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171.
Efficient QoS support in a slotted multihop WDM metro ring 总被引:1,自引:0,他引:1
A novel distributed access protocol for a slotted wavelength-division-multiplexing (WDM) metro ring employing all-optical packet switching and supporting quality-of-service (QoS) classes is presented and analyzed. Since we assume that there are more nodes than available wavelengths in the network, we obtain a scalable multihop WDM ring as underlying network architecture. By dividing each channel into several time slots and further applying destination release and slot reuse, data packets can be efficiently transmitted and received in a statistically multiplexed manner. In our architecture, each node is equipped with one tunable transmitter and one fixed-tuned receiver. Furthermore, as we generally consider so-called a posteriori access strategies, different packet selection schemes are proposed and compared. An analytical model based on the semi-Markov process methodology is developed to quantify the performance of one of these schemes. As a key element of the protocol, an efficient QoS support access mechanism is proposed and its performance is evaluated. The new QoS control scheme adopts a frame-based slot reservation strategy including connection setup and termination, which only slightly increases the signaling and node processing overhead. Thus, an efficient hybrid protocol combining connectionless and connection-oriented packet transmissions is proposed 相似文献
172.
Bandwidth Adaptation Algorithms for Adaptive Multimedia Services in Mobile Cellular Networks 总被引:3,自引:0,他引:3
The fluctuation of available link bandwidth in mobilecellular networks motivates the study of adaptive multimediaservices, where the bandwidth of an ongoing multimedia call can bedynamically adjusted. We analyze the diverse objectives of theadaptive multimedia framework and propose two bandwidth adaptationalgorithms (BAAs) that can satisfy these objectives. The firstalgorithm, BAA-RA, takes into consideration revenue and``anti-adaptation' where anti-adaptation means that a user feelsuncomfortable whenever the bandwidth of the user's call ischanged. This algorithm achieves near-optimal total revenue withmuch less complexity compared to an optimal BAA. The secondalgorithm, BAA-RF, considers revenue and fairness, and aims at themaximum revenue generation while satisfying the fairnessconstraint defined herein. Comprehensive simulation experimentsshow that the difference of the total revenue of BAA-RA and thatof an optimal BAA is negligible. Also, numerical results revealthat there is a conflicting relationship between anti-adaptationand fairness. 相似文献
173.
The ray-tracing (RT) algorithm has been used for accurately predicting the site-specific radio propagation characteristics, in spite of its computational intensity. Statistical models, on the other hand, offers computational simplicity but low accuracy. In this paper, a new model is proposed for predicting the indoor radio propagation to achieve computational simplicity over the RT method and better accuracy than the statistical models. The new model is based on the statistical derivation of the ray-tracing operation, whose results are a number of paths between the transmitter and receiver, each path comprises a number of rays. The pattern and length of the rays in these paths are related to statistical parameters of the site-specific features of indoor environment, such as the floor plan geometry. A key equation is derived to relate the average path power to the site-specific parameters, which are: 1) mean free distance; 2) transmission coefficient; and 3) reflection coefficient. The equation of the average path power is then used to predict the received power in a typical indoor environment. To evaluate the accuracy of the new model in predicting the received power in a typical indoor environment, a comparison with RT results and with measurement data shows an error bound of less than 5 dB 相似文献
174.
175.
We consider a space-time coded (STC) orthogonal frequency-division multiplexing (OFDM) system with multiple transmitter and receiver antennas over correlated frequency- and time-selective fading channels. It is shown that the product of the time-selectivity order and the frequency-selectivity order is a key parameter to characterize the outage capacity of the correlated fading channel. It is also observed that STCs with large effective lengths and ideal built-in interleavers are more effective in exploiting the natural diversity in multiple-antenna correlated fading channels. We then propose a low-density parity-check (LDPC)-code-based STC-OFDM system. Compared with the conventional space-time trellis code (STTC), the LDPC-based STC can significantly improve the system performance by exploiting both the spatial diversity and the selective-fading diversity in wireless channels. Compared with the previously proposed turbo-code-based STC scheme, LDPC-based STC exhibits lower receiver complexity and more flexible scalability. We also consider receiver design for LDPC-based STC-OFDM systems in unknown fast fading channels and propose a novel turbo receiver employing a maximum a posteriori expectation-maximization (MAP-EM) demodulator and a soft LDPC decoder, which can significantly reduce the error floor in fast fading channels with a modest computational complexity. With such a turbo receiver, the proposed LDPC-based STC-OFDM system is a promising solution to highly efficient data transmission over selective-fading mobile wireless channels 相似文献
176.
摩托罗拉微控制器(MCU)具有编程语言简单、外围设备齐全、存储器模型用户友好、选择广及供应多、性能价格比高等优点,被设计者评为最容易使用的产品之一。在全球顶级的原始设备制造厂商(OEM)的无数嵌入式系统和用户最终产品中都可找到摩托罗拉的MCU,包括键盘、传呼机、电子游戏机、洗衣机、安全系统及汽车等。 相似文献
177.
178.
Mountfield K. Mitchell P. Lee J.-W. Demczyk B. Artman J. 《IEEE transactions on magnetics》1987,23(5):2037-2039
A sequence of Co78 Cr22 films, 500 nm in thickness, was prepared by deposition on glass in a modified Varian D.C. magnetron S-gun sputtering system. The substrate temperature during deposition, Ts , was fixed at various values with an upper limit of 300°C. Specimens were examined by VSM, TM, FMR and TEM. Ms rises significantly with increasing Ts , peaking at 200°C at 370 emu/cm3. The effective volume-averaged anisotropy drops for Ts >110°C from +1.6 KOe to progressively negative values (-4.3 KOe at 300°C). From FMR we find indications of the presence, in addition to the transition and bulk layers, of a highly negative anisotropy constituent (sim-11.5 KOe anisotropy field). This resonance appears at Ts values of 150°C and above. TEM plane and cross-section views taken on a Ts = 150°C specimen show islands composed of tilted columns within the bulk. For vertical recording, specimens prepared at Ts values between 50 and 100°C are recommended. On the other hand, for longitudinal recording applications, films prepared at Ts values above 250°C would seem to be appropriate. 相似文献
179.
180.
I. N. Polandov V. K. Novik O. K. Gulish B. P. Bogomolov V. B. Morozov 《Measurement Techniques》1989,32(9):888-890
Translated from Izmeritel'naya Tekhnika, No. 9, pp. 34–35, September, 1989. 相似文献