 just small units of internally switchable traffic that the fabric can handle and manage properly and that these cells have absolutely nothing to do with the well-known ATM cells, either in nature and structure or in size. The queues of cells are then built up, which will need to be mapped onto and scheduled for delivery to the corre- sponding output ports that may very well lie on the same or on another line card. As instruments of advanced routing schemes that implement some sophisticated packet forward- ing, switch fabrics can handle different classes of traffic simultaneously. Consequently, multiple class- based queues (virtual queues, also known by various other names in the industry or in pieces of product literature) are being set up for scheduling at the input ports on the line cards. Some traffic classes will inevitably receive a higher priority than others based on the choice of one or more among multiple algorithms. The switch fabric calculates the desirable route and order of events that must occur on each packet. It then switches the corresponding cells to the correct output port. Virtual queues issue requests to the scheduler notifying it that they have received fresh traffic (from the packet dis- assembly unit) for switching. Based on its rotating priority decisions, the scheduling unit of the fab- ric issues back grants to the queues, SO everything is coordinated. 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 SWITCH FABRICS 281 Any potential congestion or contention will be managed in the following ways: In a different way depending on whether it is input congestion (contention for the switch fabric) or whether it is output congestion (more than one packets competing for the same output port at the same time). By an array of arbitration units that can be implemented by vendors on the line cards in a system- d