Distribution by Scientific Domains
Distribution within Engineering

Kinds of Packets

  • control packet
  • data packet
  • wave packet

  • Terms modified by Packets

  • packet delay
  • packet dropout
  • packet loss
  • packet loss probability
  • packet loss rate
  • packet network
  • packet radio service
  • packet scheduling
  • packet switch

  • Selected Abstracts

    An Educational Strategy for Teaching Standardized Nursing Languages

    Arlene T. Farren RN, CTN-A
    PURPOSE., The aim of this paper is to describe an educational strategy for teaching standardized nursing languages (SNL) used in both the classroom and clinical components of a psychiatric,mental health nursing course at the associate degree level. DATA SOURCES., Data included a review of the relevant literature, teaching experiences, and faculty and student experiences. DATA SYNTHESIS., Enhancing associate degree student nurses' competency regarding diagnosis and interventions is essential to influence positive health outcomes. Use of diagnostic, outcome, and intervention classifications for learning nursing care promotes critical thinking, individualization of nursing care, and students' fluency with SNL. One possible teaching strategy to assist students to learn and use SNL was implemented through the use of a faculty-developed Student Nurse Documentation Packet. CONCLUSIONS., The educational strategy provided students opportunities to enhance their experience with the SNL to plan and document care of individuals experiencing psychiatric,mental health problems. IMPLICATIONS FOR NURSING., The educational strategy used in this program was judged to be successful. Research is needed to provide empirical evidence of the efficacy of this pedagogical strategy for increasing knowledge and enhancing students' competency. [source]

    Higher prevalence of bipolar I disorder among Asian and Latino compared to Caucasian patients receiving treatment

    Sophia H.J. Hwang MSEd
    Abstract Introduction: There are limited data regarding relationships between race/ethnicity and bipolar disorder. This study assessed such relationships in patients receiving treatment in a university clinic. Methods: Demographic, illness characteristics, symptom severity, treatment, and care utilization data were collected from the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD) Affective Disorders Evaluation, Mini-International Neuropsychiatric Interview, and the STEP-BD Clinician Rated Packet. Data were compared among 51 Asian, 35 Latino, and 86 Caucasian patients in treatment at the Stanford University Bipolar Disorders Clinic. ,2 tests and analyses of variance were used to assess between-group differences. Results: Asian and Latino compared to Caucasian patients had significantly higher prevalence of bipolar I disorder (58.8% and 60.0% versus 37.2%, respectively). Asian and Latino patients also had a higher prevalence of history of psychosis, but this was related to the excess of bipolar I disorder, becoming non-significant after controlling for bipolar subtype. The racial/ethnic difference in bipolar subtype prevalence did not appear to be secondary to demographic or socioeconomic differences. Discussion: The higher prevalence of bipolar I disorder and thus lower prevalence of bipolar II disorder and bipolar disorder not otherwise specified in Asian and Latino patients may be related to under-diagnosis, misdiagnosis, or care underutilization of patients with milder forms of bipolar disorders. Additional research and public health efforts are warranted to further understand the effects of race and ethnicity on the management of bipolar disorders and to enhance timely and accurate diagnosis, culturally sensitive treatment, and optimal care utilization. [source]

    Split agent-based routing in interconnected networks

    Constandinos X. Mavromoustakis
    Abstract Adaptive behaviour of swarm-based agents (BT Technol. J. 1994; 12:104,113; AAMAS Conference '02, Melbourne, Australia, Month 1,2, 2002; Softcomput. J. 2001; 5(4):313,317.) is being studied in this paper with respect to network throughput for a certain amount of data traffic. Algorithmically complex problems like routing data packets in a network need to be faced with a dynamically adaptive approach such as agent-based scheme. Particularly in interconnected networks where multiple networks are participating in order to figure a large-scale network with different QoS levels and heterogeneity in the service of delay sensitive packets, routing algorithm must adopt in frequent network changes to anticipate such situations. Split agent-based routing technique (SART) is a variant of swarm-based routing (Adapt. Behav. 1997; 5:169,207; Proceedings of 2003 International Symposium on Performance Evaluation of Computer and Telecommunication Systems,SPECTS, Montreal, Canada, July 20,24, 2003; 240,247.) where agents are split after their departure to the next node on a hop-by-hop basis. Packets that are delay sensitive are marked as prioritized which agents recognize-as being a part of a packet- and try to influence the two-way routing tables. Thorough examination is made, for the performance of the proposed algorithm in the network and the QoS offered, taking into account a number of metrics. It is shown that the split agent routing scheme applied to interconnected networks offers a decentralized control in the network and an efficient way to increase overall performance and packet control reducing at the same time the packet loss concept. Copyright © 2004 John Wiley & Sons, Ltd. [source]


    R. Higgs
    The Ross Formation (Namurian, Ireland) and the near-identical Bude Formation (Westphalian, England), both amply described in the literature, are used by oil companies as deep-sea-fan reservoir analogues. However, the Ross Formation is reinterpreted here, like the Bude Formation in recent publications, to be composed of river-fed turbidites deposited on the wave-influenced northern shelf of a Variscan foreland-basin lake, which also had a southern flysch trough. Key features of these formations are: (i) two classes of thin (, 0.4m) sandstone "event bed" in shale comprising (a) structureless turbidite-like beds, and (b) rippled beds with combined-flow ripples and/or hummocky cross-stratification, neither structure having previously been reported from the Ross Formation; (ii) "trademark" tabular packets (1,10 m) of amalgamated event beds which interfinger laterally with mudstones; (iii) sharp packet bases and tops; (iv) rare sinuous channel fills; and (v) rare thick (1,10m) shale units, each containing a thin (cm-dm) fossiliferous band. The fossil bands are interpreted here as maximum flooding surfaces, reflecting glacioeustatic marine incursions over the lake spill point (sill), forcing the lake to rise and to turn marine or strongly brackish; these bands define Galloway-type depositional sequences 50,100 m thick. During eustatic falls, the lake was forced down to sill level, where it perched and turned fresh (desalination). Intervals containing sandstone packets are attributed to the falling-stage and lowstand systems tracts, each packet representing a higher-order lowstand systems tract. Packets are interpreted as tongue shaped, supplied by river-fed underflows. Packet bases (sharp) represent the storm-wave-graded equilibrium shelf profile, glacioeustatically forced to its lowstand position. On this erosion surface were deposited underflow turbidites produced by floods in the catchment. Occasional catastrophic storms on the lake shaved these turbidites and interfingering fair-weather muds back down to the equilibrium level, leaving behind a subsidence-accommodated increment whose surface was sculpted by storm wind and wave currents, forming hummocks, combined-flow ripples and erosional megaflutes. Whenever a river-fed underflow accompanied one of these storms, the resulting highly erosive combined flow carved a sinuous channel on the wave-sculpted equilibrium surface. Sandstone-shale intervals separating the sandstone packets are interpreted as transgressive- and highstand systems tracts. They contain both turbidites and wave-modified turbidites (rippled beds), deposited on the out-of-equilibrium drowned shelf. A gradual rotation in sole-mark direction with time in both formations is attributed to a reversal of Coriolis deflection as the plate drifted north across the equator, causing underflows (deflected along-shelf geostrophically) to flow first NEwards and then SWwards on an inferred SE-facing shelf. The lack of evidence for emergence in the Ross and Bude Formations, in spite of the great thicknesses (460m and 1,290m, respectively) of these shallow-water deposits, is attributed to regulation of minimum water depth firstly by the lake sill blocking eustatically-forced exposure, and secondly by storm grading, preventing emergence by sedimentation. [source]

    Ultrastructure of the differentiating zygotospores of Porphyra leucosticta (Rhodophyta)

    Ioannes Tsekos
    SUMMARY The ultrastructure of zygotosporogenesis is described for the red alga Porphyra leucosticta Thuret. Packets of eight zygotosporangia, each packet derived from a single carpogonium are interspersed among vegetative cells. Zygotospore differentiation in Porphyra can be separated into three developmental stages. (i) Young zygotospores exhibit a nucleus and a large centrally located, lobed plastid with pyrenoid. Mucilage is produced within concentric membrane structures during their dilation, thus resulting in the formation of mucilage sacs. Subsequently, these sacs release their contents, initiating the zygotospore wall formation. Straight-profiled dictyosomes produce vesicles that also provide wall material. During the later stages of young zygotospores, starch polymerization commences, (ii) Medium-aged zygotospores are characterized by the presence of fibrous vacuoles. These are formed from the ,fibrous vacuole associated organelles'. The fibrous vacuoles finally discharge their contents. (iii) Mature zygotospores are recognized by the presence of numerous cored vesicles produced by dictyosomes. Cored vesicles either discharge their contents or are incorporated into the fibrous vacuoles. There is a gradual reduction of starch granules during zygotospore differentiation. Mature zygotospores are surrounded by a fibrous wall, have a large chloroplast with pyrenoid and well-depicted phycobilisomes but are devoid of starch granules. [source]

    Using Multinomial Mixture Models to Cluster Internet Traffic

    Murray Jorgensen
    Summary The paper considers the clustering of two large sets of Internet traffic data consisting of information measured from headers of transmission control protocol packets collected on a busy arc of a university network connecting with the Internet. Packets are grouped into 'flows' thought to correspond to particular movements of information between one computer and another. The clustering is based on representing the flows as each sampled from one of a finite number of multinomial distributions and seeks to identify clusters of flows containing similar packet-length distributions. The clustering uses the EM algorithm, and the data-analytic and computational details are given. [source]

    Introductory quantum physics courses using a LabVIEW multimedia module

    Ismael Orquín
    Abstract We present the development of a LabVIEW multimedia module for introductory Quantum Physics courses and our experience in the use of this application as an educational tool in learning methodologies. The program solves the time-dependent Schrödinger equation (TDSE) for arbitrary potentials. We describe the numerical method used for solving this equation, as well as some mathematical tools employed to reduce the calculation time and to obtain more accurate results. As an illustration, we present the evolution of a wave packet for three different potentials: the repulsive barrier potential, the repulsive step potential, and the harmonic oscillator. This application has been successfully integrated in the learning strategies of the course Quantum Physics for Engineering at the Polytechnic University of Valencia, Spain. © 2007 Wiley Periodicals, Inc. Comput Appl Eng Educ. 15: 124,133, 2007; Published online in Wiley InterScience (; DOI 10.1002/cae.20100 [source]

    CAC and routing for multi-service networks with blocked wide-band calls delayed, Part II: approximative link MDP framework

    Ernst Nordström
    In this paper, we study the call admission control (CAC) and routing issue in multi-service networks. Two categories of calls are considered: a narrow-band with blocked calls cleared and a wide-band with blocked calls delayed. The optimisation is subject to several quality of service (QoS) constraints, either on the packet or call level. The objective function is formulated as reward maximisation with penalty for delay. A suboptimal solution is achieved by applying Markov decision process (MDP) theory together with a three-level approximation. First, the network is decomposed into a set of links assumed to have independent Markov and reward processes respectively. Second, the dimensions of the link Markov and reward processes are reduced by aggregation of the call classes into call categories. Third, by applying decomposition of the link Markov process, the link MDP tasks are simplified considerably. The CAC and routing policy is computed by the policy iteration algorithm from MDP theory. The numerical results show that the proposed CAC and routing method, based on the approximate link MDP framework, is able to find an efficient trade-off between reward loss and average call set-up delay, outperforming conventional methods such as least loaded routing (LLR). Copyright © 2006 AEIT. [source]

    On the efficiency of PGPS-based packet and cell switching technologies for traffic with guaranteed delay

    Fulvio Risso
    Circuit switching, suited to providing real-time services due to the low and fixed switching delay, is not cost effective for building integrated services networks because it is based on static allocation of resources which is not efficient with bursty data traffic. Moreover it cannot handle flows that are not integer multiple of 64,Kb/s, preventing the usage of low bit rate codecs. This work explores the most suitable alternatives to the circuit switching technology (i.e. packet/cell switching) from the efficiency point of view, assuming that a PGPS scheduler is deployed in the network nodes. The paper defines an index to measure the efficiency of packet telephony, i.e. the volume of real-time traffic with deterministically guaranteed quality plus the amount of data carried related to the amount of network resources used. Furthermore it determines the maximum efficiency obtainable by packet networks, compares different network technologies and explores the problems of the deploying of low bit-rate codecs. Copyright © 2003 AEI. [source]

    QoS in IntServ-based IP networks: the peak rate policing

    Lorenzo Battaglia
    In the last few years, IP has moved towards resource reservation, with the task to guarantee in the future Quality of Service (QoS). This has led to flow admission control algorithms based on the negotiation of standardised traffic parameters. QoS can be guaranteed in any network, a priori from the used technology, only if the used admission control algorithm wisely shares the network's resources among the users. Any admission control algorithm on its turn can do so, only if every user respects the negotiated traffic parameters. Since any user could, maliciously or not, send at a higher rate than negotiated, i.e. use a higher share of resources than the negotiated one, in every network in which admission control is performed, a policing algorithm is used. An ideal policer should guarantee to reject no packet of a well-behaved user and police contract violation as rigidly as possible. All this independently of the characteristics of the monitored stream and of the background traffic. This holds also for Integrated Services (IS) based IP networks. In these networks, every user negotiates a peak and an average rate. In this paper we present the solution to the peak rate policing issue. We adapt the Generic Cell Rate Algorithm (GCRA), well-known policer used in ATM networks, to police the peak rate of flows of packets with variable length. We intuitively call this modified GCRA Generic Packet Rate Algorithm (GPRA) and dimension its parameters so that independently of the characteristics of the policed flow and of the background traffic, no packets of a well-behaved user are rejected and that the flows of any misbehaving user are rigidly policed. Copyright © 2003 AEI. [source]

    A fuzzy approach to active usage parameter control in IEEE 802.11b wireless networks

    EXPERT SYSTEMS, Issue 5 2004
    David Soud
    Abstract: Usage parameter control (UPC) provides support for quality of service across heterogeneous networks. For the network operator UPC assists in limiting network usage through traffic shaping, to prevent unacceptable delay. Traditional methods to apply UPC involve the generic cell rate algorithm or ,leaky bucket' algorithm, now commonly implemented in asynchronous transmission mode networks. This paper proposes a novel form of UPC for 802.11b wireless networks. The method proposed measures the rate of individual network flows to actively manage link utilization using a fuzzy logic controller (FLC). The FLC monitors the flow rate and adjusts the sending transmissions to stabilize flows as close to the optimum desired rate as possible. Imposing UPC and using the FLC within a packet switched TCP network enforces cooperation between competing streams of traffic. After carrying out experiments within a wireless network, the results obtained significantly improve upon a ,best effort' service. [source]

    Mesalazine 4 g daily given as prolonged-release granules twice daily and four times daily is at least as effective as prolonged-release tablets four times daily in patients with ulcerative colitis

    Dr. Per G. Farup
    Abstract Background High doses of mesalazine usually result in an inconvenient dosage schedule and reduced compliance. The goal of this trial was to compare the effects of mesalazine 4 g daily given as prolonged-release granules in packets of 1 g with that of prolonged-release tablets of 0.5 g. Methods Two hundred twenty-seven patients with mild-to-moderate ulcerative colitis were randomized to treatment with two packets twice daily (Gr-b.i.d.), 1 packet four times daily (Gr-q.i.d.) or 2 tablets four times daily (Ta-q.i.d.) for 8 weeks. A disease activity index (ulcerative colitis disease activity index; UC-DAI) was calculated, and the granules were defined as noninferior to the tablets if the lower limit of the 95% CI for the differences was more than ,1 UC-DAI score unit. Results Noninferiority of the granules compared with the tablets was demonstrated. The mean improvement in the UC-DAI in the treatment groups Gr-b.i.d., Gr-q.i.d., and Ta-q.i.d. were 3.2, 2.9, and 2.4, respectively; the proportion of complete responders in the three groups 39%, 37%, and 31%, respectively. There were no differences in side effects. Conclusion Mesalazine 4 g daily given as prolonged-release granules twice and four times daily is at least as effective as prolonged-release tablets four times daily in patients with mild to moderate ulcerative colitis. The patients preferred the twice daily dosing. [source]

    Fault detection of networked control systems with packet based periodic communication

    Yongqiang Wang
    Abstract Fault detection of networked control systems (NCS) with communication constraints is discussed in this paper. A so-called packet-based periodic communication strategy is proposed and two kinds of optimal observer-based residual generators are designed. One residual generator is designed based on the lifted model of NCS, which generates residual signals every communication period. The other works at a faster rate, i.e. it generates residual signals every sampling period and is more suitable for prompt fault detection. Comparison with traditional periodic-communication-sequence-based scheduling strategy reveals superiority of this new communication strategy. Simulation results are also provided to illustrate effectiveness of the proposed method. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Joint queue control and user scheduling in MIMO broadcast channel under zero-forcing multiplexing

    Feng She
    Abstract This paper studies the problem of queue control and user scheduling in multi-antenna broadcast (downlink) systems under zero forcing beamforming transmit strategy. In the system, we assume that the data packet arrives randomly to the buffered transmitter. By taking the broadcast channel as a controlled queueing system, we deduce the property of queue control function that maximizes the weighted system throughput while guarantees the delay fairness among users. We also present a low-complexity user selection algorithm with the consideration of queue state and channel state together. Simulation results show that the joint queue control and user selection policy can achieve considerable fairness and stability among users. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    MLCC: A new hash-chained mechanism for multicast source authentication

    H. Eltaief
    Abstract Asymmetric cryptography has been widely used to generate a digital signature for message authentication. However, such a strategy cannot be used for packet authentication. Neither the source nor the receiver will be capable of handling the computational cost of asymmetric cryptography. For unicast communication, the solution adopted is based on symmetric cryptography. Solutions based on symmetric cryptography do not scale for multicast communication. Several solutions have been reported to authenticate multicast streams, with the possibility of packet losses. Proposed solutions are based on the concept of signature amortization, where a single signature is amortized on several packets. In this paper we present a new mechanism for multicast data source authentication based on signature amortization. Multi-layers connected chains divides the packet stream into a multi-layer structure, where each layer is a two-dimensional matrix. The hash of a packet is included into a forward chain of packets within the same layer as well as a downward chain of packets across multiple layers. The values of the key parameters that influence the mechanism efficiency as well as its performance are selected following a mathematical analysis. Comparisons of performance results with the well-known efficient multi-chained stream signature scheme as well as a recently reported scheme multiple connected chains model show that the proposed mechanism achieves a stronger resistance to packet losses with low overhead and high authentication probability. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Performance evaluation of GPON vs EPON for multi-service access

    T. Orphanoudakis
    Abstract Recently both ITU and IEEE have standardized solutions for passive optical networks (PONs) operating at gigabit per second line rates and optimized for the transport of packet-based traffic to improve the efficiency of previously standardized broadband PONs, which used the ATM cell as the data transport unit. The efficiency and performance of PON systems depend on the transmission convergence layer and mainly on the implemented medium access protocol. Although the latter is not part of the standards and left to the implementer, the standards describe a set of control fields that constitute the tool-set for the media access control (MAC) operation. Though starting from a common and quite obvious basis, the two standards present significant differences with the legacy of Ethernet marking the IEEE approach, while the emphasis of ITU is on demanding services. In this paper we compare the efficiency and performance of the two systems assuming the implementation of as close as possible MAC protocols. The target is twofold: assess and compare the traffic handling potential of each of the two standards and identify the range of applications they can support. Useful insight can also be gained to the MAC tools that could be designed into the next generation extra large WDM PONs. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Performance comparison between fixed length switching and variable length switching

    Chengchen Hu
    Abstract Fixed length switching (FLS) and variable length switching (VLS) are two main types of switching architecture in high-speed input-queued switches. FLS is based on a cell-by-cell scheduling algorithm, while VLS operates on the variable packet granularity. This paper aims to make a comprehensive comparison between these two switching modes to guide the industrial design and academic research. We use stochastic models, Petri net models, analysis and simulations to investigate various performance measures of interest. Average packet latency, bandwidth utilization, segmentation and reassembly overhead, as well as packet loss are the identified key parameters that influence the outcome of the comparison. The results achieved in this paper are twofold. On one hand, it is shown that FLS enables smaller packet loss and lower packet delay in case of a short packet. On the other hand, VLS favors better bandwidth utilization, reduced implementation complexity and lower average packet delay. We recommend VLS in the conclusion since its disadvantages can be compensated by some methods, while the problems in FLS are difficult to be solved. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    On the application of forking nodes to product-form queueing networks,

    Essia H. Elhafsi
    Abstract We define a ,forking node' as a service centre with one input feeding two outputs (each served by its own queue) under the control of an internal path-selection (PS) policy. We assume that both outputs lead to paths through which a packet reaches its final destination. However, the mean downstream delays on the two paths may be different and the PS policy should favour the path with the lower downstream delay. Using simulation, we compare the performance of this system under a variety of random, deterministic, state-dependent PS policies, including threshold-based and join-shortest-queue with bias (JSQ + b). We show that JSQ + b has better performance than the other alternatives. Moreover, if the input process to the forking node is Poisson, standard time series analysis techniques show that its two outputs are very close to being independent Poisson processes. Thus, if we find an accurate and efficient ,offline' analytical performance model for JSQ + b forking node, we can extend the applicability of product-form queueing networks to include such forking nodes. For this reason, we present several ways of modelling the performance of a JSQ + b node, using bounds, and compare their results on example networks. We establish a closed-form expression relating the bias b and the delays of the downstream paths. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Performance of delay-sensitive traffic in multi-layered satellite IP networks with on-board processing capability

    Suzan Bayhan
    Abstract In this article, performance of delay-sensitive traffic in multi-layered satellite Internet Protocol (IP) networks with on-board processing (OBP) capability is investigated. With OBP, a satellite can process the received data, and according to the nature of application, it can decide on the transmission properties. First, we present a concise overview of relevant aspects of satellite networks to delay-sensitive traffic and routing. Then, in order to improve the system performance for delay-sensitive traffic, specifically Voice over Internet Protocol (VoIP), a novel adaptive routing mechanism in two-layered satellite network considering the network's real-time information is introduced and evaluated. Adaptive Routing Protocol for Quality of Service (ARPQ) utilizes OBP and avoids congestion by distributing traffic load between medium-Earth orbit and low-Earth orbit layers. We utilize a prioritized queueing policy to satisfy quality-of-service (QoS) requirements of delay-sensitive applications while evading non-real-time traffic suffer low performance level. The simulation results verify that multi-layered satellite networks with OBP capabilities and QoS mechanisms are essential for feasibility of packet-based high-quality delay-sensitive services which are expected to be the vital components of next-generation communications networks. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Potential performance bottleneck in Linux TCP

    Wenji Wu
    Abstract Transmission control protocol (TCP) is the most widely used transport protocol on the Internet today. Over the years, especially recently, due to requirements of high bandwidth transmission, various approaches have been proposed to improve TCP performance. The Linux 2.6 kernel is now preemptible. It can be interrupted mid-task, making the system more responsive and interactive. However, we have noticed that Linux kernel preemption can interact badly with the performance of the networking subsystem. In this paper, we investigate the performance bottleneck in Linux TCP. We systematically describe the trip of a TCP packet from its ingress into a Linux network end system to its final delivery to the application; we study the performance bottleneck in Linux TCP through mathematical modelling and practical experiments; finally, we propose and test one possible solution to resolve this performance bottleneck in Linux TCP. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Performance analysis of IEEE 802.11 DCF with stochastic reward nets,

    R. Jayaparvathy
    Abstract In this paper, we present a performance study to evaluate the mean delay and the average system throughput of IEEE 802.11-based wireless local area networks (WLANs). We consider the distributed co-ordination function (DCF) mode of medium access control (MAC). Stochastic reward nets (SRNs) are used as a modelling formalism as it readily captures the synchronization between events in the DCF mode of access. We present a SRN-based analytical model to evaluate the mean delay and the average system throughput of the IEEE 802.11 DCF by considering an on,off traffic model and taking into account the freezing of the back-off counter due to channel capture by other stations. We also compute the mean delay suffered by a packet in the system using the SRN formulation and by modelling each station as an M/G/1 queue. We validate our analytical model by comparison with simulations. Copyright © 2006 John Wiley & Sons, Ltd. [source]

    Defining and maximizing PPT,a novel performance parameter for IEEE 802.11 DCF

    Yun Li
    Abstract Much research has been conducted on saturation throughput of IEEE802.11 DCF, and has led to some improvement. But increasing the successful transmission probability of packet is also important for saving stations' battery energy and decreasing the packet delay. In this paper, we define a new performance parameter, named Product of successful transmission Probability and saturation Throughput (PPT), for 802.11 DCF, which binds successful transmission probability and saturation throughput together. An analysis is given to maximize PPT. An expression of optimal minimum contention windows (CWmin) is obtained analytically for maximizing PPT. For simplicity, we give a name DCF-PPT to the 802.11 DCF that sets its CWmin according to this expression. The performance of DCF-PPT is simulated with different stations in terms of saturation throughput, successful transmission probability and PPT. The simulation results indicate that, compared to 802.11 DCF, DCF-PPT can significantly increase the PPT and successful transmission probability (about 0.95) on condition that the saturation throughput is not decreased. Copyright © 2006 John Wiley & Sons, Ltd. [source]

    Performance analysis of the IEEE 802.11 MAC protocol for wireless LANs

    P. Chatzimisios
    Abstract Wireless local area networks (WLANs) are extremely popular being almost everywhere including business, office and home deployments. The IEEE 802.11 protocol is the dominating standard for WLANs. The essential medium access control (MAC) mechanism of 802.11 is called distributed co-ordination function (DCF). This paper provides a simple and accurate analysis using Markov chain modelling to compute IEEE 802.11 DCF performance, in the absence of hidden stations and transmission errors. This mathematical analysis calculates in addition to the throughput efficiency, the average packet delay, the packet drop probability and the average time to drop a packet for both basic access and RTS/CTS medium access schemes. The derived analysis, which takes into account packet retry limits, is validated by comparison with OPNET simulation results. We demonstrate that a Markov chain model presented in the literature, which also calculates throughput and packet delay by introducing an additional transition state to the Markov chain model, does not appear to model IEEE 802.11 correctly, leading to ambiguous conclusions for its performance. We also carry out an extensive and detailed study on the influence on performance of the initial contention window size (CW), maximum CW size and data rate. Performance results are presented to identify the dependence on the backoff procedure parameters and to give insights on the issues affecting IEEE 802.11 DCF performance. Copyright © 2005 John Wiley & Sons, Ltd. [source]

    Split agent-based routing in interconnected networks

    Constandinos X. Mavromoustakis
    Abstract Adaptive behaviour of swarm-based agents (BT Technol. J. 1994; 12:104,113; AAMAS Conference '02, Melbourne, Australia, Month 1,2, 2002; Softcomput. J. 2001; 5(4):313,317.) is being studied in this paper with respect to network throughput for a certain amount of data traffic. Algorithmically complex problems like routing data packets in a network need to be faced with a dynamically adaptive approach such as agent-based scheme. Particularly in interconnected networks where multiple networks are participating in order to figure a large-scale network with different QoS levels and heterogeneity in the service of delay sensitive packets, routing algorithm must adopt in frequent network changes to anticipate such situations. Split agent-based routing technique (SART) is a variant of swarm-based routing (Adapt. Behav. 1997; 5:169,207; Proceedings of 2003 International Symposium on Performance Evaluation of Computer and Telecommunication Systems,SPECTS, Montreal, Canada, July 20,24, 2003; 240,247.) where agents are split after their departure to the next node on a hop-by-hop basis. Packets that are delay sensitive are marked as prioritized which agents recognize-as being a part of a packet- and try to influence the two-way routing tables. Thorough examination is made, for the performance of the proposed algorithm in the network and the QoS offered, taking into account a number of metrics. It is shown that the split agent routing scheme applied to interconnected networks offers a decentralized control in the network and an efficient way to increase overall performance and packet control reducing at the same time the packet loss concept. Copyright © 2004 John Wiley & Sons, Ltd. [source]

    A multiple access protocol with explicit and implicit reservation

    F. Davoli
    Abstract A new version of a MAC-level protocol is introduced and investigated, operating in a cellular environment, where a base station co-ordinates mobile users within each cell. The channel multiplexing structure is based on time division, and the slots in each frame are dynamically assigned to the users and their service classes by the cell base station. Decisions are taken on the basis of binary channel feedback information (collision/no collision), by assuming independence in the presence of packets at the mobile stations, and aim at maximizing the one-step throughput in the current frame. The frame is divided into two periods: the first (short) one contains a number of minislots, equal to the number of ,real' slots (i.e. those capable of containing a fixed size packet) of the second part. At the beginning of the frame, the access rights are computed and broadcast to the users; the enabled stations that have a packet to transmit respond, by sending a short burst that contains their ID in a minislot. This most recent feedback is used at the base station to update the parameters of the decision algorithm, which is then re-applied to yield the final access rights for the second part of the frame. The performance of the scheme is analysed by simulation in the presence of mixed voice and data traffic, and compared with those of a reservation random access protocol using the same algorithm in a single-phase fashion (RRA-ISA) and PRMA. Copyright © 2001 John Wiley & Sons, Ltd. [source]

    The bitter truth: Sensitivity to saccharin's bitterness predicts overactivity in highly arousable female dieters

    Melynda L. Craig
    Abstract Objective The interaction between taste sensitivity and emotionality in rats provides a provocative view of hyperactivity. Rats that have been bred selectively for their reactivity to saccharin exhibit characteristic emotionality. When placed on restrictive diets, these rats exhibit excessive activity levels, relative to rats that are not sensitive to saccharin. Because humans who are highly arousable (i.e., reactive to environmental stimuli) also exhibit an increase in sensitivity to saccharin's bitterness, the current study evaluated whether women who are highly arousable, currently dieting, and sensitive to saccharin's bitterness engage in excessive exercise. Method Participants completed a questionnaire packet, which assessed emotionality, eating patterns, and exercise patterns. On another occasion, they completed a body contour drawings handout, and their weight and height were measured. They also rated saccharin's bitterness and sweetness following a stressful event. Results and Discussion As hypothesized, sensitivity to saccharin's bitterness predicted overactivity in highly arousable female dieters, which reveals the multidimensionality of activity anorexia. © 2003 by Wiley Periodicals, Inc. Int J Eat Disord 34: 71,82, 2003. [source]

    Why do primary care doctors diagnose depression when diagnostic criteria are not met?

    Michael Höfler
    Abstract This study examines predictors of false positive depression diagnoses by primary care doctors in a sample of primary care attendees, taking the patients' diagnostic status from a self-report measure (Depression Screening Questionnaire, DSQ) as a yardstick against which to measure doctors' correct and false positive recognition rates. In a nationwide study, primary care patients aged 15,99 in 633 doctors' offices completed a self-report packet that included the DSQ, a questionnaire that assesses depression symptoms on a three-point scale to provide diagnoses of depression according to the criteria of DSM-IV and ICD-10. Doctors completed an evaluation form for each patient seen, reporting the patient's depression status, clinical severity, and treatment choices. Predictor analyses are based on 16,909 patient-doctor records. Covariates examined included depression symptoms, the total DSQ score, number and persistence of depression items endorsed, patient's prior treatment, history of depression, age and gender. According to the DSQ, 11.3% of patients received a diagnosis of ICD-10 depression, 58.9% of which were correctly identified by the doctor as definite threshold, and 26.2% as definite subthreshold cases. However, an additional 11.7% of patients not meeting the minimum DSQ threshold were rated by their doctors as definitely having depression (the false positive rate). Specific DSQ depression items endorsed, a higher DSQ total score, more two-week depression symptoms endorsed, female gender, higher age, and patient's prior treatment were all associated with an elevated rate of false positive diagnoses. The probability of false positive diagnoses was shown to be affected more by doctors ignoring the ,duration of symptoms' criterion than by doctors not following the ,number of symptoms' criterion for an ICD or DSM diagnosis of depression. A model selection procedure revealed that it is sufficient to regress the ,false positive diagnoses' on the DSQ-total score, symptoms of depressed mood, loss of interest, and suicidal ideation; higher age; and patient's prior treatment. Further, the total DSQ score was less important in prediction if there was a prior treatment. The predictive value of this model was quite good, with area under the ROC-curve = 0.86. When primary care doctors use depression screening instruments they are oversensitive to the diagnosis of depression. This is due to not strictly obeying the two weeks duration required by the diagnostic criteria of ICD-10 and DSM-IV. False positive rates are further increased in particular by the doctor's knowledge of a patient's prior treatment history as well as the presence of a few specific depression symptoms. Copyright © 2000 Whurr Publishers Ltd. [source]

    Modeling and simulation of mixed traffic on a prioritized shared medium

    Jeffrey J. Evans
    Network access systems (NAS) such as digital loop carriers (DLC) are increasingly utilizing a shared medium, such as Hybrid Fiber Coax (HFC) to provide point-to-multi-point access from the public switched telephone network (PSTN) to the end user (consumer). New services, such as direct access to the packet switched network (PSN, WWW) have been added to DLC equipment in such a way as to provide for a prioritized set of services over a shared medium in an effort to take advantage of otherwise unused bandwidth. The introduction of such services requires the modeling and analysis of these network access systems. This becomes complex when considering the variability in different service type traffic characteristics. This work identifies a traffic engineering problem of prioritized circuit switched and packet switched (PSTN/PSN) traffic over the same shared medium as it may relate to "perceived" quality of service (QoS). Copyright © 2002 John Wiley & Sons, Ltd. [source]

    Computation of STM images of carbon nanotubes

    P. Lambin
    Abstract Scanning tunneling microscopy (STM) is the only probing technique that allows for the investigation of both the topography and the electronic structure of carbon nanosystems at a subnanometer resolution. The interpretation of the STM images of carbon nanostructures involves complications that are normally absent in the study of planar crystalline surfaces. The complications typically appear from a number of quantum effects responsible for distortions in the microscope image of a nano-object. Because of these difficulties, computer simulation plays an extremely important role in the analysis of experimental data. In the current article, we report on two theoretical approaches developed for aiding in the interpretation and understanding of the formation of the STM image of a nanotube: first, the quantum mechanical dynamics of a wave packet, which allows for the modeling of the flow of the tunneling current between a tip and a nanotube supported by a substrate; and, second, a tight-binding perturbation theory that allows for the explicit calculation of realistic STM images and scanning tunneling spectra of carbon nanostructures. An atlas of computed STM images is provided for a series of 27 single-wall nanotubes with diameter around 1.3 nm. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [source]

    Kalman filtering over unreliable communication networks with bounded Markovian packet dropouts

    Nan Xiao
    Abstract We address the peak covariance stability of time-varying Kalman filter with possible packet losses in transmitting measurement outputs to the filter via a packet-based network. The packet losses are assumed to be bounded and driven by a finite-state Markov process. It is shown that if the observability index of the discrete-time linear time-invariant (LTI) system under investigation is one, the Kalman filter is peak covariance stable under no additional condition. For discrete LTI systems with observability index greater than one, a sufficient condition for peak covariance stability is obtained in terms of the system dynamics and the probability transition matrix of the Markov chain. Finally, the validity of these results is demonstrated by numerical simulations. Copyright © 2008 John Wiley & Sons, Ltd. [source]