Modeling and Simulation Environment for Satellite and Terrestrial Communications Networks

We shall give here an overview of this fast developing field of modeling and simulation for communication systems taking the subjects concerned in the order they appear in this book; however first a few words about terminology.

This tutorial paper on simulation of Communications Networks treats briefly all the important aspects of a simulation study including modeling, simulation software, model validation and accreditation as well as statistical issues in simulation, and concludes with an example of a simulation analysis of a communications network.

The development of telecommunications network modeling and simulation tools has been a long and painful process that appeared to mature and become focused during the 80’s only to require major rethinking as the totally distributed and autonomously managed network environment of the 90’s appeared. The evolution of networks from their original message store and forward and multi-drop to today’s wholly distributed and autonomously managed high speed/broadband fiber-based networks is driving major changes in the modeling and simulation environment. This paper looks at the differences in network designs that are evolving. It reviews structural differences in networks and synopsizes options in today’s modeling tools. Finally recommendations for improving the effectiveness of our modeling tools for the future will be offered for discussion.

In mobile communications an efficient utilisation of the channels is of great importance. In this paper we describe the basic principles for obtaining the maximum utilisation and study strategies for obtaining these limits. In general a high degree of sharing is efficient, but requires service protection mechanisms to guarantee the Quality of Service for all services. We study cellular systems with hierarchical cells, and the effect of overlapping cells, and we show that by call packing we obtain a very high utilisation. The models are generalisations of the Erlang-B formula, and include general arrival processes, and multi-rate (multi-media) traffic for third generation systems.

Traffic models have been at the core of teletraffic research for the last several decades because they have traditionally served as fundamental tools for network performance analysis. With the recent discovery of fractal characteristics of Internet traffic, the need for fractal traffic models that play beyond this traditional role has grown significantly. This is attributed to the fact that despite tremendous interests being focused on a wide variety issues dealing with this fractal nature of Internet traffic, there still remain much new knowledge yet to be gained for its implications for Internet traffic engineering. This requires the fractal traffic models that can quantitatively link the model parameters to the fractal statistics of Internet traffic so that a parameterizable, physical structure can be associated with the Traffic under study, rather than providing mainly qualitative and abstract explanations. To meet this daunting challenge, a library of traffic models based on Fractal Point Processes (FPPs) are described and analyzed. Compared to other fractal Traffic models, the FPPs have several key advantages. First, they yield a simple and effective traffic parameterization method that determines the model parameters from the first- and second-order statistics. This result allows for quantitatively understanding how model parameters are related to and control the range of time scales over which fractal behavior is dominant. Second, they reveal how session-level fractal dynamics such as session arrivals, duration, and volume affect packet-level fractal dynamics in a quantitative manner. Moreover, they are versatile and thus are able to capture a broad range of different fractal behaviors. Additional advantage of these models includes low modeling complexity, yielding high computational efficiency for running large-scale simulations. Demonstration of these benefits are provided based on simulation and characterization of two Web traces.

Self-similar (s-s) arrival processes are realistic models for many types of network traffic. Unfortunately, generating synthetic s-s arrivals and interarrival times for simulations can be difficult. This paper discusses several issues related to s-s traffic generation for discrete-event simulations: (i) what s-s processes are, and why they are important to network modelers; (ii) where to find a fast s-s generator; (iii) how to install the generator and synthesize s-s arrivals; (iv) how to convert s-s arrival counts to interarrival times; (v) how to use s-s arrivals in simulations; and (vi) how traffic from simulated s-s arrival processes compares with traffic from simulated bursty and Poisson arrival processes.

In this paper a telecommunication system with (virtual) channel switching, heterogeneous users and with Poisson input flow is studied in stationary state. It is assumed that unsuccessful calling attempts resulting from abandoned dialing, abandoned ringing, blocked and abandoned conversation situations are not repeated. A model for such a telecommunication system has been proposed that requires the measurement of minimum number of parameters for the determination of the number of terminals in use, and various other characteristics of the system state.

Mobility is the core feature in wireless communication networks. Mobile subscribers can make or receive calls regardless of their location as far as they are in the radio coverage area of the base station that they are attached to. The phone numbers are not tied to any particular location area. The phone numbers are dedicated to the mobile subscribers and they are portable, too. Communication between two parties continues without any interruption, when the mobile subscriber moves from one radio coverage area to another during the call. Mobility management protocols and algorithms are key to all these wireless communication network features. In this study, we introduce different wireless communication technologies and mobility management protocols. We discuss new simulation environments and their capabilities to model mobility management protocols and algorithms that include simulation of handover and simulation of mobility management protocols in IP based wireless networks.

Different objective functions for optimal coverage in CDMA cellular radio networks are investigated in this work. The basic methodology is to first determine the number of necessary base transmitter stations (BTS) from a general stochastic model. Then, the corresponding number of base stations is selected out of a pool of candidate BTS in an optimal way. Optimality is measured by different criteria, e.g., the weighted number of captured traffic nodes, and certain functions of carrier-to-interference ratios at traffic nodes. As a prerequisite, a fast and accurate radio wave propagation tool for urban environments is necessary. For the real world examples of this paper an algorithm developed by the authors is used.

Future satellite communication systems proposed use geosynchronous (GEO) satellites, medium earth orbit (MEO), and low earth orbit (LEO) constellations. Most of the next generation satellite systems will use fast packet switching with onboard processing to provide full two-way services to and from earth stations. One of the major service drivers is a high data rate internet access carried over integrated satellite-fiber networks. Provisioning of quality of service (QoS) within the advanced satellite systems is the critical requirement.

In this paper, we present broadband LEO satellite network QoS model and simulated performance results. We discuss the TCP flow aggregates performance for their good behavior in the presence of competing UDP flow aggregates in the same assured forwarding. We identify several factors that affect the performance in the mixed environments and quantify their effects using a full factorial design of experiment methodology.

We present Galileo, a simulator for the transmission of both connection-oriented and connectionless traffic over a constellation of LEO/MEO (Low/Medium Earth Orbit) satellites. Its scope is limited to the satellites and the stations accessing them, without any modelling of the terrestrial network, but inside this scope the goal is to study the performance of satellite-based communication networks from as many as possible points of view. Typical applications include simulation of access techniques, routing policies, fault management. The simulator is written in Java, and it makes use of dynamic loading to easily integrate user-written modules. A draft manual is available, and a preliminary version of the program will be published by the end of 2000.

This paper compares analytical and simulative approaches for investigation of mobile satellite systems (MSS). Main parameters for calculation of the capacity i.e. new call and handover blocking probabilities of MSSs have been identified. These parameters have been determined analytically and, where possible, by simulation. The differences in the results obtained by analysis and simulation methods have been determined and their cause clarified using different simulators. Some of the simulators reflected the real system characteristics better than the others which neglected some features in the same way that the analytical modeling would by necessity. The results show that the main differences are due to neglecting real geometrical shape of the cells and because of the edge effect. Approximations made for analysis purposes of the channel holding time (CHT) and handover arrival distributions are shown to have less impact on the results.

The technical complexities inherent in today’s optical components, systems and networks make the use of dedicated software tools for researchers and designers not a luxury, but a bare necessity [1]. One important category of such software tools provides the capability to simulate the physical behavior of optical systems by modeling time-dependent signal propagation; the results allow the user to assess the performance of a design, for instance with respect to the bit error rate to expect under certain conditions. A broad range of applications can benefit from this capability, including naturally research and design, but also product evaluation and technical marketing.

Due to the wide range of scales involved — from wavelength-sized resonators in lasers and filters, to interactions in global networks — such design tools must employ highly advanced simulation technology. This chapter outlines the status of design software which is commercially available today, covering simulator technology as well as modeling problems.

While the availability of accurate and efficient software is a prerequisite for successful modeling of optical systems, its application in a real-world engineering environment still faces many problems. The most dominant problems include a lack of qualified designer, a lack of suppor for the design peocess within the tools, and finally a lack of precise input data as required for an accurate simulation. We discuss development trends that strive to improve the practical usability by introducing concepts which sddress the problems mentioned above.

The physical layer of high-density multi-wavelength optical networks (DWDM) contains a number of elements of different electrical sizes, that need to be modeled differently by introducing different approximations for their characterization and for representing intra-module electromagnetic and quantum mechanical interactions. Given the complex network and signal environments the validation of models for DWDM networks is rather difficult task.

In this work the modeling and simulation of the transmission characteristics of the optical fiber channel is investigated. In DWDM simulation tools the optimization of the “fiber module” presents one of the most challenging problems. Due to the immense computational resources they require rigorous fiber models can not be used in system simulation tools, and one has to resort to different approximate models, all of which are based on the Nonlinear Schroedinger’s Equation (NLSE).. This paper focuses on the effects of the approximations made in obtaining the NLSE and on their impact on the validity of this model.

This paper provides concepts for the performance evaluation of wireless telecommunication protocols in complex simulation scenarios for a most realistic interference situation. As both tasks, the simulation of the protocol stack as well as the calculation of the interference calculation are quite resource-consuming for complex scenarios, special care has to be taken not to exhaust the hardware limitations of the simulating machines.

With respect to the permanent increasing importance and complexity of modeling and simulation applied for the analysis, synthesis and for training of (technical) systems, model verification, validation and accreditation (VV&A) becomes a more and more important issue.

At the begin, this presentation will summarize major demands and challenges for effective and efficient VV&A. Based on these requirements, we will present a methodological concept for introducing VV&A as part of the model development and life cycle process along which each modeling phase produces a specific intermediate result (such as a structured problem description, a conceptual model description, a formalised model description, an executable model or experimental data). Specific techniques are applied for verification and validation of these well-structured intermediate results.

This conceptual approach results from several research projects on discrete event simulation in which we have been involved over the past years. We will also present an implementation plan of the proposed VV&A concept and will conclude with an outlook towards the planned concrete prototypical realization and its application.

The results of experiments made to validate an indoor radio propagation computer tool are presented in this paper. This work includes the validation of the models for the reflection and transmission of radiowaves on building obstacles, as well as the validation of the whole radiowave propagation tool. The results of the computer simulation have been compared with the measurements taken using a wideband radio channel sounder. The comparison was made using statistical tests and procedures.

Channel correction in an OFDM (Orthogonal Frequency Division Multiplex) system affects channel noise, in such a way, that the usual assumption of gaussian white noise is no longer valid. The gaussian white noise that goes into the receptor is filtered together with the signal and the noise becomes coloured. This paper shows the new noise probability density function and the related parameters that would allow the use of quasianalytical bit error rate estimation technique. Two equivalent probability density functions valid under different channel conditions are shown.

Discrete event simulation as a method for performance evaluation has become an indispensable tool in many fields, e.g., teletraffic engineering. New communication networks and services pose extreme requirements regarding the quality of service, e.g., cell loss probabilities in ATM¹ networks in the order of 10 ^-9. Straightforward simulation for this type of rare event leads to simulation run times in the order of months and years, thus requiring new methods to be investigated and employed. In the following the state of the art in the field of Rare Event Simulation is described and examples are given for the RESTART² /LRE³ method to demonstrate the type of results that can be achieved by using these methods. In recent years these methods have been applied to relevant quality of service parameters in ATM networks. Future applications will concern the investigation of the emerging quality of service concepts for Next Generation Internets.

RESTART/LRE is a multi-step simulation approach which reduces the simulation run time by several orders of magnitude from, e.g., years to minutes, thus making simulation studies for rare events feasible. The approach can be applied to single nodes and networks. It is a so-called importance splitting method, where system states that lead to the rare event are saved and used as the starting point for new simulation sub-runs. The results of the sub-runs are multiplied with the corresponding weights which are obtained in the previous step and result in the desired probabilities. The statistical evaluation method LRE used in this combined approach has the additional advantage of evaluating the so-called local correlation function which gives insight into the correlation structure of the investigated random variable and is part of an error measure for controlling all phases of a simulation. Examples are given for the loss probability of ATM cells in G/G/1/N⁴ type systems as well as certain tandem networks that represent a model for ATM reference connections. Additional results are presented where the rare event details of the delay time distribution are the object of the investigation. The RESTART/LRE simulation offers as a result the complementary distribution function of the delay time distribution so that the probability of delays longer than a given maximum can be deduced directly from this result. This is one of the quality of service parameters defined for ATM. Furthermore, new results are available where the method has been used to investigate rare event details of the loss probabilities for handover in wireless ATM networks. The underlying model for an ATM reference connection consists of a network with a sequence of */D/1/N⁵ queues and an extra delay (propagation and switching) between these queues. Additional traffic is offered to the outgoing connections to model the interference of other ATM connections. Simulation results for the complementary distribution function of the delay time are presented.