 ordering them based on some priority schemes. First, we will mention that the various scheduling algorithms are classified into two broad cate- gories: work-conserving schedulers and non-work-conserving schedulers (see for example, a nice Motorola text at http://e-www.motorola.com/collatoral/SNDF2002_N302.pdf). The distinction is straightforward, although the need for it may seem not obvious. A work-con- serving scheduler is active when there are packets at hand, which are in need of its attention. The corollary is that when there is no traffic present that requires service, the work-conserving scheduler is not active. Conversely, a non-work-conserving scheduler may be inactive even if traffic is present which requires its assistance. Although it is counterintuitive, this is a highly desired behavior as it ultimately enables the deliberate instigation of some sense of predictability in the downstream traf- fic. This is accomplished by a decrease in the size of needed output queue buffers as well as in the delay jitter characterizing specific links. The reader may have noted that the function of the traffic shaping shown earlier is an example of scheduling done by a non-work-conserving method. Now we turn our attention to schedulers. A number of different scheduling algorithms are used in network processing and it is not surprising that they are used in several areas, such as inside switch 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. TRAFFIC MANAGERS TRAFFIC MANAGERS fabrics and in traffic managers. We will not discuss the internals or the merits of these algorithms, which are thoroughly covered by other more specialized sources3 listed in the references. Looking at data sheets of traffic manager chips requires a basic awareness of the most common algorithms, at least by name. The priority queue (PQ) or strict priority (SP) scheduler is bas