Handbook of Local Area Networks, 1998 Edition:Advanced LAN Interconnectivity Issues and Solutions Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next When a switch receives an RM cell from the source, an intermediate switch (congested or noncongested) computes a mean allowed cell rate (MACR) and a fair share, which is a fraction of the MACR average. When a congested switch receives an RM cell from the destination, it reduces ER filed to the fair share in the RM cell. MACR and a fair share are computed as follows: MACR = (1-a) MACR + aCCR Fair share = SW_DPF*MACR Here, a is the exponential averaging factor and SW_DPF is a multiplier (called switch down pressure factor) set close to, but below, 1. The suggested values of a and SW_DPF are 1/16 and 7/8, respectively. The destination monitors the EFCI bit in data cell if the last seen data cell had EFCI bit set, it marks the CI bit in the RM cell. In addition to setting the explicit rate, the switch can also set the CI bit in the returning RM cells if its queue length is more than a certain threshold. A source continuously decreases its cell rate by computing: ACR = ACR * RDF RDF is the reduction factor in this case. When a source receives an RM from the destination it increases its rate by an amount additive increase rate (AIR), if the CI bit is clear. For example: If         CI = 0 Then new ACR = min. (ACR + AIR, ER, PCR) On the other hand, its ACR is not changed if CI bit is set. EPRCA allows EFCI switches, binary-feedback switches, and the explicit feedback switches on the path. Therefore, EPRCA provides considerable flexibility to network vendors. The main problem in EPRCA is the switch congestion detection algorithm, which is based on queue length threshold. If the queue length exceeds a certain threshold, the switch is said to be congested. If it exceeds another higher threshold, it said to be very highly congested. This method of congestion detection was shown to result in unfairness. Basically, a source that started late was found to get lower throughput than one that started early. The Intelligent Congestion Control Algorithm The intelligent congestion control algorithm was proposed to resolve the ACR beat down problem. The key idea of this scheme is for each congested switch to estimate the optimal cell rate on each VC with a small number of computations and without the need of per-VC queuing or accounting. This estimated rate is used to adjust the cell rates of the sources using positive feedback mechanisms. More specifically, each source periodically sends an RM cell containing its current allowed cell rate (ACR) and explicit rate (ER), which is the maximum allowed cell rate of the source. For every data cell transmitted, a source continually decreases its ACR by additive decrease rate (ADR) until it receives an RM cell from the destination. A variable modified allowed cell rate (MACR) is defined in order to contain the value of the estimated optimal cell rate for each queue of a switch. When a noncongested switch receives an RM (ACR, ER) cell from the source, it replaces MACR by MACR +b * (ACR - MACR). When a congested switch receives an RM cell from the source, it replaces MACR by MACR + b * (ACR - MACR), only if ACR is smaller than MACR. Using a first order filter, an intermediate switch that is congested or non-congested iteratively estimates the optimal cell rate for each VC given the ACR of each VC. When a destination receives an RM (ACR, ER) cell, it returns the RM cell to the source. When a congested switch receives an RM (ACR, ER), it takes one of two actions depending on congestion status. One possible action is the switch replacing ER in the RM cell by min (ER, g * MACR) if the current queue length is greater than a certain threshold. The other possibility is the switch replacing ER in the RM cell by min (ER, MACR) if ACR is greater than MACR. Upon receiving an RM (ACR, ER) cell from the destination, a source computes new ACR according to the rate information in the RM cell. The data and control traffic of the scheme in a congested network, where VC1 is congested at switch 2, are illustrated in Exhibit 4-7-12. Exhibit 4-7-12.  Intelligent Congestion Control with Output Buffer Switches This scheme enhances the knowledge of the switches and provides considerable flexibility to network vendors to select a switch architecture with various cell queueing and traffic management options. It also significantly resolves the ACR beat down problem without the need of per-VC queueing or accounting, thus providing a fairness among all VCs. However, the scheme takes a lot of time to compute MACR at the switches even if the switch is not congested. Also, the response time to congestion is slow. Congestion originates from forwarding an RM cell to the destination and returning to the congested switch without taking any action to relieve the congestion. SUMMARY There is a considerable amount of interest surrounding high-speed communications and enabling technologies right now. This chapter summarizes recent developments in rate-based congestion control for ABR services in ATM networks, including their advantages and disadvantages. Several congestion control schemes have been proposed in the ATM Forum to provide the requirements of ABR services. However, these proposed schemes are still not well understood and have problems that require additional research effort. It is important for network designers and engineers to realize the limitations of the various approaches in order to make good decisions when considering ATM for their enterprise network. Traffic management for ATM networks is still in a state of flux and development of standards will be incomplete for several years to come. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. 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