Yunnan Wu

Minimum-energy multicast in mobile ad hoc networks using network coding

The minimum energy required to transmit a bit of information through a network characterizes the most economical way to communicate in a network. In this paper, we show that under a simplified layered model of wireless networks, the minimum-energy multicast problem in mobile ad hoc networks is solvable as a linear program, assuming network coding. Compared with conventional routing solutions, network coding not only promises a potentially lower energy-per-bit, but also enables finding the optimal solution in polynomial time, in sharp contrast with the NP-hardness of constructing the minimum-energy multicast tree as the optimal routing solution.

Allocating dynamic time-spectrum blocks in cognitive radio networks

A number of studies have shown the abundance of unused spectrum in the TV bands. This is in stark contrast to the overcrowding of wireless devices in the ISM bands. A recent trend to alleviate this disparity is the design of Cognitive Radios, which constantly sense the spectrum and opportunistically utilize unused frequencies in the TV bands. In this paper, we introduce the concept of a time-spectrum block to model spectrum reservation, and use it to present a theoretical formalization of the spectrum allocation problem in cognitive radio networks. We present a centralized and a distributed protocol for spectrum allocation and show that these protocols are close to optimal in most scenarios. We have implemented the distributed protocol in QualNet and show that our analysis closely matches the simulation results.

Network Coding for the Internet and Wireless Networks

In todays practical communication networks such as the Internet, information delivery is performed by routing. A promising generalization of routing is network coding. The potential advantages of network coding over routing include resource (e.g., bandwidth and power) efficiency, computational efficiency, and robustness to network dynamics. This tutorial article provides an overview of the theory, practice, and applications of network coding.

KNOWS: Cognitive Radio Networks Over White Spaces

FCC has proposed to allow unlicensed operations in the TV broadcast bands. This portion of the spectrum has several desirable properties for more robust data communication as compared to the ISM bands. However, there are a number of challenges in efficiently using the TV bands. For example, the available spectrum is fragmented, and its availability may vary over time. In this paper, we present a cognitive radio system, called KNOWS, to address these challenges. We present the design of our prototype, which includes a new hardware platform and a spectrum-aware Medium Access Control (MAC) protocol. We have implemented the MAC protocol in QualNet, and our results show that in most scenarios KNOWS increases the throughput by more than 200% when compared to an IEEE 802.11 based system.

Reducing repair traffic for erasure coding-based storage via interference alignment

We consider the problem of recovering from a single node failure in a storage system based on an (n, k) MDS code. In such a scenario, a straightforward solution is to perform a complete decoding, even though the data to be recovered only amount to 1/kth of the entire data. This paper presents techniques that can reduce the network traffic incurred. The techniques perform algebraic alignment so that the effective dimension of unwanted information is reduced.

Load-aware spectrum distribution in Wireless LANs

Traditionally, the channelization structure in IEEE 802.11-based wireless LANs has been fixed: Each access point (AP) is assigned one channel and all channels are equally wide. In contrast, it has recently been shown that even on commodity hardware, the channel-width can be adapted dynamically purely in software. Leveraging this capability, we study the use of dynamic-width channels, where every AP adaptively adjusts not only its center-frequency, but also its channel-width to match its traffic load. This gives raise to a novel optimization problem that differs from previously studied channel assignment problems. We propose efficient spectrum-distribution algorithms and evaluate their effectiveness through analysis and simulations using real-world traces. Our results indicate that by allocating more spectrum to highly-loaded APs, the overall spectrum-utilization can be substantially improved and the notorious load-balancing problem in WLANs can be solved naturally.

Routing with a Markovian Metric to Promote Local Mixing

Routing protocols have traditionally been based on finding shortest paths under certain cost metrics. A conventional routing metric models the cost of a path as the sum of the costs on the constituting links. This paper introduces the concept of a Markovian metric, which models the cost of a path as the cost of the first hop plus the cost of the second hop conditioned on the first hop, and so on. The notion of the Markovian metric is fairly general. It is potentially applicable to scenarios where the cost of sending a packet (or a stream of packets) over a link may depend on the previous hop of the packet (or the stream). Such scenario arises, for instance, in a wireless mesh network equipped with local mixing, a recent link layer advance. This scenario is examined as a case study for the Markovian metric. The local mixing engine sits between the routing and MAC layers. It maintains information about the packets each neighbor has, and identifies opportunities to mix the outgoing packets via network coding to reduce the transmissions in the air. We use a Markovian metric to model the reduction of channel resource consumption due to local mixing. This leads to routing decisions that can better take advantage of local mixing. We have implemented a system that incorporates local mixing and source routing using a Markovian metric in Qualnet. The experimental results demonstrate significant throughput gain and resource saving.

A Trellis Connectivity Analysis of Random Linear Network Coding with Buffering

Previous studies have proposed a practical network coding scheme for multicasting information in packet networks, which buffers received packets and outputs random linear mixtures of packets in the buffer. To analyze the performance of such a scheme, this paper introduces a continuous-time trellis, which models the packet transmissions in a practical asynchronous network. The continuous-time trellis represents an extension of a discrete-time trellis that was used in prior theoretical studies to model a synchronized network. The asymptotic throughput of the practical network coding scheme is characterized via an analysis of the asymptotic s-T connectivity in the trellis

Low complexity codes for writing a write-once memory twice

A write-once memory (wom) is a storage medium formed by a number of “write-once” binary cells, where each cell initially is in a ‘0’ state and can be changed to a ‘1’ state irreversibly. Examples of write-once memories include SLC flash memories and optical disks. This paper presents some low complexity codes for writing such write-once memories twice.

Celerity: a low-delay multi-party conferencing solution

In this paper, we revisit the problem of multi-party conferencing from a practical perspective, and to rethink the design space involved in this problem. We believe that an emphasis on low end-to-end delays between any two parties in the conference is a must, and the source sending rate in a session should adapt to bandwidth availability and congestion. We present Celerity, a multi-party conferencing solution specifically designed to achieve our objectives. It is entirely Peer-to-Peer (P2P), and as such eliminating the cost of maintaining centrally administered servers. It is designed to deliver video with low end-to-end delays, at quality levels commensurate with available network resources over arbitrary network topologies where bottlenecks can be anywhere in the network. This is in contrast to commonly assumed P2P scenarios where bandwidth bottlenecks reside only at the edge of the network. The highlight in our design is a distributed and adaptive rate control protocol, that can discover and adapt to arbitrary topologies and network conditions quickly, converging to efficient link rate allocations allowed by the underlying network. In accordance with adaptive link rate control, source video encoding rates are also dynamically controlled to optimize video quality in arbitrary and unpredictable network conditions. We have implemented Celerity in a prototype system, and demonstrate its superior performance over existing solutions in a local experimental testbed and over the Internet.