Bo Xing

Multi-constraint dynamic access selection in always best connected networks

In future generation networks, various access technologies, such as Wi-Fi, Bluetooth, GPRS and UMTS, etc., are simultaneously available to mobile devices. They vary in characteristics (communication range, power consumption, security, etc.) and QoS parameters (bandwidth, delay, etc.) The notion of always best connected (ABC) enables people to run applications over the most efficient combination of access technologies with continuous connectivity. Access selection is the key functional block in ABC solutions, as it chooses the most suitable access networks for application traffic flows. However, it is important that access selection decisions be dynamically made, minimizing the power consumption on mobile devices while satisfying QoS requirements and user/application preferences. In this paper, we model the problem of multi-constraint dynamic access selection (MCDAS) as a variant of bin packing problem. A series of approximation algorithms derived from the first fit decreasing (FFD) algorithm are proposed for finding near-optimal solutions. Simulation studies show that the algorithms we propose gradually improve performance towards quasi-optimal solutions in terms of power consumption and preference satisfaction.

CREW: A Gossip-based Flash-Dissemination System

In this paper, we explore a new form of dissemination called Flash Dissemination that involves dissemination of fixed, rich information to a large number of recipients in as short a time as possible. Key characteristics of Flash Dissemination include unpredictability in its need, scalability to large number of recipients and autonomic performance in highly heterogenous and failureprone environments. Previous work either addresses large content delivery in heterogenous networks or fault-tolerant dissemination of (streaming) events. We investigate a peer-based approach using foundations from broadcast networks, gossip theory and random networks. In this paper, we propose CREW (Concurrent Random Expanding Walkers), a scalable, lightweight, and autonomic gossip-based protocol. CREW is also explicitly designed to maximize the speed of dissemination using adaptive and intelligent intra and inter node concurrency. We implemented CREW on top of a scalable middleware environment and compared it to optimized implementations of popular gossip and peer-based systems. Our experiments show that CREW outperforms both traditional gossip and current large content dissemination systems, across a wide range of comparative metrics, even though its design is counterintuitive from a systems perspective.

An Experimental Study on Wi-Fi Ad-Hoc Mode for Mobile Device-to-Device Video Delivery

The demand for video content is continuously increasing as video sharing on the Internet is becoming enormously popular recently. This demand, with its high bandwidth requirements, has a considerable impact on the load of the network infrastructure. As more users access videos from their mobile devices, the load on the current wireless infrastructure (which has limited capacity) will be even more significant. Based on observations from many local video sharing scenarios, in this paper, we study the tradeoffs of using Wi-Fi ad-hoc mode versus infrastructure mode for video streaming between adjacent devices. We thus show the potential of direct device-to-device communication as a way to reduce the load on the wireless infrastructure and to improve user experiences. Setting up experiments for Wi-Fi devices connected in ad-hoc mode, we collect measurements for various video streaming scenarios and compare them to the case where the devices are connected through access points. The results show the improvements in latency, jitter and loss rate. More importantly, the results show that the performance in direct device-to-device streaming is much more stable in contrast to the access point case, where different factors affect the performance causing widely unpredictable qualities.

RADcast: Enabling Reliability Guarantees for Content Dissemination in Ad Hoc Networks

This paper deals with the problem of reliable and fast broadcast of mission-critical data with rich content over ad hoc networks. Existing approaches to dissemination reliability often assume network size knowledge, or that receivers know about the dissemination in advance. Without making similar assumptions, we propose a distinct approach which accommodates the varying reliability needs of applications. We develop the RADcast (reliable application data broadcast) protocol as an integration of two components: (a) Peddler, which ensures that receivers obtain the dissemination metadata, and (b) Pryer, which delivers the actual data to dissemination-aware receivers. We indicate how reliability guarantees/performance tradeoffs can be achieved by a careful instantiation of Peddler and Pryer. We implement RADcast on mobile devices inside a middleware and determine its feasibility. Furthermore, through extensive simulations, we show that RADcast achieves desired reliability in all cases, and performs consistently under varying network conditions and device mobilities. As compared to existing approaches, RADcast either incurs significantly lower latency/message overhead, or reduces latency by 50% with a tradeoff in message overhead.

Towards Reliable Application Data Broadcast in Wireless Ad Hoc Networks

Application data broadcast in ad hoc networks is an important primitive that has received little systematic research - the main focus of prior research being on control data broadcast. In this paper, we show why control data broadcast and even multicast techniques are insufficient for reliable application data broadcast; in fact their reliability degrades sharply with increasing application data size. We discover the root cause of this to be IP fragmenting the application data but not providing good reliability control on the fragments. We hence propose READ (reliable and efficient application-data dissemination), a protocol based on higher-layer fragmentation with fragment-level reliability control. READ splits a data packet into fragments, and disseminates them separately at dynamically adaptive intervals. Receivers piggyback implicit NACKs when propagating the fragments, and retrieve missing fragments from neighbors. Through experiments, we show that READ consistently achieves high delivery ratio and short latency, outperforming all other examined protocols.

Gateway designation for timely communications in instant mesh networks

In this paper, we explore how to effectively create and use “instant mesh networks”, i.e., wireless mesh networks that are dynamically deployed in temporary circumstances (e.g., emergency responses) - in addition to enabling coverage for internal on-site communications, such a network will support information flow into and out of the deployment site through its gateway (i.e., the mesh router that connects to the external backhaul). We study optimizing the performance of communications (specifically in terms of latency) in an instant mesh network by intelligently selecting the gateway. We demonstrate that designating the proper gateway significantly enhances the timeliness of communications with the external backhaul. We mathematically model the “gateway designation problem” using the notion of centrality from graph theory. We propose a distributed algorithm, FACE (Fast Approximate Center Exploration), for locating the optimal gateway. FACE is an approximate algorithm that works in an efficient manner without compromising the optimality of solutions. A thorough performance evaluation shows that the gateways designated by FACE reduce latencies by up to 92% for various types of communications, and that FACE saves transmission cost and execution time by up to 71% in finding the gateways.

PassItOn: An Opportunistic Messaging Prototype on Mobile Devices

With the increasing popularity of mobile handheld devices and the growing capability of these devices, it is becoming possible that information sharing/dissemination is carried out through human networks, as a complement to the traditional computer networks. In such human networks, people come across one another, while their mobile devices exchange and store information in a spontaneous and transparent way. Such an encounter could be established through direct device-todevice connectivity when two devices come into each other’s communication range, or be enabled by, e.g., a Wi-Fi access point, when the devices both enter its coverage. A new form of dissemination, which we call opportunistic messaging, is such an application that is based on human encounters and mobilities. When human encounters are exploited for communications, the reliance on network infrastructure access is eliminated; communications can be performed even where infrastructure is absent or infrastructure access is intermittent. By leveraging human mobilities, data delivery does not require an end-to-end path from the source to a recipient; instead, people carrying mobile devices serve as relays – they cache others’ data and forward/deliver the data when appropriate. Thus, the propagation of information is tied to people’s physical proximity when they move around, and incorporates the social aspects of communications as people tend to spend more time co-locating with their social relations. Opportunistic messaging is applicable anywhere, and is especially appealing where network infrastructure access is limited or intermittent (e.g., on cruise ships, in national parks, after disasters). Another intriguing characteristic of it is its ease of deployment – no central server is needed, but only a single piece of software on users’ mobile devices. However, as human encounters and mobilities are unpredictable, when used for social applications, opportunistic messaging is most suited for disseminating user-generated information that is non-formal, less important, and thus not time-critical. In recent years, a considerable amount of efforts have been invested in the research on opportunistic networking and delay-tolerant networking (which encompasses opportunistic networking but is a broader concept). A large portion of prior work has focused on routing issues, e.g., through whom as intermediate carriers to deliver a message to the destinations [1] [2] [3] [4]. The routing issues have been further explored in various contexts, such as in vehicular networks [5] [6] [7] and in social networking applications [8] [9] [10] [11]. However, serious real-world application development, deployment and evaluation of the opportunistic networking concepts still fall behind [12], in which many challenging issues remain to be addressed (to name a few, location-awareness, user incentives and preferences, power preservation, encounter controls, etc.). In this work, we design and prototype PassItOn, a fully distributed opportunistic messaging system. Our goal is to build up a proof-of-concept platform on real mobile devices, and thus show the feasibility and potentials of utilizing human movements for dissemination applications. Meanwhile, we seek to shed lights on the design, implementation and deployment issues in building such systems, and thus stimulate new ideas and perspectives on addressing these issues. Moreover, we aim to offer a real testbed on which new mechanisms, protocols and use cases can be tested and evaluated.

Disruption-Tolerant Spatial Dissemination

Spatial dissemination is a specific form of information dissemination that enables mobile users to send information to other mobile users who are or will appear at a specific location (a user-defined region). Such geo-messaging services are on the rise; they typically are built upon centralized solutions and require users to have reliable access to a stable backend infrastructure for storing and communicating content. In this paper, we develop a distributed solution to spatial dissemination, that can work without the need for such an infrastructure. Our solution utilizes the concepts from disruption-tolerant networking to build a flexible/best-effort service that leverages the intermittent ad-hoc connectivity between users. We propose Sticker, a spatial dissemination protocol that aims to maximize delivery reliability without incurring significant storage/transmission overheads. Sticker employs the store-carry-and- forward model, and strives to optimize dissemination performance by addressing three sub-problems - replication, forwarding and purging. Our experiments show that, Sticker achieves delivery ratios that are close to the maximum possible values; as compared to existing techniques, it either cuts down storage/transmission overheads by over 50%, or greatly enhances both delivery reliability and storage efficiency.