Convergence Study of PWM Control of ABR Traffic in ATM

In recent years a considerable amount of research [1-14] has been undertaken in the area of congestion control and avoidance of available bit rate(ABR) service. Both the International Telecommunication Union (ITU) and the ATM Forum have accepted the rate based feedback scheme as the framework for ABR traffic control [1, 6, 8 & 15]. In the ABR traffic management scenario, each source attempts to adjust its admission rate to control the buffer levels of the switches along its virtual channel (VC) from the source to the destination in congestion control and to prevent buffer overflow in the case of congestion avoidance. The ABR service model supports the virtual source (VS) and virtual destination (VD) concept where the network elements can segment the ABR control loop into many small control loops. There can be any number of VS/VD loops along the path of a VC from the source end system (SES) to the destination end system (DES) [8 & 15]. If the network is segmented so that there is a single link between each VS and VD pair, this is known as hop by hop (HBH) control. Methods used for closed-loop Rate based ABR traffic management can be divided into two categories depending on the quantity being controlled. In the first category, the queue level is measured and the source rate is calculated using this information and a target or a threshold queue level. The switch sends this rate indication to the source using a resource management (RM) cell. The intermediate switches can reduce the indicated rate according to their source rate values. The switch can also send the queue level written directly into an RM cell back to the upstream switch. In the second category, the switches calculate the explicit rate for the sources using currently available throughput information. This rate is sent to the upstream switch using RM cells. Intermediate switches may reduce this rate according to their rate calculations. It is also possible to write the throughput value into an RM cell by the VD switch. The VS calculates the admission rate using the throughput information available in the RM cell. In this paper, we use this