Interaction of Periodic Bursty Traffic Flows in ATM Networks

Paulo Martins de Carvalho

Universidade do Minho
Departamento de Informática
P-4700-320 Braga

e-mail: paulo (at)


ATM will certainly play a major role in the emerging provision of broadband communications; it is expected to integrate efficiently both real-time and non-real-time traffic, while providing quality of service guarantees per connection.

This thesis focuses primarily on the behaviour of ATM networks when carrying video traffic. It was developed in two stages. The first stage includes a series of experiments investigating a point-to-point video transmission over a prototype cell-based system. These show that the performance achieved is primarily determined by the bandwidth allocated to the connection. They also show the existence of critical times when processing isochronous traffic and the importance of policing. The second stage focuses on the implications which the periodicity inherent in certain traffic patterns may have for the performance of ATM networks. In particular, the interaction of periodic, bursty traffic flows, like MPEG video, is studied by using both simulation and measurements. The network model consists of ATM user terminals connected by a blocking, input-buffered ATM switch (Banyan-based). Each terminal is able to multiplex several sources, and has traffic shaping capabilities. MPEG traffic is generated using a slice based model with two periodicity levels. The length of the slices follows a Gamma distribution. The impact of traffic interaction is measured in terms of end-to-end delay and jitter.

The results show that when switching multiple, concurrent MPEG traffic flows, the variation of phases of frame starts in the different flows has to be taken into account, or resource requirements may be underestimated. It is shown that managing MPEG video at a slice level has a smoothing effect on the video stream, and is, therefore, a good approach to minimising the phasing effects which may occur. In terms of scheduling, experiments reveal that the use of an independent round-robin algorithm in each stage of multi-stage switching environment can lead to situations of extreme unfairness.

In summary, the behaviour studied has two consequences. To the network and from the traffic control perspective, the periodicity may lead to a reduction of the statistical multiplexing capability and possibly to a global QoS degradation. As a consequence, additional network resources might need to be provided. To the end-systems it will be important to minimise the periodic structure of the video stream before transmission and to allocate enough resources so that resulting variations in delay can be absorbed.

PhD Thesis, University of Kent, Canterbury, UK, Dec 1996