Showing papers by "AT&T Labs published in 1998"
TL;DR: In this paper, the authors consider the design of channel codes for improving the data rate and/or the reliability of communications over fading channels using multiple transmit antennas and derive performance criteria for designing such codes under the assumption that the fading is slow and frequency nonselective.
Abstract: We consider the design of channel codes for improving the data rate and/or the reliability of communications over fading channels using multiple transmit antennas. Data is encoded by a channel code and the encoded data is split into n streams that are simultaneously transmitted using n transmit antennas. The received signal at each receive antenna is a linear superposition of the n transmitted signals perturbed by noise. We derive performance criteria for designing such codes under the assumption that the fading is slow and frequency nonselective. Performance is shown to be determined by matrices constructed from pairs of distinct code sequences. The minimum rank among these matrices quantifies the diversity gain, while the minimum determinant of these matrices quantifies the coding gain. The results are then extended to fast fading channels. The design criteria are used to design trellis codes for high data rate wireless communication. The encoding/decoding complexity of these codes is comparable to trellis codes employed in practice over Gaussian channels. The codes constructed here provide the best tradeoff between data rate, diversity advantage, and trellis complexity. Simulation results are provided for 4 and 8 PSK signal sets with data rates of 2 and 3 bits/symbol, demonstrating excellent performance that is within 2-3 dB of the outage capacity for these channels using only 64 state encoders.
7,105 citations
24 Jul 1998
TL;DR: Several improvements to Freund and Schapire’s AdaBoost boosting algorithm are described, particularly in a setting in which hypotheses may assign confidences to each of their predictions.
Abstract: We describe several improvements to Freund and Schapire‘s AdaBoost boosting algorithm, particularly in a setting in which hypotheses may assign confidences to each of their predictions. We give a simplified analysis of AdaBoost in this setting, and we show how this analysis can be used to find improved parameter settings as well as a refined criterion for training weak hypotheses. We give a specific method for assigning confidences to the predictions of decision trees, a method closely related to one used by Quinlan. This method also suggests a technique for growing decision trees which turns out to be identical to one proposed by Kearns and Mansour. We focus next on how to apply the new boosting algorithms to multiclass classification problems, particularly to the multi-label case in which each example may belong to more than one class. We give two boosting methods for this problem, plus a third method based on output coding. One of these leads to a new method for handling the single-label case which is simpler but as effective as techniques suggested by Freund and Schapire. Finally, we give some experimental results comparing a few of the algorithms discussed in this paper.
2,900 citations
21 Apr 1998
TL;DR: The naive Bayes classifier, currently experiencing a renaissance in machine learning, has long been a core technique in information retrieval, and some of the variations used for text retrieval and classification are reviewed.
Abstract: The naive Bayes classifier, currently experiencing a renaissance in machine learning, has long been a core technique in information retrieval. We review some of the variations of naive Bayes models used for text retrieval and classification, focusing on the distributional assumptions made about word occurrences in documents.
2,235 citations
TL;DR: The design, implementation, security, performance, and scalability of the Crowds system for protecting users' anonymity on the world-wide-web are described and degrees of anonymity as an important tool for describing and proving anonymity properties are introduced.
Abstract: In this paper we introduce a system called Crowds for protecting users' anonymity on the world-wide-web. Crowds, named for the notion of “blending into a crowd,” operates by grouping users into a large and geographically diverse group (crowd) that collectively issues requests on behalf of its members. Web servers are unable to learn the true source of a request because it is equally likely to have originated from any member of the crowd, and even collaborating crowd members cannot distinguish the originator of a request from a member who is merely forwarding the request on behalf of another. We describe the design, implementation, security, performance, and scalability of our system. Our security analysis introduces degrees of anonymity as an important tool for describing and proving anonymity properties.
2,045 citations
31 May 1998
TL;DR: A definition of protocol divertibility is given that applies to arbitrary 2-party protocols and is compatible with Okamoto and Ohta's definition in the case of interactive zero-knowledge proofs and generalizes to cover several protocols not normally associated with divertibility.
Abstract: First, we introduce the notion of divertibility as a protocol property as opposed to the existing notion as a language property (see Okamoto, Ohta [OO90]) We give a definition of protocol divertibility that applies to arbitrary 2-party protocols and is compatible with Okamoto and Ohta's definition in the case of interactive zero-knowledge proofs Other important examples falling under the new definition are blind signature protocols We propose a sufficiency criterion for divertibility that is satisfied by many existing protocols and which, surprisingly, generalizes to cover several protocols not normally associated with divertibility (eg, Diffie-Hellman key exchange) Next, we introduce atomic proxy cryptography, in which an atomic proxy function, in conjunction with a public proxy key, converts ciphertexts (messages or signatures) for one key into ciphertexts for another Proxy keys, once generated, may be made public and proxy functions applied in untrusted environments We present atomic proxy functions for discrete-log-based encryption, identification, and signature schemes It is not clear whether atomic proxy functions exist in general for all public-key cryptosystems Finally, we discuss the relationship between divertibility and proxy cryptography
1,533 citations
TL;DR: Two approaches to build integer to integer wavelet transforms are presented and the precoder of Laroiaet al., used in information transmission, is adapted and combined with expansion factors for the high and low pass band in subband filtering.
1,269 citations
24 Jul 1998
TL;DR: A new algorithm for linear classification which combines Rosenblatt‘s perceptron algorithm with Helmbold and Warmuth’s leave-one-out method is introduced, which takes advantage of data that are linearly separable with large margins.
Abstract: We introduce and analyze a new algorithm for linear classification which combines Rosenblatt‘s perceptron algorithm with Helmbold and Warmuth‘s leave-one-out method. Like Vapnik‘s maximal-margin classifier, our algorithm takes advantage of data that are linearly separable with large margins. Compared to Vapnik‘s algorithm, however, ours is much simpler to implement, and much more efficient in terms of computation time. We also show that our algorithm can be efficiently used in very high dimensional spaces using kernel functions. We performed some experiments using our algorithm, and some variants of it, for classifying images of handwritten digits. The performance of our algorithm is close to, but not as good as, the performance of maximal-margin classifiers on the same problem, while saving significantly on computation time and programming effort.
1,227 citations
TL;DR: This work introduces, analyzes and demonstrates a recursive hierarchical generalization of the widely used hidden Markov models, which is motivated by the complex multi-scale structure which appears in many natural sequences, particularly in language, handwriting and speech.
Abstract: We introduce, analyze and demonstrate a recursive hierarchical generalization of the widely used hidden Markov models, which we name Hierarchical Hidden Markov Models (HHMM) Our model is motivated by the complex multi-scale structure which appears in many natural sequences, particularly in language, handwriting and speech We seek a systematic unsupervised approach to the modeling of such structures By extending the standard Baum-Welch (forward-backward) algorithm, we derive an efficient procedure for estimating the model parameters from unlabeled data We then use the trained model for automatic hierarchical parsing of observation sequences We describe two applications of our model and its parameter estimation procedure In the first application we show how to construct hierarchical models of natural English text In these models different levels of the hierarchy correspond to structures on different length scales in the text In the second application we demonstrate how HHMMs can be used to automatically identify repeated strokes that represent combination of letters in cursive handwriting
1,050 citations
TL;DR: A minimum mean-square-error (MMSE) channel estimator is derived, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels and can significantly improve the performance of OFDM systems in a rapid dispersion fading channel.
Abstract: Orthogonal frequency-division multiplexing (OFDM) modulation is a promising technique for achieving the high bit rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3-dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MMSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator-to-channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.
1,039 citations
01 Aug 1998
TL;DR: Experimental results show that refining the set of documents used in query expansion often prevents the query drift caused by blind expansion and yields substantial improvements in retrieval effectiveness, both in terms of average precision and precision in the top twenty documents.
Abstract: Most casual users of IR systems type short queries. Recent research has shown that adding new words to these queries via odhoc feedback improves the retrieval effectiveness of such queries. We investigate ways to improve this query expansion process by refining the set of documents used in feedback. We start by using manually formulated Boolean filters along with proximity constraints. Our approach is similar to the one proposed by Hearst[l2]. Next, we investigate a completely automatic method that makes use of term cooccurrence information to estimate word correlation. Experimental results show that refining the set of documents used in query expansion often prevents the query drift caused by blind expansion and yields substantial improvements in retrieval effectiveness, both in terms of average precision and precision in the top twenty documents. More importantly, the fully automatic approach developed in this study performs competitively with the best manual approach and requires little computational overhead.
685 citations
TL;DR: In this paper, the authors consider the Byzantine failure of data repositories and present the first study of quorum system requirements and constructions that ensure data availability and consistency despite these failures, and also consider the load associated with their quorum systems, i.e., the minimal access probability of the busiest server.
Abstract: Quorum systems are well-known tools for ensuring the consistency and availability of replicated data despite the benign failure of data repositories. In this paper we consider the arbitrary (Byzantine) failure of data repositories and present the first study of quorum system requirements and constructions that ensure data availability and consistency despite these failures. We also consider the load associated with our quorum systems, i.e., the minimal access probability of the busiest server. For services subject to arbitrary failures, we demonstrate quorum systems over n servers with a load of O(1/√n), thus meeting the lower bound on load for benignly fault-tolerant quorum systems. We explore several variations of our quorum systems and extend our constructions to cope with arbitrary client failures.
07 Jun 1998
TL;DR: A minimum mean-square-error (MSE) channel estimator is derived, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels and can significantly improve the performance of OFDM systems in a rapid dispersion fading channel.
Abstract: Orthogonal frequency division multiplexing (OFDM) modulation is a promising technique for achieving the high-bit-rates required for a wireless multimedia service. Without channel estimation and tracking, OFDM systems have to use differential phase-shift keying (DPSK), which has a 3 dB signal-to-noise ratio (SNR) loss compared with coherent phase-shift keying (PSK). To improve the performance of OFDM systems by using coherent PSK, we investigate robust channel estimation for OFDM systems. We derive a minimum mean-square-error (MSE) channel estimator, which makes full use of the time- and frequency-domain correlations of the frequency response of time-varying dispersive fading channels. Since the channel statistics are usually unknown, we also analyze the mismatch of the estimator to channel statistics and propose a robust channel estimator that is insensitive to the channel statistics. The robust channel estimator can significantly improve the performance of OFDM systems in a rapid dispersive fading channel.
TL;DR: This paper formalizes a new but related model of learning from statistical queries, and demonstrates the generality of the statistical query model, showing that practically every class learnable in Valiant's model and its variants can also be learned in the new model (and thus can be learning in the presence of noise).
Abstract: In this paper, we study the problem of learning in the presence of classification noise in the probabilistic learning model of Valiant and its variants. In order to identify the class of “robust” learning algorithms in the most general way, we formalize a new but related model of learning from statistical queries. Intuitively, in this model a learning algorithm is forbidden to examine individual examples of the unknown target function, but is given acess to an oracle providing estimates of probabilities over the sample space of random examples.One of our main results shows that any class of functions learnable from statistical queries is in fact learnable with classification noise in Valiant's model, with a noise rate approaching the information-theoretic barrier of 1/2. We then demonstrate the generality of the statistical query model, showing that practically every class learnable in Valiant's model and its variants can also be learned in the new model (and thus can be learned in the presence of noise). A notable exception to this statement is the class of parity functions, which we prove is not learnable from statistical queries, and for which no noise-tolerant algorithm is known.
01 Jan 1998
TL;DR: For the Support Vector method both the quality of solution and the complexity of the solution does not depend directly on the dimensionality of an input space, and on the basis of this technique one can obtain a good estimate using a given number of high-dimensional data.
Abstract: This chapter describes the Support Vector technique for function estimation problems such as pattern recognition, regression estimation, and solving linear operator equations. It shows that for the Support Vector method both the quality of solution and the complexity of the solution does not depend directly on the dimensionality of an input space. Therefore, on the basis of this technique one can obtain a good estimate using a given number of high-dimensional data.
24 Jul 1998
TL;DR: The PAC-Bayesian theorems given here apply to an arbitrary prior measure on an arbitrary concept space and provide an alternative to the use of VC dimension in proving PAC bounds for parameterized concepts.
Abstract: This paper gives PAC guarantees for “Bayesian” algorithms—algorithms that optimize risk minimization expressions involving a prior probability and a likelihood for the training data. PAC-Bayesian algorithms are motivated by a desire to provide an informative prior encoding information about the expected experimental setting but still having PAC performance guarantees over all IID settings. The PAC-Bayesian theorems given here apply to an arbitrary prior measure on an arbitrary concept space. These theorems provide an alternative to the use of VC dimension in proving PAC bounds for parameterized concepts.
01 Jun 1998
TL;DR: The results of experiments demonstrate that the Pyramid-Technique outperforms the X-tree and the Hilbert R-tree by a factor of up to 14 (number of page accesses) and up to 2500 (total elapsed time) for range queries.
Abstract: In this paper, we propose the Pyramid-Technique, a new indexing method for high-dimensional data spaces. The Pyramid-Technique is highly adapted to range query processing using the maximum metric Lmax. In contrast to all other index structures, the performance of the Pyramid-Technique does not deteriorate when processing range queries on data of higher dimensionality. The Pyramid-Technique is based on a special partitioning strategy which is optimized for high-dimensional data. The basic idea is to divide the data space first into 2d pyramids sharing the center point of the space as a top. In a second step, the single pyramids are cut into slices parallel to the basis of the pyramid. These slices from the data pages. Furthermore, we show that this partition provides a mapping from the given d-dimensional space to a 1-dimensional space. Therefore, we are able to use a B+-tree to manage the transformed data. As an analytical evaluation of our technique for hypercube range queries and uniform data distribution shows, the Pyramid-Technique clearly outperforms index structures using other partitioning strategies. To demonstrate the practical relevance of our technique, we experimentally compared the Pyramid-Technique with the X-tree, the Hilbert R-tree, and the Linear Scan. The results of our experiments using both, synthetic and real data, demonstrate that the Pyramid-Technique outperforms the X-tree and the Hilbert R-tree by a factor of up to 14 (number of page accesses) and up to 2500 (total elapsed time) for range queries.
01 Jun 1998
TL;DR: This paper rejects the assumption that global domains can be easily constructed, and assumes instead that the names are given in natural language text, and proposes a logic called WHIRL which reasons explicitly about the similarity of local names, as measured using the vector-space model commonly adopted in statistical information retrieval.
Abstract: Most databases contain “name constants” like course numbers, personal names, and place names that correspond to entities in the real world. Previous work in integration of heterogeneous databases has assumed that local name constants can be mapped into an appropriate global domain by normalization. However, in many cases, this assumption does not hold; determining if two name constants should be considered identical can require detailed knowledge of the world, the purpose of the user's query, or both. In this paper, we reject the assumption that global domains can be easily constructed, and assume instead that the names are given in natural language text. We then propose a logic called WHIRL which reasons explicitly about the similarity of local names, as measured using the vector-space model commonly adopted in statistical information retrieval. We describe an efficient implementation of WHIRL and evaluate it experimentally on data extracted from the World Wide Web. We show that WHIRL is much faster than naive inference methods, even for short queries. We also show that inferences made by WHIRL are surprisingly accurate, equaling the accuracy of hand-coded normalization routines on one benchmark problem, and outperforming exact matching with a plausible global domain on a second.
01 Oct 1998
TL;DR: A simple construction based on cascades that allows for a plausible physical explanation of the observed multifractal scaling behavior of data traffic and suggests that the underlying multiplicative structure is a traffic invariant for WAN traffic that co-exists with self-similarity is provided.
Abstract: In apparent contrast to the well-documented self-similar (i.e., monofractal) scaling behavior of measured LAN traffic, recent studies have suggested that measured TCP/IP and ATM WAN traffic exhibits more complex scaling behavior, consistent with multifractals. To bring multifractals into the realm of networking, this paper provides a simple construction based on cascades (also known as multiplicative processes) that is motivated by the protocol hierarchy of IP data networks. The cascade framework allows for a plausible physical explanation of the observed multifractal scaling behavior of data traffic and suggests that the underlying multiplicative structure is a traffic invariant for WAN traffic that co-exists with self-similarity. In particular, cascades allow us to refine the previously observed self-similar nature of data traffic to account for local irregularities in WAN traffic that are typically associated with networking mechanisms operating on small time scales, such as TCP flow control.To validate our approach, we show that recent measurements of Internet WAN traffic from both an ISP and a corporate environment are consistent with the proposed cascade paradigm and hence with multifractality. We rely on wavelet-based time-scale analysis techniques to visualize and to infer the scaling behavior of the traces, both globally and locally. We also discuss and illustrate with some examples how this cascade-based approach to describing data network traffic suggests novel ways for dealing with networking problems and helps in building intuition and physical understanding about the possible implications of multifractality on issues related to network performance analysis.
TL;DR: The theory and practice of a new advanced modem technology suitable for high-data-rate wireless communications and its performance over a frequency-flat Rayleigh fading channel are presented and it is concluded that STCM can provide significant SNR improvement over simple delay diversity.
Abstract: This paper presents the theory and practice of a new advanced modem technology suitable for high-data-rate wireless communications and presents its performance over a frequency-flat Rayleigh fading channel. The new technology is based on space-time coded modulation (STCM) with multiple transmit and/or multiple receive antennas and orthogonal pilot sequence insertion (O-PSI). In this approach, data is encoded by a space-time (ST) channel encoder and the output of the encoder is split into N streams to be simultaneously transmitted using N transmit antennas. The transmitter inserts periodic orthogonal pilot sequences in each of the simultaneously transmitted bursts. The receiver uses those pilot sequences to estimate the fading channel. When combined with an appropriately designed interpolation filter, accurate channel state information (CSI) can be estimated for the decoding process. Simulation results of the proposed modem, as applied to the IS-136 cellular standard, are presented. We present the frame error rate (FER) performance results as a function of the signal-to-noise ratio (SNR) and the maximum Doppler frequency, in the presence of timing and frequency offset errors. Simulation results show that for a 10% FER, a 32-state eight-phase-shift keyed (8-PSK) ST code with two transmit and two receive antennas can support data rates up to 55.8 kb/s on a 30-kHz channel, at an SNR of 11.7 dB and a maximum Doppler frequency of 180 Hz. Simulation results for other codes and other channel conditions are also provided. We also compare the performance of the proposed STCM scheme with delay diversity schemes and conclude that STCM can provide significant SNR improvement over simple delay diversity.
08 Nov 1998
TL;DR: A statistical path loss model derived from 1.9 GHz experimental data collected across the United States in 95 existing macrocells is presented, and it distinguishes between different terrain categories.
Abstract: We present a statistical path loss model derived from 1.9 GHz experimental data collected across the United States in 95 existing macrocells. The model is for suburban areas, and it distinguishes between different terrain categories. Moreover, it applies to distances and base antenna heights not well-covered by existing models. The characterization used is a linear curve fitting the dB path loss to the dB-distance, with a Gaussian random variation about that curve due to shadow fading. The slope of the linear curve (corresponding to the path loss exponent, /spl gamma/) is shown to be a random variate from one macrocell to another, as is the standard deviation, /spl sigma/ of the shadow fading. These two parameters are statistically modeled, with the dependencies on base antenna height and terrain category made explicit. The resulting path loss model applies to base antenna heights from 10 to 80 meters; base-to-terminal distances from 0.1 to 8 km; and three distinct terrain categories.
TL;DR: An algorithm for approximating a long-tail distribution by a hyperexponential distribution (a finite mixture of exponentials) is developed, proving that, in prinicple, it is possible to approximate distributions from a large class, including the Pareto and Weibull distributions, arbitrarily closely by hyperexPonential distributions.
01 Nov 1998
TL;DR: A minimum mean-squared error (MMSE) interference suppression technique for space-time block codes that can effectively suppress interference from other co-channel users while providing each user with a diversity benefit and is used to increasing the capacity and/or data rate of wireless communication systems.
Abstract: This paper presents a combined interference suppression and ML decoding scheme for space-time block codes that can effectively suppress interference from other co-channel users while providing each user with a diversity benefit. We consider a multiuser environment with K synchronous co-channel users, each is equipped with N transmit antennas and uses the space-time block coding. By exploiting the temporal and spatial structure of these codes, we develop a minimum mean-squared error (MMSE) interference suppression technique. Assuming that the receiver uses M/spl ges/K receive antennas, these technique will perfectly suppress the interference from the K-1 co-channel space-time users and provide a diversity order of N/spl times/(M-K+1) to each of the K users. Moreover, this MMSE solution tends itself to an adaptive implementation and does not require any explicit knowledge about the interference. In conjunction with this interference suppression technique, we show how space-time block codes can be used to increasing the capacity and/or data rate of wireless communication systems.
01 Aug 1998
TL;DR: This paper discusses two learning algorithms for text filtering: modified Rocchio and a boosting algorithm called AdaBoost, and shows how both algorithms can be adapted to maximize any general utility matrix that associates cost for each pair of machine prediction and correct label.
Abstract: We discuss two learning algorithms for text filtering: modified Rocchio and a boosting algorithm called AdaBoost. We show how both algorithms can be adapted to maximize any general utility matrix that associates cost (or gain) for each pair of machine prediction and correct label. We first show that AdaBoost significantly outperforms another highly effective text filtering algorithm. We then compare AdaBoost and Rocchio over three large text filtering tasks. Overall both algorithms are comparable and are quite effective. AdaBoost produces better classifiers than Rocchio when the training collection contains a very large number of relevant documents. However, on these tasks, Rocchio runs much faster than AdaBoost.
15 Apr 1998
TL;DR: The rationale for designing a simple trust-management system for public-key infrastructures, called KeyNote, based on the motivating principles are expressibility, simplicity, and extensibility is discussed.
Abstract: This paper discusses the rationale for designing a simple trust-management system for public-key infrastructures, called KeyNote. The motivating principles are expressibility, simplicity, and extensibility. We believe that none of the existing public-key infrastructure proposals provide as good a combination of these three factors.
01 Jun 1998
TL;DR: The Strudel system as mentioned in this paper applies concepts from database management systems to the process of building web sites, separating the management of the site's data, the creation and management of web site's structure, and the visual presentation of web pages.
Abstract: The Strudel system applies concepts from database management systems to the process of building Web sites. Strudel's key idea is separating the management of the site's data, the creation and management of the site's structure, and the visual presentation of the site's pages. First, the site builder creates a uniform model of all data available at the site. Second, the builder uses this model to declaratively define the Web site's structure by applying a “site-definition query” to the underlying data. The result of evaluating this query is a “site graph”, which represents both the site's content and structure. Third, the builder specifies the visual presentation of pages in Strudel's HTML-template language. The data model underlying Strudel is a semi-structured model of labeled directed graphs.We describe Strudel's key characteristics, report on our experiences using Strudel, and present the technical problems that arose from our experience. We describe our experience constructing several Web sites with Strudel and discuss the impact of potential users' requirements on Strudel's design. We address two main questions: (1) when does a declarative specification of site structure provide significant benefits, and (2) what are the main advantages provided by the semi-structured data model.
01 Apr 1998
TL;DR: Preliminary results from an in-depth, wavelet-based analysis of a set of high-quality, packet-level traffic measurements, collected over the last 6-7 years from a number of different wide-area networks, show that the changing nature of WAN traffic can be naturally accounted for by self-similar traffic models.
Abstract: In this paper, we report on some preliminary results from an in-depth, wavelet-based analysis of a set of high-quality, packet-level traffic measurements, collected over the last 6-7 years from a number of different wide-area networks (WANs). We first validate and confirm an earlier finding, originally due to Paxson and Floyd [14], that actual WAN traffic is consistent with statistical self-similarity for sufficiently large time scales. We then relate this large-time scaling phenomenon to the empirically observed characteristics of WAN traffic at the level of individual connections or applications. In particular, we present here original results about a detailed statistical analysis of Web-session characteristics, and report on an intriguing scaling property of measured WAN traffic at the transport layer (i.e., number of TCP connection arrivals per time unit). This scaling property of WAN traffic at the TCP layer was absent in the pre-Web period but has become ubiquitous in today's WWW-dominated WANs and is a direct consequence of the ever-increasing popularity of the Web (WWW) and its emergence as the major contributor to WAN traffic. Moreover, we show that this changing nature of WAN traffic can be naturally accounted for by self-similar traffic models, primarily because of their ability to provide physical explanations for empirically observed traffic phenomena in a networking context. Finally, we provide empirical evidence that actual WAN traffic traces also exhibit scaling properties over small time scales, but that the small-time scaling phenomenon is distinctly different from the observed large-time scaling property. We relate this newly observed characteristic of WAN traffic to the effects that the dominant network protocols (e.g., TCP) and controls have on the flow of packets across the network and discuss the potential that multifractals have in this context for providing a structural modeling approach for WAN traffic and for capturing in a compact and parsimonious manner the observed scaling phenomena at large as well as small time scales.
TL;DR: The author emphasizes the need to closely examine a problematic aspect of component reuse: the necessity and potential expense of validating components in their new environments.
Abstract: Components designed for reuse are expected to lower costs and shorten the development life cycle, but this may not prove so simple. The author emphasizes the need to closely examine a problematic aspect of component reuse: the necessity and potential expense of validating components in their new environments.
TL;DR: A new image compression technique called DjVu is presented that enables fast transmission of document images over low-speed connections, while faithfully reproducing the visual aspect of the document, including color, fonts, pictures, and paper texture.
TL;DR: The results show that transmit diversity with M transmit antennas provides a diversity gain within 0.1 dB of that with M receive antennas for any number of antennas, and that the same diversity benefit can be obtained at the remotes and base stations using multiple base-station antennas only.
Abstract: In this paper, we study the ability of transmit diversity to provide diversity benefit to a receiver in a Rayleigh fading environment. With transmit diversity, multiple antennas transmit delayed versions of a signal to create frequency-selective fading at a single antenna at the receiver, which uses equalization to obtain diversity gain against fading. We use Monte Carlo simulation to study transmit diversity for the case of independent Rayleigh fading from each transmit antenna to the receive antenna and maximum likelihood sequence estimation for equalization at the receiver. Our results show that transmit diversity with M transmit antennas provides a diversity gain within 0.1 dB of that with M receive antennas for any number of antennas. Thus, we can obtain the same diversity benefit at the remotes and base stations using multiple base-station antennas only.
23 Feb 1998
TL;DR: PolicyMaker trust management system, a general tool for addressing the trust management problem of emerging electronic commerce services that use public-key cryptography on a mass-market scale, is described.
Abstract: Emerging electronic commerce services that use public-key cryptography on a mass-market scale require sophisticated mechanisms for managing trust. For example, any service that receives a signed request for action is forced to answer the central question “Is the key used to sign this request authorized to take this action?” In some services, this question reduces to “Does this key belong to this person?” In others, the authorization question is more complicated, and resolving it requires techniques for formulating security policies and security credentials, determining whether particular sets of credentials satisfy the relevant policies, and deferring trust to third parties. Blaze, Feigenbaum, and Lacy [1] identified this trust management problem as a distinct and important component of network services and described a general tool for addressing it, the PolicyMaker trust management system.