ransport requirements. To the well-known demands of voice, we now add various flavors of data, video, MP3, an exponentially large variety of IP traffic, interactive real-time gaming, and a variety of other content types that place increasing demands on the network. Further, we have seen a requirement arise for a mechanism that can transparently and correctly transport the mix of various traffic types over a single net- work infrastructure, while at the same time delivering granular, control- lable, and measurable QoS levels for each service type. In its original form, ATM was designed to do exactly that, working with SONET or SDH to deliver high-speed transport and switching throughout the network- in the wide area, the metropolitan area, the campus environment, and the LAN, right down to the desktop-seamlessly, accurately, and fast. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Transport Technologies Chapter 7 Figure 7-6 Header Payload The ATM cell 5 octets 48 octets Today, because of competition from such technologies as QoS-aware IP transport, proprietary high-speed mesh networks, and Fast and Gigabit Ethernet, ATM, for the most part, has lost the race to the desktop. ATM is a cell-based technology, which simply means that the fundamental unit of transport- a frame of data, if you will-is of a fixed size; this allows switch designers to build faster, simpler devices, since they can always count on their switched payload being the same size at all times. That cell comprises a five-octet header and a 48-octet payload field, as shown in Figure 7-6. The payload contains user data; the header contains information that the network requires to both transport the payload cor- rectly and ensure proper quality of service levels for the payload. ATM accomplishes this task well, but at a cost. The five-octet header makes up nearly 10 percent of the