The IEEE 802.11 protocol is one of the most important standards for Wireless Local Area Networks. The primary MAC protocol of 802.11 is the distributed coordination function (DCF), which is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) with binary slotted exponential backoff scheme. The basic access and the RTS/CTS access mechanisms are the two channel access methods defined in DCF. In real work environments, the common channel access method of 802.11 is the coexistence of the above two mechanisms, i.e., packets longer than a given threshold RTSthreshold are transmitted according to the RTS/CTS mechanism, otherwise, they are transmitted by means of basic mechanism. The common mechanism is called as hybrid access mechanism in this paper. Few analytical models have been proposed to evaluate the hybrid access mechanism up to now. The necessary condition which enables the hybrid mechanism to work is that the packets must be variable. But, almost all analytical models proposed for 802.11 DCF assume that the packet size is fix, so they can not study the hybrid access method. In this paper, we propose a detailed analytical model to evaluate the saturation performance of the hybrid access mechanism, in the assumption that the packet lengthes are sampled from a general distribution function f(x). Both the throughput and the delay performances are concerned. Our model is validated by extensive simulations. By means of the proposed model, we compare the performances of the above three mechanisms, i.e., basic, RTS/CTS and hybrid, under different network scenarios. Numerical results show that the hybrid access mechanism is the best choice in almost all scenarios.

As audio and video applications have proliferated on the Internet, transcoding proxy caching has been considered as an important technique for improving network performance, especially for mobile networks. Due to several new emerging factors in the transcoding proxy, existing methods for proxy placement for web caching cannot be simply applied to solve the problem of proxy placement for transcoding proxy caching. This paper addresses the problem of proxy placement for coordinated en-route transcoding proxy caching for tree networks. We propose a model for this problem by including the new emerging factors in the transcoding proxy and present optimal solutions for this problem with/without constraints on the number of transcoding proxies using dynamic programming. Finally, we implement our algorithm and evaluate our model on various performance metrics through extensive simulation experiments. The implementation results show that our model outperforms the existing model for transcoding proxy placement for linear topology, as well as the random proxy placement model. The average improvements of our model over the other models are about 7.2 percent and 21.4 percent in terms of all the performance metrics considered.

In this paper, we propose a reconfigurable adaptive FEC system. In adaptive FEC schemes, the error correction capability t is changed dynamically according to the communication channel condition. If a particular error correction capability t is given, we can implement an FEC decoder which is optimal for t by taking the number of operations into consideration. Thus, reconfiguring the optimal FEC decoder dynamically for each error correction capability allows us to maximize the throughput of each decoder within a limited hardware resource. Based on this concept, our reconfigurable adaptive FEC system can reduce the packet dropping rate more efficiently than conventional fixed hardware systems. We can improve data transmission throughput for a reliable transport protocol. Practical simulation results are also shown.

In this paper, we consider the problem of multicast routing and wavelength assignment (MC-RWA) in multi-fiber all-optical WDM networks. Two main network design system comprehensively investigated here are mesh and multi-ring designs. Given the multicast traffic demands, we present new ILP formulations to solve the MC-RWA problem with an objective to determine the minimal number of fibers needed to support the multicast requests. Unlike previous studies, our ILP formulations are not only capable of finding the optimal multicast routing and wavelength assignment pattern to the light-trees, but also finding the optimal light-tree structures simultaneously. Since broadcast and unicast communications are special cases of multicast communications, our ILP models are actually the generalized RWA mathematical models of optical WDM networks. In addition to proposing the ILP models, this paper takes two main issues affecting the network capacity requirement into account, that is, the splitting degree level of optical splitters and techniques of wavelength assignment to the light-trees. Three multicast wavelength assignment techniques studied in this paper are Light-Tree (LT), Virtual Light-Tree (VLT) and Partial Virtual Light-Tree (PVLT) techniques. Due to the NP-completeness of the MC-RWA problem, the ILP formulations can reasonably cope with small and moderate networks. To work with large networks, this paper presents alternative MC-RWA ILP-based heuristic algorithms for the PVLT and LT networks and develops lower bound techniques to characterize the performance of our algorithms. Using existing large backbone networks, numerical results are reported to analyze such aspects as multiple fiber systems, the benefits of using optical splitters and wavelength converters, and the capacity difference between the mesh and multi-ring designs. Finally, this paper provides an analysis of the influence of network connectivity on the network implementation under the constraints of mesh and multi-ring design schemes.

The Hidden Markov Model (HMM) is a popular statistical framework for modeling and analyzing stochastic signals. In this paper, a novel strategy is proposed that makes use of level-building algorithm with a chain of AdaBoost HMM classifiers to model long stochastic processes. AdaBoost HMM classifier belongs to the class of multiple-HMM classifier. It is specially trained to identify samples with erratic distributions. By connecting the AdaBoost HMM classifiers, processes of arbitrary length can be modeled. A probability trellis is created to store the accumulated probabilities, starting frames and indices of each reference model. By backtracking the trellis, a sequence of best-matched AdaBoost HMM classifiers can be decoded. The proposed method is applied to visual speech processing. A selected number of words and phrases are decomposed into sequences of visual speech units using both the proposed strategy and the conventional level-building on HMM method. Experimental results show that the proposed strategy is able to more accurately decompose words/phrases in visual speech than the conventional approach.

In the formerly proposed theory for sub-linear (linear in a changed addition rule) systems, we developed a sub-linear mathematical theory and demonstrated its capability in several sub-linear systems. In this paper, we assert that we can further construct hybrid systems useful in engineering by combining sub-linear systems with linear systems, and as an example, we show the construction of a divergence-free electrodynamics. Since the energy of photon fields in engineering is tending upward, it would be desirable for us to get rid of the divergence difficulties encountered in the conventional high energy electrodynamics. The most important result is the recognition that Nature herself has a hybrid system composed of sub-linear photon fields and linear electron fields. Mathematically, our electrodynamics is formulated by only one point correction (the insertion of tanh into the electromagnetic energy density) in the conventional electrodynamics of photons and electrons (including positrons).

This paper addresses the scheduling problem of a class of automated manufacturing systems with multiple resource requests. In the automated manufacturing system model, a set of jobs is to be processed and each job requires a sequence of operations. Each operation may need more than one resource type and multiple identical units with the same resource type. Upon the completion of an operation, resources needed in the next operation of the same job cannot be released and the remaining resources cannot be released until the start of the next operation. The scheduling problem is formulated by Timed Petri nets model under which the scheduling goal consists in sequencing the transition firing sequence in order to avoid the deadlock situation and to minimize the makespan. In the proposed genetic algorithm with deadlock-free constraint, Petri net transition sequence is coded and a deadlock detection method based on D-siphon technology is proposed to reschedule the sequence of transitions. The enabled transitions should be fired as early as possible and thus the quality of solutions can be improved. In the fitness computation procedure, a penalty item for the infeasible solution is involved to prevent the search process from converging to the infeasible solution. The method proposed in this paper can get a feasible scheduling strategy as well as enable the system to achieve good performance. Numerical results presented in the paper show the efficiency of the proposed algorithm.

Application of multipled block codes (MBCs) for realizing new flexible and efficient transmission systems that feature in hunting-free reframing and asynchronous multiplexing is investigated. First, the principles of MBC are overviewed to show the capacities of filterless clock recovery. Then it is shown that modification of simple frame structure of MBC line code can be used for attaining hunting-free reframing for multiplexing systems. Two types of MBCs are developed to this end. While the one uses header blocks for hunting-free reframing, the other uses distributed frame patterns. Header design of multipled block codes (MBC) for hunting-free reframing (HFR) is investigated for frame patterns with and without violation compensation. The feasibility of hunting-free reframing is tested in an experimental system. Application of hunting-free reframing to asynchronous multiplexing is also investigated and tested in an experimental system. Finally, advantages of hunting-free multiplexing systems are discussed.

The effect of feedback delay and channel estimation error on closed-loop transmit diversity (CTD) systems is investigated in time-selective Rayleigh fading channels. Based on a minimum mean square error (MMSE) channel estimator, the variance of the estimation error is formulated in terms of fading index and the number of transmit antennas. A bit error rate (BER) expression for the CTD system is analytically derived as a function of channel estimation error, feedback delay, and fading index. It is shown that the BER performance of the CTD system improves as the length of training symbols increases and/or the frame length decreases. In the CTD system, more accurate channel estimation scheme is required to achieve its full gain as the number of employed transmit antennas increases. It is also found that the CTD system is applicable to the slowly moving channel environments, such as pedestrians, but not for fast moving vehicles.

Recently, a cluster map based blind RBF equalizer (CM-BRE) has been proposed. By utilizing the underlying structure characteristics of RBF equalizer, the CM-BRE can be implemented by the combination of neural-gas algorithm (NGA) with several sorting operations. Although the CM-BRE is able to achieve almost identical performance with the optimal RBF equalizer, the high computational load mainly caused by NGA limits it's application. In this paper, we propose a downsizing method that employs the inter-relation among RBF centers and significantly reduces the NGA's computational load. Furthermore, a method to determine the feedback vector is derived, then CM-BRE is extended to a cluster map based blind RBF decision feedback equalizer (CM-BRDFE). The proposed CM-BRDFE also shows the close performance with the optimal RBF decision feedback equalizer in simulations.

In this paper, we propose a novel traffic engineering architecture for IP networks with MPLS backbones. In this architecture, two link-disjoint label switched paths, namely the primary and secondary paths, are established among every pair of IP routers located at the edges of an MPLS backbone network. As the main building block of this architecture, we propose that primary paths are given higher priority against the secondary paths in the MPLS data plane to cope with the so-called knock-on effect. Inspired by the ABR flow control mechanism in ATM networks, we propose to split traffic between a source-destination pair between the primary and secondary paths using explicit rate feedback from the network. Taking into consideration the performance deteriorating impact of packet reordering in packet-based load balancing schemes, we propose a traffic splitting mechanism that operates on a per-flow basis (i.e., flow-based multipath routing). We show via an extensive simulation study that using flow-based multipath traffic engineering with explicit rate feedback not only provides consistently better throughput than that of a single path but is also void of out-of-order packet delivery.

One of the categories of decoding techniques for DFT codes in erasure channels is the class of iterative algorithms. Iterative algorithms can be considered as kind of alternating mapping methods using the given information in a repetitive way. In this paper, we propose a new iterative method for decoding DFT codes. It will be shown that the proposed method outperforms the well-known methods such as Wiley/Marvasti, and ADPW methods in the decoding of DFT codes in erasure channels.

One of the most important issues in spontaneous speech recognition is how to cope with the degradation of recognition accuracy due to speaking rate fluctuation within an utterance. This paper proposes an acoustic model for adjusting mixture weights and transition probabilities of the HMM for each frame according to the local speaking rate. The proposed model is implemented along with variants and conventional models using the Bayesian network framework. The proposed model has a hidden variable representing variation of the "mode" of the speaking rate, and its value controls the parameters of the underlying HMM. Model training and maximum probability assignment of the variables are conducted using the EM/GEM and inference algorithms for the Bayesian networks. Utterances from meetings and lectures are used for evaluation where the Bayesian network-based acoustic models are used to rescore the likelihood of the N-best lists. In the experiments, the proposed model indicated consistently higher performance than conventional HMMs and regression HMMs using the same speaking rate information.

This paper reviews process, device and circuit technologies of high-density flash memories, whose market has grown explosively as bridge media. In this memory, programming throughput as well as low bit costs is critical issue. To meet the requirements, we have developed multi-level AG (Assist Gate)-AND type flash memory with small effective cell size and 10 MB/s programming throughput. We clarify three challenges to the multilevel flash memory in terms of operation method, high reliability for data retention, and high-speed multilevel programming. Future trends of high-density flash memories are also discussed.

A low-band spectrum envelope reconstruction method was tested to see if it could improve the sound quality of F0 modified speech with the PSOLA (Pitch Synchronous OverLap Add) method. In the conventional PSOLA method, the extracted spectrum envelope using a Hanning window with two-pitch-period length had no reliable information in the band of frequencies lower than the original F0. This problem causes sound degradation of the F0 modified speech when the F0 is shifted downward. In the proposed method, the low-band spectrum envelope was properly modified according to the F0 modification rate. The amplitude of the F0 harmonic components in the low-band were reproduced based on the spectral tilt of the spectrum envelope. Subjective listening tests suggest the proposed method yields improved sound quality than the conventional TD-PSOLA method when the downward modification rate exceeds 0.4 octave.

Recently, dielectric lens antennas are paid attentions in ITS applications. Many lens shape designing methods were already developed. And electrical performances were estimated through a ray tracing method. Here, arbitral lens shapes were expressed by a system of power series. In the case of ray tracing, time-consuming three-coordinate root-finder programs were needed to find intersection points of rays on the lens surfaces. In order to calculate complicated structures such as zoned lenses and complicated rays such as multiple reflections between lens surfaces, simple ray tracing methods are requested. In this paper, a simple ray tracing method that utilizes directly designed discrete points of lens surfaces is developed. In this method, a refracted ray is automatically determined for a given incident ray. As for an intersecting point of a lens surface for an outgoing ray, the nearest point to the refracted vector is found out by employing a simple searching procedure. This method is time-saving compared to the previous three-coordinate root-finding program. Through calculated results of focal points and radiation patterns in wide angle beam steering, effectiveness of a developed method is ensured. Application of the developed ray tracing method of complicated multiple reflections are studied. Reflecting points are found out speedily by the same searching procedure. A calculated example of doubly reflected rays is obtained. Through comparing calculated and measured results of wide angle radiation patterns, effectiveness of a developed method is ensured.

We propose a feedback controller to efficiently control web requests especially on overloaded networks. The controller is designed based on a feedback closed loop that prevents overload of web server and enforces target CPU utilization via controlling the amount of input web requests. The main contribution of this letter is the use of feedback control theory to design the controller that delicately regulates web requests even under the dynamic changes in processing power of web server. In contrast with many previous heuristic methods, the proposed controller uses a systematic approach to adaptively control web requests taking account of the dynamic behavior of web server. Simulation results performed in overloaded cases show that the proposed controller lets web server effectively control input web requests and reach its CPU utilization to desired levels in relatively small settling times.

In this paper, we propose a new fragile watermarking scheme that can be used to verify three-dimensional (3-D) polygonal models by detecting unauthorized alterations. In order to generate a binary watermark sequence, we combine a binary random sequence generated by the user key with a binary logo sequence using the bit-wise exclusive OR operation. The binary watermark sequence is embedded into 3-D triangle strips by perturbing their vertex coordinates. The proposed watermarking scheme can identify specific locations that have been changed. Changes of the vertex coordinates in the watermarked 3-D model are reflected in the extracted watermark image, which indicates the attacked area. Experimental results demonstrate that we can detect various modifications of the watermarked 3-D model: translation, rotation, scaling, randomization of points, polygon simplification, geometry compression, and local deformation.

Since IEEE 802.11 has many problems such as hidden node, exposed node problem, larger sensing function and BEB (Binary Exponential Back-off), it is not suitable for use in multi-hop wireless ad hoc network. When an on-demand routing protocol is used with 802.11 DCF (Distributed Coordination Function), the route to transmit the packet will be formed by nodes which have less competition (fewer neighbors) than others for the medium access. This effect will make that the routing path will be longer and decrease network performance. Therefore, we propose a new MAC (Medium Access Control) protocol that makes a shorter routing path, enabling better performance in multi-hop wireless ad hoc networks. The protocol modifying IEEE 802.11 DCF gives priority to the node with more neighbors and with less transmission. Through simulations, we have demonstrated that the proposed algorithm improves performance in terms of transmission rate, transmission delay and total consumption energy.

In this paper, we examine the effect of random steering errors on the signal-to-interference-plus-noise-ratio (SINR) at the output of the recently addressed wavelet-based generalized sidelobe canceller (GSC). This new beamformer employs a set of P-regular M-band wavelet bases for the design of the blocking matrix of the GSC. We first carry out a general expression of the output SINR of the GSC with multiple interferers present. With this expression, we then examine the analysis of wavelet-based GSC by expressing the SINR in terms of parameters such as the regularity of wavelet filters, the number of bands of wavelet filters, the length of adaptive weights, and the input signal-to-noise ratio (SNR). Some simulation results verify the analytically predicted performance.