(QoS) to specific users or classes of users. Some switches are also required to decide whether they will accept an incoming call (especially in the connection-oriented case). This is handled by the switch's ability to block calls. A packet switch usually does not experience this type of call blocking (also known as admission control). However, in more recent environments, the packet switch has the equivalent of a circuit-switch's admission con- trol mechanism. It appears in the form of a service level agreement (SLA), which may be set up admin- 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. SWITCH FABRICS 266 NETWORK PROCESSORS istratively and is strictly policed and enforced by the hardware. However, buffers may overflow at the input or output when more packets are stored than available buffers can preserve. Newly incoming packets will simply be discarded or dropped as no more space is available to buffer them. Packet loss is a highly undesirable situation that designers strive to minimize. If we assume that all paths from inputs to outputs are simultaneously active, the switch has attained its theoretical capacity (or bandwidth). In practice, this is rarely the case, as only some of the possible paths are simultaneously active. Nevertheless, one of the switch designer's objectives is to maximize the available switching bandwidth for a specified budget and level of reliability. A typical switching fabric can be visualized as a combination of input and output buffers, some port-mapping mechanism for packetized switches and the core, which is also known as the crossbar. When we dive into actual switch fabric chipsets later in this chapter, we will notice that these logical subcomponents are in a hierarchy of system complexity even inside commercial off-the-shelf switch fabric chipsets. Figure 14.1 shows a simple example of the basic concept.