IMDEA Networks Institute Digital Repository
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Communication Networks of Visible Light Emitting Diodes with Intra-Frame Bidirectional Transmission
Get PDFUnlike traditional radio frequency communication of consumer devices, the "optical antenna" direction of Visible Light Communication (VLC), i.e., the Field-Of-View (FOV), varies greatly from device to device. This encompasses wide FOVs of ambient infrastructure and directional FOVs of light emitted by low-end embedded devices. This variety of light wave propagation can severely affect the transmission reliability, despite "pointing" devices to each other may seem enough for a reliable link. In particular, the fact that FOVs are unknown makes traditional access protocols in VLC unreliable in presence of interference among nodes of different FOVs and exacerbates the hidden-node problem. In this paper, we propose a Carrier Sensing Multiple Access/Collision Detection&Hidden Avoidance (CSMA/CD-HA) Medium Access Control protocol for a network where each node solely uses one Light Emitting Diode (LED) to transmit and receive data. The CSMA/CD-HA can enable in-band intra-frame bidirectional transmission with just one optical antenna. The key idea is to exploit the intra-frame data symbols without emission of light to introduce an embedded communication channel. This approach enables the transmission of additional data while receiving in the same optical channel and it makes the communication robust to different types of FOVs. We build a software-defined embedded platform running on Linux operating system, implement the CSMA/CD-HA protocol, and evaluate its performance through experiments. Results show that collisions caused by hidden nodes can be reduced and our protocol can increase the saturation throughput by nearly up to 50% and 100% under the two-node and four-node scenarios, respectively.TRUEpu
A Mechanism for Fair Distribution of Resources with Application to Sponsored Search
Get PDFAs advertisement is shifting from the traditional media to the Internet, advertising in web search engines has emerged in the form of sponsored search. Advertisers pay the search engine to show their content, usually in order to get traffic to their own websites. A large amount of the search engine’s income derives from sponsored search. In sponsored search, a number of advertisers are competing for a limited number of slots in each specific keyword search in the search engine. In order to distribute the available slots among the advertisers, search engines are starting to hold keyword auctions. The questions that appear in this setup from a mechanism design point of view are two, how to assign advertisers to slots (or vice versa), and how to price each slot. This work proposes a novel approach were distributing the slots among the advertisers is based on how much each advertiser values appearing in a keyword search slot at a specific time. The proposed approach makes this value independent of her true payment to the search engine, which can take the form of a flat fee. For this purpose we have designed a new auction mechanism that fairly distributes resources (or goods, e.g., slots) in online fashion, based on the users’ declared preferences, while being socially efficient. While the main motivation for this work was sponsored search, the proposed mechanism can be used in general for the fair distribution of resources in an online fashion among a set of users. Hence, we refer to this mechanism as Fair and Efficient Distribution of Resources (FEDoR). FEDoR can be used even when the auction is done in a distributed fashion (i.e., without central authority), and it provides fairness, social efficiency and incentive compatibility.TRUEpu
Novel Techniques to Speed Up the Computation of the Automorphism Group of a Graph
Get PDFGraph automorphism (GA) is a classical problem, in which the objective is to compute the automorphism group of an input graph. Most GA algorithms explore a search tree using the individualization-refinement procedure. Four novel techniques are proposed which increase the performance of any algorithm of this type by reducing the depth of the search tree and by effectively pruning it. We formally prove that a GA algorithm that uses these techniques correctly computes the automorphism group of an input graph. Then, we describe how these techniques have been incorporated into the GA algorithm conauto, as conauto-2.03, with at most an additive polynomial increase in its asymptotic time complexity. Using a benchmark of different graph families, we have evaluated the impact of these techniques on the size of the search tree, observing a significant reduction both when they are applied individually and when all of them are applied together. This is also reflected in a reduction of the running time, which is substantial for some graph families. Finally, we have compared the search tree size of conauto-2.03 against those of other popular GA algorithms, observing that, in most cases, conauto explores less nodes than these algorithms.pu
Detection of Reactive Jamming in DSSS-based Wireless Communications
Get PDFReactive jammers have been shown to be a serious threat for wireless communication. Despite this, it is difficult to detect their presence reliably. We propose a novel method
to detect such sophisticated jammers in direct sequence spread spectrum (DSSS) wireless communication systems. The key idea is to extract statistics from the jamming-free symbols of the DSSS synchronizer to discern jammed packets from those lost due to bad channel conditions. Our contribution is twofold. First, we experimentally evaluate new empirical models utilizing the preamble symbols of IEEE 802.15.4 packets, thus enabling the accurate prediction of the packet delivery ratio (PDR). We show that the chip error rate-based metric is superior to metrics used in the literature, offering an accurate and reactive indicator of the true PDR. Our second contribution is the design and evaluation of a detection technique relying on this metric to detect reactive jammers. We build a software-defined radio testbed and show that our technique enables the error-free detection of reactive jammers that jam all packets on links with a PDR above 0.3. To the best of our knowledge, our detector is the first to detect reactive jamming attacks targeting the physical layer header of DSSS packets, and does not require any modifications of the wireless communication system.pu
Understanding the Reachability of IPv6 Limited Visibility Prefixes
Get PDFThe main functionality of the Internet is to provide global connectivity for every node attached to it. In light of the IPv4 address space depletion, large networks are in the process of deploying IPv6. In this paper we perform an extensive analysis of how BGP route propagation affects global reachability of the active IPv6 address space in the context of this unique transition of the Internet infrastructure. We propose and validate a methodology for testing the reachability of an IPv6 address block active in the routing system. Leveraging the global visibility status of the IPv6 prefixes evaluated with the BGP Visibility scanner, we then use this methodology to verify if the visibility status of the prefix impacts its reachability at the interdomain level. We perform active measurements using the RIPE Atlas platform. We test destinations with different BGP visibility degrees (i.e., limited visibility - LV, high visibility - HV and dark prefixes). We show that the IPv6 LV prefixes (v6LVPs) are generally reachable, mostly due to a less-specific HV covering prefix (v6HVP). However, this is not the case of the dark address space, which, by not having a covering v6HVP is largely unreachable.TRUEpu
ESPRES: Easy Scheduling and Prioritization for SDN
Get PDFNetwork state is always in flux. Due to traffic engineering,
topology changes, policy updates, VM migrations, etc.,
today’s networks undergo a variety of large updates that concurrently affect many switches. Transitioning between network states can be a source of instability, leading to outages, disruptions and security vulnerabilities. Consistent network updates [7] introduces a mechanism that guarantees to preserve well defined behaviors when transitioning between states. However, a major problem for this technique is the update performance, that is, the time it takes to install a network state update onto the data-plane—the current generation of OpenFlow switches can install flows with rate as low as 40 rules/second [2].1 Even moderate-sized updates can take several seconds, during which operators are in the dark about how badly links could be congested. [5] Therefore it is desirable to
complete updates quickly. However, we note that the lowest
bound of the total time to complete the update is determined
by the switch that is last to complete.TRUEpu
Increasing Opportunistic Gain in Small Cells Through Energy-Aware User Cooperation
Get PDFTo meet the increasing demand for wireless capacity, future networks are likely to consist of dense layouts of small cells. The number of users in each cell is thus reduced, which results in diminished gains from opportunistic scheduling, particularly under dynamic traffic loads. We propose a user-initiated base station (BS)-transparent traffic spreading approach that leverages user–user communication to increase BS scheduling flexibility. The proposed scheme can increase opportunistic gain and improve user performance. For a specified tradeoff between performance and power expenditure, we characterize the optimal policy by modeling the system as a Markov decision process and also present a heuristic algorithm that yields significant performance gains. Our simulations show that, in the performance-centric case, average file transfer delays are lowered by up to 20% even in homogeneous scenarios and up to 50% with heterogeneous users. Further, we show that the bulk of the performance improvement can be achieved with a small increase in power expenditure, e.g., in an energy-sensitive case, up to 78% of the performance improvement can be typically achieved at only 20% of the power expenditure of the performance-centric case.pu