Kenneth S. Vastola

Threshold Detection in Narrow-Band Non-Gaussian Noise

The Middleton Class A narrow-band non-Gaussian noise model [9]-[12] is examined. It is shown that this noise model (which is known to fit closely a variety of non-Gaussian noises) can itself be closely approximated by a computationally much simpler noise model. It is then shown by numerical examples that, for the problem of locally optimum detection, the simplest form of this approximation yields nearly optimal (asymptotic) performance. The performance of other simple suboptimal threshold detectors in Class A noise is also examined. Finally, a useful relationship between the Class A model and the e-mixture model is developed.

Analytic models for the latency and steady-state throughput of TCP Tahoe, Reno, and SACK

Continuing the process of improvements made to TCP through the addition of new algorithms in Tahoe and Reno, TCP SACK aims to provide robustness to TCP in the presence of multiple losses from the same window. In this paper we present analytic models to estimate the latency and steady-state throughput of TCP Tahoe, Reno, and SACK and validate our models using both simulations and TCP traces collected from the Internet. In addition to being the first models for the latency of finite Tahoe and SACK flows, our model for the latency of TCP Reno gives a more accurate estimation of the transfer times than existing models. The improved accuracy is partly due to a more accurate modeling of the timeouts, evolution of cwnd during slow start and the delayed ACK timer. Our models also show that, under the losses introduced by the droptail queues which dominate most routers in the Internet, current implementations of SACK can fail to provide adequate protection against timeouts and a loss of roughly more than half the packets in a round will lead to timeouts. We also show that with independent losses SACK performs better than Tahoe and Reno and, as losses become correlated, Tahoe can outperform both Reno and SACK.

Robust Wiener- Kolmogorov theory

A minimax formulation is considered for the problem of designing robust linear causal estimators of linear functions of discrete-time wide-sense stationary signals when knowledge of the signal and/or noise spectra is inexact. The solution is given (under mild regularity conditions) in terms of a least favorable pair of spectra, thus reducing the minimax problem to a direct maximization problem which in many cases can be solved easily. It is noted that this design method leads, in particular, to robust n -step predictors, robust causal filters, and robust n -lag smoothers. The method of design is illustrated by a thorough development of the special case of one-step noiseless prediction. Further, solutions are given explicitly for the problem of robust causal filtering of an uncertain signal in white noise, and numerical examples are given for this case which illustrate the effectiveness of this design.

An integrated model for the latency and steady-state throughput of TCP connections

Abstract Most TCP connections in today’s Internet transfer data on the order of only a few kilobytes. Such TCP transfers are very short and spend most of their time in the slow start phase. Thus the underlying assumptions made by steady-state models cease to hold, making them unsuitable for modeling finite flows. In this paper, we propose an accurate model for estimating the transfer times of TCP flows of arbitrary size. Our model gives a more accurate estimation of the transfer times than those predicted by Cardwell et al. [Proceedings of the IEEE INFOCOM, Tel Aviv, Israel, March 2000, pp. 1742–1751], which extends the steady-state analysis of Padhye et al. [IEEE/ACM Trans. Networking 8 (2) (2000) 133] to model finite flows. The main features of our work are the modeling of timeouts and slow start phases which occur anywhere during the transfer and a more accurate model for the evolution of the cwnd in the slow start phase. Additionally, the proposed model can also model the steady-state throughput of TCP connections. The model is verified using web based measurements of real life TCP connections. We also introduce an empirical model which allows a better “feel” of TCP latency and the nature of its dependence on loss probabilities and window limitation. Finally, the paper investigates the effect on window limitation and packet size on TCP latency.

Reliable transmission of high-quality video over ATM networks

The development of broadband networks has led to the possibility of a wide variety of new and improved service offerings. Packetized video is likely to be one of the most significant high-bandwidth users of such networks. The transmission of variable bit-rate (VBR) video offers the potential promise of constant video quality but is generally accompanied by packet loss which significantly diminishes this potential. We study a class of error recovery schemes employing forward error-control (FEC) coding to recover from such losses. In particular, we show that a hybrid error recovery strategy involving the use of active FEC in tandem with simple passive error concealment schemes offers very robust performance even under high packet losses. We discuss two different methods of applying FEC to alleviate the problem of packet loss. The conventional method of applying FEC generally allocates additional bandwidth for channel coding while maintaining a specified average video coding rate. Such an approach suffers performance degradations at high loads since the bandwidth expansion associated with the use of FEC creates additional congestion that negates the potential benefit in using FEC. In contrast, we study a more efficient FEC application technique in our hybrid approach, which allocates bandwidth for channel coding by throttling the source coder rate (i.e., performing higher compression) while maintaining a fixed overall transmission rate. More specifically, we consider the performance of the hybrid approach where the bandwidth to accommodate the FEC overhead is made available by throttling the source coder rate sufficiently so that the overall rate after application of FEC is identical to that of the original unprotected system. We obtain the operational rate-distortion characteristics of such a scheme employing selected FEC codes. In doing so, we demonstrate the robust performance achieved by appropriate use of FEC under moderate-to-high packet losses in comparison to the unprotected system.

Design of a transport coding scheme for high-quality video over ATM networks

In this paper, we explore the design of forward error control (FEC)-based error concealment schemes for digital video transmission on ATM networks. In particular, we study the impact of code selection on the overall performance and provide a judicious code selection strategy. The use of FEC provides an active and powerful means of recovery from packet loss which is particularly useful when the encoded video material has high motion and scene changes. The best technique for applying FEC is to throttle the source coding rate so that the overall transmission rate after FEC application equals the original unprotected rate. However, the resulting performance then depends on the particular code selected. A well-chosen code provides good protection while allowing little sacrifice in quality and at the same time satisfies specified delay constraints. Our results show that a single code is generally insufficient to provide good performance under all operating conditions. However, a small group of codes can be preselected, using the efficient code-selection strategy described here, which will provide efficient and robust performance over a wide range of channel conditions. We show that this simple code selection strategy is sufficient to select codes judiciously for a wide range of operating conditions and constraints. Employing this selection strategy, we demonstrate that moderate length codes are sufficient to provide good performance while meeting the imposed delay constraint.

Traffic management and network control using collaborative on-line simulation

The complexity and dynamics of the Internet is driving the demand for scalable and effective network control. This paper proposes a collaborative on-line simulation architecture to provide pro-active and automated control functions for networks. The general model includes autonomous on-line simulators which continuously monitor/model the network conditions and execute a search in the parameter state space for better settings of the protocol parameters. The protocol parameters are then tuned by the on-line simulation system. We describe the building blocks of this architecture and investigate the implementation challenges in the areas of network modeling, on-line simulation and parameter search. We also discuss the applicability of this system and present the simulation and test results of a preliminary implementation.

FDQ: the fair distributed queue MAN

A new protocol, fair distributed queue (FDQ), suitable for very high speed metropolitan area networks (MANs), is presented. FDQ is a slotted system implemented on a unidirectional fiber bus. It has similarities to the distributed queue dual bus (DQDB) protocol. FDQ achieves full throughput efficiency independent of the bus length, the transmission speed, and the number of nodes. FDQ allocates equal bandwidth under heavy load to all active users in a time period less than or equal to the round-trip propagation delay without wasting bandwidth. Its delay characteristics were studied via simulation and compared to DQDB. FDQ has lower average delays under a Poisson load than the DQDB protocol with or without the bandwidth balancing mechanism. The implementation of priority levels and detailed timing structure of the protocol are also described. It was shown that FDQs delay and throughput characteristics were only slightly affected by increasing distances or the number of nodes.<<ETX>>

Analytic models and comparative study of the latency and steady-state throughput of TCP Tahoe, Reno and SACK

In this paper we present analytic models to estimate the latency and steady-state throughput of TCP Tahoe, Reno and SACK and validate our models using both simulations and TCP traces collected from the Internet. We also conduct a study comparing the performance of these versions of TCP under different loss scenarios. In addition to being the first models for the latency of finite Tahoe and SACK flows, our model for the latency of TCP Reno gives a more accurate estimation of the transfer times than existing models. Our models show that under the losses introduced by the droptail queues which dominate most routers in the Internet, current implementations of SACK fail to provide adequate protection against timeouts and a loss of roughly more than half the packets in a round will lead to timeouts. We also show that with independent losses, SACK performs better than Tahoe and Reno and as losses become correlated, Tahoe can outperform both Reno and SACK.

Delay analysis of the FDDI synchronous data class

An analytical model is applied to predict the performance of the FDDI (Fiber Distributed Data Interface) network in satisfying the delay constraints on the synchronous-class data frames. An example which illustrates the application of the analytic work and provides insight into the relationships between network performance and the negotiated parameter values of the FDDI protocol is given. In particular, the example demonstrates that while the current FDDI priority scheme guarantees a station will receive the token within a specified time, it does not guarantee that all waiting synchronous packets will meet their deadlines.<<ETX>>