The Effects of ABR Traffic on CBR Traffic

One recent development by the ATM Forum is a service class to carry ``bursty'' traffic, termed Available Bit Rate or ABR. ABR is a rate based service class that attempts to make optimal use of an ATM virtual circuit depending on the remaining bandwidth available along its route. The contract for an ABR circuit specifies parameters defining the maximum and minimum bandwidth available to it. Effective use of this available bandwidth is achieved by means of a rate control mechanism. If the network is experiencing congestion, the transmission rate of ABR traffic is decreased. Conversely, in an uncongested network the transmission rate is increased. The ABR source receives network information via Resource Management (RM) cells that are generated at regular intervals. These are returned by the destination to the ABR source, indicating any congestion experienced en route. In order to provide an acceptable transport mechanism, the effect of ABR traffic on other service classes, and in particular Constant Bit Rate (CBR) traffic, must be minimal. However, due to the dynamic nature of the ABR service class it is not readily apparent that this will be the case. The aim of the paper is to investigate how, and to what extent, the ABR service class influences CBR traffic. Of particular interest are the variation in delay and the absolute delay experienced by CBR cells in the presence of ABR traffic. Discrete event simulation is used to investigate the performance of CBR traffic given certain parameters which characterise the ABR service. These describe the bounds within which the rate control mechanism operates and how the rate changes in response to the reception of RM cells. The performance of ABR traffic is determined by the point at which network congestion occurs and actions taken by the switch to detect and respond to congestion.