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Asynchronous Transfer Mode (ATM) has been recommended and has been accepted by industry as the transfer mode for Broadband network. Currently, large scale effort has been undertaken both in the industry and academic environment to design and build high speed ATM networks for corporate bodies. These networks are meant to support both real-time and non-real time applications with different quality of service (QoS) requirements. The resources to support the applications QOS requirements are typically limited and therefore the need to dynamically allocate resource in a fair manner becomes inevitable. In this work, an evaluation is carried out on the performance of enterprise-wide network that its backbone is based on leased trunk. The performance of the leased trunk was evaluated when loaded with homogeneous and heterogeneous traffic. The evaluation was carried out in order to determine the exact effect of traffic overload on resources-trunk transmission capacity and buffer. The aim is to define the optimum loading level and the associated QoS parameter values. A typical network was adopted, modeled and simulated in MATLAB environment using Simulink tool and results obtained were analyzed using Microsoft Excel.


1.0 Introduction

Enterprise wide network also know as cooperate networks are private communication networks owned and run by enterprises. This kind of network provide communication platform for geographically separated site (offices) of an organization. The different offices of an enterprise could be within a locality, a state, a nation or distributed round all over the globe. An enterprise private network could also be seen as acomputer network built by a business to interconnect its various company sites (such as production sites, offices and shops) in order to share computer resources.’ Also and enterprise wide area network (WAN) is a corporate networkthat connects geographically dispersed users areas that could be anywhere in the world. Enterprise WAN links LANs in multiple locations. The enterprise in question often owns and manages the networking equipment within the LANs. However, the LANs are generally connected by a service provider through leased trunks thus providing connectivity to the geographically dispersed sites [1, 2]. Briefly we present an account of the key features of the current communication environment, namely the characterization of the communication services to be provided as well as the features and properties of the underlying communication network that is supposed to support the previous

1.1 Historical Background

The fundamental purpose of a communication system is to exchange information between two or more devices. Telecommunication has witnessed unprecedented and explosive growth overthe years in the area of services delivered and technology. The key parameters of telecommunication service cannot be easily identified, owing to the very different nature of the various services that can be envisioned. This is basically the reason for the rapidly change in the technological environment. In fact, a person living in the sixties, who faced the only provision of the basic telephone service and the first low-speed data services, could rather easily classify the basic parameters of these two services.

The tremendous push in the potential provision of telecommunication services enabled by the current networking capability makes such classification harder year after year. In fact, not only are new services being thought and network-engineered in a span of a few years, but also the tremendous progress in very large scale integrated circuit (VLSI) technology makes it very difficult to foresee the new network capabilities that the end-users will be able to exploit even in the very near future.
Digital technology is an aspect that has greatly affected the evolution of telecommunication networks, especially telephone networks. In the past, both transmission and switching equipment of telephone network were initially analogue. Transmission systems, such as the multiplexers designed to share the same transmission medium by tens or hundreds of channels, were largely based on the use of frequency division multiplexing (FDM), in which the different channels occupy non-overlapping frequencies bands. Switching systems, on which the multiplexers were terminated, were based on space division switching (SDS), meaning that different voice channels were physically separated on different wires: their basic technology was initially mechanical and later electromechanical.
The use of analogue telecommunication equipment started to reducein favor of digital system with progress in digital technology. Digital transmission systems based on time division multiplexing (TDM), in which the digital signal belonging to the different channels are timeinterleaved on the same medium, are now widespread and analogue systems are being completely replaced. After an intermediate step based on semi-electronic components, nowadays switching systems have become completely electronic and thus capable of operating a time division switching (TDS) of the received channels, all of them carrying digital information interleaved on the same physical support in the time domain.
Such combined evolution of transmission and switching equipment of a telecommunication network into a full digital scenario has represented the advent of the integrated digital network (IDN) in which both time division techniques TDM and TDS are used for the transport of the user information through the network. The IDN offers the advantage of keeping the (digital) user signals unchanged while passing through a series of transmission and switching equipment, whereas previously signals transmitted by FDM systems had to be taken back to their original baseband range to be switched by SDS equipment.



The industrial and scientific community soon realized that service integration in one network is a target to reach in order to better exploit the communication resources. The IDN then evolved into the integrated services digital network (ISDN) whose scope was to provide a unique user network interface (UNI) for the support of the basic set of narrowband (NB) services, that is voice and low-speed data, thus providing a narrowband integrated access[5]. The narrowband ISDN, although providing some nice features, such as standard access and network integration, has some inherent limitations: it is built assuming a basic channel rate of 64kbit/s and in any case, it cannot support services requiring large bandwidth (typically the video
services) thus the need for broadband integrated services digital network (B-ISDN). The approach taken by moving from ISDN to broadband integrated services digital network (BISDN) is to escape as much as possible from the limiting aspects of the narrowband environment The evolution of telecommunication networks promising to offer a wide spectrum of services has resulted in considerable research, development and standardization of B-ISDN.

B-ISDN is a broadband communication network developed by International Telegraph and Telephone Consultative Committee(CCITT) that enables the transmission of design simulations and other multimedia transmission that include text, voice, video and graphics in one network. It provides end users with increased transmission rate, up to 155.54Mbits/s on a switching basis. This is a great improvement as compared to the earlier rate of 64kbits/s employed in the ISDN which is not suitable for high definition moving pictures .
Also ISDN rigid channel structure based on a few basic channels with a given rate has been removed in the B-ISDN whose transfer mode has been chosen to be asynchronous transfer mode (ATM) due to its flexibility and efficiency [7].The ATM-based B-ISDN is a connectionorientedstructure
where data transfer between end-users requires a preliminary set-up of a virtual connection between them. ATM is a packet-switching technique for the transport of user information where the packet, called a cell, has a fixed size. An ATM cell includes a payload field carrying the user data, whose length is 48 bytes, and a header composed of 5 bytes. This format is independent from any service requirement, meaning that an ATM network is in principle capable of transporting all the existing telecommunications services, as well as future services with arbitrary requirements.
It is worth noting that choosing the packet-switching technique for the B-ISDN that supports also broadband services means also assuming the availability of ATM nodes capable of switching hundreds of millions of packets per second In the past, ATM was envisioned as the technology for future public network, this is due to the inherent benefits in it. Some of these benefits are its high performance via hardware switching, its dynamic bandwidth for busty traffic and its ability to support different class of multimedia traffic, its scalability in speed and network size, its common LAN/ WAN architecture and its international standard compliance. Currently ATM switches are used in private networks and as access node to public networks.

1.2 Problem Statement

Enterprises networks are meant to support real-time and non real-time application. The resources (common resources) available to carry theses different application/traffic generated by enterprise-wide network are limited owing to the fact that they are expensive to acquire and maintain. Adequately optimization of these limited resources which could be in the form of trunk line or switching points while ensuring that services are delivered at their desired QoS is a major issue faced by corporate networks. There is a challenge of adequately allocating the limited network resource in a fair manner. This challenge spouses out of the fact that there is no knowledge on how to exactly tell the optimum loading level of the network resource (trunk transmission capacity and buffer) and their associated QoS parameter.

1.3 Aim and Objectives of the Research



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