J. García-Hernández¹ & M. Ghanbarl²

¹ Instituto de Investigaciones Eléctricas, Gerencia de Control e Instrumentación, Calle Reforma No. 113, Col. Palmira, Cuernavaca, 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: July 15^th 2002.
Accepted: December 3^th 2002.

Abstract

The measurement of the quality of service in ATM* networks involves the activation of the ATM layer management OAM functions. Once an ATM connection is set up, in-service performance monitoring based on OAM cells can be used to measure and estimate the ATM layer QoS parameters. In-service measurement of ATM layer QoS parameters such as cell delay variation, cell transfer delay, cell loss ratio, etc., is necessary to verify on one hand, if the network is meeting the requested QoS and on the other hand, if the users are receiving their promised QoS. Congestion conditions in ATM switches may cause gradual deteriorations in the quality of virtual paths or virtual channel connections in an ATM network. QoS measurement using OAM cells can accurately measure the overall quality of monitored ATM layer QoS parameters in order to detect such deteriorations before the service quality has dropped bellow an acceptable level. Several factors such as network load, congestion, bottlenecks, and increments in network load may affect the cell delay variation of video transmissions in an ATM network. In this work, several ATM layer QoS parameters have been measured on a VBR video source using OAM cells. The simulation results obtained show that with low network loads, the contribution of each individual ATM switch to the local and to the end-to-end cdv is not significant. In the presence of a bottleneck switch, the cdv presented by this switch becomes the dominant one at both local and end-to-end sides. When the network load is between 0.5 and 0.6, small increments in network load do not alter the buffer performance of the non-congested ATM switches, and low values of CDV can be obtained. In contrast, it was found that there is a critical load between 0.65 and 0.7 where small increments in network load may force the cdv to grow abruptly.

Keywords: Operation and Maintenance, Cell Delay Variation, Variable Bit Rate, Quality of Service.

Resumen

La medición de la calidad de servicio en redes ATM involucra la activación de funciones OAM de administración de la capa ATM. Una vez que una conexión ATM es establecida, el monitoreo en-línea basado en celdas OAM puede ser activado para medir y estimar los parámetros de calidad de servicio de la capa de ATM. La medición en-línea de parámetros tales como la variación del retardo de celdas, el retardo de transmisión, la razón de pérdida de celdas, etc., es necesaria para verificar, por un lado, si la red está cumpliendo con la calidad de servicio requerida y, por el otro, si los usuarios están recibiendo la calidad de servicio solicitada. Condiciones de congestión en switches ATM pueden causar degradaciones graduales en la calidad de las trayectorias y conexiones virtuales en una red ATM.

La medición de la calidad de servicio usando celdas OAM puede medir con exactitud la calidad de los parámetros monitoreados de la capa ATM con el fin de detectar dichas degradaciones antes de que la calidad de servicio haya caído por debajo de un nivel aceptable. Varios factores, tales como la carga de la red, congestión, "cuellos de botella" e incrementos en la carga de la red, pueden afectar la variación del retardo de celdas de transmisiones de video en una red ATM. En este trabajo, varios parámetros de calidad de servicio de la capa ATM han sido medidos en una fuente de video tipo VBR usando celdas OAM. Los resultados de simulación muestran que con cargas de red bajas, la contribución individual de cada switch ATM al cdv local y al cdv terminal-a-terminal no es significativa. Cuando un "cuello de botella" en un switch está presente, el cdv presentado por este switch es el dominante en ambos casos local y terminal-a-terminal. Cuando la carga de la red está entre 0.5 y 0.6, incrementos pequeños en la carga no alteran el rendimiento de los switches ATM no congestionados y, por lo tanto, valores bajos de cdv pueden ser obtenidos. Por el contrario, se encontró que existe una carga crítica entre 0.65 y 0.7 donde incrementos pequeños en la carga de la red pueden forzar al cdv a crecer abruptamente.

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