1.0 Background of the study
Current trends in telecommunication are toward integration of voice and data services. So far these services have been available separately, requiring separate subscription, communication links, and equipment. It has long been acknowledged that the integration of these services will result in significant flexibility and cost benefits to both service users and service providers. The Integrated Service Digital Network (ISDN) is a major attempt to realize these objectives.
Increasing market demand for data rates substantially greater than those supported by ISDN motivated the International Telecommunication Union (ITU-T) Study Group 13 formerly Consultative Committee for International Telegraph and Telephone (CCITT) Study Group XVIII, in 1985 to start a new activity with the objective of defining standards for the migration of Narrowband-Integrated Service Digital Network (N-ISDN) towards a Broadband-Integrated Service Digital Network (B-ISDN). The latter was envisioned to be a highly flexible network with an integrated transport of both new broadband services as well as existing narrowband services .
However, in 1988, ITU-T standardized a fast-packet switching concept called Asynchronous Transfer Mode (ATM) as the transfer mode solution for the B-ISDN . It had the flexibility to support existing as well as future services, and required minimal switch processing while maintaining efficient use of the available network resources.
In ATM, user (or source) information is transmitted in fixed size packets called cells. An ATM cell, as defined by ITU recommendation I.361, contains 48 bytes of user information and 5 bytes of control information as shown in figure 1. The control part or “header” consists primarily of Virtual Path/ Channel Identifiers (VPI/VCI). An ATM virtual channel, as it is available to a user, is uniquely defined by a combination of VPI and VCI in every transmission system along the path taken by all cells belonging to the same connection . Groups of virtual channels with the same VPI value can be transferred as a virtual path without taking the VCI value into consideration. This simple labeling scheme allows network complexity to be kept to a minimum and hence allows very fast switching and routing of cells .
When the first prototypes of ATM switches appeared, research was mainly focused on optimal buffering strategies in the early 1990’s . Over the years, the switch architectures have matured in their design. Since it is now generally recognized that large buffer sizes are required to handle bursty traffic, research efforts are carried out towards the use of shared-buffer based designs to reduce the overall buffering requirements.
The statistical nature of the ATM traffic has made the buffering of cells in the ATM switch unavoidable. Two or more cells may address the same port at the same time. In such a situation, each output line can serve only one cell per time slot while other cells must be stored in the buffers. This process of storing the cells is called buffering or queuing. Buffers may be placed externally (that is, input ports and output ports) or internally within the switch fabric. This process is the traditional way of solving contention in the ATM switch . Some basic strategies of buffering or queuing are: Input Queuing (IQ), Output Queuing (OQ), Input Queuing Shared Buffer (IQSB) and Output Queuing Shared Buffer (OQSB)
1.2 Objectives of the Work
The objective was to establish that the variation of the queue size of an OQSB in ATM switch, largely affects the source behavior, average queue length and QoS parameters of an ATM network. Therefore, the Output Queue Shared Buffer (OQSB) ATM network model would be developed and converted into a computer simulation model using MATLAB Simulink environment. The model would be simulated, results collected and analyzed. The relationship between the offered loads (traffic intensity), buffer size, Cell Loss Ratio (CLR), average queuing delay and average queue length would be established by the results.
1.3 Purpose of the Work
The purpose of this work is to present the state-of-the-art of an output shared buffer ATM switch that can assist network service providers in updating their networks by cutting down the cost of building an ATM network through the use of shared-buffer based designs to reduce the overall buffering requirements and to carry out the simulation analysis using MATLAB Simulink.
1.4 Scope of the Work
This dissertation has a limited scope. No physical implementation of the architecture of the ATM switch is required for the analysis. Analysis is based on the computer simulation model of an output shared buffer ATM switch. Traffic source parameter details for voice, video and data were considered. The bursty traffic model is based on an ON and OFF state, with each input traffic alternating between active and idle periods of geometrically distributed durations.
1.5 Thesis outline
This thesis report is organized as follows: In chapter one, Integrated Service Digital Network (ISDN) was introduced as a major attempt to integrate voice and data services, the migration of ISDN to Broadband-ISDN (B-ISDN) and the definition of an ATM cell, as defined by ITU recommendation I.361 were also presented. In chapter two, a review of literature was carried out on the history/evolution of the ATM network, the ATM technology including its software and hardware parts, the ATM switch and buffering strategies. In chapter three, the architecture, models and simulations of the OQSB ATM network was presented. In Chapter Four, simulation results and analysis were presented. In Chapter Five, recommendations were made and conclusions drawn.
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