Handbook of Local Area Networks, 1998 Edition:Applications of LAN Technology 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 LAN TECHNOLOGIES FOR INTEGRATED VOICE AND VIDEO The LAN technologies in widespread use today—Ethernet, Fast Ethernet, FDDI and Token Ring—were not designed with the needs of real-time voice and video in mind. These LAN technologies provide “best effort” delivery of data packets, but offer no guarantees about how long delivery will take. Interactive real-time voice and video communications over the LAN require the delivery of a steady stream of packets with very low end-to-end delay, and this cannot generally be achieved with the current LAN technologies as they stand. Asynchronous Transfer Mode (ATM) At one time, there was a belief that ATM networking to the desktop would be embraced by LAN users to solve this problem. ATM is a networking technology that was designed specifically to handle a combination of the low-delay steady stream characteristics of voice and video and the bursty, intermittent characteristics of data communications. The ATM Forum, the industry body responsible for publishing ATM specifications, has developed a number of standards that enable desktops connected directly to ATM networks to support existing LAN data applications as well as voice telephony and videoconferencing. The ATM Forum standards for the support of voice and video over ATM to the desktop typically avoid the use of traditional LAN protocols such as IP, and instead place the voice or video streams directly over the ATM protocols. While it is clear that ATM to the desktop provides an elegant and effective solution for combining voice, video, and data over the LAN, this approach does imply a “forklift” to the LAN infrastructure and the end station connection. The cost and disruptive impact of such an upgrade tend to limit its appeal, and as a result desktop ATM is not expected to be widely adopted. However, the ability of ATM to provide “Quality of Service,”—that is, to deliver real-time voice or video streams with a guaranteed upper bound on delay—makes ATM an excellent choice for the LAN backbone where voice and video over the LAN is needed. Shared and Switched LANs It is generally accepted that shared LANs are unsuitable for handling real-time voice and video because of the widely varying delays that are seen when multiple stations are contending for access to the transmission medium. The CSMA/CD access method used in shared Ethernet is particularly poor in this respect. Token Ring, on the other hand, is based on a token-passing access method with multiple levels of priority. Stations waiting to send data packets can be pre-empted by other stations on the ring with higher priority voice or video packets to send. As a result, Token Ring has excellent potential to handle real-time voice and video traffic, though this potential has yet to be realized in currently available networking products. LAN switching does much to overcome the limitations of shared LANs, although today’s products are still a long way from providing an answer for voice and video over the LAN. It is now cost-effective to provide users with dedicated 10M-bps Ethernet connections to the desktop, and 100M-bps Fast Ethernet uplinks from the wiring closet to the backbone. However, despite the vast increase in bandwidth provision per user that this represents over and above a shared LAN scenario, there is still contention in the network leading to unacceptable delay characteristics. For example, multiple users connected to the switch may demand file transfers from several servers connected via 100M-bps Fast Ethernet to the backbone. Each server may send a burst of packets that temporarily overwhelms the Fast Ethernet uplink to the wiring closet. A queue will form in the backbone switch that is driving this link, and any voice or video packets being sent to the same wiring closet will have to wait their turn behind the data packets in this queue. The resultant delays will compromise the perceived quality of the voice or video transmission. 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.