Algebraic Network Coding Approach to Deterministic Wireless Relay Networks

The deterministic wireless relay network model, introduced by Avestimehr et al., has been proposed for approximating Gaussian relay networks. This model, known as the ADT network model, takes into account the broadcast nature of wireless medium and interference. Avestimehr et al. showed that the Min-cut Max-flow theorem holds in the ADT network. In this paper, we show that the ADT network model can be described within the algebraic network coding framework introduced by Koetter and Medard. We prove that the ADT network problem can be captured by a single matrix, called the "system matrix". We show that the min-cut of an ADT network is the rank of the system matrix; thus, eliminating the need to optimize over exponential number of cuts between two nodes to compute the min-cut of an ADT network. We extend the capacity characterization for ADT networks to a more general set of connections. Our algebraic approach not only provides the Min-cut Max-flow theorem for a single unicast/multicast connection, but also extends to non-multicast connections such as multiple multicast, disjoint multicast, and two-level multicast. We also provide sufficiency conditions for achievability in ADT networks for any general connection set. In addition, we show that the random linear network coding, a randomized distributed algorithm for network code construction, achieves capacity for the connections listed above. Finally, we extend the ADT networks to those with random erasures and cycles (thus, allowing bi-directional links). Note that ADT network was proposed for approximating the wireless networks; however, ADT network is acyclic. Furthermore, ADT network does not model the stochastic nature of the wireless links. With our algebraic framework, we incorporate both cycles as well as random failures into ADT network model.Comment: 9 pages, 12 figures, submitted to Allerton Conferenc

Algebraic Watchdog: Mitigating Misbehavior in Wireless Network Coding

We propose a secure scheme for wireless network coding, called the algebraic watchdog. By enabling nodes to detect malicious behaviors probabilistically and use overheard messages to police their downstream neighbors locally, the algebraic watchdog delivers a secure global self-checking network. Unlike traditional Byzantine detection protocols which are receiver-based, this protocol gives the senders an active role in checking the node downstream. The key idea is inspired by Marti et al.'s watchdog-pathrater, which attempts to detect and mitigate the effects of routing misbehavior. As an initial building block of a such system, we first focus on a two-hop network. We present a graphical model to understand the inference process nodes execute to police their downstream neighbors; as well as to compute, analyze, and approximate the probabilities of misdetection and false detection. In addition, we present an algebraic analysis of the performance using an hypothesis testing framework that provides exact formulae for probabilities of false detection and misdetection. We then extend the algebraic watchdog to a more general network setting, and propose a protocol in which we can establish trust in coded systems in a distributed manner. We develop a graphical model to detect the presence of an adversarial node downstream within a general multi-hop network. The structure of the graphical model (a trellis) lends itself to well-known algorithms, such as the Viterbi algorithm, which can compute the probabilities of misdetection and false detection. We show analytically that as long as the min-cut is not dominated by the Byzantine adversaries, upstream nodes can monitor downstream neighbors and allow reliable communication with certain probability. Finally, we present simulation results that support our analysis.Comment: 10 pages, 10 figures, Submitted to IEEE Journal on Selected Areas in Communications (JSAC) "Advances in Military Networking and Communications

A Multi-hop Multi-source Algebraic Watchdog

In our previous work "An Algebraic Watchdog for Wireless Network Coding", we proposed a new scheme in which nodes can detect malicious behaviors probabilistically, police their downstream neighbors locally using overheard messages; thus, provide a secure global "self-checking network". As the first building block of such a system, we focused on a two-hop network, and presented a graphical model to understand the inference process by which nodes police their downstream neighbors and to compute the probabilities of misdetection and false detection. In this paper, we extend the Algebraic Watchdog to a more general network setting, and propose a protocol in which we can establish "trust" in coded systems in a distributed manner. We develop a graphical model to detect the presence of an adversarial node downstream within a general two-hop network. The structure of the graphical model (a trellis) lends itself to well-known algorithms, such as Viterbi algorithm, that can compute the probabilities of misdetection and false detection. Using this as a building block, we generalize our scheme to multi-hop networks. We show analytically that as long as the min-cut is not dominated by the Byzantine adversaries, upstream nodes can monitor downstream neighbors and allow reliable communication with certain probability. Finally, we present preliminary simulation results that support our analysis.Comment: 5 pages, 2 figures, to appear in IEEE ITW Dublin 201

Modeling Network Coded TCP Throughput: A Simple Model and its Validation

We analyze the performance of TCP and TCP with network coding (TCP/NC) in lossy wireless networks. We build upon the simple framework introduced by Padhye et al. and characterize the throughput behavior of classical TCP as well as TCP/NC as a function of erasure rate, round-trip time, maximum window size, and duration of the connection. Our analytical results show that network coding masks erasures and losses from TCP, thus preventing TCP's performance degradation in lossy networks, such as wireless networks. It is further seen that TCP/NC has significant throughput gains over TCP. In addition, we simulate TCP and TCP/NC to verify our analysis of the average throughput and the window evolution. Our analysis and simulation results show very close concordance and support that TCP/NC is robust against erasures. TCP/NC is not only able to increase its window size faster but also to maintain a large window size despite losses within the network, whereas TCP experiences window closing essentially because losses are mistakenly attributed to congestion.Comment: 9 pages, 12 figures, 1 table, submitted to IEEE INFOCOM 201

When Similarity Strikes Back: The Positive and Negative Role of Character-Audience Similarity in Anti-Smoking Campaigns

As media technology evolves quickly, tailored and targeted communication has emerged as an important strategy in persuasion. One of the most widely used and easiest tactics of tailoring is using an exemplar or persuader character similar to the target audience. However, the effect of character-audience similarity may significantly differ depending on other message features such as how the similar character is shown to behave within the message. This dissertation examines the positive and negative effects of the similarity between the audience and the exemplar character in anti-smoking public service announcements (PSAs), based on demographics and smoking status. Would character-audience similarity still exert positive effect on persuasion even when the target of similarity assessment is shown to behave in socially unacceptable ways? Two secondary data analyses on video anti-smoking PSAs were conducted first. Study 1 examined the main effect of character-audience similarity, finding a significant positive effect of demographic similarity between the smoker character and the audience. Study 2 focused on the secondhand smoking (SHS)-themed PSAs, where the smoker character’s behavior may be seen as irresponsible and immoral by causing serious harm to others. The impact of character-audience similarity depended on the severity of consequences caused by SHS, so that the usually positive effects of character-audience similarity disappeared in PSAs depicting highly severe consequences of SHS. However, the effect was only marginally significant. Informed by these two studies, an experiment (Study 3) was conducted to systematically manipulate and examine the effect of character-audience similarity (Similar vs. Dissimilar), theme of the message (self-harm vs. harm of SHS), and severity of consequences (high vs. low severity). A consistently negative and significant two-way interaction effect between character-audience similarity and theme emerged, suggesting that seeing a similar smoker character harming their own health (self-harm condition) increased engagement with the message and identification with the character, which in turn was associated with greater perceived effectiveness (PE). However, seeing a similar smoker endangering others via secondhand smoking (SHS condition) decreased engagement and identification, causing a boomerang effect on message effectiveness. The results provide valuable guidelines for message design regarding the use of character-audience similarity: When the exemplar character acts in socially undesirable ways, such as endangering others via secondhand smoking, character-audience similarity might backfire, and message designers should avoid tailoring via character-audience similarity. While the studies in this dissertation were limited in the topics of smoking tobacco cigarettes, other behaviors (e.g. drunk driving) may also be subject to similar effects given that driving under the influence may lead to serious negative consequences on innocent others

Placemaking and the Loss of Place: Perceptions of Tourism-Induced Neighborhood Change in South Korea’s Disadvantaged Neighborhoods

This dissertation research uncovers how seemingly beneficial urban projects associated with tourism reinforce inequitable urban environments and loss of place by examining different perceptions and experiences of tourism-induced neighborhood change in disadvantaged neighborhoods in South Korea. I investigate how public art projects implemented by the government to regenerate daldongnes—informal hillside settlements—have brought economic and social disruption to residents and generated a series of contest outcomes. In this research, I examine how tourists’ perceptions and representation of the neighborhood in social media contribute to the (re)construction of the neighborhood, how the growth of tourism has influenced place attachment, and how residents and small-business owners experience indirect displacement induced by tourist gentrification. I use ethnographic fieldwork and qualitative methods to explore how, by whom, and with what effects the neighborhoods are reimagined and reconstructed as contradictory sites to visit and explore. I have demonstrated in my research that the coexistence of tourism and everyday life in the space of residential neighborhoods has led us to rethink a series of controversial outcomes accompanied by the process of neighborhood transformation. This includes the full understanding of perceptions and experiences of different stakeholders, a fluid and relational understanding of place attachment in touristifying neighborhoods, and an expanded understanding of displacement that includes both direct and indirect displacement. By doing so, my research contributes to a wide range of scholarship within urban and tourism geography, critical tourism studies, and Asian studies by engaging with interdisciplinary theories and concepts. My key findings are as follows. First, I contend that so-called ‘neighborhood improvement projects’ are, in fact, micro-scale projects of entrepreneurial place-making. The reproduction of daldongnes as tourism destinations primarily serves a “nostalgic fantasy” for a romanticized, fading past for outsiders, instead of properly addressing the real needs of marginalized residents. This disparity has served as the seed of a complex neighborhood conflict. Second, I claim that a fluid and relational understanding of place attachment is critical in understanding the complexity in daldongnes changing through touristification. I claim that place still matters in being an object of strong attachment, and people continuously construct, adapt, and reshape their place attachment during the process of tourism-induced neighborhood change. Thus, I contend that urban policy must recognize these dynamics of place attachment in order to address community conflicts likely to emerge with tourism development. Place attachment could not only positively bring the community together but also rupture relationships. Finally, focusing only on numbers of displaced people by excessive rent increases in a touristified daldongne presents a partial understanding of neighborhood change. This is because people can experience displacement without actual physical displacement. Thus, it is critical to engage with indirect displacement—emotional, psychosocial, and material impacts of displacement—to understand the phenomenon in a daldongne fully. While daldongnes are essential and exciting in their own right, the study of these neighborhoods enriches several bodies of literature and areas of geographic investigation. As I have demonstrated in this research, uncovering tourism-induced neighborhood change is an essential and inherently geographic phenomenon that reflects a complex people-place relationship that calls for more geographers\u27 engagement. A critical analysis of such a tourism phenomenon serves not only as a way to unpack the broader issue of urban inequality and marginalization but also as a way to discover what sustainable, just, and inclusive urban-dwelling means and to envision ideal neighborhood change

An approach to Handling Irregular Oversaturation in Urban Subway Stations

Train timetable, Passenger waiting time, Oversaturated condition, Genetic algorithmThis Theses presents a data-based approach for a train scheduling that aims to minimize passenger waiting time by controlling train departure time and the number of skipped trains. In contrast to existing approaches that rely on a statistical model of passenger arrival, we develop a model based on real-world automated fare collection (AFC) data from a metro line in Daegu, a Korean city. The model consists of decomposing the travel time for each passenger into waiting, riding, and walking times, clustering of passengers by trains they ride and calculating the number of passengers in each train for any given time. Based on this, for a given train schedule, the passenger waiting time of each passenger for the entire AFC data period can be calculated. The problem is formulated using the model under realistic constraints such as headway, the number of available trains, and train capacity. To find the optimal solution, we employed a genetic algorithm (GA). The results demonstrate that the average waiting time is reduced up to 56% in the highly congested situation. Moreover, letting the trains directly go to the congested station by skipping previous stations further reduces the maximum waiting time by up to 19%. The effect of the optimization varies depending on the passenger arrival pattern of highly congested stations. This approach will improve the quality of the subway services by reducing passenger waiting time.openⅠ. INTRODUCTION 1 II. RELATED WORK 4 2.1. Passenger Volume Estimation 4 2.2. Train Scheduling Optimization 5 III. PROPOSED APPROACH 6 3.1. Overview 6 3.2. Dataset 8 3.3. Scenario Analysis 9 3.3.1 Peak Hours Scenario 10 3.3.2 Congested Off-Peak Hours Scenario 10 IV. PROBLEM FORMULATION 13 4.1. Assumptions 13 4.2. Train Capacity 15 4.3. Passenger Volume Estimation 15 4.3.1. Passenger Volume on the Train 16 4.3.2. Passenger Volume on the Platform 20 4.4. Timetable Optimization Model 20 4.4.1. Train Departure Time Control 21 4.4.1.1. Passenger Waiting Time Minimization Problem 21 4.4.1.2. Oversaturation Time Minimization Problem 24 4.4.2. Train Skip Plan Control 24 4.5. Genetic Algorithm 27 V. EVALUATION 29 5.1. Peak Hours Scenario 30 5.2. Congested Off-peak Hours Scenario 32 5.2.1 Single Peak Oversaturation 32 5.2.2 Double Peak Oversaturation 36 5.2.3 Box-shaped Peak Oversaturation 40 5.3. Discussion 43 VI. CONCLUSION AND FUTURE WORK 44 REFERENCES 46 APPENDIX A. Optimization Results 48 요약문 81도시 지하철은 도로교통 상황의 영향을 크게 받지 않으며 대용량의 교통 수요를 처리할 수 있어 많은 승객들에게 이용된다. 혼잡한 지하철은 승객들에게 불편을 야기하며, 승객들의 승강장에서의 대기시간을 증가시킨다. 본 논문은 열차 출발 시간과 역들을 건너 뛴 열차 수를 조절하여 승객 대기 시간을 최소화하는 것을 목표로 한 열차 시간표 최적화 방안을 제시한다. 승객 도착 통계 모델에 의존하는 기존의 접근 방식과 달리, 이 연구는 대구의 지하철에서 수집된 교통카드 데이터들을 기반으로 하는 최적화 모델을 만든다. 모델은 각 승객의 여행 시간을 차량 대기 시간, 차량 탑승 시간 및 보행 시간으로 구분하고, 탑승한 기차에 따라 승객들을 군집화 시킨 후 각 차량마다 승객 수를 추정하는 것으로 구성된다. 이를 바탕으로 주어진 열차 스케줄에 대해 모든 승객 각각의 대기 시간들을 계산할 수 있다. 최적화 문제는 이용 가능한 열차 수, 열차가 수용 가능한 최대 승객 수, 폐색구간과 같은 현실적인 제약 조건 하에서 구성된다. 최적의 시간표를 찾기 위한 방법으로 유전자 알고리즘이 사용되었다. 그 결과 승객 평균 대기 시간은 최대 56%까지 단축되었으며, 열차 출발시간 뿐만 아니라 일부 역을 건너뛰는 열차의 수까지 최적화하면 매우 혼잡한 상황에서 승객의 차량 대기 시간을 더욱 줄일 수 있었다. 혼잡한 상황에서 기차가 일부 역을 건너뛰었을 때, 그렇지 않을 때보다 승객 최대 대기 시간은 19%, 승객 평균 대기 시간은 15% 정도 더욱 단축되었다. 또한 혼잡한 상황에서 승객 도착 패턴에 따라 최적화의 효율이 달라진다는 것을 확인하였다. 본 방안은 승객 평균 대기시간을 감소시킴으로써 지하철 서비스를 향상시킬 것이다.MasterdCollectio

Network Coding for Speedup in Switches

We present a graph theoretic upper bound on speedup needed to achieve 100% throughput in a multicast switch using network coding. By bounding speedup, we show the equivalence between network coding and speedup in multicast switches - i.e. network coding, which is usually implemented using software, can in many cases substitute speedup, which is often achieved by adding extra switch fabrics. This bound is based on an approach to network coding problems called the "enhanced conflict graph". We show that the "imperfection ratio" of the enhanced conflict graph gives an upper bound on speedup. In particular, we apply this result to K-by-N switches with traffic patterns consisting of unicasts and broadcasts only to obtain an upper bound of min{(2K-1)/K, 2N/(N+1)}.Comment: 5 pages, 4 figures, IEEE ISIT 200

Lost & Found

Lost and Found focuses on olfactory art and stimulates the sense of smell throughvarious means and materials. In this exhibition, diverse associations of the sense of smell are evoked through visual art. Through these fragrant works of art, personal, psychological, social, and cultural dimensions are conveyed and explored. The exhibition\u27s title, Lost & Found, refers to the restoration of the sense of smell, which many people have temporarily lost due to the physiological and social effects of COVID-19