Dong-young Kim

A Cross-Layer Approach for Per-Station Fairness in TCP over WLANs

In this paper, we investigate the issue of per-station fairness in TCP over IEEE 802.11-compliant wireless local area networks (WLANs), especially in Wi-Fi hot spot. It is asserted that the hot spot suffers from the unfairness among stations in exploiting the wireless medium. The source of this unfairness is analyzed from two aspects, TCP-induced asymmetry and MAC-induced asymmetry; the former causes TCP congestion control with a cumulative acknowledgment mechanism to prefer the sending stations to receiving stations, while the later exacerbates the unfairness problem in the hot spots. We investigate the interaction between TCP congestion control and MAC contention control, and propose a cross-layer feedback approach to assure per-station fairness and to ensure high channel utilization. In this approach, we introduce the notion of channel access cost to quantify the system-wide traffic load and per-station channel usage. The access cost is estimated at the MAC in an access point and conveyed to the TCP sender. Then, the TCP sender adjusts its sending rate based on the access cost, so as to assure per-station fairness. The simulation results indicate that the proposed approach can provide both per-station fairness and high channel utilization, irrespective of network configurations.

WLC47-4: Analysis of Unfairness between TCP Uplink and Downlink Flows in Wi-Fi Hot Spots

This paper focuses on the unfairness problem between TCP uplink and downlink flows in the 802.11 Wi-Fi hot spots and shows that the service is prone to be unfair. The cause of unfairness is analyzed from two aspects: TCP-induced asymmetry and MAC-induced asymmetry. Due to the asymmetric behavior of TCP congestion control with a cumulative acknowledgment mechanism between uplink and downlink flows, the service is biased toward the uplink flow and the downlink flow tends to starve. The contention-based channel access mechanism of 802.11 MAC exacerbates this unfairness problem because it intends to provide fair access opportunity only to the sending stations. Next, the analysis of the interaction between congestion control of TCP and contention control of MAC reveals interesting and counter-intuitive results: (i) Even when a station has a sufficiently large amount of traffic to send, it does not always participate in the MAC-layer contention, its opportunity for MAC-layer contention is controlled by the TCP congestion control, (ii) The aggregate throughput remains almost constant with respect to the number of stations sending/receiving TCP traffic. (Hi) Both TCP-induced unfairness and MAC-induced unfairness can be resolved if packet loss due to buffer overflow in an access point does not occur.

Effects of Ni Doping on the Initial Electrochemical Performance of Vanadium Oxide Nanotubes for Na-Ion Batteries

In this study, we demonstrated the intercalation of Na in hydrothermally synthesized VOx nanotubes (NTs) and Ni-doped VOx NTs. The changes induced in the structures of the two nanomaterials during the Na intercalation process were investigated through X-ray diffraction (XRD) analyses. It was observed that the initial capacity and rate performance of the Ni-doped VOx NTs were improved. The results of X-ray photoelectron spectroscopy (XPS) and conductance measurement confirmed that higher initial capacity and rate performance were attributed to an increase in the valence states of vanadium and increased conductivity after the Ni exchange process.

Silicon/Carbon Nanotube/BaTiO3 Nanocomposite Anode: Evidence for Enhanced Lithium-Ion Mobility Induced by the Local Piezoelectric Potential

We report on the synergetic effects of silicon (Si) and BaTiO3 (BTO) for applications as the anode of Li-ion batteries. The large expansion of Si during lithiation was exploited as an energy source via piezoelectric BTO nanoparticles. Si and BTO nanoparticles were dispersed in a matrix consisting of multiwalled carbon nanotubes (CNTs) using a high-energy ball-milling process. The mechanical stress resulting from the expansion of Si was transferred via the CNT matrix to the BTO, which can be poled, so that a piezoelectric potential is generated. We found that this local piezoelectric potential can improve the electrochemical performance of the Si/CNT/BTO nanocomposite anodes. Experimental measurements and simulation results support the increased mobility of Li-ions due to the local piezoelectric potential.

Na insertion mechanisms in vanadium oxide nanotubes for Na-ion batteries.

In this study, we successfully synthesized lamellar-structured Ni0.1VOx NTs by a microwave-assisted hydrothermal method and cation exchange reaction. High initial discharge capacity and 100% efficiency were obtained when the Ni0.1VOx NTs cathode was used as a cathode material for the Na battery. The intercalation mechanism and capacity fading effect were investigated in detail both experimentally using Transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses and theoretically using the ab initio simulation method. During the intercalation of Na(+) into VOx NT structures, TEM, XRD, FT-IR, and XPS data revealed the cointercalation of the solvent, resulting in the expansion of the interlayer spacing and carbon and oxygen adsorption. The experimental and simulation results suggest that solvent molecules coordinated the Na insertion mechanisms into the amine interlayer during discharging. These understandings of the Na intercalation mechanism in materials based on Ni0.1VOx NTs would be useful to design more stable and high-performance VOx-based electrodes for Na battery applications.

Tetrathiafulvalene as a Conductive Film-Making Additive on High-Voltage Cathode

Tetrathiafulvalene (TTF) is investigated as a conductive film-making additive on an overlithiated layered oxide (OLO) cathode. When the OLO/graphite cell is cycled at high voltage, carbonate-based electrolyte without the additive decomposes continuously to form a thick and highly resistant surface film on the cathode. In contrast, TTF added into the electrolyte becomes oxidized before the electrolyte solvents, creating a thinner film on the cathode surface. This film inhibits further electrolyte decomposition through cycling and stabilizes the interface between the cathode and the electrolyte. The cells containing the OLO cathode with TTF-added electrolyte afforded enhanced capacity retention and rate capability, making TTF a prospective electrolyte additive for higher energy density lithium-ion cells.

Multipotent neurogenic fate of mesenchymal stem cell is determined by Cdk4-mediated hypophosphorylation of Smad-STAT3

ABSTRACT Cyclin-dependent kinase (Cdk) in complex with a corresponding cyclin plays a pivotal role in neurogenic differentiation. In particular, Cdk4 activity acts as a signaling switch to direct human mesenchymal stem cells (MSCs) to neural transdifferentiation. However, the molecular evidence of how Cdk4 activity converts MSCs to neurogenic lineage remains unknown. Here, we found that Cdk4 inhibition in human MSCs enriches the populations of neural stem and progenitor pools rather than differentiated glial and neuronal cell pools. Interestingly, Cdk4 inhibition directly inactivates Smads and subsequently STAT3 signaling by hypophosphorylation, and both Cdk4 and Smads levels are linked during the processes of neural transdifferentiation and differentiation. In summary, our results provide novel molecular evidence in which Cdk4 inhibition leads to directing human MSCs to a multipotent neurogenic fate by inactivating Smads-STAT3 signaling.