bed earlier. The NPU is then connected through a standard interconnection or through an original equipment manu- facturer (OEM) proprietary scheme with a block of queuing logic that handles the interface with the switch fabric. Typical interfaces between the NPU and the switch fabric interface logic include CSIX, Universal Test and Operations PHY Interface for ATM (UTOPIA), System Packet Interface 4 (SPI-4), Network Processing Forum Streaming Interface (NPF-SI), and SO on. 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 276 NETWORK PROCESSORS The switch fabric card obviously contains the switching unit (also known as the crossconnect, which can be physically present in the form of a switch fabric chip or a set of complementary switch fabric chips). It also contains a control unit, which handles all scheduling and arbitration tasks. Sometimes the logical splitting of these two components places the scheduling control unit on the line card communicating with the crossconnect over the backplane. Scheduling is important in order to coordinate the traffic flow to and from the line cards as well as to and from the various stages of the switch fabric. A control CPU handles several tasks from error logging, activity monitoring, and configuration all the way to keeping track of operation parameters such as power supply voltage variation, temperature, and SO on. The switching unit provides connection paths from all its inputs to all its outputs. Through the switched backplane, every port on a line card can communicate with any port on the same card or on any other line card. Serdes stages ensure the appropriate modification of how information is pre- sented to and from specific submodules (serial or parallel). For instance, the backplane is always han- dling information in serial form, whereas framers on the line cards need to be able t