optimal service quality for real-time applications via respective access networks as well as avoiding packet loss during HO. The DDA method is also proposed to prevent a service interruption and smoothly continue a real-time service during HO. Evaluation results show that the scheme achieves the seamless service continuity from the users' perspective for HO between cellular and high-speed wireless access via the implementation of a prototype system.

We propose a velocity-based bicasting handover scheme for the efficient utilization of backhaul network resources in fourth-generation mobile systems. The original bicasting handover scheme adopts the mechanism of holding the data of a mobile station (MS) in all potential target base stations in advance, before the actual handover execution of the MS. The scheme minimizes the packet transmission delay caused by handover and achieves the goal of seamless connectivity, however, it results in an aggressive consumption of the backhaul network resources. Moreover, as the scheme gets widely adopted for high data rate real-time services and the demand for these services grows, the amount of the resources consumed due to the bicasting will increase tremendously. In this paper, we present a new bicasting handover scheme that reduces the data bicasting time, thereby improving the backhaul network resource utilization. Our scheme exploits the velocity parameter of MS and introduces a novel concept of bicasting threshold determined for the specific mobile speed groups. Simulations prove the efficiency of our scheme over the original one in overcoming the aggressive resource consumption at the backhaul network.

In this paper, we analyze existing vulnerabilities in handover for mobile WiMAX networks. To overcome these vulnerabilities, we propose a secure handover protocol that guarantees mutual authentication and forward/backward secrecy in handover. We present a formal analysis of our protocol using a logic-based formal method.

Packet delivery in Proxy Mobile IPv6 (PMIPv6) relies on an anchor node called LMA. All packets sent by a source node reach a receiver node via LMA, even though the two nodes attach to the same MAG. In some scenarios, PMIPv6 results in high delivery latency and processing costs due to this unnecessary detour. To address this issue, several PMIPv6 route optimization schemes have been proposed. However, high signaling costs and excessive delays remain when handover is performed. For this reason, we propose an enhanced PMIPv6 route optimization (EPRO) scheme. In addition, we analyze the performance of the EPRO. Analytical results indicate that the EPRO outperforms previous schemes in terms of signaling overhead and handover latency.

In this paper, we propose a new handover algorithm to guarantee handover quality in 4G mobile systems. The proposed algorithm limits the handover interruption time by improving the HARQ retransmission latency of the first packet transmitted from new serving cell. Through the simulations, we proved that our algorithm meets the requirement of handover interruption time for TCP services with high rate.

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.

This paper proposes a scheme for hard handover (HO) between base stations (BSs) that combines bicast with forwarding; it realizes packet-lossless HO as well as low HO control delay. The proposed scheme observes the status of the current channel condition and initiates bicasting, the simultaneous transfer of IP packets from the access router to both the old BS and the new BS, when the probability of HO becomes high; this reduces the control delay imposed by hard HO. When HO becomes unavoidable, only those IP packets remaining in the old BS buffer that are not shared with the new BS are forwarded to the MS; this prevents the loss of IP packets. Computer simulations show that the proposed scheme reduces the HO control delay at 95% cumulative distribution function (CDF) by approximately 1700 (300) msec, 340 (320) msec, and 170 (330) msec compared to the conventional forwarding scheme (the conventional bicast scheme) when the number of users is 80 and the maximum Doppler frequency (fdmax) is 5.55 Hz and data rate (D) on the wired propagation channel is 10, 50, and 100 Mbps, respectively. The results confirm the superiority of the proposed scheme as an IP packet loss prevention scheme for hard HO.

This letter proposes a secure and efficient handover authentication scheme that requires a light-weight Diffie-Hellman operation at mobile nodes. Our scheme provides more enhanced securities like the PFS, PBS, and so on than the existing security-context-transfer schemes. Also, the mobile node delegates the exponent operation for the DH to the access router to reduce computational cost on it.

In this paper, we propose a novel network selection algorithm considering power consumption in hybrid wireless networks for vertical handover. CDMA, WiBro, WLAN networks are candidate networks for this selection algorithm. This algorithm is composed of the power consumption prediction algorithm and the final network selection algorithm. The power consumption prediction algorithm estimates the expected lifetime of the mobile station based on the current battery level, traffic class and power consumption for each network interface card of the mobile station. If the expected lifetime of the mobile station in a certain network is not long enough compared the handover delay, this particular network will be removed from the candidate network list, thereby preventing unnecessary handovers in the preprocessing procedure. On the other hand, the final network selection algorithm consists of AHP (Analytic Hierarchical Process) and GRA (Grey Relational Analysis). The global factors of the network selection structure are QoS, cost and lifetime. If user preference is lifetime, our selection algorithm selects the network that offers longest service duration due to low power consumption. Also, we conduct some simulations using the OPNET simulation tool. The simulation results show that the proposed algorithm provides longer lifetime in the hybrid wireless network environment.

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.

This letter proposes an integrated authentication model for NGN to access the wired/wireless infrastructure. The existing authentication model based on port-based access control is not best suited for integrated network environments. The main focus of this paper is on developing a feasible authentication model using the authenticator positioned in the control function in the NGN.

Many handover techniques in the Internet have been introduced with the development of mobile computing technologies. Although many proposed handover schemes utilize multiple wireless interfaces, having multiple wireless interfaces in a mobile device increases its power consumption, device installation space, and hardware costs. We have been studying handover schemes for mobile nodes with a single wireless interface. To achieve seamless and efficient handover, we focus on Stream Control Transmission Protocol (SCTP) that offers a message-oriented, reliable and connection-oriented delivery transport service. Unlike other transport protocols like TCP, SCTP can provide an end-to-end handover mechanism with multi-homing feature. However, the handover mechanism in the current SCTP causes large handover latency particularly when a mobile node has only one single wireless interface. This paper investigates the current issues of the SCTP handover mechanism, and proposes a new efficient handover scheme based on SCTP, which identifies a communication path as a pair of source and destination address. Additionally, we modified SCTP behavior when an SCTP endpoint received a SET PRIMARY message to change primary destination of peer endpoint. This paper shows that our scheme can reduce the handover latency by two to thirty seconds.

In this paper, we discuss TCP performance in a wireless overlay network where wireless LANs and cellular networks are integrated. In the overlay network, vertical handover, where a mobile node changes its access link during a session, is one of the most important technologies. When a vertical handover occurs, throughput performance of a TCP flow is degraded due to not only packet losses during the handover, but drastic change of its bandwidth-delay product. In this paper, we propose an ACK-pacing mechanism for TCP congestion control to improve the performance degradation. The proposed system is receiver-driven, so no modification is required to the mechanism of TCP sender. In the proposed system, a TCP receiver adjusts a transmission rate of ACKs according to the relationship between bandwidth-delay products before and after a handover. Since the ACK-clocking mechanism of TCP adjusts the transmission rate of TCP segments, the TCP receiver can seamlessly adjust its congestion window size to the new bandwidth-delay product. Computer simulation results show that the proposed system can improve the TCP performance during the vertical handover.

This letter proposes a hierarchical key management scheme based on hash key chain for authentication of roaming mobile nodes in both intra-domain and inter-domain. The key management scheme uses a local master key concept for reducing the latency of the authentication procedure and the communication overhead between a home authentication server and an access point in the foreign domain. The proposed scheme also supports secure separation of the authentication key among local authentication servers using hash key chain.