15805395
How bad TCP can perform in mobile ad hoc networks
Several recent studies have indicated that TCP performance degrades significantly in mobile ad hoc networks. This paper examines how badly TCP may perform in such networks and provides a quantitative characterization of this performance gap. Previous approaches typically made comparisons by ignoring the inherent dynamics such as mobility, channel error and shared-channel contention. Our work provides a realistic, achievable TCP throughput upper bound, and may serve as a benchmark for future TCP modifications in ad hoc networks. Our simulation findings indicate that node mobility, especially mobility-induced network disconnection and reconnection events, has the most significant impact on TCP performance. TCP NewReno merely achieves about 10% of a reference TCPs throughput in such cases. As mobility increases, the relative throughput drop ranges from almost 0% in the static case to 1000% in a highly mobile scenario (mobility speed is 20 m/sec). In contrast, congestion and mild channel error (say, 1%) have less visible effect on TCP (with less than 10% performance drop compared with the reference TCP).
1919911
Robust rate adaptation for 802.11 wireless networks
Rate adaptation is a mechanism unspecified by the 802.11 standards, yet critical to the system performance by exploiting the multi-rate capability at the physical layer.I n this paper, we conduct a systematic and experimental study on rate adaptation over 802.11 wireless networks. Our main contributions are two-fold. First, we critique five design guidelines adopted by most existing algorithms. Our study reveals that these seemingly correct guidelines can be misleading in practice, thus incur significant performance penalty in certain scenarios. The fundamental challenge is that rate adaptation must accurately estimate the channel condition despite the presence of various dynamics caused by fading, mobility and hidden terminals. Second, we design and implement a new Robust Rate Adaptation Algorithm (RRAA)that addresses the above challenge. RRAA uses short-term loss ratio to opportunistically guide its rate change decisions, and an adaptive RTS filter to prevent collision losses from triggering rate decrease. Our extensive experiments have shown that RRAA outperforms three well-known rate adaptation solutions (ARF, AARF, and SampleRate) in all tested scenarios, with throughput improvement up to 143%.
1236934
A Packet Scheduling Approach to QoS Support in Multihop Wireless Networks
Providing packet-level quality of service (QoS) is critical to support both rate-sensitive and delay-sensitive applications in bandwidth-constrained, shared-channel, multihop wireless networks. Packet scheduling has been a very popular paradigm to ensure minimum throughput and bounded delay access for packet flows. This work describes a packet scheduling approach to QoS provisioning in multihop wireless networks. Besides minimum throughput and delay bounds for each flow, our scheduling disciplines seek to achieve fair and maximum allocation of the shared wireless channel bandwidth. However, these two criteria can potentially be in conflict in a generic-topology multihop wireless network where a single logical channel is shared among multiple contending flows and spatial reuse of the channel bandwidth is possible. In this paper, we propose a new scheduling model that addresses this conflict. The main results of this paper are the following: (a) a two-tier service model that provides a minimum “fair” allocation of the channel bandwidth for each packet flow and additionally maximizes spatial reuse of bandwidth, (b) an ideal centralized packet scheduling algorithm that realizes the above service model, and (c) a practical distributed backoff-based channel contention mechanism that approximates the ideal service within the framework of the CSMA/CA protocol.
8338982
Self-coordinating localized fair queueing in wireless ad hoc networks
Distributed fair queueing in a multihop, wireless ad hoc network is challenging for several reasons. First, the wireless channel is shared among multiple contending nodes in a spatial locality. Location-dependent channel contention complicates the fairness notion. Second, the sender of a flow does not have explicit information regarding the contending flows originated from other nodes. Fair queueing over ad hoc networks is a distributed scheduling problem by nature. Finally, the wireless channel capacity is a scarce resource. Spatial channel reuse, i.e., simultaneous transmissions of flows that do not interfere with each other, should be encouraged whenever possible. In this paper, we reexamine the fairness notion in an ad hoc network using a graph-theoretic formulation and extract the fairness requirements that an ad hoc fair queueing algorithm should possess. To meet these requirements, we propose maximize-local-minimum fair queueing (MLM-FQ), a novel distributed packet scheduling algorithm where local schedulers self-coordinate their scheduling decisions and collectively achieve fair bandwidth sharing. We then propose enhanced MLM-FQ (EMLM-FQ) to further improve the spatial channel reuse and limit the impact of inaccurate scheduling information resulted from collisions. EMLM-FQ achieves statistical short-term throughput and delay bounds over the shared wireless channel. Analysis and extensive simulations confirm the effectiveness and efficiency of our self-coordinating localized design in providing global fair channel access in wireless ad hoc networks.
11410811
Fair scheduling in wireless packet networks
Fair scheduling of delay and rate-sensitive packet flows over a wireless channel is not addressed effectively by most contemporary wireline fair-scheduling algorithms because of two unique characteristics of wireless media: (1) bursty channel errors and (2) location-dependent channel capacity and errors. Besides, in packet cellular networks, the base station typically performs the task of packet scheduling for both downlink and uplink flows in a cell; however, a base station has only a limited knowledge of the arrival processes of uplink flows. We propose a new model for wireless fair-scheduling based on an adaptation of fluid fair queueing (FFQ) to handle location-dependent error bursts. We describe an ideal wireless fair-scheduling algorithm which provides a packetized implementation of the fluid mode, while assuming full knowledge of the current channel conditions. For this algorithm, we derive the worst-case throughput and delay bounds. Finally, we describe a practical wireless scheduling algorithm which approximates the ideal algorithm. Through simulations, we show that the algorithm achieves the desirable properties identified in the wireless FFQ model.
23906830
PEAS: a robust energy conserving protocol for long-lived sensor networks
Small, inexpensive sensors with limited memory, computing power and short battery lifetimes are turning into reality. Due to adverse conditions such as high noise levels, extreme humidity or temperatures, or even destructions from unfriendly entities, sensor node failures may become norms rather than exceptions in real environments. To be practical, sensor networks must last for much longer times than that of individual nodes, and have yet to be robust against potentially frequent node failures. This paper presents the design of PEAS, a simple protocol that can build a long-lived sensor network and maintain robust operations using large quantities of economical, short-lived sensor nodes. PEAS extends system functioning time by keeping only a necessary set of sensors working and putting the rest into sleep mode. Sleeping ones wake up now and then, probing the local environment and replacing failed ones. The sleeping periods are self-adjusted dynamically, so as to keep the sensors' wakeup rate roughly constant, thus adapting to high node densities.
1954441
A Sufficient and Necessary Condition for the Consistency of XML DTDs
Recently, XML has emerged as a standard for representing and exchanging data on the World Wide Web. As a result, there is a trend of increasing amount of XML documents that publish information on the Web from various data sources. A Document Type Definition (DTD) describes the structure of a set of similar XML documents and serves as the schema for XML documents. The World Wide Web Consortium has defined the grammar for specifying DTDs; however, even a syntactically correct DTD might be inconsistent in the sense that there exist no XML documents conforming to the structure imposed by the DTD. In this paper, we formalize the notion of the consistency of DTDs, and identify a sufficient and necessary condition for a DTD to be consistent.
11393308
Computational Methods for Analysis of Cryptic Recombination in the Performance of Genomic Recombination Detection Software
The detection of recombination from DNA sequences is relevant to the understanding of evolutionary and molecular genetics. While programs such as GENECONV have been identified as detecting recombination more reliably than others, previous studies have not analyzed how many recombinations they fail to detect. We develop a method for testing how often such programs fail to identify recombinations and how detectability is affected by pairwise differences among the parental sequences. Recombination of sequences having a range of average pairwise differences (APD) is simulated by a stochastic method, and then the history of recombinations is compared to the recombinations identified by GENECONV. With high APD, GENECONV fails to detect ~50% of recombinations; while at a more typical intraspecies APD of 1% to 2%, >70% of recombinations are undetected. Quantitative results suggest corrections for estimating recombination rates more accurately and methods to detect evidence of recombination more consistently.
1552056
Pama: a Fast String Matching Algorithm
String matching is a fundamental operation in computer science, and its performance has great impact on many applications including database query, text processing, DNA and protein sequence analysis. In this paper, we propose a fast string matching algorithm, PAMA (PAttern MAtching). The shift rule used by PAMA not only subsumes both the bad character rule and the good suffix rule employed by the well-known Boyer-Moore algorithm, but also employs an additional key observation to enable faster shifting during the string matching process. Theoretically, we prove that from the same alignment, the next shift of PAMA will be at least as much as that of the Boyer-Moore algorithm. Experimentally, we show that PAMA indeed significantly outperforms the original Boyer-Moore algorithm in almost all cases, and outperforms other Boyer-Moore variants such as Tuned-BM, Turbo-BM and Horspool for long patterns (length ≥ 128) or for small alphabets (size < 8).
13993895
A formal framework for agent itinerary specification, security reasoning and logic analysis
Mobile agent technology supports object migration from one site to another autonomously and proactively, performing its designated location-dependent tasks. Although the concept of proactive mobility has recently been demonstrated in several research prototypes, there is a lack of formal treatment of such mobility from the perspective of a distributed programming language. How to specify, model and reason about travel itineraries of mobile agents is fundamentally important for the development of secure and reliable mobile agent systems. In this paper, first, we introduce an itinerary language, MAIL, to model the mobile behavior of proactive agents. The language is structured and compositional so that an itinerary can be constructed recursively from primitive itineraries. We then define the operational semantics of the language in terms of a set of inference rules and prove that MAIL is expressive enough for most migration patterns. Finally, we show that MAIL is amenable to formal methods to reason about mobility and verify correctness and safety properties.
7907707
Fast search in DNA sequence databases using punctuation and indexing
Exact pattern searching in DNA sequence databases has applications in identification of highly conserved regulatory sequences, the design of hybridization probes, and improving performance of approximate homology searching tools such as BLAST and BLAT. We propose a new pattern searching algorithm, Compressed-Punctuated-Boyer-Moore (cp-BM), to enhance exact pattern match searches of DNA sequences. cp-BM encodes two bits to represent each A, T, C, G character (4-character 8 bit (4C8B) compression), plus punctuator characters to indicate unambiguously the encoding frame of the compressed target sequence, thereby solving the misalignment problem in searching patterns with ordinary 4C8B compression. cp-BM searches DNA patterns at least 6 times faster than AGREP for pattern lengths ≥ 128 and between 2-fold and 5-fold faster than d-BM for all pattern lengths. cp-BM's performance is enhanced by punctuator indexing and multiple punctuators, especially for short sequences, yielding greater than 10-fold enhancements compared to d-BM and AGREP. In addition, cp-BM outperformed BLAT for sequences 64 or more bases in length, and was more than three-fold faster for 256 base sequences.