ATM Congestion control

Due to the unpredictable traffic pattern, congestion is unavoidable. When the total input rate is greater than the output link capacity, congestion happens. Under a congestion situation, the queue length may become very large in a short time, resulting in buffer overflow and cell loss. So congestion control is necessary to ensure that users get the negotiated QoS.

In this study, two major congestion algorithms are focused, which are especially for ABR source. Binary Feedback scheme (EFCI) uses a bit to indicate congestion occurs. A switch may detect congestion in the link if the queue level exceeds a certain level. Accordingly, the switch sets the congestion bit to 1. When the destination receives these data cells with EFCI bit set to 1, the destination sets the CI bit of the backward RM cell to 1 indicating congestion occurs. When the source receives a backward RM cell with CI bit as 1, the source has to decrease its rate. The EFCI only told the source increase or decrease the rate and hence the method converges slowly. The Explicit Rate Indication for Congestion Avoidance (ERICA) algorithm solves the problem by allowing each switch to explicitly tell the desired rate to the passing RM cells, the source adjusts the rate according to the backward RM cells.