Examines the transmission of variable bit-rate (VBR) H.261 video over a mixed traffic (video/inter-LAN) integrated services frame relay (FR) network. We introduce a modified H.261 codec that produces VBR output and show that parsing of the video bit stream at group of blocks (GOB) boundaries produces variable length FR packets well suited to the network. We demonstrate that GOB-level video traffic requires a more sophisticated statistical model of the resulting data stream than the frame-level models. The transform expand sample (TES) method is used to obtain an accurate model of the autocorrelation and the marginal probability distribution of the bit-rate variations at the GOB level. A simple and effective methodology is introduced for capturing the periodic components that are present in the GOB-level autocorrelation. The methodology is extended to permit simulations of VBR codecs in which the codec quantization step size is adjusted in response to prevailing network conditions. We show that the quality of service requirements of VBR video can be met by using the FR backward explicit congestion notification (BECN) facility in conjunction with a modified H.261 codec whose rate is controlled by the congestion notification. We also show that the performance of the control mechanism is significantly influenced by a subset of network threshold and codec control parameters which are identified using 2/sup k/ factorial analysis techniques. We obtain optimal ranges of values for these parameters using mean field annealing. Variable quantization rate control may be more effective for this purpose than variable frame rate control.

Indoor wireless communications will become important in providing access to networks which will increasingly use ATM technology to allow multimedia service applications. This paper presents two possible architectures for wireless access to an ATM LAN: the centralized and the distributed wireless controller cases. The detailed signaling procedures for both the cases are discussed, and the two architectures are compared in terms of their signaling and complexity implications.

Variable bit rate (VBR) compressed video is expected to become one of the major loading factors in high-speed packet networks such as ATM-based B-ISDN. However, recent measurements based on long empirical traces (complete movies) revealed that VBR video traffic possesses self-similar (or fractal) characteristics, meaning that the dependence in the traffic stream lasts much longer than traditional models can capture.In this paper, we present a unified approach which, in addition to accurately modeling the marginal distribution of empirical video records, also models directly both the short and the long-term empirical autocorrelation structures. We also present simulation results using synthetic data and compare with results based on empirical video traces.Furthermore, we extend the application of efficient estimation techniques based on importance sampling that we had used before only for simple fractal processes. We use importance sampling techniques to efficiently estimate low probabilities of packet losses that occur when a multiplexer is fed with synthetic traffic from our self-similar VBR video model.

Several services to be supported by emerging high-speed networks are expected to result in highly bursty (autocorrelated) traffic streams. A typical example is variable bit-rate (VBR) compressed video. Therefore, traffic modeling and performance evaluation techniques geared towards autocorrelated streams are extremely important for the design of practical networks.The TES (Transform - Expand - Sample) technique has emerged as a general methodology for modeling autocorrelated random processes with arbitrary marginal distributions. Because of their generality and practical applicability, TES models can be readily used to accurately characterize bursty traffic streams in ATM networks.Although TES models can be easily implemented for simulation studies, the need still exists for analytical results on the performance of queueing systems driven by autocorrelated traffic. Of particular interest are the tails of the waiting time distribution in queues driven by TES-modeled bursty traffic. Such tail probabilities, when they become exceedingly small, may be difficult to obtain via conventional simulation.In order to extend existing results, based on Large Deviations theory, to TES processes, the main difficulty is posed by the continuous state-space of the TES time-series. In this paper, we develop a general result concerning exponential bounds for the waiting time under continuous state-space Markov arrivals. We apply this result to TES/GI/1 queues, show numerical examples, and compare our bound with simulation results. Accurate estimates of extremely low probabilities are obtained by employing fast simulation techniques based on importance sampling.

The direct provision of connectionless service in BISDN calls for servers that are connected to or are part of an ATM network to provide the routing function at input speeds up to 622 Mb/s. Routing is achieved in such a server by changing the VCI/VPI headers in the ATM cells; actual switching is done by existing switches in the ATM network. The paper presents an architecture capable of executing all the functions of a server at input speeds up to 622 Mb/s, scalable to multiple inputs at that speed, making use of processors and special hardware that are available today. To avoid storing large quantities of data, the architecture routes data packets by examining routing information in the initial cell of the packet and routing subsequent cells as they arrive rather than waiting until the complete packet has arrived. It is capable of handling packets that have been multiplexed at the SAR sublayer using AAL Type 3/4 and, with minor modifications, could also handle Type 5 traffic. Arguments are also presented for the use of AAL Type 5 for the direct connectionless service.>

The authors discuss the role of an expert system for monitoring troubles in a wide-area network of circuit-switching, private, automatic computer exchanges embedded in a wide-area computer network (WAN). They present a taxonomy of diagnostic problems based on the breadth of view of the network necessary to detect and diagnose them and a further taxonomy of the types of patterns of events which must be detected. The architecture and experimental development of an expert monitor for a single exchange within such a network is presented with some emphasis on a design approach for eventual real-time implementation.>

This paper presents an approach to high-level support of office workers by embedding office knowledge in a network of distributed cooperating knowledge-based or expert “assistants” and servers. These knowledge-based systems incorporate both factual and procedural knowledge and are capable of making use of existing conventional office technology. They constitute a form of computer-supported cooperative work. We describe a common architecture for our assistants and servers that incorporates several key features. Our systems are capable of supporting concurrent multiple consultations or tasks and have facilities for the interruption and resumption of consultations as appropriate. The various assistants and servers, which may reside on different machines, cooperate in solving problems or completing tasks by passing messages. We propose a taxonomy of the general office knowledge normally used by office workers, together with a frame and rule-based knowledge representation scheme. We also describe an experimental system, written in PROLOG, that incorporates the above design principles.

Manufacturers of integrated electronic office systems have included electronic versions of the calendar in almost every system they offer. This paper describes a survey of office workers, carried out to examine their use both of paper calendars and of electronic calendars that are commercially available as part of integrated office systems. It assesses the degree to which electronic calendars meet the needs of users. Our survey shows that the simple paper calendar is a tool whose power and flexibility is matched by few, if any, of the current commercially available electronic calendars. Recommendations for features that should be included in electronic calendars and automatic schedulers are included.

Previous results on intersymbol interference in multichannel linear modulation schemes can be applied to nonlinear modulation. These are compared with new results specifically developed for the nonlinear case.

There are many instances in communication systems where bandpass signals are passed through nonlinear devices, such as traveling wave tubes, which exhibit both amplitude and phase nonlinearities. When the input signal is narrow band, the device may be characterized by measurements of its single-carrier amplitude and phase transfer functions. A sufficient model for such a device is a quadrature structure that includes two nonlinearities each of which, acting on its own, would exhibit only amplitude distortion. The outputs of the two halves of this model are linearly independent for arbitrary narrow-band input signals so that their power spectra add. Consequently, almost all previously published results for amplitude nonlinearities can be readily applied to the analysis of the general device. Emphasis is laid on practical procedures for analysis based directly on measured device characteristics rather than analytic approximations and accuracy is checked by comparison of certain intermodulation results with previous results and with measurements. A new result is the performance of an Intelsat IV tube for a large number of independent equal-power-density signals. A heuristically optimal saturating nonlinearity is introduced and analyzed and two methods of compensating an arbitrary saturating device to obtain this optimal characteristic are presented. Two methods of inverting the Chebyshev transform are used in this paper and the choice of basis functions for obtaining series representations of the measured device characteristics is discussed.