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 4-7A Survey of Congestion Control Schemes for ABR Services in ATM Networks HYUN MEE CHOIRONALD J. VETTER Broadband integrated services digital network (B-ISDN) efforts are driven by the emerging needs for high-speed communications and enabling technologies to support new integrated services. Among the available technologies, asynchronous transfer mode (ATM) has emerged as a standard for supporting B-ISDN. ATM uses short, fixed-size cells consisting of 48 bytes of payload and 5 bytes of header in order to transmit information. The fixed size allows fast processing of cells and reduces the variance of delay, making the network suitable for integrated traffic consisting of voice, video, and data. Providing the desired quality of service for these various traffic types is much more complex than what is done in data networks today. AVAILABLE BIT RATE (ABR) TRAFFIC MANAGEMENT The capability of ATM networks to provide large bandwidth and multiple quality of service (QoS) guarantees can be realized when equipped with effective traffic management mechanisms. Traffic management includes congestion control, cell admission control, and virtual path/virtual channel (VP/VC) routing. Proper traffic management helps ensure efficient and fair operation of the network in spite of constantly varying demand. This is particularly important for data traffic that has very little predictability and therefore cannot reserve resources in advance. Traffic management ensures that users will get their desired quality of service. However, it is difficult during periods of heavy load, especially if the traffic demands cannot be predicted in advance. This is why congestion control is the most essential aspect of traffic management. Mixing Low-Speed and High-Speed Networks Very high-speed networks using ATM pose a new set of challenges in congestion control. This results partly from the fact that the huge legacy of low-speed networks will continue to coexist with emerging high-speed links for quite some time. This heterogeneity of networks and the resulting mismatch of link speeds aggravate the congestion problem, which happens whenever the input rate is more than the available link capacity. More challenging issues arise in the congestion control of ATM networks for available bit rate (ABR) traffic—data traffic—because it cannot be predicted in advance. The objective of ABR service is to use the unused capacity of the network. Therefore, congestion control for ABR service must consider certain requirements: 1.  ABR traffic should never compromise the cell loss probability of constant bit rate (CBR) or variable bit rate (VBR) traffic. (CBR traffic is voice; VBR traffic is compressed video.) 2.  The network should never refuse a guaranteed traffic (CBR, VBR) connection because of the ABR traffic it is supporting. 3.  All users of the ABR service should have equal access to the bandwidth that is available. To satisfy these requirements for ABR traffic, several congestion control schemes have been proposed in the ATM Forum. However, each of these schemes cannot completely satisfy these requirements because of their own advantages and disadvantages. Therefore, more research is required to develop an efficient congestion control scheme for ATM networks using the ABR service class. ATM CONCEPTS AND PROTOCOL STRUCTURE To understand how congestion control operates within an ATM network, it is necessary tofirst review basic ATM concepts and protocol structure. ATM was chosen as the switching and multiplexing technique for B-ISDN in 1988. ATM is based on a fixed-size virtual connection-oriented packet or cell switching methodology. A cell consists of 5 bytes of header information and a 48-byte payload. ATM breaks all traffic into these 53-byte cells. The header contains control information such as identification, cell loss priority, routing, and switching information. Exhibit 4-7-1 illustrates the ATM cell structure. Exhibit 4-7-1.  ATM Cell Structure Each field in the ATM cell header defines the functionality of the ATM layer. The general flow control (GFC) field is used by the user network interface (UNI) to control the amount of traffic entering the network. The virtual channel identifier/virtual path identifier (VCI/VPI) fields are used for channel identification and simplification of the multiplexing process. The payload type indicator (PTI) field distinguishes between user cells and control cells. The cell loss priority (CLP) field indicates whether a cell may be discarded by a switch during periods of network congestion. In connection-oriented ATM networks, communication from higher layers is adapted to the lower ATM defined layers, which in turn pass the information on to the physical layer for transmission over a selected physical medium. The protocol reference model is divided into three layers: the ATM adaptation layer (AAL), the ATM layer, and the physical layer. The physical layer defines a transport method for ATM cells between two ATM entities. The ATM layer mainly performs switching and multiplexing functions. The AAL defines a set of service classes to fit the needs of different user requests and converts incoming user requests for services into ATM cells for transport. Exhibit 4-7-2 illustrates ATM protocol structure, which is based on standards by the ITU. Exhibit 4-7-2.  ATM Protocol Structure 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.