Zhongcheng Li

Performance Analysis and Optimization of Handoff Algorithms in Heterogeneous Wireless Networks

In heterogeneous wireless networks, handoff can be separated into two parts: horizontal handoff (HHO) and vertical handoff (VHO). VHO plays an important role to fulfill seamless data transfer when mobile nodes cross wireless access networks with different link layer technologies. Current VHO algorithms mainly focus on when to trigger VHO, but neglect the problem of how to synthetically consider all currently available networks (homogeneous or heterogeneous) and choose the optimal network for HHO or VHO from all the available candidates. In this paper, we present an analytical framework to evaluate VHO algorithms. Subsequently, we extend the traditional hysteresis based and dwelling-timer based algorithms to support both VHO and HHO decisions and apply them to complex heterogeneous wireless environments. We refer to these enhanced algorithms as E-HY and E-DW, respectively. Based on the proposed analytical model, we provide a formalization definition of the handoff conditions in E-HY and E-DW and analyze their performance. Subsequently, we propose a novel general handoff decision algorithm, GHO, to trigger HHO and VHO in heterogeneous wireless networks. Analysis shows that GHO can achieve better performance than E-HY and E-DW. Simulations validate the analytical results and verify that GHO outperforms traditional algorithms in terms of the matching ratio, TCP throughput and UDP throughput.

Efficient and Scalable Consistency Maintenance for Heterogeneous Peer-to-Peer Systems

Consistency maintenance mechanism is necessary for the emerging peer-to-peer applications due to their frequent data updates. Centralized approaches suffer single point of failure, while previous decentralized approaches incur too many duplicate update messages because of locality-ignorant structures. To address this issue, we propose a scalable and efficient consistency maintenance scheme for heterogeneous P2P systems. Our scheme takes the heterogeneity nature into account and forms the replica nodes of a key into a locality-aware hierarchical structure, in which the upper layer is DHT-based and consists of powerful and stable replica nodes, while a replica node at the lower layer attaches to a physically close upper layer node. A d-ary update message propagation tree (UMPT) is dynamically built upon the upper layer for propagating the updated contents. As a result, the tree structure does not need to be maintained all the time, saving a lot of cost. Through theoretical analyses and comprehensive simulations, we examine the efficiency and scalability of this design. The results show that, compared with previous designs, especially locality-ignorant ones, our approach is able to reduce the cost by about 25-67 percent.

Opportunistic Routing in Intermittently Connected Mobile P2P Networks

Mobile P2P networking is an enabling technology for mobile devices to self-organize in an unstructured style and communicate in a peer-to-peer fashion. Due to user mobility and/or the unrestricted switching on/off of the mobile devices, links are intermittently connected and end-to-end paths may not exist, causing routing a very challenging problem. Moreover, the limited wireless spectrum and device resources together with the rapidly growing number of portable devices and amount of transmitted data make routing even harder. To tackle these challenges, the routing algorithms must be scalable, distributed, and light-weighted. Nevertheless, existing approaches usually cannot simultaneously satisfy all these three requirements. In this paper, we propose two opportunistic routing algorithms for intermittently connected mobile P2P networks, which exploit the spatial locality, spatial regularity, and activity heterogeneity of human mobility to select relays. The first algorithm employs a depth-search approach to diffuse the data towards the destination. The second one adopts a depth-width-search approach in a sense that it diffuses the data not only towards the destination but also to other directions determined by the actively moving nodes (activists) to find better relays. We perform both theoretical analysis as well as a comparison based simulation study. Our results obtained from both the synthetic data and the real world traces reveal that the proposed algorithms outperform the state-of-the-art in terms of delivery latency and delivery ratio.

Design and Evaluation of Vertical Handoff Decision Algorithm in Heterogeneous Wireless Networks

The next generation Internet is expected to consist of different wireless networks with diverse performance. Given the complementary characteristics of 3G and WLAN, the combination of 3G and WLAN can achieve anywhere, anytime Internet access and bring benefits to both end users and service providers. For this purpose, an efficient vertical handoff (VHO) algorithm is needed to make intelligent handoff decision. In this paper, we analyze the signal strength model of mobile host (MH) during VHO and present a new motion model-based RSS estimation (MMRE) algorithm. MMRE can adapt to the change of MHs velocity and improve handoff efficiency. We have evaluated MMREs feasibility and effectiveness in simulations and compared its performance with existing handoff decision algorithms

A Novel Channel Assignment Algorithm for Multicast in Multi-radio Wireless Mesh Networks

With the development of Internet, many new applications with obvious multicast character come forth and become popular, which increase the importance of multicast in Internet especially in wireless access networks where the bandwidths of networks are very limited. In this paper, we consider the problem of channel assignment for multicast in multi-radio wireless mesh networks (WMNs), which have not been studied up to now. We propose a novel channel assignment strategy called UCAS based on unidirectional link model, and give an efficient greedy vertex coloring algorithm called BFVC. Finally, the results of simulations demonstrate the efficiency of the algorithms presented in this paper.

Churn-Resilient Protocol for Massive Data Dissemination in P2P Networks

Massive data dissemination is often disrupted by frequent join and departure or failure of client nodes in a peer-to-peer (P2P) network. We propose a new churn-resilient protocol (CRP) to assure alternating path and data proximity to accelerate the data dissemination process under network churn. The CRP enables the construction of proximity-aware P2P content delivery systems. We present new data dissemination algorithms using this proximity-aware overlay design. We simulated P2P networks up to 20,000 nodes to validate the claimed advantages. Specifically, we make four technical contributions: 1). The CRP scheme promotes proximity awareness, dynamic load balancing, and resilience to node failures and network anomalies. 2). The proximity-aware overlay network has a 28-50 percent speed gain in massive data dissemination, compared with the use of scope-flooding or epidemic tree schemes in unstructured P2P networks. 3). The CRP-enabled network requires only 1/3 of the control messages used in a large CAM-Chord network. 4) Even with 40 percent of node failures, the CRP network guarantees atomic broadcast of all data items. These results clearly demonstrate the scalability and robustness of CRP networks under churn conditions. The scheme appeals especially to web-scale applications in digital content delivery, network worm containment, and consumer relationship management over hundreds of datacenters in cloud computing services.

Signpost: Scalable MU-MIMO Signaling with Zero CSI Feedback

Poor scalability is a long standing problem in multi-user MIMO (MU-MIMO) networks: in order to select concurrent uplink users with strong channel orthogonality and thus high total capacity, channel state information (CSI) feedback from users is required. However, when the user population is large, the overhead from CSI feedback can easily overwhelm the actual channel time spent on data transmission. Moreover, due to spontaneous uplink traffic, uplink user selection cannot rely on the access points central assignment and needs a distributed realization instead, which makes the problem even more challenging. In this paper, we propose a fully scalable and distributed uplink MU-MIMO protocol called Signpost. Firstly, Signpost is scalable, i.e., it achieves zero CSI overhead by exploiting a novel orthogonality evaluation mechanism that enables each user to speculate its orthogonality to other users, using its own CSI only. Secondly, Signpost realizes distributed user selection through a two-dimensional prioritized contention mechanism, which can single out the best users efficiently by utilizing both the time and frequency domain resources. The contention mechanism includes a unique collision recovery scheme, which enables Signpost to achieve a collision probability as low as one-tenth compared with traditional 802.11-like mechanism. Software-radio based implementation and testbed experimentation show that Signpost significantly outperforms state-of-the-art user selection methods under various traffic patterns and node mobility.

Cross-Layer Design for Proportional Delay Differentiation and Network Utility Maximization in Multi-Hop Wireless Networks

One major problem of cross-layer control algorithms in multi-hop wireless networks is that they lead to large end-to-end delays. Recently there have been many studies devoted to solving the problem to guarantee order-optimal per-flow delay. However, these approaches also bring the adverse effect of sacrificing a lot of network utility. In this paper, we solve the large-delay problem without sacrificing network utility. We take a fundamentally different approach of delay differentiation, which is based on the observation that flows in a network usually have different requirements for end-to-end delay. We propose a novel joint rate control, routing and scheduling algorithm called CLC_DD, which ensures that the flow delays are proportional to certain pre-specified delay priority parameters. By adjusting delay priority parameters, the end-to-end delays of preferential flows achieved by CLC_DD can be as small as those achieved by delay-order-optimal algorithms. In contrast to high network utility loss in previous approaches, we prove that our approach achieves maximum network utility. Furthermore, we incorporate opportunistic routing into the cross-layer design framework to improve network performance under the environment of dynamic wireless channels.

CRCache: Exploiting the correlation between content popularity and network topology information for ICN caching

Information-centric networking (ICN) is designed to decouple contents from hosts at the network layer, using in-network caching as a key feature to improve the overall performance. However, the en-route caching strategy used in many ICN implementations generally yields redundancies in the cached contents across different routers. There are also some recent works focusing on cache optimization by respectively exploiting either application layer or network layer, which we think is not sufficient to increase cache hit rate and reduce traffic. In this paper, we propose a novel caching scheme (CRCache) that utilizes a cross-layer design to cache contents in a few selected routers based on the correlation of content popularity and the network topology. Specifically, through exploiting information available at both application and network layers, CRCache aims to improve the cache hit rate and reduce the overall network traffic. We conduct a large scale and real traces-driven simulation with an underlying real Internet topology in China, and show that by using CRCache, the overall cache hit rate is increased by 62.5% and network traffic reduction is improved by at least 42% compared with recent single layer schemes.

Mobility-Assisted Routing in Intermittently Connected Mobile Cognitive Radio Networks

In mobile ad-hoc cognitive radio networks (CRNs), end-to-end paths with available spectrum bands for secondary users may exist temporarily, or may never exist, due to the dynamism of the primary user activities. Traditional CRN routing algorithms, which typically ignore the intermittent connectivity of network topology, and traditional mobility-assisted routing algorithms, which generally overlook the spectrum availability, are obviously unsuitable. To tackle this challenge, we propose a Mobility-Assisted Routing algorithm with Spectrum Awareness (MARSA) to select relays based on not only the probability that a node meets the destination but also the chance at which there exists at least one available channel when they meet. To the best of our knowledge, this paper is the first to bring the idea of mobility-assisted routing to deal with the intermittently connected attribute of mobile ad-hoc CRNs, and the first to enhance the mobility-assisted routing by considering the temporal, spatial, and spectrum domains at the same time. Our simulation results demonstrate the superiority of MARSA over traditional algorithms in intermittently connected mobile CRNs.