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 294 NETWORK PROCESSORS output port destination can be chosen for forwarding instead with the appropriate feedback signal. The result of such an arrangement is that no packet will be blocked at the input just because another one in front of it is waiting to be admitted into the fabric's shared memory. In addition to sorting per destination port, the output queues are also sorted per priority levels (tagged on the packet headers) and when an output queue is enabled for forwarding, the highest- priority packets will always be admitted into the shared memory. Likewise, even after a packet's admission to memory, the priority-sorted pointers at the output queues will ensure that the highest- priority packets will exit the shared memory first. To ensure that the four available priority levels, which are always applied preemptively onto traf- fic, do not abuse the weak position of low-priority packets, IBM has implemented an elaborate credit table scheme. This allows the buildup of sufficient credit (in one step per packet cycle and up to 256 steps) that ultimately overrides the preemptive and strict priority-based scheduling. This method ensures deterministically that less-fortunate links will at some point get a chance to transmit some of their traffic, despite the potentially simultaneous presence of consistently high- priority traffic. For more information regarding the technique that enables a situation where no links are starved because of other traffic priorities, along with a detailed description of the feedback mech- anisms that communicate to the VOQs any output congestion through backpressure, refer to Glaise's article mentioned previously. The C192 has a throughput of 16 Gbps, which operates just like the fabric itself at 60 percent over- speed. Therefore, it can be configured as one OC-192 port, one 10 Gigabit Ethernet, or four OC-4