technology are thermal sensitivity, slower switching speeds, and limited scalability. Thermal sensitivity can be overcome with temperature controllers, better design, and improved liquid crystal fluid, but scalability is always an issue because of cost. 2.9.7 Optical Switching Optical switching is the enabling technology that makes ION possible by enabling the direct switching of constituent wavelengths, whether intrinsic to the network or customer originated. Optical switching eliminates O-E-O conversions. Figure 2-54 depicts a network application example. Like any other switching system, optical switches comprise a switching matrix and a switch control function. (We'll look at the control function again in Chap- ters 7 and 8.) Optical-switching matrices in the context of metro networking need to fulfill certain requirements. They must have proper optical characteristics such as low insertion loss, high isolation, and low back reflection. In addi- tion, they need to be highly reliable, fast, compact, low power consuming, and scalable. It is also important that the resulting switch is integrated into an end-to-end management system. Techniques that are suitable for opti- cal-switching matrices include but are not restricted to MEMS, some Chapter 2 City C Figure 2-54 Mux/demux + switching Network of optical switches. City A Mux/demux + switching Mux/demux + switching Optical switches City D Mux/demux + switching City B Thermo-optic switch Acousto-optic switch Liquid crystal switch Electro-optic (LiNb03) 2D MEMs Opto-mechanical Bubble jet MEMS-like technologies (thermo-capillary and piezo-electric), liquid crys- tals (LXT), bubbles, thermo- and acousto-optics, and holograms. These technologies offer different characteristics, which are summarized in Table 2-11. ¹ MEMS usually refers to arrays of tiny tilting mirrors sculpted from semi- conductor materials such as silicon. In 3D arrays, the mirrors can be tilted in any direction. 3D MEMS arrays can be used for large-scale OXCs, par- ticularly if groups of wavelength