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 QUALITY OF SERVICES (QOS) ATTRIBUTES Two end systems connected to an ATM network must inform all intermediate switches about their service requirements and traffic parameters before they can communicate across the network. This is similar to telephone networks where a circuit is set up from the calling party to the called party before any conversation can take place. In ATM networks, such circuits are called virtual circuits or virtual connections (VCs). The connections allow the network to guarantee the quality of service by limiting the number of VCs. Congestion control ensures that network resources are divided fairly and efficiently among competing connections. When setting up a connection on an ATM network, a user declares key service requirements and traffic parameters. These parameters as related to the desired quality of service are explained as follows: •  Peak cell rate (PCR) is the maximum instantaneous rate at which each source will transmit cells. PCR is the inverse of the minimum inter-cell interval and is illustrated in Exhibit 4-7-3. Exhibit 4-7-3.  Peak Cell Rate and Inter-cell Time •  Minimum cell rate (MCR) is the minimum rate desired by each source. Most VCs will ask for a low MCR—a value close to zero. Otherwise, the connection request may be denied if sufficient bandwidth is not available. •  Cell transfer delay (CTD) is experienced by a cell between network entry and exit points. It includes queueing delays at various intermediate switches, service times at queueing points, and propagation delays. •  Sustained cell rate (SCR) is the average rate as measured over a long interval. •  Burst tolerance (BT) is the maximum burst size that can be sent at the peak cell rate. •  Cell delay variation (CDV) is a measure of variance of cell transfer delay (CTD). High variation implies larger buffering for delay sensitive traffic such as voice and video. •  Cell loss ratio (CLR) indicates the percentage of cells that are lost in the network due to error and congestion. During congestion, the network first drops cells that have a cell loss priority (CLP) bit set in the ATM header because the loss of a CLP=0 cell is more harmful to the operation of the application. The cell loss ratio can be calculated using the following formula: Cell Loss Ratio = Lost Cells / Transmitted Cells TYPES OF TRAFFIC AND CONGESTION CONTROL STRATEGIES ATM was designed to support a number of different types of traffic including voice, video, and data. The traffic has been categorized according to its behavior and falls into two major classifications: guaranteed traffic and best-effort traffic. Guaranteed and Best-Effort Traffic. Guaranteed traffic includes variable bit rate and constant bit rate. Best-effort traffic includes available bit rate. VBR traffic is characterized by compressed video such as Motion Picture Expert Group (MPEG) video. These compression schemes involve sending an initial frame containing all the data, and then sending updates to that frame. The updates contain only the information that has been changed since the initial frame was sent. CBR traffic is used for emulating circuit switching. An example application that can use CBR is voice traffic. ABR traffic is characterized by applications that are not as sensitive to delay as voice and video. Although the standard does not require the cell transfer delay and cell loss ratio to be guaranteed or minimized, it is desirable for switches to minimize the delay and loss as much as possible. For ABR traffic, a source is required to control its rate depending on the congestion state of the network. Most of today’s computer applications fall into this category. Applications that can use ABR include file transfer and electronic mail. According to these various traffic types, two congestion control strategies for ATM networks have been proposed in the ATM Forum: open-loop control and closed-loop control. In open-loop control, each connection’s usable bandwidth is limited by the notion of a traffic contract. Open- and Closed-Loop Control. To assign bandwidth to the connection, each end system has to declare its traffic parameters to the network at connection setup time. Once the connection request is admitted, its quality of service is guaranteed throughout the session. Because a lack of network resources may cause a newly requested connections to be rejected, open-loop control is sometimes referred to as preventive congestion control. This sort of congestion control scheme can be applied to CBR and VBR traffic. Open-loop control, however, is insufficient for data communications because each connection can never emit cells exceeding its negotiated rate, not even when there is unused bandwidth in the network. Furthermore, the bandwidth requirements for data traffic are not likely to be known at connection setup time. Instead, the cell transmission rate can be adjusted using a feedback information that indicates the congestion status of the network. These are the reasons that closed-loop rate control has been chosen for data traffic and is being applied to ABR service in the ATM Forum. Closed-loop control dynamically regulates the cell emission process of each connection by using feedback information from the network and, therefore, it is especially suitable for data transfer service. It can also be called reactive congestion control. The various types of traffic, their applications, and different congestion control strategies according to the specific traffic type are further illustrated in Exhibit 4-7-4. Exhibit 4-7-4.  Types of Traffic, Application, and Congestion Control Strategies 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. Please read our privacy policy for details.