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## Abstracts Of Some Publications

**“Path Bandwidth Management for Large Scale Telecom Networks”**, by M. Logothetis, G. Kokkinakis,

*IEICE Transactions on Communications*, to be appeared.

This paper presents a Path Bandwidth Management (**PBM**) model for large-scale networks that leads to an almost optimal PB allocation, under constraints posed by the installed bandwidth in the transmission links of the network. The PB allocation procedure is driven from a traffic demand matrix and consists of three phases. In the first phase, a suitable decomposition of the whole network takes place, where the large-scale network is split to a set of one-level sub-networks. In the second phase, the optimization algorithm developed for one-level telecommunication networks is applied to each sub-network in order to define the optimal PB allocation. The criterion for optimization is to minimize the worst Call Blocking Probability (**CBP**) of all switching pairs of the sub-network. In the third phase, composition of the sub-networks takes place in a successive way, which leads to the final PB allocation of the large-scale network. As the large-scale network is built up from optimized sub-networks, an almost optimal PB allocation is anticipated. For evaluation, the worst resultant CBP of the proposed scheme is compared with that obtained by the optimal PB allocation procedure in order to prove its optimality and efficiency. We choose a set of large-scale networks whose size is not very large so that we can apply the optimization algorithm developed for one-level telecom networks for defining its optimal bandwidth allocation. Extensive evaluation of the PBM model has showed that the worst resultant CBP is about 2% above the optimal value, which is a satisfactory result. The proposed PBM scheme is explained by means of an application example.

**“Optimal Resource Management in ATM Networks”,**by M. Logothetis, Tutorial paper in

*"Performance Evaluation and Application of ATM Networks",*ed. D. Kouvatsos, Kluwer Academic Publishers, U.S.A., 2000.

In the beginning an overview of network management and traffic control in ATM networks is presented, based on the fact that traffic control is distinguished in two levels, the Call-level and the Cell-level control, according to the distinction of ATM traffic in call and cell components, respectively. Afterwards, the paper concentrates on the Call-level and mainly on traffic controls which effectively manage the network resources (bandwidth) and whose the performance is drastically influenced by the bandwidth capacities of the transmission links of the networks. Especially, the impact of Virtual Path Bandwidth **(VPB) **control on ATM network performance is discussed. Furthermore, the optimal VPB control is presented, which minimizes the worst Call Blocking Probability of all Virtual Paths **(VP)** of the network. A centralized VPB controller can readily rearrange the VP bandwidth of an ATM network having an appropriate architecture, composed of ATM Cross-Connect systems. The controller solves a large network optimization problem by a rigorous analytical procedure. The optimization model comprises bandwidth distribution schemes assuring network reliability. The demand for reliability requires considerably larger bandwidth to be installed in the backbone network and, therefore, optimal VPB control becomes essential. The procedure for optimal VPB allocation is clarified, step by step, in a tutorial application example. In a more realistic example, the optimal VPB control is applied on a model ATM network.

**"Virtual Path Bandwidth Control Versus Dynamic Routing Control",**by I. Papanikos, M. Logothetis, G. Kokkinakis,

*Performance Modelling and Evaluation of ATM Networks*, Chapman and Hall, London, UK (1996).

Virtual Path Bandwidth (**VPB**) control and Virtual Circuit Routing (**VCR**) control are competitive control schemes for traffic management in ATM networks. The objective of both controls is to minimize the Call Blocking Probability (**CBP**) of the congested end-to-end links, under constraints posed by the transmission links capacity of the network. Firstly, we compare the performance of two VCR control schemes, the **DAR** and **DCR**, well-known in the environment of STM networks, considering several trunk reservation parameters and different control intervals. Secondly, we compare the performance of VPB control schemes with that of VCR control schemes, both under static and dynamic traffic conditions. Under static traffic conditions the efficiency of the two control schemes in minimizing the worst CBP of the network is examined, whereas under dynamic traffic conditions their response time is measured by means of simulation. In short, VPB control is more effective than VCR control when the traffic fluctuation is large while VCR control has a faster response time than VPB control.

**"A Path Bandwidth Allocation Scheme for Hierarchical Telecommunication Networks",**by M. Logothetis, G. Kokkinakis,

*IEEE/IFIP 1996 Network Operations and Management Symposium, NOMS'96*, Kyoto, Japan, April 15-19, 1996.

This paper presents a model for large-scale hierarchical networks which leads to an almost optimal Path Bandwidth (**PB**) allocation, under constraints posed by the installed bandwidth in the transmission links of the network. The PB allocation procedure consists of three phases. In the first phase, a suitable decomposition of the whole network takes place, in which the hierarchical network is split to a set of sub-networks of just one level. In the second phase, the optimization algorithm developed for one-level telecommunication networks is applied to each sub-network to define the optimal PB allocation. The optimization criterion is to minimize the worst Call Blocking Probability (**CBP**) of all switching pairs of the sub-network. In the third phase, composition of the sub-networks takes place in a successive way which leads to the final PB allocation. As the hierarchical network is built up from optimized sub-networks, it is considered that it has an almost optimal PB allocation. The proposed PB allocation scheme is explained by means of an example.

**"Medium-Term Centralized Virtual Path Bandwidth Control Based on Traffic Measurements"**by M. Logothetis, S. Shioda,

*IEEE Transactions on Communications*, Vol. 43, Oct. 1995.

This paper presents a centralized VPB control scheme for ATM networks which satisfies mainly three specifications: a) Optimality during a Medium-Term control interval, b) Fast time response so as to absorb the Medium-Term traffic fluctuations, and c) Easy implementation. The paper mainly points at the impact of direct, on-line traffic measurements on bandwidth control. The control objective is to rearrange the installed bandwidth of the Virtual Paths according to the offered traffic so as to minimize the maximum Call Blocking Probability of the whole network. Network simulation shows that a sophisticated VPB controller which relies on simple measurements of the offered traffic can substantially improve the performance of an ATM network. The necessary bandwidth rearrangement time is also examined by simulation.

**"Influence of Synchronous Digital Hierarchy Paths in ATM-Network Performance",**by I. Papanikos, M. Logothetis, G. Kokkinakis,

*Proc. 5th International Conference on Advances in Communication & Control,*

*COMCON 5*, Rethymna, Crete, 1995.

In this paper, the influence of Synchronous Digital Hierarchy (SDH) Paths on Virtual Path (VP) bandwidth dimensioning is examined from both quantitative and qualitative point of view. From quantitative point of view, we examine the increase of the required bandwidth of a VP when it comprises more than one transmission paths (multi-path VP), under the same quality-of-service (QOS) requirements in comparison to the single-path case, where the VP consists of a single transmission path. From the qualitative point of view, we examine the increase of the CBP of the service-classes accommodated to single-path VP, under the same QOS requirements in comparison to multi-path VP of the same bandwidth. Moreover, we present the VP bandwidth dimensioning procedure we apply, taking into account the constraints posed by the transmission paths.

**"A Batch-Type Time-True ATM-Network Simulator",**by M. Logothetis, G. Kokkinakis,

*Proc. 5th International Conference on Advances in Communication & Control,*

*COMCON 5*, Rethymna, Crete, 1995.

This paper proposes a new type of ATM-network simulator with a data structure appropriate for its implementation by a general purpose programming language. The simulator is a pure time-true simulator but it is not a call-by-call type. It can be characterised as a batch type. The whole duration of simulation time is divided into short time intervals of equal duration T. During T, a butch processing of call origination or termination events is executed and the time-points of these events are sorted. The number of sorting executions is drastically reduced in comparison to call-by-call simulator. It results considerable time savings. Besides, the proposed simulator structure is well fitted to parallel processing techniques for further savings of execution time. The performance of the proposed Batch-type Time-true ATM-network Simulator is compared with that of a call-by-call simulator to reveal its superiority as far as the execution time is concerned.

**"Network Planning Based on Virtual Bath Bandwidth Management",**by M. Logothetis, G. Kokkinakis,

*International Journal of Communications Systems*, No. 8, Aug. 1995.

Periodical performance evaluation and adaptive resource assignment, already proposed as performance-oriented management, seems to be the most suitable strategy for network planning under demand uncertainty. In this paper, we exploit the inherent capability of ATM-networks to rearrange dynamically the already installed resources and propose performance-oriented management combined with Virtual Path Bandwidth (VPB) control for the planning of the extensions of bandwidth-capacities of Virtual Paths (VPs) and transmission links of the network. We define a large network optimization problem and solve it by a rigorous, analytical procedure. The optimization model comprises specific requirements of the network-planning problem and a bandwidth distribution scheme assuring network reliability. We reveal the efficiency of the proposed scheme by applying it on a model network, considering two realistic case-studies of network - traffic evolution. We show that in the presence of VPB control: a) the initial distribution of the total bandwidth to VPs is of no importance, since it can be adaptively rearranged according to the offered traffic, b) the network is utilized well and bandwidth investment could be saved, and c) whenever additional bandwidth must be installed in VPs which have an unanticipated bad grade-of-service, time savings result. We present the network performance in detail, in figures, and compare this with the performance of the network in the absence of VPB control.

**"Optimal Virtual Path Bandwidth Management Assuring Network Reliability",**by M. Logothetis, S. Shioda, G. Kokkinakis,

*Proc.*

*ICC'93*, Geneva, 1993.

The impact of the Virtual Path Bandwidth **(VPB)** management on the network performance in the expected environment of B-ISDN has been recognized and several VPB control schemes have been proposed. We present the optimal VPB management with objective to minimize the worst Call Blocking Probability **(CBP)** of all VPs in the network. We solve a large network optimization problem by a rigorous analytical procedure. Our optimization model comprises bandwidth distribution schemes assuring network reliability. The demand for reliability requires considerably larger bandwidth to be installed to the backbone network and therefore the optimal VPB management is essential. As an application example we examine a model ATM-network.

**"Optimal Virtual Path Bandwidth Management Assuring Network Reliability",**by M. Logothetis, S. Shioda, G. Kokkinakis,

*Proc.*

*ICC'93*, Geneva, 1993.

The impact of the Virtual Path Bandwidth **(VPB)** management on the network performance in the expected environment of B-ISDN has been recognized and several VPB control schemes have been proposed. We present the optimal VPB management with objective to minimize the worst Call Blocking Probability **(CBP)** of all VPs in the network. We solve a large network optimization problem by a rigorous analytical procedure. Our optimization model comprises bandwidth distribution schemes assuring network reliability. The demand for reliability requires considerably larger bandwidth to be installed to the backbone network and therefore the optimal VPB management is essential. As an application example we examine a model ATM-network.

**"Influence of Bandwidth Rearrangement Time on Bandwidth Control Schemes",**by M. Logothetis, G. Kokkinakis,

*Proc. 4th International Conference on Advances in Communication & Control, COMCON4*, Rhodes, 1993.

This paper deals with the required time for bandwidth rearrangement, due to already existing connections at the time-point of bandwidth rearrangement. The bandwidth rearrangement time is examined both in the one-service class environment of STM networks and in the multi-services environment of B-ISDN. A detailed aspect of the bandwidth rearrangement time is given by considering the bandwidth reduction in one end-to-end link. Furthermore, the bandwidth rearrangement time is examined in a whole ATM-network. The evaluation is done through simulation. It is shown that the bandwidth rearrangement time is not only considerably long but may drastically influence the performance of short-term bandwidth control schemes or even their applicability because of peaks which may occur.

**"Centralized Virtual Path Bandwidth Allocation Scheme for ATM Networks",**by M. Logothetis, S. Shioda,

*IEICE Trans.*

*Commun.*, Vol. E75-B, No. 10, October 1992.

This paper deals with a network architecture based on a backbone network using ATM switches (ATM-SW) and ATM Cross-Connect Systems (ATM-XC). The backbone network is efficiently utilized by multiple-routing scheme. The performance of the network is controlled, exploiting the concept of Virtual Paths (VP) in ATM technology. The network is controlled by allocating the bandwidth of VPs so as to minimize the worst call blocking probability of all ATM-SW pairs, under the constraints of the ATM-SW capacities and the bandwidths of transmission paths in the backbone network. To improve the network performance, we use a trunk reservation scheme among service classes. We propose a heuristic approach to solve the formulated problem of non-linear integer programming. Evaluation of the proposed optimization scheme, in comparison to other optimal methods, shows the efficiency of the present scheme.