Peter O'Reilly

An Efficient Simulation Technique for Performance Studies of CSMA/CD Local Network

A new simulation technique is presented for performance studies of local area networks which use CSMA/CD as the access protocol. The method requires partitioning the network stations into a few primary stations and the remainder as background stations. The background stations are modeled as a group by a background algorithm based on a dynamic CSMA/CD model which uses a set of probabilities and sampling to produce, in a recurrent manner, a stochastic sequence of busy/idle periods. The new method is more efficient than conventional discrete-event simulations when the number of network stations is reasonably large. The algorithm and the overall simulation technique are comprehensively validated. The technique is illustrated with a performance study of interactive transactions on a metropolitan CATV network.

Data Performance in Burst Switching When the Voice Silence Periods Have a Hyperexponential Distribution

The performance of data in burst switching has been analyzed in previous work with a fluid approximation of the data traffic. This study extends the previous model to the case where the silence interval between talkspurts has a hyperexponential, rather than an exponential, distribution. It is shown that data performance is extremely sensitive to the variance of the silence interval, and that, for empirical talkspurt and silence distributions, this model provides a vast improvement on models which assume that both types of intervals are exponentially distributed.

Conceptual issues for ATM

The authors discuss the asynchronous transfer mode (ATM), which has been strongly promoted as the transport structure for future broadband telecommunication networks by CCITT Study Group XVIII (internationally) and TISI (North America). ATM is a transport technique based on fast packet switching, where the information is packed into fixed-size cells of relatively short length. The authors do not question the technical feasibility of ATM at 150 Mb/s rates, but they maintain that the state of system definition is such that ATM cannot be unequivocally supported as a concept. Furthermore, they feel that there is danger in premature standardization of system parameters while the system concept itself is still in flux. They identify four major areas where the system concept needs to be clarified before standardization can proceed: grade of service, accounting, compatibility, and scaling.<<ETX>>

Performance Analysis of Data in Burst Switching

The performance of interactive data traffic in burst switching is analyzed with a model that allows a number of the channels to be reserved for data messages, assumes that voice talkspurts have preemptive priority over data messages in the shared channels, and uses a fluid-flow approximation for the data traffic. Extensive validations are provided for the model.

Burst and fast packet switching: performance comparisons

Abstract Two alternatives to the prevailing view of the ISDN backbone are burst switching and fast packet switching. Both these technologies have the potential to provide considerable savings in transmission and switching resources over systems with less integration, and treat the traffic generated by different services in a unified manner. In this paper we explore the significant differences between them from the performance perspective. Other important issues for a complete comparison of the two technologies, such as evolutionary development, processing complexity, and variable bit-rate encoding, are not considered. Burst and fast packet are first compared from a voice-only perspective. For the same TASI advantage and equal capacity, it is shown that fast packet can provide the same performance as burst switching. However, under these circumstances, the average residual capacity left for data is significantly greater for burst than fast packet. Nevertheless, using analytic models for both systems, it is shown that data performance within that residual capacity is — for 64 kb/s encoded voice — not significantly different except for regions of high utilization. A model developed and validated in earlier work is used to analyze the data performance in burst switching. To analyze the performance of data on an integrated packet-switched link, an analytic model is developed based on an earlier renewal theory model for packet voice and using generalized results for priority queues.

Routing of voice and data in burst-switched networks

The static and centralized routing of voice and data traffic in burst switched networks is addressed. It is assumed that the routing allows random bifurcation in voice and data paths and preemptive priorities for voice requirements. A study is made of routing of voice only, by using a multicommodity flow model with linearized link losses and average network loss as a minimization objective. Solving the resulting linear program, it is observed that optimal routing strategies prefer to freeze a requirement at an early stage of its path rather than those requirements that are close to their destinations. A study is made of the voice-data interaction at the link level using an available fluid-flow model, and the combined link performance is translated as a maximum flow constraint on a link. This constraint may have undesirable effects on the voice, such as introducing routes with flow absorbing loops, and unfair freezing of some requirements. All conflicting multiple objectives and constraints are included in a linear programming formulation, and it is shown how parameters can be tuned to produce desirable voice and data paths. >