he two classes of traffic. If the buffer queue size of Total is exceeded, all traffic will be dropped simply because buffers exhibit overflow. The EPD and PPD algorithms are used in connection-oriented ATM networks, and their principle of operation is similar to the RED algorithm. The main difference between the two is that PPD will discard a complete frame from buffers. This frame may be composed of multiple cells, which will all be dropped. EPD will only discard the cells that are buffered in excess of a crossed threshold level. For example, when TCP/IP runs over ATM, with PPD, once a switch on the linking path between two communicating parties drops an AAL5 cell, all subsequent AAL5 cells belonging on the same virtual 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 Probability of discarding a packet randomly tail-dropping region Maximum discard probability Graduated Graduated dropping dropping region for ("Out") region for non-prioritized ("In") traffic prioritized traffic Total Queue-Fill Percentage FIGURE 15.3 The RIO curve approach as a discarding mechanism (an example with two classes of traffic) circuit (VC) will also be dropped until the switch sees the end of the AAL packet. PPD helps the over- all throughput. However, all cells that happened to be transmitted before the specific dropped cell would definitely waste bandwidth. With EPD, the idea is that the switch drops every cell belonging to the same IP datagram after its port buffers exceed some predefined safe threshold, but before they actually overflow. In other words, an IP datagram is dropped in its entirety, thereby preventing wasted bandwidth from the transmission of dead cells as would be the case with PPD.2 Congestion avoidance and management involves a wide area of activities that monitor the avail- able resources and match them to spec