put forms the wider basis of space switching. If the input lines to a crosspoint switch happen to be TDM multiplexed, then different signals (meaning the content of different slots) may or may not need to be switched to the same output line. Vertical input Horizontal output Horizontal input Vertical Output FIGURE 14.2 Crosspoint switch and signal flow configuration. 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 268 NETWORK PROCESSORS Crosspoint switch states Bar state Cross state FIGURE 14.3 (a) The two possible states of a crosspoint switch and (b) a generalized crossbar switch with multiple inputs and outputs. A scheduler is therefore needed in the switching system. It will be responsible for switching on the appropriate crosspoints at the appropriate time slots, SO that the same input can be reliably connected to different outputs at different moments. The architect must then decide where and how to imple- ment such a scheduler. Despite its simplicity and functional appeal, an NXN crosspoint switch has some obvious short- comings. More specifically, it requires N2 elements, which can be prohibitively expensive for large switches. In integrated form, however, inside modern switch fabrics, state-of-the-art microelectronic design and semiconductor manufacturing processes allow the integration of hundreds of millions of transistors, SO this is not the obstacle per se. Ancillary circuitry must also be embedded inside the same die to handle buffering, scheduling, and arbitration and often to provide serdes functionality to con- nect with the rest of the world. The combination of all these integrated blocks shows why switch fab- rics appear in the form of complete chipsets. Most importantly, a potential failure at one crosspoint junction ensures that the specific output to which it connects is effectively isolated from the corre