Jaeseon Hwang

A receiver-centric multi-channel MAC protocol for wireless networks

We consider the issues of mitigating interference and improving network capacity in wireless networks from the viewpoint of channel diversity. Multi-channel diversity allows multiple pairs to concurrently use the wireless medium, thus increasing the achievable capacity; this multi-channel diversity can then be fully utilized by enabling wireless nodes to dynamically switch their channels. However, the process of switching channels in a wireless transceiver incurs switching overhead, resulting in the degradation of the throughput performance. We propose a receiver-centric multi-channel MAC protocol (RcMAC) that allows nodes to efficiently utilize multiple channels by reducing unnecessary channel switching. Receiver-centric channel switching enables each sender node to asynchronously and independently switch channels to one where its intended receiver resides, without requiring explicit channel negotiation. Thus, the wait time at the control channel is reduced, in addition to the number of channel switchings, thereby improving channel utilization. As this scheme requires prior knowledge of which channel a node is switched to, each node cooperatively shares its channel usage information in order to recognize the channels that its neighboring nodes are using. Through extensive simulations, we show that the proposed MAC protocol significantly improves network throughput and reduces end-to-end delay compared with other multi-channel MAC protocols.

Adaptive contention control for improving end-to-end throughput performance of multihop wireless networks

In multihop wireless networks, packets of a flow originating from a source node are relayed by intermediate nodes (relay nodes) and travel towards their destination along a multihop wireless path. Since the traffic forwarding capability of each node varies according to its level of contention, ideally, a node should not transmit more packets to its relay node than the corresponding relay node can forward. Instead, each node should yield its channel access opportunity to its neighbor nodes so that all the nodes can evenly share the channel and have similar forwarding capabilities. In this manner, nodes can utilize the wireless channel effectively, and further increase the end-to-end throughput of a multihop path. We propose a fully distributed contention window adaptation (CWA) mechanism, which adjusts the channel access probability depending on the difference between the incoming and outgoing traffic at each node, in order to equate the traffic forwarding capabilities among all the nodes in the path. We implement the proposed adaptive contention algorithm on Madwifi Linux kernel driver for Wi-Fi interface with Atheros chipset and carry out an empirical study in our division building. The experiment results demonstrate how the proposed mechanism can improve end-to-end throughput performance in the multihop wireless networks.

Distributed semi-synchronous channel coordination for multi-channel wireless networks

In multi-channel wireless networks, multi-channel diversity can increase the number of concurrent transmissions and thus improve the throughput performance as data transmission on a wireless channel does not interfere with transmissions on the other non-overlapping channels. However, multi-channel coordination may cause severe performance degradation due to hidden terminals, missing receivers, or broadcast deafness problems if the channel usage information is not properly shared among the neighboring nodes. In this paper, we devise a semi-synchronous multi-channel coordination protocol that enables wireless nodes to: (i) efficiently exchange channel and coordination information, and (ii) reduce the overhead of channel switchings. In the proposed protocol, a rendezvous interval is set up in a distributed manner depending on the traffic rate and pattern, and each node independently switches its channel when it can complete its transmissions and then returns to the control channel within the rendezvous interval. This approach makes all nodes return to the control channel at almost the same time without incurring a severe synchronization overhead. Through subsequent analyses and simulation studies, we show that the proposed protocol effectively reduces the number of channel switchings, thereby achieving higher throughput in various multi-channel networking environments.

MARS: measurement-based allocation of vm resources for cloud data centers

High performance data centers use virtualization technique which enables each physical server machine to host multiple virtual machines (VMs) to achieve highly efficient resource utilization. In this paper, we propose a measurement-based approach for efficient allocation of virtualized resources in hyper-convergence environments where virtualized computing, networking, and storage resources are unified and converged. Using real-time measurements of service performance metrics, our proposed approach identifies the VM with the worst performance resulting from over-utilized resource, and gradually adjusts the amount of resources allocated to it in order to improve its performance. The results of empirical evaluations conducted indicate that our proposed approach can realize efficient resource allocation among VMs with varying resource demands.

Power Control for Wireless Networks with a Limited Number of Channels

In wireless networks, interferences from adjacent nodes that are concurrently transmitting can cause packet reception failures and thus result in significant throughput degradation. The interference can be simply avoided by assigning different orthogonal channel to each interfering node. However, if the number of orthogonal channels is smaller than that of adjacent nodes, this simple channel assignment method does not work. In this paper, we propose a vertex coloring based power control algorithm for wireless networks with a limited number of channels. In order to maintain high data transmission rate between two nodes, the transmission power is increased as long as different orthogonal channel is assigned to each adjacent node. We show that the proposed algorithm significantly improves the network throughput performance for various wireless network topologies with different number o f orthogonal channels.

Queue-based contention control for congested multihop wireless networks

In multihop wireless networks, packets from a source node are relayed by intermediate nodes (relay nodes) toward their destination node along a multihop wireless path. However, if a relay node simultaneously receives a large number of packets and fails to forward them at the same rate as they are received, the end-to-end throughput performance of the network degrades due to packet losses at the bottleneck relay node. In this paper, we propose an adaptive contention control algorithm that adjusts the channel access probability depending on the level of congestion at each relay node in order to improve the end-to-end throughput performance in multihop wireless networks. In this algorithm, a bottleneck relay node is granted permission to increase its channel access probability by reducing its contention window size, thus enabling it to forward packets more aggressively. Through a series of simulations, we show that the end-to-end throughput is improved by about 60% in a chain topology.