Hakyung Jung

SNC: a selective neighbor caching scheme for fast handoff in IEEE 802.11 wireless networks

Mobility support is one of the most challenging issues in IEEE 802.11 networks. In the proactive neighbor caching (PNC) scheme, when a mobile host is connected to an access point (AP), its context (e.g. security association or QoS information) is propagated in advance to all of the APs neighbors to reduce handoff processing time. In this paper, we propose a selective neighbor caching (SNC) scheme, which propagates a mobile hosts context only to the selected neighbor APs considering handoff patterns. Therefore, the SNC scheme can reduce the message overhead on the links among APs. We evaluate the performance of the SNC and PNC schemes in terms of the cache hit probability and the signaling cost. Especially, we investigate the effect of mobility and cache size through extensive simulations. The results reveal that the SNC scheme provides a comparable cache hit probability while significantly reducing the signaling overhead in IEEE 802.11 networks. Moreover, although the SNC propagates relatively a small number of contexts to neighbor APs, the SNC scheme outperforms the PNC scheme when the cache size is small and the mobility is low.

How can an ISP merge with a CDN

As delivering contents has become the dominant usage of the Internet, efficient content distribution is one of the hottest research areas in the network community. In future networks, it is anticipated that network entities such as routers will be equipped with in-network storage due to the trend of ever decreasing storage cost. In this article, we propose a novel content delivery architecture called ISP-centric content delivery (iCODE) by which an ISP can provide content delivery services as well. iCODE can provide efficient content delivery services since an ISP can cache contents in routers with storage modules considering traffic engineering and the locality of the content requests. Compared to CDN and P2P systems, iCODE can offer reduced delivery latency by placing the contents closer to end hosts, and incentives to ISPs by reducing inter- ISP traffic and allowing traffic engineering. We also discuss the technical and business issues to realize the iCODE architecture.

A selective neighbor caching scheme for fast handoff in IEEE 802.11 wireless networks

Mobility support in IEEE 802.11 networks is a challenging issue. Recently, a new scheme, called proactive neighbor caching (PNC), was proposed and adopted as an IEEE standard. The PNC scheme introduces a neighbor graph, which dynamically captures the mobility topology of a wireless network for pre-positioning the context of a mobile host (MH). However, the PNC scheme may result in a significant signaling overhead because the MHs context is propagated to all neighbor access points (APs). We propose a selective neighbor caching (SNC) scheme, which propagates an MHs context to only the selected neighbor APs considering handoff frequencies between APs. When the context transfer is needed, neighbor APs with handoff probabilities equal to or higher than a predefined threshold value are selected. We also derive an optimal threshold value when the target cache hit probability is given. Simulation results reveal that the SNC scheme significantly reduces the signaling overhead while guaranteeing a comparable cache hit probability compared to the PNC scheme.

Guaranteeing the network lifetime in wireless sensor networks: A MAC layer approach

Energy is the most crucial but scarce resource in wireless sensor networks (WSNs). A wealth of MAC protocols are proposed only to prolong the network lifetime for energy-efficiency; whereas, others are tailored to reduce end-to-end latency in addition to extending the network lifetime. Since the requirements depend on the applications of the network, it would be difficult to design a single MAC that satisfies the wide range of applications. Specifically, some WSNs are required to survive for a certain lifetime because of too expensive deployment cost (e.g., harsh or hostile environments) to replace the energy-depleted sensor nodes. Furthermore, the portion of surviving sensor nodes can be a critical factor to satisfy the quality of surveillance (QoSv) requirements. In this paper, we propose a new adaptive MAC (A-MAC) protocol that not only guarantees the pre-configured network lifetime but also reduces the end-to-end latency. Basically, each sensor node adapts its duty cycle depending on the traffic load it suffers. By doing so, the energy consumption rate of each node approaches to the ideal energy consumption rate, which enables the node to survive for the pre-configured lifetime. Also, if a node suffers relatively less load, it can reduce the sleep delay by increasing its duty cycle. Analysis and simulation results exhibit that the proposed protocol shows less delay than S-MAC [W. Ye, J. Heidemann, D. Estrin, Medium access control with coordinated adaptive sleeping for wireless sensor networks, in: IEEE/ACM Transactions on Networking, vol. 12, No. 3, June 2004.] while meeting the network lifetime requirements.

REACT: Rate Adaptation using Coherence Time in 802.11 WLANs

The channel coherence time in indoor WLANs normally exceeds multiple frame transmission times. In light of this, we propose a new rate adaptation scheme, termed as the Rate Adaptation using Coherence Time (REACT), that has the following key features. First, without exchanging RTS and CTS frames, the receiver in REACT informs the transmitter of the improved channel condition via altering the ACK transmission rate, so that the transmitter increases the data rate for subsequent data frames. This enables the transmitter to adapt to the time-varying channel conditions while inducing the marginal overhead. Second, the transmitter in REACT can identify the reasons of frame losses by exploiting the feedback from the receiver and the estimated coherence time. Frame losses are assumed to be caused only by collisions for the duration of the coherence time after receiving an ACK frame with the altered bit rate. The coherence time is also used to enhance the adaptive RTS probing, so that the REACT can prevent the transmitter from decreasing its bit rate when collisions occur. Extensive simulations reveal that REACT consistently performs better than the other rate adaptation schemes (ARF, CARA, RRAA, and RBAR) in all the testing scenarios.

Testbed Experimentation of a Meshed Tree Routing with Local Link State for Wireless PAN Mesh

This paper focus on the testbed experimentation of a meshed tree routing algorithm with local link state for wireless personal area networks (WPANs) based on current IEEE 802.15.4 MAC and PHY. The meshed tree uses a block addressing scheme based on tree structure and local link state information for mesh routing. Our approach exploits the information from the global tree structure for the direction of packet forwarding and local link state for choosing the next hop toward the destination. It has two prominent features: scalability and fault tolerance. Each node maintains a local link state of k-hop(usually 2-hop) information regardless of network size, which makes the approach scalable. Being a mesh, our approach shows good fault tolerance and load balancing. Testbed experiments show that the meshed tree displays superior performance when compared with AODV and a tree based algorithm. Comparisons were made with respect to packet deliver ratio, energy consumption and memory usage. The proposed algorithm is being considered as the routing algorithm for IEEE 802.15.5 WPANMesh standard.

Vertical handovers in multiple heterogeneous wireless networks: a measurement study for the future internet

As the access patterns of mobile users are diverse and their traffic demand is growing, multiple wireless access networks become dominant and their coexistence will be the norm in the future Internet infrastructure. To evaluate protocols and algorithms in these heterogeneous wireless networking environments, testbed-based experiments are of crucial importance since mathematical modeling and simulation cannot reflect the high complexity of systems and wireless link dynamics sufficiently. Leveraging femtocell technologies, we propose and build a testbed in which WiFi access points and WiMAX base stations are integrated. We also implement the vertical handover functionality through the SIP protocol, and carry out comprehensive measurements to analyze vertical handover delays. The testbed measurements of vertical handovers reveal that the DHCP mechanism, the authentication process in WiMAX, and the probing process in WiFi incur substantial delay.

Distinguishing collisions from low signal strength in static 802.11n wireless LANs

This paper proposes how to figure out whether frame losses are due to collisions or low signal strength. We observe subframe losses have distinct patterns through experiments, and devise a simple criterion which effectively distinguishes collisions from weak signals.

A Scalable Rate Adaptation Mechanism for IEEE 802.11e Wireless

Although many rate adaptation schemes have been introduced to efficiently utilize the multiple transmission rates of the IEEE 802.11 standard, automatic rate fallback (ARF) is the most widely implemented scheme on the market because of its simplicity. Our study reveals that the ARF malfunctions severely when it is used over IEEE 802.11e WLANs. The reason is that the intolerably shortened range of contention window for voice access category induces frequent collisions even when a small number of users contend. This paper demonstrates the inefficiency of the ARF using an analytic model and proposes a new rate adaptation scheme that maintains scalability. Simulation reveals that our scheme performs consistently well compared to the ARF scheme.

Multipath transmission architecture for heterogeneous wireless networks

As growth of network technology and device technology, smart devices equipped multiple network interfaces now are increasingly emerging, and also these are able to communicate with other devices and services using the multiple networks simultaneously. Currently, transmission method using concurrent multiple paths has been studied in IETF, however there are several problems for performance degradation when traffic flows are transmitted through multiple paths over wireless networks. For instance, mobile user moving fast to different wireless network is occurring to frequently re-establish new sub-paths, so that overall throughput could be degraded even though communication session is maintained. Therefore, the paper identifies above problems and introduces new multipath transmission support architecture for heterogeneous wireless networks. The architecture introduces a novel entity ‘Multipath Anchor (MA)’, which supports not only multipath transmission but also deployabiliy by proxing and relaying packets to the receiver. Basic transmission process and handover process are describes. And we also discuss about deployment issues to realize the architecture, so MA can be installed by 3rd party. In the future works, the architecture will be implemented based on NetFPGA, and scheduling and traffic division policy will be proposed.