Asynchronous measurement of CDV in ATM networks using OAM cells

 

J. García-Hernández¹; M. Ghanbari²

 

¹ Instituto de Investigaciones Eléctricas, Gerencia de Control e Instrumentación, Av. Reforma No. 113, Col. Palmira, Temixco, Mor., México, E-mail: jgarciah@iie.org.mx

² The University of Essex, Department of Electronic Systems Engineering, Wivenhoe Park, Colchester, CO4 3SQ, United Kingdom.

 

Received: October 22^th 2001.
Accepted: June 27^th 2002.

 

Abstract

The synchronisation between source and destination clocks at network nodes to a common reference time of day is important in order to achieve accurate measurements of cells delays in ATM networks. Otherwise, direct measurement of the cell transfer delay and cell delay variation using time-stamped OAM cells would be inaccurate. In this work, a method for the accurate measurement of cell delay variation using OAM cells, which does not rely on synchronised clocks at the measurement points is presented. This is achieved by assuming that each ATM switch has the capability to process and to modify the information contained in the OAM cells. The system presented here was modelled by means of a block-oriented network simulator (BONeS), which is a graphically-oriented, general-purpose simulation language for modelling and simulating communications networks, including ATM networks. The measurements of the cell delay variation were carried out on a VBR video source generated by a real MPEG-2 encoder with a fixed quantizer step size. The simulation results obtained show that measurements of the cumulative cell delay variation carried out without synchronised clocks at the measurement points were very closed to those obtained using synchronised clocks at the measurement points.

Keywords: Operation and Maintenance (OAM), Cell Delay Variation (CDV), Variable Bit Rate (VBR), Quality of Service (QoS).

 

Resumen

La sincronización entre los relojes fuente y destino en nodos de red a una hora del día, es importante para obtener mediciones exactas del retardo de celdas en redes ATM. De lo contrario, la medición directa de la variación del retardo de celdas usando estampas de tiempo en celdas OAM resultaría inexacta. En este artículo, se presenta un método para medir con exactitud la variación del retardo de celdas en redes ATM usando celdas OAM, el cual no requiere de sincronización de relojes entre los puntos de medición. Esto es obtenido, considerando que cada switch ATM tiene la capacidad para procesar y modificar la información contenida en las celdas de monitoreo OAM. El sistema presentado aquí fue modelado por medio de un simulador de redes orientado a bloques (BONeS), el cual permite modelar y simular redes de comunicaciones, incluyendo redes ATM. Las mediciones fueron realizadas utilizando una fuente de video del tipo VBR, generado por un codificador MPEG-2 real con un cuantizador de paso fijo. Los resultados de simulación muestran que las mediciones de la variación acumulativa del retardo de celdas, realizadas sin la sincronización de relojes entre los puntos de medición, fueron muy similares a aquellas obtenidas usando relojes sincronizados entre los puntos de medición.

 

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Reference

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