Authors:
N. Golmie and D. Su

The rate-based congestion control mechanism in Asynchronous Transfer Mode (ATM) networks was selected at the ATM Forum for the flow control of the Available Bit Rate (ABR) service, a service used mainly for data traffic with no critical timing requirements in terms of cell transfer delay and cell delay variation. This mechanism controls the transmission rate of the ABR source, based on feedback information from the destination and/or the network switching nodes. It ensures that network resources are divided fairly and efficiently among competing connections. The basic algorithm operation is as follows: the source sends to the destination every Nrm data cells a Resource Management (RM) cell. Upon receipt of this RM cell, the destination returns it to the source iwth the latest network information in binary or Explicit Rate (ER) form. Furthermore, any intermediate network node may intercept this RM cell on its returning path and modify the congestion information it contains.

This paper analyzes the rate-based control algorithm as described in Draft ATM Forum Traffic Management Specification V.4.0 and evaluates its performance in terms of buffer requirements and network throughput. The analysis will proceed as follows. First, an expression for the flow controlled source rate, that is the Allowed Cell Rate (ACR), is obtained as a function of time and the algorithm's operational parameters. Basic equations for the buffer occupancy are also established. In a second stage, the oscillatory behavior of the ACR and the queue length processes is examined in terms of the nature of the feedback information provided to the ABR source. Simulation of the algorithm dynamics for some network configurations and scenarios of interest are presented. These experimental results verify the analysis described.

Key Words: ATM, Flow Control, Congestion control, Traffic Management, Rate-Based Mechanism.